]B3B^BQ^^^^^^^^^E3E I Marine Biological Laboratory Library I ffl Woods Hole, Mass. H HI [0 (D ^---^ [0 li Presented by ]] H^ The estate of Dr. Herbert W, | ^^ Rand UJ I Jan, 9, 1964 B ID ID D) Dl 0] ra 83 B CHORDATE ANATOMY ^A(ecii AND 1^/n/ CHORDATE ANATOMY BY HERBERT V. NEAL PROFESSOR OF ZOOLOGY, TUFTS COI.LFX.E AND HERBERT W. RAND ASSOCIATE PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY WITH 378 ILLUSTRATIONS LIBRARY ^^\ MASS. XX^j PHILADELPHIA P. BLAKISTON'S SON & CO., Inc. 1012 WALNUT STREET Copyright, 1939, by P. Blakiston's Son & Co., Inc. PRINTED IN U. S. A. BY THE MAPLE PRESS COMPANY, YORK, PA. PREFACE Comparative anatomy has both practical and theoretical interest. Courses in it based, like other science courses, upon laboratory work give the student first-hand acquaintance with the structure of fishes, amphi- bians, reptiles, birds and mammals. If, as psychologists assert, all our ideas derive from sense impressions, all real knowledge of animal structure must be based upon such laboratory experience. "Starve the senses and you starve the soul." In the laboratory, as the student increases his acquaintance with animals, he gains also in resourcefulness and inde- pendence. Many biologists believe that comparative anatomy affords the best approach to the understanding of human structure and function. In many American colleges and universities the course in comparative anatomy is a prerequisite to advanced courses in histology, embryology and physiology. In this way the student passes in his analysis from the general to the particular, from the gross to the microscopic. To many persons, however, the theoretical interest of comparative anatomy makes stronger appeal than does practical familiarity with animals. The major problem which faces the student of comparative anatomy is that of the genesis of the human body. The clue which gives meaning to many of the details of anatomy is found in the evolution theory. Most teachers of biology are so convinced of the truth of that theory that the issue is no longer debated by them. Each generation, however, must examine for itself the evidence which has led to the general acceptance of evolution by experts. While the record of the rocks is probably the most convincing evidence of evolution, the facts of com- parative anatomy greatly strengthen the case for evolution. As a result of the researches of several generations of comparative anatomists, it is now possible to sketch in fairly firm outlines the hypothetical past history of the human body. One of the purposes of this book is to summarize some of this evidence. To meet the laboratory needs of students, a number of excellent labora- tory guides have been written, some dealing with the dissection of a single animal, others with several animals. Most of them, however, make no attempt to deal with animals comparatively. They let the student make his own comparisons, which is possibly more than may be expected of him. The present book is intended to help him in these comparisons and to correlate and interpret the facts gathered in the laboratory. The VI PREFACE material presented has been selected for the light it throws on the phylo- genesis (racial history) and the physiology of the human body. The present text has been written to meet the needs of students in semester courses in comparative anatomy. It is an abbreviation of the Comparative Anatomy written by the same authors and published in 1936. In the condensation, however, no material essential to the proper understanding of the physiology and phylogenesis of the human body has been omitted. Due acknowledgment of assistance in the preparation of the book has been made in the earlier work, to which the reader is referred. LIBRARY CONTENTS Page Preface v 1. The Animal Kingdom i The Linnaean System 2 The Animal Phyla 3 Summary of Classification 22 Sequence of Organisms in Geologic Time 25 2. Reproduction 26 The Germinal Bodies 26 Oviparity, Viviparity, Impregnation 30 Protection, Nutrition, and Respiration during Development. 31 Evolutionary Significance 3^ Development 3^ Cleavage and Blastula 3^ Gastrula 43 Third Layer, Mesoderm 49 Early Development in Placental Mammals 55 Organogenesis 5^ Organogenesis in Amphioxus 58 Organogenesis in Vertebrates 62 Relation of Yolk to Organogenesis 78 Embryonic and Fetal Membranes 80 3. Histology 85 Epithelial Tissues 86 Glands 91 Non-EpitheUal Tissues 93 Muscular Tissue 93 Nervous Tissue 96 Tissues Serving for Mechanical Support 100 Adipose Tissue 106 Blood 106 Histological Specificity 108 4. The Integumentary System 1 1 1 Evolution of the Skin m Structure of the Human Skin 113 vii ^^".^7 Vlll CONTENTS Page Development of the Skin 114 Finger-prints and Their Meaning 115 Appendages of the Integument 116 Horny Scales 117 Horns 118 Nails, Claws, and Hoofs 118 Feathers 119 Hairs 120 Pigment 123 Cutaneous Glands 125 5. Teeth 129 Evolution of Teeth 129 Evolution of Compound Teeth 133 Teeth of Mammals 134 Teeth of Man 139 Development of Teeth 141 6. The Skeletal System 145 The Axial Skeleton 146 The Appendicular Skeleton 177 Evolution of Paired Appendages 177 Summary of Skeletal Evolution . ■ 182 The Appendicular Skeleton in Man 186 Development of the Appendicular Skeleton 187 7. The Muscular System 191 Evolution of the Muscular System 193 Muscles in Man 204 Development of the Muscles 205 8. The Digestive System 215 Evolution of the Digestive System 215 The Human Digestive System 217 9. The Respiratory System 245 Introduction 245 The Branchial System 246 The Pulmonary System 253 10. The Vascular System 262 Evolution of the Blood Vessels 262 Evolution of the Heart 275 CONTENTS IX Page Evolution of the Aortic Arches 275 Evolution of Arteries 277 Evolution of Veins 277 Evolution of Lymphatics 278 Development of the Heart 279 Development of the Aortic Arches 282 The Heart in Man 283 Pulmonary Circulation in Man 288 Systemic Circulation in Man 288 The Lymphatic System in Man 288 11. The Urogenital System 290 Evolution of the Urogenital System 290 The Urogenital System of Man 304 Development of the Urogenital System 315 12. The Endocrinal Organs 324 The Pancreas 325 The Gonads 326 The Suprarenal Glands 327 The Thyroid Gland 329 The Parathyroid Glands 33^ The Ultimobranchial Bodies 333 The Thymus 333 The Pituitary Gland 334 13. The Nervous System 33^ The Elements of the Nervous System 338 The Organization of the Nervous System 345 Nervous System of Chordates 34^ Evolution of the Brain 362 Evolution of the Spinal Cord 3^7 Evolution of the Peripheral Nervous System 372 Evolution of the Cranial Nerves 374 Evolution of the Spinal Nerves 37^ The Autonomic Nervous System 377 Evolution of the Autonomic System 382 Development of the Brain 3^3 Development of the Spinal Cord 3^6 Development of Motor Nerves 3^^ Meninges 39 1 X CONTENTS Page 14. The Sense Organs 393 Evolution and Development of Sense Cells 393 Cutaneous Senses 395 Lateral Line Organs 397 Olfactory Organs 399 Taste Organs 404 Visual Organs 406 Static and Auditory Organs 41 S Muscle Spindles 426 15. The Ancestry of the Vertebrates 427 Glossary 433 Index 457 CHORDATE ANATOMY CHAPTER I THE ANIMAL KINGDOM Since some of the so-called lower animals, living or extinct, more or less resemble hypothetical ancestors of man, some knowledge of them is necessary for a proper understanding of the history of the human body. Moreover, certain highly complex and obscure organs of man are most easily understood in the light of the simpler conditions of lower forms. Even the plants, so unHke us in outward appearance, contribute something to our knowledge of ourselves. But the organic world is so enormously complex that no human mind can carry its detail adequately without some system by which facts are classified and summarized. Most useful of such systems are those based on natural relations which, therefore, exhibit the course of evolution of each species, and place it correctly in an evolutionary scheme. For evolution, nowadays, is the key to all genetic animal relationships. Such an evolutionary scheme begins by dividing all living things into plants and animals. Plants are creatures which contain chlorophyl, and therefore can produce or make their food directly out of inorganic mate- rials, or else they are, obviously, such creatures as have lost their chloro- phyl and adopted the feeding habits of the simpler animals. Animals may or may not have descended from plants; only rarely do they contain chlorophyl, hence all their structure and habits rest on other means of obtaining food. There are, however, many simple organisms, for example, the slime molds, which are as much one as the other, plants or animals indifferently. Even some of the higher plants, like the Venus's fly-trap, catch and devour insects; and also some of the unicellular algae feed like animals. . The animal kingdom as a whole is commonly divided into about a dozen phyla, the precise number and the precise definitions of which have not yet been agreed upon by taxonomists. These phyla, in turn, are spht into classes, the classes into orders, the orders into genera, and the genera into species. It is sometimes convenient also to recognize sub-orders and sub-classes, and to combine similar genera into families. Scientific naming is by genera and species, a scheme devised by the great naturaUst Linnaeus, or Linne, about the middle of the eighteenth I CHORDATE ANATOMY rentury, and called the Linnaean, or binomial, system. Thus all birches are called by their Latin name Betula, and that is their genus. White, 4AMMALS^ 17000 SPJ BIRDS 20.000 SP. INSECTS 500. 000 SP. AMPHIBIANS^ 180*0 SR :ptii iooosp; IRUSTACEAl 8000 SR Varachnidsi »,5ooosp; annelids 4boosf R0TIFERS1 850^ THREADWORMS, 1600 SR MOLLUSCS. 61.000 SR uroch6rds I40p SB |MOL^~^ LUSCOIDS l>00S /HE Ml /CHORDS FISHES. J2.000SR CYCLOSTOMES CEPHALOCHORDS 22 SR J. -SP^ECHINODERMS. 10,000 SR, FLAi;WORMS^ 4500 SR SPONGE 250 OSE (COELENTERATES V7000 SP.y >ROTOZOA] 10,000 SP.. A PHYLOGENETIC TREE OF THE ANIMAL KINGDOM. Fig. -A phylogenetic tree of the animal kingdom, showing the dichotomy animals into Prnterostomians and Deuterostomians. yellow, and black birches are therefore respectively, as species, Betula alba, Betula lutea, and Betula nigra; but the American white birch is Betula papjrrifera, that is, the paper birch; and the common gray birch THE ANIMAL KINCiDOM 3 is populifolia, because it has leaves that twinkle in the wind like those of a poplar tree. The common cat is Felis domestica; the lion, Felis leo; the tiger, Felis tigris; and there are, in all, some forty species more in the genus Felis. Linne called us Homo sapiens. We belong to the family Homi- nidae (of which we are the only living species), to the order Primates, the class Mammalia, the phylum Chordata, and the animal kingdom. In a general way, for the larger and more famiHar animals and plants, the vernacular name, such as pine or elephant, refers to the genus. On the basis of the number of cells in the body animals are divided into the two Sub-Kingdoms — (i) the unicellular Protozoa, (2) multicellular Metazoa. Since the Protozoa are the simpler organisms, it may be assumed that the first animals on earth were protozoans. The division of animals into the two phylogenetic series graphically represented in Fig. i is based upon differences in the fate of the embryonic mouth or "blastopore." The left-hand branch includes the Protero- stomians, which are the Metazoa in which the blastopore becomes the mouth or lies near the adult mouth. Most animal phyla are Protero- stomians, which include such diverse forms as the Porifera, Coelenterates, Platyhelminths, Molluscoids, Rotifers, Annelids, Molluscs and Arthro- pods. This branch of the animal kingdom reaches its climax in the arthropods and molluscs. The Deuterostomians are the animals in which the blastopore becomes the anus or lies near the anus. The group includes the Echinoderms and the Chordates. The branch reaches its cUmax in the vertebrates and man. Since our present interest centers in man and chordates, and since none of the non-chordate phyla are believed to lie in the direct line of human ancestry, the non-chordate phyla will be mentioned only when they possess structures resembUng those of the hypothetical ancestors of man. It may be assumed that students in a comparative anatomy course have some acquaintance with non-chordates. Consequently no detailed description of them is needed here. We may therefore turn our attention directly to the sub-phyla and classes of chordates. Phylum CHORDATA The chordates are animals which, at least early in life, have a supporting rod, the notochord or chorda dorsalis, between the alimentary canal and the central nervous system. In higher chordates the notochord is replaced during ontogenesis by a cartilaginous or bony vertebral column. All have a dorsal tubular nervous system. The heart is ventral, and the pharynx has functional or embryonic gill sUts. Most chordates are metameric in structure, although the metamerism may become greatly obscured in the adult. Segmental excretory organs are generally present. CHORDATE ANATOMY Nearly 50,000 species are known. Four sub-phyla are included in the phylum — Hemichorda, Urochorda, Cephalochorda, and Vertebrata. Sub-Phylum Hemichorda (Enteropneusta) The hemichordates or Enteropneusta hold a somewhat uncertain posi- tion in the animal kingdom. Morphologists are by no means agreed that their closest affinities are with the chordates. Some associate them with PENING OF PERIBRANCH.CAV. /ANUS DORS. NERVE CX«D /NOTOCHORD IBmnniiiiimiiiiiiiiiiiiiiiiimimrniiiiiiiiiniiMMiiririiiiiTiiiflfiininm A. LARVAL UROCHORDATE GILL SLfTS' /DORSAL NERVE ANUS POST OPENING OF PERIBRANCHIAL CAVITY B CEPHALOCHORDATE .DORS. NERVE CORD KCILL SUT5 'HEART LIVER *> INTESTINE ANUSi C. VERTEBRATE (CYCLDSTOME) Fig. 2. — Diagrams of A, larval urochordate, B, cephalochordate (Amphioxus), and C, vertebrate (Petromyzon), illustrating the fundamental characteristics of chordates; redrawn after Hesse- Doflein. the annelids, while the resemblance of their larval stage to that of echino- derms leads others to place them near that group. Their inclusion among the chordates rests on their possession of pharyngeal gill-sUts, enteric coelomic pouches, a notochord-Uke diverticulum of the fore-gut in the pre- oral lobe, and upon the relations of the blood-vessels and nerves. Seg- mental excretory organs are, however, absent. There are possibly 50 species. BALANOGLOSSUS, the best-known genus, may be taken as representa- tive. The body of Balanoglossus is worm-Hke, and is divided into five regions, proboscis, collar, gill region, "liver" region, and intestinal region. THE ANIMAL KINGDOM The proboscis is a hollow muscular organ with an opening, a pore, on the left dorsal side of the neck. The mouth lies on the ventral side between the proboscis and the collar. The collar, like the proboscis, contains a division of the coelom, which opens to the exterior by a pair of pores near the mid-dorsal line. Like the proboscis, the collar also is muscular, and used by the organism as a means of burrowing in the sand where it lives. PROBOSCIS COLLAR BALANOGLOSSUS. MOUTH gills PiG_ 3_ — Balanoglossus, a typical genus of the sub-phylum Hemichorda. (Redrawn after Bateson.) The pharynx is divided into a dorsal portion which contains the numer- ous gill-apertures and a ventral portion which functions as the digestive passage of the pharynx. Posterior to the pharynx, the body contains a series of gonadic sacs, each of which has a pore-like opening to the exterior. The sexes are separate. In the so-called liver region, the intestine shows a series of paired diverticula, each of which produces a corresponding bulging of the rela- LONG bUSCLE OF PROBOSCIS BLOOD VESSEL Fig. 4. — Balanoglossus, the typical genus of the hemichordates, seen in left lateral aspect. The possession of both dorsal and ventral nerve-cords links hemichordates on the one hand with invertebrates and on the other with vertebrates. (Redrawn after Stempell.) For the purposes of the diagram, the body of the animal is bent upon itself. tively thin body-wall. These diverticula are glandular and supposed to have a digestive function, hence their name. Behind the liver-region, the intestine passes without convolution directly to the posteriorly situated anus. The circulatory system resembles that of annelids, but is supplemented by a lacunar system of lymph spaces. The nervous system consists of dorsal and ventral nerve strands containing occasional giant nerve cells. There are no special sense organs. CHORDATE ANATOMY APICAL PLATE GILL SLITS^ P/IC (ANT ENDO. DIVERTICULA) COLLAR CAVITIES NOTOCHORD PROBOSCIS IRCULAR BAND OF CILIA ^TRUNK CAVITIES Fig. 5. — Balanoglossus embryos. A. A horizontal section of a young embryo, showing the origin of mesodermal pouches. McBride and others have noted the similarity of this section to that of a young Amphioxus embryo as evidence of the close affinity of these two forms. B. A young Balanoglossus larva with five pairs of gill-slits, viewed from the left side. The gill-slits of Balanoglossus bear a striking resemblance to those of Amphioxus. On the other hand the young larva of Balanoglossus is strik- ingly like the larva of echinoderms. (Redrawn after Bateson.) .GILL-SLIT A ESOPHAGEAL REGION - X.& B. HARRIMANIA - STEREOGRAM OF COLLAR REGION. Fig. 6. — Harrimania, a hemichordate. A. The dorsal portion of a cross section of Harrimania in the region of the esophag\is. The resemblance of this cross section to one of Amphioxus is striking and serves to demonstrate the close genetic affinities of these two chordates. In Harrimania the notochord is present not only in the preoral lobe as in other hemichordates but also in the collar and anterior pharyngeal regions. B. A stereogram of Harrimania in the collar and anterior pharyngeal region, showing the presence of the notochord in these regions. Such evidence tends to remove the doubt that a true notochord exists in hemichordates. (Redrawn after Ritter.) PROBOSCIS PORE ARMSr NERVE STRANO .GONAD 'PHARYNX IPHARYGEAL POUCH - STALK >^-Ji^ I 1 ; *^ " notochord' MOUTH' CEPHALOOISCUS. Fig. 7. — A diagram of Cephalodiscus viewed from the left side as if in median optical section. The presence of a notochord in the pre-oral lobe is one of the reasons for placing this animal among hemichordates. While not regarded as a form " ancestral to vertebrates, Cephalodiscus interests morphologists as a primitive chordate. (Redrawn after W. Patten.) THE ANIMAL KINGDOM 7 The so-called nolo(ln)r(l is a diverticulum of the intestine which extends with a narrow lumen into the proboscis from a point just behind the mouth. The larva of Balanoglossus, known as Tornaria, shows rather striking resemblances to the larva of echinoderms. As in echinoderms, the blastopore becomes the anus. The sub-phylum, therefore, is included in the group of Deuterostomia. Cephalodiscus and Rhabdopleura are genera which show resemblances to Balanoglossus but which have a U-shaped alimentary canal. Rhabdo- pleura is without gill apertures. Sub-Phylum Urochorda (Tunicata) The urochordates, the tunicates or sea-squirts, are so named because the notochord, absent in the sessile adult, is always limited to the tail TENTACUES CIUATED GBOOVt SUBNEURAL GLAND, BRAIN ^AMlFl, NERVE CORD ESOPHAGEAL ASCIDIA-A UROCHORDATE Fig. 8. — Ascidia, a urochordate. The animal is viewed as if cut in median longitudinal section and as seen from the right side. (Redrawn from Sewertzoff, after Boas.) region. Another character common to the group is the presence of a tunicin mantle which is secreted by the skin. Tunicin is a chemical substance that resembles cellulose. A coelom is sometimes present, but is limited to the region of the ventral heart. Nephridia or coelomoducts are wanting. The body is unsegmented, and the alimentary canal is bent on itself so that the anus lies near the mouth. The pharynx is perforated by gill-slits, the number of which varies greatly in the different species. The nervous system consists of a nerve ganglion dorsal to the pharynx, from which nerves extend to the various organs. In some forms both sexual and asexual methods of reproduction occur. Individuals, however, are usually hermaphroditic. Development generally involves metamor- 8 CHORDATE ANATOMY phosis. The sexually-produced tailed larva bears certain striking resem- blances to the larva of Amphioxus. Some systematists recognize 1400 species. ClONA is a sessile tunicate, three or four inches in length, which is attached by tunicin stolons to its substratum. A tunicin test or tunic, which is secreted by the skin, encloses the entire animal as a sac. Beneath the test and loosely connected with it, except in the region of the two apertures of the body, lies the body-wall or mantle. This consists of an external simple epithelial ectoderm, and, beneath this, connective tissue containing a network of muscle fibers which are more abundant in the BRAIN CILIATED FVJNNEC MOUTH GILL SLITS ■ATRIAL CAVITY/ SPINAL CORD \notochord intestine \ENDOsrYLE "^"^ B. METAMORPHOSIS. Fig. 9. — Diagrams of stages in the metamorphosis of a uruchordate larva. When the larva settles down and becomes fixed by its adhesive papillae, the tail is lost and the notochord disappears. Thus the chordate characters which are so evident in the larva are partly lost in the mature organism. (Redrawn from Korscheldt and Heider, after Seeliger.) region of the two apertures of the body, which they serve to close and open. Of the two external apertures, the more ventral is the inhalent or oral siphon and the other the exhalent or atrial siphon. The former leads directly to the mouth, which is surrounded by a circle of tentacles. The mouth leads into a greatly enlarged pharynx, which is perforated by numerous gill-slits or stigmata. The action of the cilia on the bars of these slits serves to maintain a current of water from the pharynx into the surrounding peribranchial or atrial cavity. Such relations resemble those of similar organs in Amphioxus. In the floor of the pharynx extends a longitudinal groove, the endostyle, which morphologists generally homologize with the thyroid gland of vertebrates. A somewhat similar groove extends also along the dorsal side of the pharynx. The alimentary THE ANIMAL KINGDOM 9 canal consists of a short esophagus, a spherical stomach, and an intestine which leads to an anus situated well forward in the atrial chamber. The heart lies ventral to the esophagus in the pericardial chamber. There are no closed blood-vessels, but the blood is pumped from the heart forward to the pharynx in lacunar spaces the relations of which resemble those of the afferent branchial vessels of vertebrates. The reproductive organs He in the loop of the intestine, posterior to the stomach. Their ducts extend forward and open into the atrial cavity near the anus. The gonads are hermaphroditic. The nervous system consists of a ganglion or brain, which lies in the body-wall between the two apertures of the body. Ventral to the brain is a neural gland which has been compared with the neural part of the pituitary gland of vertebrates. The unpaired eye and static organ con- tained in the brain vesicle of the larva degenerate in the metamorphosis. STATIC ORGANx ENOOOERM STRAND PHARYNX'' CILL SLITS OCELLUSf l^f^r Fig. 10.— Diagram of a larval urochordate. The similarity of the larval uro- chordate to the embryo of a cephalochordate (Amphioxus) suggests that a form like this lies near the main line of vertebrate ancestry. (Redrawn after von Beneden and Julin modified.) Ciona during its ontogenesis undergoes a striking metamorphosis, which indicates that the animal is a degenerate descendant of a primitive branch of the chordate tree. Of the four orders of urochordates the Larvacea are of special interest since they develop without metamorphosis, and hence show no sign of degeneration. Their caudal appendage contains a notochord and spinal cord. That they lie close to the main line of vertebrate ancestry seems not unlikely. Sub-Phylum Cephalochorda (Acrania) The cephalochordates are those chordates in which the notochord occurs not only in the head, as in Hemichorda, or in the tail, as in the Urochorda, but throughout the entire length of the body. The group is sometimes called the Acrania because, as the name suggests, a brain case is lacking. Metamerism is strikingly manifested in the muscles and nerves, which form an unbroken series from the tip of the snout to the tip of the tail. Segmental protonephridia are metamerically arranged, but lO CIIORDATE ANATOMY are limited to the gill region. As in urochordates, the gills open into a peribranchial cavity. Development involves metamorphosis. There are possibly 25 species. AMPHIOXUS. The lancelet, Amphioxus, the characteristic genus of the group and the so-called connecting link between vertebrates and invertebrates, interests morphologists because of its resemblance to the hypothetical ancestor of vertebrates. Its resemblance to the larva of cyclostomes is impressive. (Fig. 11) The Amphioxus is a lance-shaped animal, not more than two inches long, with a laterally compressed body and a median caudal fin. Its external orifices are an anterior ventrally placed mouth, an anus to the left of the caudal fin, and an atriopore somewhat behind the middle of the CIRRI ORAL HOOO! Fig. II. — Amphioxus, in ventral and side views. Metamerism, lacking in uro- chordates, and scarcely evident in hemichordates, is strikingly shown by Amphioxus. Whether this metamerism is inherited from annelid-like ancestors or is a convergent trait independently acquired, is a moot question in morphology. (Redrawn after Kirkaldy.) body. The atrial chamber which surrounds the elongated pharynx is formed by the union of paired lateral folds which meet in the mid-ventral line of the body. Such a structure seems to be an adaptation to the sand-burrowing habit of the adult animal. The atrial cavity ends blindly in front, and opens externally by the atriopore just behind the pharynx. In the region of the pharynx, a pair of ventro-lateral metapleural folds extend as far back as the atriopore. The body is covered by a thin external cuticula secreted by the simple epithelial epidermis. Beneath the skin and visible through it are sixty pairs of myotomes which alternate with one another along the two sides of the body. As in the vertebrates generally, these myotomes are greatly thickened along the dorsal side of the body. Each myotome is V-shaped with the apex of the V pointed forward. The mouth, surrounded by a circle of tentacles, leads directly into the elongated pharynx, the walls of which are perforated by numerous THE ANIMAL KINGDOM II gill-slits. A ciliated groove, which is similar in function and in relations to the endostyle of urochords, extends the entire length of the pharynx. NERVE CORD^. NOTOCHORO- -DORSAL ARTERY.~. , PHARYNX r-^ METAPLEURAL FOLD. , NERVE CORD PERIBRANCHIAL CAVITY GILL APERTURE ! VENTRAL ARTERY. GILL APERTURE VENTRAL ARTERY< DIGESTIVE GROOVE. A. AMPHIOXUS. B.PTYCHODERA. Fig. 12. — Diagrams illustrating divergent methods by which the peribranchial cavity is formed. In Amphioxus (A) the pleural folds are separated from the pharynx by paired folds which extend dorsally from the ventral side. In the hemichordate Ptychodera (B), on the other hand, the paired folds begin to form at the dorsal side of the worm and extend ventrally. The peribranchial cavity in urochordates arises in a similar manner. As frequently happens in animals, a similar end-result is attained by divergent means. (Redrawn after Gaskell.) LATERAL TRUNK MUSCLES ■SPINAL CORD NOTOCHORD DORSAL AORTA- PRECARDINALV EPIBRANCHIALGROOVE GILL LAMELLAE CARTILAGE BAR GILL-RODS ge?~;^::j — TRANSVERSE MUSCLE •VENTRAL AORTA )JS 'PERIBRANCHIAL CAVITY HYPOBRANCHIAL MUSCLE METAPLEURAL FOLD A. AMPHIOXUS B AMMOCOETES. Fig. 13. — Cross sections of A, Amphioxus and B, Ammocoetes (larval Petromyzon) through the pharyngeal region showing their fundamental resemblance. Opposite it, in the roof of the pharynx, is a somewhat similar epipharyn- geal groove. The liver is a hollow tubular sac, which opens into the 12 CHORDATE ANATOMY floor of the intestine just behind the pharynx and extends forward to the left below the pharynx. The intestine is straight. The coelom, considerably reduced in size in the region of the pharynx, extends posteriorly to the region of the anus. Ninety pairs of nephridia, limited to the gill-region, open into the atrial cavity. The solenocytes attached to the nephridia are specialized excretory cells which strikingly resemble those of annelids. Nephrostomes are absent. (Fig. 263) Sexes are separate. Two dozen or more gonadic sacs surrounded by the peritoneum project into the atrial cavity. Except for the absence of a heart, the circulatory system resembles that of fishes, but the blood contains few blood corpuscles. The nervous system, as in vertebrates, is tubular and dorsal. The brain is a simple vesicle, which may possibly be compared with the fore- brain vesicle of vertebrates. The nerves are of two kinds, dorsal (sensory and motor) and ventral (motor). The former pass directly to the skin and to visceral muscles by way of the myocommata. Dorsal and ventral nerves do not unite. Sympathetic cells and fibers are not segregated to form a sympathetic system. Sense organs comparable with those of vertebrates are wanting. A median dorsal pit at the anterior end of the brain is mistakenly spoken of as the olfactory pit. A pigment spot on the brain is likewise somewhat uncritically called the cerebral eye. Amphioxus is, however, very sensi- tive to light. There is no ear. Sub-Phylum Vertebrata (Craniota) The vertebrates or craniotes are chordates with a vertebral column and a brain-case. The evolution and perfection of a light and strong endoskeleton has been an important factor in making the vertebrates masters of the world. Exoskeletal structures also appear, as among the invertebrates, but only exceptionally are heavy enough to interfere with the activity of the animal. A many-layered epidermis with various appendages enables the vertebrates to withstand successfully the vicissi- tudes of weather met by land animals. In correlation with their activity, senses multiply and become acute and the brain is much enlarged. The original metamerism characteristic of lower vertebrates becomes much obscured in the higher. The heart is ventral and may be either two, three, or four-chambered. Some 25,000 species are known. Vertebrates may be divided into seven classes. Class Cyclostomata Cyclostomes are the round-mouthed lamprey eels and hag-fishes. They have a persistent notochord, lack a biting jaw, and the beginnings THE ANIMAL KINGDOM 13 of vertebrae appear in the form of cartilaginous neural arches. In the genus Bdellostoma there are often as many as fifteen pairs of gill-sHts. There are no scales in the skin, and the teeth are horny. Some species are hermaphroditic. Paired appendages are absent. BDELLOSTOMA C PETROMYZON Fig. 14. — Three characteristic genera of cyclostomes — Bdellostoma, Myxine, and Petromyzon. That they are the most primitive vertebrates is shown in many traits, such as a permanent notochord, absence of paired appendages and jaws, etc. (Redrawn after Dean.) The lamprey, Petromyzon, is a familiar genus which undergoes meta- morphosis during its development. Its larval stage is known as Ammo- coetes. Other genera are Myxine and Bdellostoma. CEPHALASPIS- AN OSTRACODERM Fig. 15. — Cephalaspis, an ostracoderni, appears to have affinities with cyclostomes and has been thought by W. Patten to connect vertebrates with arachnids. Class Ostracodermi The ostracoderms are fossil forms which, as Stensio and others have shown, resemble cyclostomes in some striking respects. Unlike the latter. 14 CHORDATE ANATOMY THE ANIMAL KINGDOM 15 however, their heads were covered by heavy bony armor. Like the lam- preys they lacked jaws and paired appendages. As in cyclostomes the nasal aperture was median and dorsal in position. It has been asserted but not demonstrated that the ostracoderms are the ancestors of carti- laginous fishes, which are consec^uently assumed to have lost their heavy body exoskeletons. Most morphologists, however, consider ostracoderms rather highly specialized types and not primitive ancestral forms. Cepha- laspis and Pterichthys are characteristic genera. HEPTANCHUS - AN ELASMOBRANCH „.Aii^,e^d,^_^__ Fig. 17. — Types of three sub-classes of fishes — Heptanchu,s, an elasmobranch; Polypterus, a crossopterygian ganoid; and Scomberomorus, a teleost. (Redrawn after Dean.) Class Pisces Fishes are vertebrates with usually scaly skins, permanent gills, and paired fins. The heart is two- or three-chambered. The skeletons may be cartilaginous or bony. Gill-apertures number four to seven pairs. Dorsal and ventral spinal nerves join to form mixed trunks. Sympathetic ganglia are differentiated. The liver has at least two lobes. Of special interest are the orders of fishes which are believed to be near the line of ancestry of land animals. Probably cartilaginous forms like the Elasmobranchs (sharks and skates) were the common stock from which the remaining orders of fishes were evolved. Their gills are not covered by an operculum ; their skull is devoid of covering membrane bones ; their intestine has a spiral valve. The dogfish Squalus (Fig. 16) is a familiar example. 1 6 CHORDATE ANATOMY Ganoids are "ray-finned" forms with either cartilaginous or bony skeletons. Their gills are covered with an operculum; they have a spiral valve; the tail is heterocercal; their air-bladder is connected with the pharynx or esophagus by means of an open duct; their skin has ganoid scales, or sometimes, bony scutes, or it may be naked. The order of Crossopterygii, "lobe-finned" ganoids, which make their first appearance in the Devonian period, were air-breathers and possibly the direct ancestors of land animals. The Nile " bichir, " Polypterus, is a Hving representative of this largely extinct group. The sturgeon is a famiUar example of the ganoid group. Teleosts are "ray-finned " forms with a wholly bony skeleton. Unlike the ganoids their tail is never heterocercal. They are usually scaly but may be scaleless. The air-bladder when present does not have an open duct; they lack a spiral valve. Teleosts are the most abundant of fishes. The cod and salmon are familiar types. Arthrodires are fossil fishes possibly related to the modern lung-fishes or Dipnoi. The Dipnoi are not believed to be the ancestors of land forms, but they are in many ways transitional in structure between fishes and amphibia. They may have either one or two lungs. The Dipnoi are represented by the Ceratodus of Australia. Class Amphibia Amphibians bridge the gap between land and water vertebrates. Either permanent or temporary gills occur. Lungs are usually present, NECTURUS. Fig. i8. — Necturus, a urodele amphibian, interests zoologists because more than any- other living amphibian it resembles the fossil Stegocephala, another " ancestral " group. but some are lungless. Except in some fossil forms, scales are lacking in the skin. The olfactory pits communicate with the mouth cavity by means of narial passages. The paired appendages are toed. The heart is three-chambered. A postcaval vein is present. The embryo develops without an amnion. Amphibia are subdivided into Urodela or tailed forms, the newts and salamanders, Anura or tailless forms, the frogs and toads, and the Gym- nophiona or limbless types. Besides these living orders of amphibia, the fossil order Stegocephala is important, since they appear to be the direct ancestors of reptiles. Fishes and Amphibia have been grouped together as Ichthyopsida in contrast with Sauropsida which includes reptiles and birds. The embryos THE ANIMAL KINGDOM 1 7 of the latter are protected by fetal membranes, while those of the former are without them. Class Reptilia Reptiles are horny-scaled vertebrates which breathe by lungs. The embryos develop in a liquid-filled sac, the amnion. The skull articulates with the atlas vertebra by means of a single occipital condyle. Arterial and venous blood are mixed in the dorsal aorta. Living reptiles are divided into Rhynchocephalia, Lacertilia, Ophidia (Serpentes), Chelonia, and Crocodilia. Among fossil orders, the Thero- morpha are important, since, especially in their dentition, they resemble mammals, and the dinosaurs because they are the ancestors of the birds. Rhynchocephalia are mostly fossil reptiles having very primitive char- acteristics. Like some primitive amphibians they have only two sacral SPHENODON Fig. 19. — Sphenodon has been characterized as a "living fossil." Asa "primi- tive" type of reptile it interests the student of phylogenesis. It belongs to the Order Rhynchocephalia. vertebrae. Sphenodon (Hatteria), the only living representative, lacks the external copulatory organs present in all other reptiles. The Lacertilia are the lizards. They usually have two pairs of limbs; the anus is a transverse slit; eyelids and an external ear opening are usually present. Ophidia (Serpentes) are limbless reptiles devoid of movable eyelids and external ear-opening; the tongue is forked; the scales along the ventral side of the body are specially modified to assist in locomotion. Snakes. Chelonia (Testudinata) are toothless reptiles, the broad bodies of which are enclosed by a "shell" which consists of a dorsal carapace and a ventral plastron. The eyes have lids and nictitating membrane. Turtles and tortoises. Crocodilia have their teeth set in alveoli; the anus is a longitudinal slit; the tail is laterally compressed; the bodies are large. Alligators and Crocodiles. Theromorpha are fossil reptiles which may have been the progenitors of mammals; in some the teeth are differentiated as in mammals; the quadrate 1 8 CHORDATE ANATOMY bone is attached to the cranium as in mammals; the zygomatic arch of the skull resembles that of mammals. Class Aves Birds differ from reptiles in having both feathers and scales, and in having the anterior appendages modified as wings. The heart is four- chambered, and the single aortic arch on the right. Teeth are wanting in modern forms. The body temperature is higher than in other animals. Two large divisions are recognized, the flying birds or Carinatae with a keeled sternum, and the running birds or Ratitae which have no keel on the sternum. Class Mammalia Mammals are vertebrates with hairs and mammary glands. A few, the monotremes, lay eggs, but all the rest bring forth their young well developed. Mammals have a pair of occipital condyles, a muscular diaphragm, and a chain of three ear bones. The heart is four-chambered, and the aortic arch is on the left. The jaw articulates between the dentary and squamosal bones. Two major divisions are recognized, Placentals, the embryos of which are attached to the mother by a vas- cular placenta; and the Non-Placen- tals, the monotremes and marsupials, most of which lack a placenta. ORNITHORHYNCHUS. Fig. 20. — Ornithorhynchus is a repre- sentative of the most primitive group of mammals, the monotremes. As an egg-laying mammal it bridges over the gulf separating reptiles and mammals. Fig. 21. — Opossum, the typical genus of didelphians. (Redrawn after Newman.) Sub -Class Monotremata (Prototheria) The monotremes or ornithodelphians are egg-laying mammals with a cloaca. Teats are lacking. Ornithorhynchus, the duck-bill of Australia, is the best-known genus; and there are two species of the spiny anteater. Echidna. There seem to be no more than a half-dozen species surviving for the entire sub-class.- THE ANIMAL KINGDOM 19 Sub-Class Marsi'itals The marsupials or didelphiansgive birth to their young in a most imma- ture state and nourish them for some time in an external marsupial pouch situated on the ventral side of the body of the female. The brain has no corpus callosum. A loose allantoic placenta occurs in some. Dasyurus has a yolk-sac placenta. Opossum and kangaroo are well-known examples. All the indigenous mammals of Australia are non-placental. Sub-Class Placentals The placentals or monodelphians have a placenta, a corpus callosum in the brain, and no marsupial bones. Urogenital and digestive outlets are separated. Placentals are subdivided into at least ten living orders. Order I. Insectivora. The insectivores include shrews, moles, and hedgehogs. They are flat-footed and five-toed, and their denti- tion is unspecialized, so that they are appar- ently nearest of surviving forms to the original placental. Order 2. Xenarthra. The Xenarthra include part of the group formerly included in the edentates such as the armadillos, sloths and anteaters. The teeth of adults are either absent or lack enamel and roots. Dentition is limited to a single set. Order 3. Rodentia. The rodents are gnawing animals, such as rats, rabbits, squirrels, guinea pigs, beavers, porcupines, gophers. Canine teeth are absent, and the incisor teeth in both jaws grow continuously Fig. throughout life. The cecum is very large. Order 4. Carnivora. The carnivora include the fossil creodonts, the cats, dogs, weasels, bears, raccoons, and seals. Each foot has four or five toes. The canine teeth are sharp and elongated. The clavicle is reduced or absent. Order 5. Artiodactyla. Artiodactyls are such hoofed forms as cattle, deer, swine, sheep, goats, camels, llamas, hippopotamuses, and giraffes, which usually have an even number of toes on each foot. The third and fourth toes are larger, and the second and fifth reduced or absent. The stornach is complex and the cecum reduced. 22. — Tupaia, the tree- shrew, an insectivore. 20 CHORDATE ANATOMY Order 6. Perissodactyla. The perissodactyls usually have an uneven number of hoofs on each foot. They include the horse, ass, zebra, tapir, and rhinoceros. The third toe is the largest and the only one func- tional in the horse. The enamel of the back teeth is complexly folded. Order 7. Subungulata. Hoofed forms usually with plantigrade feet. Subungulates are the elephants and mastodons, and the hyrax or cony. The proboscidians such as the elephants have on each foot five toes on which they walk. Their testes do not descend into a scrotum. Sireni- ans (Manatee and Dugong) are a suborder of this group. Order 8. Cetacea. The cetaceans include whales, porpoises and dolphins. They are aquatic mammals with fish-like bodies. Hairs and LEMUR CATTA. Fig. 23. — Lemur, a primitive Primate. (Redrawn after Shipley and McBride.) pelvic extremities are absent in the adult. There are two abdominal teats. Teeth may be replaced by whalebone. Order 9. Chiroptera. Chiroptera are the bats and flying foxes. Their anterior limbs are modified to support the wings, the fingers are joined by a web, and the sternum has a keel. Order 10. primates. The primates include lemurs, marmosets, monkeys, baboons, apes, and men. They are mostly arboreal in habit. Nearly all have five digits with flattened nails, and in aU except the lowest forms the thumb is freely opposable to the fingers. Mammary glands are usually a single pair and thoracic. Primates are divided into two sub-orders. Sub-order Lemuroidea. The lemuroids include the lemurs and tarsiers. They are arboreal and nocturnal, small and not especially monkey-like. Typical lemurs have a claw on the second digit of the THE ANIMAL KINGDOM 21 hind-foot. Thumb and great toe are not completely opposable to the other digits. The uterus is two-horned. Sub-order Anthropoidea. Anthropoids include the remainder of the primates. Hands and feet are differentiated, and either the thumb or the great toe is opposable. Finger and toe-nails are flat, except in the marmo- sets which have claws. VOUNG CHIMPANZEE. _ Pig. 24.— Young chimpanzee, a type of anthropoid. (From photograph by Fred Johnson.) Three chief sections of anthropoids are recognized : Platyrhina. The South American monkeys, with broad nasal septum, three premolar teeth in each half-iaw (except the marmosets which, like the Old World monkeys, have two), and a cUmbing foot. Catarrhina. The Old World monkeys and the great apes, with a narrow nasal septum, two premolar teeth in each half jaw, and a climbing foot. Bimana. Also with narrow nasal septum and two premolars, but with the great toe non-opposable and a walking foot. 2 2 CHORDATE ANATOMY CLASSIFICATION OF ANIMALS— SUMMARY Animal Phyla 1. Protozoa. Unicellular. Reproduce by fission. METAZOA, multicellular. 2. Porifera. Multicellular. Acoelomate. Pores in body-wall. 3. Coelenterata. Multicellular. Acoelomate. Radial symmetry may possibly disguise primitive bilateral symmetry. Two-layered body- wall. Nettling-cells. Enteron with single opening. 4. PL.A.TYHELMINTHES. Bilateral. Flat-bodied. Without coelom. Anus in a few genera. 5. Nemathelminthes. Pseudocoelomate. Cylindrical. Anus. 6. Molluscoida. Usually coelomate. U-shaped alimentary canal. Lophophore. 7. Rotifera. Pseudocoelomate. Trochophore-like worms. Cilia around mouth. 8. Echinodermata. Coelomate. Spiny-skinned. Water-vascular system. Bilateral symmetry disguised. 9. Annelida. Coelomate. Metameric. Appendages, when present, without joints. 10. MoLLUSCA. Coelomate. Non-metameric. Mantle, mantle-cav- ity, foot. 11. Arthropoda. Pseudocoelomate. Metameric. Jointed append- ages. Classes, Crustacea, Arachnida, Onychophora, Myriapoda, Insecta. 12. Chordata. Notochord. Dorsal tubular nervous system. Sub-Phyla of Chordates Hemichorda. Notochord limited to oral and pre-oral region. Worm- like. Body in three primary divisions. Balanoglossus. Urochorda. Notochord limited to tail region. Body-wall covered with cellulose sac. Appendicularia, Ascidia. Cephalochorda. Notochord in head, trunk, and tail throughout life. Metameric. Amphioxus. Hemichorda, Urochorda and Cephalochorda together are often called protochordates. Vertebrata. Chordates with brain case and vertebrae. Squalus. Classes of Vertebrates (Craniota) I. Cyclostom.\ta. Without paired appendages or biting jaw. Usu- ally hermaphroditic. Petromyzon, Myxine, Bdellostoma. THE ANIMAL KINGDOM 23 2. OsTRACODERMi. Fossil monorhine fishes related to the cyclostomes. 3. Pisces. With paired appendages and movable lower jaw. Skin usually scaly. Permanent gills. Fishes are subdivided into five subclasses : Elasmobranchii. Gills lack operculum. Skeleton cartilaginous. Sharks, skates, and rays. Crossopterygii. Fossil forms related to the ganoids. Ganoidei. With operculum. Cartilage skeleton largely replaced by bone. Garpike, sturgeon. Teleostei. With operculum and bony skeleton. Common bony fishes. Dipnoi. With gills and one or two lungs. All following are Tetrapods. 4. Amphibia. Living forms are without scales and usually have lungs. Toed appendages instead of fins. Claws or nails lacking. The Stegocephala are a group of fossil amphibians. Fishes and Amphibia grouped together as Ichthyopsida. All to this point are Anamnia. All that follow are Amniota, having the embryo protected by an amnion. 5. Reptilia. Adults scaly. Lungs only — no gills. Aortic arch on both sides. The Theromorpha are fossil reptiles. 6. AvES. Feathered. Modern forms toothless. Aortic arch on the right side only. Reptiles and birds grouped together as Sauropsida. 7. Mammalia. With mammary glands and hair. Sub-Classes of Mammals 1. Monotremata. Egg-laying mammals. 2. Marsupialia. Pouched mammals. 3. Placentalia. Mammals with a placenta. Orders of Placentalia 1. Insectivora. Insect eaters. 2. Xenarthra. Toothless or teeth without enamel. 3. Rodentia. Incisors specialized for gnawing. 4. Carnivora. Flesh eaters. 5. Artiodactyla. Generally even number of hoofs. 6. Perissodactyla. Generally odd number of hoofs. 7. SuBUNGULATA. Proboscidians, hyrax, and sirenians. 8. Cetacea. Whales. 9. Chiroptera. Winged mammals. 10. Primates. Usually a single pair of thoracic mammae. Thumb usually opposable. 24 CHORDATE ANATOMY Lemuroidea. "Half-apes." Thumb not fully opposable. Anthropoidea. Thumb or great toe, except in New World monkeys, opposable. Sub-Orders of Anthropoidea Platyrhina. With broad nasal septum. New World. Catarrhina. With narrow nasal septum. Old World. BiMANA. Great toe not opposable. Species and Genera of Bimana Pithecanthropus erectus, the Java man. EoANTHROPUS Dawsoni, the Sussex man. Sinanthropus Pekinensis, Pekin man. Homo Neanderthalensis, Neanderthal man. Homo Heidelbergensis, the Heidelberg man. Homo Rhodesiensis, the Rhodesian man. Homo sapiens, Cro-Magnon and modern man. (Negro, Mongolian, etc.) THE ANIMAL KINGDOM Sequence of Organisms in Geologic Time 25 Eras Periods Years (Barrell) Characteristic Organisms Recent I , 000 , 000 to I ,500,000 Modern races of men. Recent plants and animals. Cenozoic Pleistocene (Glacial) Early species of men and primates. Mam- mals dominant life. " Age of man." Tertiary 95 ,000,000 to 115, 000 , 000 "Age of mammals." Lemuroids and in- sectivores appear. First placentals. Cretaceous 116, 000 , 000 to 136,000,000 Mammals mostly marsupials. Reptiles highly specialized. Mesozoic C m a n - chean 120,000,000 to 150,000,000 Bony fishes abundant. Flowering plants appear. Jurassic 155, 000 , 000 to 195,000,000 Diverse reptiles. Ganoid fishes. First birds. Triassic 190,000,000 to 240,000,000 Crocodiles and dinosaurs. Reptiles dom- inant. First mammals. Permian 215, 000 , 000 to 280,000,000 Mammal-like reptiles. Trilobites disappear. Pennsyl- vanian 250,000,000 to 330,000,000 Primitive amphibians and reptiles. Conifer- ous plants. Mississip- pian 300,000,000 to 3 70 , 000 , 000 Earliest amphibian fossils. Horse-tails and club-mosses. Paleozoic Devonian 360,000,000 to 420,000,000 Amphibian foot-prints. Lungfishes. Earli- est land plants. Silurian 390,000,000 to 460 , 000 , 000 Ostracoderm (armored) fishes. Elasmo- branchs. Land plants begin. Ordovician 480,000,000 to 590,000,000 Vertebrates appear. First fishes. First insects. Cambrian 550,000,000 to 700 , 000 , 000 Invertebrate phyla abundant. First tri- lobites. CHAPTER 2 REPRODUCTION Anatomy, broadly defined, includes embryology which deals with the progressively changing anatomy of the animal in the course of its develop- ment from egg to adult. Many anatomical peculiarities of animals are unintelligible so long as only the adult is studied. Embryology gives some reason for such facts as that the chief artery emerging from the heart turns to the right in a bird but to the left in a mammal and that the diaphragm of a mammal is supplied by nerves from the neck region instead of from the neighboring trunk region of the spinal cord. The theory of evolution rests to an important extent on facts derived from the com- parative embryology of vertebrates. Sexes. Reproduction in the vertebrates always involves gonads of two types, the ovary which produces eggs (ova) and the testis which produces sperm (spermatozoa). In some tunicates (Urochorda), pre- sumably remote allies of vertebrates, alternation of sexual and asexual generations occurs. A fertilized egg becomes an asexual individual from which arise buds. These become sexual adults which are her- maphrodite, that is, they produce both eggs and sperm. In all vertebrates except a few fishes the individual is either male or female — the dioecious condition. The eel-like hag, Myxine (a cyclo- stome), and several of the bony fishes (Teleostei) are normally her- maphrodite (monoecious). Among vertebrates which are normally dioecious many abnormal cases have been reported, especially in fishes and amphibians, in which germ cells of both sexes were found in one individual. The Germinal Bodies. The spermatozoa are derived from cells in the walls of delicate tubules which are the essential part of the testis. The ova come from primordial germ cells contained within the tissues of the usually solid ovary. (Fig. 274) The "head" of the spermatozoon (Fig. 25) consists of compacted nuclear material (chromatin) derived from the primordial germ cell. A locomotor "tail" is formed from the cytoplasm (extranuclear proto- plasm) of the original cell. The "ripe" spermatozoon is essentially a motile nucleus. The egg in the course of its differentiation acquires a greatly increased body of cytoplasm within which is deposited more or less food material, 26 REPRODIKTION 27 the yolk or deutoplasm. The egg may become invested by membranes or envelopes, either protective {e.g., the vitelline or yolk membrane; the hard calcareous shell of a bird's egg; see Fig. 28) or nutritive (e.g., the albumen or "white" of a bird's egg). Eggs differ most remarkably as to the amount of contained yolk and as to their outer coverings. The microscopic egg of a mammal and the gigantic ostrich egg encased in its hard shell would seem to be hardly Fig. 25. — Spermatozoa of dogfish (Squalus), frog (Rana), parrot (Psittacus), mouse (Mus) and ape (Inuus). H, head; M, middle piece; 7", tail. The spermatozoon of the frog is about o.i mm. long. (Redrawn from Retzius.) comparable objects. The thing referred to in kitchen and market as an "egg" consists of the egg in strict sense, or ovum, plus various extraneous substances and structures. The hen's ovum, corresponding to the small egg of some fish, is merely the yellow sphere commonly called the "yolk" of the "egg," enclosed in its vitelline membrane (Fig. 28). The following data illustrate the differences in eggs in regard to size and content of yolk: Egg Amphioxus Some frogs Domestic fowl (ovum or "yolk' Approximate diameter, mm. o. I 2.0 30.0 Relative volumes 8,000 2 7 , 000 , 000 The volume of an ostrich ovum would be hundreds of millions of times greater than that of a mouse egg whose diameter is about 0.06 mm. Size of eggs is correlated primarily with the method of development. Correlation with size of body may appear when the developmental methods of the animals are similar, e.g., in reptiles and birds. CHORDATE ANATOMY '- EGG-CASE The eggs of fishes are usually relatively small, less than 5 mm. in diameter. Eggs of sharks and skates, however, contain much yolk and rival in size the eggs of birds. These large eggs are enclosed in shells consisting of a horn-like material secreted by the anterior part of the oviduct. In oviparous sharks and skates the shell is usually flat and quadrangular and has long tendrils which serve to anchor it to seaweed or other objects. (Fig. 26) The eggs of amphibians, which always contain considerable yolk, are larger than the eggs of many fishes, but smaller than the average for reptiles and birds. Eggs of various frogs range from 1.5 to 3 mm. in diameter. Eggs of large salamanders (Necturus, Cryptobranchus) are 5 or 6 mm. in diameter. The amphibian oviduct deposits upon the egg a layer of gelatinous substance which, after the egg has been extruded into the water, swells to form a thick jelly-like envelope. (Fig. 27) Reptiles and birds produce eggs con- taining an enormous amount of yolk (Figs. 28, 36). The protoplasm in these great eggs is aggregated at one spot on the surface of the egg, marking the animal pole, while the remainder of the egg is yolk nearly, if not quite, devoid of protoplasm. The local- ized protoplasm (germ-disc; Fig. 36) appears as a small white fleck on the sur- face of the yellow yolk. Before the egg is fertilized the germ-disc contains a single nucleus. These large eggs are invested by a tough vitelline membrane external to which may be more or less nutritive albumen (the "white" of a hen's egg) and an outer shell which in most reptiles is of a leathery texture, but in crocodiles, alligators and birds is highly calcified and therefore hard and brittle. Eggs of mammals, with two exceptions, are minute, containing a minimum of yolk. The exceptions are the duck-bill (Ornithorhynchus) and the spiny ant-eater (Echidna) of the Australian region. These two mammals, presumably of primitive type, lay large eggs encased in tough shells. In general these mammals are reptilian in their methods Fig. 26. — Egg-case of small shark. T, tendrils coiled around branches of a horny (gorgonian) coral. About half actual size. (Drawn from specimen in the anatomical collection of the Biological Laboratories, Harvard University.) REPRODUCTION 29 of reproduction. Otherwise mammalian eggs are of microscopic dimen- sions (0.06 to 0.3 mm. in diameter). The egg (Fig. 29) is covered by a delicate membrane (zona pellucida) external to which may be a cellular membrane (corona radiata), both contributed by the ovary. G' 'OV Fig. 27. — Amphibian eggs. A, of frog, soon after laying; B, early larva of frog, just before hatching; C, of the salamander, Cryptobranchus allegheniensis. A and C, approximately actual size; B, enlarged. G, gelatinous layer; L, larva; OV, ovum. {A and B, redrawn from Marshall, "Vertebrate Embryology"; C, after A. M. Reese.) Fertilization. Development is initiated by the "fertilization" of the egg. A spermatozoon penetrates the egg (impregnation) and the sperm chromatin becomes joined with the chromatin of the egg nucleus. nucleus of Pander neck of latebra white yolk less dense albumen yellow yolk ^ Fig. 28. — Diagram representing a section of a hen's egg cut in a plane including the long axis of the egg and passing through the blastoderm. (From Patten, "Embryology of the Chick"; after Lillie.) The "maturation" process through which all germ cells pass reduces their chromatin to approximately half that contained in body cells, so that the union of sperm chromatin and egg chromatin provides the fertilized egg with a nucleus containing the full complement of chromatic bodies (chromosomes) characteristic of all body cells of the animal. 30 CHORDATE ANATOMY Therefore the fertilized egg, although the product of two cells, possesses the mechanism of a single cell. It possesses no visible structures which would adequately account for its development into a large complex animal like the parent animals. Compared to such cells as those of muscle and nervous tissue, it is strikingly devoid of visible special mechanism. Yolk is characteristic of eggs, but yolk is an inert food sub- stance, not a mechanism. The motile and aggressive sperma- tozoon might seem to be the essen- tially "animal" body in development while the relatively large unfertilized egg, burdened with inert yolk, would appear rather as a passive and vegeta- tive thing. But in normal develop- , , ment the spermatozoon merely imparts Fig. 29. — Human ovum surrounded ^ ... by follicular cells. Actual diameter of the Stimulus which mitiates develop- ovum about 0.25 mm. C. cytoplasm ^^^^ ^^^^ provides for inheritance containing some volk; CR, corona radiata; F, follicular cells; N, nucleus; from a male parent. Expcrmienta- ZP, zona pellucida. (After Nagel.) ^^j^ ^isi^ proved that the egg is fully capable of producing a characteristic adult without the assistance of a spermatozoon. Obviously, however, such an adult inherits only from a mother. Exit of Sperm and Eggs. The sperm is usually carried by ducts which lead from the testis to the exterior, but in cyclostomes and some bony fishes it is discharged from the testes into the body-cavity and finds exit through abdominal or genital pores which pierce the body-wall. Ova are usually liberated from the surface of a solid ovary (Fig. 274) into the body-cavity whence they pass into oviducts which lead to the exterior. In cyclostomes and some bony fishes the eggs pass out through abdominal pores. In other bony fishes the ovary is hollow, eggs are liberated into its lumen and pass out by way of a duct which is an extension of the wall of the ovary. The genital ducts are usually closely associated with the duct system of the kidneys. Exceptional conditions occur in bony fishes. Oviparity, Viviparity, Impregnation. The means whereb>- ovum and spermatozoon are brought together depends on whether the animal is oviparous or viviparous; also on whether the outer envelopes of the egg can be penetrated by a spermatozoon. In most oviparous fishes the eggs are impregnated after the genital products have been discharged into the water ("external fertilization"). But oviparous sharks and skates produce eggs whose shells are impene- trable by sperm. Therefore copulation must occur and the egg must be REPRODUCTION 31 reached by the sperm before the shell is deposited. Some sharks and a few teleosts are viviparous; copulation and "internal fertihzation" are therefore necessary. Among amphibians there is much diversity. In most frogs and toads impregnation is external. In tailed amphibians (Urodela) it is commonly internal, in oviparous as well as in viviparous species, and in many cases is effected by means of a spermatophore, a mass of sperm agglutinated together by a secretion from cloacal glands of the male. The spermato- phore may be introduced into the cloaca of the female or else attached to the external surface of the female. In some cases it is merely discharged and picked up later by the female. Some reptiles are viviparous. All birds are oviparous. But in all reptiles and birds the egg-shell necessitates copulation and internal impregnation. Modern mammals, except Ornithorhynchus and Echidna, are vivi- parous. The two exceptional animals lay eggs of reptilian sort. There- fore in all mammals impregnation must be internal. In general, eggs which acquire such envelopes as a layer of albumen or a hard shell must be impregnated while in the anterior region of the oviduct and before these external coverings have been deposited. Development begins immedi- ately after fertilization. Therefore, if fertiliza- tion has actually occurred, the "egg" which is "laid" by the reptile or bird contains not an ovum but an embryo at an early stage of develop- ment. Provisions for protection, nutrition and res- piration during the period of development are most diverse. In most fishes the eggs are abandoned to the hazards of the environment. Some fishes, especially those of fresh water, arrange crude nests in gravel, sand or mud. Some fishes guard their eggs. In the sea-horse (Fig. 30) and pipe-fish, the male carries the developing eggs in a brood-pouch on the ventral drawn after Boulenger in surface of the body or tail— an arrangement Jt'to^yT^""^^' ^^'"'^^ suggestive of the marsupial pouch of a female kangaroo. The smaller fish eggs, scantily endowed with yolk, develop rapidly and soon become free-living and self-supporting while still very minute. The miniature fish then enters upon a long period concerned mainly with feeding and growth. Eggs containing larger quantities of yolk pass through a longer period of development and the young fish attains relatively large size before it is obliged to obtain food from an BROOD-POUCH- FiG. 30. — Sea-horse (Hippocampus.); male, with brood-pouch. (Re- 32 ■ • CHORDATE ANATOMY external source. The embryo and young of the viviparous fish not only receive maximum protection, but may obtain from the mother some food in addition to the initial supply of yolk. In so-called "placental" sharks the wall of the oviduct develops highly vascular folds or processes and similar folds arise on the abdominal wall of the embryo. The two sets of projecting structures, maternal and embryonic, become closely approxi- mated, thus providing for diffusion of substances from the blood of one to that of the other. Among amphibians there is, in general, better provision for protection of eggs and young than in fishes. Nests and guarding of eggs are common. Among frogs and toads occur various peculiar ways of caring for eggs and young. The male of the European "obstetric" toad carries the long strings of eggs wound about his body and legs until the tadpoles emerge. In some cases eggs are carried in the mouth or vocal pouch of the male. In the South American "marsupial" frog the eggs develop in a capacious Fig. 31. — Necturus larva of about 25 mm. length. (After Eycleshymer.) pouch formed in the skin on the back of the female. The eggs of the toad, Pipa americana, develop in individual vesicles in the skin on the back of the mother. Viviparity, affording a maximum of protection, occurs in a few amphibians, including representatives of each of the three orders, Urodela, Anura, and Gymnophiona. The amphibian egg, whether laid in the open or enclosed in some protective way, develops rapidly into a highly characteristic larva, the tadpole or "poUiwog" (Fig. 31) which, with its functional gills and locomotor tail, as well as in many features of internal anatomy, is a dis- tinctly fish-like animal and, if its environment is external water, it lives the hfe of a fish. The larval period, ranging from a few weeks in some salamanders to a year or more in some frogs, is devoted mainly to feeding and growth. It terminates in a metamorphosis in the course of which the animal acquires. the adult characteristics. The transformation is most radical in frogs and toads; legs and lungs develop, tail and gills are absorbed, gill clefts close, and other changes occur. In certain excep- tional species of frog, especially large eggs are laid on land and develop to adult form without passing through a tadpole stage. In the Urodela the changes are less marked, the tail and sometimes also the gills being retained. Adult Necturus, with its tail and functional gills, is sometimes REPRODUCTION 7,7, called a ''permanent larva." Sexual maturity is ordinarily not attained during the larval state. But the Mexican axolotyl, the larva of the sala- mander, Ambystoma tigrinum, regularly breeds in the larval state. The diversity of reproductive arrangements in amphibians is in marked contrast to the uniformity which prevails in reptiles and birds. The large yolk-mass of the eggs of these animals makes possible a long develop- mental period during which the young can attain relatively great size. A newly hatched alligator is gigantic compared to a newly hatched salmon. These circumstances, together with the fact that development takes place within a thick shell, make necessary some special provision whereby food derived from the yolk may be made accessible to all parts of the large embryo and an adequate supply of oxygen obtained from some external source. The outstanding feature of the development of the reptile or bird appears when the embryo itself goes about the business of constructing a complex system of membranes so disposed and so equipped with blood- vessels as to serve very eflEiciently not only for respiration but for some other and secondary functions. Early in development, at a time when the main organs are in process of formation (Fig. 74), the outer layer of the embryo, representing the prospective body-wall of the animal, throws up a system of folds which arch over and ultimately enclose the whole of the definitive embryo — much as if an animal should enwrap itself in a highly exaggerated fold of its own skin. Thus are formed the investing membranes known as the amnion and the chorion (serosa). The amnion is derived from the inner layer of the fold, the chorion from the outer. The amnion does not fit the embryo snugly. The intervening space is occupied by a watery solution whose chemical constitution resembles that of blood — and also resembles that of sea water. Thus the embryo during its further develop- ment is bathed by a fluid whose chemical nature is compatible with that of the embryonic tissues. Further, immersion of the embryo in watery fluid affords the best possible protection from externally caused mechanical pressures and impacts. Meanwhile the enormous yolk-mass has been enclosed (Fig. 74) by cellular layers which are prospectively the wall of the digestive tube. Then from the hinder region of the embryonic digestive tube a sac bulges out ventrally (Fig. 74) and, like a great and growing hernia, pushes beyond the ventral body-wall. Having thus attained the exterior of the embryo proper, it becomes vastly expanded (by growth) and eventually spreads out so that the greater part of its outer surface is, in conjunction with the chorion, in close relation to an extensive area of the inner surface of the egg-shell. This sac is the allantois. It becomes highly vascular, its arteries and veins communicating with the main vessels of the embryo. 34 CHORDATE ANATOMY A considerable part of the blood of the embryo is diverted into the allantoic arteries and circulates vigorously through a rich system of small vessels lying close to the inner surface of the shell. The shell is porous. Thus ready interchange of respiratory gases between the blood and the external air is provided for. The allantoic sac serves also as a. receptacle for embryonic waste. The ducts from the kidneys open into the extreme hind end of the digestive tube whence the fluid excreted by the kidneys readily passes into the cavity of the allantois. The inner cellular layer (yolk-sac; Fig. 74) immediately enclosing the yolk-mass is highly vascular and its vessels, like those of the allantois, communicate with the main arteries and veins of the embryo. The blood circulating through these vitelline vessels picks up dissolved yolk materials which are conveyed to all parts of the embryo, thus making the yolk available everywhere for metabolism and growth. In viviparous reptiles, the amnion, the allantois with its vascular system, and the yolk-sac circulation are developed as in the embryos of oviparous reptiles. The oxygen obtained by the allantoic vessels, how- ever, must be derived from the maternal blood in the wall of the oviduct. In reptiles and birds building of nests and parental care of young are much more prevalent than in fishes and amphibians, reaching high specialization and efi&ciency in birds. Correlated with the greatly increased protection afforded during development, relatively few eggs are produced. Primitive mammals, as indicated by such surviving examples as Ornithorhynchus and Echidna, must have retained reptilian methods of reproduction. The duck-bill, a burrowing animal, deposits the eggs (usually two) in the burrow. Echidna, producing usually only one egg in a season, places the egg in a fold of abdominal skin, a temporary marsupium, where it is carried and incubated by the warmth of the body until the young hatches. The embryos of these two mammals develop amnion, chorion, allantois, and allantoic and yolk-sac circulations essen- tially as do reptiles. The one new thing which these animals do is to provide the young with a convenient source of food to serve for a time immediately after hatching. Milk produced by mammary glands (see page 127) developed in and by the abdominal skin serves to prolong the period of dependence on the maternal food. All known existing mammals except the duck-bill and spiny ant-eater are viviparous. The minute eggs contain so little yolk that they could never pass beyond the very early stages of development unless additional food material were somehow provided. In the great majority of mammals this is done by means of an organ which is one of the most characteristic features of a mammal. The egg, liberated from the ovary and fertilized, becomes caught and lodged in the superficial tissue of the uterine wall. REPRODUCTION 35 Here it passes into the early phases of development and very shortly gives rise to an amnion, a chorion and an allantois, essentially similar to those structures as developed in reptiles and birds. Curiously, in spite of the absence of any considerable amount of yolk, a yolk-sac also, although devoid of yolk, is formed. This is usually interpreted as a relic of reptilian ancestors. The allantoic sac becomes greatly expanded, more or less enwrapping itself around the embryo, and certain regions of it fuse with the adjacent chorion and enter into a very peculiar relation to the uterine wall (Fig. 75). From the conjoined allantoic and chorionic membranes grow out slender extensions (villi) which penetrate more or less deeply into the adjacent uterine wall. They may become more or less branched. These villi are highly vascular, fetal blood circulating in them under the drive of the fetal heart. The surrounding uterine tissue is likewise highly vascular. There is, however, no open communication between the blood- vessels of the villi and those of the uterine wall. But the fetal and the maternal vessels are so close together that materials readily diffuse from one blood to the other. Dissolved food substances and oxygen pass from the maternal to the fetal blood; waste materials and certain special fetal substances of hormone nature pass from the fetal to the maternal blood. By means of this placenta, intervening between mother and young, the nutrition and respiration of the young animal are provided for through the usually long period of intra-uterine development. Mammals show many variations in the mode of origin and details of structure of the placenta. The marsupial mammals (Metatheria; the kangaroo and its allies) produce only a weakly developed and briefly temporary placenta or none at all. Accordingly the development of the young cannot proceed beyond what is made possible by the initial small yolk supply plus what nutritive material may be absorbed by the embryo and its investing membranes directly from the neighboring uterine tissues and fluids. The young marsupial is therefore necessarily born at an early fetal stage and while very small. The deficiency of the intra-uterine arrangements is compensated for by the marsupium, a pouch formed by a fold of abdominal skin. The mammary glands are within this pouch. The very immature and quite helpless new-born young (in the great kangaroo, Macropus major, being only about one inch long) is trans- ferred to the marsupium by the mother. The young becomes attached to one of the mammary nipples and feeds passively, the milk being pumped in by contraction of muscle about the mammary gland. This "mam- mary fetus" inhabits the marsupium for a time which is usually much longer than its period of intra-uterine development For example, in the great kangaroo the period of intra-uterine gestation is between five and six weeks, but the young kangaroo is carried in the pouch and nour- ished by mammary glands for about eight months. 36 CHORD ATE ANATOMY In placental mammals, as compared to marsupials, the young are born at a relatively advanced stage of development and growth. The mammary organs, however, are in all cases an important post-natal provision for bringing the young animal along to a degree of size and strength favorable to ultimate success. They afford the great advantage, too, that the young animal is not thrown upon the world abruptly, but may acquire independence gradually. Evolutionary Significance Surveying the whole group of vertebrates, the great diversity in the conditions and arrangements attending reproduction is most impressive. It would be difficult to imagine any practicable reproductive expedient or condition which is not exhibited by some animal. There are micro- scopic eggs and there are ostrich eggs. The quantity of yolk may be vast or it may be next to nothing. The primary food supply, yolk, may in various ways be supplemented by secondary sources of nutriment — egg albumen, maternal blood, mammary milk, pigeon "milk." One egg or millions of them may be produced at a time. They may or may not have shells. Parental care of eggs or young ranges from nothing to the human maximum. Vertebrates may be oviparous or viviparous. A primary ovi- parity may be succeeded by a secondary substitute for viviparity, as when eggs develop within a fish's mouth, an amphibian vocal sac, or integumentary pouches of various sorts. Differentiation of organs may precede growth or it may be delayed until the embryo is relatively large. The newly hatched larva of so large a fish as the Atlantic salmon is about 0.65 inch long; a new-born whalebone whale is about twenty feet long. The embryo may develop directly to the adult form or there may be a larval period terminated by a metamorphosis. The embryo may or may not produce a complex set of temporarily functional membranes — amnion, chorion, allantois. The important point to be appreciated is that the association together of any two or more of these various alternatives in a single animal is not haphazard. If one circumstance is, in itself, inadequate for the success of reproduction, it is supplemented by something else. If a large fish were to produce one single microscopic egg annually and deposit it any- where in the Pacific Ocean, the species would soon become extinct. On the other hand, there is no unnecessary duplication of highly specialized arrangements. A placental mammal does not produce a large yolky egg. The entire complex of reproductive conditions occurring in any one animal comprises a consistent grouping of alternatives such that, as a whole, it is adequate. Des[)ite the great differences in methods of reproduction, the net results are equally good, or nearly so, and generation REPRODUCTION 37 after generation the life of the world goes on with at most only very slow change in the general biological balance and scheme of things. Fishes and amphibians show this reproductive diversity most mark- edly. Assuming a genetic series from fish to bird and mammal, the evolution of reproduction has not been a direct progress along one straight and narrow path. Instead, the animals within each class, especially the lower, have tried (so to speak) a variety of methods. From the many reproductive "experiments" of the lower vertebrates finally emerge two distinct types to which the higher vertebrates fairly closely adhere. Reptiles and birds exhibit one of these types, mammals the other. Yet certain distinctive features of these finally emergent types of reproduction are anticipated by some lower vertebrates. The enormous eggs of ovip- arous sharks and skates, encased in thick shells, resemble eggs of reptiles. Some viviparous sharks produce vascular uterine structures (see page 32) suggestive of the mammalian placenta. Certain vivi- parous lizards (genus Seps) develop what is practically a placenta. But there can hardly be any direct genetic connection between these structures in sharks and the somewhat similar structures in reptiles or mammals, nor between the "placenta " of a lizard and that of a higher mammal. The exaggerated filamentous gills of the intra-uterine larvae of some viviparous salamanders and the much expanded bell-shaped gills of the larvae of the "marsupial" frog, Gastrotheca, suggest that the larva may obtain nutriment as well as oxygen from neighboring maternal sources — prac- tically a "branchial placenta." The marsupial structures of vertebrates afford another example of convergence in evolution — that is, the independent origin of functionally similar but genetically unrelated things. Defining a marsupium as a brood-pouch developed on the external surface of the body-wall, there are marsupial fishes (sea-horse; pipe-fish), marsupial frogs and marsupial mammals. Viviparity is commonly thought of as something peculiarly mam- malian. Yet there are viviparous fishes, amphibians and reptiles. The only vertebrate class which contains no viviparous members is Aves. In view of the fact that all birds and the most primitive mammals that we know are oviparous and the further fact that oviparity predominates among the lower classes of vertebrates, it is highly probable that the earliest vertebrates were oviparous and that the animals which con- stituted the main trunk of the vertebrate genealogical tree were oviparous. But viviparity has appeared on twigs of various lower branches of the tree as well as at its mammalian top. The chordate ancestors of vertebrates must have been small animals and presumably produced small eggs with little yolk. It is likely that primitive vertebrates had small eggs and that large yolk masses have 38 CHORDATE ANATOMY been secondarily acquired. But even within a small group of vertebrates the yolk content of eggs may be highly variable, being apparently easil}- susceptible to evolutionary change. In point of size and yolk content the vertebrate egg has evidently had many ups and downs. In spite of the diversity of vertebrate methods of reproduction, an evolutionary trend is clearly to be seen. There is a certain extravagance about the primitive method — millions of eggs, perhaps, in a season, but only a small percentage of survival. The evolutionary tendency has been, by introduction of efBcient protective, nutritive and respiratory arrangements, together with parental care, toward the guarantee of the survival of every potential adult. This tendency bifurcates and culminates in two very differently specialized methods, one in birds, the other in mammals. Unquestionably the high degree of efficiency which has been attained by the sauropsidan method of reproduction and also by placental reproduction in mammals is some- how correlated with the necessity of adaptation to the circumstances of living on land and in air. The primitive fish methods would obviously be impracticable. An aquatic larval stage in the development of a horse or an elephant can hardly be imagined although, developing as it does in the fluid-filled amnion, the terrestrial descendant of ancient aquatic ancestors does spend its early life in a fluid medium. With increase in chance of survival there is reduction in number of eggs produced. This result has the appearance of achieving economy but there is perhaps room for question as to just how and where the economy comes in. Does it cost a cod any more to produce seven million eggs than it costs a viviparous dogfish to bear four or five large "pups"? By either method of reproduction the numerical status of the species may be maintained and so, as remarked above, the net results of the two methods are equally good. DEVELOPMENT Cleavage and Blastula Development involves great protoplasmic activity. There must be a building up of new protoplasm, rapid dividing of cells, movement and change of form. All of this calls for rapid metabolism. Metabolism requires inter-action of nuclear material and cytoplasm and exchange of materials between the protoplasm and the external medium. The area of the nuclear membrane and area of the external surface of the cell therefore impose a limit on metabolic rate. Two cells are capable of more rapid metabolism than one cell whose nuclear and cytoplasmic volumes are respectively equal to the combined volumes of the cor- responding parts of the two cells because the limiting membranes of the two cells have greater total area than those of the single cell. REPRODUCTION 39 'I'he smallest egg cells are large compared to most tissue cells of the animal to which the egg belongs. The metabolic rate in an egg before fertilization is relatively low. After fertilization the rate increases. Before entering upon a prolonged period of activity at high metabolic rate the bulky ovum increases its surfaces by dividing into small cells — the process called cleavage. The successive divisions of the original egg nucleus are, in fact, accompanied by absolute increase in the quantity of nuclear chromatin, a substance which ^^ undoubtedly plays an important part in determining the course of develop- ment. In Amphioxus. Amphioxus is not literally a vertebrate. But it is a chordate and in many respects obvi- ously primitive. The adult is a slender fish-like animal about 5 cm. long (Fig. 11). The egg is correspond- ingly small, about o.i mm. in diam- eter, and contains very little yolk. (Fig. 32) The plane of the first cleavage (Fig. 33) of the egg corresponds to the definitive median (sagittal) plane of the future adult. The two cells re- VEG. Fig. 32. — Median section of a ferti- lized egg of AMPHIOXUS. Diameter of egg about 0.1 mm. ^ TV, animal pole; suiting from the first cleavage there- iV, male and female pronuclei ;P, polar r J. J.I. • Ui J 1 rj. body; S, remnant of spermatozoon; fore represent the right and left y^^, vegetal pole; Y, region of cyto- halves of the body. The plane of the plasm occupied by coarse granules of J , . J- ^ J. yolk. (After Cerfontaine.) second cleavage is perpendicular to that of the first and the third cleavage plane is perpendicular to both the first and second. The second and third cleavages each divide the egg slightly unequally. Further cleavages follow one another in rapid succession, their planes adhering to a fairly rigidly determined order. Meanwhile the cells gradually shift their relative positions and surfaces of contact in such a way that a space opens out at the center of the whole mass. At the thirty-two cell stage the cells are disposed to form a hollow sphere whose wall is everywhere one cell in thickness. Thus every cell of the thirty-two is in direct relation to the exterior, a most favorable position for respiration and excretion. This hollow spherical shape is retained as cleavage continues (Fig. t^^ G-I) until between two hundred and three hundred cells have been formed. This stage of the embryo is called the blastula. The name, blastocoele, is applied to the cavity. The second and third cleavages introduce inequality of size among the resulting cells. This inequality persists as cleavage goes on. It is 40 CHORDATE ANATOMY correlated with the distribution of yolk in the protoplasm, the larger cells containing the more yolk. The cells of the blastula grade from PIGMENT NUCLEUS Fig. 33. — Cleavage of egg of AMPHIOXUS. A , undivided egg; B, in process of first cleavage; C, four-cell stage, lateral view; D, four-cell stage, polar view; E, eight-cell stage, lateral view; F, sixteen-cell stage, lateral view; G, eighty-eight cells, lateral view; H. same stage as G, median section; /, later stage, lateral view. P, polar body. (After Hatschek.) minimum size at one pole (animal) of the sphere to maximum size at the pQi_^ opposite pole (vegetal). This polarity is established in the egg before cleavage begins. In Amphibians. Some amphi- bian eggs (not including the gela- tinous envelope) are about 2 mm. in diameter. Such an egg would possess a volume about eight thousand times that of an egg of Amphioxus. The greater part of the increased bulk is yolk. The egg (Fig. 34) is strongly polarized with reference to the distribution The of the volk in the protoplasm. Development of the Frog's Egg"; The From the animal polc where yolk is Macmiiian Co.) ^^ ^ minimum the quantity in- creases toward the opposite vegetal pole where the maximum occurs. VEGETAL POLE Fig. 34. — Ovarian egg of frog; median section. (Redrawn from Morgan REPRODUCTION 41 Yolk is a non-living, quite inert substance. The active material in development is protoplasm. The developmental behavior of eggs containing much yolk shows quite clearly that the yolk is an impediment to the free carrying out of developmental operations — just as the necessity of carrying a heavy burden of supplies may impede the progress of a company of explorers. Figure 35 represents the cleavage stages of a frog's egg. The successive divisions follow the same general order as in Amphioxus. Cleavages ANIMAL POLE BLASTOCOELE Fig. 35. — Cleavage of the frog's egg. A, first cleavage in process; B, two cells; C, eight cells; D, fourth cleavage complete in animal hemisphere but just beginning in the four cells at the vegetal pole; E, early blastula, median section; F, G, successively- later stages, lateral view. {D, F, G, redrawn from Morgan, "The Development of the Frog's Egg"; E, redrawn from Marshall, "Vertebrate Embryology.") succeed one another at intervals of about an hour, but the period varies with temperature. The yolk evidently hinders cleavage, especially in the vegetal hemisphere. The second cleavage begins at the animal pole before the first is completed at the vegetal pole. In fact, the third cleavage may begin while both first and second are still incomplete in the region of the vegetal pole. Further, the inequality in size of cells at animal and vegetal poles is much greater than in Amphioxus, another consequence of the greater yolk mass. After the third cleavage a cavity appears in the midst of the group of eight cells. As cleavages proceed this cavity enlarges and the embryo, 42 CHORDATE ANATOMY as in Amphioxus, becomes a hollow sphere or blastula (Fig. 35£). Its cavity (blastocoele) is excentric, occupying approximately the animal hemisphere only. Its wall is more than one cell thick. The great thick- ^,T-,^,«*w,-., ness of the wall of the vegetal hemi- Y' sphere and the consequent excen- tricity of the blastocele are obviously due to the yolk. In Reptiles and Birds. In eggs whose yolk-mass greatly exceeds that of the amphibian egg all the proto- plasm is segregated into a thin plate, the germ-disc, lying on the surface of the relatively enormous mass of yolk (Fig. 36). In such an egg, obviously, there is no mechanism for dividing the yolk. Cleavage is Fig. 36. — Cleavage of the germ-disc of the egg of a turtle (Glyptemys insculpta) ; eight-cell stage. The egg-shell is not shown. About twice natural size. A, r 1 ^ ^i ^1 r xi. albumen; C, the eight-cell blastoderm; Confined tO the protoplasm Ot the F, yolk. (Redrawn from Louis Agassiz, ggrm-disc which, following fertiliza- " Embryology of the Turtle.") ... , ,.^ .,, tion of Its nucleus, sphts up rapidly and soon consists of hundreds of small cells forming what is then called the blastoderm lying as a thin plate of cells on the surface of the yolk (Figs. 36 and 37). But there is continuity of blastoderm with yolk only around the periphery of the blastoderm. Elsewhere a thin space, the sub- germinal cavity, intervenes between blastoderm and yolk (Fig. 37). lY" lY' Fig. 37. — Early blastoderm of chick; plane of section passes through center of egg. B, blastocoele (subgerminal or cleavage cavity); C, cells of blastoderm; V, fluid- filled vesicles; F^ yellow yolk, F^ white yolk. Magnified nearly twenty diameters- (Redrawn from Duval, "Atlas d'Embryologie.") Comparing this embryo with the blastula stages of Amphioxus and frog, it seems reasonable to interpret it as a blastula whose blastocoele is the subgerminal cavity, while its blastoderm is the animal region and the yolk-mass is the vegetal region of the embryo. This recognition of a blastula stage, comparable to that of Amphioxus, in the development of a reptile or bird would hardly have been possible but for the intermediate condition exhibited by the amphibian with its moderate yolk-mass and total cleavage. REPRODUCTION 43 The blastula is an essentially one-layer stage of the embryo, the "layer" being the wall of the blastula, whether one cell thick or more than one cell thick. This stage has two-fold significance. Its immediate importance is that it gives the embryonic material increased superficial contact with the environment, thus favoring metaboHsm. Its prospective significance lies in the fact that further development is to consist, to a large extent, in the manipulation of layers of embryonic material. The adult is hollow. It has a body-cavity and other cavities. Most of its organs are hollow. The walls of the hollow structures are constituted of layers — skin, epithelium, endothelium, peritoneum, muscle layers, connective tissue layers. For the construction of such a many-layered thing, the embryo naturally proceeds as early as possible to dispose its building material in the form of layers. Gastrula In Amphioxus. The blastula stage is briefly transitory. At once changes set in which transform it to a two-layered embryo. In Amphioxus the two-layered gastrula form is attained in a very simple way (Fig. 38). The vegetal hemisphere first flattens, then becomes curved inward. The infolding (invagination) continues until the material of the original vegetal hemisphere comes into close relation with the inner surface of the wall of the animal hemisphere. The spherical blastula thus becomes an approx- imately hemispherical embryo whose wall is two layers thick (Fig. 38C). As the process goes on the blastocoele is reduced and finally obliterated. The gastrula is hollow. Its cavity, resulting from the invagination process, at first opens widely to the exterior but the width of the opening is rapidly diminished by inbending of the wall about it and it is soon reduced to a narrow blastopore. In consequence of this contraction of the wall around the blastopore, the form of the entire gastrula tends at first to become spherical, but before the contraction is completed the gastrula begins to elongate in the direction of the axis which passes through the blastopore. An important accessory activity attends this process of narrowing the blastopore. The blastoporal rim is a region of transition from the outer to the inner layer. This region is marked by very rapid proliferation of cells, especially at the dorsal edge of the blastopore (Fig. 38!)) . Cells produced within this growth zone or germ-ring are added, some to the outer layer and some to the inner layer. This growth process, then, is concerned both in the narrowing of the blastopore and the elongating of the embryo. A direct consequence of it is that the material of a certain region of the inner layer immediately adjoining the blastopore attained its internal position not as result of the primary invagination but by the secondary growth process. 44 CHORDATE ANATOMY At the close of the gastrula period (Fig. 38D) the embryo is an elongated ovoid, the slightly larger end being anterior while the now very narrow blastopore marks the posterior end of the long axis. So rapid is development that this stage is attained about seven hours after fertilization. Significance of the Gastrula. The gastrula is the animal in its bare essentials. The outer layer, ectoderm, is potentially protective and ANIMAL POLE I VEGETAL POLE EC\ Fig. 38. — Gastrulation in AMPHIOXUS. The figures represent sections through the polar axis of the embryo. A, blastula with vegetal region flattened; B and C, earlier and later stages of invagination of vegetal hemisphere; D, gastrulation completed; with elongation of the gastrula, its long axis becomes the horizontal antero-posterior axis of the embryo. A, archenteron; B, blastocoele; BP, blastopore- EC, ectoderm; EN, endoderm; P, polar body. (After Cerfontaine.) nervous. It gives rise to the essential outer part of the adult skin, which produces so many important protective structures, and to the whole nervous system, both peripheral and central. The inner layer, endoderm, is nutritive. The cavity within it is the primary digestive cavity or archenteron. It is significant that the wall of the archenteron is derived from the vegetal hemisphere of the blastula. Thus, appropriately, the greater quantity of yolk comes to lie in the lining of the embryonic diges- tive cavity. In the vertebrates the blastopore never becomes mouth and REPRODUCTION 45 rarely becomes anus. The future motor mechanism, muscle, is derived indirectly from the gastrula layers. The gastrula is strongly suggestive of the two-layer body plan of a coelenterate. A simple coelenterate such as Hydra, two-layered through- out, including even the tentacles, can be regarded as a somewhat elab- orated gastrula, the Hydra "mouth" corresponding to the blastopore (Fig. 39). This resemblance, together with the fact that a gastrula stage, modified in one way or another, occurs nearly universally in the develop- ment of metazoan animals, gave rise to Ernst Haeckel's "gastraea" theory which proposed that gastrula-like animals (essentially coelen- terates) must have been the ancestors of all Metazoa. According to this theory, the occurrence of the gastrula form in the ontogeny of a Fig. 39. — Diagrams showing structural similarity of a coelenterate and a gastrula. A, Hydra, longitudinal section; B, gastrula, axial section. A, archenteron, prospective digestive cavity; BP, blastopore'; E, enteric (digestive) cavity; EC, ectoderm; £ A'', endoderm; M, mouth; T, tentacle. vertebrate is a "repetition" of the coelenterate stage in phylogeny. This may very well be true but it is not necessary to hold this view in order to account for the gastrula stage in ontogeny for some such form as the gastrula is the necessary precursor of any adult metazoan which has a skin (ectoderm) and a digestive tube (endoderm). In Amphibians. In the amphibian the vegetal wall of the blastula (Fig. 7,$E-G) is so thick that the vegetal hemisphere is, in. effect, solid. It consists of large cells heavily laden with inert yolk. Such a wall cannot readily bend inward as does the corresponding thin and labile layer of the Amphioxus blastula. In the amphibian three processes going on simultaneously effect gastrulation. The beginning of gastrulation is seen when a crescent- shaped groove (Fig. 40 A, /) forms at a certain place on the surface of the blastula. It lies just on the vegetal side of the equator determined by the animal and vegetal poles and extends transversely to the median 46 CHORDATE ANATOMY VEG Fig. 40. — Gastrulation in the frog, stages; viewed toward the vegetal pole. A, B, C, the whole embryo at successively later A', B' and C represent, in somewhat diagram- matic way, sections of corresponding stages cut in the plane including the polar axis and bisecting the gastrular invagination, /; this plane corresponds to the median plane of the adult. During the latter part of the period of gastrulation, as result of shifting of the heavy yolk (compare B' and C), the embryo rotates so that the axis passing through the blastopore {BP) becomes horizontal (see Fig. 44-4). A, arohenteron; AN, animal pole; B, blastocoele; BP, blastopore; EC, ectoderm, EN, endoderm; 1, invagination; NP, neural plate; VEG, vegetal pole; Y, yolk; YP, yolk plug. REPRODUCTION 47 plane determined by the first cleavage. The equator and a zone extending superficially somewhat into the vegetal hemisj)her(' are marked by espe- cially rapid cell-proliferation. It is in this ])articularly active region, the germ-ring, that the groove appears. Figure 40.1' represents a section in the median plane of an embryo at this stage. The groove (/) is the result of an invagination which occurs near where the upper thin wall and lower thick wall of the blastula join. The outer layer bounding the invagination consists of smaller cells which have moved inward from the superficial germ-ring region; the deeper wall of the invagination consists of yolk cells. The groove, initiated as a slight invagination, rapidly deepens (Fig. 40B-B'), not by continued invagination, but by active growth of the upper (for later events prove it to be dorsal) lip of the groove —that is, the lip resulting from the infolding of germ-ring material. This growth process serves to build out the dorsal lip of the original invagination so that the fold is caused to extend farther and farther downward over the yolk cells. Meanwhile the groove, originally a short crescent as seen on the surface of the blastula, lengthens laterally or in the direction of the curve of the crescent (Fig. 40-B) until it describes a semicircle and, con- tinuing, finally completes a circle. As the groove progressively lengthens, the newly arisen region of its outer fold, continuous with the "dorsal lip " of the initial region of the groove, grows centripetally over the surface of the yolk cells. Therefore the radius of the curve described by the groove is ever decreasing. The groove is obviously deepest at the region where it began to form and shallower in the successively newer parts of it. Having completed the circle, the centripetal growth of the outer fold of the groove continues until the original vegetal hemisphere is completely covered except for a small aperture through which bulges a mass of yolk cells, the so-called yolk plug (Fig. 40C-C'). The result of the processes just described is the formation of a new cavity in the embryo. This cavity is bounded externally by the two layers of the overgrowing fold, internally by the yolk cells. It potentially opens to the exterior but its actual opening is partly blocked by the yolk plug. If no process other than those already mentioned were involved the cavity would be exceedingly thin. It is, in fact, greatly enlarged by another process. During the progress of the overgrowth of the vegetal hemisphere, the large yolk cells become extensively rearranged. They move into the blastocoele, finally practically obliterating it. They carry out this movement in such a way that the space left vacant by them is added to the cavity formed by invagination and overgrowth. Figure 40C' represents a median section of a frog embryo at the close of gastrulation. The embryo is two-layered throughout. The outer layer, ectoderm, is uniformly thin. The inner layer, endoderm, is very thin over approximately the dorsal half of the embryo but thick in the 48 CHORDATE ANATOMY ventral region where the greater part of the original mass of yolk cells persists. The endoderm surrounds a capacious cavity, the archenteron, whose external opening, the blastopore, is occupied by the yolk plug. The blastopore marks the posterior end of the embryo. The greater part of the original yolk is now in the endoderm. The difference between gastrulation in Amphioxus and that in the amphibian is essentially this: in Amphioxus the vegetal hemisphere (prospective endoderm) of the blastula actively moves into the interior of the embryo; in the amphibian the eventual interior position of the endoderm material is due mainly to the enclosing of the yolk-mass by overgrowth (epiboly) carried out by the fold which was initiated by invagination. In Amphioxus the endoderm goes inside; in the amphibian it is put inside by being covered over. Quite clearly the difference is the necessary consequence of the presence of the great mass of inert yolk in the amphibian blastula. Fig. 41. — Gastrulation in the pigeon. Section approximately median, showing formation of endoderm by invagination at posterior edge of blastoderm. A, archen- teron; B, blastocoele (cleavage cavity); BP, blastopore; EC, ectoderm; EN, endo- derm; V, vitelline membrane; Y, yolk. Magnified about 100 diameters. (After J. T. Patterson.) In Reptiles and Birds. A reptilian or avian embryo whose yolk-mass may be millions of times that of Amphioxus could hardly be expected to carry out a process of gastrulation similar to that of Amphioxus — if, indeed, anything comparable to gastrulation were to be recognized at all. Yet the original single layer of the blastoderm, formed by cleavage (Figs. 36, 37), must somehow give rise to additional layers. The fact is that the blastoderm does at an early period become two-layered. The details of the mode of origin of the second layer differ considerably in various members of the Sauropsida. The significant fact is that the deeper layer (endoderm) results, in part if not entirely, from an inward movement of blastoderm cells at the median region of what proves to be the posterior edge of the blastoderm (Fig. 41). This inward movement may consist in the formation of a small pit, an actual invagination, from whose bottom cells move forward and laterally underneath the original blastodermic layer to become the endoderm. In other cases there is merely an in-turning of the mid-posterior edge of the blastoderm without formation of a complete pocket or invagination. In either case the process is confined to the mid-posterior region of the edge of the blastoderm. REPRODUCTION 49 The endoderm, thus initiated, rapidly spreads over the yolk-mass and under the original layer which is now identified as the ectoderm. The growth of the endoderm may be augmented by cells which become detached from the under surface of the outer layer. It is noteworthy that the place of origin of the endoderm in the saurop- sidan embryo is always at the posterior edge of the blastoderm. If the primary blastoderm is to be regarded as corresponding to the animal hemisphere and the yolk-mass to the vegetal hemisphere of the amphibian embryo, then the formation of endoderm in the sauropsidan embryo begins at a point which corresponds very closely to the position of the primary gastrular invagination in the amphibian (Fig. 40,!', /). This fact, together with later events in the sauropsidan embryo, justifies the application of the term, blastopore, to the aperture of the little invagina- tion or the slit formed by infolding of the hind edge of the blastoderm. Comparisons. Comparison of the early development of Amphioxus, amphibian and reptile or bird compels the conclusion that, were it not for difference in volume of yolk, the several embryos would be practically alike in form, at least through the gastrula stage. It is as if the embryo with the larger yolk mass " tried " to behave like the embryo of Amphioxus but is compelled by the yolk to modify its behavior. Amphioxus with total and nearly equal cleavage; the amphibian with total but very unequal cleavage; the reptile or bird with partial cleavage; the several embryos at corresponding stages exhibiting radical differences in the configuration of their materials — ^yet analysis of the processes concerned in the develop- ment of all these animals reveals a basic similarity. The actual animal is the protoplasm. Developmental processes are its dynamic expression. Yolk, although necessary, is mere inert luggage. In all these animals its composition is essentially the same. The similarities which exist in spite of variation in yolk volume are certainly much more significant than the differences which exist because of variation in yolk volume. The method whereby the sauropsidan embryo achieves a two-layered condition is not the simplest imaginable. The easy and direct way would consist in the splitting of the original blastoderm to form two layers, an inner and an outer. Such splitting or " delamination " of layers commonly occurs at other stages in development. The fact that the sauropsidan embryo initiates endoderm formation by invagination or infolding at the posterior edge of the blastoderm is open to no better explanation than that there is some necessity of adhering as closely as possible to the developmental methods employed by amphibians and Amphioxus. Such necessity can come only through inheritance. The Third Layer, Mesoderm The greater extent of the ectoderm of the embryo persists as the essential layer, epidermis, of the adult skin. The endoderm gives rise 50 CHORDATE ANATOMY directly to the lining epithelium of the adult digestive tube. But in the adult animal a great complex of structures — muscle, skeleton, central nervous organs, lungs, liver, and the reproductive, excretory and cir- culatory organs, making up the greater part of the bulk and weight of the animal — intervenes between the epidermis and the endodermal digestive epithelium. Some of these intermediate organs take origin directly and independently from the primary ectoderm or endoderm. For example, before the close of the gastrula stage the central nervous organs begin to differentiate from the dorsal ectoderm. Later, lungs, liver and pancreas arise as separate localized outgrowths from the endoderm of the early digestive tube. Others of the intermediate organs have an indirect relation to the primary layers of the gastrula. The close of the gastrula stage is marked by the formation of a layer, or system of layers, of embry- onic material which comes to be interpolated between the outer and inner layers of the gastrula. This middle and third layer, the mesoderm, spreads extensively between the primary layers and at first appears to be quite undifferentiated throughout. Later it undergoes local differentia- tion to form muscle, skeleton, kidneys, circulatory organs and various other structures. In Amphioxus. At the close of the gastrula stage the Amphioxus embryo is approximately ovoidal, the long axis antero-posterior with the blastopore at its posterior end. The dorsal surface of the embryo is somewhat flattened. Figure 38Z) shows a sagittal section of the embryo at this stage. Figure 42.4 shows a section cutting the embryo trans- versely and within the anterior third of its length. Except for the dorsal flattening, the configuration of layers is as simple as possible. Figures B-G show transverse sections at stages successively later than that of Fig. 42/I. Several things are happening simultaneously. A broad band of dorsal ectoderm {NP), slightly thicker than the adjacent regions of the layer, becomes separated, along its right and left edges, from the neighbor- ing ectoderm. This process involves the mid-dorsal ectoderm con- tinuously from the blastopore almost to the anterior end of the embryo. The median ectoderm thus delimited from the lateral ectoderm is the material of the prospective central nervous organ, the neural tube. In this initial stage it is called the neural (or medullary) plate. The dorsal endoderm is at first flattened in conformity with the neural ectoderm but later (Fig. 42D-F) it becomes convoluted along three lines extending lengthwise of the embryo. Its median slightly thicker region becomes sharply folded upward. On either side of this median fold a longitudinal groove appears on the inner surface of the endo- derm. Then the endoderm in the region of each of these grooves assumes the form of a fold extending outward dorso-laterally. Thus arise three folds, one median and a lateral pair, all convex outward, and extending REPRODUCTION 51 .NP Pig. 42. — AMPHIOXUS. Transverse sections of embryos at successively later stages, showing origin of notochord, neural tube and mesoderm. A, section somewhat anterior to the middle of the length of an embryo slightly older than that represented in Fig. 38Z?. E, from embryo having two pairs of mesodermal pouches. G, section near the middle of the length of an embryo having nine pairs of mesodermal pouches. A, archenteron; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NP, neural plate; NT, neural tube. (After Cerfontaine.) 52 CHORDATE ANATOMY nearly the whole length of the embryo. As time goes on these folds become more emphasized, but soon a difiference arises between the median fold and the lateral folds. The median fold remains continuous through- out its entire length. The lateral folds, however, become interrupted by the formation of sharp deep transverse folds which cut from above downward through each lateral fold. This process of subdivision or segmentation begins near the anterior ends of the lateral folds. Its immediate result is a pair of approximately globular pouches lying sym- metrically either side of the median fold, each pouch having a small central cavity opening by a narrow passage into the archenteron. Later this passage is closed and then the pouch becomes detached (Fig. 42F) ^MES 1-6^ EC Fig. 43. — AMPHIOXUS. Frontal (horizontal) section of an embryo having six pairs of mesodermal somites. The section is through the notochord and just below the blastopore. At the posterior end of the section may be seen a region where the notochord, endoderm and mesoderm merge indistinguishably. A, archenteron near the blastopore; EC, ectoderm; EN, endoderm; MES i-6, mesodermal somites; NC, notochord. (After Cerfontaine.) from the archenteric wall which, at the place where the pouch had formed, closes so that nothing is left to mark the spot. Immediately behind each pouch of the first pair another similar pouch forms exactly as the first did. At this stage of development, marked by the presence of two pairs of these pouches, the embryo escapes from the egg membrane ("hatches"). The period between fertilization and hatching varies considerably, its average being probably not far from twelve hours. These two pairs of pouches derived from the dorso-lateral endodermal wall of the archenteron constitute the first definitely delimited mesodermal material. The remainder of the dorso-lateral folds, extending back to the blastoporal region, is destined to give rise, after hatching, to additional mesodermal pouches. The median endodermal fold, which has remained intact during this process of segmentation of the lateral folds, is the material of the future notochord. (Fig. 42, .VC) REPRODUCTION 55 At the time of hatching, then, the embryo has made important progress beyond the gastrula stage. ' Not only has the segregation of mesoderm begun but two important organs, the central nerve tube and the notochord are indicated. After hatching, additional pairs of mesodermal segments are cut off from the lateral mesodermal folds, the addition taking place progressively from anterior to posterior, until a total of ordinarily fourteen pairs have been produced. In several of the more posterior segments cavities do not occur, the mesodermal folds merely breaking up into a succession of solid blocks of cells. (Fig. 42G) In the formation of these fourteen pairs of mesodermal pouches the material of the original mesodermal folds is completely utilized. NP\ ,NC Fig. 44. — Sectiuns of an amphibian embryo at an early stage in the development of the notochord and mesoderm. Semi-diagrammatic. A , median longitudinal section. B, transverse section near the middle of the longitudinal axis. A, archenteron; BP. blastopore; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NP, neural plate. Later the series of segments is extended backward by addition of successive solid blocks of cells which become detached from the growth zone encircling the blastopore (Fig. 43). By this means the number of pairs of mesoder- mal segments is increased to the adult total, usually sixty-one. In Amphibians. In amphibians, as in Amphioxus, the blastoporal rim or germ-ring is the all-important source of mesoderm. The amphib- ian, however, gives little evidence of anything comparable to the paired mesodermal pouches which push out from the dorso-lateral endoderm of Amphioxus. During the process of gastrulation in the amphibian the material destined to become mesoderm lies within the advancing edge of the over- growing fold (Fig. 40) which is the chief agency in the enclosing of the yolk. As the edge of this fold, the narrowing blastoporal rim, advances, it (in effect) leaves behind it — "behind" being anterior because the fold advances posteriorly — a trail of potential mesoderm which, however, is at first in no way distinguishable from other material destined to be 54 CHORDATE ANATOMY permanently endoderm (Fig. 4.0C, EN). That is, the two materials together and in no way delimited from one another constitute the deeper layer of the overgrowing fold. Later this layer virtually splits (the process called delamination) to form two layers, an inner one abutting on the archenteric cavity and an outer one which is then recognizable as a definite mesoderm (Fig. 44B). This layer, although now distinct from the endoderm which parallels it, for a time retains continuity with its source, the proliferation zone about the blastopore (Fig. 44-4). Initiated in this way, the mesoderm extends into the lateral and anterior regions of the embryo partly by growth within itself, partly by continued con- tributions from the blastoporal growth zone and possibly augmented by the detachment of cells from neighboring surfaces of the endoderm. The mesoderm of Amphioxus is segmented at the time of its detach- ment from the primary gastrular layers and some of the more anterior segments are hollow. The amphibian mesoderm is primarily unseg- mented and solid. In view of the fact that it later acquires segmentation and hollow- ness these initial differences are outweighed by the essential similarity in the relations to the blastoporal region. In Reptiles and Birds. In reptiles and birds endoderm is initiated by a small invagination or infolding at the posterior edge of the early blastoderm (see page 48). The abortive blastopore thus produced exhibits the usual feature of a blastopore in that, in terms of germ layers, it is an in- different region where ectoderm and endoderm merge together without sharp demarcation (Fig. 41). Following gastru- lation the blastodermal lavers continue to ■t~-o ^A-V V , iit2c p -y^_^ *■ ■J' Fig 45 — Surface view of blastoderm of chick after 15 hours incubation. C, "anterior cres- cent," occasioned by an irregular fold of underlying endoderm; M, region occupied by mesoderm; spread rapidly over the surface of the yolk. the blastoderm; P, area pellucida — transparent in absence of ad- hering yolk (see Fig. 41); PS, primitive streak. X 14. (After Duval, "Atlas d'Embryologie.") O, area opaca whose opacity is , • 1 1 • 1 caused by adherence of yolk to In SO domg, the growth posteriorly causes the somewhat thickened region of the blas- toporal rim to become drawn out into a long streak, the primitive streak, lying in the median line of the blastoderm (Fig. 45). Along the whole extent of this modified blastoporal region the ectoderm and endoderm merge without sharp demarcation just as they did in the earlier blastoporal walls (Fig. 46). This primitive streak is the primary seat of mesoderm formation. Rapid proliferation of cells within the substance of the thickened streak gives rise to masses of cells which move out into the space between ecto- REPRODUCTION 55 derm and endoderm (Fig. 46, MES). These masses of cells increase by continued contribution from the streak and by growth within themselves and soon become arranged in a layer which rapidly grows laterally and forward from the primitive streak and always in the space between ectoderm and endoderm. This layer, like the early mesoderm of amphib- ians, is at first unsegmented and devoid of cavity. Y- -^.'r^r^,**,' Fig. 46. — Section transverse to the primitive streak of a chick embryo of about 15 hours incubation. The section is taken near the middle of the length of the streak. EC, ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak; y, yolk at inner margin of area opaca. X 100. (After Duval, "Atlas d'Embryologie.") In the sauropsidan embryo, then, as in the amphibian, rapid growth and cell proliferation within the blastoporal rim is the primary source of mesoderm. Early Development in Placental Mammals The early development of placental mammals exhibits features peculiar to the group and more or less difficult of comparison with any- thing in the development of lower vertebrates. The minute egg (Fig. 29) CV_- A. Fig. 47. — Early stages in development of a rabbit. A, morula stage, 47 hours after coitus; B, early blastodermic vesicle, 80 hours; C, blastodermic vesicle at 83 hours. The investing layers of the einbryo are not shown. CV, cavity of blastodermic vesicle; 7, inner cell-mass; T, trophoblast. Magnified about 285 diameters. (After Assheton.) contains a bare minimum of yolk. Cleavage is total, more or less unequal and often very irregular in respect of planes and sizes of cells (Fig. 47.-I). The cells resulting from cleavage remain in a solid cluster, the morula, until as many as sixty or seventy cells are present. Then, as the number increases further, a cavity appears within the morula (Fig. ^jB-C). Most of the cells remain in a solid group at one side of the cavity whose wall elsewhere is only one cell thick. At this stage the embryo looks like a blastula, but further development proves that the stage is not the CHORDATE ANATOMY equivalent of a blastula of a lower vertebrate. The term, blastodermic vesicle, is applied to this stage of the mammalian embryo. The definitive embryo is developed entirely from the thick cell-mass of the vesicle. The thin region (trophoblast Fig. 47, T) of the wall of the vesicle becomes concerned with the early attachment of the embryo to the wall of the uterus. The fluid-filled cavity of the blastodermic vesicle rapidly enlarges and meanwhile the thick cell-mass splits off a thin layer adjoining the cavity (Fig. 48). This inner sheet of the thick mass then extends over the inner surface of the thin wall of the vesicle and ordinarily completely lines it. The vesicle as a whole thereby becomes two-layered throughout, a condition which characterizes a gastrula stage. The fur- S - Fig. 48. — Early stage of the Fig. 49. — Embryonic area or "shield" of the blastodermic vesicle of the hedge- blastodermic , vesicle of the rabbit after about hog. EC, ectoderm; EN, endo- 172 hours development. PS, primitive streak- derm; L, lacunae, spaces occupied 5-5. position of section represented in Fig. 50. by maternal blood; T, trophoblast (After Assheton.) (trophoderm). (After Hubrecht.) ther history of the two layers identifies them as embryonic ectoderm and endoderm. However, both in mode of origin and in further history the mammalian embryo at this stage shows perplexing discrepancies as com- pared to the gastrula of a lower vertebrate. As stated above, the material which constitutes the definitive embryo is within the thick and solid cell-mass (Fig. 47, /) of the early blastodermic vesicle. As development proceeds the behavior of this cell-mass is very much like that of the blastoderm of the embryo of a reptile or bird. If the cavity of the vesicle were occupied by yolk instead of b>- a watery fiuid the whole embryonic complex would resemble closely an early reptilian embryo. The thick cell-mass, lying in relation to the vesicular cavity much as the reptilian blastoderm lies upon the surface of the yolk, flattens and thins out to form the embryonic shield (Fig. 4q) in the axis of which appears an elongated thickening similar to the primitive streak KEPKUDUCTION 57 of a sauropsidan embryo. At the anterior end of this mammalian streak is usually found a small pit or even a perforation extending through the shield into the cavity of the blastodermic vesicle — very suggestive of an abortive blastopore. It is along this mammalian primitive streak, as in the similar sauropsidan structure, that rapid proliferation of cells produces a mesoderm (Fig. 50) which progressively interpolates itself between the already separated ectoderm and endoderm and spreads eventually into all regions of the embryo. The mesoderm is at first a continuous layer — unsegmented — and devoid of cavity. VMES 'en Fig. 50. — Transverse section of the embryonic shield of a rabbit at the stage repre- sented in Fig. 49. The section is taken at the position indicated by the line 5-5 in Fig. 49. EC, ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak. X175. (After Assheton.) In a rabbit embryo the embryonic shield is established ordinarily by the fifth day of development, the entire blastodermic vesicle then having a diameter of about 1.5 mm. The early development of the placental mammal presents many perplexing features. It could be expected that the minute egg, unem- barrassed by yolk, would revert to the relatively simple and direct methods of early development which, for the most part, characterize Amphioxus. But it does not. Mammalian stages precisely comparable to the blastula and gastrula of Amphioxus or amphibians cannot be recognized. When it comes to the formation of mesoderm, the laying out of the germ layers, and the early shaping up of the embryo, the behavior of the mammal is closely similar to that of a reptile or bird. This similarity exists in spite of the absence of a large yolk-mass in the mammal. These facts point to the conclusion that the developmental behavior of the reptilian embryo had become so strongly established in the protoplasm of ancestral reptiles and primitive mammals that it persisted even though the reduction of yolk had removed the immediate necessity for many of its peculiarities. The many millions of years of primitive mammalian and of reptilian lineage constituted a barrier quite impassable by any tendency for rever- sion to the indefinitely more remote developmental methods of primitive Amphioxus-like chordates. Unquestionably the yolk content of the chordate egg is much more readily subject to evolutionary change than is the developmental mecha- nism of the germinal protoplasm. That mechanism can be changed. 50 CHORD ATE ANATOMY but there is a high degree of inertia about it. The initiation of evolu- tionary change is evidently not within the embryo itself. Its inertia is such that it tends always to follow the old methods and it changes only as it must. Organogenesis The earlier period of development is concerned with laying out the building materials, the embryonic or "germ" layers. In the later and longer period these layers are shaped into organs. The formation of the central nervous organs and the notochord may begin, however, before the mesoderm is fully established. Amphioxus, partly because it is so small and partly because it is in so many respects primitive, affords what may be regarded as a simplified and diagrammatic view of the early relations of the organs in chordates. Organogenesis in Amphioxus In the preceding account of the early development of Amphioxus the embryo has been followed to a stage where the mid-dorsal ectoderm has become delimited from the lateral ectoderm to form the neural plate, the mid-dorsal endoderm has given rise to a sharp thick upward fold which is the prospective notochord, and paired mesodermal pouches are in process of formation from the dorsal endoderm either side of the noto- chordal fold, the pouches increasing in number by addition of new pouches in successively more posterior positions. (Figs. 42 and 43) In the course of further development the thickened ectodermal neural plate becomes depressed slightly below the level of the neighboring lateral ectoderm (Fig. ^2B-D). Along the line of demarcation between neural plate and lateral ectoderm separation occurs following which the lateral ectoderm extends progressively over toward the median plane and external to the neural plate. Eventually the edges of the right and left sheets of ectoderm meet in the median plane and coalesce to form a con- tinuous layer above the neural plate (Fig. ^2E), Meanwhile the neural plate transforms itself into a tube by bending its lateral regions upward and inward until the edges meet in the median plane where they become joined. (Fig. 42F-G) The neural plate originally extends back to the blastopore. The over-arching process whereby the neural plate is covered proceeds back- ward and around the posterior margin of the blastopore. Thus neural plate and blastopore come to lie under a common roof of ectoderm and the blastopore, no longer opening directly to the exterior, opens into the small space between the neural plate and its newly acquired ectodermal roof. The resulting relation of layers and cavities are shown in Fig. 51, a sagittal section of an embryo at this stage. Upon conversion of the REPRODUCTION 59 plate into a tube, the blastopore is left in communication with the lumen of the tube. At its anterior end the closure of the neural tube is delayed so that for a time its lumen is open to the exterior by a small aperture, the neuropore. The extraordinary result of these changes is an embryo whose prospective digestive cavity, still devoid of definitive mouth and anus, communicates via the neurenteric canal (the former blastopore) with the hind end of the cavity of the prospective spinal cord and thence to the outside by the anterior neuropore (Fig. 5i,P). These relations, however, are merely temporary. Eventually neuro- pore and neurenteric canal close. The definitive enteric apertures, mouth, gill clefts and anus, arise by very similar processes. At the UP ,NC Fig. 51. — AMPHIOXUS. Median longitudinal section of an embryo having two mesodermal pouches, a stage approximately like that of the transverse section in Fig. 42E. The blastopore, roofed over by ectoderm, has become the neurenteric canal. .4, archenteron; EC, ectoderm; EN, endoderm; NC, endoderm destined to become notochord; NE, neurenteric canal; NP, neural plate; P, neuropore. X350. (Based on a figure by Hatschek.) appropriate locality enteric endoderm and superficial ectoderm approach one another and coalesce. The resulting double layer then thins out until perforation occurs. The notochord, whose development is initiated by an upward folding of mid-dorsal endoderm (Fig. 42D-F), early becomes detached from the enteric endoderm and acquires its characteristic cylindrical form. The enteric endoderm meanwhile closes in beneath the notochord and restores the integrity of the dorsal wall of the enteron (Fig. 42G). As the embryo increases in length the notochord grows within itself and receives accessions from the active blastoporal region with which its posterior end remains for some time connected (Fig. 43). The more anterior mesodermal pouches (or somites), soon after their formation and long before the more posterior somites have been developed, begin to acquire their characteristic differentiation. The pouch expands, especially ventralwards, and its cavity is correspondingly enlarged. That part of its wall lying against the notochord becomes much thickened 6o CHORDATE ANATOMY -EC -NC -M -MC -EN while elsewhere the wall remains relatively thin. The expansion of the pouches continues until the walls of right and left pouches meet in the median plane beneath the enteric endoderm. At this stage three regions of the mesoderm may be distinguished: the thickened part lying alongside the notochord; an outer thin layer contiguous to the ectoderm; and an inner thin layer similarly contiguous to the endoderm. The thick part is destined to form a segment of body-muscle and is therefore called the myotome (Fig. 52,M). The outer layer, being, in conjunction with the ectoderm, the body-wall of the em- bryo, is called the somatic or parietal layer. The inner layer, associated with the wall of the enteron, is called visceral or splanchnic. The now capacious cavity resulting from expansion of the pouch is a segment of the embryonic body-cavity or coelom. The myotome rapidly thickens and also increases its dorso-ventral extent. As it thickens, the adjacent upper portion of the coelomic space is correspondingly reduced. Pig. 72*X^MPHI0XUS. Eventually the somatic and visceral layers Transverse section midway of become joined by a horizontal septum formed the length of the body of a . ^„. x r-^^ larva with five gill clefts, just below the myotome (Fig. 52). Con- C, coelom; EC, ectoderm; sequently a lower major part of the original EN, endoderm; I, intestine; . . . j r ^^ M, myotome; MC, myocoei; coelomic space IS Separated from an upper NC, notochord; NT, neural remnant of it, the myocoele (MC) which, with tube; V, subintestinal vein. . • r .^i ^ • £ „ii, (Modified from a figure by contmued expansion of the myotome, IS hnaliy Hatschek.) obliterated, while only the lower cavity partici- pates in forming the definitive coelom (C). The thin portion of the wall of the myocoele later gives rise to connective tissue including the myoconmias which intervene between and tie together successive seg- ments of muscle. As a result of the general expansion of the mesodermal layers, not only, as stated above, are the walls of right and left pouches brought together in the mid-ventral region, but the adjacent walls of successive pouches on the same side of the embryo become closely pressed together. At this stage, then, the paired coelomic spaces of the several pouches are separated from one another by thin partitions, some transverse and others median, each consisting of two layers of cells. These partitions become progressively thinner until they perforate and finally completely disappear except that remnants of the median ventral wall may persist in connection with the development of blood-vessels. With the oblitera- tion of these partitions, the several segmentally developed coelomic REPRODUCTION 6i cavities are all thrown into free communication to form one large space, the definitive coelom, which finally shows no trace of its segmental origin. An embryo of Amphioxus, at a stage when fourteen or fifteen pairs of mesodermal pouches are present, is a delicate, colorless, transparent animal having a length of about one millimeter and a diameter of one- eighth that except at the somewhat enlarged anterior end (Fig. 53). It has a straight digestive tube (enteron, /) extending from an anterior mouth to a posterior anus. There is a single gill cleft, opening from the right side of the anterior region of the digestive tube. The mouth also is unsymmetrical at this stage, opening on the left side. Later, as numer- ous additional gill clefts are formed, they shift their positions so as to become ultimately a series of symmetrically placed paired apertures. Meanwhile the mouth shifts from its original left to a median position. Just above the digestive tube lies the median rod-like notochord (NC) extending the entire length of the animal. Immediately above the Fig. 53. — Amphioxus at beginning of larval period; 14 or 15 pairs of mesodermal somites. Actual length of larva about i.o mm. CG, club-shaped gland; /, intestine; MES, mesodermal somites; NC, notochord; NE, neurenteric canal; NP, neuropore; NT, neural tube; P, pigment spot in neural tube. (After Hatschek.) notochord is the neural tube (A^T), its somewhat enlarged anterior region suggesting a brain. At the anterior end of the neural tube the dorsal neuropore (AP) is still open. The neurenteric canal (NE), at this stage, has ordinarily become closed. In the anterior region, where the differen- tiation of the mesoderm is most advanced, a coelom intervenes between the enteric tube and the outer body-wall (Fig. 52, C). The body-wall (somatopleure) consists of the ectoderm and the somatic layer of meso- derm. The enteric endoderm together with the contiguous visceral or splanchnic layer of mesoderm constitute the wall (splanchnopleure) of the digestive tube. The somatic and visceral sheets of mesoderm provide the coelom with a continuous and complete lining, the peritoneum. The superficial ectoderm is a skin. The more anterior myotomes contain partially differentiated muscle tissue capable of feeble contraction. The animal is free-swimming but the locomotor mechanism consists merely of long cilia produced by the ectodermal layer. In its main features this young Amphioxus is like a vertebrate. If its true origin and nature were not known, it might reasonably be expected to 62 CHORDATE ANATOMY proceed to develop directly into a tvpical vertebrate. But it does not. It acquires no vertebral column; the notochord serves as definitive axial skeleton. It develops no structures morphologically similar to the heart, kidneys, specialized sense organs, or paired appendages of a vertebrate. Further, in later development it acquires, especially in the head region, a variety of unique structures which adapt the adult to its peculiar mode of living but make it conspicuously unlike any adult vertebrate. Nevertheless Amphioxus is "verte- brate" in too many features to make it credible that they could have arisen otherwise than in gene- tic relationship with those of the vertebrates. Herein, then, lies in part the justification for describing the early development of Amphi- oxus to illustrate the main features of the corresponding stages of vertebrates. Further justification is derived, as already stated, from the fact that the paucity of yolk in the egg of Amphioxus relieves the embryo of the factor which in- troduces varying degrees of com- plication into the development of vertebrates and occasions much difficulty in the study and inter- pretation of the processes. Organogenesis in the Vertebrates In the late embryo of Amphi- oxus the main lines of the body plan of a vertebrate are drawn. Brief statements concerning the embry- D V Fig. 54. — Diagrams illustrating method of origin of the neural tube of vertebrates. Transverse sections in the mid-trunk region of embryos at successively (A to D) later stages. C, neural crest; CC, canalis centralis of neural tube; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NG, neural groove; NP, neural plate; NT, neural tube; V, blood-vessel onic Origin of the major Organs of (paired dorsal aorta). vertebrates follow. Neural Tube. In Amphioxus the neural plate becomes detached from the adjacent lateral ectoderm (Fig. 42) and transforms itself into a tube not until after it has been covered by the lateral ectoderm. In vertebrates a longitudinal folding of the neural plate and adjoining ecto- derm occurs in such a way that the movement of the neural material REPRODUCTION 63 into a deep position, its conversion into a tube, and the covering of it by lateral ectoderm take place simultaneously (Fig. 54). Not until the tubular form is attained does the neural ectoderm of vertebrates become detached from the overlying superficial ectoderm. Figure 55 shows, in a diagrammatic way, the characteristic appearance of a recently formed neural tube with its neural crests, dorso-lateral extensions of ectodermal material on each side of the tube. Later the neural crest becomes detached from the tube, undergoes segmentation corre- sponding to that of the myotomes, and gives rise to spinal ganglia (Fig. 345). Cells of the crest become ganglion cells whence grow out nerve fibers which constitute the dorsal sensory root of a spinal nerve. The fibers of the other Fig. 55.— Stereogram of embryonic constituent root of a spinal nerve, the "''^'■^} ^^^^ showing the segmenting neural crest, e, superficial ectoderm; ventral motor root, grow out from cells nc, neural crest; s, central canal. within the neural tube. Some cells of (^^°"^ Kingsley, " Comparative Anat- omy of Vertebrates.") the neural crests migrate mto various visceral localities and give rise to ganglia ("sympathetic"; Fig. 345) and nerves of the autonomic system. The anterior region of the tube expands to form the brain. Three enlargements, the primary brain vesicles — fore-brain, mid-brain and hind-brain (Figs. 57, 58) — characterize the cephalic part of the tube in all vertebrate embryos. Later subdivision of the first and third vesicles results in the five brain regions universally characteristic of adult verte- brates. The nervous structures (retina and optic nerve) of the paired eye grow out from the second (numbered from the front) region but the lens of the eye is derived from neighboring superficial ectoderm (Fig. 56). The receptor (that is, stimulus-receiving) nervous structures of the ear and olfactory organ originate not from the neural tube but from super- ficial ectoderm. Notochord. The notochord in the several classes of vertebrates exhibits many variations in details of its mode of origin. The essential fact is that, in vertebrates as in Amphioxus, its material is derived from mid-dorsal endoderm and from the actively growing region about the blastopore. In amniotes the origin of the notochord is closely related to that of the mesoderm. Its material, like that of the mesoderm, usually seems to be derived from the primitive streak (see page 54), a region where ectoderm and endoderm merge indistinguishably. As cells proliferated from the streak laterally give rise to mesoderm, so proliferation forward from the anterior end of the streak produces a median cord of cells which form the notochord. It may, however, receive 64 CHORDATE ANATOMY accessions from the endoderm with which it is usually in close relation. The Enteron. Gastrulation produces a two-layered embryo whose endoderm surrounds a cavity opening to the exterior by the blastopore. This archenteric cavity is the prospective digestive cavity. As the embryo elongates, the cavity is correspondingly elongated and in later develop- ment the enteric tube increases in length faster than the embryo with result that the tube becomes bent or even coiled to adapt itself to the coelomic space. In the early embryo the ectoderm at a median anteroventral position gives rise to a shallow depression or pit, the stomo- deum, whose deeper wall meets the forward-growing endoderm to form tem- porarily a two-layered oral membrane (Figs. 57, O and -720) separating the external stomodeal cavity from the enteric cavity. Soon a perforation appears at the center of the membrane and its peripheral remnant is rapidly obliterated. The per- foration and obliteration of the membrane apparently result from progressive centrif- ugal flow or movement of its cellular sub- stance. Thus is formed the mouth. The posterior enteric aperture or embryonic "anus" develops usually by a similar process. The blastopore rarely persists as a definitive posterior aperture although it does so in cyclostomes and possibly in some urodele amphibians. Otherwise, exactly as in Amphioxus, it becomes roofed over by the neural folds and thus converted temporarily into a neurenteric canal (Fig. 57) connecting the hind ends of neural tube and enteric cavity. An ectodermal pit, the proctodeum, situated just below the neurenteric canal, perforates into the hind end of the enteric cavity to form the definitive hind aperture, either anal or cloacal (Fig. 57). As result of the mode of development of the enteric apertures, the lining of more or less of the mouth cavity is derived from stomodeal ectoderm and that of the posterior region from proctodeal ectoderm. The remaining and by far greater part of the adult enteric tube is lined by endoderm which constitutes the digestive epi- thelium, the essential secreting and absorbing layer of the tube. It is a noteworthy fact that various organs which have nothing directly to do with digestion have their origin in the enteric endoderm. The Fig. 56. — Stereogram of the developing eye. The head of the embryo is cut transversely in the region of the fore-brain. cf, choroid fissure; fb, wall of fore-brain; I, ectodermal thicken- ing which invaginates to form lens; oc, optic cup; os, optic stalk; p, outer thin wall of optic cup, becoming the pigmented epithe- lium which lies behind the defini- tive retina; r, inner thick wall of optic cup, becoming the sensory retina of the eye. (From Kings- ley, "Comparative Anatomy of Vertebrates.") REPRODUCTION 65 anterior region of the embryonic enteron — the part becoming the pharynx of the adult — is concerned particularly with the organs of respiration. Gills of fishes and amphibians develop in relation to paired apertures, the pharyngeal or visceral clefts, which pierce the lateral walls of the enteron and the ectoderm and open to the exterior. A pharyngeal cleft is developed as follows. A deep lateral pouch or furrow of the endoderm bulges outward and meets a similar but shallower pouch or furrow which EC NT NC EN Fig. 57. — Frog: median longitudinal sections of embryos; .4, just before conversion of blastopore into neurenteric canal; B, just after formation of neurenteric canal and perforation of proctodeum to form cloacal aperture. B, brain; BP, blastopore; C, cloacal aperture; EC, ectoderm; EN, endoderm; H, hypophysis; HT, heart; MES, mesoderm; NC, notochord; A'^jE, neurenteric canal; AT, neural tube; O, region where mouth will perforate; P, proctodeum; PH, pharynx; R, rectal region of enteron; Y, yolk cells of endoderm; i, fore-brain; 2, mid-brain; 3, hind-brain. A, X24; B, XiQ. (Redrawn from Marshall, "Vertebrate Embryology.") the ectoderm pushes inward. The resulting two-layered membrane is then obliterated by the same process which removes the oral membrane, leaving a free passage between the pharynx cavity and the exterior. Vascular complications of the endodermal lining of these clefts produce internal gills — although it is possible that some so-called internal gills are derived from ingrowing ectoderm. External gills are ectodermal structures developed in close relation to the external apertures of pharyn- geal clefts. In amniotes the pharyngeal pouches are merely temporary 66 CHORDATE ANATOMY embryonic features except as those of the first pair are, in a modified way, represented in the auditory passages. Lungs develop by outgrowth from the endoderm of the pharynx (Figs. 235, 238). The entire epithelial lining, being the essential respi- ratory membrane, of the adult lung, is endodermal and continuous, byway of the lining of bronchi and trachea, with the lining of the digestive tube. The air bladders (swim-bladders) of fishes are endodermal sacs which grow out from an anterior region of the embryonic enteron. They are usually dorsal, rarely lateral, or ventral as in the ganoid Polypterus. The important endocrine glands, thyroid, parathyroid and th3mius, and various gland-like bodies mostly of dubious nature and function, arise as outgrowths of the endoderm of the pharyngeal pouches or the wall of the pharynx. (Fig. 235) More posterior regions of the enteric endoderm give rise to various accessory digestive organs, most important of which are the liver and pancreas. The liver develops as a mid-ventral outgrowth, sometimes more than one, from the anterior region of the prospective intestinal portion of the enteron. The pancreas arises similarly and in close relation to the liver. Vascular and connective tissues make up a large part of the bulk of the adult organs but the essential hepatic cells and the secretory tissue of the pancreas are endodermal. The position of the opening of the bile duct into the intestine marks the point of origin of the embryonic liver. The cloaca of the adult vertebrate is a superficial chamber situated at the hind end of the body-cavity and opening ventrally to the exterior. Into it open the intestine and the ducts of the kidneys and genital organs. It is commonly present in vertebrates below mammals except in Teleostei. It is derived from the extreme hind end of the embryonic enteron. Mammalian embryos develop a cloaca but only those primitive mammals, Ornithorhynchus and Echidna, retain it in the adult. In other mammals the embryonic cloaca becomes subdivided into a dorsal part connected with the intestine and a ventral part which receives the urinogenital ducts. In course of further development these two divisions of the cloaca are separated and carried apart and acquire independent openings to the exterior, the latter being the more ventral. Therefore the more distal portion of the urinogenital passage of the adult, both male and female, is a remnant of the cloaca while another remnant of it persists in the posterior region of the rectum. The Mesoderm. The vertebrate mesoderm is at first devoid of seg- mentation and ordinarily contains no definite cavity (Fig. 44). At an early embryonic stage the mesoderm upon either side splits into two layers; an outer, lying against the ectoderm, and an inner lying against the endo- derm. The two layers remain connected, however, at the upper edge REPRODUCTION 67 of the original sheet (Figs. 58, 59.! J. At about the same time the dorsal and thicker part of the mesoderm develops transverse fissures which divide it into a series of paired blocks (somites) lying symmetrically either side of the neural tube (Fig. 58). This segmentation begins in the anterior part of the embryo and progresses backwards just as, in Amphi- oxus, the mesodermal pouches are formed successively from anterior to posterior. The process of segmentation involves only the upper part of the mesoderm. As segmentation goes on, the space between the lower thin and unsegmented layers on either side becomes wider — a space already Fig. 58. — Stereogram of the anterior region of a vertebrate embryo showing the segmentation of the mesoderm. The ectoderm has been removed from the left side of the embryo, al, endoderm of alimentary tube; c, coelom; etn, epimere;/6, fore-brain; hb, hind-brain; km, hypomere; m, myotome; mb, mid-brain; mm, mesomere; n, neural tube; nc, notochord; s, stomodeal region; sk, sclerotome; so, sp, somatic and splanchnic walls of coelom. (From Kingsley.) recognizable as the coelom bounded externally by a somatopleure con- sisting of ectoderm and the outer sheet of mesoderm, and internally by a splanchnopleure consisting of endoderm and the adjacent layer of meso- derm. The mesodermal layers upon either side grow down to the mid- ventral region, carrying with them the coelom, and meet mid-ventrally to form a double vertical layer, a ventral mesentery, extending from the enteron to the outer body wall and separating right and left coelomic cavities. (Fig. 58) The splitting of the original sheet of mesoderm extends so far dorsally as to involve the somite which accordingly contains a more or less definite cavity, the myocoele — "myo-" because the somite is mainly muscle- forming. Shortly the somites become detached from the lower somatic 68 CHORDATE ANATOMY and visceral sheets of mesoderm and the myocoeles lose continuity with the permanent coelom (Fig, 59^). Eventually, as the somite differen- tiates, the myocoele is obliterated. Fig. 59. — Diagrams, (transverse sections) showing embryonic origin of pronephric tubules. A, earlier stage; B, later, c, coelom; d, pronephric tubule and duct; e, epimere; h, hypomere; m, mesomere (cross-lined); my, myotome; n, nephrostome; so, somatic layer of hypomere; sp, visceral (splanchnic) layer of hypomere. (From Kings- .ey, after Felix.) The differentiation of the vertebrate mesoderm is more elaborate than that of Amphioxus, especially in the prospective trunk region. Here, upon each side, early arise three zones of differentiation: the epi- FiG. 60. — Diagrammatic transverse section of the body of a vertebrate embryo at an advanced stage. The muscle-forming myotome is beginning to extend into the ventral body-wall of the embryo, c, coelom; g, genital ridge; jh, muscle derived from myotome; 7nc, myocoele; p, peritoneum; pd, pronephric duct; so, somatic layer (dermatome) of somite; v, advancing ventral border of myotome; the finely dotted areas are occupied by mesenchyme. (From Kingsley.) mere, a dorsal mainly muscle-forming part; the mesomere, a kidney- forming zone situated just below the epimere; and the h5rpomere, the most ventral zone, constituting the somatic and visceral layers of peri- toneum (Figs. 58 and 59^4). REPRODUCTION 69 The epimere undergoes three kinds of diflferentiation. Its heavier inner wall is mainly converted into striated body-muscle, not only the dorsal but the ventral muscle. The myotome material grows ventral- wards, pushing its way between the ectoderm and the somatic meso- derm, until it reaches the mid- ventral plane (compare Figs. 60 and 61). The medial region of the epimere gives rise to loosely aggre- gated cellular masses (mesen- ch5mae) surrounding the notochord and neural tube (Figs. 58, 60). This material produces such supporting structures — connective tissue, carti- lage and bone — as may later be developed around these two axial organs. The thin outer wall of the epimere breaks up to form loose cellular masses, mesenchyme, which give rise to the dermis, the deeper fibrous and vascular layer of the skin. The terms myotome, sclerotome and dermatome are applied respec- tively to the muscle-forming, skeleton-forming and dermis-forming regions of the epimere. (Fig. 58) The mesomeres give rise to the tubular structures of the kidneys. The process begins in the more anterior mesomeres and progresses pos- teriorly. Certain differences in mode of development and in eventual structure compel the distinction between an earlier and more anterior system of tubules, the pronephros (Figs. 59, 62), and a later more posterior and more extensive system, the mesonephros. In anamnia the meso- nephros becomes the adult kidney and the pronephros disappears except that in a few fishes it is the definitive and only kidney. In amniotes, following development of a pronephros and a mesonephros, the tubule- forming process continues backward, but with some modifications, to form a third kidney, the metanephros, which becomes the adult kidney. The tubular epididjnnis, associated with the testis of the adult amniote, is a part of the embryonic mesonephros which otherwise disappears except for certain vestiges which are apparently of little functional importance. Fig. 61.— Diagrammatic transverse section of the body of a vertebrate. av, aorta; c, coelom; e, ectoderm; ep, epaxial (dorsal) muscle; g, gonad; ha, hemal rib; hp, hypaxial (ventral) muscle; i, intestine; mes, mesentery; n, nephrid- ium; o, omentum; r, rib; p, somatopleure; sp, splanchnopleure; v, centrum of verte- bra and, above it, neural arch containing spinal cord. (From Kingsley.) 70 CHORDATE ANATOMY SPINAL CORD SPINAL GANGLION^.. NEPHROSTOME j GLOMERULUS POSTCARDINAL VEIN ■ , ^^ 1 MESONEPHRIC TUBULES PERITONEUM \// MESENTERY PRONEPHRIC TUBULES PRIMITIVE DUCT Fig. 62. — Stereogram of the developing pronephros and mesonephros. (After Kingsley modified.) MESONEPHRIC TUBULE t lYOTOM^ DERMATOME MESONEPHRIC TUBULES ^EURAL TUBE j ', Gl Of 1ERULUS NEPHROSTOME PERITONEUM POSTCARDINAL VEIN I GENfTALRIDGE i PRIMITIVE DUCT MESENTERY COELOM Fig. 63. — Stereogram of the developing mesonephros; stage later than that of Fig. 62. (After Kingsley modified.) REPRODUCTION 71 Meanwhile, as the pronephric tubules form, the mesomere material on each side of the embryo gives rise to a longitudinal tube (Fig. 59) which extends from the pronephric region to the cloaca into which it finally opens. The pronephric tubules of each side join the corresponding longitudinal pronephric duct (Fig. 62) thus putting the coelom into com- munication with the exterior by way of the cloaca. The coelomic open- ings or nephrostomes (Figs. 59^, n and 62) of the pronephros are ciliated. The arrangement apparently serves for drainage from the coelom to the exterior. The mesonephric tubules acc|uire connection with the already-formed longitudinal duct which, as the pronephros degenerates, then serves, at least in part, as the mesonephric or Wolffian duct. In Anamnia usually each mesonephric tubule has a ciliated nephrostome opening into the coelom. In the kidneys of amniotes, nephrostomes rarely appear. Mesonephric and metanephric tubules usually form specialized excre- tory structures. The tubule (Figs. 63, 64) gives rise to a cup-shaped expansion (Bowman's capsule). The hollow of the cup is occasioned by ingrowth of a dense network of fine blood-vessels, the glomerulus. The capsule and glomerulus together constitute a renal (or malpighian) corpuscle. The part of the tubule between the corpuscle and the mes- onephric duct eventually becomes much elongated, coiled and locally differentiated. Fig. 64. — Diagram of renal (Malpighian) corpuscle, a, artery; b. Bowman's capsule; gl, glomerulus; n, nephrostome; t, nephridial tubule; v, vein. (From Kingsley, "Comparative Anatomy of Vertebrates.") In the absence of nephrostomes drainage of waste from the coelom does not occur and the function of excretion must be confined to the renal corpuscle, where the glomerulus brings blood-vessels into close relation to the lumen of a kidney tubule, and to other vascular regions of the tubule. The amniote metanephros has outlet by way of a duct, the ureter, which develops as a forward-growing branch from the cloacal end of the mesonephric duct of the same side of the embryo. The tubular struc- tures of the metanephros are formed largely by outgrowth from the anterior end of the ureter. 72 CHORDATE ANATOMY The adult kidney (Fig. 271) consists of the entire system of tubules — mesonephric or metanephric — of one side of the embryo, increased to great number by formation of secondary tubules from the primary tubules, each tubule tremendously elongated and much coiled, the tubules bound together by connective tissue with blood-vessels richly interspersed, and the whole complex ensheathed by connective tissue and thereby delimited from adjacent tissues of the body-wall. The hypomere mesoderm, later backed up by a layer of connective tissue, becomes the definitive peritoneum. Its somatic layer completely MVJ Fig. 65. — Diagrammatic transverse section of the body of a vertebrate showing relations of organs to the peritoneum and coelom. A, dorsal aorta; C, coelom; EN, endodermal epithelium of digestive tube; G, gonad; /, integument; K, kidney ;L, liver; M, musclelayer of digestive tube; MD, dorsal muscle of body-wall; MV, ventral muscle of body-wall; NC, position of embryonic notochord; NT, neural tube (spinal cord); PP, parietal peritoneum; PV, visceral peritoneum; R, rib; VC, vertebral column. lines the body-wall. Its visceral layer covers the coelomic surfaces of the digestive tube and of all other organs which occupy the coelom. In the median plane at all regions not occupied by median organs (Fig. 58) the right and left visceral layers of the hypomere meet one another to coalesce and become membranes or mesenteries which connect and support the viscera. In later stages of development the mesenteries undergo considerable reduction, especially those between the digestive tube and the ventral body wall (Fig. 60). Figure 65 shows the ideal relations of REPRODUCTION 73 the peritoneum and mesenteries to the coelomic organs. It is clear that no organ can be said to lie in the coelom except as the peritoneum investing that organ is regarded as a part of the organ. In strict sense, median organs lie between the peritoneal sheets of the right and left halves of the body. The peritoneum plays a part in the development of the gonads although it is not necessarily the source of the germ cells. The prospective gonads first appear as longitudinal thickenings or genital ridges in the dorsal peritoneum, one on each side and between the dorsal mesentery and the mesonephros (Figs. 60, 61 and 66). The earlier belief that the germ cells Fig. 66. — Section of genital ridge of a chick of five days incubation, e, peritoneal epithelium of ridge; c, genital cords; o, primordial germ cells. (From Kingsley, after Semon.) • are derived from the peritoneal layer has been shaken by evidence that the primordial germ cells first appear in the mid-dorsal enteric endoderm whence they migrate into the genital ridge. The deeper substance of the definitive gonad is derived either from the thickened peritoneum of the genital ridge or, especially in the male, from the mesoderm of the closely adjacent mesonephros. The gonads find outlet by way of ducts which arise in relation to the kidneys. The seminiferous tubules of the testis acquire connection with the neighboring mesonephric tubules and thereby gain exit by way of the Wolffian duct which therefore, in Anamnia, serves as a urinogenital duct. In amniotes the adult male retains, in the epididymis, that part of the embryonic mesonephros which provided connection between the testis and the Wolffian duct. With metanephros and ureter serving the urinary function,' the Wolffian duct is left as a vas deferens or sperm duct only. 74 CHORDATE ANATOMY Fig. 67. — Transverse sec- tion through the urinogeni- tal region of a four-day chick embryo, g, mesoder- mal epithelium (peritoneum) of genital ridge; w, infolding The oviducts in elasmobranchs and probably some amphibians arise by longitudinal splitting of the pronephric duct, one portion of it serving thereafter as the mesonephric duct while the other portion acquires, by fusion of several pronephric nephrostomes, a wide anterior opening into the coelom in the vicinity of the ovary. In other vertebrates, the oviduct develops as a fold of peritoneum (Fig. 67, m) closely parallel to the Wolffian duct but independent of it. The Mesenchyme. Reference has been made (page 69) to the fact that certain regions of the mesodermal somite, the sclerotome and the dermatome, are the source of cellular material which becomes detached from the somite and aggregates in the spaces between the somite and neighboring organs or layers where it produces skeletal, connective and integumentary tissues. This secondary meso- derm ("derm" implying a sheet or layer), being usually not disposed in definite layers, is called mesenchyme. But the somite is not the only of peritoneum to form Miil- source of mesenchyme. Quantities of it are lerian duct; ms, mesentery; Q^UCed in all rCgionS of the CmbryO. s, mesenchyme cells which i^ ° ■' give rise to the stroma (non-genital tissue) of gonad; t, mesonephric tubules; W, Wolffian duct. (From visceral layers of the hypomere are a prolific Kingsiey, after Waideyer.) ^^^^^^ ^^ -^^ numcrous cells becoming detached from the outer (next the ectoderm) surface of the parietal layer and from the inner (next the endoderm) surface of the visceral layer. Also the endoderm contributes to the mesenchyme which accumulates between the enteric wall and the adjacent layers of mesoderm. The ectoderm plays a minor part but evidence has been found indicating that mesenchyme of ectodermal origin, "mesectoderm," participates in the development of parts of the skeleton of the pharyngeal region. Mesenchyme spreads from its place of origin and eventually is found in all parts of the embryo. Although late in origin, its importance is by no means secondary. Chief among its derivatives are the following materials and structures. Fibrous connective tissue is omnipresent in the adult vertebrate. It invests, supports, connects, separates or cushions parts of the body. Every location where cartilage or bone is destined to develop is occu- pied by mesenchyme. The deeper parts of the skull, the vertebral column, ribs, sternum and the skeleton of the paired appendages are first constructed of cartilage. The entire endoskeleton is permanently Beyond question, most of the mesenchyme comes from the mesoderm. The parietal and REPRODUCTION 75 cartilaginous in elasmobranchs. Cartilage is a direct product of mesen- chyme. Cells of the mesenchyme become cartilage cells (Fig. 68) and deposit the ground substance or matrix of the cartilage. In the great majority of vertebrates the primary cartilaginous skeletal structures are, Mes. PrcCart. Cart. A'- m. 0': '"^^^^^j|^ Fig. 68. — Diagrams illustrating formation of cartilage by mesenchyme. A, in fishes, according to Studnicka; B, in mammals, according to Mall. Cart., cartilage; Mes., mesenchyme; Pre. Cart., precartilage. (From Bremer, "Text-book of Histology.") in later development, more or less completely replaced by bone. The process of replacement (Fig. 178) involves the destruction of the greater part of the cartilage. The remnants of the cartilage are in form of a spongy meshwork whose strands become calcified and serve as a frame- work upon which bone-producing cells, osteoblasts, build up bone. Osteoblasts. Calcifying connective-tissue bundles. Bone matrix. Bone cells. Fig. 69. — Development of dermal (secondary) bone from mesenchyme. From a section of the mandible of a human embryo of four months. X240. (From Bremer, "Text-book of Histology.") In the development of certain of the more superficial bones of the cranium, the outer bones of the jaw skeleton and some parts of the shoulder girdle, no cartilage is formed. Mesenchyme cells, becoming osteoblasts, build up bone directly on the surfaces of strands of calcified connective 76 CHORDATE ANATOMY tissue (Fig. 69). Most of the bones which develop in this manner are derived from the embryonic mesenchyme of that same general superficial layer which otherwise gives rise to the dermis of the skin. They are accordingly called dermal bones. Bone resulting from replacement of cartilage is called cartilage bone. Mesenchyme is the source of nearly all unstriated or "smooth" muscle, whether in the walls of viscera or in the body-wall. Most visceral organs are hollow. In their early embryonic stages their primary and essential walls are either endoderm as in the case of the digestive tube, lung or urinary bladder; or mesoderm as in the urinogenital ducts. The outer surfaces of these primary walls are always adjacent to regions occupied by mesenchvme. The unstriated muscle fibers of these organs are differen- FiG. 70. — Diagrammatic transverse sections of developing heart. In A the descend- ing right and left mesodermal hypomeres have nearly met; mesenchyme cells appear between them. In B the layers have met ventrally forming the ventral mesocardium; the enclosed mesenchyme has formed the endocardium. In C the layers have met dorsally to form a dorsal mesocardium; meanwhile the ventral mesocardium has dis- appeared and the right and left coelomic spaces have become the pericardial cavity. c, coelom; ec, ectoderm; en, endoderm; end, endocardium; m, ventral wall of hypomere; p, pericardial cavity; v, mesenchyme cells. (From Kingsley, "Comparative Anatomy of Vertebrates.") tiated from cells of the adjacent mesenchyme. Unstriated muscle fibers occur in the walls of larger blood-vessels and of some integumentary glands where they serve to expel the contents of the gland. Hairs and feathers are erected by contraction of delicate muscles, usually unstriated. The dilators fibers in the iris of the human eye, however, are apparently of ectodermal origin. The statement that blood-vessels are derived from mesenchyme is probably admissible although some vessels seem to arise fairly directly from the mesoderm. They may arise as solid cords of cells, later becoming hollow, or may be hollow from the beginning. The essential wall or endothelium having been established, the outer layers of connective tissue and unstriated muscle are provided by adjacent mesenchyme. The heart develops in the region just behind that where the pharyngeal clefts are forming. The right and left hypomeres of the mesoderm push ventralwards and in the median ventral space between them (Fig. 70) accumulate cells derived from the adjacent hypomeres, therefore essen- tially mesenchymal. These cells arrange themselves to form a very thin layer which becomes the endothelial lining or endocardium of the prospec- REPRODUCTION 77 tive heart. In some cases, at first two endothelial tubes are formed, lying side by side, later coalescing into one. The thick muscular layer (myocardium) and the outer layer (epicardiimi) of the wall of the heart, also the pericardium lining the pericardial cavity, are derived from the adjacent hypomeric mesoderm. The heart muscle, however, unlike that of blood-vessels, is striated. The transverse septum, separating pericardial from abdominal cavity, consists of pericardium in front and peritoneum behind, with connective tissue between. The diaphragm of the mammal is not the exact equivalent of the transverse septum of other vertebrates (Fig. 71). That part of the coelomic space lying on the cephalic side of the diaphragm is subdivided into three cavities, the pericardial and the right and left pleural cavity containing the corresponding lobes of the lungs. The Fig. 71. — Diagrams showing the relations of the coelomic cavities (black) in fishes (^4), amphibians and sauropsida (B), and mammals (C). L, liver; P, lungs; S, septum transversum; D, diaphragm. In B the lungs lie in the peritoneal (or pleuroperitoneal) cavity; in C they occupy special pleural subdivisions of the coelom. (From Kingsley.) diaphragm is muscular. Its muscle is striated and, like body-wall muscle, is derived from epimere mesoderm. Strangely, however, it is mesoderm which shifts backward from somites of the neck region. This accounts for the innervation of the diaphragm by cervical spinal nerves. Head, Neck, Tail. The mesoderm of the head is less definitely seg- mented than that of the trunk. The six muscles, consisting of striated fibers, which effect the movements of the eyeball in its orbit are developed from head mesoderm which is probably the equivalent of three somites or epimeres of the trunk. There is nothing corresponding to the mesomere of trunk mesoderm. The neck region, whether or not differentiated externally, corresponds approximately to that of the embryonic pharyngeal pouches. In this region the dorsal mesoderm forms epimeres which give rise to neck muscles. The lateral mesoderm, remaining unsegmented,* corresponds to the hypomere of the trunk. Whereas the trunk hypomere forms onl}- 78 CHOKDATE ANATOMY the unstriated muscle of the digestive tube and other visceral parts, the pharyngeal h>^omeric mesoderm produces striated muscle which dififeren- tiates into an elaborate system of muscles (branchiomeric muscles) related to the skeleton of the jaws and gill region (Fig. 189). The tail is produced by growth of ectodermal and mesodermal parts backward from the region of the blastopore. Growth of the mesoderm keeps pace with that of the neural tube and notochord. The mesoderm forms somites which produce the segmental striated caudal muscle and the mesenchyme which gives rise to skeletal, vascular and connective- tissue structures of the tail. Relation of Yolk to Organogenesis Cleavage, gastrulation and the mode of origin of the mesoderm and the notochord are necessarily much affected by the presence of the bulky and inert yolk. Once the germ layers have been established, however, the development of organs proceeds in vertebrates of all classes with only minor differences in details of the processes. Apparently each germ layer is capable of producing certain structures and no others and those particular structures arise from that layer in all vertebrates, whether fish or man. Yet at early stages of development the embryonic material may not be so rigidly determined. By appropriate operations at suffi- ciently early stages of embryos, both vertebrate and invertebrate, it has been proved that a certain region of germ material may be caused to produce structures other than those which it would have produced normally. Yolk is food. The appropriate place for food is in the enteron. In an amphibian embryo the yolk is contained within cells. Gastrulation having established the enteron, the greater part of the embryonic food is then present, not in the enteric cavity but, even better than that, within the cells which constitute the wall of the enteron where it may be directly acted upon by the endodermal protoplasm and made available, as the blood system develops, for transportation to all parts of the growing embryo. The enormous yolk of the egg of a shark, reptile or bird is mor- phologically a part of the original ovum. But by the time cleavage of the germ-disc has progressed so far as to produce a many-celled blastoderm spreading out thin and flat on the surface of the yolk, the cells of the blastoderm can be regarded as, at most, merely joint proprietors of the food supply and the yolk is essentially e.xtra-cellular. As development proceeds, the blastoderm differentiates into the typical germ layers, the mesoderm splits to form somatic and visceral sheets with coelomic space between them, and all -of these layers progressively spread over and around the non-living yolk until eventually it is entirely enclosed (Fig. 74) by REPRODUCTION 79 splanchnopleure and somatopleure with coelom between them. The embryo is put to the necessity of building not only its enteron but its body-wall around its prospective food. ectodsrm o( neural plat ctodcrm of blastoderm fore-gut illantoic bud yolk- stalk Fig. 72. — Diagrams representing median longitudinal sections of chick embryos after incubation for approximately one day, A ; two days, B; three days, C; four days, D. The four stages show progressive differentiation of the regions of the enteron and pro- gressive constriction between the yolk-sac and the shaping body of the embryo. (From Patten, "Embryology of the Chick.") In course of development the yolk is assimilated and utilized in the building of new protoplasm. It therefore steadily decreases in bulk both relatively and absolutely. As the body of the embryo begins to 8o CHORDATE ANATOMY take form, a constriction involving both somatopleure and splanchno- pleure (Figs. 72, 74) appears between the yolk-sac and the remainder of the embryo. The constriction deepens until the embryo presents the appearance of a small animal having a narrow-necked globular sac sus- pended from the under side of the body (Figs. 72Z), 73). In amniotes the amnion is concerned in this constriction (Fig. 74). As the embryo increases in size the shrinking yolk-sac is drawn up into the body. The inner wall (splanchnopleure) of the sac finally constitutes a small region of the wall of the intestine. In elasmobranchs the somatopleure of the yolk-sac finally flattens out and persists as a part of the abdominal wall. In reptiles and birds at the time of hatching the somatopleure is ruptured Fig. 73.- -Young dogfish shortly before birth. The yolk-sac, containing a remnant of the yolk of the egg, protrvides from the ventral body-wall. at the constriction between the definitive body and the extra-embryonic structures and everything external to the rupture is abandoned. Embryonic and Fetal Membranes In the description (pages 33-36) of the reproductive arrangements in vertebrates a general account of the embryonic membranes, amnion, chorion and allantois, of reptiles, birds and mammals was given. The foregoing account of the origin of the germ layers and the shaping up of the embryonic body now makes it possible to appreciate the manner of formation of these membranes in terms of germ layers. All eggs are invested by protective coverings which are either pro- duced by the ovum itself or are secreted about the egg by the oviduct. Such membranes consist of material which is not cellular and not in any sense living. They have merely passive functions. The amnion, chorion and allantois are produced by the germ layers at a relatively advanced stage of the embryo. They are constituted of living cellular material and they are actively concerned with such important functions as nutrition, respiration, excretion and circulation. REPRODUCTION 8l SOhMTOFLEURE AMNION CAVITY\ EXTRA-EMBRYON, COELOM SEROSA ^CHORION I AMNION .COELOM A. AMNION CAVITX AMNION, ENTERON BRAIN SEROSA FOETAL PLACEfNfTA ALLANTOIS COELOM YOLK-STALK YOLK SPLANCHNOPLEURE Fig. 74. — Diagrams illustrating the development of the amnion and allantois. Upper figure, earlier stage; section transverse to long axis of embryo. Lower figure, later stage; longitudinal section of embryo. (After Kingsley, modified). CHORDATE ANATOMY The amnion and chorion are simultaneously produced by an up-rising fold of the somatopleure (ectoderm accompanied by mesoderm) or embryonic body-wall (Fig. 74). The embryo becomes completely sur- rounded by such a fold which then grows in centripetally from all direc- tions and finally encloses the embryo. Where opposite edges of the fold meet above the embryo they coalesce. Reference to Figs. 74 and 75 will serve better than description to make clear the resulting relations of layers and spaces. Fig. 75. — Diagram of the fetal structures of a mammal. (The broken lines represent mesoderm.) A, amnion; AL, cavity of allantois; B, brain; C, chorion; E, enteron; EX, extra-embryonic coelom; H, heart; NC, notochord; NT, neural tube; P, placental region of allantois and chorion; SM, somatopleure; SP, splanchnopleure; V, chorionic villi; YS, cavity of yolk-sac. The somatopleural folds which give rise to the amnion and chorion are, at the time of their formation, a living part of the embryo. The statement that the folds eventually enclose the embryo anticipates the fact that the amnion and chorion do not become any part of the adult. Therefore "the embryo" which the folds enclose is the definitive body region of the embryo. Everything else is conveniently referred to as extra-embryonic. The allantois, an outgrowth from the hind region of the enteron (Figs. 72Z), 74, 75), is a product of the splanchnopleure and is lined by endoderm. In the region of its fusion with the chorion the apposed meso- dermal layers of the two membranes develop a rich network of fine blood- vessels which are connected bv the allantoic arteries and veins to the main REPRODUCTION 83 blood-vessels of the embryo. This allantoic circulation in a reptile or bird provides for respiration (see pages 33, 34). Before the time of hatching the shrinking yolk-sac is drawn up into the growing body. The umbilical stalk — that is, the whole complex of connexions between the definitive body of the embryo and the extra- embryonic membranes — becomes narrowly constricted. At time of hatching the amnion and the slender neck of the allantois are ruptured at the umbilicus. As the young animal emerges, the amnion and chorion and the extra-embryonic part of the allantois are abandoned. The proximal portion of the allantois, remaining within the body, becomes enlarged and serves as the urinary bladder of such adult reptiles as possesses that organ. In birds, the adult having no urinary bladder, the proximal remnant of the allantois degenerates. Among mammals there is some diversity as to the manner of origin of the amnion and chorion. Once established, however, these membranes possess the same relations to the germ layers and to the definitive body of the embryo as in reptiles. The main facts concerning the development of a placenta, by the chorio-allantoic membrane have already been stated (see page 35). The highly vascular vilU produced by the chorio-allantoic membrane (Fig. 75) may be merely lodged in depressions in the uterine wall or they may pierce more or less deeply into its tissues. In extreme cases (e.g., in man) there is destruction of walls of uterine blood-vessels and the extravasated blood fills large sinuses in the uterine wall. The villi project into these sinuses so that the villous surfaces are directly bathed by maternal blood, an arrangement providing maximum efficiency in the exchange of materials between fetal and maternal blood. Mammals exhibit various types of placenta, depending on the dis- tribution of villi in the chorionic surface. When the villi are uniformly distributed over the chorion, as in the horse, pig and other ungulates, the placenta is called diffuse. In most ruminant ungulates, such as cattle, the villi are localized in numerous patches or clusters of varying sizes — the cotyledonary placenta. In carnivores the placenta usually takes the form of a broad band or zone encircling the chorion at a position about midway between head and tail of fetus — the zonary placenta (Fig. 76). A discoidal placenta, in which villi are restricted to a single relatively large area of the chorion, occurs in insectivores, bats, rodents and higher primates including man. A fetal placenta whose villi do not penetrate deeply into the uterine wall separates from it readily and without loss of uterine material. Such a placenta, called non-deciduate, occurs in most ungulates, in the whale and dugong, and in lemurs. When, however, the fetal villi are deeply imbedded in the uterine wall, at time of birth the involved layer of the 84 CHORDATE ANATOMY Uterus is split ofif and discharged with the fetal placenta. This deciduate condition occurs in carnivores, in the elephant, and commonly in animals having a discoidal placenta. In certain marsupials (Dasyurus) it is the splanchnopleure of the yolk-sac which joins the chorion and forms a placenta-like vascular area which is apposed against the uterine wall. Possibly in early mammals both the yolk-sac and the allantois were potentially placenta-forming. In higher primates, the allantoic sac is rudimentary and the fetal portion Fig. 76. — Fetus of cat, removed from uterus without rupturing chorionic sac (C), showing zonary distribution of placental villi. of the placenta is of chorionic origin only; yet the allantois develops far enough to bring its blood-vessels into connexion with the chorionic vessels of the placenta. The umbilical cord is the much elongated and attenuated connexion between the body of the fetus and the extra-fetal membranes. At time of birth the amnion and chorion are ruptured and the young mammal is expelled, along with the amnionic fluid, by muscular contrac- tion of the uterine walls. The amnion, chorion, allantois, fetal placenta, and more or less uterine tissue in a placenta of the deciduous t>^e are dis- charged later as the "after-birth." The umbilical cord is severed. That portion of the allantois remaining within the body becomes the urinary bladder. CHAPTER 3 HISTOLOGY Animals are constituted of ''living substance" or protoplasm together with various non-living materials which are produced by protoplasm. It is chemically complex and possesses a definite, elaborate and minute physical structure. Its basic activities as "living" substance are nutri- tion, respiration and excretion. For the adequate carrying on of these processes, every particle of protoplasm must be in close relation to an environment containing food and oxygen and providing for removal of wastes. Therefore protoplasm cannot exist in indefinitely large con- tinuous masses. The protoplasm of larger animals is subdivided into minute (usually microscopic) structural and physiological units called cells. Circulation of fluid in intercellular spaces provides for the meta- bolic requirements of the individual cell. Animals, e.g., most of the Protozoa, may be so small as to be organized as single cells. The body of a large animal is locally differentiated for the carrying on of various functions. The specialized regions, more or less definitely delimited from one another and each characterized by a configuration which is consistent with its special function, we call organs. These organs, in contrast to the organs of a protozoan, comprise many cells, and the cells of any one organ, so far as they are concerned in carrying on one common function, all exhibit intracellular differentiation of the same kind. Such a group or system of cells, coordinated in one common function and alike in their internal differentiation, constitutes a tissue. An ideally simple organ would consist of only one tissue. As a matter of fact, nearly all organs are concerned with more than one function. An organ's primary function usually demands certain accessory functions, and a corresponding diversity of tissues enters into the constitution of the organ. In a stomach the primary tissue is the lining layer or digestive epithelium. Muscular, nervous, vascular and connective tissues play accessory but nevertheless necessary roles. Vascular and connective tissues enter into the constitution of all major organs. Anatomy deals with organs as such. Histology concerns itself with the internal and specific structure and organization of tissues. Since the tissue is constituted of cells, histology is necessarily concerned with them. Cytology, narrowly defined, deals with cells as such — that is, with that fundamental cell mechanism which is common to all cells and inde- pendent of tissue specialization. 85 86 CHORD ATE ANATOMY Most vital functions involve the surface between protoplasm and the medium immediately external to it. Food enters from without. Respiratory gases pass in and out. Waste is expelled from the surface. Special secretions are produced at the surface. External forces impinge upon the surface. Further, most of the organs of the adult animal are hollow. They contain something or they convey something — food, air, blood. Even such organs as the liver and pancreas, upon casual inspection apparently quite solid, are minutely hollow. Muscles, how- ever, are solid. Connective and skeletal tissues may form bulky solid masses — solid, that is, except insofar as they are penetrated by blood vessels. Bone may contain cavities, but these cavities have a merely passive mechanical significance. The occupation of bone cavities by a blood-forming marrow makes advantageous use of what might otherwise be mere waste space in the animal, but this marrow tissue has no direct relation to the skeletal function of the bone. Such nervous organs as brains, ganglia, central nerve cords, and nerves need not be hollow and ordinarily are not. Every surface of the animal, whether apposed directly to the external medium or to some internal cavity, is a critical region. It is a surface on the one side of which is living substance while on the other side of it may be food, water, air, blood or something else between which and the protoplasm is being carried on some vitally necessary activity — digestion, respiration, absorption, secretion, excretion, diffusion. Or it may be a surface at which the underlying protoplasm deposits a protective non- living substance. Provision for the adequate carr3dng on of these essential and diverse surface activities can be afforded only by the presence of a superficial membrane constituted of living material and specialized appropriately for the functional requirements of the particular surface. Consequently, with very rare exceptions, every free surface of an animal, external or internal, is the surface of a more or less specialized cellular la_ver, an EPITHELIUM. EPITHELIAL TISSUES Epithelia are tissues of primary importance. They are, in double sense, the most primitive of tissues. The smaller simpler coelenterates consist merely of an outer and an inner epithelium. The gastrula of animal embryos consists of two epitheha. It is evident, then, that epithelium provides for all animal needs, and therefore all-epithelial animals may and do exist. The outer layer of the vertebrate gastrula, while it is the source of various structures which attain a deeper position, otherwise persists as the epidermis which is the external epithelium of the adult body. The HISTOLOGY 87 inner layer of the gastrula, giving rise to various organs such as the liver, pancreas and lungs which grow outward from the enteron, otherwise persists as the lining of the digestive tube, the digestive epithelium, which is the innermost epithelium of the adult body. By far the greater Term. bar. CS^ M iii*!"*^*?.*" ^r-l Top plat -^-'- 3 7-, © ., ^.& ai"^ Conn, tissue. Squam.epith. Fig 77. — At left, section of the allantois and amnion of a pig embryo at a region where the mesodermal layers of the two membranes have coalesced. The section is perpendicular to the surfaces of the allantois (above) and the amnion (below). At right, surface view of allantois. The allantoic epithelium is cuboidal, the amnionic epithelium is squamous. The "top plate" is a superficial denser layer of the cell; "terminal bars" are thickenings of intercellular substance just beneath the surface of the epithelium. (From Bremer, "Text-book of Histology.") part of the massive adult has been inserted between the two primary layers. The term endothelium is commonly applied to the lining layer of blood-vessels and lymphatics. Mesothelium may be used for the peri- toneal epithelium. Cells may form a layer resembling an epithe- lium but not abutting upon a cavity. The tissues of some endo- crine glands are of this nature. To such tissues is applied the adjec- tive epithelioid. Fig. 78. — Types of epithelia. B. simple squamous; C, simple columnar; D, strati- fied columnar, ciliated a.t E; F, stratified polyhedral, upper cells squamous. (From Kingsley.) Epithelia carry on functions of most diverse kinds. The diversity is reflected in the structure of epithelia. Only a few of the more general features of structure can be mentioned here. s& CHORDATE ANATOMY Simple Epithelium. An epithelium only one cell in thickness is termed simple. There is, however, great variation in the thickness of ^/wM noTz.^ (»44« Fig. 79. — Columnar ciliated epithelium from human trachea. Most of the cells are slender, with axes more or less curved, and extend from the basement membrane to the free surface of the epithelium. Occasional short cells, basal cells, lie at or near the basement membrane and do not extend to the free surface. Several swollen mucous cells ("goblet" cells) are shown. (From Bremer, "Text-book of Histology.") simple epithelia. The cells, seen in sections perpendicular to the surface, may be approximately square in outline. Such an epithelium is called cuboidal (Fig. 77), but incorrectly for the cells are usually hexagonal prisms. A simple epithelium consisting of tall prismatic cells (Fig. 78C) is called columnar. At the extreme of thinness are epithelia (flat or squamous) each of whose cells is a broad flat plate, hexag- onal in outline. (Figs. 77, 78S) Stratified Epithelium. On Amphi- oxus, a slender marine animal only four or five centimeters long, an epidermis one cell thick afi'ords adequate protection. On an elephant it would not. Surfaces of large heavy animals are exposed to excessive mechanical friction and impact. Loss of material at the surface is best compensated for by a stratified epithelium whose lower layers persistently grow to replace the loss. A stratified epithelium may be two or several or many cells in thickness (Fig. 'jSD-F). In all vertebrates the epidermis is stratified (Fig. 80). Fig. 80. — Skin of lung-fish, Proto- pterus; section perpendicular to sur- face; much enlarged, c, dermis (corium); e, epidermis; g, multi- cellular gland; u, unicellular gland. (From Kingsley.) HISTOLOGY 89 Its thickness varies with the size and habits of the animal, and, in a particular animal, it varies locally depending upon the degree of exposure to mechanical wear. In a thick stratified epithelium the cells of the bottom layer are usually columnar and those of the outer layers are more or less flattened. The intermediate cells have a form such as would result from crowding tightly together a mass of compressible spheres, that is, polyhedral. Yet the cells are not actually packed tightly together. They are separated by excessively thin intercellular lymph spaces through which seeps lymph Stratum comeum. Stratum germinativum. Corium (Tunica pronria.) Fig. 81. — Epidermis from the sole of the foot of an adult man. Section perpendicu- lar to surface of skin. External to the stratum germinativum, the strata show successive stages in the production of the stratum corneum. X360. (From Bremer, "Text-book of Histology.") derived from underlying blood-vessels and serving to provide for the metabolic needs of the individual cells. Cells on opposite sides of the intercellular space are connected by delicate strands of solid, or at least dense, substance. Presumably protoplasmic, the strands are called protoplasmic bridges or plasmodesms. Many epithelia, although "simple" in the sense of being only one cell thick, are not the ideally simple tissue of the definition (page 85), con- stituted of cells all "alike in their internal differentiation." Among the special functions of an epithelium are the following: (i) production of a 90 CHORDATE ANATOMY superficial covering of non-living, mechanically protective substance; (2) production of special secretions such as mucus; (3) reception of external stimuH; (4) provision for motile activity. Two or more of these functions may be carried on by one "simple" epithelium or by a stratified epi- FiG. 82. — Developing scales of dogfish, Squalus; sections perpendicular to surface of skin; much enlarged, c, upper layers of epidermis; d, dentine of scale, deposited by dermal cells beneath it; ee, enamel-forming organ of scale — a specialized region of the germinative layer (w) of the epidermis; p, "pulp", the dentine-forming organ. (From Kingsley.) thelium. Within the epithelium, then, cells will exhibit differentiation of as many types as there are functions. (i) Most epithelia produce a protective covering at the free surface. A cuticula is a dense, tough or hard nitrogenous material deposited on the exposed surface of an epi- thelium. The cells which produce it and underlie it remain alive. Keratin is a nitrogenous organic substance which is formed within some epithelial cells. It is the basis of the homy structures of the vertebrate skin. Tha "horny layer" (stratum comeum; Fig. 81) developed on the skin of vertebrates other than fishes, consists of one or more of the outer strata of the epidermis, the cells more or less filled with keratin and strongly adherent to one another so that the whole layer acquires a high degree of mechanical resistance. Completely keratinized cells are dead. Hair, feathers, reptilian scales, claws, nails and hoofs are horny structures. Calcareous material may be deposited by an epithelium, either at its outer surface {e.g., shell of a mollusk) or, exceptionally, at its inner surface {e.g., enamel of teeth; Figs. 82, 129). Fig. 83. — Sensory cells. A, cell from the sense organ (crista acustica) of an ampulla of the ear; B, rod cell from the retina; C, cell from the olfactory epithelium. (From Kingsley, After Furbringer.) HISTOLOGY 91 (2) A glandular epitheliiun is one in which secreting cells are scattered more or less abundantly throughout the layer. (Fig. 80, 11) Gustatory Pore Sustentacular cell canal cell Pore canal Sustentacular Pore Gustatory (cut obliquely) cell canal cell Connective tissue of tunica propria Fig. 84. — Taste-buds from a vallate papilla of the human tongue; as seen in section perpendicular to the surface of the epithelium. S is a diagrammatic representation of the structure of one "bud." X475. (From Morris, "Human Anatomy.") (3) In a sensory (or neuro-) epithelium certain cells are specialized for reception of stimulation by some agency in the cell's environment (Fig. 83). EpitheHal sensory cells may be grouped in clusters to form sense organs (Fig. 84). An epithelium may be rendered sensory by free nerve termina- tion, that is, the terminal twigs of a nerve fiber ramifying amongst the epithe- lial cells (Fig. 85). These nerve fibers, however, are not produced by the epithe- lium itself but invade it from adjacent tissue. (4) Cilia are extremely delicate motile ' filaments borne by the free ends of epitheHal ation in the epiderm^rof^'^S^la- Cells. A single cell mav carry from one ^^ndra. (From Kingsley. after ' ... Retzius.) to over a hundred. A cmated epithelium is one in which some or all of the cells carry cilia. (Figs. ^SE, 79) Cilia and mucous glands commonly occur in the same epithelium. The simple external epithelium of an earthworm and the stratified epider- mis of a fish combine cuticular, glandular and sensory specializations. Glands " Glands " whose products are as different as are sweat, eggs and blood- cells hardly merit the same name. Accepting the name, it is necessary 92 CHORDATE ANATOMY to distinguish different types of gland: (i) secretory glands whose products are retained at least temporarily and serve some useful purpose — e.g., mucous, salivary and thyroid glands; (2) excretory glands which eliminate waste — e.g., kidneys; (3) cytogenic glands which produce living cells— e.g., reproductive glands producing eggs or sperm, various lymph and blood glands in which white blood-cells are produced. Secretory glands may be unicellular (Figs. 79, 80, u) or multicellular (Fig. 80, g). Nearly all multicellular glands develop directly from epithelia and retain their epithelial character. Some endocrine glands are epithelioid. Most secretory glands develop from either the ectodermal or the endo- dermal epithelium and discharge at the surface of their native epithelium. Such are the many kinds of skin glands and digestive glands. The Fig. 86. — Types of multicellular glands. A-D, tubular; E, F, alveolar or acinous. A, simple; B, coiled; C-F, branched. The duct pierces the epithelium from which the gland has been produced. (From Kingsley.) mesoderm gives rise to some secretory glands, especially in connexion with the reproductive system — e.g., the albumen glands and shell glands of oviducts and the mucous glands of the mammalian uterus. Multicellular glands may be tubular (Fig, S6 A-D) , or alveolar (acinous; Fig. S6E,F). Glands of either type, complicated by branch- ing, are called compound (Fig. S6C-F). The larger multicellular glands, and especially those which are com- pound, require certain accessory structures. A good blood supply must be provided. Therefore the gland may have an outer investment of connective tissue containing blood-vessels and lymphatics. A thin layer of unstriated muscle fibers may be present on the wall of a gland which discharges its contents abruptly. The muscle would be accom- panied by nerve fibers and in some glands nerves may be traced to the secretory cells. Secretory glands in vertebrates range from unicellular mucous glands in the skin of fishes and amphibians and in the digestive epithelium of all vertebrates to such massive compound multicellular glands as the mammary glands and the liver. HISTOLOGY NON-EPITHELIAL TISSUES 93 The primarily essential parts of a metazoan animal are the epidermal epithelium and the enteric epithelium. Certain of the organs which, in the adult, lie between these two layers consist of tissues which do not retain the epithelial character of the embryonic tissues from which they are derived but give rise to more or less bulky and solid masses of material. The important types of adult non-epithelial tissues are the following: (i) muscular; (2) nervous, exclusive of neuro-epithelial structures; (3) tissues serving for mechanical support — the connective and skeletal tissues; (4) adipose tissue or fat; (5) blood. Muscular Tissue Locomotion in some protozoans is effected by beating of ciHa. The movements of large animals depend on contractile mechanisms. Con- tractility is inherent in protoplasm. The least specialized protoplasm is apparently able to contract in the direction of any of its axes. When protoplasmic mechanism for effecting vigorous, quick or long continued contracting is established, the ability to contract becomes restricted to one axis. The protoplasmic structures which seem to be somehow immediately concerned with contraction are exceedingly fine fibrils, the myofibrils, which extend through the cell parallel to the axis of contraction. Fig. 87. — A, unstriated ("smooth") muscle cell with single nucleus; B shows a small portion of the length of a multinucleate striated fiber. (From Kingsley.) Among invertebrates the usual type of muscle element is a much elongated cell having a single nucleus, more or less numerous myofibrils extending through the protoplasm lengthwise of the cell, and having the usual cell-wall devoid of any special membranous covering. Such cells, associated together to form layers, bundles or masses, constitute the muscles of the body-wall and the viscera. Certain invertebrates, how- ever, whose muscles are, in one way or another, especially efficient have muscle cells or more complex sort. The myofibrils become strongly developed and each fibril exhibits an alternation of darker and lighter zones. The zones of either type lie exactly alongside one another on adjacent fibrils so that they give the impression of transverse bands or 94 CHORDATE ANATOMY striations extending continuously across the cell. Muscle cells of this sort are called striated. Uninucleate striated fibers occur in the heart of some moUusks. In arthropods, especially insects, striated fibers Fig. 88. — Striated muscle; human. Above, in longitudinal section, showing small portions of several fibers; below, section transverse to the length of the fibers. Nuclei lie at the surface of the fiber. (From Bremer, "Text-book of Histology.") attain great length, are multinucleate, and exhibit a complex system of transverse striations. Vertebrates possess both striated and unstriated (or "smooth") muscle (Fig. 87). In general, the muscle of the bodv-wall is striated and HISTOLOGY 95 from A Fig. 89. — Motor guinea-pig; surface view of muscle fiber: B, from hedgehog; section perpendicular to surface of muscle fiber, g, granular substance of the motor plate; m, striated muscle; n, nerve fiber; t.r., terminal rami- fication of the nerve fiber. (From Bremer, "Text-book of Histology"; after Bohm and Davidoff.) visceral muscle is unstriated. But unstriated muscle occurs in the walls of blood-vessels which lie in the body-wall, in connexion with some skin structures such as hair and certain glands, and also in the iris of the eye. The muscles in the walls of the mouth, pharynx and at least the upper part of the esophagus are striated, and it is said that striated muscle occurs in the wall of the stomach of some fishes. Also the external anal muscle is striated. The muscular part of the diaphragm is derived from the embryonic body- wall and its muscle is accordingly striated. And in all vertebrates the muscle of the wall of the heart is striated. Unstriated muscle fibers in vertebrates are much like those of inver- tebrates. They are ordinarily not over a fraction of a millimeter in length and, in man, much less than a hundredth of a millimeter in diameter. ^^^^»„^^^ They are usually spindle-shaped (Fig. 87^!) ^'"^'^ , lying in the tissue with their tapering ends li,--'" ^ overlapping. B,.- " The somatic striated fibers of vertebrates are enormously larger than unstriated fibers (Fig. 87^, 88). Their diameter may approach a millimeter and their length, not accurately known, doubtless reaches several or many millimeters. But these great fibers are not, in strict sense, single cells. They contain scores or hundreds of nuclei. The myofibrils of striated fibers are much coarser than those of unstriated fibers. They are imbedded in a peculiar fluid sarcoplasm which is probably a nutrient medium rather than ordinary cytoplasm. The wall of the fiber, much more prominent than an ordinary cell-wall, is called the sarcolemma. The alternate dark and light bands on the individual fibril are due to physical differences such that, in polarized light, the dark bands are doubly refractive (anisotropic) while the lighter bands are singly refractive (isotropic). Both the dark and the light bands are traversed by finer markings seen only under high magnification. Fig. 90. — Human car- diac muscle; a very small portion seen under high magnification, d, intercal- ated disc; Z, Krause's mem- branes which lie transversely at regular intervals along each myofibril, bisecting each light band. The dis- tinction between light and dark bands does not appear in the figure. (From Bremer, "Text-book of Histology"; after Heidenhain.) 96 CHORDATE ANATOMY The relation of an unstriated fiber to its nerve is apparently of the simplest sort. A terminal twig of nerve merely attaches to the surface of the fiber, the end of the nerve often showing a knot-like enlargement. Presumably every striated fiber has a nerve connected to it. The nerve, however, enters a small flat plate of nucleated protoplasm lying super- ficially on the muscle fiber. Within this motor plate (Fig. 89) the nerve ramifies into fine twigs which seem to terminate in the substance of the plate. Striated fibers are bound together in bundles enwrapped by a con- nective-tissue perimysium. Thick muscles consist of several or many such bundles wrapped together. Cardiac muscle has striations which resemble those of somatic muscle but the fibers are relatively short and are branched. The sarcolemma is less strongly developed than in somatic fibers. A peculiar feature of the cardiac fiber is the presence of conspicuous transverse bands, the inter- calated discs (Fig. 90) which are quite distinct from the ordinary stria- tions. Their significance is not known. Nervous Tissue All nervous functions are carried on by protoplasm organized, as always, in cells. To say, as is often done, that nervous tissues consist of nerve cells and nerve fibers is inaccurate. So far as known, every fiber which conducts nervous impulses is developed as an outgrowth from a cell and can function and survive only so long as it remains in physical and physiological continuity with the nucleated region of the cell of which it is an integral part. Any cell engaged in nervous operations, together with all conducting fibers which have grown out from it, is called a neuron. A central nervous organ is a more or less complex system of physiologi- cally related neurons serving for the proper association, coordination and integration of nervous impulses. A ganglion is a minor localized nerve center consisting of the cell-bodies of neurons together with the adjacent regions of their nerve processes. Neurons are of various types depending on the form of the cell-body and the number of nerve processes (Figs. 91, 92). Unipolar cells, of comparatively rate occurrence, have a single process; bipolar neurons are usually spindle-shaped and have a process at each end; multipolar cells have several processes of which one, the neuraxon (axon or neurite), is relatively long, while the short dendrites branch out into fine twigs which end within a short distance of the cell-body. The neuraxon may give off lateral branches (collaterals) and its distal extremity breaks up into fine branches forming the terminal arborization. HISTOLOGY 97 Most types of receptor neurons are epithelial. In some of these the receptor cell itself produces a nerve fiber which conducts to the central Fig. 91. — Types of nerve cells. A, multipolar cell; B, portion of nerve fiber with sheaths; C, unipolar cell (such a cell may arise by modification of a bipolar cell as shown in Fig. 93); D, pyramidal cell (from cerebral cortex), a, axon; c, collateral; cb. cell- body; d, dendrites; m, medullary sheath; n, nucleus of cell of Schwann's sheath; r, node of Ranvier; s, sheath of Schwann; t, telodendron. (From Kingsley.) Fig. 92. Cell-bodies of neurons showing arrangement of neurofibrils. .4, from human spinal ganglion; two cut fragments of the neuraxon lie near the cell-body. B, "giant pyramidal cell" from human cerebral cortex. Highly magnified, a, neuraxon. (From Morris, "Human Anatomy.") organ— e.g., an olfactory cell and its fiber (Figs. 83C, 3485). In such cases, one neuron serves as both receptor and conductor. In other 98 CHORD ATE ANATOMY cases, as in the auditory organ and taste-buds (Figs. S4B, 348Z)), the epithelial receptors do not produce nerve fibers but are intimately related to the terminal twigs of afferent nerve fibers whose cell-bodies lie in some deep ganglion such as the acoustic ganglion or a spinal ganglion. Nerve cells vary greatly in size, but in general are relatively large. They are often the largest cells in the body exclusive of eggs. The most striking characteristic of the body of a neuron is the presence of large masses of a granular substance which has a strong affinity for the anilin dye, methylen blue. These Nissl's bodies (Fig. 94) have been shown to become reduced in neurons which have been excessively active, indicating that the bodies contain something which is a source of energy for nervous activity. Less conspicuous are the neurofibrils (Fig. 92), Bipolar cells. Fig. 93. — Diagram showing how an embryonic bipolar nerve cell is transformed into a unipolar cell ("T-cell") such as occurs in ganglia of the dersal roots of spinal nerves. (From Bremer, "Text-book of Histology.") Fig. 94. — Nerve cell, with processes cut short; from human spinal cord. X430. (From Bremer, "Text-book of Histology.") extremely fine fibrils which are ordinarily seen only after use of special staining methods. Such neurofibrils may form an elaborate system within the body of the neuron and may be traced into the neuraxon and larger dendrites. The appearance and arrangement of these neurofibrils strongly suggest that they are specialized avenues for conduction of impulses. The neuraxon is a delicate thread consisting of a probably modified protoplasm in which, as just mentioned, neurofibrils may be demonstrated. It may be surrounded by one or two special ensheathing layers. The medullary or myelin sheath is a relatively thick layer of fat-like substance, myelin, fitting the neuraxon closely. The neurilemma or sheath of Schwann is an exceedingly thin cellular layer wrapped around the neu- raxon. (Figs. 91, 304) A neuraxon may possess either, both, or neither of these two sheaths. When both are present the myelin sheath is always next the nerve fiber and, at fairly regular intervals (in man averaging about 0.5 mm.) along HISTOLOGY 99 the fiber, it seems to be nearly or quite interrupted so that the neurilemma there comes into close relation with the nerve fiber (Fig. giB). The neuraxon therefore presents a segmented appearance due to these nodes of Ranvier. Nerves whose individual fibers possess the myelin sheath appear more nearly white than do non-medullated nerves. The so-called " white " parts of the brain and spinal cord consist mainly of medullated nerves. Non-medullated fibers and the cell-bodies of neurons are the chief con- stituents of "gray matter." The sheaths doubtless serve for the protection, insulation and nutrition of the nerve fiber. The source of the myelin is not definitely known. Bundles of nerve fibers Epineurium. Perineurium. Endoneurium. Fig. 95. — Structure of a nerve. The figure represents a small part of a transverse section of a large nerve constituted of many bundles of medullated fibers. X20. (From Bremer, "Text-book of Histology.") A nervous organ is constituted of neurons supported by connective tissues accompanied by vascular tissues. In the brain and spinal cord of vertebrates occurs not only the usual mesenchymal connective tissue but another which is unique in that its cells have ectodermal origin in common with the nerve cells. Some of the cells of this neuroglia possess branched processes which make them confusingly similar in appearance to nerve cells. The neuroglia cells form, by means of their processes, a supporting network for the nerve cells. A nerve is a bundle of neuraxons, each of which may be ensheathed as described above, and all wrapped together within a sheet of connective tissue, the perineurium (Fig. 95) extensions of which (endoneurium) may penetrate into the bundle. Larger nerves consist of several or many bundles all tied together by connective tissue and enwrapped by a rela- tively thick epineurium. Small blood-vessels traverse the connective- tissue lavers of the nerve. loo chord ate anatomy Tissues Serving for Mechanical Support Protoplasm is a substance of semi-fluid or gelatinous consistenc\-. An elephant constituted of protoplasm only is a mechanical impossibility. Large animals, especially if they are land animals, require mechanical support. Protoplasm provides such support by appropriating various materials from the environment and building them into non-living struc- tures which are external to the cells and physically adapted to the mechanical needs of the animal as a whole and of its parts. The basis of the material of these supporting structures consists of various nitrogenous or protein substances. By impregnation of the /. f \ \ ^ A B Fig. 96. — -.4, mesenchymal tissue from, embryo of the amphibian, Ambystoma. B, pigment cells from Ambystoma; below, a cell with pigment dispersed in numerous branched processes of the cell; above, a "contracted" cell with pigment concentrated, the transparent processes not shown. (From Kingsley.) material with inorganic salts, chiefly those of calcium, hard or rigid supporting structures are produced. The protoplasmic or cellular agencies concerned in building the supporting tissues are mesenchyme cells, except in the cases of the notochord and the ectodermal neuroglia (page 99) of nervous organs. The embryonic precursor of supporting tissues other than the excep- tions mentioned is a more or less spongy mesenchyme (Fig. 96.4) whose individual cells have branching processes by means of which the cells are joined together The spaces within the meshwork of cells is filled by a homogeneous fluid substance, the matrix. Presumably the cells are the source of the matrix. Connective Tissue The essential mechanical structures in connective tissue are relatively coarse white fibers consisting of an albuminoid substance, collagen, HISTOLOGY lOI the source of gelatin and glue. These collagenous fibers are only slightly elastic. They may be branched. Each fiber is a bundle of very delicate fibrils. Exceedingly flattened cells with flat nuclei appear as if clinging closely to the surface of a fiber. These connective-tissue cells or fibro- cytes are presumably the agencies which have brought about the produc- tion of the fiber in the intercellular matrix. Elastic fibers are much finer than collagenous fibers and difi"er from them chemically in being composed of elastin which is not a source of gelatin. An occasional elongated fibrocyte may be seen stretching along the surface of a fiber (Fig. 97). Elastic fibers commonly occur inter-mingled with collagenous fibers. Connective tissue forming a loose open mesh-work, as does the subcutaneous tissue lying between the skin and the muscle of the body, is called areolar tissue. Tendons and ligaments are connective- tissue structures highly adapted to resisting tensile strain. They consist of coarse collagenous fibers arranged in compact bundles. Tendons are inelastic. Chroma tophores, pigment cells (Fig. 96S), may occur in connective tissue, especially in the dermal layer of the skin. The specific pigment appears as granules lying in the cytoplasm. Black pigment (melanin) is most common and cells con- taining it are called melanophores. Chro- matophores are usually richly branched. The pigment may at one time be distributed throughout the processes ("expanded" phase), at another time densely massed in the central part of the cell ("contracted" phase). Some pigment cells are migratory. Fig. 97. — A, elastic fibers of the subcutaneous areolar tissue of a rabbit. B, cells related to elastic fibers, as seen after treat- ment with acetic acid; from sub- cutaneous tissue of a pig embryo. (From Bremer, Text-book of Histology: A, after Schafer; B, after Mall.) Skeletal Tissues Notochord. The essential notochord material consists of cells each of which contains a relatively enormous vacuole occupied by a substance of fluid, or possibly gelatinous, consistency. The cytoplasm of the dis- tended cell is so stretched that it appears as the thinnest possible layer surrounding the vacuole. The very flat nucleus occasions a bulge in the contour of one side of the cell (Fig. 98). The outer cell-membrane, while very thin, is probably of semi-rigid consistency. Seen under the microscope, this tissue looks like a mass of soap bubbles crowded closely I02 CHORDATE ANATOMY together, the cytoplasm and cell-membrane of each cell being the wall of a bubble. The vacuolated notochord tissue is enclosed by sheaths which differ in number and nature in various animals. There is commonly an inner elastic sheath (Fig. 98, ei) composed of material secreted by an outer epithelioid layer of the notochord tissue, and a thick outer sheath of dense fibrous connective tissue. Mechanically, the notochord resembles a length of rubber tubing, closed at the ends, and filled with liquid under pressure. Fig. 98. — Developing vertebrae of the amphibian, Ambystoma; I, earlier; II, later. Longitudinal sections. Cartilage and bone are forming around the notochord. cc, cartilage in center of vertebra; ci, epithelioid internal elastic sheath of notochord; /, incisure cutting through ic, intercentral (intervertebral) cartilage; n, notochord; ns, outer notochordal sheath; v, developing bone (black) of centrum of a vertebra. (From Kingsley.) Cartilage. In development of cartilage, mesenchyme cells become densely massed and then produce an abundant intercellular substance whose accumulation causes the cells to become more or less widely sepa- rated from one another (Figs. 68, 99). The intercellular matrix becomes solid and acquires a firm or even hard consistency. Chemically it is a complex of collagenous, albuminoid and other protein substances. The cartilage cells remain imbedded in the matrix, each occupying a close- fitting space, a lacuna. In some cartilages have been described exceed- ingly fine canals penetrating the matrix and putting any one lacuna into communication with neighboring lacunae. HISTOLOGY 103 The external surface of cartilage is invested by a connective-tissue membrane, the perichondrium (Fig. 99) which contains blood-vessels but they do not penetrate into the cartilage. Hence cartilage cannot occur in thick masses. In growing cartilage, cells from the perichondrium become cartilage cells and add cartilage to the exterior of the mass already formed. At »• -"S^ Capsule. o? :*■-. <^;^. . Matrix. '^erichond. -Bl.v. ^ Fig. 99. — Hyaline cartilage, with perichondrium; from human trachea. Bl.v., blood-vessel; x, cartilage cell whose nucleus is not in section; y, new matrix forming between two cells resulting from a recent division of a cartilage cell. (From Bremer, "Te.xt-book of Histology.") the same time deep cartilage cells divide. The resulting cells secrete matrix substance whereby they become separated, each to lie in a lacuna of its own. Hyaline cartilage (Fig. 99), usually bluish and clear, is nearly devoid of fibrous material. In fibrocartilage the matrix contains fibers similar to those of ordinary connective tissue. Elastic cartilage contains numer- ous elastic fibers. Calcified cartilage is rendered white and relatively hard by deposit of calcium salts in the matrix. I04 CHORDATE ANATOMY Bone. Cartilage and bone are similar in that their essential skeletal material is a non-living matrix within which are imbedded living cells. Bone differs from cartilage in that the matrix is highly calcified and cor- respondingly hard and also in that it never exhibits the apparent homo- geneity of the matrix of hyaline cartilage but is disposed in very thin -'BLOOD VESSEL ^COMPACT BONE ^MARROW ARTICULAR UGAMENT Fig. 100. — Diagram of the structure of a long bone. (Redrawn from Kahn's " Der Mensch," Albert Miiller, Zurich.) parallel layers. Usually the deeper substance of a bone (Fig. loo) is of a porous or spongy texture (cancellous bone) while the outer region is dense or solid (compact bone). A section of fully developed compact bone, seen under high magnifica- tion, shows the matrix layers or lamellae arranged in parallel or concentric order (Fig. loiB and 102). Between adjacent lamellae are minute Fig. loi.- — A, stereogram representing a sector of the shaft of a long bone. B, transverse section, much more enlarged, showing part of one Haversian system, bl, bone lamellae; c, canaliculi; /;, Haversian canal; I, lacuna. (From Kingsley.) cavities, the lacunae. Exceedingly fine canals, the canaliculi, extend between each lacuna and neighboring lacunae, piercing the intervening lamellae. In bone of a living animal each lacuna is occupied by a living bone-cell (osteoblast) from which processes extend into the adjoining canaliculi. All external surfaces of bone are covered by a membrane, the perios- teum (Fig. 100), of dense fibrous connective tissue well supplied with blood-vessels which enter the bone and branch throughout it. Most HISTOLOGY 105 bones, notably the long bones of the appendages, have internal cavities (Fig. 100) occupied by a more or less vascular soft tissue, the marrow. The "yellow marrow" of long bones contains much fat. "Red marrow" is highly vascular, contains little fat and may be a source of blood cells of various types. In long bones the larger blood-vessels lie approximatel}- parallel to the long axis of the bone. Around such vessels the bone lamellae are arranged in concentric order (Figs. loi and 102) forming so-called Haver- FiG. 102. — Section, highly magnified, of compact bone from the shaft of the human humerus. The section, cut transversely to the long axis of the bone, shows four Haversian systems with their central canals, concentric lamellae of bone, lacunae between adjacent lamellae, and canaliculi extending between lacunae. (From Bremer, "Text-book of Histology"; after Sharpey.) sian systems. These concentric systems are much less prominently developed in fiat bones. The matrix of bone consists of commingled organic and inorganic materials. Collagenous and other protein substances constitute the organic part while various salts of calcium, mostly the phosphate and carbonate, are the most important inorganic ingredients. Bone, because of the rigidity of its calcified matrix, is incapable of such interstitial growth as occurs in cartilage. A further difference between cartilage and bone lies in the fact that the cartilage cell produces matrix in all directions and thus surrounds itself by its own product, whereas the osteoblast produces matrix only at such part of its surface as is adjacent to the already- formed bone. A layer of bone cells building up lamella upon lamella io6 CHORDATE ANATOMY of bone may be likened to a group of masons laying course upon course of stone at the unfinished top of a wall. But, in the case of the bone, every now and then one of the masons, an osteoblast, is left behind and buried between successive courses of the wall, remaining there in his little lacuna as a permanent bone cell. Adipose Tissue Adipose tissue or "fat" consists of cells each of which contains a globule or vacuole of oil so large that the cytoplasm appears as merely an bl.v. i bl.v. Fig. 103. — Fat cells in subcutaneous tissue of a human embryo of four months. bl.v., blood-vessel; c.t., white connective-tissue fibers; fib., young fibrocyte; mes., mesen- chymal cell ; X, young fat cell, nucleus not in section; 1,2,3, developing fat cells. (From Bremer, "Text-book of Histology.") exceedingly thin layer surrounding the vacuole (Fig. 103). The flat nucleus lies in the peripheral layer of cytoplasm. The irregular poly- hedral form of the cells is doubtless the result of their mutual pressures. Blood The circulatory function of blood requires that it be fluid but various special services are rendered by cells suspended in the fluid, some of them passively carried by it, others capable of independent motion somewhat like that exhibited by an ameba. HISTOLOGY 107 The fluid part of blood, the plasma, is water containing all the other substances which enter into the constitution of protoplasm together with various hormones and the waste products of metabolism. In its inorganic chemical constitution, the plasma resembles sea water. In the coagulation of blood, on exposure to air or under some other circumstances, a nitrogenous substance, fibrinogen, carried by the plasma in solution, becomes transformed into fine solid filaments of fibrin (Fig. 104). The uncoagulated portion of the plasma is called serum. The "clot" is a mass of fibrin with blood cells caught in its meshes. Fig. 104. — Coagulated blodd. Biconcave red corpuscles arranged in "rouleaux"; filaments of fibrin radiating from minute blood plates. (From Bremer, "Textbook of Histology"; after Da Costa.) Blood cells are of two main kinds, red corpuscles or er5rthroc)rtes and white corpuscles or leucocytes. The red cells are much more numerous. In human blood the red cells outnumber the white in the ratio of five or six hundred to one. Erythrocytes (Figs. 104 and 105) are relatively small and usually have the form of flat discs with elliptical outlines. These blood cells are the oxygen-carriers, being heavily loaded with hemoglobin, a complex protein substance containing iron and having a strong afl^nity for oxygen which the cells pick up at the respiratory surfaces of the animal. Their color is due to the hemoglobin. The mature erythrocytes of all verte- brates except mammals are nucleated. In adult mammals, the red cells in course of their differentiation lose their nuclei, thereby acquiring the form of biconcave discs. (Fig. 104) Io8 CHORD ATE ANATOMY Leucocytes are permanently nucleated and do not carry hemoglobin. Several types are recognized. (Fig. 105) L5rmphocyte: usually small, cytoplasm scanty and usually non- granular, nucleus spherical. Large mononuclear leucocyte (monocyte) : more abundant and non- granular cytoplasm, nucleus excentrically placed. Polymorphonuclear leucocyte : large, with conspicuous granules in cytoplasm, nucleus indented, lobulated, irregular or separated into two or more parts. Several kinds are distinguished on the basis of the reaction of their granules to anilin dyes. Basophiles have granules which take basic stains; eosinophiles have an afl&nity for eosin, an acid dye; the granules of neutrophiles take both basic and acid dyes. Most leucocytes are capable of active ameboid motion. Many are phagocytic. Blood plates (Fig. 105) are minute bodies which seem to be proto- plasmic and yet are not nucleated. They probably result from fragmen- tation of cells in bone marrow or elsewhere. They seem to have some relation to the clotting of blood as indicated by the fact that the filaments of fibrin (Fig. 104) tend to radiate from blood plates. Lymph resembles blood but lacks erythrocytes and is therefore color- less. The fluids occupying the coelomic spaces and the cavities of brain and spinal cord, the aqueous humor of the eye and the amnionic fluid are all of the general nature of lymph but contain relatively few cells and differ from one another in details of chemical constitution. HISTOLOGICAL SPECIFICITY In general, histological differences are less conspicuous than the corresponding anatomical differences. Unstriated muscle fibers appear much the same whether they are in the wall of a stomach or of a lung. Nevertheless tissues and cells usually exhibit characteristics which mark them as belonging to a particular organ or animal. The nerve cells of a spinal ganglion differ from the motor nerve cells in the spinal cord of the same animal. Epidermal tissue of a fish differs from that of a reptile. It follows, therefore, that the individual tissue cell may, in its visible structure, exhibit characteristics reflecting as many as four grades of Fig. 105. — Cells from smear preparation of normal human blood; Wright's stain. In the center: adult red blood corpuscles, blood platelets and a polymorphonuclear neutrophile. At left above: two polymorphonuclear basophiles and two polymorpho- nuclear eosinophiles. At right above: three large and four small lymphocytes. At left below: polymorphonuclear neutrophiles; two of these cells, the uppermost and lower- most of the group, are young, with merely crooked nuclei; the mature cells have multi- lobed nuclei. At right belov/: six monocytes; in the younger cells the nuclei tend to be rounded, in the adult cells they arc horseshoe-shaped, indented or lobed. (From Bremer, "Text-book of Histology'.') HISTOLOGY -m ^\ 109 :.-J> «* *^ £: T^oTtr Fig. 105. — ("See page ro8 for description.') no CHORDATE ANATOMY organization. First there are those cell organs, such as nucleus and chromatin bodies, which represent the fundamental organization of protoplasm as cells. Then there are those intracellular structures such as myofibrils or neurofibrils which mark the cell as belonging to a particular tissue — muscular or nervous. Thirdly, there may be features which identify the tissue as that of a certain organ; for example, the inter- calated discs in the heart muscle of vertebrates. Finally, the individual tissue element may have peculiarities which are specific for animals of a certain group; for example, the striated muscle fiber of an insect differs in details of structure from that of a vertebrate. Behind all of this visible dilTerentiation and specificity of structure must be chemical specificity. CHAPTER 4 THE INTEGUMENTARY SYSTEM EVOLUTION OF THE INTEGUMENT Since all life involves continual adjustment of processes within the organism to conditions outside, the skin and its appendages which mediate this relation are highly important organs. Even among the Protozoa, an external semipermeable membrane separates the living protoplasm from the surrounding medium. Most Protozoa have in addition an outer differentiated layer of clearer cytoplasm, the ectosarc, analogous in function to the skin of the higher animals though without genetic relation. A true multicellular skin appears first in sponges and coelenterates, the ectodermal layer of Hydra being a familiar example. Even in so simple a skin as this, there is some differentiation among cells. Most are epithelial covering cells, each commonly prolonged at its base into a contractile thread. But among these are gland cells, which by their various secretions in different coelenterates indicate a wide difference in metabolic processes. The secretion of lime salts by the skin of coelenterates may be regarded as the beginnings of the exoskeleton of many higher invertebrates. Most invertebrates retain essentially unaltered the simple epitheUal ectoderm of coelenterates. Some have a ciliated epidermis which aids locomotion. Many secrete an external cuticula, in which lime may or may not be present. The evolution of a simple epithelium into a stratified epidermis, such as occurs in vertebrates, results, presumably, from a change in the direction of cleavage planes during cell multiplication. So long as cell walls form perpendicular to the surface, a simple epithehum results. When, however, cleavage planes form parallel to the surface, the membrane becomes stratified. The outer layers of cells serve to protect the lower layer where growth and cell multiplication take place. In animals exposed to dry air, an outer layer of dead cells is obviously adaptive. Yet the beginnings of a protective outer layer appear in the exoskeletons of water-dwelling invertebrates. Among invertebrates appears also, though exceptionally, a connective-tissue layer or corium beneath the epidermis. The lowest chordates (Balanoglossus, Ciona, Amphioxus) have both an outer epidermis and an inner corium; but the epidermis is only a single I 12 CHORDATE ANATOMY layer of cells. Gland cells are numerous in the epidermis of Amphioxus. The layer secretes a thin cuticle like that of annelids. The corium in Amphioxus is gelatinous. Although the epidermis is stratified in all vertebrates, such low forms as cyclostomes do not have the outer dead horny layer, and they do have the thin cuticular layer of the invertebrates and Amphioxus. The skin of fishes is like that of cyclostomes, except for differences in gland secre- tions. See Fig. io6. A. AMPHIOXUS a PETROMYZON C SQUALUS. 0. RANA. Fig. io6. — Cross sections of the skin of four chordates, Amphioxus, Petromyzon, Squalus, and Rana, showing the fundamental differentiation of the skin into corium and epidermis. The difTerentiation of the epidermis into a dead outer layer and an inner liv- ing layer began in aquatic animals. (Redrawn mainly after Plate and Schimkewitsch.) The outer dead horny layer of the epidermis, the corneum, appears first in Amphibia, correlated apparently with the land habit, since most land animals have it. As the amphibian skin is fundamentally like that of higher vertebrates, the evolution of the skin beyond the amphibians presents no serious difficulties. The striking differences are in the secre- tions of the glands. It is indeed difficult to imagine how the skin mucus of amphibians could have evolved into the milk of mammals. It should, however, be remembered that sUght chemical differences often result in striking differences in properties; so that we should not be surprised to find chemical differences in the skin secretions of vertebrates that are far greater than any morphological differences in the glands themselves. THE INTEGUMENTARY SYSTEM 113 STRUCTURE OF THE HUMAN SKIN The skin of man, together with its appendages, hair, nails, teeth, mem- brane bones, and glands, is only about four per cent of the body weight. Like that of other mammals, it consists of two tissues, an outer epidermis and an inner connective-tissue corium. A cross section of the epidermis shows under the microscope a many- layered epithelium, which varies greatly in thickness in different parts of GERMINATIVUM :\ «^ SWEAT GLANO'^ Fig. 107. — A cross section of the thickened skin of the sole. The stratum corneum is especially thickened on the sole and on the palm of the hand. the body. Even where it is thinnest, as for example on the back, at least two layers of cells are distinguishable, an inner, growing stratum germina- tivum and an outer, horny stratum corneum. The cells of the stratimi germinativimi are columnar in shape; those of the stratum corneum are flattened and scale-like. The former are alive and, by their constant proliferation on division planes parallel to the surface of the skin, they make continual additions to the stratum corneum. The living cells in their turn, as by the wearing off of the outer layers they come nearer and 114 CHORD ATE ANATOMY nearer the surface, replace their Uving protoplasm by keratin, and become the horny scales of the outer epidermis. In man, as in most mammals, the stratum corneum wears away as rapidly as it is formed and never becomes greatly thickened on most parts of the body. Amphibia, however, shed the stratum corneum in sheets, sometimes sloughing off the entire covering of the body at once. Serpents do the same thing, scales and all. Sections of the thick epidermis of the palms and soles show between the stratum germinativum and the stratxim corneum, two intermediate layers, a stratum granulosum and a stratum lucidum. These, however, are merely transitions between the inner growing layer and the outer lifeless horn. See Fig. 107. Coriimi. The deeper layer of the skin, the corium, cutis, or dermis, is connective tissue, with a much greater variety of cell elements than the epidermis, and, unUke the epidermis, richly supplied with blood-vessels. Where it touches the epidermis, especially on the palms and soles, the corium is thrown up into many fine papillae, the capillaries of which feed the cells of the stratum germinativum. In some of these papillae are tactile corpuscles and other nerve terminations. Cutaneous glands and the roots of hairs, both derived from the epidermis, become embedded in the corium, and from it they are fed. Fat cells are numerous, especially in the lower layers. The greater portion of the corium is made up of connective-tissue libers, both elastic and non-elastic. Most of these lie parallel to the surface, interwoven like the fibers of felt; but some bundles are perpendicu- lar to the surface. The fibers are more compactly set in the outer parts of the corium than in the inner. The deepest layer is the loose or areolar connective tissue by which the entire skin is attached to the underlying muscle or bone. Skin muscles are few, and are mostly connected with the bases of the hairs. The elasticity of the skin decreases with age. Leather is made from the outer, compact layer of the corium of animals. The epidermis is removed by maceration, and the connective-tissue fibers are toughened by tanning. DEVELOPMENT OF THE SKIN Notwithstanding the close connexion between the two main layers of the skin, their origin in the embryo is diverse, the epidermis developing from the ectoderm, the corium from mesenchyma. Since the mesen- chyma is derived chiefly from the mesoderm, this contrast in origin is fundamental. Epidermis. The embryonic epidermis arises directly from the ectoderm, and is at first a simple cuboidal epithelium. By the end of the first month, as the result of cell divisions in a plane parallel to the surface, THE INTEGUMENTARY SYSTEM II5 this epithelium becomes two-layered, the outer and thinner layer being the periderm. By the continued multiplication of the basal cells, the number of layers gradually increases until, by the fourth month, all four layers of the thicker parts of the adult skin have appeared. The cells of the stratum comeum contain a fatty or waxy substance, which helps to form the pasty vemix caseosa which covers the body of the new-born infant. Developing hairs, instead of penetrating this layer, lift it as a continuous sheet, the epitrichial layer. Coriimi. In most parts of the body, the mesenchyma which produces the corium is derived from cells which have migrated from the parietal layer of the mesoderm. For this reason, that part of the epimere which forms the corium is called the dermatome. In some vertebrate embryos, if not in all, the ectoderm also contributes to the mesenchyma of the head and possibly, therefore, to the corium. Embryonic mesenchyma consists of scattered, stellate cells, separated by wide spaces. It becomes the connective tissue of the corium by secret- ing intercellular fibers, both elastic and non-elastic. By the fourth month, the compact fibrous layer of the corium is distinguishable from the loose areolar tissue under it. Blood-vessels and nerves invade the corium from below, hairs and glands grow into it from the epidermis. Abnormalities in the distribution of blood-vessels cause birthmarks. FINGER-PRINTS In all primates, the entire surface of the palms and soles, but no other portion of the body, is marked with fine parallel ridges separated by equally fine grooves. At definite places on hands and feet, these ridges form concentric lines or loops. Eleven distinct "friction-ridge patterns" have been distinguished, five on the finger tips, four at the base of the fingers, two near the wrist or ankle. See Fig. 108. Those of the finger tips are most familiar, since they are used for identification. Since no function has ever been proved for these designs, their presence and their constant position has stimulated much interest and discussion. To useless organs, the hypothesis of special creation gives no clue. Are they, then, rudiments of structures functional in the lower animals? The significant fact about these patterns is that they match precisely, both in number and position, the concentric rows of horny scales on the foot-pads of insectivores, a group which, for various reasons, is thought to be near the direct line of man's ancestry. In the insectivores, the posi- tion and arrangement of the scaly ridges is clearly adaptive and the best possible one to prevent slipping in any direction. These finger-print pat- ii6 CHORDATE ANATOMY terns, therefore, serve to convict men of animal ancestry, as they have on occasion served to convict them of crime. On the sides of the fingers, the friction-ridges merge into rows of wart- like elevations. This has been interpreted as confirming the opinion that the ridges are remnants of rows of horny scales. That the ancestors ^ FRICTION ^DIGITAL PADS^ _^^RIDGES m TRIRADIl TRIRADII MNTERDIGITALjl, PADS ^1- Y THENAR.< PAD \ .WPOTHENAR N, PAD HYPOTHENAR ^ PAD A. INSECTIVORE B. MONKEY C. MAN Fig. io8. — Friction-ridge patterns in three mammals — insectivore, monkey, and man. The presence of such useless and rudimentary concentrically arranged ridges in the human hand receives its only reasonable interpretation in the light of the evolution theory. (Redrawn after Wilder.) of the mammals were scaly is, however, supported by more convincing evidence than this. APPENDAGES OF THE INTEGUMENT Throughout almost the entire animal kingdom the skin tissues form various calcareous, chitinous, or horny structures — shells, spines, teeth. SPINE OENTI IE EriAriEL ORCAN COMPACT CONNECTIVE TISSUE LOOSE 'connective TISSUE BASAL PLATE B. D. Fig. 109. — A vertical section of the skin of an elasmobranch, showing five stages in the development of a placoid scale. The development of a placoid scale is essentially like that of a tooth. This fact taken together with the similarity of their structure sug- gests that teeth may have evolved from placoid scales. (Redrawn after Schimkewitsch, modified.) bones, scales, hair, feathers, horns — which serve for defense, support of tissues, or attachment of muscles. The limy shells of molluscs and the chitinous exoskeletons of arthropods serve all three purposes. Among vertebrates, the placoid scales, which first appear in certain sharks of the Upper Devonian, are especially important because of their THE INTEGUMENTARY SYSTEM 117 further evolution. Each of these scales has a flat basal plate of dentine embedded in the skin, and each has commonly also a projecting spine coated, like a tooth, with hard enamel. From these minute placoid scales of ancient sharks have evolved all the multiform teeth of all the higher vertebrates. From these and other types of scale have evolved also, by simple enlargement, the heavy continuous dermal armor of ganoids and other fishes. These same bony plates survive also in man and the higher Fig. 110.— The scales are scales of the li -ENAMEL SPINE -The imbricated pattern of placoid scale arrangement in elasmobranchs. arranged in rows and usually each scale is in line with the interval between nes in front and behind. (Redrawn after Klaatsch.) vertebrates as " membrane bones " which, unlike most parts of the skeleton, are not pre-formed in cartilage but develop directly in connective tissue. HORNY SCALES Vertebrates, besides bony scales, have also horny; but these have played a much less important part in evolution, and are confined to amniotes, more especially reptiles. In reptiles, the stratum corneum forms a continuous scaly layer over the entire body, the separate scales being local thickenings which con- tinue to grow by the addition of new keratin from underneath. Serpents commonly shed this scaly coat twice a year. But the rattlesnake retains bits of the old skin at the tip of the tail. These become the rattle, which therefore grows two rings a year. Il8 CHORD ATE ANATOMY Most reptiles have substituted horny scales for the bony scales charac- teristic of fishes. But crocodiles have both sorts on the same individual. On the ventral side of some snakes, large horny scales are attached to muscles and become organs of locomotion. The largest reptilian scales are those of Chelonia, in which horny scales fuse with the bony carapace and plastron. In birds horny scales cover the feet. Among mammals, the East Indian Manis, and the tails of rats and mice are scaled. Fig. III. — Section of developing scales of lizard, Sceloporus. c, papilla of corium; f, outer layer of epidermis which later becomes cornified; /, fibrous layer of skin; ni. Malpighian (stratum germinativum) layer; p, periderm; ts, tela subjunctiva. (From Kingsley's "Comparative Anatomy of Vertebrates.") It is a curious fact that, while horny scales are purely epidermal structures, their development is initiated, like that of bony scales, by the corium. HORNS To produce such diverse structures as hairs, feathers, scales, nails, and hoofs, demands most exceptional evolutionary potentiaUties on the part of the horny layer of the skin. Among the surprising developments of keratin-forming tissues are the horns of ruminants and rhinoceros. Those of the rhinoceros are formed wholly of keratin produced by the stra- tum corneum on the snout. The hollow horns of cattle have, in addition to external keratin, a bony base and core, which extends from the frontal bone into the cavity of the horn. The antlers of the deer tribe are bony out- growths with no covering of horn, but only the skin or "velvet" which is soon lost. Horns are best interpreted as weapons of defense and offense. NAILS, CLAWS, AND HOOFS Nails are scale-like thickenings of the stratum corneum at the ends of the fingers and toes, formed of homogeneous keratin identical with that of the stratum lucidum from which they develop. Nails and claws are strikingly alike except in form. Both develop from a matrix at the base, which in man appears as the whitish "lunula." Both have their bases surrounded by a fold of skin, the "nail wall." In both, a convex "outer plate" on the upper side of the digit may be dis- THE INTEGUMENTARY SYSTEM 119 tinguished from a concave "ventral plate" on the under side, each being morphologically a reptihan scale. The ventral plate in man is reduced to a narrow fold of skin between the nail and the finger pad. Claws appear first in urodele Amphibia. In some Anura, they are limited to certain hind toes. But certain male frogs, at mating time, develop horny papillae on their thumbs, which serve to hold a slippery female. Reptiles have claws on all toes. Those of mammals are like those of reptiles, except where the mammalian claw has altered into a hoof, or become retractile, as in cats, which walk on foot-pads and keep their claws sharp by raising them off the ground. Claws of mammals intergrade with nails, so that it is difficult to draw a line between the two. Some primates have both claws and nails on the same foot. Nails are Fig. 112. — Diagrams of (A) nails, (B) claws, and (C) hoofs, e, unmodified epidermis; M, unguis (outer plate); s, subunguis (ventral plate). (From Kingsley, after Boas.) then rudimentary claws, modified to correlate with the increased sensi- bility of the ends of the digits and their use as organs of touch. Some mammals, such as the horse and deer, which run on their toes, have hoofs instead of claws. The structure, development, and relations of hoofs, however, prove that they are nothing more than enlarged and modified claws. Both have dorsal and ventral plates. The attempt to divide mammals into hoofed and clawed types encounters the difficulty that at least one animal, Hyrax, has both claws and hoofs. FEATHERS Feathers, which are characteristic of birds, are modified scales, and their early development is the same. A corium papilla initiates both; but the feather anlage, instead of flattening to a scale, becomes an elon- gated cylinder, which spUts into the barbs and barbules of the developed feather (Fig. 113). A down feather, in fact, suggests an elongated and frayed-out scale. I20 CHORDATE ANATOMY Birds, which have descended from reptihan ancestors, still have scales on their feet, and even their bills and claws are presumably enlarged and modified scales. HAIRS Hairs, which are characteristic of mammals, are not comparable morph- ologically with either feathers or scales, since the development of hairs is CORIUM PAPILLA^ BARBULES P[JLP.^s:^=C^^ — ^PAPILLA C. "FOLLICLE' D. Fig. 113. — Four stages in the development of a feather. A, B, and C represent stages in the development of a down feather. D shows a contour feather in the feather- sheath. A-B and C-D are sections of a young contour feather at the levels indicated in D. In contrast with a hair the development of a feather is initiated by a corium papilla. (Redrawn from Ihle, after Biitschli.) initiated by the epidermis and not by the corium. When, as in Manis, hairs and scales occur together, the hairs are at the apices of the scales. That scales are older phylogenetically than hairs, is indicated by the fact that scales develop earUer in the embryo; and fossil evidence demon- strates that mammals have evolved from some scaly stegocephalan-Hke cotylosaurian. But since neither the skin nor its non-bony appendages are commonly fossiUzed, their history has to be made out chiefly from embryology and comparative anatomy. All mammals have hair; and man's relative hairlessness is by no means a distinctive human trait, since some mammals, for example the whales, have less hair than man. It is well known that changes in the secretion of the endocrine glands affect profoundly the growth of hair, and man's loss of hair may have been thus brought about. THE INTEGUMENTARY SYSTEM 121 Hair Structure. The hair of all mammals is essentially similar. There are, however, such differences of detail as enable an expert to identify different species. Each hair consists of a "root" buried in the skin, and an external shaft. Microscopic examination shows a multicellular structure, with the cells in three layers, an outer cuticle, a cortex, and a central medulla. The cells of the cuticle are scale-like, overlapping one another like shingles on a roof. Cortex cells, greatly elongated, make up the greater portion of each hair. The medulla occurs only in the "contour hairs" and is wanting in the finer hairs. It is made up of cuboidal cells usually in a double row. Fig. 114. — Diagram of structure of hair, b, blood-vessels; ct, cuticle of hair; ex, cortex; g, gland; h, hair; he, Henle's layer; hf, hair follicle; hx, Huxley's layer; w, medulla; p, papilla; sg, stratum germinativum of epidermis. (From Kingsley's "Comparative Anatomy of Vertebrates.") The root is surrounded by epithelial and connective-tissue sheaths. It ends in a swollen "bulb," from which it grows and which contains a connective-tissue papilla, with capillaries which feed the hair. Hairs of different human races differ in cross section. In general, the rounder the hair, the straighter it is; the more compressed, the curlier. It has not been shown that these differences have been developed either by natural or by sexual selection. Hair Direction. Hairs, instead of projecting vertically from the skin, emerge at an acute angle, have a slant in some special direction, and thus form streams in various parts of the body. Where such currents meet, either "rhomboids" or "vortices" may form, the latter being commonly called "cowlicks." The fact that such rhomboids and vortices appear on the human body in regions where the hair is short, has been interpreted to mean that man's hairy covering was once longer than at present. Although in general the direction of hair growth is such as to make gravity the determining influence, it is a curious fact that the hair on 122 CHORDATE ANATOMY the human forearm suggests his animal ancestry. The hair of the fore- arm slants from the wrist toward the elbow, in the reverse direction to the slant on the upper arm. Man shares this pecuHarity with the apes alone. All other mammals have the same hair direction on both parts of the limb. Why this resemblance of man to the apes unless they share a common ancestry? The pecuharity is not adaptive, and it is not easy to see why, if man and apes were independently created, they should resemble one another in this detail. Hair Arrangement. That the arrangement of hairs on the human body has any evolutionary meaning is, to say the least, surprising. Indeed, since such patterns can have no use, we should hardly expect to find them at all. No less surprising is an arrangement of hair in mammals that indicates descent from scaly ancestors. In most mammals, the hairs occur in Fig. II';. — Arrangement of the , rn, hairs in groups of threes and fives groups of three.or more. These groups in the human embryo, with the g^j-g arranged in parallel rows in such wise l^t:^L'"Trot KlSsTe^'aft:' that each cluster Ues opposite an interval Stohr.) in the rows in front and behind. In short, the arrangement is imbricated, Hke the universal arrangement of scales. This arrangement, though quite useless, is precisely what we should expect if mammals have descended from scaly ancestors. See Fig. 115. Histogenesis of Hairs. Hairs are, in origin, epidermal, and therefore ectodermal. Each begins as a minute epidermal papilla, which has arisen by local cell proliferation in the stratum germinativimi. See Fig. 116. Continued proUferation gradually converts this papilla into a cellular column, which extends obliquely downward into the underlying mesen- chyma which is to become the corium. The growing end swells into a bulb, in which later develops the corium papilla from which the hair is to grow. Cellular differentiation of the hair column results in an inner sheath and the hair-shaft, all surrounded by an outer sheath. From the bulb to the point in the hair column where the sebaceous gland develops, the cells of the hair-shaft become cornified. Above this point the central cells degenerate to form a canal in which the hair-shaft grows towards the surface. Continued cell multiplication of the stratimi germinativum of the papilla elongates the central hair-shaft to extend beyond the skin. Each hair thus formed continues to elongate throughout its life of several months or years, the rate of growth varying greatly in different THE INTEGUMENTARY SYSTEM I 2- parts of the body. But finally growth ceases, the hair dies, and is shed. If the hair papilla retains its stratum germinativum, a new hair grows. Each hair column, in addition to producing a hair, may form as lateral outgrowths one or more sweat or sebaceous glands. Muscle cells devel- oped from the mesenchyma of the corium attach themselves to the hair- roots and become arrectores pilorum. The human foetus before birth has a hairy covering, the ''lanugo," which is shed shortly before or soon after birth. The coat persists, SEBACEOUS GLAND EPITHELIAL BED ROOT OF HAIR Fig. ii6. — A vertical section of skin of a five month human embryo, showing four early stages in the development of a hair. The growth of a hair is initiated by the forma- tion of an epidermal papilla projecting (down) into the underlying corium. (Redrawn from Bremer after Stohr.) however, in certain types of "hairy men." The evolution theory affords the only rational explanation of the lanugo. PIGMENT Skin color in man is due in part to the blood in the capillaries of the corium. In addition, there are two pigments in the skin and hair, a brown, sometimes darkened to a black, both in granules; and a yellow, that may strengthen to a red, both diffused in the tissues. All are prod- ucts of cell metaboUsm.^ The pigments of the hair are confined to the cortex. The epidermis and the outer parts of the corium are both pigmented. Not until shortly after birth do pigment granules appear in the stratum germinativum, so that even Negroes are born white. Moles and freckles involve excessive local pigmentation. Freckles are small local patches of excess pigmentation, which are more likely to occur in light-skinned individuals who have been exposed to strong sun- light. A mole or nevus is an elevation of the skin due to local prolifera- T24 CHORDATE ANATOMY tion of epidermis and corium, and is usually excessively pigmented. When a mole is congenital and involves blood capillaries, it forms a "birthmark." Since pigments like those of vertebrates are found also in invertebrates, there is no reason to question their common origin. Many animals below the mammals have their pigments in special cells, the chromato- phores, which "expand" or "contract" (see page loi) under the influence of hormones and thus alter the color of the skin. The colors of lizards, which are often brilliant, are not in their scales, but in chromatophores of the underlying corium. Widely among vertebrates, pigments of scales, skin, hair, or feathers often show striking and elaborate patterns that serve for protection, warning, recognition, or sexual allure; but in man chiefly the region of the nipples and the external genitals are slightly darker than the rest of the body. In man and some other hairless mammals, such as the elephant, the function of the skin pigment is to check ultraviolet light before it penetrates to living cells. Everyone has observed the effect of the sun's rays upon unwonted skin, and the promptness with which the skin responds by tanning. Lacking skin pigment, men could not live in some parts of the earth. Color in Races and Individuals. The blue of the iris of human children and new-born kittens is an interference color, like the blue of the sky or the "eyes" of a peacock's tail. Later, as the iris fibers thicken, the inter- ference is less perfect, and the eye is gray. Brown pigment in some fibers only, gives hazel. Brown eyes are evenly pigmented. Dark brown eyes are called black. There is also the yellow pigment which, nearly free from brown, gives the amber eyes of some blondes. The same color intensified makes the red iris that sometimes accompanies red hair. The interference blue slightly masked by yellow, gives that rarest of all eye colors, green. In general, among Europeans, the eyes are less pigmented than the hair, so that dark hair with gray-blue eyes is common. But some blondes have a striking color scheme, eyes darker than the hair. Hair is colored by the same two pigments, both usually present, with the brown-black, masking the red-yellow, except in strong light. But some dark hair lacks the red and is blue-black. Some blondes have no brown pigment, and little yellow. Most have brown also, along with varying amounts of yellow. The tow head with a touch of dark, is the ash blond. Yellow with some red is golden; and starting from this, the red may strengthen to a rather unadmired carrot or orange. More brown carries the red over into auburn; still more gives bronze. Hair that has lost its pigment is white, for the same reason that snow is; the crystal faces of the one and the cell walls of the other scatter the light. THE INTEGUMENTARY SYSTEM 12 5 Skin color is like hair color except that the blood color below the pig- ment may show through, and that sunUght, which fades the Hfeless hair, stimulates the living skin to turn dark. Primitive man was dark as the ape ancestor was, and as most races are still. Reducing the black pigment, with the yellow retained, gives the Mongolian skin color. The stronger yellow, together with a good deal of brown, is the traditional hue of the Red Man, though as a matter of fact, most Indians are brown, like most White Men. The blond White Man is a local race that originated in some region near the Baltic, apparently since the last Ice Age. Being highly energetic and uncommonly well endowed, the descendants of these blond giants have made their way all over the world, and, much diluted with darker blood, still appear in most civilized countries of the world. Why their blondness, nobody knows. It cannot be due to chmate, for the Eskimos, Samoyads of Siberia, the Patagonians, and the people of northern China are all dark. Naturally, a blond race could hardly survive in the tropics; but a white skin is no obvious advantage anywhere. Yet certain studies of Chicago children show that the highly pigmented Italians are more Uable than lighter stocks to rickets, which is a sunlight- deficiency disease. So it may be that in high latitudes, in a wooded coun- try or one that has much cloud and fog, a fair skin that is still able to tan may have a selective value and be accounted for on Darwinian principles. Skin color plays queer tricks. Any two parents, even two Negroes, may have an albino child; two dark-haired parents may somehow miss with the brown pigment and have red-haired offspring. The most that anyone can say is that "Nordic" man probably began as a mutant from a dark stock. Possibly, after the mutation appeared, it was admired and selective mating kept it to the fore. CUTANEOUS GLANDS Since among invertebrates most glands are unicellular, it has generally been assumed that the multicellular glands of vertebrates have evolved from such beginnings, an increase in the size of the secreting cells tending to carry them into the underlying corium, where groups of such epidermal cells become multicellular organs. Be this as it may, cutaneous glands develop, much as hairs do, from solid cords, which are proliferated from the stratum germinativum and grow downward into the underlying corium. The lumen of the gland forms later, to connect with the exterior, and the gland anlage differen- tiates into a secretory portion and a lining for the duct. The secretory cells become intimately associated with blood-vessels and nerves. Sweat Glands. In man sweat glands occur in most regions of the body, and are especially abundant on the palms and soles. They are, for the most part, of the simple tubular type, much coiled to increase the 126 CHORDATE ANATOMY secreting area; but those of the axillae are branched and greatly enlarged. They are of the "vitally secretory" type, that is, the cell protoplasm merely produces the secretion, but is not converted into it, and the cell continues alive indefinitely. The sweat is usually oily but, in man, becomes watery under the influence of the nerves. See Fig. 117. ASSOCIATION NEURONE EFFERENT NERVE SWEAT GLAND CAPILLARIES Fig. 117. — A diagram illustrating the nervous mechanism of temperature regulation in man. The quantity of secretion of tubular glands (and consequently the amount of sweat which may evaporate to cool the body) depends upon the quantity of blood in the capillaries associated with the glands and dermal papillae. Through a reflex arc the circulation is regulated by the temperature of the skin. (Redrawn after Hough and Sedgwick.) Sebaceous Glands. Sebaceous glands in man occur on most parts of the body, but are wanting on the palms and soles. Most hairs have sebaceous glands connected with their follicles. They are of the acinous type, and necrobiotic, that is, their protoplasm forms the fatty secretion, which the cell extrudes, and then dies. Other Glands. Besides the sebaceous and sweat glands, there are other highly specialized cutaneous glands, the lacrimal glands of the eye and the Meibomian of the eyehds, the wax glands of the auditory meatus, besides preputial, vaginal and anal glands, which occur in most mammals, and mammary glands in all mammals. Of the organs which have evolved from glands, none are more sur- prising than the luminous organs or photophores of deep-sea fishes. These are true dark lanterns since they have a condensing lens and a reflecting membrane. The light is produced by the oxidation of luciferin secreted by the gland. No carbonic acid and little heat are evolved in the process. THE INTEGUMENTARY SYSTEM 127 Mammary Glands. Mammary glands first appear in monotremes as a pair of milk-secreting organs on the ventral side of the body. They are without nipples, and in Echidna they pour their secretion into a CUT LIMB BUD I POSITION OF DEFINITIVE GLAND 'I?^|f1"^^^ MILK LINE GLAND ANLAGE NIPPLE EPIDERMIS V u-^' SMOOTH MUSCLE.ltj_ CORIUM- ■ MUSCLE 13 5 MM. HUMAN EMBRYO B MUSCLE C FAT Fig. 118. — A figure illustrating the development of the mammary gland in man. A 13.5 mm. embryo shows the "milk line", a ridge which extends from the a.xillary region to the groin. The definitive gland develops only from the anterior portion of this line. Taken with the evidence of supernumerary teats in man, the line is interpreted as proof that the ancestors of man had more than a single pair of mammary glands. (Redrawn from Arey, after Kollmann.) A, B, and C are sections of the definitive mammary gland in successive stages of ontogenesis. A is from a two-months embryo, B from a four-months embryo, and C from a seven-months embryo. From its development the mammary gland is seen to be a compound tubtilar gland. (Redrawn from Arey, after Tourneux.) Fig. 119. — Scheme of different kinds of nipples. Single line, ordinary integument; double line, that of primary mammary pocket. A, primitive condition, found in Echidna; B, human nipple; D, Didelphys before lactation; C, same at lactation; E, embryonic, F. adult conditions in cow. B and C are true nipples, F, a pseudo-nipple (teat). (Based on figures by Weber from Kingsley.) depression, the "mammary pocket", surrounded by a fold of skin. From this condition in monotremes, the teats of the higher mammals have evolved, either by elevating the milk-field at the bottom of the pocket 128 CHORDATE ANATOMY into a "true teat," as in man, or else by elevating the surrounding ridge to form the "false teat" of ruminants. The number of teats corresponds roughly to the number of young in a litter. Certain abnormalities in the milk glands of man, however, confirm strongly the theory of the animal origin of the human body. Super- numerary nipples appear in man with a certain statistical frequency. But these extra teats, instead of being placed at random, are usually set Fig. 120. — The presence of supernumerary teats (polymastism) in man supports the theory of the animal origin of the human body. Their repeated occurrence has received no other rational explanation. They are reversions or atavisms. (Redrawn after Wiedersheim.) in two ventral rows, precisely like two rows of nipples which form the milk lines of lower mammals. They are, then, best interpreted as rever- sions to an animal ancestor. The theory of special creation gives no clue whatever to their occurrence in human beings. See Fig. 120. Functional differences among the glands of vertebrates are much greater than morphological, and their physiological evolution is a difficult problem in biochemistry. CHAPTER 5 TEETH No invertebrate has teeth at all comparable, save in function, position, and material, with the teeth of vertebrates. Some annelids, like Nereis, have horny pharyngeal teeth that act like pincers. A circle of calcareous teeth surrounds the mouth of the vegetarian sea-urchin to form the "lantern of x\ristotle," but each tooth has its own muscles and there is no jaw. Some snails have a radula with which they rasp their food. Many arthropods, notably the biting insects, have their appendages modified into hard mouth-parts that are both jaws and teeth. But a series of independent teeth operated by a movable jaw is peculiar to chordates. Nor do all chordates possess such teeth. The protochordates have no teeth of any sort. Cyclostomes have within the oral hood horny ecto- dermal teeth, with which they cling to their prey or bore their way into its flesh. In this absence of calcareous teeth attached to jaws, as in so many other characteristics, the cyclostomes exhibit their primitive nature. The larvae of some amphibia have upon their jaws horny teeth in the form of papillae. Most reptiles have true teeth; but the Chelonia have replaced those of their ancestors with horny beaks. So, too, have all modern birds, although their ancestors of the Jurassic and Cretaceous had typical reptihan teeth. The embryo of the duck-billed platypus has rudimentary teeth which it does not use. Even among the placental mammals, the edentates either have no teeth or have them without enamel. In the toothless whales, teeth are present in the embryo, but the adult has only whalebone strainers. EVOLUTION OF TEETH Since the protochordates are without teeth, the cyclostomes have horny ones, and all attempts to discover any sort of rudimentary cal- careous teeth in cyclostomes have proved unsuccessful, it seems clear that teeth of the vertebrate type are a new acquisition with no homologs anywhere among invertebrates. Typical or "true" vertebrate teeth have their beginning in the innu- merable, minute placoid scales which so roughen the skin of sharks that in former time shark skin, under the name shagreen, was widely used as an abrasive. 129 I30 CHORDATE ANATOMY In elasmobranchs, on the edges of the jaws, these minute scales become enlarged into formidable biting teeth or sometimes, inside the mouth, into bony pavements that are used for grinding the food. That the biting tooth of elasmobranchs is a modified placoid scale is obvious from inspection, since the two look alike, and there is a transition in size and form between them. This identity is further borne out by OLFACTORY PIT -- TASTE PAPILLA L:_.uPHAGtAL VILLUS Fig. 121. — The pharynx of an elasmobranch (Squalus) laid open to show the double row of teeth in both upper and lower jaws. Such teeth differ only in size from the placoid scales of the pharynx and skin. Elasmobranch teeth, like scales, are fastened in the skin and are not attached to the jaw cartilages. (After Cook.) other evidence. Like true teeth, placoid scales have a base of dentine, which contains a pulp-cavity filled with connective tissue. Both scales and teeth have a spinous process, covered with enamel, which protrudes through the skin. Moreover, their development is similar in that, in both, the enamel is secreted by the ectoderm and the dentine by mes- enchyme, and both arise in that portion of the mouth where the ectoderm has invaginated to line the digestive tube. See Fig. 122. TEETH 13 r Originally, in vertebrates, the teeth were for seizing and holding prey. Grinding and cutting teeth, tusks, and fangs, are all modifications of the primitive mouth trap. The number of these holding teeth is indefinite in elasmobranchs, which may have as many as one hundred. They are not attached to the jaws, but merely imbedded in the skin of the mouth. They are all about aUke; and when one is lost, another moves forward into its place. In teleosts, the number of teeth is somewhat reduced, although all parts of the mouth and even the pharynx may carry them. The special advance made by the teleosts is to set the teeth more firmly by fusing their bases with the membrane bones of the mouth. Fig. 122. — Comparison in development and structure between a placoid scale and a tooth, a, b, and c represent the scale; d, e, and / the tooth. In all the figures the epidermis is dotted, but its stratmn germinativiitn is represented by a layer of large cells with nuclei; and the cutis is presented as composed of fibers with scattered cells. X, enamel membrane; y, cutis papilla; e, enamel; d, dentine; p, pulp cavity. (From Wilder's "History of the Human Body," Henry Holt & Co.) Amphibia still farther reduce the number of teeth, but retain them on premaxilla, maxilla, mandible, vomer, and palatine bones, and more rarely on the parasphenoid. But toads have no teeth whatever. A strik- ing feature of certain ancient and long extinct amphibians, the labyrin- thodonts, which arose in the Coal Period and survived into the Triassic, was the enormously complicated folding of the tooth enamel and dentine, which anticipated, yet went far beyond, the similar arrangement in some mammals. Reptiles make two important advances toward the condition in mammals. Some of them, like some of the amphibians, have their teeth set on a ledge on the inner side of the jaw— pleurodont dentition. Or they may have the tooth set directly on the bone, acrodont dentition. But the crocodiles and some fossil reptiles attain to a thecodont dentition, in 1,3 • CHORX)ATE ANATOMY which each tooth is fixed in a separate socket, as in mammals. In addi- tion, some lizards and numerous fossil reptiles abandon the original homodont dentition, with all teeth about alike, and have their teeth more or less differentiated into incisors, canines, and molars, as in mam- mals, a heterodont dentition. The differentiation of teeth is obviously, an adaptive division of labor, the incisors acting as cutters or chisels, the canines as daggers, and the molars as grinders. The reptiles, more- over, limit their teeth to the two jaws. An especially elongated tooth occurs in the lower jaw of lizard and snake embryos, which is used to break through the tough membranous shell. A hardened tip of the horny beak of birds is used for the same pur- pose. The two structures are, however, morphologically quite different. A C Fig. 123. — Jaws of some primitive Jurassic mammals. The resemblance of these jaws and teeth to those of the theriodont reptiles of the same period suggests a similar genetic origin. (Redrawn from Romer, after Simpson.) Especially remarkable in reptiles are the highly specialized poison fangs of certain snakes. These are modified from the ordinary conical tooth, first by a folding of the tooth to form a groove along which the venom from a modified salivary gland flows into the wound. In other snakes, by a still further folding the edges of the groove unite, and the tooth becomes a hollow needle. One pair only is functional at any one time, others up to nearly a dozen pairs being held in reserve to take the place of the large fangs when these are lost. All the vipers, including the rattlesnakes, fold back the functional pair of fangs when the mouth is closed, and only in the act of striking pull them erect by special muscles. Mammals, besides having nearly always a heterodont dentition, with incisors, canines, and molars well differentiated, have acquired also a definite succession of sets of teeth, a set of "milk teeth" developed in early life being later replaced by a "permanent" set. The elasmobranchs, indeed, do have a certain succession of teeth, but only one at a time as single teeth are lost. Lower vertebrates, reptiles conspicuously, have a somewhat indefinite number of sets, and are said therefore to be polyphyodont. Only the mammals have two definite TEETH 133 sets, and are therefore diphyodont. But monotremes, sirenians, and toothed whales retain their milk teeth throughout life, have no second set, and are said to be monophyodont. In general, then, the course of evolution has been from a large and indefinite number of simple teeth all alike, not fixed firmly in place, and borne by any part of the mouth, to a reduced and definite number, set firmly in alveoU, confined to the jaws only, and differentiated into three sorts. Along with this has gone a shortening of the jaws and a change of food habits. But whether the change in diet caused the change in teeth, or the change in teeth made possible the change in foods, is still an unsolved problem. EVOLUTION OF COMPOUND TEETH Compound teeth resembling the molars of mammals first appear in certain late Permian and early Triassic reptiles, the theromorphs. Since amphibians and the earlier reptiles had simple conical teeth, the conclusion has been drawn that compound teeth are derived from conical teeth, and morphologists have advanced two theories as to how this evolution came about. The differentiation theory of Cope and Osborn assumes that the teeth of vertebrates were originally of the simple conical type found in most reptiles. Such were the teeth of the premammalian Stegocephala and of primitive theromorph reptiles. The first multitubercular type of molars of modern mammals appears in such a Triassic mammal as Dromatherium, the teeth of which had a large median cone or protocone in line with two smaller cones, a paracone in front and a metacone behind. Corresponding parts in the teeth of the lower jaw are called protoconid, paraconid and metaconid. Teeth of this sort are known as triconodont. Besides the three cones, triconodont teeth have a basal rim, the cingulum, which forms part of the crown. Marsupial-Uke mammals of the Tertiary had teeth of this triconodont sort. See Fig. 124. The secondary tubercles of such teeth show a tendency to enlarge to the size of the protocone. A further advance occurs when the three cones assume a triangular relation to one another, the secondary cones of the upper jaw migrating inwards, those of the lower jaw outwards. Teeth of this tritubercular sort occur in Amphitherium of the Jurassic period. Later, in mammals, appeared a posterior projection or talon, and a fourth tubercle, the hypocone and hypoconid. With these additions, the molar teeth assumed more and more the modern form with six cusps. It took many million years to accomplish these changes, which were 134 CHORDATE ANATOMY naturally based upon change in the form of the tooth-germ and involved budding of that organ. The concrescence theory accounts for the multitubercular molar teeth of mammals by supposing a fusion of the anlagen of conical teeth, the number of cusps corresponding with the number of conical teeth involved. Some observers claim to have found evidence of fusion of tooth-germs in vertebrate embryos, but most investigators are sceptical. It must be said, however, that tooth fusion is known to occur in the case of the massive pavement teeth of dipnoi. At the present time the con- crescence theory seems to have less factual support than does the differen- tiation theory. According to Bolk, in a modified form of the concrescence theory, compound teeth are formed by the fusion of the germs of successive sets. A B C D E Fig. 124. — A, triconodont tooth of Dromatherium; B, tritubercular tooth of Spalaco- therium; C, interlocking of upper (dark) and lower (light) tritubercular molar teeth (after Osborn) ; D, molar of Erinaceus; E, of horse (selenodont type); c, cingulum; m, metacone (metaconid) ; pa, paracone (paraconid) ; pr, protocone (protoconid) ; I, talon. (From Kingsley's "Comparative Anatomy of Vertebrates.") His theory assumes that the ancestors of mammals had more than two generations of teeth Hke the milk and permanent sets, that is, their denti- tion was polyphyodont. Under these conditions, the germs of successive sets might fuse with one another. The factual foundations of the theory, however, are weak. TEETH OF MAMMALS Teeth of mammals are especially important for the paleontologist, partly because they are hard and therefore Hkely to be preserved, but more because mammalian teeth are closely correlated with feeding habits. But feeding habits, in their turn, are correlated with the entire bodily structure, so that teeth are a key to the whole organism. Moreover, mammalian teeth are so highly specialized and so diverse in size and structure, that a single one is often sufficient to identify a species. In general, the tendency has been to reduce the number and to do away with the division into two sets, and at the same time to specialize and elaborate individual teeth. An ideally complete set for a placental TEETH 135 mammal would consist of three incisors, one canine, four premolars, and four molars in each half-jaw. The distinction between premolars and molars is that the premolars replace milk teeth, and are therefore of the second set, like the teeth in front of them ; but the molars have no predeces- sors, and are therefore really of the first set. Functionally, however, and often in size and shape, there is little difference, and the two groups are conveniently lumped together as cheek teeth. But while 3-1-4-4, 48 teeth in all, is ideal (conceptual), no placental mammal conforms to it, 44 being the usual limit in any actual animal and 3-1-4-3 a common formula. Nevertheless, it is convenient to think of each actual tooth as one particular member of the ideal set. Thus can the history of each tooth be followed throughout all the placental mammals, and each be identified wherever it occurs. But the marsupials are aber- rant, opossums, for example, having four and five incisors, so that their teeth cannot always be homologized with those of placentals. Starting, then, with the ideal dental formula: ^'3, c\, p\, m\, the little hyrax or cony, allied to the ungulates, is one of the few mammals to retain the full eight cheek teeth. But its incisors are reduced to one in each half-jaw and it has no canines. On the other hand, the ungulates tend to have the typical four front teeth, but to lack one molar and sometimes a premolar also. They usually have the canine like the incisors and so have practically, though not morphologically, four incisors. But all hollow-horned ungulates lack upper canines, and many, like domesticated sheep, have lost all four incisor-form teeth from the upper jaw. Their 0-0-^-3 dental formula is, therefore, in brief form: The pigs, with forty-four teeth in all, are peculiar in having the canines in both jaws grow throughout life as fast as they wear away. They are kept sharp by whetting against one another. The walrus makes tusks of the upper pair only, which also are unrooted. The narwhal, for a like pur- pose, uses one incisor, its mate remaining rudimentary, and has no other teeth. The Carnivora make the canines, especially the upper pair, into long curved daggers, which reach their extreme development in the extinct saber-toothed tiger of the Pliocene but are noteworthy even in the domestic cat. With each canine, in the flesh eaters, goes a "carnassial" tooth, especially developed in the cats, a premolar above and a molar below. Other cheek teeth, especially in the cats, tend to be reduced almost to rudiments. Moreover, the Carnivora, though uniform as to incisors and canines, 3-1-4-2 differ somewhat widely in the cheek teeth. Thus, while the dog is 3-1-4-3 the cat is reduced to The lynx is made a separate genus from 136 CHORDATE ANATOMY the cats because it has lost the minute first premolar of the upper jaw and ■2-J-2-I brought its dentition down to On the other hand, some of the ° 3-1-2-1 whales have gone back to primitive conical teeth used only for holding, are virtually or quite homodont, and have fifty or more pegs in each jaw. Characteristic of rodents is the complete absence of canines, and the reduction of the incisors to one functional pair in each jaw. The single pair, however, is a remarkable tool. Each tooth grows from a permanent germ that is set far back in the jaw, so that each passes under all the cheek teeth before it emerges at the front of the mouth. Enamel coats the front surface only, so that as the tooth wears, the dentine wears most, and the thin plate of enamel remains always sharp. Since these teeth grow throughout life, if they are not worn away by gnawing they become too long and the animal cannot feed. No rodent has more than six cheek teeth, many have only four, and an Australian mouse so far depends on its incisors, that it has brought its dentition down to But the hares and rabbits, and some other 1-0-0-2 rodents, as if to exhibit their affinities with other mammals, have two more incisors, very minute, behind the large pair in the upper jaw. Proboscidians. The most specialized of all teeth are those of elephants. Incisors and canines are completely lacking in the lower jaw. In the upper jaw, one pair only of incisors become the tusks, but the other two pairs have so completely vanished that it is not known certainly which pair remains. The tusks are rootless, and grow from far up in the skull. They elongate throughout fife, growing faster than they wear away, until in some instances they have reached a length of eight feet and a weight of more than 150 pounds each. Certain extinct elephants had tusks even larger, up to twelve feet and two hundred pounds. In the Indian ele- phant, only the males have tusks. But the larger African species, which uses the tusks for digging roots, has them in both sexes. The famous African elephant Jumbo, in a fit of rage, broke off both tusks inside his cheeks. When they grew out again, they made new holes through the flesh, but the original holes remained for the rest of the animal's life. Two extinct proboscidians, Tetrabelodon and Dinotherium, had tusks on the lower jaw also, those of Tetrabelodon nearly parallel with the upper pair, those of the Dinotherium turned downward like those of a walrus. Curiously, the small milk tusks of the young elephants, which are shed early, are rooted like ordinary teeth — another illustration of Von Baer's law that the young in a speciaUzed group tend to resemble general- ized ancestors. The cheek teeth of proboscidians, less conspicuous than the tusks, are even more remarkable. There are six in each half jaw, i.e., twenty- TEETH 137 eight teeth in all including the tusks and a pair of evanescent incisors. But of the six grinders not all are in use at one time. As the foremost wears down and is shed, a second and larger moves into its place, only to be followed by the remaining teeth in succession. Thus an old animal, since there are no canines, may have only the two tusks and four grinders. The grinders themselves are remarkable for their enormous size, the largest being more than a foot from front to rear and four inches wide. Each tooth is highly complex, with intricate folding of enamel and dentine, so that as the softer dentine wears away faster, the tooth keeps always its sharp grinding ridges. The same arrangement on a smaller scale appears in various other vegetarian mammals, notably in the horse. The -PALATINE A. CAUCASIAN B. NEGRO C. ORANG OUTAN Fig. 125. — Dental arcades of ape, Negro and Caucasian. The fcjrm of the Negro arcade is transitional between that of the ape and white man. With the shortening of the human jaw the diastema between incisor and canine teeth seen in the ape jaw is lacking. The refinement of the face is one of the most striking results of primate evolution. (Redrawn after Wiedersheim.) huge single teeth coming successively into use are a device for prolonging the life of the animal long after any set of teeth all functional at once would wear away. Apparently, as a result, the elephants are among the longest lived of mammals. Primates. The primates, except for their hands, feet, and brains, are a somewhat unspecialized group, and their teeth, though reduced in number to conform to the shortened jaws, are little differentiated and the enamel is not folded. The dental formula for the Old World monkeys is 2-1-2-3 ill both jaws. But the New World monkeys have another premolar, and sometimes lack one of the three molars. It is a curious fact, which no special creationist has attempted to explain, that man, also an Old World primate, has exactly the dental formula of the others. The canines in monkeys are somewhat longer than the other teeth, and in the male gorilla are much like those of the less specialized carnivores. 138 CHORDATE ANATOMY Significantly, in man, although even the upper canines are hardly larger than incisors, they have nevertheless the long roots of the animal tusk. That general tendency to shorten the mammalian face, which has brought down the cats to three and four cheek teeth and the higher primates to five, continues in man by a general reduction in size of all the teeth and by closing the diastema, the open space next the canines. Consequently, human teeth are a continuous series and no tooth is very much larger than another, for the canines ceased to be tusks at the begin- fCANINE INCISORS I PREMOLARS A^^^^^l— v .--" 'MOLARS \ "PREMOLARS INCISORS '^CANINE Fig. 126. — Human teeth viewed from the left side. The human dental formula is: is, c'l, pmS, mh. As a result of the shortening of the human jaws the third molars fre- quently do not erupt. The elongated root of the canine tooth suggests that as in lower primates the ancestors of man may have had fangs. (Redrawn after Braus.) ning of human evolution. Along with these, has gone a change in the direction of the incisors, correlated with the appearance of a chin. In the apes the incisors protrude, in man they stand upright. Incidentally, the human "bite" becomes horseshoe-shaped, with the rows of cheek teeth no longer parallel, as in all lower forms, even the apes. In addition, the triangular upper molars of the apes, with three cusps, become in man quadrangular with four, and, correlated with the reduced size of the single teeth, their pulp cavities become relatively still farther reduced, not to sacrifice unduly the thickness of the tooth wall. Some fossil human molar teeth, however, are taurodont, having a relatively large pulp cavity as in Neanderthal man. All these differences between apes and men are, however, bridged by various fossil creatures, on the whole human, some of the genus Homo TEETH I3Q but not our species, others quite outside the genus, but still within the family. TEETH OF MAN Human teeth are in structure substantially like those of most other mammals, and very like indeed to those of other primates. In each tooth three parts are distinguishable, an external enamel- capped crown, a root buried in a bony socket or alveolus, and a neck or constricted region between root and crown. The number of cusps or tubercles on the crown varies in the different teeth. The incisor and canine teeth have a single cusp, the premolars have two, and hence are known as bicuspids, and the molar teeth may have as many as five. The number of roots also varies in the different teeth. Incisors, canines, and premolars have but one, although the roots of the premolars are some- times divided into two. The lower molars have two roots, and the upper molars three. The finer structure of a tooth may be best seen in a thin longitudinal section. See Fig. 127. The central portion, the pulp cavity, is filled with connective tissue containing blood capillaries and nerve fibers, which enter the tooth through a minute foramen at the end of the root. The larger mass of the tooth is formed by a bone-like substance, the dentine or ivory. Unlike bone, however, dentine is devoid of cells. In section, the dentine takes on a somewhat fibrous appearance from the presence of parallel tubes, the dental canaliculi, which radiate from the pulp cavity through the dentine. At their peripheral terminations in the dentine, the canaliculi branch profusely. The sensitivity of the dentine to the dentist's drill is probably due to the living protoplasm in these canaliculi, which acts in the manner of nerve fibers. The larger part of the dentine, approximately 75%, consists of inorganic mineral salts such as calcium phosphate and calcium carbonate. The remaining 25% is organic material. At no place on the tooth does the dentine reach the surface, since the crown and neck are covered with enamel, while the root is surrounded by a heavy cement. Enamel is the hardest substance in the human body, since it contains only three and a half per cent of organic substance. It is thickest at. the apex of the crown, and thins out towards the neck and root. High magnification shows that the enamel consists of minute parallel hexagonal prisms which rest on the dentine and extend to the outer surface of the crown. Increase in the amount of enamel toward the outside of the crown is effected by means of increase in the number of enamel prisms and not by their enlargement or branching. In this way, the solidity of the enamel is maintained throughout the crown of the tooth. The mineral constituents of enamel are identical with those of dentine. 140 CHORDATE ANATOMY growth lines in enamel pulp chamber growth lines in dentine root canal oral epithelium osteoblasts of periosteum of alveolus connective- tissue fibers cementoblasts ( from dental sac) bone of mandible blood vessels and nerves Fig. 127. — Schematic diagram showing the topography of a tooth and its relations to the bone of the jaw. The numbered zones indicate empirically the sequence of deposition of the dentine and enamel. The so-called growth lines in the dentine and enamel follow the general contours indicated by the dotted lines in the figure but are much more numerous. (From Patten's "Embryology of the Pig.") /DENTINE CANAL ODONTOBLASTS Fig. 128. — Diagrams illustrating the difference in the secretion of dentine, .4; and of bone, B. The functional polarization of the odontoblasts and osteoblasts is, how- ever, similar. (Redrawn after Braus.) TEETH 141 Cement is a bone-like substance covering the root of the tooth as a thin layer which becomes thickest at the apex. Like other bone, the cement contains lacunae connected with one another by canaliculi. The mineral constituents are identical with those of bone. Surrounding the cement is a connective-tissue dental sac or membrane continuous with the periosteum of the alveolus and at the neck connected with the covering of the gum, gingiva. Development of Teeth When the human embryo has attained a length of about 11 mm., that is, by the end of the sixth week, the ectodermal epitheHum covering the upper and lower jaws grows rapidly down into the underlying con- ERDERMIS CORIUM BONY ALVEOLU LINGUAL LAMINA ENAMEL ORGAN OF PERMANENT TOOTH Fig. 129. — Diagrams of three stages in the development of a mammalian tooth as seen in sections of the jaw. The anlage of the permanent tooth Mes on the lingual side of that of the milk-tooth. (Redrawn after O. Hertwig and Arey.) nective tissue to form a horseshoe-shaped ridge or lamina extending along the edge of the jaw. As growth continues, the lamina divides into an outer labial lamina and an inner lingual lamina. The two ingrowths, however, soon separate, one growing in labially, the other lingually. The latter forms the dental ridge or lamina. As in development of a hair, the dental ridge is formed by cell multiplication in the stratum germinativum of the epidermis. Early in the development of the dental lamina, a series of bell-shaped enlargements, ten in each jaw, appear along its labial border (Fig. 129). These are known as enamel organs since they secrete the enamel covering of the crowns of the teeth. Each of the twenty milk teeth has a separate enamel organ, and all of them are present in a 2^^ months embryo. Each enamel organ contains a mesenchymatous dental papilla, the outer cells of which, the odontoblasts, secrete the dentine of the tooth. The remaining cells of the papilla become the pulp of the tooth. As develop- 142 CHORDATE ANATOMY ment proceeds, each enamel organ recedes from the dental lamina with which it retains a transient connexion by means of a "neck" or cord of cells. The free edge of the dental lamina, losing connexion with the anlagen of the milk teeth, forms a second set of enamel organs lying on the lingual side of the primary set. In this way, the anlagen of the thirty-two permanent teeth come to lie embedded in the connective tissue of the jaws on the hngual side of the primary set. The permanent teeth are, however, relatively slow in development, the third molar usually not forming in the jaw before the fifth year. blood-vessel \ in pulp dentine enamel blood-vessel in mesenchyme odontoblast ' dentinal Tbme's ameloblast outer epithelium fiber process layer of enamel organ Fig. 130. — Projection drawing of small segment of developing incisor from 130 mm. pig embryo to show formation of enamel and dentine. X350. (From Patten's "Embryology of the Pig.") Soon after the enamel organs emerge from the dental lamina, they become differentiated into three layers, an inner ameloblast layer which secretes the enamel, a mesenchyme-like enamel pulp, and a layer of outer enamel cells. The ameloblast cells which line the enamel organs are columnar epitheHal cells derived directly from the stratum germinativum of the epidermis. Viewed from the inner surface, each ameloblast cell is hexagonal and each secretes a simple hexagonal prism of enamel. As the enamel increases in thickness, the multiphcation of ameloblast cells results in an increase in the number of enamel prisms. The twisting and curvature of the prisms in the developed tooth are a consequence of the torsion of the ameloblast layer during active secretion. While the enamel grows by addition from the outside, the dentine increases in thickness from within. Consequently as the tooth is formed the amelo- TEETH 143 blast and odontoblast layers are pushed farther and farther apart. During the secretion of the dentine, protoplasmic strands from the odonto- blasts are retained within the dentine thus forming the dental canaliculi. The odontoblast cells persist throughout life, and by their continued secretion may in old age entirely obliterate the pulp cavity of the tooth. The crown of the tooth is the first to develop, and for a while the tooth resembles a silver-plated thimble, the thin enamel coating cor- ENAMEL PULP PERMANEhq:, TOOTH PAPILLA' ■»^p?e!i. Fig. 131. — A section of the jaw of a nine-months human embryo, showing the anlage of a canine tooth. The enamel organ of the permanent incisor is seen on the lingual side of the milk-tooth. (Redrawn after Corning.) responding to the silver plate, the dentine to the underlying metal. As the tooth grows, it increases in length as well as in thickness, adding first a neck and later a root. The opening into the inner pulp cavity becomes more and more restricted as the root elongates until finally only a minute foramen remains to admit blood-vessels and nerves. The nerves grow into the pulp and acquire free terminations among the odontoblast cells. The cement layer is the last to be added. Cement is secreted by bone-cells which penetrate the connective-tissue sac enclosing the tooth. 144 CHORDATE ANATOMY Membrane bone is formed around the root of the teeth to form the alveoli of the jaw-bone and to hold the teeth firmly in place. The mechanics of the eruption of teeth is a problem which needs further elucidation. Among the factors which operate is the elongation of the root, although teeth erupt before the root has completed its growth. The eruption of the deciduous teeth begins during the seventh month after birth, and is usually completed by the end of the second year. Of PERMANENT- INCISORS DEC IDUOUS — c — r^3 INCISORS. \ \V SECOND PERMANENT- MOLAR. PERMANENT- PREMOLARS PERMANENT- CANINE PERNMNENT MOLAR. '^^^f^^y^ PERMANENT INCISORS- FiG. 132. — The teeth of a five-year-old child. Portions of the jaws have been removed so as to expose the roots of the milk teeth and the anlagen of the permanent teeth. The latter are stippled in the figure. (Redrawn after Sobotta.) the permanent set, the first to erupt are the first molars which appear during the sixth year. The last to erupt are the third molars, which frequently become impacted in the jaw-bone so that eruption is impossible. The shape of a tooth is determined by that of the tooth-germ. If the layer of ameloblasts is folded, the enamel is correspondingly modified, and teeth such as those of ruminants and elephants, which become ridged by wear as the result of the difference in hardness of enamel and dentine, owe this adaptive characteristic to the folding of the ameloblast and odontoblast layers. The multipHcation of roots as in molar teeth is produced by the budding of the odontoblast layer of the dental papilla. CHAPTER 6 THE SKELETAL SYSTEM Some creatures, jelly fish for example, have no skeleton. In some, as in many molluscs and more conspicuously the corals, the skeleton is heavier than all the soft parts combined. In man, the bones make up about a fifth of the entire weight of the body, and this is not far from the average for active air-breathing vertebrates that do not have dermal armor. All skeletons support or protect the softer parts. Supporting skeletons occur even in such lowly creatures as protozoans and sponges. Protective skeletons are conspicuous in echinoderms and molluscs, are universal among the arthropods, and are found among vertebrates in such diverse groups as the Paleozoic ostracoderms, ancient and modern ganoids, dinosaurs, turtles, and armadillos. Skeletal parts, which are also jointed levers used in locomotion, occur in arthropods and vertebrates alone. Arthropods solve the problem of locomotion by means of a chitinous exoskeleton with the muscles inside it. Such a skeleton is highly efficient as attachment for muscles, and it has the further advantage of providing armor at the same time. Its disadvantage is that it cannot grow, so that all the arthropods, by one device or another, shed their exoskeletons as their bodies enlarge. This leaves them for a time helpless. Furthermore, since the tissues of the molting arthropod are unsupported, no arthropod can attain any considerable size. Among arthropods the largest air- dwellers are foot-long centipedes; and although among water-dwellers the euripterids of the lower Paleozoic and earlier were more than a yard long, a twenty-pound lobster is about the limit for a modern form. The typical arthropod is a tiny insect. The endoskeleton of vertebrates, light and strong, and capable of indefinite growth, has the single disadvantage that skeletal armor must be developed independently. But vertebrates have for the most part abandoned armor. Their success as a group has depended not a little on their admirable endoskeleton. To its usual functions, the vertebrates add the production of blood cells by the marrow, especially in the long bones. The Two Parts of the Skeleton. Historically, the vertebrate skeleton consists of two parts, which began independently, have evolved separately, 145 146 CHORDATE ANATOMY and not even in the 'higher forms have become completely integrated. These are the appendicular skeleton of the four limbs with their girdles; and the axial skeleton, which includes the skull with the jaws, and the vertebral column, the sternum, and the ribs. The individual bones number, in man, sixty-four for the shoulder girdle and the arms, sixty-two in the pelvic girdle and the legs, twenty-three in the skull, twenty-six in the backbone, and twenty-five for ribs and sternum, with six ear bones besides, over two hundred in all. TARSALS METATARSALS PHALANGES Fig. 133. — A diagram of the vertebrate skeleton, showing the division of the skeleton into axial, visceral, and appendicular. Membrane bones are shown in black, cartilage bones stippled. THE AXIAL SKELETON Evolution of the Vertebral Colunin. Nothing like a vertebral column appears in any invertebrate, so that the earlier portions of its history are unknown; though, if Amphioxus gives the clue, it was once no more than a medial dorsal fold of the ahmentary canal. Its first certain beginnings are the notochord of the lower chordates, the Hemichorda, Urochorda, and Cephalochorda. In the cyclostomes, the notochord is still the main part of the axial skeleton. Since the cyclostomes have cartilaginous neural arches, it is probable that neural arches are the earhest vertebral elements. Elasmobranchs, both fossil and modern, show a considerable advance over the cyclostomes. Cartilaginous haemal arches and centra appear, with both neural and haemal spinous processes. The anterior trunk vertebrae of elasmobranchs have short lateral or "costal" processes which extend between the myotomes and which suggest the future ribs of mammals. Since in fossil and living forms two centra may occur in each body segment, and since each centrum usually develops in ontogenesis by the fusion of antero-posterior anlagen, it is possible that two centra in each segment (diplospondyly) may have been the original arrange- ment in vertebrates. Elasmobranchs, moreover, begin the long process of vertebral differentiation, the vertebrae of the tail being unlike those THE SKELETAL SYSTEM 147 of the trunk, the difference correlated with a difference in the relation of the coelom to the vertebrae. In the trunk region, where the body-cavity Fig. 134. — Diagram of the skeletogenous tissue in the caudal region of a vertebrate. bv, blood-vessels; d, corium; epmu, epaxial muscles; hs, horizontal septum; hytny, hypaxial muscles; msd, msv, dorsal and ventral median septa; mys, myosepta; n, spinal cord; nc, notochord. (From Kingsley's " Comparative Anatomy of Vertebrates.") . CARnL>CE3 /NEURAL PROCESS B CESTRACION, A SHARK. Fig. 135. — A, The skeleton of a cyclostome. Petromyzon; B, The skeleton of an elasmobranch, Cestracion. Elasmobranchs were the first animals to invent paired appendages and the skeletal elements to support them. Marked differences in the axial and branchial skeletons of cyclostomes and elasmobranchs also appear. (Redrawn after Dean.) lies, the haemal arch of each vertebra is incomplete, while in the caudal region each arch is complete with a median spinous process. The noto- chord persists intervertebrally and the centra are biconcave. The 148 CHORDATE ANATOMY skeleton is still cartilaginous, but the cartilage is often hardened with lime. Bony vertebrae make their appearance in ganoid fishes, some of which however retain a cartilagin- ous vertebral column. Ball-and- ^^^^^^ socket joints between the centra are developed in Lepidosteus (gar- pike) as in some Amphibia. Am- phicoelous or biconcave vertebrae, however, predominate in all groups of fishes. Centra are wanting in Fig. 136. — Sagittal section of Sqaulas the Dipnoi, vertebrae, cut surfaces obliquely lined. ^-^j^ ^-^^ amphibians, bone c, calcifications of centra; cd, caudmeurals; _ ^ cdh, caudihemals; cr{i), cranineurals SUCCecds cartilage; and the VCrte- (intercalaria) ; d exits of dorsal nerve roots; ^^^^ ^^^ differentiated intO Cervical, crh, cranihemals; n, notochord; v, exits or ventral nerve roots. (From Kingsley's trunk, Sacral, and caudal. The "Comparative Anatomy of Vertebrates.") ^-^g^^ ^^^^^^ vertebra is but slightly modified for attachment to the pelvic girdle. A single atlas represents the cervical series of higher forms. Zygapophyses, for articulating each vertebra with its two neighbors, first appear in this group. Articulation Fig. 137. Fig. 138. Fig. 137. — Diagrams of (.4 and B) fish vertebrae and (C) vertebra from higher groups, b, basal stumps; c, centrum; ch, capitular head of rib; rf, diapophysis; ha, hemal arch; hr, hemal rib; n, notochord; na, neural arch; p, parapophysis; r, rib; /, tubercular head. (From Kingsley's "Comparative Anatomy of Vertebrates.") Fig. 138. — Two caudal vertebrae of alligator, c, centrum; ha, hemapophysis; hs, hemal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses; t, transverse process. The arrow passes through the neural arch. (From Kingsley's "Comparative Anatomy of Vertebrates.") with the ribs is effected by two sorts of processes, diapophyses from the neural arches and parapophyses from the centra. Lumbar vertebrae are first differentiated in reptiles, which also have two sacral vertebrae. Here also appear vertebrae with centra flattened THE SKELETAL SYSTEM 149 on both anterior and posterior sides and with the centrum of the atlas fused with the axis, as in mammals. The vertebral column of mammals shows little advance beyond that of reptiles. A few Insectivora have intercentra in the lumbar region — a diplospondylous condition reminiscent of elasmobranchs. Parapophyses are reduced to shallow pits for articulating the heads of the ribs. The human spine differs little from that of other mammals, except that the tail is reduced to a COCC50C with a few variable muscles attached. Man's only distinctive feature is the sigmoidal curve, which bends his spine in two directions, instead of one only as in other creatures. In addition to the two main spinal curvatures, thoracic and lumbar, man has two lesser curvatures, cervical and sacral, in the region of the neck and sacrum respectively. The Vertebral Column in Man. In the backbone of a child there are thirty-three vertebral elements. During growth the last nine fuse to form Fig. 139. — Diagrammatic sagittal sections of (A) amphicoelous; (B) procoelous; (C) opisthocoelous; and (D) amphiplatyan vertebrae. The head is supposed to be at the left. Cut surfaces obliquely lined. (After Kingsley modified.) two adult bones, the sacrum and the coccyx. The other twenty-four vertebrae remain separate throughout life and become differentiated into seven cervical vertebrae, twelve dorsal or thoracic, and five lumbar. These are sometimes called "true" vertebrae in contra-distinction from those of the sacrum and coccyx which are called "false" vertebrae. Although the vertebrae are separate bones, they are nevertheless so firmly fastened together by ligaments and fibrous cartilages as to make the backbone a fairly rigid column. Four curvatures appear in the adult — cervical, thoracic, lumbar, and sacral. The Structure of a Vertebra. A typical vertebra consists of a cylin- drical body, the centrum, which is flattened on its superior (cranial) and inferior (caudal) surfaces. A neural arch arises from the dorsal side of the centrum and surrounds a vertebral canal. That part of the neural arch which connects with the centrum is the pedicle. A spinous process extends backwards and downwards from the mid-dorsal side of the neural arch. That part of the neural arch between the spinous process and the pedicle is the lamina. Anterior and posterior notches or incisures con- strict the pedicles so that the incisures of two successive vertebrae form 15° CHORDATE ANATOMY EPISTROPHEUS CERVICAL ,' CURVATURE^ }'CERV1CAL the foramina for the spinal nerves which pass out between the vertebrae. Articular processes or zygapophyses project forwards and backwards from the neural arches. A postzygapophysis of one vertebra overlaps a prezygapophysis of the next vertebra and the two are bound together by ligaments: thus the backbone is strengthened, but at the same time made less flexible. On each side a transverse process projects from the neural arch laterally into the muscles of the body wall. The Kinds of Vertebrae. There are five kinds of vertebrae, cervical, thoracic, lumbar, sacral, and caudal or coccygeal. A distinguishing fea- ture of cervical vertebrae is a trans- verse foramen which in the upper six vertebrae transmits the vertebral artery. The lateral border of this foramen is formed by the fusion of a rudimentary rib with the vertebra. The first two cervical vertebrae are the atlas and the axis or epistro- pheus. A pecuUarity of the two is that the centrum of the atlas fuses with that of the axis to form the odontoid process upon which the atlas rotates. The forms and arrangement of the cervical verte- brae permit greater freedom of move- ment than is possible in other parts of the column. The spinous process of each cervical vertebra except the last is forked or bifid. Only the twelve thoracic verte- brae carry ribs. A pit in the cen- trum articulates with the head of the rib and a similar pit at the extremity of the transverse process articulates with the tubercle of the rib. The head of most ribs articulates with two adjacent centra. The five lumbar vertebrae are the largest. Short ribs fuse with them to form conspicuous transverse processes. The neural arches of these vertebrae have mammillary and accessory processes in addition to articular. Coccygeal. Fig. 140. — The human vertebral col- umn viewed from the left side. (Redrawn after Sobotta.) THE SKELETAL SYSTEM 151 The sacrum is a spade-shaped bone formed by the fusion of five vertebrae. Its lateral wings are modified ribs fused together and articulated with the hip bones. Spinous processes are much reduced. Between the costal processes four pairs of sacral foramina provide exit for nerves and blood-vessels. The sacral canal is the continuation of the vertebral canal. The coccyx consists of four fused centra which lack neural arches and processes. Frequently the first of these vertebrae fuses with the sacrum, and only the last three form the coccyx. Successive vertebrae are connected to form a continuous column by intervertebral discs of fibrous cartilage. Interconnexions are further strengthened by numerous vertebral ligaments. — EPIDERMIS I— SCLEROTOME NOTOCHORD -MYOTOME DERMATOME Fig. 141. — A horizontal section of an elasmobranch embryo, showing the differentia- tion of the mesoderm (epimere) into sclerotome, myotome and dermatome. The sclerotome surrounding the notochord gives rise to the centrum of the vertebra. Development of the Vertebral Column. In man as in other verte- brates the primary axial skeleton is the notochord. Around this the definitive axial skeleton is built; and the notochord disappears, slight traces only being left as nuclei pulposi of the intervertebral cartilages. The processes involved in this replacement are complicated, beginning with the appearance of mesenchyma cells around the notochord and the neural tube. In this mesenchymal matrix, cartilage develops only to be destroyed in its turn and replaced by bony vertebrae. The mesenchyma from which the vertebrae arise is produced by proliferation of the sclerotome, the cells of which migrate into the space between the mesoderm and the notochord. Later, by a continuation of the same migration, the neural tube becomes completely surrounded by mesenchyma. Before cartilage is secreted in the mesenchyma, the sclerotome median to each myotome becomes differentiated into a denser posterior portion 15^ CHORDATE ANATOMY and a less dense anterior half. As the definitive vertebrae are formed, the posterior half of each vertebral anlage fuses with the anterior half of the following one. By this process the definitive vertebrae come to lie intersegmentally, alternating with the myotomes. The result is obvi- ously adaptive, since only by this arrangement could each myotome become connected with two vertebrae and with two successive ribs. The Ribs. Man has twelve pairs of ribs, which form a basket sur- rounding the thoracic cavity. Each rib is a curved flat bone ending ventrally in a costal cartilage. By means of these costal cartilages the first seven pairs connect directly with the sternum and are therefore EPIDERMIS DERMATOME MYOTOME -SCLEROTOME NOTOCHORD -SCLEROTOME °o"0 MYOTOME DERMATOME .Ssils^^^^^s^j^SsSCsSs^i^^SjS^is^^s^^s^tfR^^J^EPIDERMIS Fig. 142. — A diagram showing the relations of myotome and sclerotome as seen in a horizontal section of a vertebrate embryo. The upper half of the figure shows the relations in an earlier .stage of development, while the lower half represents a later stage. The posterior half of each sclerotome unites with the anterior half of the following sclerotome to form a centrum which thus alternates in position with the adjacent myotome. Thus each myotome becomes attached to two vertebrae. called sternal or true ribs while the five remaining pairs are distinguished as "false" ribs. The last two pairs, the eleventh and twelfth, do not connect with others, and are known as floating ribs. Each rib has a head or capitulum, which articulates with the vertebral centrum, and a tuber- culum, which articulates with the transverse process. As the rib basket rises and falls in breathing, each rib rotates on an a.xis running through the tuberculum and the capitulum. Each rib has a costal groove extending along its lower or posterior border. To the ridges which border this groove are attached the external and internal intercostal muscles. The Development of Ribs. Ribs develop in the embryo as costal processes of the vertebrae in the intermuscular septa or myocommata. Primarily, the cartilaginous anlagen of the ribs are continuous with the THE SKELETAL SYSTEM 153 cartilaginous vertebrae. The short costal processes in the cervical, lumbar, and sacral regions unite with the transverse processes and are indistinguishable from them in the adult. In the thoracic region, separate centers of ossification in the ribs are formed and articulations with the vertebrae develop. Epiphyses at the capitulum and tuberculum make possible the elongation of thoracic ribs. The ventral extremities of the ribs do not ossify but remain throughout Kfe as the costal cartilages. The Evolution of Ribs. Ribs are wanting in chordates below the elasmobranchs, and even in elasmo- branchs they occur only in the anterior trunk region as short cartilaginous processes lying in the horizontal septum separating epaxial and hypaxial muscles. Such true ribs should not be con- fused with the hemal arches of fishes which are median to the lateral trunk muscles and adjacent to the peritoneal lining of the body cavity. See Fig. 143. The ribs of modern Amphibia show little advance above those of the elasmobranchs, and in many Anura continue as short cartilagin- ous processes of the vertebrae. But bony ribs are present in urodeles such as Necturus and the attachment to the vertebrae is, as in the higher vertebrates, by means of tubercular and capitular proc- esses. In some fossil Amphibia the ribs were elongated and extended around the body to the ventral side. Abdominal ribs were also present, as in some modern reptiles. In reptiles, ribs increase in number, and in some forms encircle the abdominal cavity. Abdominal ribs are common. The ribs of snakes are especially numerous. In mammals and man, ribs which articulate with the vertebrae and extend around the body-cavity are limited to the thoracic region. The Sternum. The sternum is a flat, dagger-shaped bone lying mid- ventrally of the chest. Three parts are distinguished, i. The manubrium or presternum, triangular, the widest portion and the most anterior. It articulates with the clavicle. 2. The gladiolus or mesosternum, the longest portion. Fig. 143. — Diagrammatic section of a vertebrate to show the relation of ribs to the muscles of the body wall, av, aorta; c, coelom; e, ectoderm", ep, epaxial muscles; g, gonads; -ha, hemal rib; hp, hypaxial muscles; i, intestine; mes, mesentery; n, nephridium; o, omentum; r, true rib; p, somatopleure; sp, splanchno- pleure; v, vertebra. (From Kingsley's ' ' Comparative Anatomy of Vertebrates. ' ' ) 154 CHORDATE ANATOMY formed by the fusion of four sternal elements or sternebrae. 3. The posterior metasternum, xiphoid or ensiform process. The xiphoid PHALANGES . -METACARPALS. CARPALS . CERVICAL VERTEBRAE ACROMION, PROCESS PHALANGES Fig. 144. — Human skeleton viewed from behind. (Reproduced in modified form from "The Human Body" by Dr. Logan Clendening, (Copyright ig27, 1030 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) process is sometimes perforated by a foramen and is sometimes forked posteriorly. THE SKELETAL SYSTEM 155 Development of the Sternum. The sternum arises from connective tissue which is afterwards chondrified to become a pair of cartilaginous PHALANGES.-j^ METACARPALS, - MANDIBLE. /CLAVICLE. ACROMION PROCESS. CORACOID. MANUBRIUM. STERNUM. XIPHOID PROCESS. -HUMERUS. RADIUS. Stt-carpals. PHALANGES. Fig. 145. — Human skeleton viewed from in front. (Reproduced in modified form from "The Human Body" by Dr. Logan Clendening, (Copyright 1927, 1930 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) bars, which secondarily unite in the mid-ventral line and only later connect with the costal cartilages. Ossilication begins in a series of paired 156 CHORDATE ANATOMY centers, but the manubrium usually has one center only. Ossification of the metasternum or xiphoid process remains incomplete until very late in life. Evolution of the Sternum. Opinion is divided as to the beginnings of the sternum. Some morphologists take the median portion of the elasmobranch pectoral girdle to be the homologue of the mammalian presternum, notwithstanding the fact that in some urodeles the sternum is a midventral plate of cartilage quite unconnected with the pectoral girdle. Since, however, the median ventral portion of the elasmobranch pectoral girdle is limited to a single intersegment, while the sternum of higher vertebrates extends through several segments and in mammals is I A Fig. 146. — Scheme of development of mammalian sternum. A, early stage; B, cartilage, the halves uniting; c, coracoid(?) procartilage; cl, clavicle; co, centers of ossification; w, mesosternal parts; mn, manubrium; p, presternum; st, sternebrae; x, xiphisternum. (From Kingsley's "Comparative Anatomy of Vertebrates.") clearly metameric, this hypothesis leaves the metamerism of the sternum unexplained. To meet this difficulty, it would be necessary to assume an antero-posterior extension of the sternum along the mid-ventral line, and a secondary segmentation. But the fact that in urodeles, where the sternum makes its first appearance in the vertebrate series, the sternum is independent of the pectoral girdle, and the additional fact that the sternum develops in ontogenesis independently of the pectoral girdle, make it difficult to accept this hypothesis. See Fig. 147. A second and more plausible hypothesis assumes that the sternum arose by the fusion of the ventral ends of a series of ribs. In favor of this opinion it is pointed out that in such a primitive amphibian as Necturus the sternum is represented by a series of four or five pairs of cartilages near the mid-ventral line. Like ribs these cartilages are intermyotomic. While in Necturus ribs do not extend from the vertebrae to the ventral side of the body, it is believed that there were primitive amphibians THE SKELETAL SYSTEM 1 57 in which the ribs were so extensive. The hypothesis that the sternum is a rib-sternum has at least so much foundation. The facts of mammalian ontogenesis, however, do not appear to support this view. As stated above, the mammalian sternum arises independently of the ribs by the union of a pair of longitudinal cartilages which arise near the mid-ventral line. The connexion of these cartilages with the ribs is secondary. If an opinion were to be based upon the relation of the sternum in Necturus and of the ontogenesis of the mammalian sternum alone, we NTERCUkVICLE XIPHOID PROCESS Fig. 147. — Types of vertebrate sterna. A, Squalus; B, Salamandra; C, Necturus; D, Rana; E, Felis; F, Crocodilus; G, Homo. The sternum is shown in black. While there is no doubt of the homology of the various amniote sterna, their homology with those of anamnia is in dispute. should have to conclude that the sternum arose from paired segmented cartilages formed near the mid-ventral line independently both of the girdle and of the ribs. Under the circumstances, and until more decisive evidence is discovered, suspension of judgment is necessary. In the reptiles the sternum is converted into a metameric structure composed of a series of sternebrae and connected with the ribs as in mammals. The mammahan sternum differs little from that of reptiles. It is divided into the same three elements as those of reptiles and man, pro- meso-, and meta-sternum. 158 CHORDATE ANATOMY The Skull. There are two chief parts to the skull, which have different origins and a different history. One of these is the cranium or brain-case, together with the bones of the face except the two jaws. The other is the visceral skeleton, that is to say, the two jaws, the hyoid bone, the ear bones, and the cartilages of the larynx. 3ASISPHENOI0 ORBITOSPHENOID \ ALISPHE>JOID PRESPHENOID \ \ \ PARIETAL FRONTAL VOMER PREFRONTAL \ \ J^>Cpp@>W NASAL ^ ^ \ ^< 3 < a t3 Quadrate Pterygoid*t Palatine Quadrato-jugal Jugal Maxillary Premaxillary X X X ? X X X X X X X X X X X X X X X OT 0-3. t: rt c 5 SftOJ OJ (OX- Incus X X Malar X X INCUS Pterygoid processes on sphe- noid X MALAR 1 Fused to form ( MAXILLA < < W M 0! >— 1 is ►J Articular Dentary Splenial Coronoid Supra-angular A na,ular X X X X X X X X X X X X X X X X X Malleus X ? ? Tympanic MALLEUS X Adult lower jaw is a single dermal bone, the — MANDIBLE Tympanic fused with temporal < w > 9^ Hyomandibular Symplectic Interhyal Epihyal X X X X Columella Columella ossify in_ doi embryonic ' Stapes and stapes sal part of lyoid arch. STAPES Intermediate part of hyoid arch becomes STYLOID PROCESS of temporal bone, and STYLO-HYOID LIGAMENT HYOID BONE m. v. Ceratohyal Hypohyal Basihyal m, v X X X A variable ventral hyoic number of elements BRANCHIAL ARCHES Maximum number in adult 7 421 Additional cartilages of I and more pc larynx and ] I sterior arches are represented in perhaps of trachea. OPERCULUM OF FISHES Operculum Preoperculum I nleroperculum Suboperculum X X X X Probably nc jt represente d in verteVjrates above fishes Cartilage bones are in bold-face type; dermal bones in italics. d, dorsal; v, ventral; 1, bones which are lateral and paired; m, a median bone. All bones not otherwise designated are lateral and paired. * Cartilage bone more or less augmented by addition of dermal bone. t A name which (with appropriate prefixes) applies to two or more bones which are closely related in origin and position: e.g., mesethmoid, ectethmoid; entopterygoid, metapterygoid, etc. X Homology of alisphenoid and vomer of mammals with alisphenoid and vomer, respectively, of lower vertebrates is questionable. (Modified from Rand, Comparative Anatomy of Vertebrates, Harvard University Press, 1929.) 1 68 CHORDATE ANATOMY 1400. The discovery of the cranium of the Java man with a brain capacity of between 800 and 900 cc. helps to reduce this contrast. The striking fact revealed by fossil human skulls is that the characteristics which distinguish them from the skulls of modern man tend to bridge over the gap between man and apes. In other words, all fossil skulls, except that of the Cro-Magnons which is like that of modern man, are more ape-like than those of modern races. The dental arch of Negroid races is intermediate between that of apes and Europeans. The chin which is such a striking feature of the modern human jaw is lacking in some fossil men as in the great apes. See Fig. 158. Another contrast between the skull of man and apes is in the relation of the skull to the backbone. The skull of modern man is poised on the occipital condyles at about its center of gravity, but the condyles of apes lie far behind the center of gravity of the head. Therefore are the neck muscles of modern man relatively weak, those of the ape massive. It is an interesting fact from the evolutionary point of view that the skull of Neanderthal man shows an intermediate condition. The Human Skull. The human skull consists of twenty bones of which eight form the cranium or brain-case and the remaining twelve make the facial skeleton. The eight bones of the cranium are the frontal, occipital, ethmoid, sphenoid and the paired parietals and temporals. The facial skeleton includes the mandible and vomer, and the paired maxillaries, zygomatics (malars), nasals, lacrimals, and palatines. Since the turbinal bones of the nose are extensions of the ethmoid they are not counted as separate bones. A comparison of the mammal skull with that of man reveals that the bones are homologous. Development of the Cranium. The history of the vertebrate skull revealed by the study of its comparative anatomy is amply supported by its embryology. Both prove it to be a complex formed by the union of diverse elements, capsules that contain the sense organs, supports for the gills, and underpinning and protective covering for the brain, while in the occipital region vertebrae appear to have fused with the brain case. The primordial cartilaginous cranium of the human embryo arises as a pair of parachordal cartilages and a pair of prechordal cartilages or trabeculae. These fuse together; later they combine with two pairs of sensory capsules, the olfactory and the auditory. This formation of the cartilaginous basis of the cranium, the chondrocranium, takes place during the second month of intra-uterine Ufe. Ossification from separate centers begins with the third. See Fig. 149. Evidence from comparative anatomy proves that the bones of the human skull correspond to a much larger number of separate bones which appear in the fishes and have been progressively reduced by fusion with THE SKELETAL SYSTEM 169 one another during the course of evolution. This also is borne out by the development of the human cranium. Ossification of the occipital bone, for example, begins in four centers corresponding with the basioccipital, the paired exoccipitals with their condyles, and a supraoccipital of lower vertebrates. To these are later added a membranous interparietal. Ossification of the occipital begins in the third month but is not completed until the seventh year. NASAL LACRIMAL MAXILLA ZYGOMATIC VECKEL'S CART MANDIBLE NASAL CONCHA (fR; >J^s3jr- OCCIPITAL - -A^-i^jf TEMPORAL MASTOID PROCESS 'AUDITORY MEATUS ZYGOMATIC ARCH Fig. 159. — The human skull, embryonic (A) and adult (B-D). In the fetal skull (14 weeks) membrane bones are black, and the cartilage cranium (chondrocranium) stippled. Figure B shows the adult skull in basal aspect, figure C in frontal aspect, and figure D in left lateral aspect, approximately one quarter natural size. (Redrawn after Sobotta.) The development of the sphenoid bone is even more complex; no fewer than ten centers of ossification are recognized. Six of these arise in the body of the bone and four more in the two paired wings. Membrane bone is added both to the pterygoid processes and to the great wings. Fusion of the separate elements is completed before the second year. Ossification of the ethmoid remains throughout life incomplete. Three centers of ossification corresponding to the pro-, epi-, and opisthotic 170 CHORDATE ANATOMY bones of lower vertebrates develop in the otic capsule and help to form the petrosal and mastoid portions of the temporal bone. The styloid process of the temporal is an ossified portion of the hyoid cartilage which fuses with the temporal. The squamous portion of the temporal is mem- branous in origin. An outgrowth of the epitheUum of the middle ear penetrates the mastoid process to form a cavity or antrum. The rest of the cranium is membrane bone. Because the roofing bones of the brain case ossify slowly and expand from centers, uncovered regions or fontanelles persist for some months after birth as "soft-spots" between the frontal, parietal, and occipital bones. lo Idm ^° ^9 INCUSCQUADRATEJ— MALLEUS CARTICULAR) MECKEL'S CARTILAGE CMANDIBULAR)^ HYOID DENTARY- STYLOHYOID LIGAMENT CHYOID) - MANDIBULAR A ELASMOBRANCH HYOMANDIBULAR (COLUMELLA) QUADRATE ARTICULAR DENTARX. MANDIBULAR - C AMPHIBIAN AND REPTILil. D. MAMMAL. Fig. 164. — Diagrams of the first and second visceral arches in A, Elasmobranch; B, Teleost; C, Amphibian and Reptile; and D, Mammal, illustrating the transformation of the hinge of the jaw of lower vertebrates into the malleus and incus of the mammal. The third earbone, the stapes, comes from the hyomandibular. (Redrawn after Gegenbaur and Stempell.) The three ear bones are named malleus, incus, and stapes from their fancied resemblance to hammer, anvil, and stirrup. Within the cavity of the middle ear they extend in the order given from the ear drum or tympanum to the oval window or fenestra vestibuli of the internal ear. Thus they serve to carry vibrations from the ear drum to the liquids of the internal ear. See Fig. 369. 176 CHORDATE ANATOMY Development of the Visceral Skeleton. In the human embryo a series of visceral arches separated by pharyngeal pouches appear in relations corresponding to those of aquatic vertebrates. In the first of those arches the maxilla of the upper jaw and the mandible of the lower jaw develop Fig. 165. — Early chondrocranium of Squalus. (The brain in outline.) als, alisphenoid cartilage; ch, anterior end of notochord; h, hyoid arch; ?na, mandibular arch, not yet divided into pterygoquadrate and Meckelian; oc, otic capsule; t, trabecula; 1-5, branchial arches; cartilages dotted. (From Kingsley's " Comparative Anatomy of Vertebrates," after Sewertzoff.) as membrane bones. The mandible, however, surrounds a cartilage, Meckel's cartilage, which corresponds to the mandibular cartilage of the lower jaw of elasmobranchs. While most of Meckel's cartilage disappears during ontogenesis, that portion which extends into the cavity of the middle ear ossifies in two centers, one of which forms the malleus and the other Fig. 166. — Diagram of early elasmobranch skull, bp, basal plate; c, trabecular cornu;^, foramen lacerum; ga^-\ gill arches; gc, gill cleft; /;, hyale; lim, hyomandibular; ii, i^, upper labials; II, lower labials; m, Meckel's cartilage; nc. nasal capsule; oc, otic capsule; of, orbital foramen; ov, occipital vertebrae; pq, pterygoquadrate; s, suspensor ligament; sp. spiracle; si, sphenolateral; t, trabecula; v, vertebrae; I-VII, visceral arches; i-io, cranial nerves. (From Kingsley's "Comparative Anatomy of 'Vertebrates.") the incus. The so-called Meckel's cartilage of the mammalian embryo appears therefore to correspond not only to the mandibular or Meckel's cartilage of lower vertebrates, but with their cjuadrate element also. The quadrate element develops into the incus while the articular portion of the mandibular ossifies as the malleus. THE SKELETAL SYSTEM 177 The cartilages of ihe remaining visceral arches in the human embryo have a diversified fate. The dorsal part of the second, the hyoid, ossifies to form the stapes of the middle ear, while the ventral portion forms the lesser horn and a part of the body of the hyoid bone. The intermediate portion of the hyoid cartilage forms the stylohyoid ligament by which the hyoid bone is suspended from the petrosal bone of the cranium. The cartilage of the third visceral arch fuses with that of the second, and later ossifies to form the greater horn and part of the body of the hyoid bone. The cartilages of the fourth and fifth arch persist as the thyroid and aryte- noid cartilages of the larynx, and form also the cuneiform and corniculate cartilages. The cartilage of the fifth arch is said to form the cricoid cartilage also. Other observers claim that the cartilage of the sixth arch contributes to the formation of the cricoid. See Fig. 163. II. THE APPENDICULAR SKELETON Evolution of Paired Appendages. The cyclostomes have no paired appendages and, so far as the evidence goes, never have had them. We are, consequently, forced to conclude that the paired appendages of vertebrates have no genetic connexion with those of invertebrates, but have arisen independently as vertebrate novelties. Unfortunately for the speculative morphologist, the beginnings of these appendages are obscure. Those of elasmobranchs are the simplest in living vertebrates, but even these are highly differentiated. The most promising attempt to solve the problem of the origin of paired fins is the so-called fin-fold theory. See Fig. 167. According to this, paired fins began as paired folds of skin extending from the region posterior to the gills back to the anus. The paired metapleural folds of Amphioxus are often mentioned, with dubious justification, as structures which suggest how the fin-folds may have had their origin. Pectoral and pelvic fins are supposed to be formed by enlarging the end portions of these folds and suppressing the intermediate region. In favor of this hypothesis is the presence of longitudinal paired "Wolffian" folds in vertebrate embryos, and the fact that the anlagen of the appendages extend through more segments of the embryonic body than do the appendages of the adult, the bases of the appendages becoming constricted dming ontogenesis. Some morphologists have used the continuous fin-folds of skates as evi- dence supporting the fin-fold theory, while others doubt their significance. The second step in this evolution was the invasion of connective tissue and muscle into the fin-folds. A similar migration actually occurs in ontogenesis. In elasmobranchs the muscle buds which invade the fin- folds are metamerically arranged. The third step in this evolution was the appearance of a series of inter- myotomic cartilage rays like those which support both median and paired lyS CHORDATE ANATOMY fins of elasmobranchs. The universal occurrence of skeletal material in connexion with muscle, and indeed wherever in an organism stresses occur, may possibly be taken as explaining these radial cartilages. The position between the myotomes is obviously adaptive, as is also their position between the dorsal extensor muscles of the appendage and the ventral flexor muscles. m'////my//////////y^^ MEDIAN FIN-FOLD PECTORAL FIN ^^,.T PELVIC FIN ANUS PRIMITIVE RADIALS PELVIC GIRDLf Fig. 167. — Diagram illustrating the hypothetical evolution of the paired fins and their skeletal supports. A represents the primitive stage of continuous fin-folds. The dorsal fin and the ventral fin posterior to the anus are median and unpaired. B is the definitive elasmobranch stage. The paired fin-folds persist only in the region of pectoral and pelvic fins. The median fins also become discontinuous. C-E illustrate hypo- thetical stages in the evolution of the skeleton of the pelvic fins of elasmobranchs. The right side of C and E represents a later stage in phylogenesis than the left. In E the skeletons of the girdle and extremity are differentiated. (After Wiedersheim.) Further steps in the evolution of the appendicular skeleton involve the thickening and fusion of the basal or proximal portions of the radial cartilages and the extension of the basal cartilages thus formed into the body-wall and towards the mid-ventral hne. The result of this appears today in the pelvic fin of elasmobranchs. The beginnings of a girdle are seen in a ventral cartilaginous plate, the ischio-pubis. A doubtful begin- ning of the ilium may be seen in the so-called iliac process. Evolution in thp: skeletal system 179 the pectoral girdle seems to have been more rapid than in the pelvic girdle, if we may base our conclusion on the fact that in elasmobranchs the scapula or dorsal arm of the pectoral girdle is already well developed when there is little, if any, indication of a dorsal arm, the ilium, of the pelvic girdle. In both girdles in the elasmobranch, however, a ball-and-socket articulation between girdle and free extremity has already made its appearance. An advance towards the pectoral girdle of higher vertebrates appears in living and fossil ganoids in which a membrane bone, the clavicle, is added to the pectoral girdle. There is no structure in the pelvic girdle homologous with the clavicle. PREAXIAL BORDER TO RIGHT ABCD HYPOTHETICAL STAGES Fig. 168. — Diagrams illustrating the hypothetical evolution of the extremities of dipnoan (/), ganoid (H), and elasmobranch (G) from a fin-fold supported by a series of similar radial cartilages. By fusion basal elements are differentiated. The skeletal supports of fins eventually differ in the relations of the basal elements to the radialia. (Redrawn after A. Brazier Howell.) A tripartite pectoral girdle makes its first appearance in amphibians. The ventral arm, which in fishes was single and undivided, becomes in amphibians differentiated into posterior and anterior moieties, the coracoid and precoracoid. The dermal clavicle becomes closely apposed to the precoracoid . The dorsal scapula and suprascapula remain undivided as in fishes. The dorsal arm of the pelvic girdle, the ilium, articulates with the transverse process of a single sacral vertebra. In its most primitive form in amphibians, the ventral portion of the pelvic girdle resembles that of ganoid fishes, and consists of a broad cartilaginous plate with which the femur articulates. See Fig. 169. Centers of ossification corresponding with the ischium and pubis of reptiles arise successively in this plate. i8o CHORDATE ANATOMY The girdles of reptiles are essentially like those of amphibians. In the turtle they become definitely Y-shaped. The clavicle fuses with the pre- coracoid and becomes indistinguishable from it. The ilium connects with JO-PUBO-lSOflAWC CARTCLAGE- E. DACTYLETHRA a NECTURUS. Fig. 169. — A series of six appendicular skeletons illustrating the gradual emergence of the elements of the pelvic girdle found in reptiles and mammals. They probably represent fairly well stages in the evolution of the human pelvis. First came the separa- tion of girdle and extremity (A and B) ; then the fusion of the paired elements of the girdle into a median ventral cartilaginous plate (C and D) ; the differentiation of bony ischium, pubis, and ischium (D and £); and finally the appearance of the obturator foramen (F). There is no essential difference between the reptile and mammal girdle. (Redrawn after Wilder, "History of the Human Body"; Henry Holt & Co.) SUPRASCAPULA| SCAPULA' -OMOSTERNUM CLAVICLE^ ACROMION- EPIPHYSIS OF CLAVICLEi INTER- \ .clavicular " 'ligament HUMERUS ■'^PRECORACOID 'HUMERUS -i--rnDarr,in COSTOCOR ACO I D/ y-CORACOID LIGAMENT ^ --EPICORACOID CARTILAGES'^ LIGAMENT -STERNUM 'sternum NSCAPULA /OSSA SUPRASTERNALIA B Fig. 170. — Diagrams illustrating the fundamental similarity of the human (B) and amphibian (A) pectoral girdle. In man the coracoid element has degenerated into a process (coracoid) and a connective-tissue ligament containing occasional cartilage nodules. (Redrawn after Huntington.) two sacral vertebrae. In pythons, a rudimentary hip girdle connects with a pair of rudimentary claws in the anal region. Both are useless; both go to prove the descent of serpents from tetrapod ancestors. THE SKELETAL SYSTEM i8i In mammals, the coracoid is reduced to a process fused to the scapula. In man, in addition to the coracoid process, a remnant of the coracoid bone survives in the coracoid ligament which extends from the coracoid process to the sternum, and in which occasional pieces of cartilage are found as rudiments of the coracoid. The clavicle has supplanted the precoracoid, remnants of which, however, usually occur within the clavicle. See Fig. 170. The mammalian hip bone differs Httle from that of reptiles. The number of sacral vertebrae to which the coxal bone is attached increases in mammals. In man there are five sacral vertebrae, to three of which the hip bone is attached. Evolution of the Free Extremities. Two contrasting types of free extremity appear in vertebrates, the fins characteristic of fishes and the VLRTEBRAL MARGIt 5-STERNAL EXTREMITY Fig. 171. — Human pelvic and pectoral girdles in lateral aspect. A is the pelvic girdle of the right side and B-C the pectoral girdle of the same side. toed appendages such as are found in the remaining classes from amphibians to man. The conversion of the one into the other continues to be a vexed cjuestion of vertebrate morphology. Technically stated, the problem has been to determine how the evolution of the ichthyopterygium into the cheiropterygium has occurred. Interest has centered especially in the transformation of the skeleton. Primarily the fish fin, like that of the fossil shark Cladoselache, was supported by radial cartilages which articulated with basalia, of which one or more articulated with the girdle. In the pectoral fin of modern elasmobranchs three basalia, propterygium, mesopterygium, and meta- pterygium (Fig. 172^), connect the fin with the girdle. Morphologists, however, disagree as to the skeleton of the primitive extremity, the archipterygium. While some suppose it to have been uniserial, i.e., the radial cartilages were limited to one side of the basal cartilages or axis as l82 CHORDATE ANATOMY in elasmobranchs, other morphologists regard the biserial fin skeleton of Dipnoi as the more primitive. Conclusions in regard to the evolution of the skeleton of the extremity differ, therefore, as one or other of these two types of fish-fin skeleton is assumed as more primitive. Summary of Skeletal Evolution. Most animal phyla, even the Protozoa, have some sort of skeletal structures. But there seems to be no genetic connexion between the skeletons of invertebrates and those of vertebrates. In the evolution of a skeleton, vertebrates have been given RADIAUA A. ICHTHYOPTERYGIUM PHALANGES l\ n METTACARPALS , / I ICMETATARSALS5 "^ Ji RADIUS ^ffl!»->. MS>> |C TIBIA 5Gt^ is a nine-week human embryo in left lateral view. hand. In the lower hmb they are hip, thigh, leg or shank, and foot. A comparison of the human skeleton (Figs. 144 and 145) with that of the gorilla (Fig. 175) shows that, bone for bone, the two correspond. The dif- THE SKELETAL SYSTEM 187 ferences are those of pro{)ortion only. The facts are in harmony with the assumption that the two have evolved from a common ancestry. Homologies of the Limb Bones. The striking similarity of the bones of the upper and lower limbs, notwithstanding their great diversity of function, is interpreted by morphologists as indicating a primary similarity in use. Their present differences in form, size, and function have arisen secondarily and adaptively. Development of the Appendicular Skeleton. The paired appendages of vertebrates arise from two Wolffian folds, which extend along the sides of the embryo at approximately the level where the hypomere NCURAL PROC£33. 1 1 MM. EMBRYO Fig. 177. — Stages in the development of the appendicular skeleton of man. A, left lateral aspect of arm in 11 mm. embryo; B, left lateral aspect of arm in 16 mm. embryo; C, left lateral aspect of arm in 20 mm. embryo; D, left lateral aspect of leg in 11 mm. embryo; E, left lateral aspect of leg in 14 mm. embryo; F, left lateral aspect of leg in 20 mm. embryo. (Redrawn after Bardeen and W. H. Lewis from Keibel and Mall.) connects wath the mesomere. Only the end portions of these folds, how- ever, go to form the Umbs; the intermediate region atrophies and disap- pears. The e\ddence accords with expectation from the standpoint of the fin-fold theory of the origin of the extremities. See p. 177. The Wolffian folds consist of an external covering of ectoderm and a core of mesenchyma, which in the human embryo is of uncertain origin. In their early development, both arms and legs take the form of shovel- shaped outgrowths (Fig. 176), which gradually elongate. The cartilagi- nous anlagen of the bones arise in the mesenchyma (Fig. 176.4) and are slowly converted into bone through complex processes partly suggested in CHORDATE ANATOMY Fig. 178. In the human embryo fingers and toes make their appearance at the ends of the extremities as early as the second month. (Fig. 176) Four stages in the development of a long bone are shown in Fig. 179. The connective-tissue membrane or perichondrium which surrounds the cartilage anlage of the bone secretes a cylinder of bone around the shaft of CARTILAGE S<>> CELLS SUCCESSIVE STAGES BLOOD. CELLS ' vM'^^^^iT^''^^ ■'^ l"^? DEGENERATION OSTEOBLAST OSTEOCLASTS BONE Fig. 178. — Endochondral bone formation at the end of a long bone. Destruction of cartilage is followed by the secretion of lime in the form of thin lamellae. Osteoblasts then lay down bone upon these lamellae. In this way cancellous bone replaces cartilage. (Redrawn after Dahlgren and Kepner.) the anlage. Thus the perichondrium is converted into a periosteum which persists throughout the life of the bone. Other perichondria! cells penetrate the cartilage and destroy it. Eventually a fatty marrow takes the place of the cartilage within the bony cylinder. The cylinder becomes the diaphysis of the adult bone. Since the diaphysis is formed THE SKELETAL SYSTEM 189 outside and around ihe original cartilage this mode of bone formation is known as perichondral bone formation. The diaphysis of the infant is GROWTH DISC DIAPHYSIS—^ PERICHONDRIAL BONE EPIPHYSIS-J GROWTH DISC EPIPHYSIS DIAPHYSIS - MARROW a_ PERICHONDRIAL BONE i ENDOCHONDRAL BONE GROWTH DISC Fig. 179. — Diagrams illustrating four stages in the development of a long bone. Perichondral bone cross-hatched; endochondral bone stippled; maiTow black; cartilage unshaded. (Redrawn from Coming's "Human Embryology," after Duval.) --'BLOOD VESSEL >COMPACT BONE »MARROW CAVITY ARTICULAR LIGAMENT Pig. 180. — Diagram of the structure of a long bone. (Redrawn after Fritz Kahn, "Der Mensch," Albert Muller, Zurich.) converted into that of the adult by the continuous addition of new bone on the outside of the shaft. At the same time the marrow cavity of the shaft is enlarged through the destruction of bone on the inside of the shaft. In igO CHORDATE ANATOMY this way the marrow cavity of an adult bone becomes large enough to contain the entire bone of the infant. The long bone grows in length at the two ends. Through endo- chondral bone formation centers of ossification are formed within the cartilage at the ends of the shaft. The bony centers are known as epiphy- ses. During growth new bone is added between epiphysis and diaphysis in a cartilaginous ''growth-disc." When adult size is attained the growth- disc is converted into bone, the epiphyses fuse with the diaphysis, and growth ceases. The regulation of size is influenced through the action of endocrinal secretions of the pituitary and thyroid glands. CHAPTER 7 THE MUSCULAR SYSTEM The muscular system of an active vertebrate makes up nearly half the entire body-weight, in man slightly more than forty per cent. A muscle can do only one thing — contract. It cannot expand and, having once contracted, must be pulled out to its resting length by one or more antagonistic muscles. Each skeletal muscle consists of a fleshy part or belly, each end of which is attached to a bone or cartilage, either directly to the periosteum or indirectly by means of a tendon. The attachment TENDON- BELLY- -- 'ARTERY VEIN NERVE TENDON Fig. i8i. — A diagram of the biceps muscle taken as a typical muscle, showing its nervous and vascular relations. Each skeletal muscle is attached to a bone either directly to the periosteum or indirectly — as in the case of the biceps — by means of tendons. (Redrawn after Keith.) which moves most when the muscle contracts is its insertion; the other is its origin. Each muscle is surrounded by a connective-tissue membrane or perimysium, from which septa may grow into the muscle and divide it into several muscle shps, each of which has a separate function. Muscles vary greatly in shape according to the arrangement of their fibers and the way these are attached. Muscles may be segmented into a series of similar units such as appear in the body muscles of fishes. igi 192 CHORDATE ANATOMY Fig. 182. — Diagrammatic outlines to illustrate various types of muscle architecture and the relations of the main nerve branches to the fiber-bundles of the muscle, a. Two segments of the rectus abdominis muscle of a small mammal, b. Portion of sheet-like muscle with two nerve-branches and intramuscular nerve plexus, c. Typical quadri- lateral muscle with nerve passing across the muscle about midway between the tendons. d and e. Two triangular muscles with different types of innervation. /, Long ribbon-like muscle with interdigitating fiber-bundles, g, Unipenniform muscle, h, Bipenniform muscle, i. Typical fusiform muscle. The main intramuscular nerve branches are distributed to the fiber-bundles about midway between their origins and insertions. (From Morris' "Human Anatomy.") THE MUSCULAR SYSTEM IQ3 They may spread out in thin sheets that are ribbon-Hke, triangular, pinnate, or fan-like. Appendicular muscles are more frequently spindle- shaped and massive. Each muscle is well supplied with capillaries and with both motor and sensory nerves. As to origin, muscles are sharply divided into two kinds: skeletal (epimeric or myotomic), derived from the dorsal or epimeric portion of the mesoderm; and visceral (hypomeric) derived from the hypomere. In the trunk region, in contrast with the head, visceral muscles arise from the splanchnic layer of mesoderm only. Smooth visceral muscle fibers are found not only in the wall of the intestines, but also in the walls of blood-vessels, in the lungs, the bladder, the genital organs, and the skin. Skeletal muscles are composed of striped fibers whose response to stimulation is a rapid contraction. Most visceral muscles, on the other hand, consist of slow-acting smooth or non-striped fibers. The former are voluntary (in man, "under control of the will"), the latter are usually involuntary. Exceptions are found in the heart muscle which is visceral and involuntary, but formed of striped fibers, and in the chewing and facial muscles which are visceral and at the same time striped and voluntary. EVOLUTION OF THE MUSCULAR SYSTEM The muscles of man and other mammals are the last term in the series of transformations of the mechanism of contraction, the evolution of which it is now possible to sketch in fairly firm outlines. Contractility appears to be one of the original properties of living cells. Touch an amoeba and it responds by drawing together into a sphere. There is no single axis, but contraction takes place from all directions towards a center. In some Protozoa, however, progressive advance in the function appears in the differentiation of contractile fibrils. A cluster of such fibrils in the stalk of Vorticella is so arranged as to contract in one direction only, like a muscle fiber. True muscle cells first appear in the animal series in the sponges. The primary independence of muscle and nerve is indicated by the presence of muscle cells in this group which lacks nerves altogether. The epithelio-muscular cells of coelenterates are essentially similar to those of sponges. The next step in the evolution of muscles appears in the flatworms, in which muscle cells are aggregated into clusters. The bilateral sym- metry characteristic of the muscles of higher animals also appears in this group. Transitional evolutionary stages between flatworms and chordates are, it must be admitted, highly speculative. Even if we accept the assump- tion that annelids resemble the ancestors of vertebrates, there still remains 194 CHORDATE ANATOMY a wide gulf to be filled between flatworms and annelids. In certain characteristics, the muscles of annelids, it is true, strikingly resemble those of vertebrates. Among these are the segmentation of the muscles, and their separation by a body-cavity into somatic and visceral divisions. It is impossible, however, to be sure that these similarities are not cases of convergence. The eyes of cuttle-fish and of man are similar in many respects, but this does not prove a genetic connexion. While the pre-chordate history of muscles is obscure, the evolutionary changes of muscles in chordates are fairly clear. Since the lower chor- / MOUTH A AMPHIOXUS. METAPLEURAL FOLD V LATERAL TRUNK CSOMATIC) MUSCLES GONADS QLL APERTURES A B. PETROMYZON. HYPOBRANCHIAL MUSCLE LATERAL TRUNK MUSCLES ANUS XJRSAL CONSTRICTORS LEVATOR WAXILLAE f^ _^ SPIRACLE. I ^P^""^ pAXIAL TRUNK MUSCLES HYPAXIAL MUSCLES CLOACAL-ANAL ORIFICE Fig. 183. — The lateral trunk muscles of a cephalochordate, a cyclostome, and an elasmobranch, showing their striking metamerism, and fundamental similarity. A, Amphioxus; B, Petromyzon; C, Squalus. dates, the Hemichorda and Urochorda, are non-metameric, we must assume that the metamerism of Amphioxus and vertebrates is a new acquisition in the group. The trunk muscles of Amphioxus form an unbroken series of segments extending throughout the entire length of the animal. Each muscle segment or myotome is a mass of muscle tissues which extends around the side of the body nearly to the mid-dorsal and mid-ventral line. Each myotome terminates anteriorly and poste- riorly in connective-tissue septa, the myocommata, which separate suc- cessive myotomes. A sharp bend near the middle of each myotome gives it in side view the shape of a letter V. All alike are innervated by somatic motor nerves. THE MUSCULAR SYSTEM 195 The viscera] muscles in the wall of the intestine are non-metameric, and are differentiated into an inner circular and an outer longitudinal MYOTOME I MYOTOME 10 ANTERIOR/. CAVITIES ENDOSTYLE' A.AMPHIOXUS EMBRYO. isT perm.gill-slit FACIALIS GANGLION LENS OTIC CAPSULE MYOTOME 4- myotome 10 SPIRACULAR POUCH 1ST GILL-SUT HYPOBRANCHIAL B. CYCLOSTOME EMBRYO. ^^^^""^ OTIC CAPSULE LATERAL TRUNK MUSCLE MYOTOME 4 IST GILL -SLIT HYPOBRANCHIAL MUSCLE C. ADULT CYCLOSTOME. Fig. 184. — Diagrams illustrating the origin of the hypobranchial muscles of verte- brates. Lacking in cephalochordates (Amphioxus), hypobranchial muscles make their first appearance in cyclostomes in the form of muscle buds from post-branchial myo- tomes. They become the tongue muscles of tetrapods and are innervated by the hypoglossal (XII) nerve. In cyclostomes as in higher vertebrates myotomes i, 2, and 3 form eye muscles. layer. In the region of the gills, the visceral muscles are connected with the gill cartilages, and are dififerentiated into levators, depressors, and constrictors of the gills. 196 CHORDATE ANATOMY The lateral trunk muscles of cyclostomes strikingly resemble those of Amphioxus. In the region of the body-cavity, on the ventral side, an external layer of oblique muscles is differentiated. The most important evolutionary advance, however, appears in the differentiation of six eye muscles. Paired eyes first appear in this group, and with them six eye muscles like those found in all vertebrates up to man. All six are formed from the first three embryonic myotomes. Like the eye muscles of higher vertebrates, they are innervated by the 3rd, 4th, and 6th cranial nerves. Since in cyclostomes the fourth myotome of the embryo forms the first permanent trunk myotome, all the myotomes of the embryo persist in the adult. Of none of the higher vertebrates is this true. (Figs. 184, 185, 186) Hypobranchial muscles, lacking in Amphioxus, first appear in cyclo- stomes. They arise from postbranchial myotomes which send myotomic SOP. RECTUS^ 0CUU3M0T0RN. N. TBOCHLCARIS^ EXT RECTUS (CUT) M. 0BLI0UU5 SUR. Cxr. RECTUS — NF OBLfQUE INF RECTUS Fig. 185. — Diagrams of the eye muscles of man. A shows the left eye-ball and associated muscles viewed from the outer side. B is the left eye-ball with associated muscles and nerves viewed from the median side. (Redrawn after Warren and Car- michael. Courtesy of Houghton Mifflin & Co.) buds ventrally and anteriorly below the gills as far forward as the mouth. The development and nerve relations of this hypobranchial musculature prove that it is the homologue of the tongue and throat muscles which, in higher vertebrates, are innervated by the twelfth nerve, the hypoglossal. Cyclostomes, however, have no true tongue. The hypobranchial muscles function as a part of the lateral trunk muscles. (Fig. 184, C) The embryos of elasmobranchs provide a clue to the history of the eye muscles, by demonstrating that the differentiation of the three anterior myotomes into the six eye muscles involves primarily a longitudinal splitting of the myotomes into dorsal and ventral moieties such as happens also in the first and second post-otic myotomes of cyclostomes. The facts suggest that the spUtting occurred along the series of lateral-line sense organs, which at one time may have included the lens of the eye and the ear vesicle. Each of the two divisions of the first myotome splits again lengthwise, thus making the four eye muscles innervated by the oculo- motor nerve. The dorsal of the two moieties of the second myotome forms the superior oblique muscle innervated by the trochlear nerve, THE MUSCULAR SYSTEM 197 FACIALIS GANGLION 0^,^ CAPSULE MYOTOME 3 MYOTOME 2 MYOTOME I. MYOTOME MYOTOME 2 A. PETROMYZON. HYPOBRANCHIAL MUSCLE 1ST PERM. GILL-SLIT FACIALIS GANGLION MYOTOME 3V MYOTOME 2 MYOTOME OTIC CAPSULE ST PERM. META-OTIC MYOTOME THYROID ANTERIOR CAV. HYPOBRANCHIAL MUSCLE MYOTOME B. SQUALUS. SUP. RECTUS SUP. OBLIQUE INT. RECTUS EXT. RECTUS 1ST META-OTIC MYOTOME OLF. PIT INF. OBLIQUE INF. RECTUS C. ADULT SQUALUS. HYPOBRANCHIAL MUSCLE Fig. 186. — Diagrams based upon cyclostome and elasniobranch embryos illustrating the phylogenesis of the six eye muscles. The eye muscles develop from the first three embryonic myotomes. Myotomes are cross-hatched. Those which degenerate in ontogenesis are cross-hatched with broken lines. In C the eye muscles are shown as if viewed from the median side of the eye. 198 CHORDATE ANATOMY while the ventral portion unites with the third myotome to form the external rectus muscle innervated by the abducent nerve. The dorsal division of the third somite breaks up into loose mesenchyme to form connective tissue. The myotomes of the fourth, fifth, and sixth somites also break up into connective tissue, so that the first persistent trunk myotome is the seventh. In this way, a hiatus occurs in the series of myotomes, and the eye muscles are left as an isolated group which owe their persistence to the fact that they become functionally connected with the eyeball. (Fig. 186) If we may draw phylogenetic conclusions from these facts of onto- genesis, we must consider the eye muscles not as relatively young muscles LATERAL UNE ADDUCTOR OF MANDIBLE' Fig. 187. — The superficial muscles in the shoulder region of Squalus. From such relatively simple beginnings have evolved the complex muscles of the arm and shoulder of man. The flexor protractor muscle which corresponds to the deltoid muscle in mammals is covered in the figure by the posterior gill constrictor. (Redrawn after A. Brazier Howell.) or as post-otic muscles which have migrated into the pre-otic region, but as the first three myotomes of the vertebrate body. Their present isola- tion may be interpreted as a consequence of the enlargement of the otic capsules. The ontogenesis of cyclostomes and elasmobranchs supports the assumption that in the ancestors of vertebrates, as in Amphioxus today, the myotomes formed an unbroken series extending throughout the entire length of the body. The history of the eye muscles sums up as the transformation of the first three myotomes of an Amphioxus-like ancestor into the six eye muscles of the vertebrates. In elasmobranchs the metamerism of the body muscles, which is such a characteristic feature of the musculature of cyclostomes, is retained with slight modification. A more elaborate folding of the myo- tomes of elasmobranchs, however, greatly compUcates their form. The cause of this folding is unknown. The total amount of muscle remains the same; and, although the myocommata are folded along with the THE MUSCULAR SYSTEM 199 i|i^ ^^^--^ FiM-rniX"^ MYOTOMIC buds' \ ^'^-"-^ h, FIN-FOLD "•" "Sypogldssus muscle buds B. \ COELOM,/;- Fig. 188. — Diagram of bvidding of hypoglossal and pectoral fin muscles from trunk myotomes in an elasmobranch embryo. A. Lateral view after Braus. 2-6, visceral arches. B. Cross section in region of pectoral fin-fold. LEVATORES ARCUORUM C(-7) VISCERAL SKELETAL ARCHES Cl-7) \ DEPRESSORES ARCUORUM Cl-7^ LEVATORES [-4 DIGASTRICUS MASSETER TEMPORALIS DORSO-LARYNGIS AND DORSO- TRACHEALIS B. INTERMANDIBULARES HYO-PHARYNGEI Fig. 189. — Diagrams illustrating the hypothetical evolution of the branchiomeric muscles. A. Hypothetical ancestral form. B. Branchiomeric muscles in urodele amphibian. (Redrawn after Wilder's "History of the Human Body," Henry Holt & Co.) 200 CHORDATE ANATOMY myotome so that the surface of attachment of the muscles is increased, it has not been proved that this increase is adaptive. See Fig. i86, C. A novelty first appearing in this group is the division of the lateral trunk myotomes by a horizontal connective-tissue septum into epaxial D NECTURUS. OCPffESSOft MANOIBUl>e SPHINCTER COLLI TRAPEZIUS DOnSAUS SCAPULAE EPAXIN. MUSCLES HYRUUAL UUSXCS Fig. 190. — Superficial lateral trunk muscles in an amphibian, a reptile, and a mam- mal. D, Necturus. E, Sphenodon. F, Felis. The metamerism of the lateral trunk muscles which is such a striking feature of the lower vertebrates is retained in urodeles and reptiles, but disappears in mammals. The factors in this change are chiefly the increasing dominance of the appendicular muscles and the fusion of the primarily metameric embryonic trunk muscles. The primitive metamerism, however, appears in mammalian embryos and hypaxial groups of muscles. Five post-branchial myotomes send buds anteriorly into the floor of the pharynx to form the hypobranchial musculature innervated by the hypoglossal nerve. THE MUSCULAR SYSTEM 20I ,. The most important advance, however, made by the elasmobranchs is the first appearance in vertebrates of the muscles of pectoral and pelvic fins. As the myotomes extend ventrally in the body- wall, hollow epithe- lial buds branch off laterally into the fin anlagen. See Fig. i88. The appendicular muscles are thus seen to be derivatives of lateral trunk muscles. Differentiation of the muscles thus formed takes place in two directions in elasmobranchs and higher animals. First, the appendicular muscles are subdivided into intrinsic muscles which lie within the fin and extrinsic muscles which are connected with the fin but He within the body-wall. Both groups are subdivided into levators and depressors. On the anterior side of the fin, a muscle is formed which pulls the fin forward M. TRANSVERSOSPI NALIS, TRA^4SVERSE PFOCESS- --M. LONGISSIMUS DORSl. M. RHOMBOIDEUS. /M. SERRATUS POSTERIOR. SCAPULA. M. L4TISSIMUS DORSl GLENOID CAVITY. ^^ 'ACROMION. M EXTERNAL'yO^ ^S^f ''''^'?''' ~"^^i^( ^^"^ " '| ' ' " ^,e^^J<^-^^ tORACDID. M. RECTUS ABDOMINIS^ ^ / ^ M BRACHIALIS MANUBRIUM STEBNl! ^~~^^ ' '^^ PECTDRALIS MAJOR. INFERIOR. CL^VVICLE' ^M. TRAhJSVERSUS THORACIS. Fig. 191. — Thoracic and lumbar muscles of man as seen in cross section. Thoracic muscles on the right, lumbar on the left. The muscle arrangement is fundamentally like that of any mammal. (Redrawn after Braus.) towards the head. No special antagonistic muscle is differentiated in elasmobranchs, the adduction of the fin being effected by the combined action of the posterior part of the levator and depressor groups acting together. The extension of the extrinsic muscles of the fins in fan-like form over the lateral trunk muscles tends to obscure the metamerism of these in the region of the appendages. The trapezius muscle, which extends from the scapula anteriorly above the gills, makes its first appear- ance in this group. In the head region, the visceral muscles become specialized in relation to the jaws. The levators of the first two visceral arches form the jaw muscles, including the masseter, temporalis, and pterygoids, while the depres- sors of these arches become the intermandibularis muscles. The muscles of the remaining visceral arches remain relatively unmodified. (Fig. 189) In the urodeles, the metamerism of the lateral trunk musculature persists as a striking characteristic. The extrinsic muscles of the append- 202 CHORDATE ANATOMY ages, however, become widely extended anterior and posterior to the legs. Such definitive muscles as the pectoralis and the latissimus dorsi now appear, and the intrinsic muscles subdivide into those of the arm and thigh, the forearm and shank, and the feet. By further spHtting of the original muscle mass within the limb, many new muscles arise, some of which may be homologized with those of man. On the sides of the body, the lateral trunk muscles become delaminated into layers, some amphibians -M. PIRIFORMIS 'M. ABDUCTOR CAODAE VENT M. EXTENSOR CAUOAE LATERALIS M ABDUCTOR CAUDAE DORSAL IS DORSAL Fig. 192. — Human caudal muscles viewed from A. ventral and B. dorsal side. These rudimentary muscles are the last remnants of the powerful caudal muscles of the lower vertebrates. The presence of such useless rudiments receives its best interpreta- tion in the evolution theory. (Redrawn from Wilder's " History of the Human Body," Henry Holt & Co.; after Lartschneider.) having as many as four. The epaxial muscles of the trunk divide into longitudinal bundles connected with the head. A further novelty in amphibians is a movable tongue. Its intrinsic muscles are those which, as we have seen, grow from occipital myotomes into the floor of the throat and are innervated by the hypoglossal nerve. In this group also we find differentiated sternohyoid and geniohyoid muscles, which connect sternum and lower jaw respectively with the hyoid. No very striking developments affect the muscles of reptiles. The three sets of epaxial muscles of the trunk, — trans verso-spinalis, lumbo- costalis, and ilio-costalis, appear. The fusion of the lateral trunk THE MUSCULAR SYSTEM 203 myotomes and the consequent loss of metamerism leads towards the con- ditions in mammals. An extreme degree of delamination affects the lateral trunk muscles, some reptiles having as many as eight layers in the body-wall. FRONTAL- M. ORBICULARIS OCULI NASAL- ZYGOMATIC AUR I CULO- LABIAL SUR AURICULO-LABIAL INF. TRIANGULAR A. ATELES. SUR AURICUI-AR OCCIPITAL POST. AURICULAR 1-. / M. ANTITRAGICUS ANT. AURICULAR M. PLATYSMA- ANT. AURICULAR M. ORBICULARIS OCULI M. OUADRATUS LAB 1 1 SUR CANINE- M. ORBICULARIS ORIS M. RISORIUS M. QUADRATUS LABI! INF M. MENTAL IS SUR AURICULAR POST. AURICULAR TRIANGULAR' B. HOMO. Fig. 193. — Mimetic muscles in monkey (Ateles) and man. A, Ateles (redrawn from Wilder after Ruge) and B, Homo. The similarity of these muscles both in function and relations attests their similar genetic derivation. With the great enlargement of the appendages of mammals, there appears a corresponding increase in the appendicular musculature and trunk muscles become relatively reduced. Subdivision and migration of muscles increase. Caudal muscles dwindle with the reduction of the tail. In the trunk region, metamerism is preserved only in the intercostals, the rectus abdominis, and the intervertebral muscles. 204 CHORDATE ANATOMY Integumental (dermal or cutaneous) muscles in the form of a panni- culus carnosus group appear suddenly in monotremes and marsupials only to disappear in the higher primates except as rudiments. In the head and neck region, however, the platysma and facial muscles persist in man and apes. In the trunk region, these integumental muscles are outgrowths of the pectoralis minor complex. In the head region, however, they are visceral in origin. The most important muscular novelty contributed by mammals is the diaphragm. Its innervation by branches of cervical spinal nerves proves that it is a derivative of cervical myotomes. Muscles in Man There is no essential difference between the muscles of man and those of other mammals. The presence in man of such useless muscle rudi- ments as the sacro-coccygeal and ear muscles suggests a mammalian derivation. The evolutionary process of subdivision, fusion, migration, and sphtting of muscles reaches its cUmax in primates, forearm and hand being especially noteworthy. The human body has nearly four hundred paired or bilaterally sym- metrical muscles, of which forty-seven pairs are visceral and the rest skeletal. In addition to these, four unpaired muscles are recognized. Each part of the body — head, neck, back, abdomen, thorax, diaphragm, shoulder and chest, upper arm, forearm and hand, hip, thigh, lower leg and foot, pelvis — has its intrinsic set of muscles. Space does not permit the description of all these muscles. There is no question that in fundamental pattern the muscles of vertebrates and of man are alike. (See Fig. 190.) Comparison of the superficial muscles of man (Figs. 194 and 195) with those of the cat (Fig. 190) reveals a surprising degree of resemblance. On account of their exact homology many of these muscles in the two forms are given identical names. The same is true of many of the deeper muscles. When the muscles of man are compared with those of another primate, the similarity is much greater. There is no reason to doubt that the similarity of the mimetic muscles in man and monkey (Fig. 193) has genetic signifi- cance. Few, if any, muscles in man are without homologues among primates. The evolution theory in its appHcation to the human body derives much support from the comparative anatomy of the muscles. The presence in man of useless muscle rudiments such as those of the coccyx and ear mentioned above (page 204) receives its only adequate interpretation in this theory. Pointing in the same direction is the existence in man of inconstant and variable muscles, the homologues of which are functional in THE MUSCULAR SYSTEM 205 lower animals. The pyramidalis abdominis muscle is an example. When present in man the pyramidalis arises from the pubic bone anterior (ven- trad) to the rectus abdominis muscle. Its length varies greatly in individu- als. It may occur on one or both sides or may be wanting. In non-placental mammals the pyramidalis is powerfully developed in con- nexion with the marsupial bones which it serves to support. Even in insectivores in which the marsupium has disappeared the pyramidalis muscle is well developed. The presence in man of such a useless rudiment suggests the animal origin of the human body. Rudimentary integumentary muscles occasionally appear in individu- als. Among these are the stemalis muscle of the chest and the axillary muscle connected with the pectoralis in the axillary region. They are normally present in apes, but occur in human individuals only excep- tionally. They are interpreted as remnants of the panniculus camosus of lower mammals. Of similar significance is the fact that, although metamerism is evident in few adult mammalian muscles (intercostals, intervertebrals and the rectus abdominis), nevertheless all the skeletal muscles arise from the metameric somites of the embryo. Why are embryonic myotomes meta- meric when the muscles which develop from them are not metameric? The primary metamerism corresponds with the muscular metamerism of the skeletal muscles of the lower vertebrates. Does this fact not give the clue to the muscular metamerism of the human embryo? Developm.ent of the Muscles. Classified on the basis of their onto- genetic development, muscles are of two kinds: (i) Somatic Muscles, derived from the mesodermal epimere, and (2) Visceral Muscles, which develop from the hypomere. With the exception of the smooth muscles of the eyeball, which are of ectodermal origin, all muscles are mesodermal. We may then describe first the development of the Somatic Muscles, derived from the Epimere or Somite. Very early in ontogenesis the mesoderm becomes divided into a metameric series of "somites." In all chordates above the cephalo- chordates (Amphioxus) the metamerism affects only the dorsal portion of the mesoderm, that is, the portion known as the epimere. In the trunk region — but not in the head — of vertebrate embryos the adjacent mesomere becomes segmented as the nephrotome. The epimere becomes later differentiated into (i) myotome which forms muscle, (2) sclerotome which forms skeletal material, and (3) dermatome which gives rise to loose connective tissue. In the embryos of the lower vertebrates, as shown in Fig. 197, the somites extend in an unbroken series throughout head and trunk. In embryos of the higher vertebrates, however, the metamerism of the mesoderm in the head region disappears. Only in the embryos of Amphioxus and cyclo- 2o6 CHORDATE ANATOMY stomes (Petromyzon) do all the somites produce myotomes. In the higher forms the series is broken in the ear region. FLEX. CARPI ULNARIS FLEX. CARPI RAD. LOXB.s BRACHIORADIAL.' Da CARPI RAD LONGUS- PALMARIS LONGUS. BRACHIALIS TRICEPS •■'- CORACO BRACHIALIS-' TERES MAJOR' LATISSIMUS DORS I SERRATUS ANTERIOR LINE A ALBA RECTUS ABDOMINIS EXTERNAL OBLIQUE ~ ILIUM TENSOR FASCIA LATA~ ILIOPSOAS RECTI NEUS ■ PUBIS '■ RECTUS FEMORIS- SARTORIUS ~Mljl ADDUCTOR LONGUS - -"" "' ADDUCTOR MAGNUS - VASTUS LATERALIS PERONEUS LONGUS GASTROCNEMIUS^-^- PERONEUS BREVIS- EXT. HALLUCIS LONG.- TRANS. CRURAL LIGAMENT. FRONTAL . ORBICULARIS OCULI. -AURICULAR .' ^ZYGOMATIC. -MASSETER. OMOHYOID. STERNOCLEIDOMASTOID. SCAUENES h^c-z /^^-3^-T RA PEZ I US . ::^^^=\ -CLAVICLE ^■""■"^PECTORALIS MAJOR. ' ■■ -DELTOID- TRICEPS . ••^/BICEPS . ' it-BRACHIAL- EXT CARPI RADIALIS. BRACHIORADIAL. SUPER. FLEXORS. FLEXOR CARPI ULNARIS. -PATELLAR LIGAMENT. J r(,jj,i,t> INSERTION OF SARTORIUS. ■'JJ- TIBIALIS ANTERIOR. _ 'ML., 40-TiBIA wt-^-EXT OIGITORUM COM. LONG. -FLEX. DIGITORUM COM. LONG. CRUCIATE LIGAMENT. Fig. 194. — Superficial nniscles of man; front view. (Reproduced in modified form from "The Human Body" by Dr. Logan Clcndening (Copyright 1927, 1930 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) The serial homology of the head cavities or somites with trunk somites, which for many years was a controverted problem, has now been demon- THE MUSCULAR SYSTEM 207 strated by the fact that, in the embryos of lower vertebrates, the head somites form a series of mesodermal segments continuous wilh the trunk TEMPORAL -f- - OCCIPITAL^t^"-*' STERNOCLEIDOMASTOID -'*•- TRAPEZIUS^:;^ SPINE OF SCAPULA >^^-\ \ DELTOID -i^-.> ~0, , EXT.POLLICIS BREVIS. ABD. POLLICIS LONGUS. GASTROCNEMIUS CRUCIATE LIGAMENT. ACHILLES TENDONt Fig. 195. — Superficial muscles of human body ; back view. (Reproduced in modified form from "The Human Body" by Dr. Logan Clendening (Copyright 1927, 1930 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) somites. Like the latter, they become differentiated into myotome and sclerotome, are innervated by somatic motor nerves, and are dorsal 208 CHORDATE ANATOMY to notochord and dorsal aorta. Furthermore, their segmentation is independent of that of the visceral arches. Another point of resemblance is that the first and second head cavities divide during ontogenesis into dorsal and ventral moieties precisely as do the first and second post-otic myotomes in Petromyzon. The fusion of portions of two myotomes, the second and the third, to form the external rectus muscle of the eye resem- bles the fusion of trunk myotomes such as occurs in the formation of the tongue muscles. THE MUSCULAR SYSTEM I5T SOMITE ANTERIOR CAVITY ENDOSTYLE CLUB-SHAPED GLAND A. AMPHIOXUS EMBRYO OMfTE 10 1ST PERM. GILL-SLIT 209 FACIALIS GANGLION OTIC CAPSULE 1ST SOMITE LENS, SOMITE 10 THYROID ST PERM.GILL-SUT B. PETROMYZON VI FACIALIS GANGLION 1ST SOMITE SOMITE 10 ANTERIOR CAVITY HEART C.SQUALUS Pig. 197. — Diagrams of the mesodermal (somatic) segmentation in the head region of embryos of lower chordates as viewed from the left side. In embryos of these lower vertebrates, just as in the adult Amphioxus, the somites (myotomes) extend in unbroken succession throughout head and trunk. Roman numerals number the brain " neur omeres . " 2IO CHORDATE ANATOMY Lateral Trunk Muscles. The lateral trunk muscles of man develop from myotomic segments which first appear in the fourth week of onto- genesis, and by the end of the second month have increased to nearly SCLEROTOME- HYPOCHORDA NEURAL CREST NEURAL TUBE ■ jil^EPIMERE /wiL 'SOMATIC MOTOR NERVE— r^^^\^~--— NOTOCHORD-''" MESOMERE-y^' ECTODERM j ^ — ENDODERM CRMATOME MYOTOME SCLEROTOME HYPOCHORDA HYPOMERE COELOM SUBINTESTINAL, BLOOD VESSEL A. HEAD B. TRUNK Fig. 198. — Diagrams of cross sections in A, head and B, trunk regions of an elasmo- branch embryo showing the fundamental similarity of the two regions. The discovery that the coelom of elasmobranch embryos extends throughout head and trunk and that in this respect the two regions are alike was made by the English embryologist, Francis Balfour. EVTREMITV SOMATIC MUSCLE INTESTINE ENDODERM COELOM NEURAL CREST SOMATIC MOTOR NERVE SPINAL CORD SPINAL GANGLION RAMUS DORSALIS NOTOCHORD CENTRUM NEPHROTOME CORIUM VISCERAL MUSCLE RAMUS VENTRALIS •r /" DERMATOME MYOTOME SCLEROTOME PARIETAL MESODERM ISCERAL MESODERM Fig. 199. — A stereogram of the trunk region of a vertebrate embryo, based upon elasmobranch embryos. The figure shows an earlier stage of development on the right side, a later stage on the left. The extension of the myotome to form the lateral trunk musculature is shown. The lateral trunk musculature of the ventral half of the body- wall thus arises as a secondary invasion. (Redrawn after Braus.) forty pairs. The original metamerism of the myotomes, which persists even in the adults of the lower vertebrates, becomes largely lost in adult man and mammals as the result of a number of processes among which THE MUSCULAR SYSTEM 211 fusion is the most important. As the myotomes grow in size and thick- ness through cell multiplication, the connective-tissue septa between them disappear. In this way are formed such elongated muscles as the spinalis and iliocostalis. Among the processes tending to obscure the original metamerism is the degeneration of myotomes into connective-tissue fasciae and aponeuroses, which may be very extensive. Migration of muscles may accompany their fusion. Among the other ontogenetic changes in trunk myotomes is tangential splitting of muscles into sheets. One of the most characteristic ontogenetic processes affecting the trunk muscles SOMITES lO-l-f 'MCXJTH I I MliOTOMIC BUDS YPOPHYSIS HYPOGLOSSUS' VISCERAL ARCH A CYCLOSTOKC NERVE '■" 'OLR«CTORY PIT VISCERAL ARCH 4 B ELASMOBRANCH C. REPTILE D. MAMMAL Fig. 200. — Diagrams illustrating the mode of origin of hypoglossal (hypobranchial) muscles in A. Cyclostome, £. Elasmobranch, C. Reptile, and/?. Mammal. \t\A,B, and C cervical myotomes send myotomic buds into the hypobranchial region. In mammals such buds are not formed but a migration of mesenchyme cells from cervical myotomes provides material for these muscles. The number of myotomes which participate is usually four or five. is the subdivision of a muscle mass into a number of bellies each of which acquires an independent origin or insertion, or both. The original seg- mentation of the trunk myotomes is, however, retained in such muscles as the transversospinalis, intercostalis, and rectus abdominis. By the growth of a horizontal connective-tissue septum which extends laterally from the transverse processes of the vertebrae, the lateral trunk muscles become divided into epaxial and hypaxial portions, of which the former are innervated by dorsal rami of the spinal nerves, the latter by ventral rami. The muscles of the diaphragm, which are peculiar to man and mammals, migrate into the chest from the neck, as is evidenced by the fact that they are innervated by branches of the third, fourth, and fifth cervical nerves. 212 CHORD ATE ANATOMY Tongue Muscles. The origin of the h\'poglossal muscles in the human embrvo is somewhat uncertain. Since, however, they have the same innervation as in lower vertebrates, it is generally assumed that their development is essentially similar. In all vertebrates below mammals, muscle buds grow from four or five occipital myotomes ventrally into the floor of the throat. From the mass of cells thus formed arise the intrinsic muscles of the tongue, innervated by the twelfth nerve, the h>-poglossus. In man and mammals e\adence is lacking of muscle buds in the formation of the hypoglossal muscles. It may be assumed that cell migration takes the place of bud formation and extension. Appendicular Muscles, In the embryos of lower vertebrates, elasmo- branchs to reptiles, as the myotomes grow ventrally in the body-wall and reach the level of the lateral folds from which the appendages develop, they give off lateral buds into the appendicular folds. After they have entered the folds, these buds lose their connexion \\dth the trunk muscles, although they still retain their epithelial character. Within the anlage of the appendage, the appendicular muscle buds subdi\dde into dorsal and ventral moieties, from which develop respectively the levator and depressor muscles of the appendage. The appendicular muscles of man and mammals, on the contrary, do not develop from myotomic buds, but arise by cell migration. The two methods are after all not radically different. In fishes, for example, where most of the appendicular muscles arise from myotomic buds, some muscles which develop later than the others come from migrant mesen- chymatous cells as they do in mammals. Similarity of innervation, however, attests the homology of the appendicular muscles throughout the vertebrate series. The fact that the arm muscles of man are innervated by the last four cer\'ical and the first thoracic nerves further justifies the assumption that they are derived from the myotomes of these segments. To the group of muscles derived from this source, are added others, such as the trapezius, sterno-cleido-mastoid, and levator scapulae. The pectoralis and latissi- mus dorsi muscles spread out from the arm. Most of the muscles of the shoulder, chest, and arm appear early in the second month, and are differentiated by the beginning of the third. From the connexion of the muscles of the lower leg with spinal nerves, including the last four lumbar and first three sacral, it may be assumed that their cellular anlagen are derived from the corresponding myotomes. In all essentials their development resembles that of the muscles of the arm. A common mass of cells within the Hmb-bud differentiates into dorsal and ventral muscle anlagen. The muscles from the ventral group become innervated by the femoral nerve while the dorsal group are con- nected with the obturator. The subdivision of the primary muscle THE MUSCULAR SYSTEM 213 mass into the separate muscles of the adult limb is mostly completed by the end of the second month. Visceral Muscles, Derived from the Hypomere. The visceral or hypomeric muscles include those of the heart and main blood vessels as well as those associated with the alimentary canal. While most of them consist of smooth muscle fibers, the visceral muscles of the head and heart are striped. The muscles of the wall of the alimentary canal are formed from mesenchymatous cells proliferated from the visceral layer of the hypomere. Such cells fill the space between the mucous epithehum lining the aU- miwmmniiimimmrj ffi •/MYOTOMES 1-4 1ST. CERVICAL MYOTOME ANL M TRAPEZIUS* "M. STERNOCLEIDO- MASTOID ^SPINAL /GANGLIA MANDIBULAR MUSCLES' ANLACE DIAPHRAGM 1ST. THORACIC MYOTOME' _ Fig. 201. — The anlagen of the cranial muscles with their nerve relations as seen in a 7 mm. human embryo. (Redrawn from Keibel and Mall, after W. H. Lewis.) mentary canal and the adjacent hypomere. They also differentiate into both the connective tissues and the blood-vessels of the wall of the ali- mentary canal and into its circular and longitudinal muscles. The circular layer of muscles is formed before the longitudinal layer. The fate of the hv-pomere in the head is much more complex than in the trunk. Besides forming the heart and pericardium, the head hypo- mere gives rise to the chewing muscles, the muscles of expression, and the pharyngeal and laryngeal muscles. In general, the processes involved are similar in lower and higher vertebrates. In embryos of lower vertebrates, e.g., elasmobranchs, the coelom extends throughout head and trunk. In the head region, as a result of the outpocketing of pharyngeal pouches, the h^-pomere becomes divided into a series of pouches each of which lies in a visceral arch. This hypo- meric segmentation (branchiomerism) is independent of the segmentation 214 CHORDATE ANATOMY of the epimere (mesomerism) , and should not be confused with this, although it is possible that the two types of segmentation may originally have coincided. From the mesoderm of the visceral arches arise the muscles, connective tissues, and blood-vessels of the arches. In the fishes, these muscles are differentiated into levators, depressors, and constrictors of the gills. In the process of conversion the epithelium of the hypomere breaks up into mesenchyme and the coelomic cavity disappears. In mammals and man, the coelom is absent in the visceral arches and the muscles are formed from masses of mesenchymatous cells. From the first visceral arch arise the muscles innervated by the mandibular branch of the fifth nerve, the masseter, temporalis, pterygoid, mylohyoid, and tensor veH palatini. (Fig. 201) From the same source come the tensor tympani of the ear and the anterior belly of the digastricus. The muscles innervated by the facial nerve are derived from the second visceral arch, the hyoid. They include the muscles of expression, the stylo-hyoid, stapedius, and the posterior belly of the digastricus. From the third visceral arch arise the stylopharyngeus muscle innervated by the glossopharyngeal nerve, and the constrictors of the pharynx innervated by the vagus nerve. The laryngeal muscles, innervated by the vago- accessory nerve, originate from the fourth and fifth visceral arches. As ah-eady explained, the muscles of the tongue and throat innervated by the hypoglossal nerve are myotomic, not visceral, in origin. CHAPTER 8 THE DIGESTIVE SYSTEM Life depends upon an unceasing intake and outgo of matter. Each living thing takes in food or the raw materials for food, assimilates this into its own peculiar sorts of protoplasm, and after forming these chemical substances, promptly burns them up into simpler chemical substances, which finally leave the body as the wastes and ashes of life. Upon this fundamental chemical process of metabolism, all other vital functions depend. The foundations of life are chemical. The products of plant metabolism, on their way back to the inorganic world, become, directly or indirectly, the food of animals. Thus all animals are parasites on the green plants. But their feeding habits are varied. Some marine organisms live on the mud as well as in it; earth- worms pass through their digestive tract enormous quantities of soil for the sake of the organic matter which they extract from it. But leeches live chiefly on blood. Oysters sweep bacteria into their mouths by ciliary action. Barnacles kick food into their mouths by means of their six pairs of legs. Some insect larvae feed on cellulose, some on fur and wool. Some whales eat minute swimming crustaceans, which they strain out by means of the whalebone. Others live chiefly on gigantic cuttle-fish. Some mammals are herbivorous; some are carnivorous; others, like man, are omnivorous. Man alone cooks his food. Digestion. The first chemical change which ingested foods undergo is a process by which insoluble substances are made soluble, so that they may be absorbed through the lining membranes of the small intestine. The agents in this chemical process are certain remarkable enzymes which, like other and inorganic catalyzers, are able to bring about chemical changes without appreciable effect on themselves. During digestion, these enzymes split up the huge molecules of colloids into simpler mole- cules, small enough to pass through animal membranes. Their composi- tion is unknown; but they are thought to be rather simple colloids derived from proteins. The specificity of their action is remarkable, each enzyme affecting only one food substance. All are secreted by glands connected with the alimentary canal. EVOLUTION OF THE DIGESTIVE SYSTEM The Protozoa have no digestive system. The single cefl merely engulfs the food particle, surrounds it with protoplasm, digests and assim- 215 2l6 CHORDATE ANATOMY ilates it, and extrudes what remains. The Porifera, though they have a cloacal cavity, do their feeding essentially like Protozoa, each cell for itself. The first real step in evolving a proper digestive system is taken by the coelenterates. These, as their name affirms, have a cavity or enteron which is the digestive tract. This has but one opening to the exterior, which serves both as mouth and anus. See Fig. 375. Fig. 202. — Diagram of a vertebrate, a, anus; h, brain; df, dorsal fin; h, heart; i, intestine; /, liver; m, mouth; n, nephridia; p, pancreas; pc, pericardium; pf, pectoral fin; 5, stomach; sc, spinal cord; sp, spleen; vf, ventral fin. (From Kingsley's "Com- parative Anatomy of Vertebrates.") Most flatworms, like coelenterates, have a single opening to the digestive cavity (enteron), and this opening serves as both mouth and anus. A few species of flatworms, however, possess an anus — some indeed have two ani — the invention of which therefore should be credited to flatworms. Threadworms, with few exceptions, have both mouth and Fig. 203. — Spiral valve of Raia. Cartilaginous fishes increase the absorbing surface of their intestine not by elongation, as is done by higher animals, but by a spiral fold in theintestine. (From Kingsley's " Comparative Anatomy of Vertebrates," after Mayer.) anus, and their alimentary canal is separated from the muscular body- wall by a space, a false body-cavity or pseudocoelom. The digestive tube in threadworms is purely epithelial and non-muscular. A muscular digestive tube, one of the important steps in animal evolu- tion, is contributed by the annelids. In these for the first time in the phylogenesis of animals an epithelium-lined coelom or ''body-cavity" THE DIGESTIVE SYSTEM 217 NASAL PHARYNX proper separates the alimentary canal from the body-wall. In annelids, as in all the higher animals, there is no connexion between the two cavities, enteron and coelom. The single tube that forms the body of lower forms has become double, and the muscular activities of the alimentary canal are carried on independently of those of the body-wall. Among the forms which lie near the main line of human ancestry, pharynx, esophagus, and stomach are first differentiated in urochor- dates. A liver arises in the cepha- lochordates. The cyclostomes contribute a pancreas and a bilobed liver. Elasmobranchs, utilizing dermal scales as teeth and transforming a visceral arch into a jaw, convert the sucking mouth into a biting one. To increase surface for ab- sorbing digested food they develop a spiral fold or "valve" in the intestine. (Fig. 203) They de- velop also a new cavity, the cloaca, to receive the wastes and secretions of the urogenital and digestive systems. The amphibians fasten their teeth in a groove in the jaw bone, VFJiMIFORM PROCESS Fig. 204. — Diagram of the alimentary canal. (From Morris' "Human Anatomy.") invent salivary glands, utilize hypobranchial muscles to make a mobile tongue, and differentiate small from large intestine. Mammals greatly elongate the intestine and, by suppressing the cloaca, separate the rectum from the urogenital sinus. The result is a muscular, epithelium-lined alimentary canal, differentiated into nearly a dozen different organs, and having about the same number of different glands associated with it. THE HUMAN DIGESTIVE SYSTEM Mouth The mouth cavity is divided into an anterior vestibule or labial cavity lying between the lips and the teeth, and a posterior mouth cavity proper or buccal cavity underlaid by the tongue and extending to the posterior margin of the soft palate. The roof of the mouth cavity proper is formed 2l8 CHORDATE ANATOMY by the hard and soft palates, which separate the mouth cavity from the nasal passage above. Development. At a relatively late state of ontogenesis, at the anterior end of the fore-gut where the mouth is to break through, the ectoderm invaginates to form the stomodeum. At the bottom of the stomodeum, ectoderm and endoderm are in contact as a two-layered membrane, which ruptures and disappears leaving no trace in the adult. The covering of the lips and gums is derived from the ectodermal stomodeum, while that of the rest of the mouth is endodermal. Evolution. There is no doubt that the mouths of all vertebrates are homologous, the sucking mouth of cyclostomes being no exception. Cyclo- epiphysis lateral telencephalic vesicle hyoid arch visceral arch III Fig. 205. — Drawing to show the external appearance of the structures in the oral region of a four-day chick. Ventral aspect. (From Patten's "Embryology of the Chick.") stome and gnathostome mouths have the same fundamental structure, development, and relations to other parts, and must therefore be considered homologous. Beard and Kupffer, however, are persuaded that vertebrates have had two mouths — an old paleostoma and new neostoma. The paleostoma, in their opinion, may be represented by the hypophysis, which in some cyclostomes, e.g. Bdellostoma, opens directly into the pharynx. (See Fig. 206, A) According to Kupffer, the hypophysis of vertebrates repre- sents a paleostoma which functioned as a mouth in prechordates, following their abandonment of the original blastoporic mouth. In support of this assumption, he points out that the definitive mouth of vertebrates arises late in ontogenesis in such relation to the series of gill-slits that it might have been formed from a pair of coalesced gill-sUts; that the presence of a THE DIGESTIVE SYSTEM 219 pre-oral gut in vertebrate embryos suggests that the alimentary canal formerly extended anterior to the present mouth; and, finally, that in the myxinoids and the embryonic sturgeon the hypophysis actually opens into the pharynx and, like the mouth of urochordate larvae, has a dorsal external opening. (Fig. 208) Whatever view is held of the origin of vertebrates, we must believe that there have been at least two mouths in the course of vertebrate phylogene- sis. The reason for this conclusion is that the original coelenterate mouth becomes the mouth in no vertebrate, while only in cyclostomes, dipnoans, and possibly some amphibians does it become the anus. The coelenterate HYPOPHYSIAL DUCT A.BDELLOSTOMA. PHARYNX/ ,1 ST GILL APERTURE HYPOBRANCHIAL MUSCLE HYPOPHYSIAL ^ DUCT ^=-^ (RESPIRATORY TUBE NOTOCHORD I ST GILL APERTURE B. PETROMYZON. HYPOBRANCHIAL MUSCLE Fig. 206. — Diagrams of median longitudinal sections of the heads of Bdellostoma and Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the former the hypophysial duct opens posteriorly into the pharynx, suggesting the possi- bility that it may once have served as a mouth (paleostoma). In Petromyzon the hypo- physis fails to open into the pharynx and is converted into a pipette-like organ into which the olfactory pits open. On the basis of this difference cyclostomes are divided into two sub-classes, Hyperotreta and Hyperoartia. mouth becomes the blastopore of chordate embryos. And the blastopore of chordates lies at the posterior end of the body and forms the neurenteric canal, which connects the neural tube with the enteron, while the chordate mouth develops at the anterior end of the enteron. Consequently, it seems indisputable that there have been at least two mouths in the history of vertebrates. While, however, all agree that the vertebrate mouth is not the primary animal mouth, and that at least two mouths have successively appeared, some rnorphologists believe that there have been at least three mouths, Delsman (1922), reviving an earlier suggestion of Kowalevsky (1877), claims that " in the ontogeny of vertebrates we see three successive mouths appear in the same succession as they appeared in phylogeny, viz., the blastopore (Urmund), the neuropore (the annelidan mouth), and finally 220 CHORDATE ANATOMY .BLASTOPORE NEUROPORE. ^""^<^^"' A. B. C. Fig. 207. — A diagram illustrating the way in which, according to Delsman, the blasto- poric mouth of coelenterates is in chordates converted into the neurenteric canal. Delsman homologizes the chordate neural tube with the ectodermal f oregut of annelids. NEURAL NOTOCHORD BLASTOPORE NEUROPORE TUBE | BLASTOPORE ENDODERM A.GASTRULA GILL POUCHES B.AMPHIOXUS EMBRYO ENTERON MOUTI NEUROPORE NOTOCHORD GILL POUCHES C.UROCHORDATE LARVA ENDODERM STRAND NEUROPORE OTIC CAPSULE notocho^Jd'^'^nte^ic^canal^^ POST-ANAL GUT ANUS GILL POUCHES DEFINITIVE MOUTH HYPOPHYSIS D. VERTEBRATE Fig. 208. — Diagrains illustrating the hypothetical phylogenesis of the vertebrate mouth. The primitive animal mouth, the blastopore, is converted in vertebrates either into an anus or a neurenteric canal. The definitive mouth of vertebrates therefore is a secondary mouth. But the relations of the neuropore are such that at one time in the ancestry of chordates this may have served as a mouth and the neural tube as a f oregut. It is also possible that the mouth of urochordates is not homologous with the definitive mouth of vertebrates. The evidence of a paleostoma or hypophysial opening suggests that this may once have been a functional mouth. Thus the definitive mouth may have been the last in a series of four mouths. THE DIGESTIVE SYSTEM 221 the definitive mouth." According to this view, the neural tube was for- merly a part of the digestive system, and its anterior embryonic external opening, the neuropore, once functioned as a mouth. For a part of the digestive system to become nervous in function is indeed a surprising change, which is no greater, however, than others which have occurred in phylogenesis. If we add to the three mouths mentioned by Delsman the hypophysial "paleostoma " mentioned by Beard and Kupffer, then there have been four mouths in the phylogenesis of vertebrates, the present mouth being the fourth and last. Diagrams showing the position of the four mouths mentioned are shown in Fig. 208. The objection to this idea that there have been a series of mouths in the course of animal phylogenesis, on the ground that the chances are against the appearance of more than one ingestive opening into the enteron, loses much of its weight in view of the fact that many openings into the aUmentary canal, such as the gill-slits, have made their appearance in the course of phylogenesis. The phylogenesis of the vertebrate mouth remains, therefore, an unsolved problem. That there have been at least two mouths in the course of animal evolution, all morphologists agree. These are the coelenterate mouth, w^hich is the blastopore, and the definitive vertebrate mouth. Evidence is, however, not wanting that the embryonic neuropore and the hypophysis may have served as mouths. But such assumptions are considered to have a relatively insecure foundation. The Salivary Glands in Man As food enters the mouth, it is moistened by the secretion of a number of salivary glands, in addition to which are lingual, labial, buccal, palatine and molar mucus-secreting glands. Besides moistening the food, the chief salivary glands contain serous cells which secrete the starch-splitting enzyme ptyalin and the sugar-spUtting enzyme maltase. The sublingual and submaxillary glands secrete mucus also. The largest of the salivary glands is the parotid, which lies below the ear. It is a serous tubulo-acinous gland, and empties by Stenon's duct into the vestibule of the mouth opposite the second upper molar tooth. The submaxillary is a mixed (mucous and serous) tubulo-acinous gland located in the floor of the mouth near the angle of the lower jaw. Its secretions are carried by Wharton's duct which opens near the frenulum at the front margin of the tongue. The sublingual is also a mixed tubulo-acinous gland lying below the tongue in the front of the mouth near the median line. Mucus and serous cells are about evenly distributed. The openings of the sublingual ducts lie in front of the tongue near those of Wharton's ducts. 222 CHORDATE ANATOMY Development. From their position and the relations of their ducts, it is generally assumed that the chief saHvary glands are of ectodermal origin. The numerous glands of the tongue, however, are formed by the local prolification of the stratum germinativum of the endodermal mucous lining of the mouth. History of Salivary Glands. Salivary glands are not unknown among the invertebrates. Multicellular mucus glands connected with the mouth are present in molluscs. Malaria is transmitted by the saliva of mosquitoes. It is doubtful, however, if the saUvary glands of inverte- brates have any genetic relation with those of vertebrates. Part of an excretory duct A crescent consisting of eight serous cells. Lumen Fig. 209. — Section of a human sublingual gland, X252. Histology.") Tangential section of serous cells. Mucous cells and thick membrana propria . Connective tissue. (From Bremer's "Text Book of Lower chordates have no salivary glands, and fishes only unicellular mucus glands. It has generally been assumed that the multicellular glands of the higher vertebrates have their beginnings in such unicellular glands. Multicellular oral glands appear in Amphibia. Besides the mucus- secreting cells of the tongue, most amphibians have an intermaxillary gland, the duct of which opens between the intermaxillary bones. In some amphibians, e.g., Rana, mucus glands are located also in the posterior nasal passages. That enzymes are secreted by the mucus cells of fishes and amphibians has, however, not been demonstrated. In the reptiles, there are serous cells in the oral glands, and lingual, sublingual, and palatine glands occur. Glands connected with the teeth are differentiated as the poison glands of some snakes. THE DIGESTIVE SYSTEM 223 True salivary glands secreting enzymes are limited to mammals. There seems no good reason to doubt, however, that the salivary glands of mammals are derived from the oral glands of reptiles. Labial and buccal glands become abundant in mammals, and possibly the parotid is an enlarged buccal gland. In addition to the Ungual and palatine glands, TUBULAR GLANO> TUBULAR GLAND iCOMPOUND TUBULAR GLAND Fig. 210. — Various types of digestive and endocrinal glands which develop from the endodermal (mucous) lining of the alimentary canal. The endocrine glands are duct- less. The digestive glands may be simple or compound, tubular or alveolar (acinous). (Redrawn after Braus.) the sublingual and submaxillary glands are present; and in general, the glands of man resemble those of other primates. The Tongue The tongue is a muscular organ of miscellaneous functions — digestive, sensory, conversational — lying in the floor of the mouth cavity and attached to the hyoid bone. It consists of an apex or body directed towards the teeth of the lower jaw, a root or muscular attachment, a dorsum divided by the sulcus terminalis into an anterior papillated por- tion and a posterior tonsillar and glandular portion, and an inferior surface below the apex. The sulcus terminahs is a V-shaped groove with the apex of the V pointing backwards and marking the position of the foramen coecum. See Fig. 211. The dorsum of the tongue anterior to the sulcus is covered with numerous papillae which give the tongue its characteristic rough appear- ance. Four kinds of papillae are distinguished, vallate, filiform, foliate, and fungiform. The vallate papillae are the largest, and are distinguished also by the deep depression or fossa which surrounds each of them. On their sides they bear numerous taste-buds. Their number varies from six to twelve, and they occur in a V-shaped row just in front of the sulcus terminahs. Of the various forms of papillae on the tongue the filiform papillae are the most numerous. Each filiform papilla is covered with filamentous processes. Foliate papillae are three to eight parallel folds on each side of the tongue. Like the vallate papillae, the fohate papillae have taste-buds. The fungiform papillae are scattered over the entire 224 CHORDATE ANATOMY dorsum of the tongue, and are distinguished by their reddish color and their globular mushroom shape. They also bear taste-buds. No papillae occur on the posterior and inferior surfaces of the tongue. (Fig. 211) Most of the mass of the tongue consists of striated muscle. In the connective-tissue corium of the tongue, both mucus and serous glands are abundant. The lingual tonsils lie on the posterior dorsum. Development of the Tongue. The apex and root of the tongue, which develop from separate anlagen, remain throughout life divided by the EPIGLOTTIS. 'v . A^ FORAMEN CAECUM \/ Fig. 211. — The dorsal surface of the tongue. The sulcus terminalis divides the body or apex of the tongue from the root. The two regions have a different embryonic origin. (Redrawn after Sobotta.) sulcus terminalis. The apex of the tongue is formed by the union of a median tuberculum impar with the basal portions of the two halves of the mandibular arch. (Fig. 212) The root of the tongue arises from por- tions of the second, third and fourth visceral arches. The tongue muscles, however, are not formed from those of the visceral arches, but from post- occipital myotomes which send buds downward and forwards into the tongue. History of the Tongue. None of the lower chordates has a tongue, so that the vertebrate tongue seems to be an emergent organ like the notochord. The so-called tongue of cyclostomes is a muscular piston THE DIGESTIVE SYSTEM 225 associated with the sucking mouth and cannot be compared with the tongue of higher vertebrates since the hypobranchial muscles which form the mass of tongue muscles in higher vertebrates, though present in cyclostomes, have no connexion with the so-called tongue. Gnathostome fishes have an immovable tongue, which .forms a swelling in the floor of the mouth and is supported by the basihyal or os ento- glossum. Although it lacks muscles, this fish tongue is generally regarded as homologous with the root of the tongue of tetrapods. The tongue of tetrapods, beginning with amphibians, consists of an apex and root as in man. While the root is derived from the tongue of BODY ''sft. TUBERCULUM TUBERCULUM' \ IV IMPAR •■ EPIGLOTTIS Fig. 2 12. — Two stages in the development of tongue and pharyngeal floor of man. The body of the tongue comes from paired and unpaired anlagen of the mandibular arch ; the root from second and third visceral arches. That the fourth arch is involved is doubtful. (After Kallius.) fishes, the body is a new formation derived from the mandibular arch united with a median outgrowth from the floor of the mouth. The tetrapod tongue is further modified by the ingrowth of hypo- branchial muscles by which it attains a high degree of mobility. Con- sequently, in addition to its other functions of moving food in mouth and swallowing, it serves as a means of capturing food. Its gustatory function continues throughout the entire vertebrate series. Some have assumed that the primary function of the tongue muscles was that of squeezing secretions out of the lingual glands. Papillae appear first in amphibians, but become more highly diiTerentiated in mammals. The Pharynx The pharynx is that part of the alimentary canal where the respiratory and digestive passages cross one another. It is bordered by the soft palate above, the tongue below, and the glossopalatine arch on each side. The glossopalatine arch partially covers the palatine tonsil, a mass of 226 CHORDATE ANATOMY adenoid tissue pitted with numerous crypts which tend to be a source of infection. The hypertrophy of adenoid tissue, especially that of the soft palate in childhood, interferes with breathing and often requires surgical treatment. The soft palate is a muscular partition separating digestive and respiratory portions of the pharynx. From its posterior border hangs the uvula. (Fig. 213) Seven cavities open into the pharynx — the mouth, the two nasal passages, the two Eustachian tubes, the larynx, and the esophagus. PARIETAL BONP GYRUS CINGULI PITUITARY, SUBDURAL CAVITY- FRONTAL LOBE- FRONTAL BONE- RIGHT CEREBRAL HEMISPHERE ORPUS CALUDSUM NASAL BONE SPHENOID BONE- NASAL CONCHAE EUSTACHIAN TUBE MAXILLA MOUTH CAVITY- PALATINE BONE- VESTIBULE- SOFT PALATE M. GENIOGLOSSUS MANDIBLE- M. GENIOHYOID M. MYLOHYOID FORNIX PINEAL GLAND DURA MATER -OCCIPITAL BONE EREBELLUM PHARYNX- EPIGLOTTIS- LARYNX>==^ SPINAL CORD SPLENIUS TRAPEZIUS SEMISPINALIS CERVICIS ESOPHAGUS' Fig. 213. — A median longitudinal section of the human head sho-wing the relations between digestive and respiratory passages in the pharyngeal region. (Redrawn after Braus.) Three divisions may be distinguished, oral, nasal, and laryngeal. The palatine tonsils lie in the oral portion, the nasal passages and Eustachian tubes open into the nasal portion, while the larynx opens into the laryngeal portion. When food enters the pharynx the entire pharynx is raised by the contraction of the stylo-pharyngeal muscles while the constrictor muscles of the pharynx squeeze the bolus towards the esophagus. The nerve supply of the pharynx comes chiefly from the glossopharyngeal. Since the pharyngeal region is closely associated with the respiratory organs of vertebrates, the description of the evolution and development of the pharynx is omitted here and will be found in the following chapter. THE DIGESTIVE SYSTEM 227 The Esophagus The esophagus is that portion of the alimentary canal which extends from the pharynx to the stomach. It is nearly ten inches in length and is the narrowest part of the digestive tract. From the pharynx it passes just beneath the backbone through the mediastinum and diaphragm to the cardiac region of the stomach. The wall of the esophagus consists of the four layers characteristic of the digestive tract, tunica mucosa, tunica submucosa, tunica muscularis, MUCOSA > i-»SUBMUCOSA MUSCULARIS MUCOSAE MUCOUS GLAND CIRCULAR MUSCLES ,_ '(^LONGITUDINAL i,'' MUSCLES ^ADVENTITIA VAGUS NERVE Fig. 214. — The esophagus as seen in cross section. A is a section of the entire esophagus. J5 is a small portion much enlarged. The layers of tissue characteristic of the entire alimentary canal are found in the esophagus. (Redrawn after Braus.) and tunica adventitia ; but the serous layer which covers the stomach and intestine is wanting in the esophagus, since the body-cavity Uned by the serosa does not extend into the neck. The tunica mucosa includes not only the stratified squamous epithelium which lines the esophagus, but also a connective-tissue tunica propria and a muscularis mucosae, a thin layer of longitudinal muscle fibers. The muscular coat of the esophagus consists of striped fibers in the upper third, while those of the lower two- thirds are smooth. (Fig. 214) 2 28 CHORDATE ANATOMY The submucosa is a layer of loose connective tissue containing glands and many blood and lymph vessels. The tunica muscularis consists of an inner layer of circular muscles and an outer longitudinal layer. The connective tissue between them contains a plexus of sympathetic nerve fibers. By the wave-like peristalsis of the circular muscles food is con- veyed from the pharynx to the stomach. Development of the Esophagus. Beginning with the fourth week, the esophagus develops as an elongation of the fore-gut between pharynx and stomach. Its single-layered columnar epithelium becomes gradually converted into a stratified squamous epithelium like that which lines the pharynx. History of the Esophagus. There is little to distinguish the esophagus of a fish from its stomach, except the relative scarcity of glands, and the fact that its muscle fibers, like those of the pharynx, are striated, while those of the stomach are smooth. In amphibians, the esophagus becomes slightly elongated. Its considerable elongation in reptiles and mammals is correlated with the elongation of the neck. In these groups, it becomes constricted in diameter and most of its muscle fibers become smooth. The Stomach The stomach, lying between the esophagus and small intestine, is the most expanded part of the alimentary canal. Its shape in man varies greatly, depending upon the quantity of food contained. The human stomach lies almost transversely across the abdominal cavity with a greater curvature on the left side of the body and a lesser curvature to the right. The opening of the esophagus into the stomach is the cardiac orifice, that into the small intestine is the pylorus. The anterior more enlarged portion of the stomach is the cardiac portion, the posterior more constricted region is the pyloric portion. The pyloric portion of the stomach diminishes in size towards the pylorus, which is reduced to a small aperture by a local ring-like thickening of the mucous lining and of the layer of circular muscle. The wall of the stomach contains the same four layers of tissue as are seen in the esopha- gus, plus an external serous layer. The tunica muscularis contains three layers of muscle, longitudinal, circular, and obUque. By their combined action under the stimulus of the sympathetic nerves, the stomach maintains a peristaltic churning action as long as food is present. The simple mucous epithelium which lines the stomach joins abruptly the stratified epithelium of the esophagus. Viewed with a hand-lens, the inner surface of the stomach appears to be filled with minute pores, which are the apertures of the ducts of the gastric glands. Three kinds of stomach glands are distinguished, cardiac, gastric, and pyloric. The THE DIGESTIVE SYSTEM 229 '■greater CURVATUIte PYLORIC STOMACH Fig. 215. — The right half of the human stomach, viewed from within. (Redrawn from Braus, after Elze.) MUCOUS EPITHELIUM- ^OASTRIC PIT<.' MUCUS GLANDS<' ^MUCUS GLANDS ETAL CELLS MUSCULAR I Sc Fig. 216. — Cross sections of the wall of the human stomach, showing A, the struc- ture of the gastric (fundus) glands, and B, that of the pyloric glands. While the secre- tions of gastric glands are chiefly digestive (gastric juice), the pyloric glands secrete mucus chiefly. (Redrawn after Braus.) 230 CHORDATE ANATOMY cardiac glands occupy a relatively small area near the cardiac orifice and resemble closely the glands of the esophagus. Each cardiac gland consists of a group of parallel tubules opening into a single duct or pit. The walls of the tubules are formed of cells which secrete zymogen or pepsinogen granules, of parietal cells which secrete the chemical precursor of hydrochloric acid, and of mucus-secreting cells. Most of the glands of the stomach are gastric each of which, Hke the cardiac glands, consists of a duct or pit connected with a group of straight pORSAL PANCREAS ^•VENTRAL PANCREAS INTESTINE -CAECUM PHARYNX- PERITDNEAL CAVITV ^MESONEPHRIC PANCREAS VENTRAL PANCREAS URO SINUS UMBIUCAL CORO' Fig. 217. — Stages A-F in the ontogenesis of the alimentary canal and associated structures in the human embryo. A, early embryo; B, three-weeks embryo; C, three to four weeks embryo; D, four weeks embryo; E, five weeks embryo; F, seven weeks embryo. Notable among the changes represented are the great elongation of the canal, the outgrowth of numerous appendages, and in the cloacal region the separation of the organs of excretion and digestion. (Redrawn after Thompson, Ingalls, F. T. Lewis and Arey.) or slightly curved tubules. The pits are relatively short and are Hned with mucous gland cells like those which cover the inner surface of the stomach, while the tubular glands are relatively elongated and their walls are formed of granular chief cells and of peripheral parietal cells. The chief cells secrete two kinds of zymogen granules— pepsinogen and prochymosin. When mixed with hydrochloric acid secreted by the parietal cells, pepsinogen becomes pepsin, which spUts the molecules of albumen into peptones, and the prochymosin becomes chymosin or rennin, which changes casein into paracasein. It is also asserted that the gastric glands secrete lipase, a fat-splitting enzyme. THE DIGESTIVE SYSTEM 23 I The pyloric glands are limited to the pyloric portion of the stomach. Their chief secretion is mucus, but the presence of some chief and parietal cells suggests that they may also secrete some gastric juice. They differ from gastric glands also in having relatively long pits and short, branched and twisted tubules. Thus they resemble duodenal glands. Development of the Stomach. During ontogenesis, beginning with the fifth week, the stomach arises as a local enlargement of the fore-gut. Its lining, therefore, together with the glands derived from it, is endo- dermal. The external peritoneal membrane is mesodermal ; the remainder of the stomach wall, including the submucous and muscularis layers, is mesenchymatous. The more rapid growth of the dorsal wall produces the greater curvature of the stomach. The lesser curvature develops from the ventral side. The original dorsal side shifts to the left side of the body, while the primitive ventral side comes to lie towards the right. Gastric glands begin to appear as local proliferations of the lining epi- thelium during the seventh week. History of the Stomach. Since stomachs are not unknown among invertebrates, it might be assumed that the stomach of vertebrates is derived directly from that of invertebrates. However, among the proto- chordates, the hemichordates and some urochordates possess a stomach, while the cephalochordates do not, the pharynx passing immediately into the intestine. The liver of Amphioxus develops as a ventral outgrowth a short distance behind the pharynx. Consequently, if we consider Amphioxus as an ancestral type, the stomach of vertebrates must have arisen from the short portion of the alimentary canal which in cephalo- chordates lies between pharynx and liver. The esophagus must likewise have developed from this region. In the cyclostomes, the stomach is a slight enlargement of the ali- mentary canal. As in the Dipnoi, there is no flexure. In most fishes, however, the stomach becomes J-shaped by the bending of the pyloric region, and this curvature persists throughout the vertebrate series. The complications of stomachs such as are found in ruminants are of considerable importance and interest. The stomach of the cow, for example, is divided into four functional divisions, rumen, reticulum, omasum (psalterium) , and abomasum. Since, however, such adaptations to a special diet throw no light on the problem of human phylogenesis, detailed description is omitted. The Intestine The intestine is the portion of the alimentary canal from the pylorus to the anus. Its length averages about thirty feet, of which five feet are included in the large intestine and the remainder in the small intestine. 232 CHORDATE ANATOMY Small Intestine. The small intestine extends, gradually diminishing in diameter, from the pylorus to the ileocolic valve of the colon. The small intestine is distinguished not only by its smaller diameter but also by the presence of numerous villi which cover its inner surface and give it a velvety appearance. Somewhat arbitrarily three regions are dis- tinguished, duodenum, jejunum, and ileum. The duodenum, the anterior portion of the small intestine, averages about nine inches in length, and "VILLI ---PLICA CIRCULARIS S^- LYMPHATIC BLOOD VESSELS ■VILLUS CRYPT MUSCULARIS MUCOSAE ■SUBMUCOSA CIRC. MUSCLE "LONG. MUSCLE -SEROSA Fig. 2i8. — A longitudinal section of the human jejunum, showing in cross section one of the circular plicae (valvulae conniventes). XiS- is characterized by the presence of tubulo-acinous glands located in the submucosa and known as duodenal or Brunner's glands. The duodenal glands secrete an alkaline mucus which neutralizes the acidity of the food which enters the duodenum from the stomach. Zymogenic cells are also found in the duodenal mucosa. The jejunum, which forms two-fifths of the remainder of the small intestine, contains numerous transverse crescentic folds, the plicae or valvulae conniventes, covered with large villi. See Fig. 218. These plicae serve to retard the passage of food and also to increase the absorptive THE DIGESTIVE SYSTEM 233 surface. In ihe ileum, Ihe crescentic folds disappear, and villi become smaller and more scattered. The four layers of tissue characteristic of the alimentary canal are present in the small intestine. Throughout the entire length of the intestine are numerous tubular mucus-secreting glands, perpendicular to the surface of the intestine, the intestinal glands or crypts of Lieberkiihn. Goblet-shaped cells distended with mucus are abundant in the walls of these glands. The secretions of these glands are said to stimulate peristal- sis of the intestine as well as lubricate its surface. (Figs. 218-219) r -MUCOUS EPITHELIUM "CRYPT -LYMPHOID NODULE -_ MUSCULAR! S MUCOSAE *^r^4^<*;^t*:?:^s^-SUBMUC0SA ■CIRCULAR MUSCLE i- LONGITUDINAL MUSCLE "^ "" "^ — SEROSA Fig. 219. — A longitudinal section of the human colon. Xi5- Each villus is covered with a mucus epithelium containing numerous goblet-cells, and each villus has a core of connective tissue filled with capillaries and lymph vessels. A single lymphatic or lacteal occupies the center of each villus, and a network of capillaries lies just below the base- ment membrane of the mucous epithelium. Each villus is therefore a mechanism admirably adapted for absorbing the digested food which bathes it. Besides the peristaltic waves which pass along the intestine squeezing the food backwards towards the large intestine, divisive or churning movements are also carried on, bringing the digested food into contact with the villi. 234 CHORDATE ANATOMY Absorption takes place in the small intestine in accordance with the law of osmosis. The dissolved foods pass through the lining membranes, are taken up by the blood capillaries and the lymphatics, enter the general circulation, and are absorbed into the cells of the various tissues. Large Intestine. The large intestine or colon differs from the small not only in its great diameter but also in the absence of villi in the adult. The walls of the large intestine are sacculated, and they bear externally numerous fatty appendages, the appendices epiploicae. The longitudinal muscles do not form a continuous layer as in the small #ffi>r- .^^„ LONGITUDINAL MUSCLE ~'^f'^:;7r'^~" CIRCULAR MUSCLE - <5--v «< -iS "* %. ^^^^^ /' >\^ ^ ^^^'^'plx w:t^?^;:^~^T'^^ CRYPT •i^jh #A. "^ :'^^"T1GERM. CENTER vV% X . '-~---;|^;^^^-SUBMUCOSA . ^^* -^^''?;:; ;- serosa Pig. 220. — A cross section of the human vermiform appendix. Xi5« intestine, but are arranged in three longitudinal bands, the teniae. Trans- verse crescentic folds, the plicae semilunares, are abundant. Between these the wall of the colon bulges out to form haustra. The large intestine is divided into cecum, vermiform appendix, colon, rectum, and anus. The cecum is a blind sac, about two and a half inches in length, lying near the ileocolic valve in the right ihac fossa. The vermiform appendix of the cecum is an elongated worm-shaped tube between three and four inches in length, attached to the apex of the cecum. The structure of the appendix is similar to that of the large intestine in having numerous Lieberkiihn's glands and lymph nodules. In the majority of persons, the lumen becomes occluded in later life. The THE DIGESTIVE SYSTEM 235 appendix appears to be a rudiment of a more extended cecum functional in the ancestors of man. The colon is divided into four regions, ascending, transverse, descend- ing and sigmoid colon. The ascending colon passes up the right side of the abdominal cavity as far as the liver, where it bends to the left to form the transverse colon. Reaching the lower end of the spleen on the left side, it curves sharply downward, to become the descending colon. Passing down the left side to a point below the kidney, the descending colon bends toward the median plane of the body and enters the pelvic cavity, where it forms the sigmoid flexure. The rectum is continuous with the sigmoid colon and extends to the anus. In the rectum, a number of transverse folds of the wall tend to prevent fecal matter from pressing into the anal canal. In the anal region, the layer of circular muscles is thickened to form the sphincter ani, which, unlike that of the lower rectum, is non- striated and not under control of the will. The external sphincter of the anus, however, is striated and voluntary. Development of the Intestine. Except in the mouth and anal regions, the mucous lining of the alimentary canal and the secretory epithelium of the glands connected with it develop from the endoderm. Primarily, the endoderm of the embryonic area is continuous with that which lines the yolk-sac, (Fig. 221) In correlation with the development of head- fold and tail-fold, a fore-gut and hind-gut are formed in connexion with the yolk-sac by means of anterior and posterior intestinal portals (Fig. 72). From the fore-gut develop pharynx, esophagus, stomach, and the anterior part of the small intestine; from the hind-gut the remainder of the intestine. Early in development, an allantois arises as a ventral outpocketing of the hind-gut, with which it retains connexion by an allantoic stalk. The cloaca is the posterior portion of the hind-gut into which allantois and intestine open, and which is closed to the exterior by the cloacal membrane (Fig. 72Z}). The later development of the intestine involves its elongation and twisting. The opening into the yolk-sac becomes reduced to a slender vitelline duct, which disappears during the second month. Becoming at first too long for the body-cavity, a loop of the intestine pushes down into the umbilical cord. In a six weeks' embryo, the beginning of a cecum is indicated by a swelling posterior to the vitelline duct. Later a horizontal septum grows backward to divide the cloaca into a dorsal rectum and a ventral urogenital sinus. The septum forms the perineum of the adult. During the second month, an anal canal is formed by the invagination of an ectodermal proctodeum and the rupture of the cloacal membrane. (Fig. 217) The four layers of the intestinal wall develop as has been described for the stomach. History of the Intestine. The intestine as a region for the digestion and absorption of food is present in the great majority of animals from 236 CHORDATE ANATOMY flatworms to man. An anal aperture makes its first appearance in flat- worms. Vertebrate morphologists generally regard the anus of verte- brates as homologous throughout the group notwithstanding differences in ontogenetic development in different groups. The post-anal gut may be interpreted as a special modification correlated with the elongation of the tail, and not as a primitive trait. The assumption of a partial homology of the vertebrate anus with the blastoporic mouth of invertebrates seems to be in harmony with all known facts. The uncertainty of pre-chordate homologies will explain why most vertebrate morphologists take the intestine of Amphioxus as the starting point for intestinal evolution. The intestine of Amphioxus extends as a straight tube from the region of the liver directly to the left-sided anus. The intestine of cyclostomes is almost as simple. A spiral fold projecting into the cyclostome intestine, however, suggests the beginning of intestinal differentiation. The intestine of elasmobranchs contains a more elaborate spiral valve. Intestinal elongation has its inception in the sigmoid flexure of elasmobranchs. Increase of intestinal surface is effected in elasmo- branchs and ganoids mainly by the development of a spiral valve. A finger-hke rectal gland makes its appearance in elasmobranchs near the anus. A cloaca also makes its first appearance in this group. A further step in advance is seen in the teleosts, which have a convoluted small intestine, intestinal ceca, and a somewhat enlarged colon. Most amphib- ians except Gymnophiona differentiate smaU and large intestines. All have a cloaca. Some have, in their small intestine, intestinal glands, valvulae conniventes, and villi. The intestine of reptiles is relatively short. Their large intestine is short, and they retain a cloaca. In mammals, the small intestine becomes greatly elongated and differ- entiated into duodenum, jejunum, and ileum. Valvulae conniventes, vflh, and intestinal glands become very numerous. Duodenal glands make their appearance. Colon and rectum are differentiated. In many mammals, especially herbivorous forms, the cecum becomes much elon- gated and forms an important organ of absorption. In others, as in man, it degenerates in size and serves as an adenoid organ. Mesenteries and Omenta The peritoneum lining the abdominal cavity is a serous membrane formed from the embryonic hypomere. It not only lines the body-wall, but is reflected over the viscera, so that parietal and visceral portions are distinguishable. The complex relations of the peritoneum are due chiefly to the complications of the ahmentary canal with which it is connected. These are best understood by tracing their development in the embryo. In the region of the pharynx the splanchnic layers of meso- THE DIGESTIVE SYSTEM 237 derm unite in the median plane to form the tubular heart and the meso- cardial membranes in which the embryonic heart is suspended. In the abdominal part of the coelom the splanchnic layers of mesoderm unite dorsal mesoderm intermediate mesoderm lateral mesoderm Fig. 221. Schematic diagrams of cross sections at various stages to show the establishment of the coelom and mesenteries. (From Patten's "Embryology of the Chick.") above and below the alimentary canal to form dorsal and ventral mesen- teries. (Fig. 221) The dorsal mesentery persists throughout life, but the greater part of the ventral mesentery disappears in ontogenesis. Only the anterior portion which connects stomach, liver and ventral body- wall 238 CHORDATE ANATOMY is retained. With the differentiation of the successive regions of the aHmen- tary canal, corresponding portions of the dorsal mesentery are recognized as mesogaster, mesentery, mesocolon, and mesorectum. The mesenteries serve not only as means of attachment of the intestine to the body-wall, but also as a passage for the blood-vessels of the ahmentary canal. In the adult the mesenteries become very complex in relations as the result of the elongation of the intestine, formation of omenta, and local adhesions. As the stomach develops its greater curvature, it rotates on its long axis so that its left side becomes ventral and the right side dorsal. As a result the dorsal mesogaster is stretched to the left and a pouch or bursa between the mesogaster and the right side of the stomach is formed. /MESOGASTER 'GREATER OMENTUM < ^1^^^ |t~-CAECUM-^ j/ APPENDIX--^ j SMALL INTESTINE '/-COLON •YOLK STALK ESOPHAGUS - CURVATURE STOMACH '— GREATER OMENTUM COLON CAECUM ^j VPPENDIX -■!^ SMALL INTESTINE' Fig. 222. — Diagrams illustrating the development of the mesenteries and omentum in the human embryo. An arrow marks the opening (foramen of Winslow) of the greater omentum. (Redrawn after Hertwig.) As the sacculation of the mesogaster progresses, dorsal and ventral layers become distinguishable. The two-layered sac thus formed grows ventrally and posteriorly between the viscera and the ventral wall of the abdomen as an apron-like membrane, the greater omentum. Much of the original cavity of the omentum is lost through the fusion of dorsal and ventral layers. In the region of the stomach, however, the cavity persists as the bursa omentalis, which opens by the foramen epiploicum into the coelom of the right side. The omentum becomes the seat of deposit of con- siderable fat and serves as a blanket to keep the viscera warm. The Liver The functions of the Hver are diverse. During early ontogenesis, it forms red blood corpuscles. Later in life, it becomes an agent in the eUmination of blood cells. It transforms both sugar and protein into a polysaccharid, glycogen, which it stores in its cells for later use. It also THE DIGESTIVE SYSTEM 239 secretes bile, which aids in the emulsification of fats, in the activation of lipase secreted by the pancreas, and in the stimulation of peristalsis of the intestine. The Hver is a reddish-brown organ l>ing between the stomach and the diaphragm and is the largest gland in the body, weighing between two and three pounds. It is wedge-shaped, and divided into a smaller left lobe DIAPHRAGM ' ESOPHAGUS ■SPINAL CORD DORSAL AORTA LIVER FORAMEN EPIPLaCUM PANCREAS STOMACH ?. MESENTERrCART. ^_rri_LTRANSVERSE COLON ili=irriI\OL.i J.INR MESENTERIC ART. — ^ |5~r5 1 — MESENTERY rV^— --J- A_0MENTAL BURSA BLADDER SYMPHYSIS UTERUS RECTUM Fig. 223. — A median section of the abdominal cavity, showing the relations of the omental bursa. The diaphragm is cross-hatched. The course of the duodenum- jejunum and of the colon is shown by means of arrows. (Redrawn after Braus.) and a larger right lobe. The two lobes are separated by the falciform ligament, which is developed from the ventral mesentery and attaches the liver to the diaphragm and the ventral body-wall. Two smaller lobes, the caudate and quadrate, He between the right and left lobes on their inferior surface. The gall bladder lies below the right lobe near the duo- denum. The postcaval vein passes through the right lobe. 240 CHORDATE ANATOMY Secretions pass from each lateral lobe by a single duct, the two uniting to form the hepatic duct. Nearer the intestine, the hepatic duct joins the cystic duct from the gall bladder to form the common bile duct or ductus choledochus, which opens into the duodenum at a point about three or four inches from the pylorus. The liver is a compound tubular gland, the tubules of which are arranged radially around branches of the hepatic vein. Each cluster of tubules around a central intralobular vein forms a lobule. (Fig. 225) The numerous lobules of the Hver are bound together by interlobular connective tissue containing interlobular veins which are branches of the POST CAVA VEIN CUT EDGE OF PERITONEUM HEPATIC ARTERY RIGHT LOBE QUADRATE LOBE/ GALL BLADDER/ \BILE DUCT Fig. 224. — The human liver viewed from below. (Redrawn after Sobotta.) portal vein, interlobular ducts carrying bile, and branches of the hepatic artery. Connexions between intralobular and interlobular veins are effected by means of intralobular capillaries or sinusoids, which bathe the liver tubules and supply them with the materials for secreting the bile. The relations may best be understood by examination of the diagram (Fig. 226). While branches of the vagus nerve reach the liver, most of its nerves belong to the sympathetic system. The gall bladder is a pear-shaped muscular sac between three and four inches in length, holding about 30 cc. Its inner surface is lined by a mucous epithelium which is thrown into folds. Crescentic folds in the neck of the bladder and in the common bile duct form a sort of spiral valve. When food enters the duodenum from the stomach, the muscles of the gall bladder squeeze the bile into the intestine. Development of the Liver. The anlage of the liver appears in a 2.5 mm. human embryo as a ventral outpocketing of the fore-gut near the anterior intestinal portal, between the two vitelline veins. See Fig. 217. THE DIGESTIVE SYSTEM 241 b o rl C Fig. 225. — Liver of a pig. The lobules have artiticially shrunken from the inter- lobular tissue, a; b, bile duct; c, hepatic artery; d, interlobular vein (a branch of the portal) ;f, trabeculae;/, central vein. Highly magnified. (From Bremer's "Text Book of Histology," after Radasch.) fr--_PORTAL VEIN- LOBULE-- -HEPATIC VEIN A B C Fig. 226. — A, B, and C — diagrams of successive stages in the development of the lobules of the liver. The subdivision of the liver anlage into lobules is correlated with branching of the portal and hepatic veins. The branches of the hepatic vein are intralobular, and those of the hepatic portal vein — shown in black — are interlobular. (Redrawn after Mall.) 242 CHORDATE ANATOMY The liver diverticulum projects into the ventral mesentery and the meso- derm of the septum transversum which separates the pericardial cavity from the abdominal cavity. The outgrowth soon becomes differentiated into an anterior mass of branching cords surrounded by branches of the vitelline veins, and a posterior hollow sac which later becomes the gall bladder. The multiphcation of the cords, correlated with that of the blood capillaries associated with them, produces the lobules. Mesenchyme cells form the interlobular connective tissue. Bile capillaries appear within the cell cords, which thus become hepatic tubules, and the blood capillaries acquire endothelial walls. As a result of this, the lumen of each bile capillary is separated from that of each blood capillary by a layer of gland cells and a layer of endothelial cells. (Figs. 217, 221, 226) The multiplication of tubular cords and of blood spaces results in a rapid enlargement of the liver, which begins to bulge out from the septum transversum and the ventral mesentery and to push into the abdominal cavity between the septum and the stomach. In this way the liver becomes covered by the peritoneum. Meanwhile it acquires its two chief lobes. The ventral mesentery into which it originally grew forms the falciform ligament. (Fig. 221) History of the Liver. The vertebrate liver has no homolog among invertebrates, though many of these have organs which are called livers. The liver of Amphioxus is generally regarded as representing the beginning of that of vertebrates. This is a ventral outpocketing of the intestine immediately behind the pharynx. It grows ventrally and forwards beneath the pharynx, and remains a hollow sac throughout life. Its rela- tions to the blood-vessels resemble those of the liver of vertebrates. The liver becomes bilobed in cyclostomes and elasmobranchs, and a gall bladder is differentiated. In the higher vertebrates and man no important morphological changes occur. The form, however, varies with the shape of the abdominal cavity and the pressure of surrounding organs. The Pancreas The pancreas is a light pinkish organ about five inches in length, extending across the abdominal cavity from a loop of the duodenum on the right side to the left colic flexure. In man the pancreas usually has two functional ducts. One of these, the pancreatic or Wirsung's duct, generally opens into the common bile duct; the other, the accessory or Santorini's duct, opens into the duodenum about an inch above the opening of the bile duct. The pancreas secretes trypsinogen, which is converted into trypsin through the action of enterokinase secreted by the intestinal glands. Tryj)sin splits proteins into amino-acids. The enzyme amylopsin secreted THE DIGESTIVE SYSTEM 243 by the pancreas splits starch into monosaccharids. Another enzyme, lipase or steapsin, when activated by enterokinase breaks fats into fatty acids and glycerine. Another enzyme, ereptose or erepsin, splits pro- teoses and peptones. The digestive activity of the pancreas is stimulated through the endocrinal effect of secretions poured into the blood by the intestinal glands when the acid chyme enters the intestine from the stomach. Besides this digestive function, the pancreas acting as an endocrine gland regulates the sugar metabolism of the body by means of the hormone insulin. The histological structure of the pancreas strikingly resembles that of the parotid gland, both being compound acinous glands divided into lobes and lobules by connective-tissue septa which contain interlobular ducts, ..--STOMACH DUCT OF SANTORIN RSAL PANCREIAS DORSAL PANCREAS "VENTRAL R^NCREAS' DUODENUM BILE DUCT WIRS UNO'S DUCT A. EARLIER STAGE. B. LATER STAGE. Fig. 227. — A and B, two stages in the development of the pancreas. The duct of the dorsal pancreas, Santorini's duct, may degenerate in ontogenesis. The two gland anlagen unite into a single organ in the adult. (Redrawn after Broman.) blood-vessels, and nerves. The acini of the pancreas, instead of being hollow, contain central cells. Scattered irregularly among the acini of the pancreas are clusters of lightly-staining cells. The area of each cluster in section is considerably greater than that of a single acinus. These are the islands of Langerhans, endocrinal organs which secrete insulin. (Fig. 289) Development of the Pancreas. Like the liver, the pancreas develops from the endoderm. It is formed by the fusion of two separate out- growths of the intestine, a ventral bilobed outpocketing from the bile- duct, and a dorsal evagination of the intestine slightly anterior to that of the liver. By the proHferation of the cells of these anlagen, two pan- creases are formed. They secondarily unite, but retain usually the two primary connexions with the intestine, the ventral becoming Wirsung's duct and the dorsal Santorini's, the two connecting within the body of the gland. The dorsal pancreas grows much faster than the ventral, and forms the body and tail of the gland and part of the head. 244 CHORDATE ANATOMY History of the Pancreas. The pancreas seems to be an emergent trait of vertebrates, since no comparable structure is found in the invertebrates or even in the lower chordates. In cyclostomes, the pancreatic tissue remains buried in the substance of the Uver or in the wall of the small intestine. Since no duct appears in these forms, it is assumed that the pancreas was primarily endocrinal and not digestive. Other verte- brates, beginning with the elasmobranchs, have both dorsal and ventral pancreases. CHAPTER 9 THE RESPIRATORY SYSTEM Introduction. Living protoplasm, that is to say a living organism, burns slowly and continually. When oxidation ceases, life ceases also. Galen in the second century saw the similarity between respiration and burning. But it was many centuries before Lavoisier (1771-1780) proved its chemical nature. Breathing is but a subordinate part of respiration. Respiration is the process of gaseous exchange which occurs in a living body through the oxidation of carbon compounds. This exchange involves an intake of oxygen and an outgo of carbon dioxide. The process requires uncombined oxygen, which forms one fifth of the air. Aquatic organisms obtain their oxygen from air dissolved in water. Two kinds of respiration may be distinguished, external and internal. In external respiration, animals make use either of a moist skin or of specialized respiratory organs such as lungs and gills in which blood capillaries are brought into intimate relation with moist membranes. Under these conditions, intake of oxygen goes on in accordance with the law of diffusion of gases separated by semipermeable membranes. In inter- nal respiration, in accordance with the same law, gaseous exchange takes place within all the tissues of the body which are bathed with blood or lymph. Cells draw on the oxygen in these just as a burning match gets its oxygen from the air. The living cell, however, unUke the match, is the master of the oxidative process and not its servant. The necessity for two kinds of respiratory organs, one adapted to aquatic and the other to land and aerial life, has produced in chordates two distinct but possibly not entirely independent respiratory systems to complicate evolutionary history. These are the pharyngeal gills of the lower and the lungs of the higher classes. Chordates have not inherited their respiratory system from their invertebrate forbears, but have invented new ones of their own. Fortunately for the land vertebrates, their fish ancestors were already prepared for the transition from water to land life before the event occurred. By a change of function and some modifications of structure and relation, the bilobed air bladder of the crossopterygian fishes was made to serve as a lung. Furthermore the advantage of nasal passages in air breathing was probably already anticipated by the fish ancestors of amphibians. This assumption seems justified by the fact that some fishes, such as the 245 246 CHORDATE ANATOMY Dipnoi, have narial passages. But it is not generally believed that the Dipnoi are in the direct line of amphibian ancestry. The story of gills is one of great multiplication in number in forms like the protochordates which use the pharynx both for obtaining food and for gaseous exchange. In the fishes and amphibians, however, the gills are considerably modified, are reduced in number and finally in higher verte- brates disappear. Startling changes of function occur. Supporting skeletal elements are converted into a sound-conducting apparatus. Gill-slits degenerate into blind pharyngeal pouches, which in higher verte- brates become endocrinal glands. The transformation of a ventral air bladder into lungs is sufficiently well attested to be plausible. The chief evolutionary change which lungs undergo is an enormous increase of respiratory surface so that, even within the limits of the mammahan chest, they expose many square yards of moist surface for gaseous exchange. To meet respiratory needs, two sorts of organs have emerged in animals, branchial organs or gills found in aquatic animals and pulmonary organs characteristic of land forms. A. The Branchial System. The fact that lungs are wanting in all classes of protochordates, as well as in the more primitive groups of verte- brates, proves that the primary respiratory system is the series of paired pharyngeal gills which form the branchial system of chordates. Rem- nants of this system persist in all higher vertebrates. The transition between gilled and lunged forms occurs in the amphibians most of which, at least at some time in their individual development, have both gills and lungs and which thus bridge the gap between aquatic and terrestrial life. Gills, like lungs, function as respiratory organs by bringing a network of blood capillaries in close contact with moistened membranes through which gaseous exchange takes place. Their efficiency is increased either by the activity of cilia which cover the surface of the gills or by the con- traction of muscles which pump a stream of water through the pharynx, or by waving the gills to and fro as in Necturus. Gills are not the pharyngeal openings through which water passes in respiration; these are gill-slits or gill-clefts. Two sorts may be distin- guished, internal gills within the body-wall and external gills. Those of most animals are internal; a few fishes and amphibians have external. The gills of elasmobranchs may be taken as typical. They are modifica- tions of the branchial bars or arches which alternate with the gill-slits and serve to keep them open. Each branchial arch consists of an inter- branchial septum of connective tissue which is covered on the surface of the body by skin, and which includes near the pharyngeal lining a car- tilaginous arch as a support. Within the septum are branches of the dorsal and ventral aortae which supply the gills with blood. The septa THE RESPIRATORY SYSTEM 247 are further supported by skeletal gill-rays extending from the skeletal branchial arch laterally towards the skin. Each interbranchial septum bears on each surface a half-gill or hemi- branch, which together constitute a holobranch. Each hemibranch is a mucous membrane folded into minute parallel lamellae or branchial filaments, each of which has parallel secondary folds containing a capillary network. Between the capillaries and separating them are pilaster cells peculiar to the gill filaments. In the ganoids and teleosts the inter- FiG. 228. — Diagram of gill clefts in (A) elasmobranchs and (B) teleosts. A' and B', a single gill of each, a, artery; ba, branchial arch; br, branchial ray; d, demibranchs; gc, atrial chamber; gr, gill raker; o, operculum; oe, esophagus; 00, opercular opening; 5, spiracle, in A', septum; v, veins. (From Kingsley's "Comparative Anatomy of Vertebrates.") branchial septum becomes reduced and tends to disappear, leaving only the portion containing the skeletal arch and branchial blood-vessels. In these forms, the gill-slits do not open separately to the exterior as in elasmobranchs but are covered by an operculum formed by the backward growth of the septum of the hyoid arch. (Fig. 228) The mechanism of breathing diflfers considerably in fishes which, like the elasmobranchs, have modified the first gill-slits into spiracles, and those which have not. In all fishes, through the action of antagonistic pharyngeal muscles, the cavity of the pharynx is alternately expanded and contracted, so that water is sucked in through the mouth or the spiracles and forced out through the gill-slits. In forms with an opercu- 248 CHORDATE ANATOMY lum, this functions as a valve, and prevents the entrance of water through the gill-shts. Gaseous exchange takes place through the thin mucous epithelium which covers the gill lamellae. The gills of fishes function also as excretory organs, excreting nitrogenous waste as do the kidneys. Fig. 229. — Diagram of the reUitiuns of external and internal gills in the anuran tadpole, ab, eb, afferent and efferent branchial arteries; h, heart; o, ear cavity; ph, pharynx; ra, radix aortae. (From Kingsley's " Comparative Anatomy of Vertebrates," after Maurer.) External gills are of two sorts, external gill filaments such as occur in elasmobranch embryos as prolongations of the posterior gill lamellae, and external gills which characterize some adult urodeles and the larvae of some fishes and amphibians. The evidence on the whole supports the opinion that they are secondary derivatives of the gill system, developed DIENCEPHALON LENS ESOPHAGUS ECTODERM SPINAL CORD PRONEPHROS NOTOCHORD MYOTOMES DORSAL AORTA PETROMYZON. 16-DAY EMBRYO -FRONTAL SECTION. Fig. 230.- — Frontal (horizontal) section of a 16-day Petromyzon embryo, showing seven pairs of gill pouches (1-7) formed as lateral diverticula of the pharynx. Slight invaginations of the ectoderm to meet the gill pouches are seen. By the rupture of the double (ectoderm-endoderm) membrane each gill pouch is converted into a gill cleft. Between the successive gill pouches the mesoderm is divided into a series of branchiomeric segments, from which the muscles and skeletal arches of the gills develop. in adaptation to special conditions. They have no genetic relation to any human structure. Development of Gills. Gill-slits develop from a series of paired endo- dermic diverticula of the pharynx which meet corresponding invaginations of the ectoderm. (Fig. 230) By the disappearance of the double mem- THE RESPIRATORY SYSTEM 249 BRAIN , MYOTOME brane thus formed the pouches are converted into gill-shts. The branchial arches develop from the regions between the gill-slits. Each arch has an endodermal pharyngeal lining and an external ectodermal covering. The core of each arch is mesodermal. The levator and depressor muscles of the gills are developed from the hypomeric mest)derm enclosed in each branchial arch. The connective tissue, the cartilage or bone, and the blood-vessels of each arch are derived from the mesenchyma. History of the Gills. Pharyn- geal gills are peculiar to chordates and are one of the most constant characteristics of the group. This should not be understood to imply that invertebrates are without structures from which gills might have evolved. The origin of gills from endodermic diverticula sug- gests the possibility that their be- ginnings may be seen in the intestinal diverticula of fiat worms. Were these diverticula to meet the skin and become perforate, aper- tures similar to gill-slits would be formed. Gill-slits first appear in the hemichordates. Rhabdopleura has none, but Cephalodiscus has a single pair. In most hemichordates the number is considerable and in- creases throughout life. Early in their development, their number is doubled by the growth of "tongue- bars" which extend from the dorsal side of the gill aperture to the ventral side. Later the gill bars thus formed become intercon- nected by cross rods or sjmapticulae such as occur also in urochordates and cephalochordates. In urochordates, the number of gill-slits varies from a single pair in Appendicularia to the many characteristic of most genera. The gill-slits of this group open into an atrial cavity developed by an ectodermal ingrowth along the dorsal side of the body. COELOM"-- ESOPHAGUS Fig. 231. — The pharyngeal regio 1 of a young Squalus embryo, showing the visceral arches and clefts. !5o CHORDATE ANATOMY SOMITES S0M3 MIDBRAIN SOMITE 7 I rOREGUT SOMITE A \ » SOMITE I HEART PERICARDIAL CAVITY POST -ANAL GUT NEURENTERIC CANAL' Fig. 232. — A 7 mm. Squalus embryo viewed as a cleared specimen from the left side. The yolk-sac has been mostly removed. Two gill-slits are open. Cranial nerve anlagen are indicated by Roman numerals. ANTERIOR CAVITY MYOTOME 2 CHORDA ENTERON MOUT ENDOSTYLE / LEFT 1ST (TRANSIENT)GILL-POUCH CLUB-SHAPED GLAND A. EARLIER LARVA. ENDOSTYLE RIGHTIST PERMANENT GILL-SLIT PREORAL PIT CLUB-SHAPED GLAND' TRANSIENT GILL-SLIT LE FT 1ST PERMANENT GILL-SLIT B. LATER LARVA. MEDIAN VENTRAL BLOODVESSEL Fig. 233. — A, yottng Amphioxus larva viewed from the left side as a translucent object. (Redrawn after Hatschek.) 5, later larva. (Redrawn after van Wijhe.) The mouth of Amphioxus becomes enormously enlarged and, by its growth backward on the left side, interferes with the symmetry of development of the gill-slits. The gill-slits of the left side of the body develop before those of the right side. The median line of the ventral side of the pharynx is indicated by the median ventral blood-vessel. Modifi- cation of function and degeneration affect the anterior three pairs of gill-slits. The first pair become the endostyle. The second pair form the transient larval club- shaped gland. The left third slit has no mate and soon disappears. The fourth pair of slits form the first permanent pair. THE RESPIRATORY SYSTEM 2!; I Amphioxus, the typical genus of cephalochordates, has as many as one hundred and eighty paired openings or stigmata. As in hemichor- dates, the original number is doubled by formation of secondary gill-slits. Before metamorphosis the number of primary gill-slits in the larva of Amphioxus is nineteen pairs. The large number of gill-slits in the proto- chordates is apparently an adaptation, since these organisms use their gills not only for respiration but also as a mechanism for obtaining food by ciliary action. The history of gills in vertebrates is one of continuous reduction in number and modification in function. The transformation of their skeletal supports has been described in the history of the skeletal system. Even in Amphioxus, some of the gill pouches of the embryo are modified or lost. (Fig. 233) The first pair become the endostyle while the second pair form the larval club-shaped gland. The third left slit never has a corresponding right slit and disappears early in ontogenesis. The first Fig. 234. — Diagram of relations of esophagus and respiratory tracts in (A) Myxine and Ammocoetes, and {B) Petromyzon; b, branchial duct ("bronchus"); oe, esophagus; t, thyroid gland. (From Kingsley's " Comparative Anatomy of Vertebrates.") permanent gill-slit of Amphioxus is therefore the fourth of the ontogenetic series. There are cogent reasons for homologizing this sHt with the spiracle of elasmobranchs, but there is httle agreement among morpholo- gists in regard to the exact homology of serial organs in chordates. The popular belief among morphologists that the vertebrate mouth has been formed by the coalescence of a pair of gill-sHts is supported by the mode of development of the mouth in Amphioxus. The endoderm takes the initiative in the development of the mouth of Amphioxus, as would be the case if it were a gill-slit. In this respect, the mouth of Amphioxus differs from that of vertebrates, in which the ectoderm initiates development. The question whether or not gills are metamenc structures nas been an open one. The metamerism of chordates is manifested primarily in the mesodermal somites. Since there are none of these in hemichordates and urochordates, it is impossible to demonstrate in these forms a cor- respondence between mesomerism and branchiomerism, and thus to estabHsh the metamerism of the latter. The case is different, however, in Amphioxus, where the mesodermal segmentation is one of the most striking features. In the adult animal, there is no correspondence 252 CHORDATE ANATOMY between gills and myotomes. But in the larva, the gill-slits not only take an intermetameric position in relation to the myotomes, but also are innervated by metameric nerves. A similar metameric correspondence is strikingly shown in the embryos of cyclostomes. The conclusion drawn is that mesomerism and branchiomerism correspond. The number of gills varies greatly in different cyclostomes. In the genus Bdellostoma, the number ranges from fourteen to six pairs. The number in Myxine and Petromyzon is respectively six and seven, or one more counting the spiracular pouch which does not become perforate. By the backward growth of the hyoid septum, the external apertures in Myxine becomes reduced to a single pair, a condition not unlike that in bony fishes. Among elasmobranchs, Heptanchus has seven pairs of gill-slits in addition to the spiracles, which are evidently modified gill-slits since they bear rudimentary hemibranchs. Hexanchus and PHotrema have six pairs of gill-slits. Most elasmobranchs have five pairs of gill-slits plus spiracles. In bony fishes the number is reduced to four pairs and the spiracle is absent. Gill-slits disappear in adult tailless amphibians, but are present in some aquatic urodeles. The number however is reduced. Some adult urodeles have three pairs of gill-slits, some two, and some only one. In the newts they disappear entirely. Cutaneous respiration is common in the group, and some respire by means of a highly vascular pharynx. Nevertheless, even in those adult forms which are devoid of functional gills, gill pouches occur in the embryo, and the embryos of Gymnophiona may have as many as six such pouches, suggesting a corresponding number of functional gills in their ancestors. Most amphibian larvae have functional gills. Functional gills are lacking in amniotes, but rudiments of gills are represented by transient embryonic pharyngeal pouches and their inter- mediate visceral arches. In the embryos of reptiles, some of the gill-slits usually become perforate and later close. The perforation of gill-slits in mammals is abnormal. Pharyngeal pouches are, however, always formed in the human embryo, and when these become perforate fistulae in the throat, they may persist and require surgical treatment. The presence of five pharyngeal pouches and six visceral arches alternating with them in the human embryo receives its most reasonable interpretation in the evolution theory. As has already been explained, the disappearance of the visceral arches in man and mammals is incomplete. The skeletal elements are converted into ear bones, attachment for the tongue, and support of the larynx. Three of the aortic arches also persist, as will be shown in the next chapter. Moreover, in addition to these rudiments there are certain derivatives of the gill pouches which require special discussion. THE RESPIRATORY SYSTEM 253 Pharyngeal Derivatives. From the epithelial lining of the embryonic pharyngeal pouches arise some of the important endocrinal glands, thyroid, parathyroid, thymus, and the ultimobranchial bodies. In addition to these which occur in man, some vertebrates have also epithelial bodies and suprapericardial bodies. From the second pair of pharyngeal pouches come the palatine tonsils. The history and development of these m. HYPOPHYSIS -MOUTH -,POUCH 1 . iTeustachian tube) 1 -thyroid gland -palatine tonsil -POUCH 2 PARATHYROID 1 ^ -POUCH 3 _j%-THYMUS 1 gp-PARATHYROID 2 ^"THYMUS 2 POSTBRANCHIAL BODY POUCH 4: POUCH 5--^ TRACHEA" ^.-LUNG LOBE ESOPHAGUS Fig. 235. — Ventral view of pharyngeal region of a human embryo showing the pharyn- geal pouches and their glandular derivatives; semidiagrammatic. pharyngeal derivatives will be taken up in the chapter on endocrinal organs. B. The Pulmonary System. The respiratory system of man and mammals includes lungs, larynx, trachea, bronchial tubes, nasal passages, and diaphragm. Lungs. Lungs are the essential respiratory organs of land vertebrates. Man, like virtually all land animals except snakes, has two, the left having two lobes and the right three. The lungs lie within the rib basket, and when expanded obliterate the potential pleural cavities. They are separated from one another by the mediastinum or interpleural space, 254 CHORDATE ANATOMY which contains the heart, esophagus, and the great blood-vessels which leave the heart. In childhood, the color of the lungs is pinkish, but may become slaty grey in the adult as the result of the accumulation of soot. The structure of the lungs is admirably adapted to the need of exposing to the air a large amount of surface, estimated to equal that of a balloon S-ALVEOLAR SAC Fig. 236. — Diagram of a lung lobule showing the subdivision of a bronchiolus into alveolar ducts, sacs and alveoli. Respiratory epithelium may extend into the bronchioli. (Redrawn after Bremer.) ten feet in diameter, and a section of the lungs shows that the volume of air space greatly exceeds that of solid tissue. The required moisture is supplied by mucous glands. The trachea or wind-pipe subdivides into bronchi, both structures having cartilaginous supports. The bronchi divide into bronchioli, the THE RESPIRATORY SYSTEM 255 bronchioli into alveolar ducts, the alveolar ducts into atria, alveolar sacs, and alveoli which form the ultimate subdivisions (Fig. 236). Exchange of gases occurs chiefly in the alveoU, although the thin respira- tory epithelium is found also in the atria and alveolar sacs and may extend even into the bronchioli, which in general are lined with a simple cuboidal non-respiratory epithelium. There is an elaborate network of capillaries in the walls of the alveoli, so that only two extremely thin membranes separate the blood in the capillaries from the air in the alveoK. Lungs are very elastic, and their elasticity is increased by the smooth muscle fibers which extend into the connective tissue of the lungs as far as the alveolar sacs but not into the walls of the alveoli. The respiratory blood-vessels of the lung are branches of the pulmonary arteries and veins. The bronchial artery and vein supply the connective tissues of the lungs. The innervation of the lung is through branches of the vagus and of the sympathetic. On the outside of the lung the pleura, corresponding to the peritoneal lining of the abdominal cavity, consists of a subserous connective tissue which extends into the walls of the lobules of the lung, and an external epithelial serosa. The pulmonary pleura is reflected back on the inside of the chest as the parietal pleura. Larynx. The larynx or voice-box lies between the root of the tongue and the trachea, and opens into the pharynx by the glottis. Nine carti- lages support it, the unpaired epiglottic, thyroid, and cricoid cartilages, and the paired arytenoids, corniculate, and cuneiform cartilages. Small paired triticeous cartilages also sometimes are found. Numerous muscles are attached, some extrinsic and some intrinsic. The extrinsic muscles are chiefly to lift the larynx in swallowing. Among the intrinsic muscles are the thyro-arytenoid or vocalis and the cricothyroid, which affect the pitch of the voice. At puberty in the male, the larynx becomes enlarged and the vocal cords within it elongated so that the voice is deepened. The epiglottis and vocal cords are covered with the same kind of squamous stratified epitheUum as that which lines the pharynx, but the rest of the larynx is lined with ciliated columnar epithelium similar to that of the trachea. The action of the cilia is such as to carry the secretions of the mucous glands of the lungs, together with particles of dust, out into the pharynx. Mucous glands are numerous. The nerve supply is from the vagus and the sympathetic. Trachea and Bronchi. The human trachea or wind-pipe is a membra- nous tube, four to five inches long, supported by fibrous connective tissue and incomplete U-shaped rings of cartilage. It carries air to and from the lungs. The cartilages vary in number from sixteen to twenty and are incomplete on the side next to the esophagus. The trachea divides to form the right and left bronchi. The lining of the trachea is a mucous 2^6 CHORDATE ANATOMY PARIETAL BONE' GYRUS CINGULI PITUITARY, SUBDURAL CAVITY- FRONTAL LOBE FRONTAL BONE- yRIGHT CEREBRAL HEMISPHERE iRPUS CALU3SUM NASAL BONE SPHENOID BONE NASAL CONCHAE EUSTACHIAN TUBE MAXILLA MOUTH CAVITY- PALATINE BONE VESTIBULE SOFT PALATE M. GENIOGLOSSUS MANDIBLE M. GENIOHYOID M. MYLOHYOID •FORNIX PINEAL GLAND CORPORA QUADRIGEMINA dura mater -occipital bone :erebellum ESOPHAGUS' '\:ENTRA Fig. 237.- — A median longitudinal section of the human head showing the relations between digestive and respiratory passages in the pharyngeal region. (Redrawn after Braus.) MCXTTH INVAGINATION ILL POUCHES LUNG SAC anlage of lung PHARYNX pronephros Fig. 238. — Stages in the development of lungs in vertebrates. /I is a hori- zontal section of a salamander embryo showing the series of paired pouches which form the gill-slits; after Goette. The last pair of pharyngeal pouches are the anlagen of the lungs. Such evidence suggests that lungs may have arisen in phylogenesis from a pair of gill pouches which failed to reach the surface. B and C are earlier and later stages in the development of the lungs in an amphibian. D is a cross section of the lung anlage in a reptile; after Wiedersheim. (Redrawn from Ihle.) THE RESPIRATORY SYSTEM 257 ciliated stratified columnar epithelium. Below this is a submucous connective tissue containing many mucous glands derived from the mucous layer. Between the cartilage and the mucosa is a layer of circular muscle fibers. Nasal Passages. Air is taken in and expired through the nasal passages. The external orifices are the external nares and the openings into the pharynx are the choanae. The paired nasal passages are sepa- rated from one another by the nasal septum and the median plates of the maxillary and vomer bones, and from the cavity of the mouth by maxillary and palatine bones. They are hned with a ciliated columnar epithelium containing many mucus-secreting goblet cells. Diaphragm. Air is drawn into the lungs under atmospheric pressure as the result of the contraction of the muscles of the diaphragm and ribs. Their contraction raises the rib-basket and flattens the dome-shaped dia- phragm. As a result, the size of the pleuroperitoneal cavity is increased. To fill the enlarged space thus formed, air enters the lungs and inflates them to the size of the chest cavity. The diaphragm is a -muscular partition which divides the cavity of the chest from that of the abdomen and which occurs only in man and other mammals. Lacking a diaphragm, the amphibians must swallow their air. The phrenic nerve, a branch of the cervical plexus of nerves, innervates the diaphragm. Development of the Lungs. During the fourth week of the human embryo a laryngo-tracheal groove is formed in the floor of the pharynx immediately behind the fourth gill pouch. Externally this groove appears as a ridge which is bordered on either side by a groove or furrow. By the approximation of these paired lateral grooves and their union in the median plane, the lung anlage is separated from the pharynx, except anteriorly where connexion with the pharynx is retained. The posterior blind end of the diverticulum swells to form the lung anlage while the less expanded anterior portion becomes the larynx and trachea (Fig. 235). The lung anlage later divides into two lateral buds which, by successive subdivision, gradually assume the adult structure. (Fig. 239) The cartilages which support the larynx correspond exactly with those which in aquatic vertebrates support the fourth and fifth branchial arches. The muscles of these arches form the laryngeal muscles. Vocal cords appear during the eleventh week. Beginning with the fifth week, the paired lung-buds branch in the manner of a compound tubular gland. In this way, the entire lining of the lungs is derived from the pharyngeal endoderm. The connective tissue develops from the surrounding mesenchyma. The splanchnic mesoderm forms the serosa, which covers the lungs and lines the chest cavity. As the two lungs enlarge, they push laterally into the body-cavity and by their ventral extension nearly surround the heart, from which they 258 CHORDATE ANATOMY remain separated by the pericardium. With the development of the diaphragm, the pleural cavity containing the lungs becomes separated from the more posterior peritoneal cavity. According to Broman the diaphragm arises from four sources, the septum transversum anterior to the liver, the pleuroperitoneal membranes, the body-wall, and the dorsal mesentery. The nasal passages of lower vertebrates, such as the Dipnoi and Amphibia, develop from nasobuccal grooves similar to those seen in some adult elasmobranchs. In the embryo an ectodermal groove extends from each olfactory pit to the corner of the mouth. Later the groove deepens, Fig. 239.^ — Stages in the development of the trachea, bronchi and lungs in the pig. The pulmonary arteries are shown in black; the veins are cross hatched. Ep, bud of eparterial bronchus. (From Patten's "Embryology of the Pig," after Flint.) its edges meet and fuse together and convert the groove into a tubular passage which connects the pit with the mouth cavity (Fig. 352). The development of the nasal passage in the human embryo is slightly different. In the month-old embryo a similar nasobuccal groove makes its appearance. The nasal passage, however, is not formed by the closure of this groove, but by the backward extension of the epithehum of the olfactory pit, which thus acquires a secondary connexion with the mouth (Fig. 353). History of the Pulmonary System. Invertebrates have no organs comparable with the human pulmonary system, the so-called lungs of pulmonate molluscs being modifications of the mantle and not out- growths from the alimentary canal. Opinions are divided as to the origin of the lungs. According to some, a pair of gill pouches which failed to reach the skin have been converted into lungs. Others suppose that lungs have evolved from the air bladder of fishes. Some seek to reconcile these THE RESPIRATORY SYSTEM 259 two divergent opinions by asserting that the air bladder is itself derived from a pair of modified gill pouches. Goette (1875) was the first to suggest that lungs are modified gill pouches, on the ground that in some amphibian embryos the lungs develop from a pair of posterior endodermal pouches in series with the gill pouches. (Fig. 238) A number of observers have confirmed this observation and reached the same conclusion. In support of Goette's hypothesis is the fact that the pulmonary arteries develop from the sixth pair of aortic arches. Furthermore, it is obvious that, if a gill pouch were to fail to reach the skin and were to grow backwards into the body-cavity, it would assume the relations of a lung. On the other hand, supporters of the air-bladder hypothesis emphasize the fact that the air bladder of such a fish as the Nile bichir (Polypterus) develops, like the lung, as a median ventral outgrowth of the pharynx. Its bilobed adult form is secondary, as is also its vascular connexion with the sixth aortic arch. Basing the homology of air bladder and lung upon their similar development as median ventral outgrowths from the pharynx, the supporters of this view are skeptical of the attempt to com- pare a median organ with paired structures such as gill pouches. To meet this difficulty, it may be pointed out that the transformation of a paired organ into a median one is not unknown. For example, the thyroid gland in all vertebrates develops as a median ventral outpocketing of the pharynx, yet all morphologists agree in homologizing the thyroid with the endostyle of Amphioxus. The endostyle, however, in Amphioxus develops from a pair of gill pouches. It may be doubted whether we have any adequate explanation of the substitution of lungs for gills as respiratory organs. The fact that lungs are much better adapted to the needs of land animals than gills, which tend to dry in air, does not explain their origin. It is to be noted, however, that in this change of life animals have "played safe." Even before they Fig. 240. — Diagrams of air bladder in fishes. A, Physostomous fishes; B, Lepidosteus and Amia; C, Erythrinus; D, Ceratodus. The air bladder of the crossopterygian fish, Polypterus, is, like the lungs of amphibians, bilobed and connected with the floor of the pharynx. (From Kingsley's "Comparative Anat- omy of Vertebrates," after Dean.) 26o CHORDATE ANATOMY abandoned the water for a land life, they had acquired an organ, the air bladder, which would serve as a substitute for gills. While some uncertainty remains in regard to the origin of the lungs, the facts on the whole seem to accord with the gill-pouch hypothesis. If it is assumed that the crossopterygian air bladder is a pair of modified gill pouches, the rest of the problem of the history of the lungs is easily solved, since there are among living vertebrates all intergradations in Fig. 241. — Diagrams of stages in the phylogenesis of the lungs. The respiratory surfaces are stippled, and conductory passages cross-hatched. Embryological stages corresponding with the comparative anatomical series shown in A-E occur in the ontogenesis of lungs in mammals. See Fig. 239. (Redrawn after Huntingrton.) complexity between the simple air bladder of Polypterus and the mam- malian lung. (Fig. 240) The evolutionary changes which occur involve