HX64084310 QM601 .P92 A laboratory manual RECAP JSL ! ISP IBffi I WW • '.;'■ H Uli '';'': ,;:>;i' BiKHHttfHHn ttfiilttttt Rflray HK RUmM Columbia WLnibzv&it? in tfie Cttp of Jkto §?orfe College of ^fjpsictans anb gmtseons Reference library /* A LABORATORY MANUAL AM) TEXT- BOOK. of EMBRYOLOGY By CHARLES WILLIAM PRENTISS, A.M., Ph.D. PROFESSOR OF MICROSCOPIC ANATOMY IN THE NORTHWESTERN UNIVERSITY MEDICAL SCHOOL, CHICAGO WITH 368 ILLUSTRATIONS MANY OF THEM IN COLORS PHILADELPH] \ \\!> LONDON W. B. SAUNDERS COMPANY 1915 Copyright, iqis by W. B. Saunders Company PRINTED IN AMEHICA PRE8S OF W. B. 8AUN0ER6 COMPANV PHILADtLPHIA PREFACE This book represents an attempt to combine brief descriptions of the verte- brate embryos which are studied in the laboratory with an account of human embryology adapted especially to the medical student. Professor Charles Sedg- wick Minot, in his laboratory textbook of embryology, has called attention to the value of dissections in studying mammalian embryos and asserts that " dissection should be more extensively practised than is at present usual in embryological work " The writer has for several years experimented with methods of dissecting pig embryos, and his results form a part of this book. The value of pig embryos for laboratory study was first emphasized by Professor Minot, and the development of my dissecting methods was made possible through the reconstructions of his former students, Dr. F. T. Lewis and Dr. F. W. Thyng. The chapters on human organogenesis were partly based on Keibel and Mall's Human Embryology. We wish to acknowledge the courtesy of the pub- Ushers of Kollmann's Handatlas, Marshall's Embryology, Lewis-Stohr's Histology and McMurrich's Development of the Human Body, by whom permission was granted us to use cuts and figures from these texts. We are also indebted to Professor J. C. Heisler for permission to use cuts from his Embryology, and to Dr. J. B. De Lee for several figures taken from his 'Principles and Practice of Obstetrics." The original figures of chick, pig and human embryos are from preparations in the collection of the anatomical laboratory of the Northwestern University Medical School. My thanks are due to Dr. H. C. Tracy for the loan of valuable human material, and also to Mr. K. L. Vehe for several reconstruc- tions and drawings. C. W. Prentiss. Northwestern University Medical School, Chicago, III., January. 1915. CONTENTS PAGE Introduction 1 1 Chapter I I'm: Germ Cells 17 The ( )vum ■ 17 Ovulation and Menstruation 20 1'he Spermatozoon .1 Mitosis and Amitosis 22 Maturation J4 Fertilization 30 The Chromosomes in Heredity The Determination of Sex ji Chapter II. — Segmentation and Formation of the Germ Layers 3$ Segmentation in Amphioxus, Amphibian, Bird, and Reptile $$ Segmentation of the Rabbit's Ovum 36 ( )rigin of the Ectoderm and Entoderm 37 Origin of the Mesoderm, Notochord and Neural Tube 42 Origin of the Mesoderm in Mammals 45 The Notochord 41 , Chapter III. — The Study of Chick Embryos 48 Chick Embryo of Twenty-five Hours 48 Transverse Sections of Twenty-five-hour Chick Embryo 50 Origin of the Primitive Heart 52 Chick Embryo of Thirty-six Hours ( 18 segments) 54 Central Nervous System 54 Digestive Tube 55 Heart and Vessels '56 Mesodermal Segments 61 Ccelom 63 Mesenchyma 63 Derivatives of the Germ Layers 64 Chick Embryo of Fifty Hours (27 segments) 65 Nervous System 66 Digestive Organs 67 Blood System 67 Study of Transverse Sections 69 Amnion, Chorion 74 Yolk-sac and Allantois 75 Chapter IV. — The Fetal Membranes and Early Human Embryos 77 Early Human Embryos 77 Fetal Membranes of Pig Embryos 7 7 Umbilical Cord 79 Fetal Membranes of Early Human Embryos 80 Chorion 81 Amnion s ^ Allantois N | Yolk-sac and Stalk 86 Anatomy of a 4.2 mm. Human Embryo 88 Central Nervous System 80 Digestive Canal 80 Urogenital and Circulatory Organs 02 Age of Human Embryos 95 Chapter V. — The Study of Pig Embryos Q7 The Anatomy of a six mm. Pig Embryo 07 External Form 07 Internal Anatomy 00 The Study of Transverse Sections 1 1 1 8 CONTENTS PAGE The Anatomy of a 10 mm. Pig Embryo 120 External Eorm 1 20 Central Xervous System and Viscera 122 Heart and Blood Vessels 128 The Study of Transverse Sections 132 Chapter VI. — Methods of Dissecting Pig Embryos: Development of the Face, Palate, Tongue, Teeth and Salivary Glands 145 Directions for Dissecting Pig Embryos 145 Lateral Dissections of 6-35 mm. Embryos 146 Median Sagittal Dissections 148 Ventral Dissections 153 Development of the Face in Pig and Human Embryos 153 Development of the Hard Palate 155 Development of the Tongue 158 Development of the Salivary Glands 161 Development of the Teeth 162 Chapter \TI. — Entodermal Canal and its Derivatives 168 The Pharyngeal and Cloacal Membranes 168 Pharyngeal Pouches and their Derivatives 168 The Thymus Gland 171 The Epithelial Bodies or Parathyreoids 172 The Thyreoid Gland 172 Larynx. Trachea and Lungs 173 Digestive Canal 177 Digestive Glands : Liver 183 Pancreas 186 Body Cavities, Diaphragm and Mesenteries 188 Primitive Ccelom and Mesenteries 188 Septum Transversum 191 Pleuro-pericardial and Pleuro-peritoneal Membranes 192 Diaphragm and Pericardial Membrane 195 Omental Bursa, or Lesser Peritoneal Sac 197 The Mesenteries 200 Chapter VIII. — Urogenital System I 203 Pronephros 203 Mesonephros 205 Metanephros .- 207 Nephrogenic Tissue 21c Cloaca, Bladder, Urethra and Urogenital Sinus 213 Genital Glands and Ducts 216 Testis 218 Ovary 221 Union of Genital Glands and Mesonephric Tubules 225 Uterine Tubes, Uterus and Vagina 226 Ligaments of the Internal Genitalia 228 Descent of Testis and Ovary 230 External Genitalia 232 Male and Female Genitalia Homologizcd 235 The Uterus during Menstruation and Pregnancy 238 The 1 >eciflual Membranes 239 Chorion Laeve and Frondosum 243 Decidua Vera 243 Do idua Capsularis 244 The Placenta 245 The Relation of Fetus to Placenta 249 Chapter K. — Vascular System 251 The Primitive Blood-vessels and Blood-cells 251 Erythrocytes 252 Leu< 01 ytes 253 Blood plates 254 The I development of the Heart 255 Primitive Blood Vascular System 267 I development and Transformation of the Aortic Arches 270 Vertebral and Basilar Arteries 272 Segmental Arteries 274 Umbilical and Iliac Arteries 275 CONTENTS g PAOI Arteries of i In- Extremities zj$ Vitelline and Umbilical Veins: Vena Porta Anterior ( Cardinal Veins: Superior Vena ( lava . 279 Posterior Cardinal Veins: Inferior Vena Cava _\So The Veins of the Extremities ->s^ The Petal ( Circulation The Lymphatic System 286 Lymph Glands 287 Ikemolvmph Glands and Spleen . 288 Chapteb X. Histogenesis 290 The Entoderms! Derivatives Tile Mesodermal Tissues. 290 291 Si lerotomes and Mesenchyme 29 Supporting Tissues 29 Cartilage . 294 Bone 205 Joints 298 Muscle 298 The Ectodermal Derivatives 303 Epidermis '^04 Hair 3o5 Sweat Glands 307 Mammary Glands 307 Nails 308 The Nervous Tissues 309 The Differentiation of the Neural Tube 310 Neurones of the Ventral Roots 313 Spinal Ganglia and their Neurones . 313 The Neurone Theory . 315 The Supporting Tissue of the Nervous System 316 Chapter XI. — Morphogenesis of the Central Nervous System 31Q The Spinal Cord 320 The Brain 325 The Differentiation of the Subdivisions of the Brain 330 Myelencephalon . 330 Metenccphalon 333 Cerebellum 334 Mesencephalon ^^^ Diencephalon 336 Hypophysis 337 Telencephalon 33g Chorioid Plexus of Lateral Ventricles 340 Cerebral Hemispheres 346 Chapter XII. — The Peripheral Nervous System ^ t The Spinal Nerves • - 1 Brachial and Lumbo-sacral Plexuses 353 Cerebral Nerves . 355 Special Somatic Sensory Nerves 335 Somatic Motor Nerves 338 Visceral Mixed Nerves 35g The Sympathetic Nervous System . 364 Chromaffin Bodies: Suprarenal Gland . 366 The Sense Organs . 368 General Sensory Organs . 368 Taste Buds . 368 Olfactory Organ . 369 Eve. 373 Ear 381 Index 389 TEXT-BOOK OF EMBRYOLOGY INTRODUCTION The study of human embryology deals with the development of the individual from the origin of the germ-cells to the adult condition. To the medical student human embryology is of primary importance because it affords a comprehensive understanding of gross anatomy. It is on this account that only recently a prominent surgeon has recommended a thorough study of embryology as one of the foundation stones of surgical training. Embryology not only throws light on the normal anatomy of the adult, but it also explains the occurrence of many anomalies, and the origin of certain pathological changes in the tissues. From the theoretical side, embryology is the key with which we may unlock the secrets of heredity, of the determination of sex and, in part, of organic evolution. There is unfortunately a view current among graduates in medicine that the field of embryology has been fully reaped and gleaned of its harvest. On the contrary, much productive ground is as yet unworked, and all well- preserved. human embryos are of value to the investigator. An institute of embryology for the purpose of collecting, preserving and studying human embryos has re- cently been established by Professor F. P. Mall of the Johns Hopkins Medical School. Aborted embryos and those obtained by operation in case of either normal or ectopic pregnancies should always be saved and preserved by immersing them intact in 10 per cent, formalin or Zenker's fluid. The science of embryology is a comparatively new one, originating with the use of the compound microscope and developing with the improvement of micro- scopical technique. Chick embryos had been studied by Malpighi and Harvey previous to Leeuwenhoek's report of the discovery of the spermatozoon by Dr. Ham in 1677. At this period it was believed that the spermatozoa were both male and female and developed in the ovum of the mother; that the various part s of the adult body were preformed in the sperm-cell. Dalenpatius (,1699) believed that he had observed a minute human form in the spermatozoon. Previous to this period, many animals were believed to be spontaneously generated from slime and decaying matter as asserted by Aristotle. The preformation theory was first combated by Wolff (1759) who saw that the early chick embryo was differentiated 1 2 INTRODUCTION from unformed living substance. Tins theory, known as epigenesis, was proved correct when, in 1827, von Baer discovered the mammalian ovum and later demonstrated the germ-layers of the chick embryo. When, after the work of Schwann and Schleiden (1839), the cell was recognized as the structural unit of the organism, the ovum was regarded as a typical cell and, in 1843, Barry ob- served the fertilization of the rabbit's ovum by the spermatozoon. Hence- forth all multicellular organisms were believed to develop each from a single fertilized ovum, which by continued cell-division eventually gives rise to the adult body. In the case of vertebrates, the segmenting ovum differentiates first three primary germ-layers. The cells of these layers are modified in turn to form tissues, such as muscle and nerve, of which the various organs are composed, and the organs together constitute the organism, or adult body. Primitive Segments — Metamerism. — In studying vertebrate embryos we shall identify and constantly refer to the primitive segments or metameres. These segments are homologous to the serial divisions of an adult earth- worm's body, divisions which are identical in structure, each containing a ganglion of the nerve cord, a muscle segment, or myotome and pairs of blood- vessels and nerves. In vertebrate embryos the primitive segments are known as mesodermal segments, or somites. Each pair gives rise to a vertebra, to a pair of myotomes, or muscle segments, and to paired vessels; each pair of mesodermal segments is supplied by a pair of spinal nerves, consequently the adult verte- brate body is segmented like that of the earth-worm. As a worm grows by the formation of new segments at its tail-end, so the metameres of the vertebrate embryo begin to form in the head and are added tailwards. There is this dif- ference between the segments of the worm and the vertebrate embryo. The seg- mentation of the worm is complete, while that of the vertebrate is incomplete ventrally. GROWTH AND DIFFERENTIATION OF THE EMBRYO A multicellular embryo develops by the division of the fertilized ovum to form daughter cells. These are at first similar in structure and, if separated, any one of them may develop into a complete embryo, as has been proved by the experiments of Driesch on the ova of the sea-urchin. The further development of the embryo depends (1) upon the multiplication of its cells by division; (2) upon the growth in size of the individual cells; (3) upon changes in their form and structure. The first changes in the form and arrangement of the cells give rise to three GROWTH AND DIFFERENTIATION OF THE EMBRYO 13 definite plates, or germ-layers, which arc termed from their positions the ectoderm (outer skin), mesoderm (middle skin) and entoderm (inner skin). In function the ectoderm, as it covers the body, is primarily protective, and gives rise to the nervous system through which sensations are received from the outer world. The entoderm, on the other hand, lines the digestive canal and is from the first nutritive in function. The mesoderm, lying between the other two layers, naturally per- forms the functions of circulation, of muscular movement and of excretion; it gives rise also to the skeletal structures which support the body. While all three germ-layers form definite sheets of cells known as epilhelia, the mesoderm takes also the form of a diffuse network of cells, the mesenchymes. The Anlage. — This German word is the term applied to the first ag- gregation of cells which will form any distinct part or organ of the embryo. The various anlages are differentiated from the germ-layers by a process of un- equal growth. At points where multiplication of the cells is more rapid than in the circular area surrounding them, outgrowths or ingrowths of the germ-layer will take place. The outgrowths or cvaginations are illustrated by the development of the finger-like villi from the entoderm of the intestine; ingrowths or invagina- tions by the formation of the glands at the bases of the villi. According to Minot, the development of cvaginations and invaginations, due to unequal rapidity of growth, is the essential factor in moulding the organs, and hence the body of the embryo. Differentiation of Tissues. — The cells of the germ-layers which form organic anlages may be at first alike in structure. Thus the evagination which forms the anlage of the arm is composed of a single layer of like ectodermal cells, surrounding a central mass of diffuse mesenchyma (Fig. 131). Gradually the ectodermal cells multiply, change their form and structure and give rise to the layers of the epidermis. By more profound structural changes the mesenchymal cells also are transformed into the elements of connective tissue, tendon, cartilage, bone and muscle, aggregations of modified cells which are known as tissues. The development of modified tissue cells from the undifferentiated cells of the germ-layers is known as histogenesis. During histogenesis the struc- ture and form of each tissue cell are adapted to the performance of some special function or functions. Cells which have once taken on the structure and func- tions of a given tissue can not give rise to ceils of any other type. In tissues like the epidermis, certain cells retain their primitive embryonic characters throughout life and, by continued cell-division, produce new layers of cells which are later comified. In other tissues all of the cells are differentiated into the adult type and, during life, no new cells are formed. This takes place in the case of the nervous elements of the central nervous svstem. 14 INTRODUCTION Throughout life, tissue cells are undergoing retrogressive changes. In this way the cells of certain organs like the thymus gland and mesonephros degenerate and largely disappear. The cells of the hairs and the surface layer of the epider- mis become cornified and eventually are shed. Tissue cells may thus normally constantly be destroyed and replaced by new cells. The Law of Biogenesis. — Of great theoretical interest is the fact, con- stantly observed in studying embryos, that the individual in its develop- ment recapitulates the evolution of the race. This law of recapitulation was asserted by Meckel in 1881 and was termed by Haeckel the law of Mo genesis. According to this law, the fertilized ovum is compared to a unicellular organism like the amoeba; the blastula embryo is supposed to represent an adult Volvox; the gastrula, a simple sponge; the segmented embryo a worm-like stage, and the embryo with gill-slits may be regarded as a fish-like stage. The blood of the human embryo in development passes through stages in which its corpuscles resemble in structure those of the fish and reptile ; the heart is at first tubular, like that of the fish; the kidney of the embryo is like that of the amphibian, as are also the genital ducts. Many other examples of this law may readily be observed. A more complete account of the general conceptions of embryology is given in Minot's "Laboratory Textbook of Embryology." Methods of Study. — Human embryos not being available for individual laboratory work, we employ instead the embryos of the lower animals which best illustrate certain points. Thus the ova of Ascaris, a parasitic round worm, are used to demonstrate the phenomena of mitosis; the larvae of echinoderms, or of worms, are frequently used to demonstrate the segmentation of the ovum and the development of the blastula and gastrula larvae; the chick embryo af- fords convenient material for the study of the early vertebrate embryo, of the formation of the germ-layers and of the embryonic membranes, while the struc- ture of a mammalian embryo, similar to that of the human embryo, is best ob- served in the embryos of the pig, which are very readily obtained. An idea of the anatomy of the embryos is obtained first by examining the exterior of whole embryos and studying dissections and reconstructions of them. Finally, each embryo is studied in serial sections, the level of each section being determined by ( omparing it with figures of the whole embryo. Along with his study of the embryos in the laboratory, the student should do a certain amount of supplementary reading. Only the gist of human organo- -is is contained in the following chapters. A very complete bibliography of the subject is given in Keibel and Mall's "Human Embryology," to which GROWTH AND DIFFERENTIATION "I THE EMBRYO [5 the student is referred. Below are given the titles of some of the more impor- tant works on vertebrate and human embryology, to which the student is referred and in some of which supplementary reading is required. I 1 I IIS FOR REFERENi E Duval, M. Atlas D'Embryologie. Masson, Paris. His, W. Anatomii' menschlicher Embryonen. Vogel, Leipzig, 1885. Keibel, F. Normentafel zur Entwicklungsgeschichte der Wirbelthiere. Bd. I. Fischer. Jena, 1897. Keibel and Elze. Normentafel zur Entwicklungsgeschichte des Menschen, Jena. [908. Keibel and Mall. Human Embryology. Lippincott, 1910-1912. Kollmann, J. Handatlas der Entwicklungsgeschichte des Menschen. Fischer, Jena. 1907. Lee, A. B. The Microtomist's Vade Mecum. Blakiston. Philadelphia. Lewis, F. T. Anatomy of a 12 mm. Pig Embryo. Amer. Jour. Anat., vol. 2. Minot, C. S. A Laboratory Textbook of Embryology. Thyng, F. W. The Anatomy of a 7.8 mm. Pig Embryo. Anat. Record, vol. 5. Wilson, E. B. The Cell in Development and Inheritance. Macmillan, New York. CHAPTER I THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION THE GERM CELLS The human organism with its various tissues composed each of aggregations of similar cells is, like that of all other vertebrates, developed from the union of two germ cells, the ovum and spermatozoon. The Ovum. — The female germ cell or ovum is a typical animal cell pro- duced in the ovary [for structure of typical cell see histologic texts]. It is nearly spherical in form and possesses a nucleus with nucleolus, chromatin network, chromatin knots, and nuclear membrane (Fig. i). The cytoplasm of the ovum is distinctly granular, containing more or less numerous yolk granules and a minute centrosome. The nucleus is essential to the life, growth, and reproduction of the cell. The function of the nucleolus is unknown; the chromatin probably bears the hereditary qualities of the cell. The yolk granules, containing a fatty substance termed lecithin, furnish nutrition for the early development of the embryo. A relatively small amount of lecithin is found in the ova of mammals, the embryo developing within, and being nourished by, the uterine wall of the mother. It is much larger in amount in the ova of fishes, amphibia, reptiles, birds, and the primitive mammalia, the eggs of which are laid and develop outside of the body. The so-called yolk of the hen's egg (Fig. 2) is the ovum proper and its yellow color is due to the large amount of lecithin which it con- tains. The albumen, egg-membrane, and shell of the hen's egg are secondary envelopes of the ovum. The human ovum is of small size, measuring from 0.22 to 0.25 mm. in diam- eter (Fig. 1 A) . The cytoplasm is surrounded by a relatively thick radially striated membrane, the zona pellucida. The striated appearance of the zona pellucida is said to be due to fine canals which penetrate it and through which nutriment is carried to the ovum by smaller follicle cells during its growth within the ovary. The origin and growth of the ovum within the ovary are known as oogenesis, and will be described in Chapter VIII. We may state here that each growing ovum is at first surrounded by small nutritive cells known as follicle cells. These increase iS THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION Fig. i A. Vitelline '.'/ membrane Nude, Zona. 2y pellucida. 8*"^ asm 'foyiart Fig. i B. Fig. i. — A, Human ovum examined fresh in the liquor folliculi (Waldeyer) . The zona pellucida is seen as a thick, clear girdle surrounded by the cells of the corona radiata. The egg itself shows a central granular deutoplasmic area and a peripheral clear layer, and encloses the nucleus in which is seen the nucleolus; B, ovum of monkey. X 430. I III: GERM CELLS 19 Fig. 2. — Diagrammatic longitudinal section of an un- incubated hen's egg (after Allen Thomson, in Heisler). (Somewhat altered) : b.l, germinal area; w.y, white yolk, which consists of a central flask-shaped mass, and a num- ber of concentric layers surrounding the yellow yolk (y.y.); v.t, vitelline membrane; x, a somewhat fluid al- buminous layer which immediately envelops the yolk; IP, albumen, composed of alternating layers of more and less fluid portions; ch.l, chalazae; a.ch, air-chamber at the blunt end of the egg — simply a space between the two layers of the shell-membrane; i.s.m, inner, s.m, outer layer of the shell-membrane; s, shell. Fig. 3. — Section of human ovary, including cortex; a, germinal epithel- ium of free surface; b, tunica albugi- nea; c, peripheral stroma containing immature Graafian follicles (d . well-advanced follicle from whose wall membrana granulosa has partially separated; /, cavity of liquor folli- culi; g, ovum surrounded by cell-mass constituting cumulus oophorus (Pier- sol). tfi'It^M^a^W" ■i: -'mf Fig. 4. — Section of well-developed Graafian follicle from human embryo (von Herff ) ; the enclosed ovum contains two nuclei. Fig. 5. — Ovary with mature Graafian follicle about ready to burst (Ribcmont- Dessaignes). 20 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION in number during the growth of the ovum until several layers surround it (Fig. 3). A cavity appearing between these cells becomes filled with fluid and thus forms a sac, the Graafian follicle, within which the ovum is eccentrically located. The cells of the Graafian follicle immediately surrounding the ovum form the corona radiata (Fig. 1) when the ovum is set free. Ovulation and Menstruation. — When the ovum is ripe, the Graafian follicle is large and contains fluid, probably under pressure. The ripe follicles form bud-like projections at the surface of the ovary (Fig. 5), and at these points Ovum Follicle cells Zona pellucida Fig. 6. — Immature follicle containing several ova. From the ovary of a young monkey. X 43°- the ovarian wall has become very thin. It is probable that normally the bursting of the Graafian follicle and the discharge of the ovum are periodic and associated with the phenomena of menstruation. That ovulation or discharge of the ovum from the ovary may occur independent of the menstrual periods has been proven by the observations of Leopold. Also in young girls ovulation may precede the inception of menstruation and it may occur in women some time after the menopause. At birth, or shortly after, all of the ova are formed in the ovary of the female thk germ cells 2T Head End ring Mam segment of Tail > Galea capitis intend of Knob Post end of Knob it fiber Sheath of ax/a/ thread child. Hensen estimates that a normal human female may develop in each ovary 200 ripe ova. Most of the young ova, which may number 50,000, degenerate and never reach maturity. At ovulation but one ovum is normally ripened and dis- charged from the ovary. Several ova, however, may be produced in a single follicle in rare cases. Such multiple follicles have been ob- served in human ovaries and are of frequent occurrence in the ovary of the monkey. Fig. 6 shows such a follicle containing live immature ova. The Spermatozoon. — The male cell or spermatozoon is a minute cell 0.05 mm. long, specialized for active movement. Because of their active movements, spermatozoa were, when first discovered, regarded as para- sites living in the seminal fluid. The sperm cell is composed of a flattened head, indistinct neck piece, and thread- like tail (Fig. 7). The head is about 5 micra in length. It appears oval in side view, pear-shaped in profile. When stained, the anterior two-thirds of the head may be seen to form a cap, and the sharp border of this cap is the perforatorium by means of which the spermatozoon penetrates the ovum. The head contains the nu- clear elements of the sperm cell. The neck is said to be disc-shaped and to contain the centrosorhes as the anterior and posterior centro- some bodies. The tail is divided into a short connecting piece, aflagellum which forms about four-fifths of the length of the sperm cell and a short end-piece (Fig. 7). The connecting piece is marked off from the flagellum by the annulus. -Ax/a/ thread' -Capsule Terminal filament Fig. 7. — Diagram of a human spermato- zoon, highly magnified, in side view (Meves, Bonnet). 22 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION It is traversed by the axial filament (rilum principale), and surrounded (i) by the sheath common to the flagellum; (2) by a sheath containing a spiral filament; and (3) by a mitochondria sheath. The flagellum is composed of an axial filament surrounded by a cytoplasmic sheath and the end-piece is the naked continuation of the axial filament. The spermatozoa are motile, being propelled by the movements of the tail. They swim always against a current at the rate of about 25 micra per second, or 1 mm. every forty seconds. This is important, as the outwardly directed cur- rents induced by the ciliary action of the uterine tubes and uterus direct the sper- matozoa by the shortest route to the infundibulum. Keibel has found sperma- tozoa alive three days after the execution of the criminal from whom they were obtained. They have been found motile in the vagina twelve to seventeen days after coitus. They have been kept alive eight days outside the body by arti- ficial means. It is not known for how long a period they may be capable of fer- tilizing ova but, according to Keibel, this period would be certainly more than a week. MITOSIS AND AMITOSIS Before the discharged ovum can be fertilized by the male germ cell, it must undergo a process of cell division and reduction of chromosomes known as matu- ration. As the student may not be familiar with the processes of cell division, a brief description may be necessary. (For details of mitosis see text-books of histology and E. B. Wilson's "The Cell".) Amitosis. — Cells may divide directly by the simple fission of their nuclei and cytoplasm. This process is called amitosis. Amitosis is said to occur only in moribund cells. It is the type of cell division found in the epithelium of the bladder. Mitosis. — In the reproduction of normally active cells, complicated changes take place in the nucleus. These changes give rise to thread-like structures, hence the process is termed mitosis (thread) in distinction to amitosis (no thread). Mitosis is divided for convenience into four phases (Fig. 8). Prophase (Fig. 8, I— III) . — 1. The centrosome divides and the two minute bodies resulting from the division move apart, ultimately occupying positions at opposite poles of the nucleus. 2. Astral rays appear in the cytoplasm about each centreole. They radiate from it and the threads of the central or achromatic spindle are formed between the two asters, thus constituting the amphiaster (Fig. 8, II). MI I'nSIS AM) AMI IOSIS 23 III. 3. The nuclear membrane and nucleolus disappear, the nucleoplasm and cytoplasm becoming continuous. 4. During the above changes the chromatic network of the resting nucleus resolves itself into a skein or spireme, the thread of which soon breaks up into distinct, heavily-staining bodies, the chromosomes. A ._. , .. ... i. ,'•■ "•-.^ ii-..-'' "••.. definite number of chromo- somes is always found in the cells of a given species. The chromosomes may be block- shaped, rod-shaped, or bent in the form of a U. 5. The chromosomes ar- range themselves in the equa- torial plane of the central spindle. If U-shaped the base of each U is directed toward a common center. The am phi aster and the chro- mosomes together constitute a mitotic figure and at the end of the prophase this is called a monaster. Metaphase. — The longi- tudinal splitting of the chro- mosomes into exactly similar halves constitutes the meta- phase (Fig. 8. IV. V). The aim of mitosis is thus accom- plished, an accurate division of the chromatin between the nuclei of the daughter cells. Anaphase. — At this stage the two groups of daughter chromosomes separate and move up along the central spindle fibers, each toward one of the two asters. Hence this is called the diastcr stage (Fig. 8, VI). At this stage, the centrioles may each divide in preparation for the next division of the daughter cells. Telophase (Fig. 8, VII. VIII). — 1. The daughter chromosomes resolve them- K\ VII. VIII. .--' Fig. 8. — Diagram of the phases of mitosis (Schafer). 24 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION selves into a reticulum and daughter nuclei are formed. 2. The cytoplasm di- vides in a plane perpendicular to the axis of the mitotic spindle. Two complete daughter cells have thus arisen from the mother cell. The complicated processes of mitosis, by which cell division is brought about normally, seem to serve the purpose of accurately dividing the chromatic sub- stance of the nucleus in such a way that the chromatin of each daughter cell may be the same qualitatively and quantitatively. This is important if we assume that the chromatic particles of the chromosomes bear the hereditary qualities of the cell. The number of chromosomes is constant in the sexual cells of a given species. The number for the human cell is in doubt. It has been given as 16, 24, and 32. According to Winiwarter's recent work, the number of chromosomes in each immature ovum or oocyte is 48, in each spermatogone 47. Wiemann (Amer. Jour. Anat., vol. 14, p. 461) finds the number of chromosomes in various human somatic cells varies from 34 to 38. In species of A scaris megalocephala, a parasitic worm, but two or four chromosomes are found and in their cells the processes of mitosis are most easily observed. We have seen that reproduction in mammals is dependent upon the union of male and female germ cells. The union of two germinal nuclei (pronuclei) would necessarily double the number of chromosomes in the fertilized ovum and also the number of hereditary qualities which their particles are supposed to bear. This multiplication of hereditary qualities is prevented by the processes of matu- ration which take place in both the ovum and spermatozoon. MATURATION Maturation may be defined as a process of cell-division during which the number of chromosomes in the germ cells is reduced to one-half the number characteristic for the species. The spermatozoa take their origin in the germinal epithelium of the testis. Their development, or spermatogenesis, may be studied in the testis of the rat; their maturation stages in the testis tubes of Ascaris. Two types of cells may be recognized in the germinal epithelium of the seminiferous tubules, the sustentacu- lar cells (of Sertoli), and the male germ cells or spermatogonia (Fig. 9). The spermatogonia divide, one daughter cell forming what is known as a primary spermatocyte. The other daughter cell persists as a spermatogone and, by con- tinued division during the sexual life of the individual, gives rise to other primary spermatocytes. The primary spermatocytes correspond to the ova before matu- ration. Each contains the number of chromosomes typical for the male of the species. The process of maturation consists in two cell divisions of the primary MATURATION 25 Fig. 9. — Diagram showing cycle of phases in the spermatogenesis of the rat (Schafcr, Brown). The numbered segments of the circle represent portions of different seminiferous tubules, a, spermato- gonia; a', sustentacular cells; b, spermatocytes actively dividing in 5; c, spermatids forming an irregular clump in 1,6, 7 and 8 and connected to sustentacular cell a' in 2, 3, 4 and 5. In 6, 7 and 8 advanced spermatozoa of one generation are seen between spermatids of the next generation, s', parts of sperma- tids which disappear when sperms are fully formed; s, seminal granules representing disintegration of s'; a", in 1 and 2 are atrophied sustentacular cells. C JE F Fig. 10. — Diagrams of the development of spermatozoa (after Meves in Lewis-Stohr) ; a.c, an- terior centrosome; a.f., axial filament; c.p., connecting piece; ch.p., chief piece; g.c, galea capitis; n, nucleus; nk., neck; p., protoplasm; p.c, posterior centrosome. 26 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION spermatocytes, each producing first, two secondary spermatocytes, and these in turn four cells known as spermatids. During these cell divisions the number of chromosomes is reduced to half the original number, the spermatids possessing just half as many chromosomes as the spermatogonia. Each spermatid now be- Mm® $%&$m mm r = • H : c.. o i. A' i. Fig. ii. — Reduction of chromosomes in spermatogenesis in Ascaris megalocephala (bivalens) (Brauer, Wilson). A-G, successive stages in the division of the primary spermatocyte. The original reticulum undergoes a very early division of the chromatin granules which then form a doubly split spireme (B). This becomes shorter (C) and then breaks in two to form two tetrads (D) in profile, (E, in end). F, G, H, first division to form two secondary spermatocytes, each receiving two dyads. /, secondary spermatocyte. /, K, the same dividing. L, two resulting spermatids, each containing two single chromosomes. comes transformed into a mature spermatozoon (Fig. 10), the nucleus forming the larger part of the head, the centrosome dividing and lying in the neck or middle piece. The posterior centrosome is prolonged to form the axial filament, and the cytoplasm forms the sheaths of the middle piece and tail. M Ml R Wins 27 The way in which the number of chromosomes is redui ed may be seen in the spermatogenesis of Ascaris (Fig. n). Four chromosomes are typical fir Ascaris megaloccphala bivalcns and each resting primary spermatocyte contain-, this number. When the first maturation spindle appears only two chromosomes are formed, but each of these is double, so four are really present. Each represent- the union of two chromosomes, shows a quadruple structure, and is termed a tetrad (Fig. 11 -E, F). At the metaphase (G) the two tetrads split each into two chromosomes which already show evidence of longitudinal fission and are termed dyads. One pair of dyads goes to each of the daughter cells, or secondary sper- Spermatogonium Proliferation period Growth period Maturation period 12 3 4 1234 Fig. 12. — Diagrams of maturation, spermatogenesis and oogenesis (Boveri). matocytes (Fig. n G, I). Before the formation of a nuclear membrane, the second maturation spindle appears at once, the two dyads split into four monads, and each daughter spermatid receives two single chromosomes, or one-half the number characteristic for the species. A diagram of maturation in the male As- caris is shown in Fig. 12 A. The first maturation division is reductionaL each daughter nucleus receiving two complete cJiromosomes of the original four, whereas in the second maturation division as in ordinary mitosis, each daughter nucleus receives a half of each of the two chromosomes, these being split lengthwise. In the latter case the division is equational, each daughter nucleus receiving chro- mosomes bearing similar hereditary qualities. In many insects and some ver- 28 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION tebrates it has been shown that the number of chromosomes in the oogonia is even, the number in the spermatogonia odd, and that all the mature ova and half the spermatids contain an extra or accessory chromosome (see p. 32). Previous to fertilization, the ova undergo a similar process of maturation. Two cell divisions take place but with this difference, that the cleavage is un- equal and, instead of four cells of equal size resulting, there are formed one large ripe ovum or oocyte and three rudimentary or abortive ova known as polar bodies or polocytes. The number of chromosomes is reduced in the same manner as in the spermatocyte, so that the ripe ovum and each polar body contain one-half the number of chromosomes found in the immature ovum or primary oocyte. The female germ cells, from which new ova are produced by cell division, are called oogonia and their daughter cells after a period of growth within the ovary are the primary oocytes, comparable to the primary spermatocytes of the male. During maturation the ovum and first polocyte are termed secondary oocytes (comparable to secondary spermatocytes) , the mature ovum and second polocyte, with the daughter cells of the first polocyte, are comparable to the spermatids (see diagram B, Fig. 12). Each spermatid, however, may form a mature sper- matozoon, but only one of the four daughter cells of the primary oocyte becomes a mature ovum. The three polocytes are abortive and degenerate eventually, though it has been shown that in the ova of some insects the polar body may be fertilized and segment several times like a normal ovum. The maturation of human ova has not been observed, but such a process probably takes place. The reduction of the chromosomes may be best observed in the ova of Ascaris and of insects. The mouse offers a favorable opportunity for studying the maturation of a mammalian egg as the ova are easily obtained. Their maturation stages have recently been studied by Mark and Long (Carnegie Inst. Publ. No. 142). Maturation of the Mouse Ovum. — The nucleus of the mature ovum is known as the female pronucleus. When the spermatozoon penetrates the mature ovum it loses its tail and its head becomes the male pronucleus. The aim and end of fertilization consists in the union of the chromatic elements contained in the male and female pronuclei and the initiation of cell division. In the mouse, the first polocyte is formed while the ovum is still in the Graafian follicle. In the forma- tion of the maturation spindle no astral rays and no typical centrosomes have been observed. The chromosomes are V-shaped. The first polar body is seg- mented from the ovum and lies beneath the zona pellucida as a spherical mass about 25 micra in diameter (Fig. 13). Both ovum and polar body (secondary oocytes) contain 10 or 12 chromosomes, or half the number normal for the mouse. MATURATION 29 (According to Mark and Long, the chromosomes number 20.) The first matura- tion division is the reductional one and the chromosomes take the form of tetrads. After ovulation has taken place, the ovum lies in the ampulla of the uterine tube. If fertilization takes place, a second polocyte is cut off, the nucleus of the /•; G Fig. 13. — Maturation and fertilization of the ovum of the mouse. A.C-J, X 500; B X 750. (after Sobotta). A-C, entrance of the spermatozoon and formation of the second polar body. D-E, develop- ment of the pronuclei. F-J, successive stages in the first division of the fertilized ovum. ovum forming no membrane between the production of the first and second polar bodies (Fig. 13 A-D). The second maturation spindle and second polar body are smaller than the first. Immediately after the formation of the second polar body, the chromosomes resolve themselves into a reticulum and the female pro- nucleus is formed (Fig. 13 D). 30 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION Fertilization of the Mouse Ovum. — Normally, a single spermatozoon enters the ovum six to ten hours after coitus. While the second polar body is forming, the spermatozoon penetrates the ovum and loses its tail. Its head is converted into the male pronucleus (Fig. 13 D). The pronuclei, male and female, approach each other and resolve themselves into a spireme stage, then into two groups of 12 chromosomes. A centrosome, possibly that of the male cell, appears between them, divides into two, and soon the first segmentation spindle is formed. The 12 male and 12 female chromosomes arrange themselves in the equatorial plane of the spindle, thus making the original number of 24 (Fig. 13 I). Fertilization is now complete and the ovum divides in the ordinary way. The fundamental results of the process of fertilization are (1) the union of the male and female chromosomes, (2) the initiation of cell division or cleavage of the ovum. These two factors are separate and independent phenomena. It has been shown by Boveri and others that fragments of sea-urchin's ova containing no part of the nucleus may be fertilized by spermatozoa, segment and develop into larvae. The female chromosomes are thus not essential to the process of segmentation. Loeb, on the other hand, has shown that the ova of invertebrates may be made to segment by chemical and mechanical means without the cooperation of the spermatozoon. It is well known that the ova of certain invertebrates develop normally with or without fertilization (parthenogenesis). These facts show that the union of the male and female pronuclei is not the means of initiating the development of the ova. In all vertebrates it is, nevertheless, the end and aim of fertilization. Lillie {Science, vols. 36 and 38, pp. 527-530 and 524-528) has recently shown that the cortex of sea-urchin's ova produces a substance which he terms fertilizin. This substance he regards as an amboceptor essential to fertilization with one side chain which agglutinates and attracts the spermatozoa, another side chain which activates the cytoplasm and initiates the segmen- tation of the ovum. Spermatozoa may enter the mammalian ovum at any point. If fertilization is delayed and too long a period elapses after ovulation, the ovum may be weak- ened and allow the entrance of several spermatozoa. This is known as poly- spermy. The fertilization of the human ovum has not been observed, but probably takes place in the uterine tube some hours after coitus. Ova may be fertilized and start developing before they enter the uterine tube. If they attach themselves to the peritoneum of the abdominal cavity, they give rise to abdominal pregnancies. If the ova develop within the uterine tube tubular pregnancies result. Normally, the embryo begins its development in the uterine tube, thence passes into the uterus and becomes embedded in the uterine mucosa. The time required for the passage of the ovum from the uterine tube to the uterus is unknown. It probably varies in different cases and may occupy a week or more. The ovum may in some cases be fertilized within the uterus. Fertilization is favored by the fact that the spermatozoa swim always against a current. As the cilia of the uterus and uterine tube beat downward and outward the sperms are directed upward and inward. They may reach the uterine tubes within two hours of a normal coitus. DETERMINATION OF SEX 31 Usually but one human ovum is produced and fertilized at coitus. The de- velopment of two or more embryos within the uterus may be due to the ripening and expulsion of an equal number of ova at ovulation, these being fertilized later. Identical twins are regarded as arising from the daughter cells of a fertilized ovum, these cells having separated, and each having developed like a normal ovum. The Significance of Mitosis, Maturation and Fertilization. — It. is assumed by students of heredity that the chromatic particles of the nucleus bear the hereditary qualities of the cell. During the course of development these particles are probably distributed to the various cells in a definite way by the process of mitosis. The process of fertilization would double the number of hereditary qualities and they would be multiplied indefinitely were it not for maturation. At maturation not only is the number of chromosomes halved, but it is assumed also that the number of hereditary qualities is reduced by half. In the case of the ovum, this takes place at the expense of three potential ova, the polocytes, which degenerate, but is to the advantage of the single mature ovum which retains m^re than its share of cytoplasm and nutritive yolk. Mendel's Law of Heredity. — Experiments show that all hereditary characters fall into two opposing groups, which alternate with each other and are termed allelomorphs. As an example, we may take the hereditary tendencies for black and blue eyes. It is supposed that there are paired chromatic particles in the germ cells which bear these hereditary tendencies. Each pair may be composed of similar particles, both bearing black-eyed tendencies or both blue- eyed tendencies, or opposing particles may bear the one black, the other blue-eyed tendencies. It is assumed that at maturation these paired particles are separated, and that one only of each pair is retained in each germ cell, in order that new and favorable combinations may be formed at fertilization. In our example, either a blue-eyed or a black-eyed tendency bearing partide would be retained. At fertilization the segregated tendency-bearing particles of one sex may enter into new combinations with the allelomorphs of the other sex, combinations which may be favorable to the offspring. Three combinations may be possible. If the color of the eyes is taken as the hereditary character, (1) two "black" germ cells may unite; (2) two "blue'* germ cells may unite; (3) a "black" germ cell may unite with a "blue" germ cell. The result- ing individual will be in (1) black-eyed; in (2) blue-eyed; in (3) either black-eyed or blue-eyed, according to whether one or the other tendency predominated. Were the black-eyed tendency in (3) predominant and the resulting individual black-eyed, there would still be blue-eyed bearing chromatin particles in his or her germ cells. In the next generation these recessive blue-eyed qualities may unite with similar qualities of another black-eyed individual. The offspring would be blue-eyed, though both the parents were black-eyed. DETERMINATION OF SEX The assumption that the chromosomes are the carriers of hereditary ten- dencies is borne out by the observations of cytologists on the germ cells of insects and some vertebrates. It has been shown that in some forms the nucleus of the spermatogonia contain 23 chromosomes, while those of the oogonia contain 24. When maturation and reduction of the chromosomes take place, half of the sper- matids contain 12 chromosomes, the other half only eleven, while all the oocytes 32 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION and polocytes contain 12. There is thus one extra chromosome in each mature ovum and in each of half the spermatozoa. This chromosome is larger than the others in some insects, and is termed the accessory chromosome. McClung was the first to assume that the accessory chromosome was a sex determinant. It has since been shown by Wilson, Davis, and others that the accessory chromosome carries the female sexual characters. When the spermatozoan with 12 chromo- somes fertilizes an ovum, the resulting embryo is a female, its somatic nuclei containing 24 chromosomes. An ovum fertilized by a sperm cell containing only n chromosomes (without the accessory chromosome) produces a male with so- matic nuclei containing only 23 chromosomes. Winiwarther (Arch. d. Biol. Bd. 27) has recently made similar observations on the human germ cells but they have yet to be confirmed by other investigators. It is probable, however, that sex is transmitted by the human chromosomes in much the same way as in insects. CHAPTER II SEGMENTATION OF THE FERTILIZED OVUM AND ORIGIN OF THE GERM LAYERS SEGMENTATION The processes of segmentation, not having been observed in human -ova, must be studied in other vertebrates. It is probable that the early development of all vertebrates is, in its essentials, the same. It is modified, however, by the presence in the ovum of large quantities of nutritive yolk. In many vertebrate ova the yolk collects at one end, the vegetal pole. Such ova are said to be tdolccithal. Examples are the ova of Amphioxus, the frog and bird. When very little yolk is present, the ovum is said to be alccitlial (no yolk). Examples are the ova of the higher mammals and man. The typical processes of cleavage may be studied most easily in the fertilized ova of invertebrates (Echinoderms, Annelids, and Mollusks). Among Chordates, the early processes in develop- ment are primitive in a fish-like form Amphioxus. The yolk modifies the development of the amphibian and bird's egg, while the early structure of the mammalian embryo can be explained only by assuming that the ova of the higher Mammalia at one time contained a considerable amount of yolk like the ovum of the bird and of the lower mammals. Amphioxus. — The ovum is telolecithal, but contains little yolk (Fig. 14). About one hour after fertilization it divides vertically into two nearly equal daugh- ter cells. The process is known as cell cleavage, or segmentation and takes place by mitosis. Within the same interval of time the daughter cells cleave in the same plane, forming four cells. Fifteen minutes later a third segmentation takes place in a horizontal plane. As the yolk is more abundant at the vegetal poles of the four cells the spindle lies nearer the animal pole. Consequently in the eight- celled stage the upper tier of four cells is smaller than the lower four. By suc- cessive cleavages, first in the vertical, then in the horizontal plane a 16- and 32- celled embryo is formed. The upper two tiers are now smaller and a cavity, the blastocoeL is enclosed by the cells. The embryo is called a morula (mulberry). In subsequent cleavages, as development proceeds, the size of the cells is di- minished while the cavity enlarges (Fig. 14). The embryo is now a blast ula, 3 33 34 SEGMENTATION OF THE FERTILIZED OVUM nearly spherical in form and about four hours old. The cleavage of the Amphioxus ovum is thus complete and somewhat unequal. PB Fig. 14. — Segmentation of the egg of Amphioxus, X 220 (after Hatschek). 1. The egg before the commencement of development; only one polar body, P.B, has been seen, but from analogy with other animals it is probable that there are really two present. 2. The ovum in the act of dividing, by a vertical cleft, into two equal blastomeres. 3. Stage with four equal blastomeres. 4. Stage with eight blastomeres; an upper tier of four slightly smaller ones and a lower tier of four slightly larger ones. 5. Stage with sixteen blastomeres in two tiers, each of eight. 6. Stage with thirty-two blastomeres, in four tiers, each of eight; the embryo is represented bisected to show the segmentation cavity or blastocoel, B. 7. Later stage: the blastomeres have increased in number by further division. 8. Blastula stage bisected to show the blastocoel, B. The Ovum of the Frog. — The ovum contains so much yolk that the nucleus and most of the cytoplasm lies at the upper or animal pole. The first cleavage spindle lies in this cyto plasm. The first two cleavage planes are vertical and the four resulting cells are nearly equ SEGMENTATION 35 (Fig. 15). The spimlks for the third cleavage are located near the animal pole and the cleavage. takes place in a horizontal plane. As a result, the upper four cells are much smaller than the lower four. The large yolk-laden cells divide more slowly than the upper small cells. At the blastula stage, the cavity is small, and the cells of the vegetal pole an- each many times larger than those at the animal pole. The cleavage of the frog's ovum is thus complete hut unequal. 4 5 Fig. 15. — Segmentation of the frog's ovum (Hatschek in Marshall). B, segmentation cavity; U, nucleus. Ova of Reptiles and Birds.— The ova of these vertebrates contain a large amount of yolk. There is very little pure cytoplasm except at the animal pole 36 SEGMENTATION OF THE FERTILIZED OVUM and here the nucleus is located (Fig. 2). When segmentation begins, the first cleavage plane is vertical but the yolk, being lifeless matter, does not cleave. The segmentation is thus incomplete or meroblastic. In the hen's ovum the cy- toplasm is divided by successive vertical furrows into a mosaic of cells which, as it increases in size, forms a cap-like structure upon the surface of the yolk. These cells are separated from the yolk beneath by horizontal cleavage furrows, and successive horizontal cleavages give rise to several layers of cells. The space between cells and yolk mass may be compared to the blastula cavity of Am- phioxus and the frog (Fig. 17). The cellular disc or cap is termed the germinal area or disc. The yolk mass which forms the floor of the blastula cavity and the greater part of the ovum may be compared to the large yolk-laden cells at the vegetal pole of the frog's blastula. The yolk mass never divides, but is gradu- ally used up in supplying nutriment to the embryo which is developed from the cells of the germinal area. Round the periphery of the germinal area new cells constantly form until they surround the yolk. The Ovum of the Rabbit. — The ovum of all the higher mammals, like that of man, is microscopic in size and nearly alecithal (no yolk). Its segmentation has been studied in several mammals but we shall take the rabbit's ovum as an example. The cleavage is complete and nearly equal (Fig. 16), a cluster of nearly equal cells being formed within the zona pellucida. This corresponds to the morula stage of Amphioxus. Next an inner mass of cells is formed which corre- sponds to the germinal area, or blastoderm, of the chick embryo (Fig. 16). The inner cell mass is overgrown by an outer layer which we term the troph-ectoderm because, in mammals, it supplies nutriment to the embryo from the uterine wall. Between the outer layer and the inner cell mass fluid next appears, separating them except at the animal pole. As the fluid increases in amount, a hollow vesicle results, its wall composed of the single-layered troph-ectoderm except where this is in contact with the inner cell mass. This stage is known as the germinal or blastodermic vesicle. It is usually spherical or ovoid in form, as in the rabbit, and probably this is the form of the human ovum at this stage. In the rabbit it is of macroscopic size before it becomes embedded. Among Ungulates (hoofed animals) the vesicle is greatly elongated and attains a length of several centi- meters, as in the pig. If we compare the mammalian blastodermic vesicle with the blastula stages of Amphioxus, the frog and the bird, it will be seen that it is to be homologized with the bird's blastula, not with that of Amphioxus (Fig. 17). In each case there is an inner cell mass of the germinal area. The troph-ectoderm of the Out,-, cell Outer cells. Polar bint it Outer cell- h titer cells. Outer cells. Inner cell ( luter cells. FlG. 16. — Diagrams showing the segmentation of the mammalian ovum and the formation of the blasto- dermic vesicle (Allan Thomson, after van Beneden). THE FORMATION <>!•' THE l< rODERM AND ENTODERM 37 mammal represents a precocious development of cells which, in the bird, later envelop the yolk. The cavity of the vesicle is to be compared, not with the blastula cavity of Amphioxus and the frog but with the yolk mass plus the rudi- mentary blastoeoel of the bird's ovum. The mammalian ovum, although almost devoid of yolk, thus develops much like the yolk-laden ova of reptiles and birds. It- segmentation, however, is complete and the early stages in its development arc- abbreviated. A B Blastula cavity D Yolk cavity Fig. 17. — Diagrams showing the blastula?: A, of Amphioxus; B, of frog, and C of chick; D, blastodermic vesicle of mammal. In Primates, but one stage in segmentation has been observed. This, a four-celled ovum of Macacus nemestrinus figured by Selenka, shows the cells nearly equal and oval in form. This ovum was found in the oviduct of the monkey and shows that, in Primates and probably in man, segmentation as in other mammals takes place normally in the oviducts. THE FORMATION OF THE ECTODERM AND ENTODERM The blastula and early blastodermic vesicle show no differentiation into layers. Such differentiation takes place later in all vertebrate embryos and the three primary germ layers, ectoderm, entoderm and mesoderm, are formed. From these three layers all of the body tissues and organs are derived. Gastrulation. — In the case of Amphioxus and amphibia the entoderm is 38 SEGMENTATION OF THE FERTILIZED OVUM formed by a process termed gastrulation. The larger cells at the vegetal the blastula either fold inward (invaginate) or are overgrown by the more dividing micromeres. Eventually the invaginating cells obliterate the blastula cavity and come into contact with the outer layer of cells (Fig. 18). The new cavity formed is the primitive gut, or archenteron. The mouth of this cavity is the blastopore. The outer layer of cells is the ectoderm, the inner, newly formed layer is the ento- derm. The entodermal cells are henceforth concerned in the nutrition and metabolism of the body. The embryo is now termed a Gastrula (little stomach). The Origin of the Entoderm in Reptilia, Birds and Mammals. — Here the entoderm arises in quite a differ- ent manner. Instead of a process of gastrulation by in- vagination of cells we have first a process of delamination. Cells are split off or delaminated from the under side of the germinal area, arrange themselves in a definite inner layer, and thus the yolk entoderm is formed. This layer is already apparent in a longitudinal section through the germinal area of a chick (Tig. 19). In mammals like the pole of rapidly y<* -ulation of amphioxus (modified from Hatschek). .1, Blastula; az, animal rolls; vz, vegetative cells; fh, cleavage-cavity. ginning invagination of vegetative pole. C, Ciastrula stage, the -nation of the v etative cells being complete; ak, outer germ- ik, inner germ-layer; nd, archenteron; u, blastopore (Heisler). "3 '-: ** «V: M 0 fa CHE FORMATION' ()F THE ECTODERM AND ENTODERM 39 rabbit, the entoderm is split from the under side of the germinal area ami the cells soon grow around the inside of the blastodermic vesicle, and form an inner entodermal sae (Fig. 16). In Tarsius, a creature classed by Hubre. ht Ectoderm of embryonic disc Blastopore Ectoderm Yolk entoderm Blastopore Ectoderm 'Completion plate " Protentoderm Yolk entoderm Blastopore ■J , < o • • * i> t *,. i • ' - — Peristomal mesoderm .ff,'.,*.'. • •-•-!£££ ■ .' *fc ] / " Completion plate " Peristomal mesoderm Completion plate" Peristomal mesoderm "Completion plate " Fig. 20. — From medial vertical sections through embryonic disk of lizard, showing five successive in gastrulation (Wenckeback, Bonnet;. with the Primates, the entoderm cells, after splitting off, do not grow around the wall of the vesicle as in the rabbit, but soon form an entodermal sac separated by a space from the troph-ectoderm layer. Just how the vesicle is 40 SEGMENTATION OF THE FERTILIZED OVUM Fig. 21. — Two germ-discs of hen's egg in the first hours of incubation (after Roller in Heislerj : df, area opaca; /;/, area pellucida; s, crescent; sk, crescent-knob; es, embryonic shield; pr, primitive groove. formed is not known. It is attached only to the cells of the germinal area. Although this stage has not been observed in the human embryo it is probable from the structure of the youngest known human embryo that an entodermal vesicle is formed in much the same way as in Tarsius. Gastrulation in Reptiles and Birds. — After the formation of the primary or yolk entoderm in reptiles and birds, a process of invagination takes place which has been compared to gastrulation in Amphioxus and Amphibia. In the lizard after the primary entoderm has developed by delamination a curved depression is formed at the posterior border of the germinal area. There is a true invagina- tion of cells at this point (Fig. 20). The cells grow cephalad and contain an invagination cavity. Later the floor of the cavity fuses with the yolk entoderm and disappears. The cells of the roof persist as the dorsal or notochordal plate. The crescentic depression, at which point invagination occurs, may be compared a to the blastopore of Amphioxus. The invagination cavity is the gastrula cavity or archenteron and the dorsal plate represents the entodermal roof of the gastrula cavity. In the bird, invagination takes place at a crescentic groove (Fig. 21), but the ingrowing cells form a solid plate, at first without an invagination cavity. The result is the same, however, the formation of a notochordal plate which lies beneath the ectoderm. The crescentic groove is interpreted as representing the blastopore (Fig. 19). Formation of the Primitive Streak in Birds. — The germinal area increases in siz< by growth about its periphery, new cells constantly being formed here until eventually the germinal area surrounds the yolk. According to the interpreta- tion of Duval and Hertwig, as the periphery of the germinal area extends itself, a middle point in the cranial lip of the crescentic groove remains fixed while the "Ns Jl f/Y if \\\\ Fig. 22. — Diagram elucidating the forma- tion of the primitive groove (after Duval). The increasing size of the germ-disc in the course of the development is indicated by dot- ted circular lines. The heavy lines represent the crescentic groove and the primitive groove which arises from it by the fusion of the edges of the crescent (Heisler). TIIK FORMATION OF THE ECTODERM AND ENTODERM 41 edges of the lip on each side arc carried caudad and brought together. Thus the crescent is transformed into a longitudinal slit, as in Fig. 26. The lips of the slit fuse, and the line of fusion is marked by the longi- tudinal primitive groove. This interpretation of the primitive groove and streak, shown in Fig. 22, is known as the con- crescence theory. According to the theory, thecrescentic groove of reptiles and birds is homo- logous with the blastopore of Amphioxus. As it is trans- formed into the primitive streak and groove, these represent a modified blastopore. Accord- ing to this view, a large part of the entoderm of birds, rep- tiles, and mammals is formed by gastrulation, as in Amphi- oxus. Gastrulation in Mammals. — As in reptiles and birds so also in mammals, a process resembling gastrulation takes place but after the formation of the xolk entoderm. A primitive streak appears at the posterior border of the germinal Fig. 23. — The primitive streak of pis,' embryos (Kei- bel). A, embryo with primitive streak and primitive node; B, a later embryo in which the medullary groove is also present, cephalad in position. Post opening of notochord canal Pri.-nitivf streak /Intopenina of /Int. persisting portion of notochord canal notochordal canaJ Fig. 24. — Median longitudinal section through the blastoderm of a bat (Vcspcrlilio minimis) (after Van Beneden). area (Fig. 23), with a crescentic opacity corresponding to the crescentic groove of birds. Longitudinal sections (Fig. 24) of the germinal area of the bat show the formation of a dorsal or notochordal plate which has replaced, and is fused later- ally with, the yolk entoderm. A blastopore or notochordal canal is present lead- 42 SEGMENTATION OF THE FERTILIZED OVUM ing from the dorsal surface of the germinal area into the space beneath the ento- derm, the archenteron. No gastrulation stage for the human embryo has yet been observed but the primitive streak may be recognized in later stages (Fig. 73 A). There is also evidence of an opening, the notochordal or neat -enteric canal, leading from the exterior into the cavity of the primitive gut (archenteron). According to the view of Keibel and Hubrecht, the invagination of cells to form the noto- chordal plate in reptiles, birds and mammals is a secondary process not to be compared with formation of the entoderm by gastrulation, as in Amphioxus. The notochordal plate is not entodermal but ectodermal, and the primitive streak cannot be compared in its entirety to the blastopore of Amphioxus. THE ORIGIN OF THE MIDDLE GERM LAYER (Mesoderm), NOTOCHORD, AND NEURAL TUBE Amphioxus. — The dorsal plate of entoderm, which forms the roof of the archenteron, gives rise to paired lateral diverticula or ccelomic pouches (Fig. 25). These separate both from the plate of cells in the mid-dorsal line (which form the notochord), and from the entoderm of the gut, and become the primary mesoderm. Fig. 25. Origin of the mesoderm in Amphioxus (after Hatschek). n.g., neural groove; n.c, neural Mesoderm Primitive node Entoderm Ectod Primitive sTreaK Mesoderm Fig. 28. — Transverse sections through the embryonic area of a twenty-hour chick. A, through the head process; B, through the primitive node; C, through the primitive streak. X 165. laterally ('Fig. 28 C). In the midline, a depression in the ectoderm is the primitive groove. In this region there is no line of demarcation between ectoderm and mesoderm. A transverse section through the primitive node (Fig. 28 B. guide line H. Fig. 26) shows in this region the marked proliferation of cells, whir b an- growing cephalad to form the notochordal plate (head process). A transverse section through the notochordal plate just beginning to form at this stage ('Fig. 28 A, guide line A, Fig. 26) shows the thickening near the midline which will separate from the lateral mesoderm and form the notochord. THE ORIGIN OF THE MIDDLE GERM LAYER 45 After the notochordal plate becomes prominent at twenty hours the dif- ferentiation of the germinal area is rapid. A curved fold, involving the three layers of the germinal area, is formed cephalad to the notochordal process. This is the head-fold and is the anlage of the head of the embryo (Fig. 27). The ecto- derm has thickened on each side of the mid-dorsal line, forming the neural folds. The groove between these is the neural groove. The closure of this groove will form the neural tube, the anlage of the central nervous system. The notochord is now differentiated from the mesoderm and may be seen in the mid-dorsal line through the ectoderm. In the mesoderm lateral to the notochord and cephalad to the primitive node, transverse furrows have differentiated a pair of mesodermal segments. As development proceeds these increase in number, successive pairs being developed caudally. They will be described in detail later. To sum up, in the chick the mesoderm appears with the formation of the primitive streak. It originates from the primitive streak and node and spreads in all directions between the other germ layers as an undivided plate of cells. It grows cephalad in the midline as the notochordal process or plate from which the notochord is developed. As the mesoderm is derived from the entoderm in Amphioxus, its origin is generally regarded as entodermal in birds and mammals. This would certainly be the case if we interpret the notochordal process and entoderm as formed by a process of gastrulation. Keibel (in Kernel and Mall, vol. I), however, holds that the mesoderm and notochordal plate are derived from the ectoderm, and that any relation which they bear to the entoderm is of secondary origin. The Origin of the Mesoderm in Mammals. — As we have seen, the primitive streak is formed on the surface of the germinal area in mammalian embryos as in the chick. It has been described as due to a keel-like thickening of the ecto- derm, and the knob-like mass of cells at its cephalic end, the primitive node, is the first to appear. The mesoderm is formed precisely as in the chick, growing out in all directions from the primitive streak and node between the other two layers. Its extent in rabbit embryos is shown in Fig. 29 A and B. Cranial to the primitive node the notochord is differentiated in the midline, the meso- derm being divided into two wings. The mesoderm rapidly grows round the wall of the blastodermic vesicle until it finally surrounds it and the two wings fuse ventrally (Fig. 30 A and B). The single sheet of mesoderm soon splits into two, the cavity between being the co:lom or body cavity. The outer mesodermal layer (somatic), with the ectoderm, forms the somatopleure or body wall, the inner splanchnic layer, with the entoderm, forms the intestinal wall or splanclinopleure. The neural tube having in the meantime been formed from the neural folds of the 46 SEGMENTATION OF THE FERTILIZED OVUM ectoderm, we have the ground plan of the vertebrate body, the same in man as in Amphioxus. The origin of the mesoderm in the human embryo is unknown, but in Tarsius it has two sources, (i) The primary mesoderm derived by delamination from the Fig. 20. — Diagrams showing the extent of the mesoderm in rabbit embryos (Kolliker). In .4 the mesoderm is represented by the pear-shaped area at the caudal end of the embryonic area; in B by the circular area which surrounds the embryonic area. ectoderm at the caudal edge of the germinal area. This forms the extra-embryonic mesoderm and takes no part in forming the body of the embryo. (2) The secondary or intraembryonic mesoderm, which gives rise to body tissues, takes Ectoderm Mesoderm Entoderm Mesodermal segment Ectoderm Archenteron Entoderm Archenteron Neural iube /Vephrotome Notochord Splanchnic mesoderm Coelom I i'.. jo.— Diagrams showing the origin of the germ layers of mammals as seen in transverse section (modified from Bryce). its origin from the primitive streak and node as in the chick and lower mammals. The origin of the mesoderm in human embryos is probably much the same as in Tarsius. The Notochord. — In mammals and in man the notochordal plate is described THE ORIGIN OF THE MIDDLE GERM LAYER 47 as taking its origin directly from the entoderm. Keibel points out that this con- nection of the notochord is only secondary. The notochordal process grows cephalad from the primitive node and the tissue from which it is derived is of ectodermal origin, according to Keibel's view. In later stages, the notochord extends in the midline beneath the neural tube from the tail to a dorsal out-pocket- ing of the oval entoderm known as Seessel's pocket. It becomes enclosed in the centra of the vertebrae and in the base of the cranium, and eventually degenerates. In Amphioxus, it forms the only axial skeleton and it is persistent in the axial skeleton of fishes and Amphibia. In man, traces of it are found as pulpy masses in the intervertebral discs. CHAPTER III THE STUDY OF CHICK EMBRYOS In the following descriptions we shall use the terms dorsad and ventrad to indicate "towards the back" or "towards the belly"; cephalad and cranially to denote "headwards," caudad to denote "tailwards," and laterad when the loca- tion is at the side. As there is no single word in English to express the primi- tive cellular germ of a structure, the German word anlage has been adopted by embryologists and will be used here. Chick embryos may be studied whole and most of the structures identified up to the end of the second day. The eggs should be opened in normal saline solution at 400 C. With scissors, cut around the germinal area, float the embryo off the yolk and remove the vitelline membrane. Then float the embryo dorsal side up on a glass slide, remove enough of the saline solution to straighten wrinkles, and carefully place over the embryo a circle of tissue paper with opening large enough to leave the germinal area exposed. Add a few drops of fixative (5 per cent, nitric acid gives good fixation) and float embryo into a covered dish. After fix- ing and hardening, stain in acid Hematoxylin (Conklin) or in acid Carmine. Extract sur- plus stain, clear, and mount on slide supporting cover-slip to prevent crushing the embryo. Acid Haematoxylin gives the best results for embryos of the first two days. For a detailed ac- count of embryological technique see Lee's "Microtomist's Vade Mecum." EMBRYO OF SEVEN SEGMENTS (TwTiNTY-FIVE HOURS' INCUBATION) In this embryo (Fig. 31) there is a prominent network of blood-vessels and blood-cells in the caudal portion of the area opaca. In its cranial portion isolated groups of blood and blood-vessel forming cells are seen as blood-islands. To- gether, they constitute the angioblast from which arises the blood vascular system. The area pellucida has the form of the sole of a shoe with broad toe directed for- ward. The head-fold has become cylindrical and the head of the embryo is free for a short distance from the germinal area. The mesoderm extends on each side beyond the head leaving a median clear space, the proamniotic area. The en- toderm is carried forward in the head-fold as the fore-gut, from which later arise the pharynx, esophagus, stomach and a portion of the small intestine. The opening into the fore-gut faces caudad and is the fovea cardiaca. The way in which the entoderm is folded up from the germinal disc and forward into the head is seen well in a longitudinal section of an older embryo (Fig. 32). The 48 EMBRYO OF SEVEN SEGMENTS 49 tubular heart lies ventral to the fore-gut and cranial to the fovea cardiaca. In later stages it is bent to the right. Converging forward to the heart on each side of the fovea are the vitelline veins just making their appearance at this stage. The lips of the neural folds have met throughout the cranial two-thirds of the embryo but have not fused. The neural tube formed thus by the closing of the ectodermal folds is open at either end. Cephalad, the neural tube has begun to interior neuropore Forebram ■ ■: m Pharynx Left vitelline M *t vein Me so derma Segment 3 Irea \ pellucida Nolochord Area opaca Primitive sfreaK Free portion of head ^ Fovea cardiaca Bight vitelline vein \~y yf — &- Neural groove Segmental ■L Primitive node island Fig. 31. — Dorsal view of a twenty-hve-hour chick embryo with seven primitive segments. X 20. expand to form the brain vesicles. Of these only the fore-brain is prominent, and from it laterally the optic vesicles are budding out. The paraxial mesoderm is divided by transverse furrows into seven pairs of primitive segments. Caudally between the segments and the primitive streak there is undifferentiated meso- derm, but new pairs of segments will develop in this region. Looking through the open neural tube (rhomboidal sinus), one may see in the midline the chorda dor- salis extending from the primitive node cephalad until it is lost beneath the 4 5° THE STUDY OF CHICK EMBRYOS neural tube in the region of the primitive segments. The primitive streak is still prominent at the posterior end of the area pellucida, forming about one-fourth the length of the embryo. Transverse sections through the primitive streak and open neural groove show approximately the same conditions as in the twenty-hour embryo (Figs. 26 and 28). A Transverse Section through the Fifth Primitive Segment (Fig. 33) is characterized by the differentiation of the mesoderm, the approxi- mation of the neural folds and the presence of two vessels, the descending aortae on each side between the mesodermal segments and the entoderm. The neural folds are thick and the ectoderm is thickened over the embryo. The notochord is a sharply de- fined oval mass of cells. The mesodermal segments are somewhat triangular in outline and connected by the intermediate cell mass with the lateral meso- derm. This is partially divided by irregular flat- tened spaces into two layers, the dorsal of which is the somatic, the ventral the splanchnic layer of mesoderm. Later, the spaces unite on either side to form the ccelom or primitive body cavity. Transverse Section Caudal to the Fovea Cardiaca (Fig. 34). — The section is characterized (1) by the closing together of the neural folds to form the neural tube; (2) by the dorsad and laterad folding of the entoderm which, a few sections nearer the head end, forms the fore-gut or pharynx; (3) by the presence of the vitelline veins laterally be- tween the entoderm and mesothelium; (4) by the wide separation of the somatic and splanchnic mesoderm and the consequent increase in the size of the ccelom. In this region, it later sur- rounds the heart and forms the pleuro- pericardial cavity. The neural tube in this region forms the third brain vesicle or hind-brain. The neural folds have not yet fused. Mesodermal segments do not develop in Fig. 32. — Median longitu- dinal section of a thirty-six-hour chick embryo (Marshall). AN, amnion fold; BF, fore-brain; BH, hind-brain; BM, mid-brain; CII, notochord; CP, pericardial cav- ity; GF, fore-gut; //, entoderm; .V.S', spinal cord; NT, neurenteric canal; PS, primitive streak; RV, ventricle of heart; SO, somato- pleure; SP, splanchnopleure; TA, allantois. KMI'.KYu <»|' SIA'KN SKCMKNTS 51 this region, instead a diffuse network of mesoderm partly fills the space between ectoderm, entoderm and mesothclium. This is termed mesenchyma and will be described later. Neural plate Neural groove Ectoderm \ / .Mesodermal segment Oomatic mesoderm \ jfex 'iS^. / Coelo Splanchnic mesoderm \ Notochord Descending aorta ' Entoderm Fig. 33. — Transverse section through the fifth pair of mesodermal segments of a twenty-five-hour chick embryo. X 90. Neurci enst Fig. 34. — Transverse section caudal to the fovea cardiaca of a twenty-five-hour chick embryo. X 90. Neural tube Deseending aorta Notochord Oplanchnic mesoderm (Myocardium) Coelo, Ectoderm O0/71. mesoderm Splanchnic mesoderm // Endothelium of heart tube Entoderm FlG. 35. — Transverse section through the fovea cardiaca of a twenty-five-hour chick embryo. X 90. Transverse Section through the Fovea Cardiaca (Fig. 35).— This section is marked by a vertical layer of the entoderm at the point where it is folded into the head as the fore-gut. The entoderm is thickened laterally and forms a continuous mass of tissue between the vitelline veins. The splanchnic mesoderm 52 THE STUDY OF CHICK EMBRYOS is differentiated into a thick walled pouch on each side lateral to the endothelial layer of the veins. Transverse Section through the Heart (Fig. 36). — As we pass cephalad in the series of sections the vitelline veins open into the heart just in front of the fovea cardiaca. The entoderm in the head-fold now forms the crescentic pharynx or fore-gut separated by the heart and splanchnic mesothelium from the entoderm of the germinal disc. The descending aortas are larger, forming conspicuous spaces between the neural tube (hind-brain) and the pharynx. The heart, as will be seen, is formed by the union of two endothelial tubes, similar to those which form the walls of the vitelline veins in the preceding sections. The median walls of these tubes disappear at a slightly later stage to form a single tube. Thick- Ectoderrr, Somatic mesoder Notochord Myocardium Entoderm ■Neural tube Descending aorta Pharynx. Endocardium Splanchnic rnes. Fig. 36. — Transverse section through the heart of a twenty-five-hour chick embryo. X 90. ened layers of splanchnic mesoderm which, in the preceding section, invested the vitelline veins laterally, now form the mesothelial wall of the heart. In the median ventral line, the layers of splanchnic mesoderm of each side have fused, separated from the splanchnic mesothelium of the germinal disc and thus the two pleuro-pericardial cavities are in communication. The mesothelial wall of the heart forms the myocardium and epicardium of the adult. Dorsally, the splanch- nic mesoderm is continuous with the somatic mesoderm and forms the dorsal mesocardium. Origin of Primitive Heart. — From the two sections just described, it is seen that the heart arises as a pair of endothelial tubes lying in the pockets of the splanchnic mesoderm. Later, the endothelial tubes fuse to form a single tube. The heart then consists of an endo- thelial tube within a thick-walled tube of mesoderm. The origin of the endothelial cells of the heart is not surely known. They may be split from the entoderm, or arise from the mesoderm. According to another view, the endothelium arises in the vascular area and grows into the body EMBRYO OF SKVKN SKOMKMS 53 of the embryo. The vascular system is primitively a paired system, the heart arising as a double tube with two veins entering and two arteries leaving it. Origin of the Blood-vessels and Blood. We have Been that in the area opaca a network of blood-vessels and blood-islands are differentiated as the angioblast. This tissue gives rise to all of the primitive blood-vessels and blood-cells and probably is derived from the splanchnic mesoderm. The vessels arise first as reticular masses of cells, the so-called blood- islands. These cellular thickenings undergo differentiation into two cell types, the innermost becoming blood-cells, the outermost forming a flattened endothelial layer which encloses the blood-cells. All the primitive blood-vessels of the embryo are composed of an endothelial layer only. The endothelial cells continue to divide, forming vascular sprouts and in this way new vessels are produced. The first vessels arising in the vascular area of a chick embryo form a close network, some of the branches of which enlarge to form vascular trunks. One pair of such trunks, the vitelline veins, is differentiated opposite, and later connects with, the posterior end of the heart. Another pair, the vitelline arteries, are developed in connection with the aortae of the embryo. The vessels of the vascular area thus appear before those of the embryo have developed, probably arise from the splanchnic mesoderm, and, both arteries and veins, are com- posed of a simple endothelial wall. As the ccelom develops in the region of the vascular area of the embryo soon after the differentiation of the angioblast the anlages of the blood-vessels are formed only in the splanchnic layer. (For the development of the heart and blood-vessels see Chapter IX.) ectoderm Afesenclyma Moto chord . Pharynqeal membrane— Entoderm '^p-^'^^^"**'55^ ""^^r^g^ of proamnion XT ^* Bag—*"'''!!!— .»i'Aji' ' \S Fig. 37. — Transverse section through the pharyngeal membrane of a twenty-five-hour chick embryo. X 90. Transverse Section through the Pharyngeal Membrane (Fig. 37). — This section passes through the head-fold and shows the head free from the underlying germinal area. The ectoderm surrounds the head and near the mid-ventral line is bent dorsad, somewhat thickened, and in contact with the thick entoderm of the pharynx. The area of contact between ectoderm and pharyngeal entoderm forms the pharyngeal plate or membrane. Later, this membrane breaks through and thus the oral cavity arises. The expanded neural tube is closed in this region and forms the middle brain vesicle or mid-brain. The dorsal aortae appear as small vessels dorsal to the lateral folds of the pharynx. The germinal area in the region beneath the head is composed of ectoderm and entoderm only. This is the proamniotic area. Laterad may be seen the layers of the mesoderm. ryrrgea/ membrane— tj4 THE STUDY OF CHICK EMBRYOS Transverse Section through the Fore-brain and Optic Vesicle (Fig. 38). — The neural tube is open here and constitutes the first brain vesicle or fore-brain. The opening is the anterior neuropore. The ectoderm is composed of two or three layers of nuclei and is continuous with the much thicker wall of the fore-brain. The lateral expansions of the fore-brain are the optic vesicles, which eventually give rise to the retina of the eye. The two ectodermal layers are in contact Ectoderm Optic vesicle ^ V?»w '^ Neuropore Neural tube "roamrtion <4&. Pr Fig. 38. — Transverse section through the fore-brain and optic vesicles of a twenty-five-hour chick. X 90. with each other except in the mid-ventral region, where the mesenchyma is beginning to penetrate between and separate them. The proamnion consists of a layer of ectoderm and of entoderm. CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS (THIRTY-SIX HOURS) The long axis of this embryo is nearly straight (Fig. 39), the area pellucida is dumb-bell shaped and the vascular network is well differentiated throughout the area opaca. The tubular heart is bent to the right, and opposite its posterior end the vascular network converges and becomes continuous with the trunks of the vitelline veins. Connections have also been formed between the descending aortae and the vascular area, but as yet the vitelline arteries have not appeared as distinct trunks. The proamniotic area is reduced to a small region in front of the head, which latter is now larger and more prominent. In the posterior third of the vascular area blood-islands are still prominent. Central Nervous System and Sense Organs. — The neural tube is closed save at the caudal end where the open neural folds form the rhomboidal sinus. In the head the neural tube is differentiated into the three brain vesicles marked off from each other by constrictions. The fore-brain (prosencephalon) is charac- terized by the outgrowing optic vesicles. The mid-brain (mesencephalon) is undifferentiated. The hind-brain (rhombencephalon) is elongated and gradually merges caudally with the spinal cord. It shows a number of secondary constric- tions, the neuromeres. The ectoderm is thickened laterally over the optic ves- CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 55 icles to form the lens placode of the eye (Fig. 41). The optic vesicle is flattened at this point and will soon invaginate to produce the inner, nervous layer of the retina. In the hind-brain region, dorso-laterally the ectoderm is thickened and invaginated as the auditory placode (Fig. 43). This placode later forms the otocyst or otic vesicle from which is differentiated the epithelium of the internal ear (membranous labyrinth). Forsbram Mid-brain \ Hind -bra in Vitelline Vein _^ Mesodermal', segment 8 Notochord ~± Proamnion Optic vesicle Free portion of he-ad Heart Neural tube Bhomboidal sinus Primitive streaK Fig. 39. — View of the dorsal surface of a thirty-six-hour chick embryo. X 20. Digestive Tube. — The entoderm is still flattened out over the surface of the yolk caudal to the fovea cardiaca. In Fig. 40 the greater part of the entoderm is cut away. The flattened fore-gut, folded inward at the fovea, shows indications of three lateral diverticula, the pharyngeal pouches. Cephalad the pharynx is closed ventrally by the pharyngeal membrane. 56 THE STUDY OF CHICK EMBRYOS Heart and Blood-vessels. — As seen in the dorsal view of the embryo, the heart tube is bent to the right. Viewed from the ventral side, the bend is to the left (right of embryo) (Fig. 40). After receiving the vitelline veins cephalad to the fovea cardiaca the double-walled tube of the heart dilates and bends ventrad Optic vesicle- Paired ventral aorta' Ventral aorta Bulbus cordis Ventricle Splanchnic mesoderm Fovea cardiaca R. descending aorta K2T]|*~1 Vascular plexus — vyf I 'a Splanchnic mesoderm Xotochord \ / [• \ Mes. segment Segmental zone Fore-brain Phar. pouch I Descending aorta Phar. pouch II Somatopleure Left vitelline vein Entoderm Section medullary tube Somatopleure Descending aorta Medullary tube Splanchnic mesoderm Capillary plexus Somatopleure Neural groove In.. 40. — Ventral reconstruction of a thirty-six-hour chick embryo. The entoderm has been removed save about and caudal to the fovea cardiaca. X 38. and to the right (left of Fig. 43). It then is flexed dorsad and to the median line, and narrows to form the ventral aorta. The aorta lies ventrad to the pharynx and divides at the boundary line between the mid- and hind-brain into two ventral aorta. These diverge and course dorsad around the pharynx. Before CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 57 reaching the optic vesicles they bend caudad, and as the paired descending aortae may be traced to a point opposite the last primitive segments. In the region of the fovea cardiaca they lie close together and have fused to form a single vessel, the dorsal aorta. They soon separate and opposite the last primitive segments they are connected by numerous capillaries with the vascular network. In this region at a later stage the trunks of the paired vitelline arteries will be differen- tiated. The heart beats at this stage, the blood flows from the vascular area by way of the vitelline veins to the heart, thence by the aortae and vitelline arteries back again. This constitutes the vitelline circulation and through it the embryo receives nutriment from the yolk for its future development. Lens an/aye Fig. 41. — Transverse section through the fore-brain of a thirty-six-hour chick embryo. X 75. In studying transverse sections of the embryo the student should not only identify the structures seen, but should locate the level of each section by compar- ing with Figs. 39 and 40, and trace the organs from section to section in the series. Transverse Section through the Fore-brain and Optic Vesicles. (Fig. 41). — The optic stalks connect the optic vesicles laterally with the ventral portion of the fore- brain. Dorsally the section passes through the mid-brain. We have alluded to the thickening of the lens placode. Note that there is now a considerable amount of mesenchyma between the ectoderm and the neural tube. In the germinal area the layers of mesoderm are present. Transverse Section through the Pharyngeal Membrane and Mid-brain (Fig. 42). — In the mid-ventral line the thickened ectoderm bends up into contact with the entoderm of the rounded pharynx. At this point the oral opening will break through. On either side of the pharynx a pair of large vessels are seen; the ventral pair are the ventral aorta-. Two sections cephalad their cavities open into those of the dorsal pair, the descending aorta:. The section is thus just caudad to the point where the ventral aorta? bend dorsad and caudad to form the descending aorta?. The section passes through the caudal end of the mesencephalon which is here thick walled with an oval cavity. Note the large amount of undifferentiated 58 THE STUDY OF CHICK EMBRYOS mesenchyma in the section. The structure of the germinal area is complicated by the presence of collapsed blood-vessels. Transverse Section through the Hind-brain and Auditory Placodes (Fig. 43). — Besides the auditory placodes already described as the anlages of the internal ear, this section is characterized (1) by the large hind-brain, somewhat flattened dorsad; (2) by the broad dorso-ventrahy flattened pharynx, above which on each side he the dorsal aortce; (3) by the Ectoderm Mesenchyma Descending aorta Neural tube Notochord Forequt Pharyngeal membrane Splanchnopleure Fig. 42. — Transverse section through the pharyngeal membrane of a thirty-six-hour chick embryo. X7S- Ectoderm Notochord Descending Qorta Pericardial cavity Somatic fne Neural tube Ant Cardinal Vein Auditory placode Forego.! Ectoderm Endothelium of heart vv toderm Endothelium of ventral aorta Myocardium Fig. 43. — Transverse section through the hind-brain and auditory placodes of a thirty-six-hour chick embryo. X 75. presence of the ventral aorta and bulbar portion of the heart. The descending aortae are located on each side dorsal to the pharynx. The ventral aorta is suspended dorsally by the mesoderm, which here forms the dorsal mesocardium. The bulbus of the heart lies to the left in the figure fright of embryo) and a few sections caudad in the series is continuous with the ventral aorta (see Fig. 40). Between the somatic and splanchnic mesoderm is the large pericardial cavity. It surrounds the heart in this section. CHICK EMBRYO OF EIGHTEEN PRIMITIVI SEGMENTS 59 Transverse Section through the Caudal End of the Heart (Fig. 44. )— The section passes through the hind-brain. The descending aorta- are separated only by a thin septum which is ruptured in this section. The mesothelial wall of the heart is continuous with the somatic mesoderm. On the right side of the section there is apparent fusion between the myocardium of the heart and the somatic mesoderm. Lateral to the aorta." are the anterior cardinal Ectoder, Mes. Ant. cardinal Vein 'eural tube Foreou-V Splanchnic mesoderm Entoderm Vitelline vein / """> — ^ Splanchnic mesoderm Heart I tyocardium Fig. 44. — Transverse section through the caudal end of the heart of a thirty-six-hour chick embryo. X75- sSomdtopleure Edoderr Mes Segment Dorsal aorta. Coelon Neural tube chord Extra embryonic coelom ^Entoderm L.vit. ve/' _^~- A^es. segment Archenkron — 7^^£^S Splanchnic — iff tiff mesoderm III Ijff "9p»Si^§n?^^^— ^ Ectoderm ^n&. \^ Somatic \&. ^* mesoderm Entoderm — 1| jp |i vs^* 1/ with the mesonephros, and as the kidney of the adult (metanephros) is partly developed as an outgrowth of the primary excretory duct, we may regard the intermediate cell mass as the anlage of the urogenital glands and their ducts. They are thus of mesodermal origin. Somatopleure and Splanchnopleure. — In the embryo of seven primitive seg- ments we saw that the mesoderm split laterally into two layers, the somatic (dorsal) and the splanchnic (ventral) mesoderm. These layers persist in the adult, the somatic mesoderm giving rise to the pericardium of the heart, to the parietal pleura of the thorax and to the peritoneum of the abdomen, while the splanchnic layer forms the epicardium and myocardium of the heart, visceral pleura of the lungs, the mesenteries and mesodermal layer of the gut. The somatic mesoderm and the ectoderm with the tissue developed between them con- stitute the body wall, which is termed the somatopleure. In the same way the splanchnic mesoderm and the entoderm with the mesenchymal tissue between them constitute the wall of the gut, termed the splanchnopleure. Ccelom. — The cavity be- tween the somatopleure and splanchnopleure is the ccelom (body cavity). With the splitting of the mesoderm, isolated cavities are produced. These unite on each side and eventually form one cavity — the ccelom. With the extension of the mesoderm, the ccelom surrounds the heart and gut ventrally (Fig. 52). Later, it is subdivided into the pericardial cavity of the heart, the pleural cavity of the thorax and the peritoneal cavity of the abdominal region. In the stages we have studied, the embryo is flattened on the surface of the yolk and the somatopleure and splanchnopleure do not meet ven- trad. If this were the case we should have the structural relations as in Fig. 52 which is essentially the ground plan of the vertebrate body. Mesenchyma. — In the sections through the head of this embryo and through that of the preceding stage, we have found but three primitive segments present. The greater part of the mesoderm in the head appears in the form of an undif- Fig. 52. \Coelom -Diagrammatic transverse section of a vertebrate embryo (adapted from Minot). 64 THE STUDY OF CHICK EMBRYOS ferentiated network of cells which fill in the spaces between the definite layers (epithelia). This tissue is mesenchyma (Fig. $$). The mesoderm may be largely converted into mesenchyma as in the head, or any of the mesodermal layers may contribute to its formation. Thus it may be derived from the primitive segments and from the somatic and splanchnic mesoderm. The cells of the mesenchyma form a syncytium or network, and are at first packed closely to- gether. Later, they may form a more open network with cytoplasmic processes extending from cell to cell (Fig. 53). The mesenchyma is an important tissue of the embryo, as from it are differentiated the blood and lymphatic systems, together with most of the smooth muscle, connective tissue, and skeletal tissue of the body. The body of the embryo is now com- posed (1) of cells arranged in layers — epi- thelia, and (2) of diffuse mesenchyma. The term "epithelium" may be used in a general sense or restricted to layers cover- ing the surface of the body or lining the digestive canal and its derivatives. Layers lining the body cavities are termed meso- thelia, while those lining the blood-vessels and heart are called endothelia. Derivatives of the Germ Layers. — The tissues of the adult are derived from the epithelia and mesenchyma of the three germ layers as follows: Ectoderm Mesen- chyma Fig. 53. — Mesenchyma from the head of a thirty-si.\-hour chick embryo. X 495. Ectoderm Mesoderm Entoderm I. Epidermis and its deriva- A Mesothelium. 1. Epithelium of digestive tives (hair, nails, glands). 1. Pericardium. tract. 2. Conjunctiva and lens of eye. 2. Pleura. 2. Liver. 3- Sensorj' epithelia of organs 3- Peritoneum. 3- Pancreas. of special sense. 4- Serous layer of intestine. 4- Epithelium of pharynx. 4- Epithelium of mouth, enam- el of teeth, oral glands. 5- Epithelium of uro- gans. genital or- Eustachian tube. Tonsils. Pituitary body. 6. Striated muscle. Thymus. 5- Epithelium of anus. 1. Skeletal. Thyreoids. 6. Epithelium of amnion and 2. Cardiac. Epithelial bodies. | OaA^V. -oa^ Epithelium of respiratory chorion. B Mesenchyma. 5- 7- Nervous, neuroglia and 1. Blood-cells. tract. chromaffin cells of nervous 2. Bone marrow. Larynx. system. Retina and optic 3- Endothelium of blood-vessels. Trachea. nerve. 4- Endothelium of lymphatics and Lungs. 8. Xotorhord (?) spleen. 6. Notochord (?) 9- Smooth muscle of sweat 5- Supporting tissues. (Connect- glands and of iris. ing tissue, cartilage and bone.) 6. Smooth muscle. CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 65 For the histological development (histogenesis) of the various tissues from the primary germ layers see Chapter X. CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS (FIFTY HOURS) This embryo, which is taken as a type of the forty-eight to fifty-two-hour stage, lies in the center of the vascular area and is peculiar in that the head is twisted qo° to the right. We therefore see the right side of the head but the dorsal side of the body. In the region of the mid-brain is the very marked head-bend mm Hind-brain Otic vesicle * Branchial clefts Amnion fold. M num w •^ Midbrain Ap-v-'Ni Fort-brain ^ n t- ■ i -•■ \ v^f'S Optic vesicle P^P^ Lens vesicle Ventricle of heart Vitelline vein I. Vitelline artery Neural tube AH Primitive streak (_fj^.i r A Vl Fig. 54. — Dorsal view of a fifty-hour chick embryo, stained and mounted in balsam. X 14. or cephalic flexure. Below the head and ventral in position lies the tubular heart, now bent in the form of a letter S. Dorsal to the heart in the region of the pharynx, three transverse grooves or slits may be seen. These are the branchial clefts or gill slits. The head of the embryo is now covered by a double fold of the somatopleure, the head-fold of the amnion. It envelops the head like a veil. 66 THE STUDY OF CHICK EMBRYOS Caudally a fold and opacity mark the position of the tail-fold from which de- velops the caudal end of the body. The curved fold embracing this is the tail- fold of the amnion which will eventually meet the head-fold and completely enclose the embryo. Mid-brain Optic Aperture of lens n Fore-brai Phary Bulb of heart Ventricle R. vitelline vein Fore-gut Splanchnopleure Splanchnic mesoderm Dorsal aorta R. vitelline artery Mes. segment Segmental zone Neural plate Entoderm Primitive node nd-brain Notochord Otocyst Aortic arches i, 2, 3 Ant. cardinal vein Atrium Common cardinal vein Post cardinal vein Descending aorta Liver anlage Fovea cardiaca Entoderm Somatopleure Spinal cord L. vitelline artery Edge of splanchnic mesoderm Mes. segment Vascular plexus Notochord Hind-gut FlG. 55. — Semi-diagrammatic reconstruction of a fifty-hour chick embryo, ventral view. X 22. The entoderm has been removed save in the region of the fovea cardiaca and of the hind-gut. The cranial third of the embryo is seen from the left side, the caudal two-thirds in ventral view owing to the torsion of the embryo. Central Nervous System and Sense Organs (Fig. 55). — The neural tube is divided by constrictions cephalad into four vesicles. The fore-brain of the previous stage is now subdivided into two regions, the telencephalon and dien- cephalon. The cephalic flexure has been established in the region of the mesen- cephalon. The hind-brain is as yet undivided and is as long as the other three CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 67 vesicles. The lens of the eye has invaginated, pushing in the wall of the optic vesicle and thus forming a double-walled structure, the optic cup. The audi- tory placode has become a sac, the otocyst, which overlies the hind-brain opposite the second branchial groove and is still connected with the outer ectoderm, cut away in Fig. 55. The rhomboidal sinus is still open at the caudal end of the neural tube (Fig. 54). Digestive Canal. — In a reconstruction from the ventral side the digestive canal shows differentiation into three regions. Of these, the fore-gut we have seen in earlier stages; the mid-gut is without ventral wall and overlies the yolk. A greater part of the mid-gut has been cut away to show the underlying struc- tures. Caudad, a small fovea leads into the hind-gut which is just beginning to evaginate into the tail-fold. The pharyngeal membrane now lies in a consider- able cavity, the stomodceum, formed by the invaginated ectoderm. The median ectodermal pouch next the brain-wall is known as Rathke's pocket and is the an- lage of the anterior lobe of the hypophysis. The pharynx shows laterally three out pocketings, of which the first is wing like and is the largest. These pharyngeal pouches occur opposite the three branchial grooves and at these points entoderm and ectoderm are in contact. Between them are developed the branchial arches, in which course the paired aortic arches. Towards the fovea cardiaca the fore-gut is flattened laterally and before it opens out into the mid-gut there is budded off ventrally a bilobed structure, the anlage of the liver (Fig. 60). It lies between the vitelline veins and in its later development the veins are broken up into the sinusoids or blood spaces of the liver. Just as the entoderm grows out into the head-fold to form the fore-gut so it grows into the tail-fold and forms the hind-gut. This at once gives rise to a tubular outgrowth which becomes the allantois, one of the fetal membranes to be described later. Blood Vascular System. — The tubular heart is flexed in the form of a letter S when seen from the ventral side. Four regions may be distinguished: (1) The sinus venosus, into which open the veins; (2) a dilated dorsal chamber, the atrium; (3) a tubular ventral portion flexed in the form of a U, of which the left limb is the ventricle, the right limb (4) the bulbus cordis. From the bulbus is given off the ventral aorta. There are now developed three pairs of aortic arches which open into the paired descending aortae. The first aortic arch passes cranial to the first pharyngeal pouch and is the primitive arch seen in the thirty-six-hour embryo. The second and third arches course on either side of the second pharyngeal pouch. They are developed by the enlargement 68 THE STUDY OF CHICK EMBRYOS of channels in primitive capillary networks between ventral and descending aortae. Opposite the sinus venosus the paired aortic trunks fuse to form the single dorsal aorta which extends as far back as the fifteenth pair of primitive segments. At this point the aortae again separate and opposite the twentieth segments each connects with the trunk of a vitelline artery which was developed in, and conveys the blood to, the vascular area (Fig. 55). Caudal to the vitelline arteries the dorsal aortae rapidly decrease in size and soon end. Hind-brain Notochord Ectoder Lens vesicle Cavity of . forebrcvn Ant.CardinaJ vein Aortic arch I Optic vesicle Prosencephalon Fig. 56. — Transverse section through the fore-brain and eyes of a fifty-hour chick embryo. X 50. As in the previous stage, the blood is conveyed from the vascular area to the heart by the vitelline veins, now two large trunks. In the body of the embryo there have developed two pairs of veins. In the head have appeared the anterior cardinal veins, already of large size and lying lateral to the ventral region of the brain vesicles (Fig. 58). Caudal to the atrium of the heart, two smaller posterior cardinal veins are developed. They lie in the mesenchyma of the somatopleure laterad in position (Fig. 60). Opposite the sinus venosus the anterior and pos- terior cardinal veins of each side unite and form the common cardinal veins (ducts CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 69 of Cuvier) which open into the dorsal wall of the sinus venosus. The primitive veins are thus paired like the arteries, and like them develop by the enlargement of channels in a network of capillaries. The following series of transverse sections from an embryo of this stage shows the more important structures. The approximate plane and level of each section may be seen by referring to Figs. 54 and 55. Blastoderm Hind-brain Mesenchy, Amnion Chorion Fig. 57. — Transverse section through the optic stalks and hypophysis of a fifty-hour chick embryo. X50. Section through the Fore-brain and Eyes (Fig. 56). — The section passes cranial to the optic stalks, consequently the optic vesicles appear unconnected with the fore-brain. The thickened ectoderm is invaginated to form the anlages of the lens vesicles. The thicker wall of the optic vesicles next the lens anlage will give rise to the nervous layer of the retina, the thinner outer wall becomes the pigment layer of the retina. Ventrad in the section are the wall and cavity of the fore-brain, dorsad the hind-brain with its thin dorsal ependymal layer. Be- tween the brain vesicles on either side are sections of the first aortic arches and lateral to the hind- brain are the smaller paired anterior cardinal veins, which convey the blood from the head to the heart. Section through the Optic Stalks and Hypophysis (Fig. 57). — The section passes just caudal to the lens which does not show. The optic vesicles are connected with the wall of the fore-bra in by the optic stalks which later form the path by which the fibers of the optic nerve pass from the retina to the brain. Both the ventral and the descending aorta: are seen in section 7° THE STUDY OF CHICK EMBRYOS about the cephalad end of the pharynx. Between the ventral wall of the fore-brain and the pharynx is an invagination of the ectoderm, Rathke's pocket. Section through the Otocysts and Second Aortic Arch (Fig. 58). — The otic vesicles are sectioned caudal to their apertures and appear as closed sacs lateral to the wall of the hind-brain. The cavity of the pharynx is somewhat triangular and its dorsad wall is thin. The anterior cardinal veins pass between the otocysts and the wall of the hind-brain. Ventral to the pharynx the bulbus cordis is sectioned obliquely where it leaves the heart, and at this level gives off laterad the second pair of aortic arches which connect dorsad with the descending aorta?. Surrounding the bulbus cordis is the large pericardial cavity. Between the first and second aortic arches (Fig. 58) is the first pair of pharyngeal pouches, lateral diverticula of the entoderm. The student should note that in the sections of this stage so far studied, the tnesen- Aartitarch 2 Ventral aorta. Endothelium of bulbus Fig. 58. — Transverse section through the otic vesicles and second aortic arches of a fifty-hour chick embryo. X 50. chyma of the head is undifferentiated, the tissues peculiar to the adult not yet having been formed. Section through the Sinus Venosus and Common Cardinal Veins (Fig. 59). — At this level, the common trunk formed by the anterior and posterior cardinal veins opens into the thin- walled sinus venosus. The sinus receives all of the blood passing to the heart and is separated only by a slight constriction from the larger atrium. The muscle plates of the first mesoflermal segments are seen, and the descending aorta; have united to form a single dorsal vessel. On either side of the pharynx are seen subdivisions of the ccelom which will form the pleural cavities. These cavities are separated from the pericardial cavity by the septum trans- versum in which the common cardinal veins cross to the sinus venosus. The folds of the amnion envelop the right side of the embryo and the ectoderm of these CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 71 folds now forms the outer layer of the chorion and the inner layer of the amnion. The meso- dermal folds of the amnion have not united. Chorion Spinal cord Mes. segment Common . card- vein Entoderm Myocardium Coeton Atrium Endothelium of heart Fig. 59. — Transverse section through the sinus venosus and common cardinal veins of a fifty-hour chick embryo. X 50. Chorion Fig. 60. — Transverse section through the anlage of the liver of a thirty-six-hour chick embryo. X 50. Section through the Anlage of the Liver (Fig. 60). — In this section the cavity of the fore-gut is narrow, the gut being flattened from side to side. Yentrad there are evaginated from the entoderm two elongate diverticula which form the anlages of the liver. On either side 72 THE STUDY OF CHICK EMBRYOS of the anlages of the liver are sections of the vitelline veins on their way to the sinus venosus at a higher level in the series. Note the intimate relation between the entodermal epithelium of the liver and the endothelium of the vitelline veins. In later stages, as the liver anlages branch, there is, as Minot aptly expresses it, "an intercrescence of the entodermal cells consti- tuting the liver and of the vascular endothelium" of the vitelline veins. Thus are formed the hepatic sinusoids of the portal system, which surround the cords of hepatic cells. The septum transversum is still present at this level and lateral to the fore-gut are small body cavities. Lateral to the body cavities appear branches of the posterior cardinal veins. Section through the Cranial Portion of the Open Intestine (Fig. 61). — The intestine is now open ventrad, its splanchnopleure passing directly over to that of the vascular area. The folds of the amnion do not join, leaving the amniotic cavity open. The dorsal aorta Splanchnic mesoderm Ectoderm Mes.segment Descending/ aorta Coelom Somato phure Fig. 6i. — Transverse section through the cranial portion of the open intestine of a fifty-hour chick embryo. X 50. is divided by a septum into its primitive components, the right and left aortcc. The ccelom is in communication with the extra-embryonic body cavity. Section through the Seventeenth Pair of Mesodermal Segments (Fig. 62). — The body of the embryo is now no longer flexed to the right. On the left side of the figure the mesodermal segment shows a dorso-lateral muscle plate. The median and ventral portion of the segment is being converted into mesenchyme. On the left side appears a section of the primary excretory or mesonephric duct. The embryonic somatoplcure is arched and will form the future ventro-lateral body wall of the embryo. The fold lateral to the arch of the somato- pleure gives indication of the later approximation of the ventral body walls, by which the embryo is separated from the underlying layers of the blastoderm. Section through the Origin of the Vitelline Arteries (Fig. 63). — At this level the embryo is more flattened and simpler in structure, the section resembling one through the mid-gut region of a thirty-six-hour chick (Fig. 47). The amniotic folds have not appeared. On the left side of the figure the vitelline artery leaves the aorta. On the right side the con- CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 73 nection of the vitelline artery with the aorta docs not show, as the section is cut somewhat obliquely. The other structures were described La connection with Fig. 47. Section Caudal to the Mesodermal Segments (Fig. 64). The mesodermal seg- ments are replaced by the segmental zone, a somewhat triangular mass of undifferentiated mesoderm from which later are formed the segments and nephrotomes. The notochord is larger, Afes. Segment" Descending aorta <5omatopleure Som. mes Spinal cord Ectoderm Notochord Somatic mesoderm Splanchnopleure Coelo -Splanchnic mesoderm ^Entoderm Fig. 62. — Transverse section through the seventeenth pair of mesodermal segments of a fifty-hour chick. embryo. X 50. Mes.segment. Nephrotome ■Sp'wa I cord Ectoderm Nephrotome Somatic mesoderm Somatopleure. tJplanchnopleurc Aorta Jj- Vitelline artery Fig. 63. — Transverse section of a fifty-hour chick embryo at the level of the origin of the vitelline arteries. X 50. Descending aorta Sim. mesoderm Spinal cord Ectoderm ^Splanchnic mesoderm Coelom Entoderm Notochord Fig. 64. — Transverse section of a fifty-hour chick embryo through the last pair of mesodermal segments. X50. the aortse smaller, and a few sections caudad they disappear. Laterally the somatoplcurc and splanchnopleurc are straight and separated by the slit-like ccelom. Section through the Primitive Node Cranial to the Hind-gut (Fig. 65). — 'With the exception of the ectoderm, the structures near the median line are merged into an undiffer- entiated mass of tissue. The cavity of the neural tube and its dorsal outline may still be seen, 74 THE STUDY OF CHICK EMBRYOS but its ventral portion, the notochord, mesoderm and entoderm, blend in a dense mass of tissue which is characteristic of the primitive node. Laterally the segmental zone and the various layers are differentiated. Section Passing through the Hind-gut (Fig. 66). — In this embryo the caudal evagination to form the hind-gut has just begun. The section shows the small cavity of the hind-gut in the mid-line. Its wall is composed of columnar entodermal cells and it is an out- growth of the entodermal layer. Dorsal to the hind-gut may be seen undifferentiated cells of the primitive streak continuous dorsad with the ectoderm, ventrad with the entoderm of the hind-gut and laterally with the mesoderm. Neural tube Bctoderm Segmental stone pan nop u ^oderm Nohchordal folate Fig. 65. — Transverse section of a fifty-hour chick embryo through the primitive node cranial to the hind-gut. X 50. In the chick embryos which we have studied there are large areas developed which are extra-embryonic, that is, lie outside the embryo. The splanchnopleure of the area vasculosa, for instance, forms the wall of the yolk-sac, incomplete in the early stages. The amnion, chorion and allantois are extra-embryonic mem- branes which make their appearance at the fifty-hour stage. These structures are important in mammalian and human embryos and a description of their further development in the chick, where their structure and mode of develop- Somcttic mespnerm Primitive node Ectoderm omatopleure Entoderm Splanchnopleure ■ Hind -gut Fig. 66. — Transverse section passing through the hind-gut of a fifty-hour chick embryo. X S°- ment is primitive, will lead up to the study of mammalian embryos in which the amnion and chorion are precociously developed. Amnion and Chorion. — These two membranes are developed in all Amniote Vertebrates (Reptiles, Birds and Mammals). They are derived from the extra- embryonic somatopleure. The amnion is purely a protective structure, but the chorion of mammals has a trophic function, as through it the embryo derives its nourishment from the uterine wall. Fig. 67 A shows the amnion and chorion CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 75 developing. The head-fold of the somatopleure forms first and envelops the head, the tail-fold makes its appearance later. The two folds extend laterad, meet and fuse (Fig. 67 B). The inner leaf of the folds forms the amnion, the remainder of the extra-embryonic somatopleure becomes the chorion. The actual appear- ance of these structures and their relation to the embryo we have seen in Figs. 60 and 61. The amnion, with its ectodermal layer inside, completely surrounds the embryo by the fourth day, enclosing a cavity filled with amniotic fluid (Fig. 68). In this the embryo floats and is thus protected from injury. The chorion is of little importance to the chick. It is at first incomplete but eventually entirely surrounds the embryo and its other appendages. Yolk-sac and Yolk-stalk. — While the amnion and chorion are developing during the second and third day, the embryo grows rapidly. A£ The head- and tail-folds elon- gate and the trunk expands lat- erally until only a relatively nar- row stalk of the splanchno- pleure connects the embryo with the yolk. This portion of the splanchnopleure has grown more slowly than the body of the embryo and is termed the yolk-stalk. It is continuous with the splanchnopleure which envelops the yolk and forms the yolk-sac. The process of un- equal growth by which the em- bryo becomes separated from the yolk has been described as a process of constriction. This, as Minot points out, is an error. The splanchno- pleure at first forms only an oval plate on the surface of the yolk but eventually encloses it. In Fig. 67, C and D, the relation of the embryo to the yolk-sac is seen at the end of the first week of incubation. The vitelline vessels ramify on the surface of the yolk-sac and through them all the food material of the yolk is conveyed to the chick during the incubation period (about twenty-one days). Allantois. — We have seen that in the fifty-hour chick a ventral evagination, the hind-gut, develops near its caudal end (Fig. 66). From it develops the anlage Fig. 67. — Diagrams showing the development of the amnion, chorion and allantois (Gegenbaur in McMurrich's ''Human Body")- Af., amnion folds; A!., allantois; A ;»., amniotic cavitv; Ds., volk-sac. 76 THE STUDY OF CHICK EMBRYOS of the allantois, and, as it is an outgrowth of the splanchnopleure, it is lined with entoderm and covered with splanchnic mesoderm (Fig. 67). It develops rapidly into a vesicle connected to the hind-gut by a narrow stalk, the allantoic stalk. At the fifth day it is nearly as large as the embryo (Fig. 68). Its wall flat- tens out beneath the chorion and finally it lies close to the secondary egg mem- brane (shell) but is attached only to the embryo. The functions of respiration and excretion are ascribed to it. In its wall ramify the allantoic vessels, which have been compared to the umbilical arteries and veins of mammalian embryos. The chick embryo is thus protected by the amnion which develops from the Fig. 68. — Diagram of a chick embryo of the fifth day showing amnion, chorion and allantois (Mar- shall). AN , inner or true amnion; A V, outer margin of the area vasculosa; AZ, outer or false amnion (chorion); EM, embryo; SH, shell of egg; SM, shell membrane; SV, air chamber; TA, allantois; YS, yolk-sac. inner leaf of the folded somatopleure and is composed of an inner ectodermal and an outer mesodermal layer. Nutriment for the growth of the embryo is supplied by the yolk-sac and carried to the embryo by the vitelline veins. The allantois, which takes its origin from the splanchnopleure of the hind-gut and is composed of an inner layer of entoderm and an outer layer of splanchnic mesoderm, func- tions as an organ of respiration and serves as a reservoir for the excreta of the embryonic kidneys. As we shall see, the allantois becomes more important, the yolk-sac less important in some mammals, while in human embryos both yolk - sac and allantois are unimportant when compared to the chorion. CHAPTER IV THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS The fetal membranes of mammals include the amnion, chorion, yolk-sac and allantois, structures which we have seen are present in chick embryos. Most important in mammals is the manner in which the embryo becomes attached to the uterine wall of the mother and in this regard mammalian embryos fall into two groups. Among the Ungulates or hoofed mammals (example the pig) the fetal membranes are of a primitive type, resembling those of the chick. Among Unguiculates (clawed animals like the bat and rabbit) and Primates (example Man) the fetal membranes of the embryo show marked changes in development and structure. FETAL MEMBRANES OF THE PIG EMBRYO The amnion and chorion develop very much as in the chick embryo (Fig. 67 A, B). A fold of the somatopleure forms very early about the whole embryo. The amnion is closed in embryos with but a few pairs of segments, but for some time remains attached to the chorion by a strand of tissue (Keibel). The yolk- sac develops early as in all mammals. In the pig it is small and the greater part of it soon degenerates. It is important only in the early growth of the embryo, its functions then being transferred to the allantois. Branches of the vitelline vessels ramify in its wall, as in that of chick embryos, but soon degenerate. The trunks of the vitelline vessels, however, persist within the body of the em- bryo. The allantois, developing aS in the chick from the ventral wall of the hind- gut (Fig. 67 A-D), appears when the embryo is still flattened out on the germinal area. In an embryo 3.5 mm. long it is crescent shaped and as large as the em- bryo. It soon becomes larger and its convex outer surface is applied to the inner surface of the chorion. As these surface layers are composed of splanchnic mesoderm they fuse more or less completely. A pair of allantoic veins and ar- teries branch in the splanchnic layer of the allantois. These branches are brought into contact with and invade the mesodermal layer of the chorion. The outer ectodermal layer of the chorion in the meantime has closely applied itself to the uterine epithelium, the ends of the uterine cells fitting into depressions in the 77 78 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS chorionic cells (Fig. 69) . When the allantoic circulation is established, waste prod- ucts given off from the blood of the embryo must pass through the epithelia of Spinal cord Mes.segment Amniotic cavity Upper limb bud FbjtamJinaJ vein Dorsal ClorTa Glomerulus Rumbilical Vein Hind-gut Somatic and Splanchnic mesoderm YolK-sac Entoderm -Chorionic mesoderm Chorionic ectoderm Uterine 1 epithelium ■ Tunica propria of uterus Fig. 69.— A, Transverse section through the yolk-sac and stalk of a 5 mm. pig embryo showing attachment of amnion. B, Diagram of the fetal membranes and allantoic placenta of a pig embryo in median sagittal section (based on figures of Heisler and Minot). UMBILICAL CORD 79 both chorion and uterus to be taken up by the blood of the mother. In the same way nutritive substances and oxygen must pass from the maternal blood through these layers to enter the allantoic vessels. This exchange does take place, how- ever, and thus in Ungulates the allantois has become important not only as an organ of respiration and excretion but as an organ of nutrition. Through its vessels it has taken on a function belonging to the yolk-sac in birds, and we now see why the yolk-sac becomes a rudimentary structure in the higher mammals. Excreta from the embryonic kidneys are passed into the cavity of the allantois which is relatively large. The name is derived from a Greek word meaning saus- age-like, from its form in some animals. The chorion is important only as it brings the allantois into close relation to the uterine wall, but in man we shall see that it plays a more important role. UMBILICAL CORD In their early development the relation of the amnion, allantois and yolk- sac to each other and to the embryo is much the same as in the chick of five days (Fig. 68). With the increase in size of the embryo, however, the somato- pleure in the region of the attachment of the amnion grows ventrad. As a result, it is carried downward with the ccelom about the yolk-sac and allantois, forming the umbilical cord. Thus in a pig embryo 10 to 12 mm. long the amnion is attached at a circular line about these structures some distance from the body of the embryo. The ccelom at first extends ventrad into the cord, but later the mesodermal layers of amnion, yolk-stalk and allantois fuse and form a solid cord of tissue. This is the umbilical cord of fetal life and its point of attachment to the body is the umbilicus or navel. The cord is covered by a layer of ectoderm con- tinuous with that of the amnion and of the embryo and contains, embedded in a mesenchymal (mucous) tissue (1) the yolk-stalk and (in early stages) its vitelline vessels; (2) the allantoic stalk; (3) the allantoic vessels. These, two arteries and a single large vein, are termed from their position the umbilical vessels. At certain stages (Figs. 117 and 118) the gut normally extends into the ccelom of the cord, forming an umbilical hernia. Later, it returns to the ccelom of the embryo and the cavity of the cord disappears. The umbilical cord of the pig is very short. Human Umbilical Cord. — This develops like that of the pig and may attain a length of more than 50 cm. It becomes spirally twisted, just how is not kno%vn. In embryos from 10 mm. to 40 mm. long the gut extends into the ccelom of the 80 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS cord (Fig. 172). At the 42 mm. stage, according to Lewis and Mall, the gut re- turns to the ccelom of the body. The mucous tissue peculiar to the cord arises from mesenchyme. It contains no capillaries and no nerves, but embedded in it are the large umbilical vein, the two arteries, the allantois and the yolk-stalk. The umbilical cord may become wound about the neck of the fetus, causing its death and abortion, or by coiling about the extremities it may lead to their atrophy or amputation. EARLY HUMAN EMBRYOS AND THEIR MEMBRANES Referring to the blastodermic vesicle of the mammal (Figs. 16 and 17), we find it consists of an outer layer, which we have called the trophectoderm, and the inner cell mass. The trophectoderm forms the primitive ectodermal layer of the chorion in the higher mammals and probably in man. From the inner cell mass are derived the primary ectoderm, entoderm and mesoderm. In the earliest known human embryos described by Teacher, Bryce, and Peters, the germ layers and amnion are present, indicating that they are formed very early. We can only guess at their early origin by what we know from other mammals. The diagrams (Fig. 70 A and B) show two hypothetical stages seen in median longi- tudinal section. In the first stage (A) the blastodermic vesicle is surrounded by the trophectoderm layer. The inner cell mass is differentiated into a dorsal mass of ectoderm and a ventral mass of entoderm. Mesoderm more or less completely fills the space between entoderm and trophoderm. It is assumed that as the embryo grows (Fig. 70 B) a split occurs in the mass of ectoderm cells, giving rise to the amniotic cavity and dividing these cells into the ectodermal layer of the embryo and into the extra-embryonic ectoderm of the amnion. At the same time, a cavity may be assumed to form in the entoderm, giving rise to the primitive gut. About this stage the embryo embeds itself in the uterine mucosa. In the third stage, based on Peter's embryo (Fig. 70 C), the extra-embryonic mesoderm has extended between the trophectoderm and the ectoderm of the amnion and the extra-embryonic ccelom appears. The amniotic cavity has increased in size and the embryo is attached to the trophectoderm by the unsplit layer of mesoderm between the ectoderm of the amnion and the trophectoderm of the chorion. The latter shows thickenings which are the anlages of the chorionic villi surrounded by trophoderm cells. In the fourth stage, based on Graf Spee's embryo (D), the chorionic villi are longer and branched. The mesoderm now remains unsplit only at the posterior end of the embryo, where it forms the body-stalk peculiar EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 8l to Unguiculates and Primates. It connects the mesoderm of the embryo with the mesoderm of the chorion. Into it there has grown from the gut of the em- bryo the entodermal diverticulum of the allantois. The Chorion. — The human chorion is derived directly from the outer troph- ectoderm layer of the blastodermic vesicle and from the extra-embryonic somatic mesoderm. Its early structure resembles that of the pig's chorion. The troph- Ectode Amniotic cavity Coelom Trophecto derm Archenteron Entoderm Mesoderm D Ectoderm of amnion Ectoderm of embryo Amniotic cavity Troph ectoderm YolK-Sac Entoderm Splanchnic mesoderm Ectoderm of embryo Cavity of amnion Mesoderm of amnion Ectoderm ofchonon Cavity of 'yolK sac Entoderm of yol K-jac Mesoderm of Allantoi s BodystalK S Extraembryonic yolKSac a — rnelam ' Extraembryonic coelom Chorionic mesoderm Mesoderm of Ck Trophoderm Chor Fig. 70. — Four diagrams showing hypothetical stages of early human embryos (based on figures of Robinson and Minot). ectoderm of the human embryo early gives rise to a thickened outer layer, the trophoderm (syncytial and nutrient layer) . When the developing embryo comes into contact with the uterine wall the trophoderm destroys the maternal tissues. The destruction of the uterine mucosa serves two purposes: (1) the embedding and attachment of the embryo, it being grafted, so to speak, to the uterine wall; and (2) it supplies the embryo with a new source of nutrition. To obtain nutri- 6 82 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS ment to better advantage, there grow out from the chorion into the uterine mucosa branched processes or villi. The villi are bathed in maternal blood, and in them blood-vessels are developed, the trunks of which pass to and from the embryo as the umbilical vessels. The embryo receives its nutriment and oxygen, Inner eel '/-mass Entode Trophoblast Inner eell-mass Trophoblast Embryonic ectoderm Entoderm Maternal bloodvessels Syncytiotrophoblast Cijtotrophoblast Embryonic ectoderm Entoderm Fig. 71. — Section showing three stages in the formation of the amnion of bat embryo (after Van Beneden). and gets rid of waste products through the walls of the villi. The region where the attachment of the chorionic villi to the uterine wall persists during fetal life is known as the placenta. It will be described later with the decidual membranes of the uterus. We saw how the allantois of Ungulates had assumed the nutritive EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 83 functions performed by the yolk-sac in birds, with a consequent degeneration of the ungulate yolk-sac. In man and Unguiculates, the functions of the allantois are transferred to the chorion and the allantois in turn becomes a rudimentary structure. The Amnion. — This is formed precociously in Unguiculates and in a manner quite different from its mode of origin in Ungulates and birds. It is assumed that its cavity arises as a split in the primitive ectoderm of human embryos, as in bat embryos (Fig. 71). Later, a somatic layer of mesoderm envelops its ectodermal layer, its component parts then being the same as in birds and Ungulates, an inner layer of ectoderm and an outer layer of mesoderm (Fig. 70 D). It becomes — — «9 c»> f> J °- Fig. 72. — Section of embryonic rudiment in Peters' ovum (first week) (after Peters), cct, ectoderm of chorion; mes, mesoderm; am, amnion; cm. pi., embryonic plate; y.s.. yolk-sac; cut. entoderm; ex. cce., portion of extra-embryonic ccelom limited by a strand of the magma reticulare. a thin, pellucid, non- vascular membrane and about a month before birth is in contact with the chorion. It then contains from one-half to three-fourths of a liter of amniotic fluid, the origin of which is unknown. During the early months of pregnancy the embryo, suspended by the umbilical cord, floats in the amniotic fluid. The embryo is protected from maceration by a white fatty secretion, the vernix caseosa. At birth the amnion is ruptured either normally or artificially. If not ruptured, the child may be born enveloped in the amnion popularly known as a veil or ''caul." The amniotic fluid may be present in excessive amount, the condition being known as hydr amnios. If less than the normal amount of fluid is present, the amnion may adhere to the embryo and produce malformations. It has been found, too, that fibrous bands or cords of tissue may extend across the amniotic cavity and, pressing upon parts of the embryo 84 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS during its growth, may cause scars and splitting of eyelids or lips. Such amniotic threads may even amputate a limb or cause the bifurcation of a digit producing a type of polydac- tylism. The Allantois.— The allantois appears very early in the human embryo be- Yolk-sac Neural groove — j — r~r Neurenteric canal Primitive streak Body-stalk r^u. $&y- --! Villi of chorion Chorion t^- Mesoderm Body-stalk Primitive streak Allantois Yolk-sac Mesoderm Fig. 73.— Views of a human embryo 1.54 mm. long. A, dorsal surface; B, median sagittal section (Graf Spee). EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 85 fore the development of the fore-gut or hind-gut. In Peter's embryo the amnion, chorion and yolk-sac are present but not the allantois (Fig. 72). In an embryo 1.54 mm. long, described by Von Spee (Fig. 73 A, B), there is no hind-gut, but the allantoic diverticulum of the entoderm has invaded the mesoderm of the body- stalk. This embryo, seen from the dorsal side with the amnion cut away, shows a marked neural groove and primitive streak. In front of the primitive knot a pore is figured leading from the neural groove into the primitive intestinal cavity, hence called the neurenteric canal. The fore-gut and head-fold have formed at this stage and there are branched chorionic villi. A reconstruction by Dandy of Mali's embryo, about 2 mm. long with seven pairs of seg- ments, shows well the embryonic appendages (Fig. 74). The fore- and hind-gut are well developed, the amniotic cavity is large, and the yolk-sac still communicates with the gut through a wide open- ing. The allantois is present as a long curved tube somewhat di- lated near its blind end and em- bedded in the mesoderm of the body-stalk. As the hind-gut de- velops, the allantois comes to open into its ventral wall. A large umbilical artery and vein are present in the body-stalk. In an embryo of 23 somites 2.5 mm. long, described by Thompson, the allantois has elongated and shows three irregular dilatations (Fig. 75). A large cavity never appears distally in the human allantois as in Ungulates. When it becomes included in the umbilical cord its distal portion is tubular and it eventually atrophies. That part of the allantois extending from the umbilicus to the cloaca of the hind-gut takes part in forming the urogenital sinus, the bladder and the urachus, a rudiment extend- ing as a solid cord from the fundus of the bladder to the umbilicus. According to Felix, the allantois forms only the urachus and a portion of the bladder. Fig. 74. — A human embryo of 2 mm. in median sagittal section (adapted from reconstructions of Mall's embryo by F. T. Lewis and Dandy). 86 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS The human allantois is thus small and rudimentary as compared with that of birds and Ungulates. As we have seen, the cavity is very large in the pig, and Haller found an allantoic sac two feet long connected with a goat embryo of two inches. In human embryos it appears very early and is not free but embedded in the body-stalk. Its functions, so important in birds and Ungulates, are in man performed by the chorion. Neural folds Neurenteric canal Fig. 75. — Median sagittal section of a 2.5 mm. human embryo showing digestive tract (after Thompson). X 40. All., allantois; CI., cloaca; C. per., pericardial cavity; Div.hep., hepatic diverticulum; D. v., ductus vitellinus (yolk-stalk); gl. th., thyreoid gland; Men. cl., cloacal membrane; Ph., pharynx; Sept. lr., sep- tum transversum. Fig. 76. — Human embryo of 2.1 1 mm. (Eternod). Yolk-Sac and Yolk-Stalk.— In the youngest human embryos described (Peters) the entoderm forms a some- what elongated vesicle. With the development of the fore-gut and hind- gut in embryos of 1.54 and 2 mm. (Figs. 73 and 74), the entodermal vesicle is divided into the dorsal intestine and ventral yolk-sac, the two being connected by a somewhat narrower region. This condition persists in an embryo of 2.5 mm. long (Fig. 75). In the figure most of the yolk-sac has been cut away. An embryo with 9 pairs of segments, with three brain vesicles and with the amnion cut away is seen in Fig. 76. The relation of the fetal appendages to the embryo shows well in the embryo of Coste (Fig. EARLY HUMAN EMBRYOS AND THEIR MEMBRA \ I - 87 77). The dorsal convexity is probably abnormal. A robust body-stalk attaches the embryo to the inner wall of the chorion. With the growth of the head- and tail-folds of the embryo, there is an apparent constriction of the yolk-sac where it joins the em- bryo. This will become more marked in later stages and form the yolk-stalk. His's embryo, 2.6 mm. long, shows the relative size of yolk-sac and embryo and the yolk-stalk (Fig. 78). The relations of the fetal membranes to the embryo are much the same as in the chick embryo of five days, save that the allantois of the human embryo is embedded in the body-stalk. The embryo shows a regular convex dorsal curvature, there is a marked cephalic bend in the region of the mid-brain and there are three gill clefts. The head is twisted to the right, the tail to the Fig. 77. — Human embryo at the commencement of the third week (from His, after Coste). X 15. A, inner or true amnion; A.s., body-stalk; //, heart; V, blood-vessel on yolk-sac; Y.s., yolk-sac. Amnion Branchial clefts 1-3 Body-stalk Maxillary process Mandibular process Heart Yolk-sac Fig. 78. — Human embryo 2.6 mm. long showing amnion, yolk-stalk and body-stalk (His). 88 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS left. At the side of the oral sinus are two large processes; the dorsal of these is the maxillary, the ventral the mandibular process. The heart is large and flexed in much the same way as the heart of the fifty-hour chick embryo. Fig. 70. — Yolk-sac and-stalk of a 20 mm. human embryo. X 11. Mid-brain In later stages, with the development of the umbilical cord, the yolk-stalk becomes a slender thread extending from the dividing line between the fore- and hind-guts to the yolk-sac or umbilical vesicle (Fig. 114). It loses its attach- ment to the gut in 7 mm. embryos. A blind pocket may persist at its point of union with the intestine and is known as Meckel's diverticulum, a struc- ture of clinical importance because it may telescope and cause the oc- clusion of the intestinal lumen. The yolk-stalk may remain embedded in the umbilical cord and extend some distance to the yolk-sac which is found between the amnion and chorion (Fig. 79). The yolk-sac may be persistent at birth. Fore-brain ~~i Stomodaum Mandibular process I/cart Hind-brain uditory vesicle Branchial arches ■Amnion (cut) Body-stalk THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO Fig. 80. — Left side of a human embryo of 4.2 mm (His). This embryo, studied and de- scribed by His, is probably not quite normal. It shows a concave dorsal flexure which Keibel regards as due to distortion. Viewed from the left side (Fig. 80), with the amnion cut away close to its line of attachment, there may be seen the yolk-stalk, and a portion of the yolk-sac and of the THE ANATOMY OF A 4.2 MM. IH'MAN EMBRYO 89 body-stalk. There is an indication of the primitive segments along the dorso- lateral line of the trunk. The head is bent ventrad almost at right angles in the mid-brain region (cephalic flexure). There are also marked cervical and caudal llexures, the trunk ending in a short blunt tail. The heart is large and flexed as in the earlier stage. Three gill clefts separate the four branchial arches. The first has developed two ventral processes. Of these the maxillary process is small and may be seen dorsal to the stomodaeum. The mandibular process is large and has met its fellow of the right side to form the mandible or lower jaw. Dorsal to the second gill cleft may be seen the position of the oval otocysl, now a closed sac. Opposite the atrial portion of the heart and in the region of the caudal flexure bud-like outgrowths indicate the anlages of the upper and lower extremities. Central Nervous System and Sense Organs. — The neural tube is closed throughout its extent and is differentiated into brain and spinal cord. The brain tube or encephalon is divided by constrictions into four regions or vesicles as in the fifty-hour chick (Fig. 55). Of these, the most cephalad is the telenceph- alon. It is a paired outgrowth from the fore-brain, the persisting portion of which is the dicnccplialon. The mid-brain or mesencephalon located at the cephalic flexure is not subdivided. The hind-brain, or rhombencephalon, which is long and continuous with the spinal cord, later is subdivided into the metencephalon (region of the cerebellum and pons) and myelenccphalon (medulla oblongata). The spinal cord forms a closed tube extending from the brain to the tail and containing the neural cavity, flattened from side to side. The eye is represented by the optic vesicles and the thickened ectodermal anlage of the lens. Its stage of development is between that of the thirty->ix and fifty-hour chick embryos. The otocyst is a closed sac, no longer connected with the outer ectoderm as in the fifty-hour chick. Digestive Canal. — In a reconstruction of the viscera viewed from the right side (Fig. 81), the entire extent of the digestive canal may be seen. The pharyn- geal membrane which we saw developed in the chick between the stomodaeum and the pharynx has broken through so that these cavities are now in communi- cation. The fore-gut. which extends from the oral cavity to the yolk-stalk is differentiated into pharynx, trachea and lungs, esophagus and stomach, small intestine and digestive glands (pancreas and liver). The gut is suspended from the dorsal body wall by the dorsal mesentery. The ectodermal limits of the oral cavitv are indicated dorsad bv the diverti- 9° THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS culum of the hypophysis (Rathke's pocket). The fore-gut proper begins with a shallow out-pocketing known as Seessel's pocket. As the pharyngeal mem- brane disappears between these two pockets, it would seem that Seessel's pocket represents the persistence of the blind anterior end of the fore-gut. No other significance has been assigned to it. MetencephaJo, /lortic arches /.Z.3.4.6 A/ofochord Hind-gut /Mesencephalon % cephalic flexure Hypophysis Diencephalon Int. Carotid artery Optic vesicle Trosen cephalon Mouth cavity Pharyngeal pouches l-*r Ventral aorta- Atrium of heart Umbilical Vein Liver an/age •Jplanchmc mesoderm Mid-gut Entoderm of yolk-stalk Tail aut Umbilical artery Meson ephric duct Joaca. Allantois Fie. 81. — Diagrammatic reconstruction of a 4.2 mm. human embryo, viewed from the right side (adapted from a model by His). The pharynx is widened laterally and at this stage shows four pharyngeal pouches. Later a fifth pair of pouches is developed (Fig. 82). The four pairs of pharyngeal pouches are important as they form respectively the following adult structures: (1) the Eustachian tubes; (2) the palatine tonsils; (3) the thymus anlages; (4) the parathyreoids or epithelial bodies. Between the pharyn- geal pouches are the five branchial arches in which are developed five pairs of aortic arches. Between the bases of the first and second branchial arches, on THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 91 the floor of the pharynx, is developed the tuberculum impar which may form a portion of the anterior part of the tongue. Posterior to this unpaired anlage of the tongue there grows out ventrally the anlage of the thyreoid gland. From the caudal end of the trachea have appeared ventrally the lung buds. The trachea is still largely a groove in the ventral wall of the pharynx and esophagus. Caudal to the lungs a slight dilation of the digestive tube indicates the position Mouth cavity Pharyngeal pouches /-4- Trachea Luna bud Hepatic diverticulum Ventral pancreas Mesonephric tubule with glomerulus Hind-gut Allantois Tail- gut Thyreoid anlage Esophagus Stomach Dorsal pancreas YolK-stalK Mesonephros Mesonephric duct a oaca. FIG. 82. — Diagrammatic ventral view of pharynx, digestive tube and mesonephroi of a 4-5 mm. embryo (based on reconstructions by Grosser and His). The liver and yolk-sac are cut away. The tubules of the right mesonephros are shown diagrammatically. of the stomach. The liver diverticulum has grown out from the fore-gut into the ventral mesentery cranial to the wall of the yolk-stalk. It is much larger than in the fifty-hour chick, where we saw its paired anlage cranial to the fovea cardiaca, and is separated from the heart by the septum transvcrsum. The small intestine between the liver and yolk-stalk is short and broad. In later stages it becomes enormously elongated as compared with the rest of the diges- tive tube. The yolk-stalk is still broad and wide. The region of its attachment 92 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS to the gut corresponds to the open mid-gut of the chick embryo. The hind-gut and tail-fold of this embryo are greatly elongated as compared with the chick embryo of fifty hours. The hind-gut terminates blindly in the tail. Near its caudal end it is dilated to form the cloaca. Into the ventral side of the cloaca opens the stalk of the allantoic. Dorso-laterally the primary excretory (Wolffian) ducts which we saw developed in the fifty-hour chick have connected with and open into the cloaca. Caudal to the cloaca on the ventral side is the cloacal mem- brane, which later divides and breaks through to form the genital aperture and anus. That part of the hind-gut between the cloaca and the yolk-stalk forms the rectum, colon, caecum, and appendix, with a portion of the small intestine (ileum) . Urogenital Organs.— We have seen that the primary excretory (Wolffian) ducts open into the cloaca. These are the ducts of the mid-kidney or meso- nephros. At this stage the nephrotomes, which in the chick embryos formed the anlages of these ducts, are also forming the kidney tubules of the mesonephros which open into the ducts (Fig. 82). The mid-kidneys project into the peri- toneal cavity as ridges on each side. A thickening of the mesothelium along the median halves of the mesonephroi forms the anlage of the genital glands or gonads (Fig. 213). Circulatory System. — The heart is an S-shaped double tube as in the fifty- hour chick. The outer myocardium is confined to the heart, while the inner endothelial layer is continuous, at one end with the veins, at the other end with the arteries. The disposition of the heart tube is well seen in a ventral view of a younger embryo (Fig. 83). The veins enter the sinus venosus just cranial to the yolk-sac. Next in front is the atrium with the convexity of its flexure di- rected cephalad. The ventricular portion of the heart is U-shaped and is flexed to the right of the embryo. To the left is the ventricular limb, to the right is the bulbus. The arteries begin with the ventral aorta which bends back to the midline and divides into five branches on each side of the pharynx (Figs. 83 and 84) . These are the aortic arches and they unite dorsally to form two trunks, the descending aorta. The aortic arches pass around the pharynx between the gill clefts in the branchial arches. The arrangement is like that of the adult fish which has its gill slits, branchial arches and aortic arches to supply the gills. The descending aortas run caudad and opposite the lung buds unite to form a single median dorsal aorta. This in the region of the posterior limb buds divides into the two umbilical arteries, which, curving cephalad and ventrad, enter the body-stalk on each side of the allantois and eventually ramify in the villi of the chorion. The vitelline arteries, large and paired in the chick, are Ventricle Liver R. vitelline vein Aortic arches 1-4 Atrium I 'itrllo-umbilical vein L. umbilical vein Fig. 83. — Ventral reconstruction of a 3.2 mm. embryo, showing vessels (His). Fig. 84. — Lateral view of human embryo of 4.2 mm., showing aortic arches and venous trunk- His . m.x. Maxillary process; mn, mandibular arch; d.C, common cardinal vein; jv.. anterior cardinal vein; c.v., posterior cardinal vein; ;.;.. vitelline vein; u.a., umbilical artery; u.v , umbilical vein ; all., allantois; pl., placental attachment of body-stalk; olf.. olfactory pit; ot, otocyst. THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 93 represented by a single small trunk which branches on the surface of the yolk- sac (Fig. 84). Compared with the arterial circulation of the chick of lift)' hours the important differences are (1) the development of the fourth and the fifth pairs of aortic arches, and (2) the presence of the chorionic circulation by way of the umbilical arteries in addition to the vitelline circulation found in the fifty- hour chick. The veins are all paired and symmetrically arranged (Figs. 83 and 84). There are three sets of them: (1) The blood from the body of the embryo is drained, from the head end by the anterior cardinal veins; from the tail end of the body by the posterior cardinal veins. These veins on each side unite dorsal to the heart and form a single common cardinal vein which joins the umbilical vein of the same side. (2) Paired vitelline veins in the early stages of the embryo drain from the yolk-sac the blood carried to it by the vitelline arteries. The trunks of these veins pass back into the body on each side of the yolk-stalk and liver and with the paired umbilical veins form a trunk which empties into the sinus venosus of the heart. As the liver develops it may be seen (Fig. 83) that the vitelline veins break up into blood spaces called by Minot sinusoids. When the liver becomes large and the yolk-sac rudimentary the vitelline veins receive blood chiefly from the liver and intestine. (3) A pair of large umbilical veins which drain the blood from the villi of the chorion and are the first veins to appear. These unite in the body-stalk and, again separating, enter the soma- topleure on each side. They run cephalad to the septum transversum where they unite with the vitelline veins to form a common vitello-umbilical trunk which empties into the sinus venosus. The veins of this embryo are thus like those of the fifty-hour chick save that the umbilical vessels are now present and take the place of the allantoic veins of later chick embryos. The veins, like the heart and arteries, are primitively paired and symmetrically arranged. As development proceeds their symmetry is largely lost and the asymmetrical venous system of the adult results. The later stages of the human embryo can not be described in detail here. The student is referred to the texts of Minot, Keibel, and Mall. Two embryos will be compared with the pig embryos described in Chapter V. Figs. 85 and 86 show the human embryos described by His, the age of which was estimated by him at from two weeks to two months. The figures show as well as could any description the changes which lead to the adult form when the embryo may be called a, fetus. The external metamorphosis is due principally: (1) to changes in the flexures of the embryo; (2) to the development of the face; (3) to the development of the external structure of the sense organs (nose, eye and ear); THE FETAL MEMBRANES AND EARLY HUMAN EMBRYO (3) U:>Ly^ (4-) \ (5) Jtg. 85. — Embryos of His' NormentafeL The embryos figured are of the first month (Keibel and Elze). X 5- 94 THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 95 (4) to the development of the extremities and disappearance of the tail. The more important of these changes will be dealt with in later chapters. Fig. 86. — Embryos of the second month from His' Xormentafel (Keibel and Elze). Age of Human Embryos.— The ages of the human embryos which have been obtained and described can not be determined with certainty, because fer- tilization does not necessarily follow directly after coitus. It has been shown g6 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS also that ovulation does not always coincide with menstruation so that the menstrual period cannot be taken as the starting point of pregnancy. In 1868, Reichert, from studying the corpus luteum in ovaries obtained during menstrua- tion, concluded that ovulation takes place as a rule just before menstruation and that if the ovum is fertilized the next menstruation does not occur. Reichert then decided that a human embryo of 5.5 mm., which he had obtained from a woman two weeks after menstruation failed to occur, must be two weeks, not six weeks, old. His accepted Reichert's views and since then the ages of embryos have been estimated on this basis. According to this view, Peter's ovum, ob- tained thirty days after the last period, is only three or four days old. This does not agree at all with what is known of the age of other mammalian embryos. From the observations of Mall and obstetricians of the present day, we must conclude that ovulation does not immediately precede menstruation but that most pregnancies take place during the first or second week after the menstrual period. It is therefore more correct to compute the age of the embryo from the end of the last menstruation or, according to Grosser, from the tenth to the twelfth day before the first missed menstrual period. Peter's embryo then would be about fifteen days old. To compare an embryo with one of known age, the length from vertex to breech is usually taken. Embryos of the same age vary greatly in size so that their structure must be taken into account. At the present time, the exact relation of ovulation to menstruation is not known nor the exact time required for the fertilized ovum to reach the uterus. The computed age of the embryo can be thus only approximate. The period of gestation of the human fetus is usually computed from the beginning of the last menstrual period. Forty weeks or two hundred and eighty days is the time usually allowed. As some women menstruate once or more often after becoming pregnant this is not a certain basis for computation. The following are the estimated ages, lengths, and weights of human em- bryos according to Mall, Schroeder, and Fehling: Length in Weight Age Millimeters in Grams Eighteen to twenty-one days 0.5 Twenty-four to thirty days 2.5 Thirty-one to thirty-five days 5-5 Thirty-eight to forty-two days no Fifty days 20.0 Second lunar month 30.0 Third lunar month 7°-°~ ^o-0 2° Fourth lunar month 180.0 120 Fifth lunar month 250.0 285 Sixth lunar month 3x5-° °35 Seventh lunar month 37°° 1220 Eighth lunar month 425-° 1700 Ninth lunar month 470.0 2240 Tenth lunar month 5°°.o 325° CHAPTER V THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS A. THE ANATOMY OF A 6 MM. PIG EMBRYO In its early stages the pig embryo is flattened out on the surface of the yolk- sac like a chick embryo (Fig. 87), but as the head- and tail-folds elongate the body becomes flexed and twisted spirally, making it difficult to study. In embryos 5 to 7 mm. long the twist of the body begins to disappear and its structure may be seen to better advantage. External Form of 6 mm. Em- bryo.— When compared with the form of the 4 mm. human embryo, its marked difference is the convex dorsal flexure which brings the head and tail regions close together (Fig. 88) . The flexure at the mesen- cephalon forms an acute angle and there is a marked neck or cervical flexure. As a result, the head is somewhat triangular in form. The body is bent dorsad in an even convex curve and the tail is flexed sharply dorsad and to the right side. Lateral to the dorsal line may be seen the segments, which become larger and more differen- tiated as we go from tail to head. At the tip of the head a shallow depression marks the anlage of the olfactory pit. The lens vesicle of the eye is open to the exterior. Caudal to the eyes at the sides of the head are four branchial arches separated by three grooves, the branchial clefts. The fourth arch is partly con- cealed in a triangular depression, the cervical sinus (see Fig. 92). The first, or 7 97 Fig. 87. — Pig embryos 04) of seven and (B) of eleven primitive segments, dorsal view, with amnion cut away (Keibel, Normentafel). X 20. 98 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS mandibular arch, forks ventrally into two processes, a smaller maxillary and a larger mandibular process, and the latter with its fellow forms the mandible or lower jaw. The position of the mouth is indicated by the cleft between these processes. The groove between the eye and the mouth is the lacrymal groove. The second or hyoid arch is separated from the mandibular arch by a hyo- mandibular cleft which persists as the external auditory meatus. About the dorsal end of the cleft develops the external ear. The heart is large and through the transparent body wall may be seen the dorsal atrium and ventral ventricle. Caudal to the heart, a convexity indicates Cephalic flexure Olfactory pit Yo/K-Sac Maxillary process ^^^ process Br. arch II Br. arch III Cervical sinus Alrium of heart Fig. 88. — Pig embryo of 6 mm., viewed from the left side. The amnion has been removed and its cut edge is shown in the figure. X 12. the position of the liver. Dorsal to the liver is the bud of the anterior extremity, now larger than in the 4 mm. human embryo. Extending caudal to the anlage of the upper extremity, a curved convexity indicates the position of the right mesonephros. At its caudal end is the bud of the lower limb. The amnion has been dissected away along the line of its attachment ventral to the mesonephros. There is as yet no distinct umbilical cord and a portion of the body-stalk is at- tached to the embryo. As the term of its development is shorter, a young pig embryo is somewhat precocious in its development as compared with a human embryo of the same se ("Fig. 89). In a human embryo 7 mm. long the head is larger, the tail shorter. I.ATKRAI. DISSF.i HON OF Till-. VISCKRA 99 The cervical flexure is more marked, the olfactory pits larger and deeper. The liver is more prominent, the tnesonephros and segments less so than in the 6 mm. pig. Lateral Dissection of the Viscera To understand the sectional anatomy of an embryo, a study of dissections and reconstructions is essential. For methods of dissection see p. 146, Chapter Myelencephiilun Spinal cord Cervical seg- ment 8 Future milk line Thoracic seg- ment 12 Metencephalon Mesencephalon — Diencephalon Yolk-sac and umbilical cord Lumbar segment 5 Fig. 89. — A human embryo 7 mm. long, viewed from the right side (Mall in Kallmann's Hand- atlas). /, 77, III, branchial arches 1, 2 and 3; H, Hi, heart; L, liver; L ', otic vesicle; R, olfactory placode; Tr, semi-lunar ganglion of trigeminal nerve. VI. Before studying sections, the student should become as well acquainted as possible with the anatomy of the embryo and compare each section with the figures of reconstructions and dissections. Nervous System. — Fig. 90 shows the central nervous system and viscera exposed on the right side of a 5.5 mm. embryo. The ventro-lateral wall of the head has been left intact with the lens cavity, olfactory pit, and portions of the maxillary and mandibular processes, second and third branchial arches and IOO THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS cervical sinus (see Fig. 88). The brain is differentiated into the five regions, telencephalon, diencephalon, mesencephalon, metencephalon and myelencephalon. The spinal cord is cylindrical and gradually tapers off to the tail. The anlages of the cerebral and spinal ganglia and the main nerve trunks are shown. The oculomotor nerve begins to appear from the ventral wall of the mesencephalon. Ventro-lateral to the metencephalon and myelencephalon occur in order the Sup. gang. n. Q Otocyst Acustic ganglion Geniculate gang. n. 7 Semilunar gang. n. 5 Metencephalon Mesencephalon Jugular gang. n. 10 Gang, nodosum n. 10 V. accessorius Gang. Froriep Gang. cerv. i N. hypoglossus Cervical sinus Atrium Ventral lobe liver Dorsal lobe liver Thoracic gang. 1 Mesonephros Small intestine N. oculomotorius Diencephalon Petrosal gang. n. g Lens opening Olfactory pit Telencephalon Yolk-sac Allantoic Ventricle Allantoic stalk Hind-gut Fig. 90. — Dissection of a 5.5 mm. pig embryo, showing the nervous system and viscera from the right side. X 18. semilunar ganglion and three branches of the trigeminal nerve, the geniculate ganglion and nerve trunk of the n. facialis, the ganglionic anlage of the n. acusticus and the otocyst. It will be observed that the nerve trunks are arranged with reference to the branchial arches and clefts. Caudal to the otocyst a continuous chain of cells extends lateral to the neural tube into the tail region. Cellular enlargements along this neural crest represent developing cerebral and spinal LATERAL DISSECTION OF THE VISCERA IOI ganglia. They are in order the superior or root ganglion of the glossopharyngeal nerve with its distal petrosal ganglion; the ganglion jugulare and distal ganglion nodosum of the vagus nerve; the ganglionic crest and proximal portion of the spinal accessory nerve; and the anlage of Froriep's ganglion, an enlargement on the neural crest just cranial to the first cervical ganglion. Between the vagus and Frojiep's ganglion may be seen the numerous root fascicles of the hypoglossal nerve, which take their origin along the ventro-lateral wall of the myelencephalon and unite to form a single trunk. The posterior roots of the spinal ganglia are very short; their anterior or ventral roots are not shown. The position of the heart with its ventricle, atrium and sinus venosus are shown. The liver is divided into a small dorsal and a large ventral lobe. The fore-gut emerges from between the liver lobes and curves ventrad to the yolk- stalk and sac. The hind-gut is partly hidden by the fore-gut; it makes a U- shaped bend from the yolk-stalk to the caudal region. The gut is attached to the dorsal body wall by a double layer of splanchnic mesoderm which forms the mesentery. The long slender mesonephros lies ventral to the spinal cord and curves caudad from a point opposite the eighth cervical ganglion to the tail re- gion. The cranial third of the mesonephros is widest and its size diminishes tailwards. Between the yolk-sac and the tail the allantois is seen, its stalk curving around from the ventral side of the tail region. Digestive Canal. — The arrangement of the viscera may be seen in median sagittal and ventral dissections (Figs. 91 and 92), also in the reconstruction shown in Fig. 100. The mouth lies between the mandible, the median nasal process of the head, and the maxillary processes at the sides. The diverticulum of the hypophysis, flattened cephalo-caudad and expanded laterad, extends along the ventral wall of the fore-brain (Fig. 99). Near its distal end, the wall of the brain is thickened and later the posterior lobe of the hypophysis will develop from the brain wall at this point. The pharynx is flattened dorso-ventrally and is widest near the mouth. Its lateral dimension narrows caudad, and opposite the third branchial arch it makes an abrupt bend, a bend which corresponds to the cervical flexure of the embryo's body (Figs. 99 and 100). In the roof of the pharynx just caudal to Rathke's pocket is the somewhat cone-shaped pouch known as SeesseVs pocket, which may be interpreted as the blind cephalic end of the fore-gut. The lateral and ven- tral walls of the pharynx and oral cavity are shown in Fig. 93. Of the four arches the mandibular is the largest and a groove partly separates the processes of the two sides. Posterior to this groove and extending in the median line to the 102 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS hyoid arch is a triangular rounded elevation, the tuberculum impar, which later forms a part of the tongue. At an earlier stage the median thyreoid anlage grows out from the mid-ventral wall of the pharynx just caudal to the tuberculum impar. The ventral ends of the second arch fuse in the mid-ventral line and form a prominence, the copula. This connects the tuberculum impar with a Anlage of tongue R. atrium Esophagus Interatrial foramen Lung bud Stomach Hepatic diverticulum Ventral pancreas Cranial limb intestine Genital ridge Pharynx Metamere 4 Rathke's pocket Isthmus Mesencephalon Diencephalon - Bulbus cordis Telencephalon Ventricle Septum trans- versum Liver Yolk-sac Allantois Tail-gut Cloaca Metanephros Spinal cord / I Caudal limb of intestine Mesonephros Mesonephric duct Fig. 91. — Median sagittal dissection of a pig embryo of 6 mm., to show viscera and neural tube. X 18. rounded tubercle derived from the third and fourth pairs of arches, the anlage of the epiglottis. Its cephalic portion forms the root of the tongue (compare Fig. 151 A and B). Caudal to the epiglottis are the arytenoid ridges and a slit between them, the glottis, leads into the trachea. The branchial arches converge caudad and the pharynx narrows rapidly before it is differentiated into the trachea and esophagus (Figs. 99 and 100). Laterally and ventrally between the arches are the four paired outpocketings of LATERAL DISSECTION OF THE VISCERA IO3 the pharyngeal pouches. The pouches have each ;i dorsal and ventral diverti- culum (Fox, Thyng). The dorsal diverticula are large and wing-like (Fig. 99), meet the ectoderm of the gill clefts, fuse with it and form the closing plates. Between the ventral diverticula of the third pouch lies the median thyreoid anlage. The fourth pouch is much smaller than the others. Its dorsal diverticu- Eye Maxillary process Month Br. arch 3 Br. arch 4 Upper limb bud Hepatic diverticulum Yolk-sac Body-stalk Allanlois Umbilical artery Mesonephric duct Ironlo-nasal process Olfactory pit Mandibular process Br. arch 2 Aortic bulb Trachea Lung bud Stomach Cephalic loop of intestine Mesonephros Mesonephric duct Caudal loop of intestine Lower limb bud 'Rectum Dorsal aorta and umbilical artery Fig. 92. — A ventral dissection of a 6 mm. pig embryo. The head has been bent dorsally. Br. arch 2, j and 4, branchial arches, 2, 3 and 4. lum just meets the ectoderm, its ventral portion is small, tubular in form and is directed parallel to the esophagus (Fig. 99). The groove on the floor of the pharynx caudal to the epiglottis is continu- ous with the tracheal groove. More caudally opposite the atrium of the heart the trachea has separated from the esophagus. The trachea at once bifurcates to form the primary bronchi, and the anlages of the lungs. The lungs consist merely of the dilated ends of the bronchi surrounded by a layer of splanchnic mesoderm. They bud out laterally on each side of the esophagus near the cardiac end of the 104 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS stomach, and project into the pleural ccelom. The esophagus is short and widens dorso-ventrally to form the stomach. The long axis of the stomach is nearly straight, but its entodermal walls are flattened together and it has revolved on its long axis so that its dorsal border lies to the left, its ventral border to the right, as seen in transverse section (Fig. 106). Caudal to the pyloric end of the stomach, and to its right is given off from the duodenum the hepatic diverticulum. Its opening into the gut is seen in the ventral dissection (Fig. 92). The hepatic diverticulum is a sac of elongated oval form which later gives rise to the gall bladder, cystic duct and common bile duct. It is connected by several cords of cells with the trabecular of the liver. The liver is divided incompletely into four lobes, a small dorsal and a large ventral lobe on each side (Figs. 90 and 107). The lobation does not show in a median sagittal section. The pancreas is represented by two outgrowths. The Lateral Ungual anlage- i Tuberculinn impar '~--w_., ~-*-~ — j Branchial arch II Arytenoid ridge Branchial arch I Epiglottis \f 7^>S5i "s*dL S^m Branchial arch III *gjr jfl \ ' Branchial arch 71' Glottis Fig. 93. — Dissection of the tongue and branchial arches of a 7 mm. pig embryo, seen in dorsal view. ventral pancreas takes origin from the hepatic diverticulum near its attachment to the duodenum. It grows to the right of the duodenum and ventrad to the portal vein. The dorsal pancreas takes origin from the dorsal side of the duo- denum caudal to the hepatic diverticulum and grows dorsally into the substance of the gastric mesentery (Figs, ioo and 108). It is larger than the ventral pan- creas, and its posterior lobules grow to the right and dorsal to the portal vein and in later stages anastomose with the lobules of the ventral pancreas. The intestine of both fore-gut and hind-gut has elongated and curves ven- trally into the short umbilical cord where the yolk-stalk has narrowed at its point of attachment to the gut. As the intestinal tube grows ventrally, the layers of splanchnic mesoderm which attach it to the dorsal body wall grow at an equal rate and persist as the mesentery. The cloaca, a dorso-ventrally expanded portion of the hind-gut gives off LATERAL DISSECTION OF THE VISCERA 105 cephalad and ventrad the allantoic stalk. This is at first a narrow tube but soon expands into a vesicle of large size, a portion of which is seen in Fig. 90. Dorso- laterad the cloaca receives the primary excretory {Wolffian ducts). The hind-gut is continued into the tail as the tail-gut (post-anal gut) which dilates at its ex- tremity as in the 7.8 mm. pig described by Thyng. The mid-ventral wall of the cloaca is fused to the adjacent ectoderm to form the cloacal membrane. In this region later the anus arises (Fig. 100). The post-anal gut soon disap- pears. The urogenital organs consist of the mesonep/iroi, the mesonephric ducts, the anlages of the metanephroi, the cloaca and the allantois. The form of the mesonephroi is seen in Figs. 90 and 92. Each consists of large vascular glom- eruli associated with coiled tubules lined with cuboidal epithelium and opening into the mesonephric duct (Figs. 107 and 109). The Wolffian ducts beginning at the anterior end of the mesonephros curve at first along its ventral, then along its lateral surface. At its caudal end each duct bends ventrad and to the midline, where it opens into a lateral expansion of the cloaca. Be- R^tric/e fore this junction takes place, an evagination into the mesenchyme from the dorsal wall of each mesonephric duct gives rise to the FlG- 94-— Ventral and cranial sur- face of the heart from a 6 mm. pig anlages of the metanephroi, or permanent embryo. X 14. kidneys. A slight thickening of the meso- thelium along the median and ventral surface of each mesonephros forms a light-colored area, the genital fold (Fig. 91). This area is pointed at either end and confined to the middle third of the kidney. It is the anlage of the genital gland from which either testis or ovary is developed. Blood Vascular System. — The heart lies in the pericardial cavity as seen in Fig. 91. The atrial region (Fig. 94), as in the 4.2 mm. human embryo, has given rise to two lateral sacs, the right and left atria. The bulbo-ventricular loop has become differentiated into right and left ventricles much thicker walled than the atria. The right ventricle is the smaller and from it the bulbus passes between the atria and is continued as the ventral aorta. Viewed from the caudal and dorsal aspect (Fig. 95), the sinus venosus is seen dorsal to the atria. It opens into the right atrium and receives from the right side the right common cardinal vein, from the left side the left common cardinal. These veins drain the blood from the body of the embryo. Caudally the sinus venosus receives the two io6 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS vitelline veins. Of these, the left is small in the liver and later disappears. The right vitelline vein, now the common hepatic, carries most of the blood to the heart from the umbilical veins, from the liver sinusoids, gut and from the yolk-sac. Transverse sections of the embryo through the four chambers of the heart show the atria in communi- Bulbus cordis cation with the ventricles through the atrio-ventricular canals (Fig. 104), and the sinus venosus opening into the right auricle. This open- ing is guarded by the right and left valves of the sinus venosus. Septa incompletely separate the two atria and the two ventricles. In Fig. 104 the atrial septum {septum primum) appears complete. t. common Cardinal vein Left ventricle R. Atrium R. vitelline vein .R.Ventricle Fig. 95. — Dorsal and caudal view of the heart from a 6 mm. pig embryo. X 21. Bulbus cordis In Fig. 96, from a slightly smaller embryo, it is seen that the septum primum grows from the dorsal atrial wall of the heart and does not yet meet the endocardial cushions between the atrio-ventricular canals. This opening between the atria is known as the interatrial foramen. Before it closes, another opening appears in the septum, dorsal in position. This is the foramen ovale and persists during fetal life. In Fig. 96 these two openings may be seen, as may also the dorsal and ventral endo- cardial cushions. The outer mesothelial layer of the ventricles has become much thicker than that of the atria. It forms the epicardium and the myocardium, the sponge-like meshes of which are now being developed. The arteries begin with the ventral aorta, which takes origin from the bulbus cordis. From the ventral aorta are given off five pairs of aortic arches. These run dorsad in the five branchial arches (Figs. 99 and 100) and join the paired dorsal or descending aortce. The first and second pairs of aortic arches are very small and take origin from the small common trunks formed by the bifurcation Fofamm oyafe WallofLaJrium Interatrial foramen Endocardial cushionr Wall of.l.ventricle Fig. 96. — Dissection of a 5.5 mm. pig's heart from the left side, showing the septum primum and two interatrial foramina. X 14. LATERAL DISSECTION OF THE VISCERA 107 of the ventral aorta just caudal to the median thyreoid gland. The fourth aortic- arch is the largest. From the fifth arch small pulmonary arteries are developing. al to the first pair of aortic arches, the descending aortae are continued ird into the maxillary processes as the internal carotids. Caudal to the aortic arches the descending aortae converge, unite opposite the cardiac end of the stomach and form the median dorsal aorta. From this vessel and from the mding aorta? paired dorsal intersegmental arteries arise. From the seventh pair of these arteries (the first pair to arise from the median dorsal aorta), there are developed a pair of lateral branches to the upper limb buds. These vessels are the subclavian arteries. From the median dorsal aorta there are also given off ventro-lateral arteries to the glomeruli of the mesonephros, and median ventral arteries. Of the latter the coeliac artery arises opposite the origin of the hepatic diverticulum. The vitelline artery takes origin by two or three trunks caudal to the dorsal pancreas. Of these trunks the posterior is the larger and persists as the superior mesenteric artery. Thyng (Anat. Record, vol. 5, 191 1) has figured three trunks of origin in the 7.8 mm. pig. These unite and the single vitelline artery branches in the wall of the yolk-sac. Opposite the lower limb buds the dorsal aorta is divided for a short distance. From each division arises laterad three short trunks which unite to form the single umbilical artery on each side. The middle trunk is the largest and apparently becomes the common iliac artery. A pair of short caudal arteries, much smaller in size, continue the descending aortae into the tail region. The Veins. — The vitelline veins, originally paired throughout, are now repre- sented distally by a single vessel, which, arising in the wall of the yolk-sac, enters the embryo coursing cephalad to the intestinal loop (Figs. 97, 99 and 100). Cross- ing to the left side of the intestine and ventral to it, it is joined by the superior mesenteric vein which has developed in the mesentery of the intestinal loop. The trunk cranial to the union of these two vessels becomes the portal vein. It passes along the left side of the gut in the mesentery. Opposite the origin of the dorsal pancreas it gives off a small branch, a rudiment of the left vitelline vein, which courses cephalad and in earlier stages connects with the sinusoids of the liver. The portal vein then bends sharply to the right dorsal to the duo- denum and as the right vitelline vein, passing between the dorsal and ventral pancreas to the right of the duodenum, it soon enters the liver and connects with the liver sinusoids. The portal trunk is thus formed by persisting portions of both vitelline veins, and receives a new vessel, the superior mesenteric vein. The middle portions of the vitelline veins are connected with the network of liver ioS THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS sinusoids. Their proximal vitelline trunks drain the blood from the liver and open into the sinus venosus of the heart. The right vitelline trunk is much the larger and persists as the proximal portion of the inferior vena cava (for the de- velopment of the portal vein see Chapter IX) . The umbilical veins, taking their origin in the walls of the chorion and allan- toic vesicle, he caudal and lateral to the allantoic stalk and anastomose (Figs. 97 and 99). Before the allantoic stalk enters the body, the umbilical veins sepa- Spinal cord Ant. cardinal vein Cervical sinus Pericardial cavity Atrial junction sinus-, venosus Sinus venosus Right vitelline vein Liver Large venous sinusoid of Liver Hepatic diverticulum {cut) Yolk-stalk Portal vein Cephalic limb intestinal loop Right umbilical vein Vitelline artery Nolochord Ant. cardinal vein Pharynx Pericardial cavity Left common cardinal vein Left horn of sinus venosus Left vitelline vein Ductus venosus Ant. limb bud Inf. vena cava Dorsal pancreas - — Left vitelline vein Common vitelline vein Left umbilical vein Sup. mesenteric vein sLeft umbilical artery Caudal limb •' intestinal loop Right umbilical artery Dorsal aorta ' Fig. 97. — Reconstruction in ventral view of a 6 mm. pig embryo to show the vitelline and umbilical veins, the latter opened (original drawing by Mr. K. L. Vehe). Post, limb bud Spinal cord rate and run lateral to the umbilical arteries. The left vein is much the larger. Both, after receiving branches from the posterior limb buds and from the body wall, pass cephalad in the somatopleure at each side. Their course is first cephalad, then dorsad, until they enter the liver. The left vein enters a wide channel, the ductus venosus, which carries its blood through the liver, thence to the heart by way of the right vitelline trunk. The right vein joins a large sinu- soidal continuation of the portal vein in the liver. This common trunk drains into the ductus venosus. LATERAL DISSECTION OF THE VISCERA IO9 The anterior cardinal veins are formed by the plexus of veins on each side of the head which are drained by two trunks (Figs. 98 and 99). These extend caudad and lie lateral to the ventral portion of the myelencephalon. Each an- terior cardinal vein receives branches from the sides of the myelencephalon, then curves ventrad, is joined by the linguo-facial vein from the branchial arches and at once unites with the posterior cardinal of the same side to form the common cardinal vein. This, as we have seen, opens into the sinus venosus. Spinal cord Anterior cardinal vein Cervical sinus Pericardial cavity R. common cardinal vein Post, cardinal vein Esophagus Large venous sinusoid Liver Anterior limb bud —M Inf. vena, cava * Post. Post, cardinal vein Mesonepliros (cut surface) R. subcardinal vein — Venous sinusoid on dorsum of mesonc phros Dorsal aorta Notochord Xotochord Pharynx Trachea L. common cardinal vein Lung Liver Stomach (cut edge) Omenta! bursa Mesogastriuni Mesonepliros (cut surface) Capillary anastomosis between subcardinal veins Vitelline artery in dorsal mesentery Capillary anastomosis between subcardinal veins Venous sinusoid on dorsum of mesonepliros Spinal cord Fig. 98. — Reconstruction of the cardinal and subcardinal veins of a 6 mm. pig embryo showing the early development of the inferior vena cava (K. L. Vehe). The posterior cardinal veins develop on each side in the mesonephric ridge, dorso-lateral to the mesonephros (Figs. 98 and 99). Running cephalad, they join the anterior cardinal veins. When the mesonephroi become prominent, as at this stage, the middle third of each posterior cardinal is broken up into sinusoids (Minot). Sinusoids extend from the posterior cardinal vein veritrally around both the lateral and medial surfaces of the mesonephros. The median sinusoids anastomose longitudinally and form the subcardinal veins, right and left. The no THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS subcardinals lie along the median surfaces of the mesonephroi, more ventrad than the posterior cardinals with which they are connected at either end. There A. car. V. card Meten. jnt. ant. Ph Ph. R2 Ph. R3 Mesen. Ao. v. V. oph. Ch. d. Ao. desc. Med. sp. V. card. post. Aa. mes. Mes. Fig. 99. — Reconstruction of 7.8 mm. pig embryo showing veins and aortic arches from the left side (Thyng). X 15. Ao. desc, descending aorta; Ao. v., ventral aorta; A. car. int., internal carotid artery; Aa. mes., mesonephric arteries; A. pul., pulmonary artery; At. d., right atrium; D. v., ductus venosus; Ph. P. 1, 2, 3, 4, pharyngeal pouches; 5. v., sinus venosus; Th. med., thryeoid gland; V. card, ant., anterior cardinal vein; V. card. com. d., right common cardinal vein; V. card. com. s., left common cardinal vein; V. card, post., posterior cardinal vein; V. hep. com., common hepatic vein; V. is., inter- segmental vein; V. ling-fac, linguo-facial vein; V. oph., ophthalmic vein; V. p., portal vein; V.scard. d., right subcardinal vein; V. scl. d., right subclavian vein; V. umb. d., right umbilical vein; Vcn. d., ri^'ht ventricle. is a transverse capillary anastomosis between them, cranial and caudal to the permanent trunk of the vitelline artery. The right subcardinal is connected with the liver sinusoids through a small vein which develops in the mesenchyme TRANSVERSE SECTIONS III of the plica venae cavas (caval mesentery) located to the right of the mesentery ( Fig. 107). This vein now carries blood direct to the heart from the right pos- terior cardinal and right subcardinal, by way of the liver sinusoids and the right vitelline trunk (common hepatic vein). Eventually these four vessels form the unpaired inferior vena cava. (For the development of the inferior vena cava see Chapter IX). - First aortic arch Seessel's pocket Second aortic arch Pharynx Thyreoid Third aortic arch Xolochord Fourth aortic arch 102 Fifth aortic arch and pulmonary artery 103 Esophagus ■ 104 Trachea 105 R. lung 106 107 Stomach 108 Cccliac artery log Ventral pancreas Dorsal pancreas Gall bladder L. umbilical vein Vitelline artery Isthmus Int. carotid artery ■)i Pituitary bodv (pharyngeal lobe) Optic recess 102 Telencephalon Ventral aorta Bulbus cordis Interventricular foramen L. horn of sinus venosus io5 s-L. umbilical vein Tail-gut Cloaca 107 Spinal cord in 112 108 Melanephric anlage iog L. umbilical artery 'Anastomosis between dorsal aorta Allantoic stalk L. dorsal aorta Mesonephric duct Cephalic limb of intestinal loop Dorsal aorta I Artery to mesonephros Mesentery Caudal limb of intestinal loop Fig. 100. — Reconstruction of a 6 mm. pig embryo in the median sagittal plane, viewed from the right side. The numbered heavy lines indicate the levels of the transverse sections shown in Figs. 101-112. The broken lines indicate the outline of the left mesonephros and the course of the left um- bilical artery and vein. The latter may be traced from the umbilical cord to the liver where it is sec- tioned longitudinally. (Original drawing and reconstruction by Mr. K. L. Vehe). X i63 ■>■ Transverse Sections Having acquainted himself with the anatomy of the embryo from the study of dissections and reconstructions, the student should examine serial sections cut in the plane indicated by guide lines on Fig. 100. Refer back to the external structure of the embryo (Fig. 88), to the lateral dissection of the organs (Fig. 112 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 90) , and note the plane of each section and the structures which would appear in Fig. 100. Sections typical of certain regions should be drawn. The various structures may be recognized by referring to the figures of sections in the text, and they should be traced through the series as carefully as time will allow. Transverse Section through the Myelencephalon and Otocysts of a 6 mm. Embryo (Fig. 101). — As the head is bent nearly at right angles to the body, this section passes lengthwise through the myelencephalon. The diencephalon is cut transversely. The cellular walls of the myelencephalon show a series of six pairs of constrictions, the neuromeres. Lateral Fourth ventricle Neur. 6 Gang, jugular n.io Neur. 5 Otocyst Neur. 4 Neur. 3 Neur. 2 Neur. 1 Int. carotid artery Prosencephalon Myelencephalon Gang, superior n. g Ant. cardinal vein Gang, acust. n. S Gang, geniculat. n. 7 Gang, semilunar. Vein Vein Fig. ioi. — Transverse section through the myelencephalon and otocysts of a 6 mm. pig embryo. X 26.5. Ant. cardinal vein, anterior cardinal vein; Gang, acust. n.S, acustic ganglion of acustic nerve; Gang, geniculat. n.7, geniculate ganglion of the facial nerve; Int. carotid artery, internal carotid artery; Neur. 1, 2, 3, 4, neuromeres 1, 2, 3, and 4. to the fourth pair of neuromeres are the otocysts, which show a median outpocketing at the point of entrance of the endolymph duct. The ganglia of the nn. trigeminus, facialis, acus- ticus and the superior ganglion of the glossopharyngeal nerve occur in order on each side. Sections of the anterior cardinal vein and its branches show on the left side. Ventral to the diencephalon are sections of the internal carotid arteries. Passing along down the series into the pharynx region, observe the first, second and third pharyngeal pouches. Their dorsal diverticula come into contact with the ectoderm of the branchial clefts and form the closing plates. Transverse Sections through the Branchial Arches and the Eyes (Fig. 102). — The section passes lengthwise through the four branchial arches, the fourth sunken in the cervical sinus. Dorsad is the spinal cord with the first pair of cervical ganglia. The pharynx is cut across between the third and fourth branchial pouches. In its floor is a prominence, TRANSVERSE SECTIONS 113 the anlage of the epiglottis. Ventral to the pharynx the ventral aorta gives off two pairs of vessels. The larger pair are the fourth aortic arches which curve dorsad around the pharynx to enter the descanting aorlcc. The smaller third aortic arches enter the third branchial ur< hes on each side. A few sections higher up in the series the ventral aorta bifurcates and the right and left trunks thus formed give off the first and second pair of aortic, arches. Craniallv in the angle between their common trunks lies the median thyreoid anlage. The anterior cardinal veins are located lateral and dorsal to the descending aorta?. The end of the head is cut through the telencephalon and the optic vesicles. On the left side of the figure the lens vesicle may be seen still connected with the ectoderm. The optic vesicle now shows a thick inner and a thin outer layer; these form the nervous and pigment layers of the retina respectively. Spinal gang. Xotochord A nl. cardinal vein Pharynx Phar. pouch 3 Aortic arch 3 Phar. pouch 2 ^*sj&& Neural tube Myotome Lens of eye Prosencephalon Optic vesicle Fig. 102. — Transverse section through the branchial arches and eyes of a 6 mm. pig embryo. X 26.5. Disc, aorta, descending aorta; Br. arch 2, 3, 4, branchial arch 2, 3 and 4; Phar. pouch 2, j, pharyngeal pouches 2 and 3; A', aortic arch 4. Transverse Section through the Tracheal Groove, Bulbus Cordis and Olfactory Pits (Fig. 103). — The ventral portion of the figure shows a section through the tip of the head. The telencephalon is not prominent. The ectoderm is thickened and slightly invagi- nated ventro-latcrad to form the anlages of the olfactory pits. These deepen in later stages and become the nasal cavities. In the dorsal portion of the section may be seen the cervical portion of the spinal cord, the notochord just ventral to it, the descending aortcr, and ventro- lateral to them the anterior cardinal veins. The pharynx now is small with a vertical groove in its floor. This is the tracheal groove and more caudad it will become the cavity of the trachea. The bulbus cordis lies in the large pericardial cavity. On either side the section cuts through the cephalic portions of the atria. These will become larger as we go caudad in the series. 8 H4 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS Transverse Section through the Heart (Fig. 104.) — The section passes through the bases of the upper limb buds. Lateral to the descending aortse are the common cardinal veins. The right common cardinal opens into the sinus venosus which in turn empties into the right atrium, its opening being guarded by the two valves of the sinus venosus. The trachea has now separated from the esophagus and lies ventral to it. Both trachea and esophagus are sur- rounded by a condensation of mesenchyme. The myocardium of the ventricles has formed a spongy layer much thicker than that of the atrial wall. An incomplete interventricular septum leaves the ventricles in communication dorsad. The septum primum is complete in this section but higher up in the series there is an interatrial foramen (see Fig. 96). The/or- amen ovale is not yet formed. Spinal chord Notochord^ Ant cardinal vein Ratrium Somatopleure Olfactory pit. Myotome Descending aorta arynx Pericardial cavity Bui bus cordis Telencephalon Fig. 103. — Transverse section through the bulbus cordis and olfactory pits of a 6 mm. pig embryo. X 26.5. Transverse Section through the Lung Buds and Septum Transversum (Fig. 105). — The tips of the ventricles lying in the pericardial cavity still show in this section. Dor- sally the pericardial cavity has given place to the pleuro-peritoneal cavity. Into this cavity project ventrad the Wolffian ridges in which the posterior cardinal veins partly lie. Into the floor of the pleuro-peritoneal cavities bulge the dorsal lobes of the liver, embedded in mesen- chyma. This mesenchyma is continuous with that of the somatopleure, and forms a complete transverse septum ventrally between the liver and heart. This is the septum transversum which takes part in forming the ligaments of the liver and is the anlage of a portion of the diaphragm. Passing through the septum are the two proximal trunks of the vitelline veins. Projecting laterally into the pleuro-peritoneal cavities are ridges of mesenchyma covered by splanchnic mesoderm in which the lungs develop as lateral buds from the caudal end of the trachea. The right lung bud is shown in the figure. Between the esophagus and the lung is a crescent-shaped cavity, the end of the lesser peritoneal sac. TRANSVERSE SECTIONS 115 Transverse Section through the Stomach (Fig. to6). — The section passes through the upper limb buds and just caudal to the point at which the descending aorta unite to form the median dorsal aorta. As the liver develops in early stages, it comes into relation with the plica venoz cavee along the dorsal body wall to the right side of the dorsal mesogastrium. The Myotome Descending aorta Esophagus Sinus venosus Valve of sinus Venosus R. atrium Atrioventricular opening Inter ventricular Septum R.Ventncle Oomalopleure Spinal cord L Common Cardinal Vein Jntervenlrlc- ular foramen Peri cardial cavity Fig. 104. — Transverse section through the four chambers of the heart of a 6 mm. pig embryo. X 26.5. Spinal ganglion Spinal nerve Descending aorta. Pleuro-peri- toneal Cavity R.lung bud Rvitelline vein Septum transversum Pericardial Cavity R. Ventricle Spinal cord Upper limb bud Post, cardinal Vein esophagus Dorsal lobe liver Lesser sac L. vitelline L. ventricle Fig. 105. —Transverse section through the right lung bud and septum transversum of a 6 mm. pig embryo. X 26.5. i6 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS space between the liver and plica to the right, and the stomach and its omenta to the left, is a caudal continuation of the lesser peritoneal sac. The dorsal wall of the stomach is rotated to the left, its ventral wall to the right. The liver shows a pair of dorsal lobes and contains Spinal gang. Nohchord Dorsal aorta. teritoneal cavity Lesser sac Common hepatic Vein (R.vitel line) ff. ventral lobe liver R. ventricle Spinal cord Spinal nerve Post, card. Vein Upper limb bud Otomach L.ventral lobe fiver Lventricle Fig. 106. — Transverse section through the stomach of a 6 mm. pig embryo. X 26.5. Spinal cord Nolochord Posf. card, vein Dorsal aorta inf. vena cava. Portal vein n. umbilical vein Hepatic diverticulum Myotome Postcard vein Upper limb bud — Dorsal rnesogastrium Dorsal lobe Liver {..Vitelline \/ein I. umbilical Vein Peritoneal Cavity Fig. 107. — Transverse section through the hepatic diverticulum of a 6 mm. pig embryo. X 26.5. large blood spaces and networks of sinusoids lined with endothelium. Ventral to the liver, the tips of the ventricles are seen. Transverse Section through the Hepatic Diverticulum (Fig. 107).— The upper limb buds are prominent in this section. The mesonephric folds show the tubules and glomeruli TRANSVERSE SECTIONS 117 of the mcsoncphroi and the posterior cardinal veins arc connected with the mesonephric sinu- soids. To the median side of the right mesonephros shows the right subcardinal vein. From the dorsal attachment of the liver there is continued down into this section a ridge on the dorsal body wall just to the right (left of figure) of the mesentery. In this ridge lies a small vein which connects cranially with the liver sinusoids, caudally with the right subcardinal vein. As it later forms a portion of the inferior vena cava, the ridge in which it lies is termed the plica vena cava; or caval mesentery. The right dorsal lobe of the liver contains a large blood space into which opens the portal vein. The duodenum has curved ventral to the position occupied by the stomach in the previous section. There is given off from it ventrad and to the right the hepatic diverticulum. In the sections higher up small ducts from the liver tra- becular may be traced into connection with it. In the left ventral lobe of the liver, a large- blood space indicates the position of the left umbilical vein on its way to the ductus venosus. Dorsal aorta R. post, cardinal Vein Glomerulus of Mesonephros Inf. vena cava Portal vein ff. umbilical Vein Distal end hepatic diverticulum Spinal cord L. post, cardinal vein Mesonephros Upper limb bud Mesentery Dorsal pancreas [..vitelline Vein Duodenum L.umbilical Vein Ventral pancreas Fig. 108. — Transverse section through the dorsal pancreas of a 6 mm. pig embryo. X 26.5. Transverse Section through the Dorsal Pancreas (Fig. 108). — At this level the upper limb buds still show, the mesenephroi are larger and marked by their large glomeruli. The right posterior cardinal vein is broken up into mesonephric sinusoids. The vein in the plica Venae cavae will, a few sections lower, connect with the right subcardinal vein. The anlage of the dorsal pancreas is seen extending from the duodenum dorsad into the mesenchyme of the mesentery. It soon bifurcates into a dorsal and right lobe, of which the latter is slightly lobulated. Yentro-latcral to the duodenum, the anlage of the ventral pancreas is seen cut across. It may be traced cephalad in the series to its origin from the hepatic diverticulum. To the right of the ventral pancreas (left of figure) lies the portal vein (portion of right vitelline). To the left of the dorsal pancreas is seen the remains of the left vitelline vein. The ventral lobes of the liver are just disappearing at this level. In the mesenchyme which connects the liver with the ventral body wall lie on each side the umbilical veins, the left being the larger. Between the veins is the extremity of the hepatic diverticulum. The body wall is continued ventrad to form a short umbilical cord. n8 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS Transverse Section at Level of the Origin of the Vitelline Artery and Umbilical Arteries (Fig. 109). — As the posterior half of the embryo is curved in the form of a half circle, sections caudal to the liver, like this one, pass through the lower end of the body at the level of the posterior limb buds. Two sections of the embryo are thus seen in one, their ventral aspects facing each other and connected by the lateral body wall. In the dorsal part of the section the mesonephroi are prominent with large posterior cardinal veins lying dorsal Spinal cord Noto chord ff.post card Vein Dorsal aorta R.sub, card. vein Mesentery Cephalic limb of intestine /?. umbilical l/em Caudal Jirnb of intestine Vein „ Tail . Lower limb bud mesonephric duct Dorsal aorta Spinal Cord Spina,/ nerve Post. card. vein Mesonephros I. sub, card. Vein Lviteltine Vein - L.umbilical l/ein went Fig. 109. — Transverse section of a 6 mm. pig embryo at the level of the origin of the vitelline artery. The lower end of the section passes through the posterior limb buds. X 26.5. Mes. tubule, mesonephric tubule; R. post. card, vein, right posterior cardinal vein. to them. The trunk of the vitelline artery takes origin ventrally from the aorta. It may be traced into the mesentery, and through it into the wall of the yolk-sac. On either side of the vitelline artery are the subcardinal veins, the right being the larger. In the mesentery may be seen two sections of the intestinal loop (the small intestine being cut lengthwise, the large intes- tine transversely), and also sections of the vitelline artery and veins. In the lateral body walls ventral to the mesonephros occur the umbilical veins. The left vein is large and cut length- wise. The right vein is cut obliquely twice. TRANSVERSE SECTIONS 119 In the ventral portion of the section, the lower limb buds are prominent laterally. A large pair of arteries, the common iliacs, are given off from the aorta and may be traced into connection with the umbilical arteries. The large intestine supported by a short mesentery lies in the ccelom near the midline. On each side are the mesonepkric folds, here small and each show- ing a section of the mesonephric duel and a single vesicular anlage of the mesonephric tubules. The mesonephric ducts are sectioned as they curve around from their position in the dorsal portion of the section. Section through the Primitive Segments and Spinal Cord (Fig. no).— This section is near the end of the series and as the body is here curved it is really a longitudinal section. At the left side of the spinal cord the oval cellular masses are the spinal ganglia cut across. The ectoderm, arching over the segments, indicates their position. Each segment shows an outer dense layer, the cutis plate, lying just be- neath the ectoderm. This plate curves lateral to the spindle shaped muscle plate which gives rise to the volun- tary muscle. Next comes a diffuse mass of mesenchyma. the sclerotome, which, eventually, with its fellow of the opposite side, surrounds the spinal cord and forms the Spinal gang. Inlerseg- mental artery M itscle plate Cutis plate Sclerotome Ectoderm Spinal cord Fig. no. — Transverse sec- tion through the primitive seg- ments and spinal cord of a 6 mm. pig embryo. X 45- Vein Rjjmbilicol artery. Tail Mesonephric duct ^SkBuBEm Spinal cord jVotochord Fig. iii. — Transverse section through the umbilical vessels, allan- tois and cloaca of a 6 mm. pig embryo. X 45. anlage of a vertebra. From it is developed also connective tissue. A pair of spinal nerves and spinal ganglia are developed opposite each somite, and pairs of small vessels are seen between the segments. These are dorsal intersegmental arteries. Section through the Umbilical Vessels, Allantois and Cloaca (Fig. in). — We have now studied sections at various levels of the 6 mm. embryo to near the end of the series. We shall next examine sections through the caudal region and study the anlages of the uro- genital organs. Owing to the curvature of the embryo, we will now be going cephalad in our series. The first section passes through the bases of the limb buds at the level where the allantoic stalk, curving inward from the umbilical cord, opens into the cloaca. At either side of the allantoic stalk may be seen oblique sections of the umbilical arteries and lateral to these the large left and small right umbilical vein. The mesonephric ducts occupy the mesonephric ridges which project into small caudal prolongations of the ccelom. Midway between the ducts lies the hind-gut, dorsal to the cloaca. The tip of the tail is seen in section to the left of the figure. 120 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS Section through the Anlages of the Metonephroi, Cloaca and Hind-gut (Fig. 112). — The metonephroi are seen as dorsal evaginations from the mesonephric (Wolffian) ducts just before their entrance into the cloaca. Each consists of an epithelial layer surrounded by a condensation of mesenchyme. Traced a few sections cephalad the mesonephric ducts open into the lateral diverticula of the cloaca, which, irregular in outline, because it is sectioned obliquely, lies ventral to them and receives dorsad the hind-gut. Caudal to the cloaca in this embryo, the tail bends abruptly cephalad and to the right. The blind prolongation of the hind-gut may be traced out into this portion of the tail until it ends in a sac-like dilatation. n. umbilical vein n.umbi lied artery Tail Mesonephric duct Metonephric onlag Spinal Ventral body wall Lumbilical artery Lumbilical vein Allantoic stalk; Cloaca Hind-gut Notochord Fig. 112. — Transverse section through the anlages of the metanephroi of a six mm. pig embryo. X 45. B. THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS The study of embryos at this stage is important as they possess the anlages of most of the organs. The anatomy of a 12 mm. pig embryo has been carefully studied and described by Lewis (Amer. Jour. Anal., vol. 2, pp. 211-225, 1903). External Form (Fig. 113).- -The head is now relatively large on account of the increased size of the brain. The third branchial arch is still visible in the embryo, but the fourth arch has sunken in the cervical sinus; usually both have disappeared at a slightly later stage. The olfactory pits form elongated grooves on the under surface of the head and the lens of the eye lies beneath the ectoderm surrounded by the optic cup. The maxillary and mandibular processes of the first branchial arch are larger and the former shows signs of fusing with the median nasal process to form the upper jaw. Small tubercles, the anlages of the external ear have developed about the first branchial cleft which itself becomes the external auditory meatus. At the cervical bend the head is flexed at right angles with the body bringing the ventral surface of the head close to that of the trunk and it is probably owing to this flexure that the third and fourth branchial arches buckle inward to form the cervical sinus. Dorsad the trunk forms a long curve more marked opposite THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 121 the posterior extremities. The reduction in the trunk flexures is due to the in- creased size of the heart, liver and mesonephroi. These organs may be seen Myelcnccphalon Br Hyoid arch Cervical flexure Br. arch III Cervical sinus - Upper / limb bud 1 Milk line - — i-i~ Mes. segment Cephalic flexure Eye — Maxillary process Mandibular process Olfactory pit Yolk-sac Umbilical cord Lower limb bud' Fig. 113. — Exterior of a 10 mm. pig embryo viewed from the right side. X 7. Br. arch III, branchial arch three; Br. cleft I, first branchial cleft; mes. segment, mesodermal segment. Cervical flexure Ext. Ear Cephalic flexure through the translucent body wall and the position of the septum transfer sum may be noted between the heart and the diaphragm, as in Fig. 115. The limb buds are larger and the umbilical cord is prominent ventrad. Dor- sally the mesodermal segments may be seen and extending in a curve between the bases of the limb buds is the milk line, a thickened ridge of ectoderm which forms the an- lages of the mammary glands. The tail is long and tapering. Between its base and the umbilical cord is the genital eminence (Fig. 115). Human embryos of this stage or slightly older, vary considerably in size (Fig. 1 14) . They differ from pig embryos in the greater size of Yolk-sac ,'Q of 12 Fig. 114. — Exterior of a human ( mm., viewed from the right side, showing attachment of amnion (cut away) and yolk-stalk and -sac. X 5. 122 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS the head, the shorter tail, the much smaller mesonephric region, the longer umbilical cord and the less prominent segments. The yolk-sac is pear-shaped with long slender yolk-stalk. Central Nervous System and Viscera. — Dissections show well the form and relations of the organs (Figs. 115, 116 and 117). Directions for preparing dis- sections are given in Chapter VI. Metencephalon N. trochlearis Gang. n. 5 \ I Mesencephalon Gang. nn. 7 and 8 N. facialis Gang, superior n. 9 \ Gang, jugular e n. 10 Gang, petrosal n. 9 Gang. Froriep Gang, nodos. n. 10 N. accessorius X. l:\poglossus Atrium Lung Gang. cerv. 8 Septum transversum Liver Mesonephros Gang, tliorac. 10 N. oculomotorius Diencephalon Ophthalmic r. n. 5 •N. opticus Maxillary r. n. 5 Telencephalon Mandibular r. n. 5 Chorda tymp. n. 7 Ventricle Umbilical cord Genital eminence Fig. 115. — Lateral dissection of a 10 mm. pig embryo, showing the viscera and nervous system from the right side. The eye has been removed and the otic vesicle is represented by a broken line. The ventral roots of the spinal nerves are not indicated. X 10.5. n., nerve; r., ramus. Brain. — Five distinct regions may be distinguished (Figs. 115 and 117): (1) The telencephalon with its rounded lateral outgrowths, the cerebral hemispheres. Their cavities, the lateral ventricles communicate by the interventricular foramen with the third ventricle. (2) The diencephalon shows a laterally flattened cavity, the third ventricle. Ventro-laterally from the diencephalon pass off the optic stalks and an evagination of the mid-ventral wall is the anlage of the posterior THE WAIuMY «>F TEN TO TWELVE MM. PIG EMBRYOS 123 hypophyseal lobe. (3) The mesencephalon is undivided but its cavity becomes the cerebral aqueduct leading caudally into the fourth ventricle. ( 4 1 The melencephalon is separated from the mesencephalon by a constriction, the isthmus. Dorso- laterally it becomes the cerebellum, vent rally the pons. (5) The elongated myclcnccphalon is roofed over by a thin non-nervous ependymal layer. Its ventro- lateral wall is ..thickened and still gives internal indication of the ncuromcrcs. The cavity of the metencephalon and myelencephalon is the fourth ventricle. Cerebral Nerves. Of the twelve cerebral nerves all but the first (olfactory) and sixth (abducens) are represented in Fig. 115. For a detailed description . 1 ccessory gang. 1 Accessory gong. 2 Ace. gang. 3 r — Myelencephalon Ace. gang 4 Cent. gang. 2 Gang, nodosum X. 12 Fig. 116. — Dissection of the head of a 15 mm. pig embryo from the right side to show the accessory vagus ganglia with peripheral roots passing to the hypoglossal nerve. of these nerves see Chapter XII. (2) The optic nerve is represented by the optic stalk cut through in Fig. 115. (3) The oculomotor, a motor nerve to the eye muscles, takes origin from the ventro-lateral wall of the mesencephalon. (4) The trochlear nerve fibers, motor, to the superior oblique muscle of the eye, arise from the ventral wall of the mesencephalon, turn dorsad and cross at the isthmus, thus emerging on the opposite side. From the myelencephalon arise in order (5) the trigeminal nerve, mixed, with its semilunar ganglion and three branches, the ophthalmic, maxillary, and mandibular; (6) the ;/. abducens. motor, from the ventral wall to the external rectus muscle of the eye; (7) the n. facialis, mixed, 124 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS with its geniculate ganglion and its superficial petrosal, chorda tympani and facial branches; (8) the n. acusticus, sensory, arising cranial to the otocyst, with its acustic ganglion and sensory fibers to the internal ear; (9) caudal to the otocyst the n. glossopharyngeus, mixed, with its superior and petrosal ganglia; (10) the vagus, sensory, with its jugular and nodose ganglia; (11) accompanying the vagus the motor fibers of the spinal accessory which take origin between the jugular and sixth cervical ganglia from the lateral wall of the spinal cord and myelencephalon; the internal branch of the n. accessorius accompanies the vagus; the external branch leaves it between the jugular and nodose ganglia and supplies the sternocleidomastoid and trapezius muscles; (12) the n. hypoglossus, motor, arising by five or six fascicles from the ventral wall of the myelencephalon, its trunk passing lateral to the nodose ganglion and supplying the muscles of the tongue. From the jugular ganglion of the vagus extends a nodular chain of ganglion cells. These have been interpreted as accessory vagus ganglia. They may, however, be continuous with Froriep's ganglion which sends sensory fibers to the n. hypoglossus. In pig embryos of 15 to 16 mm. this chain is frequently divided into four or five ganglionic masses, of which occasionally two or three (including Froriep's ganglion) may send fibers to the root fascicles of the hypo- glossal nerve. Such a condition is shown in Fig. 116. Spinal Nerves. These have each their spinal ganglion, from which the dorsal root fibers are developed (Figs. 115 and 131). The motor fibers take origin from the ventral cells of the neural tube and form the ventral roots which join the dorsal roots in the nerve trunk. In Fig. 115 the heart with its right atrium and ventricle, the dorsal and ven- tral lobes of the liver, and the large mesonephros are prominent. Dorsal, and somewhat caudal to the atrium, is the anlage of the right lung. The septum transversum extends between the heart and the liver. Pharynx and Its Derivatives. — Dorsally the anterior lobe of the hypophysis is long and forks at its end (Figs. 117 and 118). In the floor of the pharynx are the anlages of the tongue and epiglottis (Fig. 151 A) . From each mandibular arch arises an elongated thickening which extends caudal to the second arch. Be- tween, and fused to these thickenings, is the triangular tuberculum impar. The opening of the thyreoid duct between the tuberculum impar and the second arch is early obliterated. A median ridge, or copula, between the second arches con- nects the tuberculum impar with the epiglottis, which seems to develop from the bases of the third and fourth branchial arches. On either side of the slit-like glottis are the arytenoid folds of the larynx. (For the development of the tongue, THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 125 see p. 158.) The pharyngeal pouches are now larger than in the 6 mm. pig (Fig. 118). The first pouch persists as the Eustachian tube and middle ear cavity, the closing plate between it and the first branchial cleft forming the tympanic membrane. The second pouch later largely disappears. About it, develops the palatine tonsil. The third pouch is tubular, directed at right angles to the pha rvnx MetencephaUm Tela choroidal Ncuromcrcs of myelencephalon Notochord Tongue Spinal cord Dorsal Pancreas Hepatic diverticulum Duodenum L. genital fold L. mesonepkros Dorsal aorta Mesencephalon Diencephalon Post, lobe hypophysis Optic recess Telencephalon A nt. lobe hypophysis Bulb us cordis Ventricle Yolk-sac Septum transversum Yolk-stalk Liver Caecum 'Small intestine Allanlois Urogenital sinus I 'relcr Mesonepkric duct Colon Umbilical artery (cut away) Metanephros Rectum Fig. 117. — Median sagittal dissection of a 10 mm. pig embryo, showing the brain, spinal cord and viscera from the right side. X 10.5. and meets the ectoderm to form a " closing plate." Median to the plate, the ventral diverticulum of the third pouch is the anlage of the thymus gland. Its dorsal diverticulum forms an epithelial body, or parathyrcoid. The fourth pouch is smaller and its dorsal diverticulum gives rise to a second parathyreoid body. Its ventral diverticulum is a rudimentary thymus anlage. A tubular 126 THE STUDY OP SIX AND TEN MILLIMETER PIG EMBRYOS outgrowth, caudal to the fourth pouch, is regarded as a fifth pharyngeal pouch in human embryos and forms the post-branchial body on each side (see p. 172). The thyreoid gland, composed of branched cellular cords, is located in the mid- line between the second and third branchial arches (Fig. 118). Trachea and Lungs. — Caudal to the fourth pharyngeal pouches the eso- Ganq.n.s Gang.nnj Otbcyst Phar. pouch J, Gancj.juauJare n- Aortic an Phar. pouch Caudal root n hypoglossus Ganyfr Aortic arch*- Gany.cervT PhaxpouthTJ Aortic arch 5 R descend. Aort, Esophagus Trachea- Vertebral arte Subclavian artery R.Lu/ICJ R. Atrium Stomach I Dorsal pan ere i Vitelline artery Ventral pahcreas I Descending Jlorti tiokihor Fig. 118. — Reconstruction of a 10 mm. pig to show the side. The veins are not indicated. Broken lines indicate positions of the limb buds. X 10. Post lobe hypophysis nt. lobe hypophysis Eye Phar. pouch I Maxillary process Thyreoid gland Pulmonary art: Aorta Yolksac RVentrielt Septum transversum Liver diverticulum Cloaca. Allantois Rectum Ureter 'etartephros Umbilical artery Mesonephric duct Cephalic limb, infest, loop position of the various organs from the right the outline of the left mesonephros and the phagus and trachea separate and form entodermal tubes (Figs. 117 and 118). Be- fore the trachea bifurcates to form the primary bronchi there appears on its right side the tracheal bud of the upper lobe of the right lung. This bronchial bud is developed only on the right side and appears in embryos of 8 to 9 mm. Two secondary bronchial buds arise from the primary bronchus of each lung, and form the anlages of the symmetrical lobes of each lung (Fig. 119). CHE ANATOMY OP TEX TO TWELVE MM. PIG EMBRYOS 127 Esophagus and Stomach. — The esophagus extends as a narrow tube caudal to the lungs, where it dilates into the stomach. The stomach is wide from its greater to its lesser curvature and shows a cardiac diverticulum (Lewis). The pyloric end of the stomach has rotated more to the right, where it opens into the duodenum, from which division of the intestine develop the liver and pancreas. The liver, with its four lobes, fills in the space between the heart, stomach and duodenum (Fig. 117). Extending from the right side of the duodenum along Lat. nasal process Lacrymal groove Maxillary process Mandibular process Cervical sinus Trachea Tracheal lung bud Upper limb bud Septum transversum Hepatic diverticulum Yolk-sac Yolk-stalk Allantois R. umbilical artery Olfactory pit Eye Median nasal process Br. arch 2 Br. arch 3 Br. arch 4 L. lung Esophagus Stomach Mesonephric duct Ventral pancreas Mesonephros Cephalic limb of intestine Caudal limb of intestine Rectum Mctanephros Lower limb bud ' Mesonephric duct / Spinal cord Rectum Fig. 1 19. — Ventral dissection of a 9 mm. pig embryo. The head is represented as bent dorsally. the dorsal and caudal surface of the liver is the hepatic diverticulum. It lies to the right of the midline and its extremity is saccular. This saccular portion becomes the gall bladder. Several ducts connect the diverticulum with the liver cords. One of these persists as the hepatic duct which joins the cystic duct of the gall bladder. The proximal portion of the diverticulum becomes the common bile duct, or ductus cholcdochus. The ventral pancreas arises from the common bile duct near its point of origin (Fig. 118). It is directed dorsad and caudad to the right of the duodenum. The dorsal pancreas arises more caudally from the 128 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS dorsal wall of the duodenum and its larger, lobulated body grows dorsally and cranially (Figs. 118 and 135). Between the pancreatic anlages courses the portal vein. In the pig, the duct of the dorsal pancreas persists as the functional duct. Intestine. — Caudal to the duodenum, the intestinal loop extends well into the umbilical cord (Figs. 117 and 118). At the bend of the intestinal loop is the slender yolk-stalk. The cephalic limb of the intestine lies to the right, owing to the rotation of the loop. The small intestine extends as far as a slight enlarge- ment of the caudal limb of the loop, the anlage of the ccecum, or blind gut. This anlage marks the beginning of the large intestine (colon and rectum). The intestinal loop is supported by the mesentery which is cut away in Fig. 117. The cloaca is now nearly separated into the rectum and urogenital sinus. The cavity of the rectum is almost occluded by epithelial cells (Lewis). Urogenital System. — The mesonephros is much larger and more highly dif- ferentiated than in the 6 mm. embryo (Figs. 115 and 119). Along the middle of its ventro-median surface the genital fold is now more prominent (Fig. 117). In a ventral dissection (Fig. 119) the course of the mesonephric ducts may be traced. They open into the urogenital sinus, which also receives the allantoic stalk. The metanephros, or permanent kidney anlage, lies just mesial to the umbilical arteries where they leave the aorta (Fig. 118). Its epithelial portion derived from the mesonephric duct is differentiated into a proximal slender duct, the ureter, and into a distal dilated pelvis. From this grow out later the calyces and collecting tubules of the kidney. Surrounding the pelvis is a layer of con- densed mesenchyma, or nephrogenic tissue, which is the anlage of the remainder of the kidney. Blood Vascular System.— The Heart. — In Fig. 120 the cardiac chambers of the right side are opened. The septum primum between the atria is perforated dorsad and cephalad by the foramen ovale. The inferior vena cava is seen opening into the sinus venosus, which in turn communicates with the right atrium through a sagittal slit guarded by the right and left valves of the sinus venosus. The right valve is the higher and its dorsal half is cut away. The valves were united cephalad as the septum spurium. The aortic bulb is divided distally into the aorta and the pulmonary artery, the latter connecting with the fifth pair of aortic arches. Proximally the bulb is undivided. The interventricular septum is complete except for the interventricular foramen, which leads from the left ven- tricle into the aortic side of the bulb. Of the bulbar swellings which divide the THE ANATOMY OF TEN TO TWELVE MM. PIG EMIiKVoS 129 Sept. n Left valve si Inf. vena ca bulb into aorta and pulmonary trunk, the left joins the interventricular septum, while the right extends to the endocardial cushion. These folds eventually fuse and the partition of the ventricular portion of the heart is completed. The en- docardium at the atrio-ventricular openings is already undermined to form the anlages of the tricuspid and bicuspid valves. From the caudal wall of the left atrium is given-off a single pulmonary vein. The Arteries. — As seen in Fig. 118, the first two aortic arches have dis- appeared. Cranial to the third arch, the ventral aortas become the external carotids. The third aortic arches and the cephalic portions of the descending aorta^ constitute the internal carotid arteries. The ventral aortae between the third and fourth aortic arches persist as the common carotid arteries. The de- scending aorta) in the same region are slender and eventually atrophy. The fourth aortic arch is largest and on the left side will form the aortic arch of the adult. From the right fourth arch caudad, the right descending aorta is smaller than the left. Opposite the eighth segment, the two aortae unite and continue caudally as the median dorsal aorta. The fifth aortic arches (the sixth of human embryos) are connected with the pulmonary trunk, and from them arise small pulmonary arteries to the lungs. Dorsal interseg- mental arteries arise, six pairs from the descending aortae, others from the dorsal aorta. From the seventh pair, which arise just where the descending aortae fuse, the subclavian arteries pass off to the upper limb buds and the vertebral arteries to the head. The latter are formed by a longitudinal anastomosis between the first seven pairs of interseg- mental arteries on each side, after which the stems of the first six pairs atrophy. Ventro-lateral arteries from the dorsal aorta supply the mesonephros and genital ridge (Fig. 118). Ventral arteries form the cceliac artery to the stomach region, the vitelline or superior mesenteric artery to the small intestine, and the inferior mesenteric artery to the large intestine. The umbilical arteries now arise laterally from secondary trunks which persist as the common iliac arteries. 9 Tricuspid Valve Fig. 120. -Heart of 12 mm. embryo dissected from the right side. 13 O THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS The Veins. — The cardinal veins have been reconstructed by Lewis in a 12 mm. pig (Fig. 121). The veins of the head drain into the anterior cardinal vein, which becomes the internal jugular vein of the adult. After receiving the ex- ternal jugular veins and the subclavian veins from the upper limb buds the anterior cardinals open into the common cardinal veins (duct of Cuvier) . The posterior cardinal veins arise in the caudal region, course dorsal to the Fig. 121 A. — Reconstruction of a 12 mm. pig embryo to show the veins and heart from the left side. For names of parts see Fig. 121 B on opposite page (F. T. Lewis). X 13-5- mesonephroi, and drain the mesonephric sinusoids. The subcardiyial veins anastomose just caudal to the origin of the superior mesenteric artery and the posterior cardinals are interrupted at this level. The caudal portion of the right posterior cardinal vein now anastomoses with the right subcardinal vein and with it forms a part of the inferior vena cava. The proximal portions of the posterior cardinals open into the common cardinal veins as in the 6 mm. embryo. THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 131 Of the two subcardinal veins, the right has become very large through its con- nection with the right posterior cardinal vein and the common hepatic vein, and now forms the middle portion of the inferior vena cava. For the development of this vein, see Chapter IX. Geti Fig. 121 B. — Reconstruction of a 12 mm. pig embryo to show the veins from the left side (Lewis). X 13.5. A., umbilical artery; Ao., aorta; Au.. right auricle ^atrium); Card.', Card.", superior and in- ferior sections of posterior cardinal vein; d. left common cardinal vein; D.C., right common cardinal vein; D.V.. ductus venosus; Jug.', Jug.", jugular or ant. cardinal vein; L., liver; L.s., anlage of lateral sinus; mx, transverse vein; P., pulmonary artery; Sc, subcardinal vein; Scl., subclavian vein; Sis., anlage of sup. longitudinal sinus; L*m. d.. right umbilical vein; Yen., right ventricle; Y.H.C., common hepatic vein; Y.op., ophthalmic vein; Y.P., portal vein; X, anastomosis between the right and left subcardinal veins. The Umbilical Veins (Figs. 121 and 122) anastomose in the umbilical cord, separate on entering the embryo, and course in the ventro-lateral body wall of each side cranially to the ventral lobe of the liver. The left vein is much the larger and, after entering the liver, its course is to the right and dorsad. After connecting with the portal vein, it continues as the ductus venosus and joins the 132 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS proximal end of the inferior vena cava. The smaller right umbilical vein after entering the liver breaks up into sinusoids. It soon atrophies, while the left vein persists until after birth. The Vitelline Veins. — Of these a distal portion of the left and a proximal portion of the right are persistent. The left vitelhne vein, fused with the right, courses from the yolk-sac cephalad to the intestinal loop. Near its dorsal anas- tomosis with the right vein just caudal to the duct of the dorsal pancreas, it receives the superior mesenteric vein, a new vessel arising in the mesentery of the Notochord Pharvnx R. ant. cardinal, vein Pericardial Cai/iry Lvitelline Vein Small inlesT. Sup. mesenteric vein Spinal COrd Ant cardinal vein Esophagus Trachea. Upper limb Common Cardinal Vein Ductus Venosus Liver Pyloric stomach Hepatic diverticulum Dorsal pancreas Duodenum Lumbilical Vein Allantois R. umbilical Vein /?. umbilical artery Fir;. 122. — Reconstruction of a 10 mm. pig embryo to show the umbilical and vitelline veins from the ventral side. X indicates sinusoidal connection between left umbilical vein and portal vein. intestinal loop. Cranial to its junction with the sup. mesenteric vein, the left vitelline with its dorsal anastomosis and the proximal portion of the right vitel- line vein form the portal vein, which gives off branches to the hepatic sinusoids and connects with the left umbilical vein. For the development of the portal vein, see Chapter IX. Transverse Sections of a 10 mm. Pig Embryo Figures are shown of sections passing through the more important regions and should be used for the identification of the organs. The level and plane of TRANSVERSA". SECTIONS OF A TKN MM. PK; F.MIiRYO 133 each section is indicated by guide lines on Fig. 123. The student should compare this with Figs. 113 and 118, and orient each section with reference to the embryo as a whole. Keep in mind the fact that the transverse sections are drawn from the cephalic surface so that the right side of the figure is the left side of the em- bryo. Transverse Section through the Eyes and Otocysts (Fig. 124). — The brain is sectioned twice, lengthwise through the myelencephalon, transversely through the fore-brain. The brain wall shows differentiation into three layers: (1) an inner epcndymal layer densely Myelencephalon Ganyjup n 9. Metencephalon Mesencephalon Dicncephalon Telencephalon Olfactory pit Fig. 123. — Reconstruction of a 10 mm. pig embryo, showing the chief organs of the left side. The numbered lines indicate the levels of transverse sections shown in the corresponding figures (124-138). For the names of the various structures not lettered see Fig. 118. X 8. Gang, and n. access., ganglion and n. accessorius; Gang. sup. n. p., superior ganglion of glossopharyngeal nerve; Pulm. artery, pul- monary artery. cellular; (2) a middle mantle layer of nerve cells and fibers; (3) an outer marginal layer chiefly fibrous. These same three layers are developed in the spinal cord. A thin vascular layer differentiated from the mesenchyma surrounds the brain wall and is the anlage of the pia mater. The myelencephalon shows three neuromeres in this section. The telencephalon is represented by the paired cerebral hemispheres, their cavities, the lateral ventricles, connecting through the interventricular foramina with the third ventricle of the dicncephalon. Close to the ventral wall of the dicncephalon is a section of the anterior lobe of the hypophysis (Rathke's pocket). Lateral to the dicncephalon is the optic cup and lens vesicle of the eye, which are sec- tioned caudal to the optic stalk. The outer layer of the optic cup forms the thin pigment layer; the inner thicker layer is the nervous layer of the retina. The lens is now a closed vesicle dis- tinct from the overlying corneal ectoderm. The large vascular spaces are the cavernous sinuses, which drain by way of the w. capitis 134 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS lateralis into the internal jugular veins. Transverse sections may be seen of the maxillary and mandibular branches of the n. trigeminus; the n. abducens is sectioned longitudinally. Ven- tral to the otocyst are seen the geniculate and acustic ganglia of the mi. facialis and acusticus. The wall of the otocyst forms a sharply defined epithelial layer. More cephalad in the series the endolymphatic duct lies median to the otocyst and connects with it. Dorsal to the oto- cyst the n. glossopharyngeus and the jugular ganglion of the vagus are cut transversely while the trunk of the n. accessor ius is cut lengthwise. Section through the First and Second Pharyngeal Pouches (Fig. 125). — The end of the head, with sections of the telencephalon and of the ends of the olfactory pits, is now Fourth ventricle Gany.juaulare n. 10 -M CcuiQ.acust.ri.8 Mandibular ramus 77. .5 Maxi/lary ramus '■n-5 Ant lobe hypophysis Lens vesicle Wall of Myelencephalon M accessories N. glossophar- yngeus Otocyst Gang, genicul. n. 7 N. abducens Basilar artery Sinus cavern. Int. carotid- art erv Optic vesicle Foramen intervenr. Third venlricle of. telencephalon Lat.venTricle of telencephalon Fig. 124. — Transverse section passing through the eyes and otocysts of a 10 mm. embryo. X 22.5. distinct from the rest of the section. The pharynx shows portions of the first and second pharyn- geal pouches. Opposite the first pouch externally is the first branchial cleft. A section of the tuberculum impar of the tongue shows near the midline in the pharyngeal cavity. The neural tube is sectioned dorsally at the level of Froriep's ganglion. Between the neural tube and the pharynx may be seen on each side the several root fascicles of the n. hypoglossus, the fibers of the nn. vagus and accessor ius and the petrosal ganglion of the n. glossopharyngeus. Mesial to the ganglia are the descetuling aorta; and lateral to the vagus is the internal jugular vein. Section through the Third Pharyngeal Pouches (Fig. 126). — The tip of the head i= now small and shows on either side the deep olfactory pits lined with thickened olfactory epi- TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 135 thelium. The first, second and third branchial arches show on cither side of the section, the third being slightly sunken in the cervical sinus. The dorsal diverticula of the third pharyngeal pouches extend toward the ectoderm of the third branchial cleft. The ventral diverticula or thymic anlages may be traced caudad in the series. The floor of the pharynx is sectioned through the epiglottis. Ventral to the pharynx are sections of the third aortic arches and the solid cords of the median thyreoid gland. Dorsally the section passes through the spinal cord and first pair of cervical ganglia. Between the cord and pharynx, named in order, are tin- internal jugular veins, the hypoglossal nerve, and the nodose ganglion of the vagus. Lateral to the ganglion N.accessonus Cangfroriep Myelencepholm Basfa.ra.rt ► Noto chord Ganaptfrosal Phar pouch Z Pharpouchl- Oral cavity Olfactory pii Telencephalon Fig. 125. — Transverse section passing through the first and second pharyngeal pouches of a 10 mm. pig embryo. X 22.5. Neural cavity Roots of n. hypoglossus Int. jugular vein N.n. vagus el accessorius Descend Aorta rV. facialis Br. arch Z Tongue Mandible Maxillary process is the external branch of the n. accessorius, and mesial to the ganglia are the small descending aorta:. Section through the Fourth Pharyngeal Pouches (Fig. 127). — This region is marked by the disappearance of the head and the appearance of the heart in the pericardial cavity. The tips of the atria are sectioned as they project on either side of the bulbus cordis. The bulbus is divided into the aorta and pulmonary artery, the latter connected with the right ventricle, which has spongy muscular walls. The pharynx is crescentic and continued laterally as the small fourth pharyngeal pouches. Into the mid- ventral wall of the pharynx opens the vertical slit of the trachea. A section of the vagus complex is located between the descending Op ma I ganalion. Epiglottis Branch arch 3 Spinal cord Intjuqular vein N. hypogtossus Gang, nodos. n.W Pharyngeal pouch 3 Aortic arch3 Branch arch 2 MandibU Thyreoid anlage, Olfactorypit Fig. 126. — Transverse section through the third pharyngeal pouches of a 10 mm. pig embryo. X 22.5. Spinal, qana. R. desc. aorta Pharynx Hvagus Tracheal groove R. atrium Aorta Rilmonary artery Spinal cord L.desc aorta* Int. jugular vein 'Pharyngeal pouch 4 L atrium Pericardial cavity L.]/entrlc!e ft.venTricle Fig. 127. — Transverse section through the fourth pharyngeal pouches of a 10 mm. pig embryo. X 22.5. 136 TRANSVKKSK SECTIONS OF A TEN MM. PIG EMBRYO 137 aorta and the internal jugular vein. At this level the jugular vein receives the linguo-facial vein. The left descending aorta is larger than the right. The ventral aorta may be traced cranially in the scries to the fourth aortic arches. '1'hc pulmonary artery, if followed caudad, connects with the fifth aortic arches as in Fig. [28. Section through the Fifth Aortic Arches (Fig. 128). The fifth aortic arch is complete on the left side. From these pulmonary arches small pulmonary arteries may be I caudad in the series to the lung anlages. The cavity of the pharynx forms a curved horizontal slit. All four chambers of the heart are represented, but the aorta and pulmonary artery are incompletely separated by the right and left bulbar swellings or folds. Section through the Sinus Venosus and the Heart (Fig. 120).- The section is marked by the symmetrically placed atria and ventricles of the heart and by the presence of £pmal qana Nohchord Kdesc aorlc Esophagus Trachea Aorta /?. atrium Cavity of Bulbus fi. ventricle Spinal cord L .Descending aorta Anf. cardinal Vain N. vagus L. atnum Pulmonary artery L.ventrick Fig. 12S. — Transverse section through the fifth pair of aortic arches and bulbus cordis of a 10 mm. pig embryo. X 22.$. the upper limb buds. Dorsal to the atria are the common cardinal veins, the right vein forming part of the sinus venosus. The sinus venosus drains into the right atrium through a slitdike opening in the dorsal and caudal atrial wall. The opening is guarded by the right and left of the sinus venosus. which project into the atrium. The septum primum completely divides the right and left atria at this level, which is caudal to the foramen ovale and the atria-ventricular openings. The septum joins the fused endocardial cushions. Xote that the esophagus and trachea are now tubular and that the left descending aorta is much larger than the right. Around the epithelium of both trachea anil esophagus are condensations of mesenchyma, from which their outer layers are differentiated. Section through the Foramen Ovale of the Heart (Tig. 1^0). — The level of this section is cranial to that of the previous figure and shows the septum primum interrupted dor- sally to form the foramen ovale. Each atrium communicates with the ventricle of the same i*8 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS side through the atrio-vcntricular foramen. Between these openings is the endocardial cushion, which in part forms the anlages of the tricuspid and bicuspid valves. The atria are marked off dpinal gong. A/otocnord R. c/esc. Aorta. Sinus venosus R. valve sinus Venosus Pericardial cavi/y R. Ventricle Intervent Septum tSj)'mal cord Upper limb bud Esophagus L com. card, vein Trachea L Afr/'um Endocardial cushion Body wall Fig. 129. — Transverse section through the sinus venosus of the heart in a 10 mm. pig embryo. X 22.5. L. com. card, vein, left common cardinal vein; R. desc. Aorta, right descending aorta. Foramen ovale R.AtriUm L. Atrium Septum 1 Endocardial cushion L.atrio-vent foramen L.Ventricle Intervent Septufr Fir,. 130. — Transverse section through the foramen ovale of the heart in a 10 mm. pig embryo. X 22.5. />. atrio-vcnt. foramen, R. alrio-vent. foramen, left and right atrio-ventricular foramen; Intervent. septum, interventricular septum. externally from the ventricles by the coronary sulcus. Between the two ventricles is the inter- ventricular septum. The ventricular walls are thick and spongy, forming a network of muscular TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 139 cords or trabecular surrounded by blood spaces or sinusoids. The trabecule are composed of muscle cells, which later become striated and constitute the myocardium. They are surrounded by an endothelial layer, the endocardium. From the blood circulating in the sinusoids the mammalian heart receives all its nourishment until, later, the coronary vessels of the heart wall are developed. The heart is surrounded by a layer of mesothclium, the epicardium. which is continuous with the pericardial mesothelium lining the body wall. Section through the Liver and Upper Limb Buds (Fig. 131). — The section is marked by the presence of the upper limb buds, the liver and the bifurcation of the trachea to form the primary bronchi of the lungs. The limb buds are composed of dense undifferentiated mesenchyme surrounded by the ectoderm which is thickened at their tips. The seventh pair <5pinalqojiqlion Spinal nerve Post, cardinal vein Mesonephros Pleural cavity Upper limb bud U ver Spinal cord Noto chord Descending dorta. Esophagus Bifurcation of trachea Fig. 131- -Transverse section through the liver and upper limb buds of a 10 mm. pig embryo at the level of the bifurcation of the trachea. X 22.5. Inf. vena cava, inferior vena cava. of cervical ganglia and nerves are cut lengthwise showing the spindle-shaped ganglia with the dorsal root fibers taking origin from their cells. The ventral root fibers arise from the ventral cells of the mantle layer and join the dorsal root to form the nerve trunk. On the right side, a short dorsal ramus supplies the anlage of the dorsal muscle mass. The much larger ventral ramus unites with those of other nerves to form the brachial plexus. The descending aorta: have now fused and the seventh pair of dorsal intersegmental arteries arise from the dorsal aorta. From these intersegmental arteries the subclavian arteries are given off two sections caudad in the series. Lateral to the aorta are the posterior cardinal veins. The esophagus, ventral to the aorta, shows a very small lumen, while that of the trachea is large and continued into the bronchi on either side. The lung aidages project laterally into the crescentic pleural cavities, of which the left is separated from the peritoneal cavity by the 140 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS septum transversum. The liver, with its fine network of trabecule and sinusoids, is large and nearly fills the peritoneal or abdominal cavity. The liver cords are composed of liver cells surrounded by the endothelium of the sinusoids. Red blood-cells are developed in the liver Mfocnord Esoplmy, Lesser peri- toneal sac' Inf. vena cava Qanq. Post card, vein Mesonephrt'c tubule Peritoneal cavity D.hbe of Liver- Sinusoids of liver Ductus venosus/ Fig. 132. — Dorsal half of a transverse section through the lung buds cranial to the stomach in a 10 mm. pig embryo. X 22.5. Post. card, vein, posterior cardinal vein. Spinal cord Notocnord Dorsal aorta Plica venae cai/ae Inf. Vena cava lesser omentum Spinal gano. Base of. , upper limb Glomerulus of Tneso nephros Greater omentum Stomach D. lobe of liver Ductus venosus l/.lobe of liver Ventral attachment, of Liver FlG. i 5.3. — Transverse section through the stomach and liver of a 10 mm. pig embryo. X 22.5. TRAXSVKRSK SKl TK>\S ()F A TK.Y MM. PIG EMBRYO 141 at this stage. The large vein penetrating the septum transversum from the liver to the heart. is the proximal portion of the inferior vena cava, originally the right vitelline vein. Ventral to the bronchi may Ik- seen sections of the pulmonary veins. Section through Lung Buds Cranial to Stomach I Fig. 132). — The lungs are sectioned through their caudal ends ami the esophagus is just beginning to dilate into the stomach. On either side of the circular dorsal aorta are the mesoncphroi. The pleural cavities now communicate freely on both sides with the peritoneal cavity. A section of the peritoneal sac appears as a crescent-shaped slit to the right of the esophagus. In the right dorsal lobe of the liver is located the inferior vena cava. Near the median line ventral to the lesser sac is the large ductus venosus. Spinal cord Notochord Mesoneph Plica venae cavae Inf. vena cava Lesser peritoneal Sac Portal vein Hepatic diverticulum ft. Umbilical vein Sympathetic ramus Dorsal aorta Dorsal mesoaastnum Mesonephr'ic duct Stomach Ventral lobe Liver L.Urnbi lical Vein Fig. 134. — Transverse section through the hepatic diverticulum of a 10 mm. pig embryo. X 22.5. Section through the Stomach and Liver (Fig. 133 ). — Prominent in the body cavity are the mesoncphroi and liver lobes. The mesonephroi show sections of coiled tubules lined with cuboidal epithelium. The glomeruli, or renal corpuscles, are median in position and develop as knots of small arteries which grow into the ends of the tubules. The thickened epithelium along the median and ventral surface of the mesonephros is the anlage of the genital gland. The body wall is thin and lined with mesothelium continuous with that which covers the mesenteries and organs. The mesothelial layer becomes the epithelium of the adult peri- toneum, mesenteries and serous layer of the viscera. The stomach lies on the left side and is attached dorsally by the greater omentum, ventrally to the liver by the lesser omentum. The right dorsal lobe of the liver is attached dorsally to the right of the great omentum. In the liver ventral to this attachment courses the inferior vena cava and the attachment forms the plica voice cavec. Between the attachments of the stomach and liver, and to the right of the stomach, 142 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS is the lesser peritoneal sac. In the liver to the left of the midline is the ductus venosus, sectioned just at the point where it receives the left umbilical vein and a branch from the portal vein. The ventral attachment of the liver later becomes the falciform ligament. Section through the Hepatic Diverticulum (Fig. 134). — The section passes through the pyloric end of the stomach and the duodenum near the attachment of the hepatic diverti- culum. The great omentum of the stomach is larger than in the previous section and to its right, in the plica venae cavae, lies the inferior vena cava. Ventral to the inferior vena cava is a section of the portal vein. The ventral and dorsal lobes of the liver are now separate and in the right ventral lobe is embedded the saccular end of the hepatic diverticulum, which forms the gall bladder. To the right of the stomach, the diverticulum is sectioned again just as it enters the duodenum. Ventrally the left umbilical vein is entering the left ventral lobe of the liver. It is much larger than the right vein, which still courses in the body wall. On the left side of the embryo the spinal nerve shows in addition to its dorsal and ventral rami a sympa- thetic ramus, the fibers of which pass to a cluster of ganglion cells located dorso-lateral to the Dorsal Aorta Inf. Vena cava R. vitelline or portal vein Mesonephric duct Post cardinal l/ein Mesonephros L. Vitelline I vein Dorsal pancreas Liver Duodenum \Jentral pancreas- Fig. 135. — Portion of a transverse section through the pancreatic anlages of a 10 mm. pig embryo. X 22.5. aorta. These cells form one of a pair of sympathetic ganglia and are derived from a spinal ganglion. Section through the Pancreatic Anlages (Tig. 135). — The lesser peritoneal sac just above the level of this section has opened into the peritoneal cavity through the epiploic foramen (of Winslow). The mesonephric ducts are now prominent ventrally in the mesonephroi. The duct of the dorsal pancreas is sectioned tangentially at the point where it takes origin from the duodenum. From the duct the lobulated gland may be traced dorsad in the mesentery. To the right of the dorsal pancreatic duct is a section of the ventral pancreas, which may be traced cephalad in the series to its origin from the hepatic diverticulum. Dorsal to the ventral pancreas is a section of the portal vein. The inferior vena cava appears as a vertical slit in the dorsal mesentery. Section through the Urogenital Sinus and the Lower Limb Buds (Fig. 136). — The figure shows only the caudal end of a section, in the dorsal portion of which the mesonephroi were sectioned at the level of the subcardinal anastomosis. A portion of the mesentery is shown with a section of the colon. In the body wall are veins which drain into the umbilical veins, and on each side are the umbilical arteries, just entering the body from the umbilical cord. Be- tween them, in sections cranial to this, the allantoic stalk is located. Here it has opened TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 143 into the crescent ic urogenital sinus. Dorsal to the urogenital sinus (dorsal now being at the bottom of the figure owing to the curvature of the caudal region) is a section of the rectum, separated from the sinus by a curved prolongation of the ccelom. From the ends of the uro- Muenterf CaudaJ limh. of Intestine Lower limb bud Mesonephric duct Notochord Vein in body wall L. umbilical artery Allantoit and uro- genital sinus Rectum Spinal cord Fig. 136. — Transverse section through the urogenital sinus and rectum of a 10 mm. pig embryo. X 22.5. Lowerlimb bud P.umb'ilicaJ artery Notochord Caudal limb of Intestine Fig. 137. — Transverse section of a 10 mm. embryo passing through the lower limb buds at the level of the openings of the ureters into the mesonephric ducts. X 22.5. *&&£M t^feT>^@KI the fourth ventricle, which is now spread out laterally and flattened dorso-ven- trallv. About the notochord mesenchymal anlages which form the centra of the vertebra are prominent. Turning to the alimentary tract, observe that the primitive mouth cavity Pedunc. cerebri Cerebellum Chorioidal plexus veti'-> Tela of ventricle 4 Myelencephalon Epiglottis Esophagus - Spinal cord Trachea A orta R. atrium R. bronchus Dorsal acrta >ia cava Stomach Pancreas Suprarenal gland Genital gland Duodenum — \ \ Metanephros Colon L. mesonephri, I 'reter / Urogenital sinus with mesonephric duct Epiphysis Thalamus M ■ it ncephalon Tela chorioidea Lai. chorioid plexus Corpus striatum Hypophysis Lobus ol/actorius Turbinate anlage Palate Pulmonarx arlerv Gall bladder Small intestine Rectum Fig. 142. — Median sagittal dissection of a 35 mm. embryo. is now divided by the palatine folds into the upper nasal passages and lower oral cavity. In the lateral walls of the nasal passages develop the anlages of the turbinate bones. On the floor of the mouth and pharynx, the tongue and epiglottis become more prominent. The trachea and esophagus elongate and the lungs He 152 THE DISSECTION OF PIG EMBRYOS FOR STUDY Anla-qe . Mulleria., duct more and more caudad. The dorsal portion of the septum transversum, the anlage of a portion of the diaphragm, is thus carried caudad and although origi- nally, when traced from the dorsal body wall, it was directed caudad and ventrad now it curves cephalad and ventrad, bulging cephalad into the thorax. The proximal limb of the intestinal loop elongates rapidly and, beginning with the duodenum, becomes flexed and coiled in a characteristic manner. The distal limb of the intestinal loop is not coiled, but its diverticulum, the ccecum, is more marked. Caudally the rectum, or straight gut, has completely separated from the urogenital sinus and opens to the exterior through the anus. Of the urogenital organs, the genital folds have become the prominent genital glands attached to the med- ian surfaces of the meso- nephroi. The metanephroi have increased rapidly in size and have shifted ceph- alad. The proximal portion of the allantoic stalk has di- lated and, with the adjacent part of the urogenital sinus, forms the bladder. As the urogenital sinus grows it takes up into its wall the proximal ends of the meso- nephric ducts, so that these and the ureters have sepa- rate openings into the sinus. Owing to the unequal growth of the sinus wall, the ureters cpen near the base of the bladder, the mesonephric ducts more caudally into the urethra. The phallus now forms the penis of the male or the clitoris of the female. Cranial to the metanephros a new organ, the suprarenal gland, has developed. These are ductless glands and are much larger in human embryos. The heart, as may be seen by comparing Figs. 91 and 142, although at first pressed against the tip of the head, shifts caudally until in the 35 mm. embryo it lies in the thorax opposite the first five thoracic nerves. Later it shifts even further caudad. The same is true of the other internal organs, the metanephros excepted. As the chief blood-vessels are connected with the heart and viscera, /ll/anfoi CoeLom Fig. 143. — Ventral dissection of a 15 mm. embryo, show- ing lungs, digestive canal and mesonephroi. The ventral body wall, heart and liver have been removed and the limb buds cut across. X 6. DEVEI.oIWIEN I of I HE FACE 153 profound changes in the positions of the vessels are thus brought about, for the vessels must shift their positions with the organs which they supply. Ventral Dissections. — Ventral dissections of the viscera are very easily made. With the safety razor blade, start a cut in a coronal plane through the caudal end of the embryo and the lower limb buds (Fig. 143). Extend this cut laterad and cephalad through the body wall and the upper limb bud. The head may be cut away in the same plane of section and the cut continued through the body wall and upper limb bud of the opposite side back caudally to the start- ing point. Section the embryo in a coronal plane, parallel with the first section and near the back, so that the embryo will rest upon the flattened surface. With forceps, now remove the ventral body wall. By tearing open the wall of the umbilical cord along one side it may be removed, leaving the intestinal loop in- tact. Pull away the heart, noting its external structure. The liver may also be removed, leaving the stomach and intestine uninjured. A portion of the septum transversum covering the lungs may be carefully stripped away and the lungs thus laid bare. Dissections made in this way show the trachea and lungs, the esophagus, stomach and dorsal attachment of the septum transversum, the course of the intestinal canal, and also the mesonephroi and their ducts. Favor- able sections through the caudal end of the body may show the urogenital sinus, rectum and sections of the umbilical arteries and allantois (Figs. 92, 119 and 143). In late stages, by removing the digestive organs the urogenital ducts and glands are beautifully demonstrated (Figs. 216 and 217). DEVELOPMENT OF THE FACE The heads of pig embryos have long been used for the study of the devel- opment of the face. The heads should be removed by passing the razor blade between the heart and adjacent surface of the head, severing the neck. Xext cut away the dorsal part of the head by a section parallel to the ventral surface, the razor blade passing dorsal to the branchial clefts and eyes. Mount ventral side up three stages from embryos 6, 12 and 14 mm. long as shown in Figs. 92 and 144 A and B. In the early stages (Figs. 92 and 119) the four branchial arches and clefts are seen. The third and fourth arches soon sink into the cervical sinus, while the mandibular processes of the first arch are fused early to form the lower jaw. The frontal process of the head is early divided into lateral and median nasal processes by the development of the olfactory pits. The processes are distinct 154 THE DISSECTION OF PIG EMBRYOS FOR STUDY and most prominent at 12 mm. (Fig. 144 A). Soon, in 13 to 14 mm. embryos, the median nasal processes fuse with the maxillary processes of the first arch and constitute the upper jaw (Fig. 144 B). The lateral nasal processes fuse with the maxillary processes and form the cheeks, the lateral part of the lips and the alae of the nose. Later, the median nasal processes unite and become the median part of the upper lip and the columna nasi. The early development of the face is practically the same in human embryos Lat. nasal process Olfactory pit Med. nasal process Mandible Br. arch II Ventral aorta Eye Lacrymal groove Maxillary proc. Br. cleft. I Br. cleft II Lat. nasal process Maxillary process Mandible Br. cleft I Ext. naris Eye Med. nasal process Oral cavity Ext. ear Fig. 144. — Two stages showing the development of the face in pig embryos. A, Ventral view of face of a 12 mm. embryo; B, of a 14 mm. embryo. (Fig. 145). At the end of the fourth week, the lateral and median nasal processes have developed. During the sixth week, the maxillary processes fuse with the nasal processes, and at the end of the second month the median nasal processes have united. The mandibular processes fuse at the sixth week and from them a median projection is developed which forms the anlage of the chin. The lips begin to appear as folds at the sixth week. As the median nasal processes and the maxillary processes take part in their development, the failure of these parts to fuse may produce hare lip. The lips of the new-born child are peculiar in that their proximal surfaces are covered with numerous villi, finger-like processes which may be a millimeter or more in length. DKVKLOPMEXT OF THE HARD PALATE !55 The external car is developed around the first branchial cleft by the appearance of small tubercules which form the auricle. The cleft itself becomes the external auditory meatus and the concha of the ear. (For the development of the external ear see Chapter Xll). Fig. 145. — Development of the face of the human embryo (His). A, embryo of about twenty-nine days. The median frontal process differentiating into median nasal processes or processus globulares, toward which the maxillary processes of first visceral arch are extending. B, embryo of about thirty- four days: the globular, lateral nasal, and maxillary processes are in apposition; the primitive naris is now better defined. C, embryo of about the eighth week: immediate boundaries of mouth are more definite and the nasal orifices are partly formed, external ear appearing. D, embryo at end of second month. DEVELOPMENT OF THE HARD PALATE This may be studied advantageously in pig embryos of two stages: (a) 20 to 25 mm. long; (b) 28 to 35 mm. long. In the first stage, the jaws are close together and the mandible usually rests against the breast region. The palatine processes are separated by the tongue and are directed ventrad (Fig. 146 A). In embryos of 26 to 28 mm., the jaws open and the tongue lies ventral to the palatine processes which now approach each other in a horizontal plane (Fig. 146 B). Dissections may be made by carrying a shallow incision from the THE DISSECTION OF PIG EMBRYOS FOR STUDY angle of the mouth back to the external ear on each side. The incisions are then continued through the neck in a plane parallel to the hard palate. Before mount- ing the preparation, remove the top of the head by a section cutting through the eyes and nostrils parallel to the first plane of section. Transverse sections through the snout may also be prepared to show the positions of tongue and palatine folds before and after the fusion of the latter. In the human embryo of two and a half months, three palatine anlages are Nasal septum Tongue Lat. palatine process —X Xasal septum Lat. palatine process Mandible Turbinate cnlage Tongue Fig. 146. — Sections through the jaws of pig embryos to show development of the hard palate. A, 22 mm; B, 34 mm. X 8. developed: a small median process developed from the fused median nasal pro- cesses, and paired lateral palatine processes developed from the maxillary pro- cesses, and extending from the line of fusion of the median nasal process and of the maxillary process caudally along the wall of the pharynx (Fig. 148). In pig embryos (Fig. 147 A and B), the median process forms a single heart-shaped structure. The lateral palatine processes lie at first lateral and ventral to the dorsum of the tongue and their edges are directed ventrad and mesially (Fig. 146 A). Before these processes can fuse, the tongue is withdrawn from between DEVELOPMENT OF THE HARD PALATE 157 them owing to a change in the position of the mandible due to the development of its arch (Fig. 146 B). With the withdrawal of the tongue the edges of the Median palatine process Lateral palatine process Int. choanal Oral cavity Med. palatine process Raphe of lat. palatine process Nasal passage A nlage of inula Fig. 147. — Dissections to show the development of the hard palate in pig embryos. A, ventral view of palatine processes of a 22 mm. pig embryo, the mandible having been removed; B, same of 35 mm. embryo showing fusion of palatine processes. />•;'•' /'•;;■ palatine folds are approximated and soon fuse, thus cutting off the nasal passages from the primitive oral cavity dorsad (Fig. 147 B). At the point in the median line where the lateral and median palatine processes meet, fusion is not complete, leaving the incisive fossa, and laterad between the two processes openings persist for some time, which are known as the incisive canals (of Sten- son). After the withdrawal of the tongue, the lateral palatine pro- cesses take up a horizontal posi- tion and their edges are approxi- mated, because the cells on the ventral sides of the folds prolifer- ate more rapidly than those of the dorsal side (Schorr, Anat. Hefte, Bd. 36, 1908). That the change in position of the palatine folds is not mechanical, but due to unequal growth, may be seen in Fig. 149, a section through the palatine folds of a pig Fig. 148. — The roof of the mouth of a human embryo about two and a half months old. showing the develop- ment of the palate, p.g., processus globularis; p.g.' . pala- tine process of processus globularis; mx, maxillary pro- cess; mx', palatine fold of maxillary process. Close to the angle between this and the palatine process of the processus globularis on each side, the primitive choansc. (After His.) 158 THE DISSECTION OF PIG EMBRYOS FOR STUDY embryo which shows the right palatine fold in a horizontal position, although the left fold projects ventral to the dorsum of the tongue. A region of cellular pro- liferation may be seen on the under side of each process. At the end of the second month the palatine bones begin to develop in the lateral palatine folds and thus form the hard palate. Caudally the bones do not develop and this portion of the folds forms the soft palate and the uvula. The un- Nasal septum Fig. 149. — Section through the jaws of a 25 mm. pig embryo to show the change in the position of the palatine processes with reference to the tongue. fused backward prolongations of the palatine folds give rise to the arcus pharyn- go-palatini, which is taken as the boundary line between the oral cavity proper and the pharynx in adult anatomy. , The lateral palatine processes occasionally fail to unite in the middle line, producing a defect known as cleft palate. The extent of the defect varies considerably, in some cases involv- ing only the soft palate, while in other cases both soft and hard palates are cleft. THE DEVELOPMENT OF THE TONGUE The tongue develops as two distinct portions, the body and the root, separated from each other by a V-shaped groove, the sulcus terminalis. Its development may be studied from dissections of pig embryos 6, 9 and 13 mm. long. As the pharynx is bent nearly at right angles, it is necessary to cut away its roof by two pairs of sections passing in different planes. The first plane of section cuts through the eye and first two branchial arches just above the cervical sinus (Fig. 150, I). From the surface, the razor blade should be directed obliquely dorsal in cutting toward the median line. Cuts in this plane should be made from either side. In the same way make sections on each side in a plane forming an obtuse angle with the first section and passing dorsal to the cervical sinus (Fig. 150, II). Now sever the remaining portion of the head from the body by a transverse sec- THE DEVELOPMENT OF THE TONGUE 159 tion in a plane parallel to the first (Fig. 150, III). Place the ventral portion of the head in a watch-glass of alcohol and, under the dissecting microscope, remove that part of the preparation cranial to the mandibular arches. Looking down upon the floor of the pharynx, remove any portions of the lateral pharyngeal wall which may still interfere with a clear view of the pharyngeal arches as seen in Fig. 151. Permanent mounts of the three stages mentioned above may be made and used for study by the student. In both human and pig embryos, the body of the tongue is developed from three anlages which are formed in front of the second branchial arches. These are the median, somewhat triangular tubcrculum impar, and the paired lateral thicken- ings of the mandibular arches, both of which are present in human embryos of 5 mm. (Fig. 152 A). At this stage, a median ventral elevation formed by Fig. 150. — Lateral view of the head of a 7 mm. pig embryo. The three lines indicate the planes of sections to be made in dissecting the tongue as de- scribed in the text. Br. arch I Tuberculum impar Br. arch II Br. arch III Br. arch TV Arytenoid ridge ! Lateral lingual anlage Br. arch I Lateral lingual anlage Br. arch III-^ Br. arch IV Arytenoid ridge Tuberculum impar Br. arch II Epiglottis Glottis Fig. 151. — Dissections showing the development of the tongue in pig embryos. A, 9 mm. embryo; B, 13 mm. embryo. i6o THE DISSECTION OF PIG EMBRYOS FOR STUDY the union of the second branchial arches forms the copula. This, with the por- tions of the second arches lateral to it, forms later the base or root of the tongue. Between it and the tuberculum impar is the point of evagination of the median thyreoid gland. The copula also connects the tuberculum impar with a rounded prominence which is developed in the mid- ventral line from the bases of the third and fourth branchial arches. This is the anlage of the epiglottis. In later stages (Fig. 151 A and B) the lateral mandibular anlages increase rapidly in size, are bounded laterally by the linguo-alveolar grooves, and fuse with the tuberculum impar which lags behind in development and is said to form the median septum of the tongue. According to Hammar, it completely atrophies. The epiglottis becomes larger and concave on its ventral surface. Caudal to it, and in early Lateral tongue Thyreoid swellings diverticulum Lateral tongue swellings Entrance to larynx ntrance to larynx rytenoid swellings Fig. 152. — The development of the tongue in human embryos. A, 5 mm.; B, 7 mm. (modified from Peters). stages continuous with it, are two thick rounded folds, the arytenoid folds. Be- tween these is the slit-like glottis leading into the larynx (see p. 174). The musculature of the tongue is supplied chiefly by the hypoglossal nerve, and both nerve and muscles develop caudal to the branchial region in*which the tongue develops. The mus- culature migrates cephalad and gradually invades the branchial region beneath the mucous membrane. At the same time, the tongue may be said to extend caudad until its root is cov- ered by the epithelium of the third and fourth branchial arches. This is shown by the fact that the sensory portions of the nn. trigeminus and facialis, the nerves of the first and second arches, supply the body of the tongue, while the nn. glossopharyngeus and vagus, the nerves of the third and fourth arches, supply the root and the caudal portion of the body of the tongue. In embryos of 50 to 60 mm. the fungiform and filiform papillae may be dis- tinguished as elevations of the epithelium. Taste buds appear in the fungiform papillae of 100 mm. embryos and are much more numerous in the fetus than in the adult. The vallate papillae (Fig. 153 A) appear at go mm. as a V-shaped epi- DEVELOPMENT OF THE SALIVARY GLANDS 161 thelial ridge, the apex of the V corresponding to the site of the median thyreoid evagination. At intervals along the epithelial ridges circular epithelial down- growths develop which take the form of inverted and hollow truncated cones (Fig. 153 B). During the fourth month circular clefts appear in the epithelial downgrowths, thus separating the walls of the vallate papilla; from the surround- ing epithelium and forming the trench from which this type of papilla derives its name. At the same time, lateral outgrowths arise from the bases of the epi- thelial cones, hollow out and form the ducts and glands of Elmer. The taste buds of the vallate papillae are also formed early, appearing in embryos of three months. Fig. 153. — Diagrams showing the development of the vallate papilhe of the tongue (Graberg in McMurrich's "Human Body"). DEVELOPMENT OF THE SALIVARY GLANDS The glands of the mouth are all regarded as derivatives of the ectodermal epithelium. Of the salivary glands, the parotid is the first to appear. Its anlage has been observed in 8 mm. embryos as a furrow in the floor of the alveolo-buccal groove. The furrow elongates and, in embryos of 17 mm., separates from the epithelial layer, forming a tubular structure which opens into the mouth cavity near the cephalic end of the original furrow. The tube grows back into the region of the external ear, branches and forms the gland in this region, while the unbranched portion of the tube becomes the parotid duct (Hammar, Anat. Anzeiger, Bd. 19, 1901). The submaxillary gland arises as an epithelial ridge in the alveolo-lingual groove, its cephalic end located caudal to the frenulum of the tongue. The caudal end of the ridge soon begins to separate from the epithelium and extends caudad and ventrad into the submaxillary region where it enlarges and branches to form the gland proper, its cephalic unbranched portion persisting as the duct which soon hollows out. The sub-lingual and alveolo-lingual glands develop as several solid evaginations of epi- thelium from the alveolo-lingual groove, appearing from the eighth to the twelfth week (Fig. 162 THE DISSECTION OF PIG EMBRYOS FOR STUDY 157). Of the alveolo-lingual glands nine or ten may develop on either side in embryos of 40 mm. (McMurrich in Keibel and Mall, vol. 2, p. 348-349.) The branched anlages of the salivary glands are at first solid and hollow out peripherally. The glands continue growing and enlarging until after birth. Mucin cells may be distinguished by the sixteenth week and acinus cells in the parotid glands at five months (McMurrich). THE DEVELOPMENT OF THE TEETH The development of the teeth is described in all the standard textbooks of histology and only a brief account of their origin and structure will be given here. The enamel organs, which give rise to the enamel of the teeth and are the moulds, so to speak, of the future teeth, are of ectodermal origin. There first appears in embryos of 10 to 12 mm. an ectodermal downgrowth, the dental ridge or lamina on the future alveolar portions of the upper and lower jaws (Fig. 154). These uL. Fig. 154. — Early stages in the development of the teeth. A, 17 mm.; B, 41 mm. (Rose). LF., LFL., labial groove; Pp., dental papilla; UK., lower jaw; uL., lower lip; ZL., dental ridge. laminae parallel and are mesial to the labial grooves, being directed obliquely toward the tongue. At intervals, on each curved dental ridge or lamina a series of thickenings develop, the anlages of the enamel organs (Fig. 155). Soon the ventral side of each enamel organ becomes concave (embryos of 40 mm.) forming an inverted cup and the concavity is occupied by dense mesenchymal tissue, the dental papilla (Figs. 154 B and 156). An enamel organ with dental papilla forms the anlage of each decidual or milk tooth. Ten such anlages are present in the upper jaw and ten in the lower jaw of a 40 mm. embryo. The connection of the dental anlages with the dental ridge is eventually lost. The position of the tooth anlage between the tongue and lip is shown in Fig. 157. The anlages of those permanent teeth which correspond to the decidual, or milk teeth, are developed in the same way along the free edge of the dental lamina median to the decidual teeth. In addition, the anlages of three perma- nent molars are developed on each side, both above and below from a backward or THE DEVELOPMENT OF THE TEETH 163 aboral extension of the dental lamina, entirely free from the oral epithelium. The anlages of the first permanent molars appear at seventeen weeks (180 mm.), those of the second molars at six months after birth, while the anlages of the third permanent molars or wisdom teeth are not found until the fifth year. The per- manent dentition of thirty-two teeth is then complete. The internal cells of the enamel organs are at first compact, but later by the Oral epithelium Enamel organs Dental groove Dental lamina Necks of enamel organs Free edge of the dental lamina Dental lamina Labial groove Milk molar I Aboral prolonga- tion of dental lamina Fig. 155. — A, B, C, D. diagrams showing the early development of three teeth. One of the teeth is shown in vertical section (Lewis and Stohr). E, dental lamina and anlages of the milk teeth of the upper jaw from a fetus of 105 mm. (Rose in Kollmann's Handatlas). development of an intercellular matrix the cells separate forming a reticulum resembling mesenchyma and termed the enamel pulp (Fig. 156). The outer enamel cells, at first cuboidal, flatten out and later form a fibrous layer. The inner enamel cells bound the cup-shaped concavity of the enamel organ. Over the crown of the tooth these cells, the ameloblasts, become slender and columnar in form, producing the enamel layer of the tooth along their basal ends (Fig. 158). i64 THE DISSECTION OF PIG EMBRYOS FOR STUDY The enamel is laid down first as an uncalcified fibrillar layer which later becomes calcified in the form of enamel prisms. From the ends of the cells project cutic- Denfal lamina Epidermis Fig. 156. — Section through the upper first decidual incisor tooth from a 65 mm. human embryo. X 70. *f^*S*vi- Tip of tongue Submaxillary duct Sublingual duct Epidermis of lip Enamel organ of tooth Dental papilla Meckel's cartilage Bone of mandible Fig. 157. — Parasagittal section through the mandible and tongue of a 65 mm. human embryo showing the position of the anlage of the' first incisor tooth. X 14- ular fibers known as Tomes' processes (Fig. 158). The enamel is formed first at the top of the crown of the tooth and extends toward the root over the crown. TIIK 1)K\ KI.OPMKN'T OF Till'. TEETH i6« The enamel cells about the future root of the tooth remain cuboidal or low col- umnar in form, come into contact with the outer enamel cells and the two layers constitute the epithelial sheath of the root which does not produce enamel prisms. The Dental Papilla. — The outermost cells of the dental papilla at the end of the fourth month arrange themselves as a definite layer of columnar epithelium. Since they produce the dentine, or dental bone, these cells are known as odonto- blasts. When the dentine layer is developed, the odontoblast cells remain in- ternal to it and branched processes from them (the dental fibers of Tomes) extend into the dentine and form the dental canaliculi. Internal to the odontoblast layer the mesenchymal cells differentiate into the dental pulp, popularly known Fig. 158. — Section through a portion of the crown of a developing tooth showing the various layers (Tourneux in Heisler). 1,2, cells of enamel pulp; j, ameloblast layer of enamel-forming cells; 4, 5, enamel prisms; 6, layer of dentine containing processes of 7, odontoblast cells; S, cells of dental pulp. as the "nerve" of the tooth. This is composed of a framework of reticular tissue in which are found blood-vessels, lymphatics and nerve fibers. The odonto- blast layer persists throughout life and continues to secrete dentine so that eventually the root canal may be obliterated. Dental Sac. — The mesenchymal tissue surrounding the anlage of the tooth gives rise to a dense outer layer and a more open inner layer of fibi .mective tissue. These layers form the dental sac (Fig. 159). Over the root of the tooth a layer of osteoblasts or bone forming cells develops, and, the epithelial sheath formed by the enamel layers having disintegrated, these osteoblasts deposit about the dentine a layer of bone which is known as the substantia ossea or cement. The cement layer contains typical bone cells but no Haversian canals. As the i66 THE DISSECTION OF PIG EMBRYOS FOR STUDY teeth grow and fill the alveoli the dental sac becomes a thin vascular layer con- tinuous externally with the alveolar periosteum, internally with the periosteum of the cement layer of the tooth. Dental sac Outer I aver Inner layer r^hm :, S^lHllii Outer enamel cells Enamel pulp m^Inner enamel cells & Enamel Dentine Odontoblasts /^j^SS Epithelial sheath Dental papilla {future pulp) Blood-vessel Bony trabecula of the lower Fig. 159. — Longitudinal section of a deciduous tooth of a newborn dog. X 42- The white spaces between the inner enamel cells and the enamel are artificial, and due to shrinkage (Lewis and Stohr). When the crown of the tooth is fully developed the enamel organ disinte- grates, and as the roots of the teeth continue to grow their crowns approach the surface and break through the gums. The periods of eruption of the various Permanent second molar Deciduous molars Mandibular canal Permanent first molar Permanent premolars Permanent canine Mental foramen Permanent incisors Fig. 160. — The skull of a five-year-old child showing positions of the decidual and permanent teeth (Sobotta-McMurrich. Atlas of Human Anatomy). TILE DEVELOPMENT OF THE TEETH 167 milk or decidual teeth vary with race, climate and nutritive conditions. Usually the teeth are cut in the following sequence: [dual 'ik Mm.k Teeth Median Incisors sixth to eighth month. Lateral Incisors eighth to twelfth month. First Molars twelfth to sixteenth month. Canines^- seventeenth to twentieth month. Second Molars twentieth to twenty-fourth month. The permanent teeth are all present at the fifth year. They are located mesial to the decidual teeth (Fig. 160), and, before the permanent teeth begin to erupt, the roots of the milk teeth undergo absorption, their dental pulp dies and they are eventually shed. The permanent teeth are "cut" as follows: (McMurrich in Keibel and Mall, vol. 2, p. 354). First Molars seventh year Median Incisors eighth year. Lateral Incisors ninth year. First Premolars tenth year. Second Premolars eleventh year. c j,r, J thirteenth to fourteenth year. Second Molars J Third Molars seventeenth to fortieth year. Dental anomalies are frequent and may consist in the congenital absence of some or all of the teeth, or in the production of more than the normal number. Defective teeth are fre- quently associated with hare lip. Cases have been noted in which, owing to defect of the enamel organ, the enamel was entirely wanting. Many cases in which a third dentition occurred have been recorded and occasionally fourth molars may be developed behind the wisdom teeth. The teeth of vertebrates are homologues of the placoid scales of elasmobranch fishes (sharks). The teeth of the shark resemble enlarged scales, and many generations of teeth are produced in the adult fish. In some mammalian embryos three or even four dentitions are present. The primitive teeth of mammals are of the canine type and, from this conical tooth, the incisors and molars have been differentiated. CHAPTER VII THE ENTODERMAL CANAL AND ITS DERIVATIVES: THE BODY CAVITIES When the head- and tail-folds of the embryo develop, there are formed both cranially and caudally from the spherical vitelline sac blind entodermal tubes, the fore-gut and hind-gut respectively (Fig. 161 A). The region between these Pharynx Pharyngeal membrane Pericardial cavity Fore-gut Hepatic diverticulum Yolk-stalk Hind-gal Cloacal membrane Allantois Cloaca Pharynx Pharyngeal membrane Thyreoid gland Pericardial cavity Fore-gut Hepatic diverticulum Yolk-stalk Allantois Cloacal membrane Cloaca Hind-gut Fig. 161. — Diagrams showing in median sagittal section the alimentary canal, pharyngeal and cloacal membranes. A, 2 mm. embryo, modified after His; B, 2.5 mm. embryo, after Thompson. intestinal tubes, open ventrally into the yolk-sac, is known as the mid-gut. As the embryo and the yolk-sac at first grow more rapidly than the connecting re- gion between them, this region is apparently constricted and becomes the yolk- stalk, or vitelline duct. At either end the entoderm comes into contact ventrally with the ectoderm. Thus there are formed the pharyngeal membrane of the fore- 168 PHARYNGEAL POUCHES 169 gut, the cloacal membrane of the hind-gut. In 2 mm. embryos the pharyngeal membrane separates the ventral ectodermal cavity, or slomodceum, from the pharyngeal cavity of the fore-gut. Cranial to the membrane is the ectodermal diverticulum, Rathkcs pocket. In 2.5 to 3 mm. embryos (Fig. 161 B) the pharyn- geal membrane is perforated and the stomodaeum and pharynx are continuous. The blind termination of the fore-gut probably forms SccsseVs pocket. The fore-gut later forms part of the oral cavity and is further differentiated into the pharynx and its derivatives; into the esophagus, respiratory organs, stomach, duodenum, jejunum and a portion of the ileum. From the duodenum arise the liver and pancreas. The hind-gut, beginning at the attachment of the yolk-stalk extends caudally to the cloaca, into which opens the allantois in 2 mm. embryos. The hind-gut is differentiated into the ileum, caecum, colon and rec- tum. The cloaca is subdivided into the rectum and urogenital sinus (for its de- velopment see Chapter VIII). At the same time, the cloacal membrane is separated into a urogenital membrane and into an anal membrane. The latter eventually ruptures and this opening is the anus. The yolk-stalk usually loses its connection with the entodermal tube in embryos of 7.5 mm. (Fig. 172). We have seen how the palatine processes divide the primitive oral cavity into the nasal passages and mouth cavity of the adult, and have described the development of the tongue, teeth and salivary glands, organs derived wholly or in part from the ectoderm. It remains to trace the development of the pharynx and its derivatives. PHARYNGEAL POUCHES There are developed early from the lateral wall of the pharynx paired out- growths which are formed in succession cephalo-caudad. In 4 to 5 mm. embryos, five pairs of pharyngeal pouches are present, the fifth pair being rudimentary. At the same time there appears in the mid- ventral wall of the pharynx, between the first and second branchial arches, a small rounded prominence, the thyreoid anlage. This constricts off and forms a stalked vesicle (Fig. 82). Its stalk, the thyreo-glossal duct, opens near the aboral border of the tuberculum impar. Meantime, the pharynx has been flattened dorso-ventrally and broadened later- ally and cephalad so that it is triangular in ventral view (Figs. 82 and 162). From each pharyngeal pouch develop small dorsal and large ventral diverticula. The first four pouches come into contact with the ectoderm of the branchial clefts, fuse with it and form the closiiig plates. Only occasionally do the closing plates become perforate in human embryos. The first and second 170 THE ENTODERMAL CANAL AND ITS DERIVATIVES pharyngeal pouches soon connect with the pharyngeal cavity through wide open- ings. The third and fourth pouches grow laterad and their diverticula com- municate with the pharynx through narrow ducts in 10 to 12 mm. embryos (Fig. 162). When the cervical sinus is formed the ectoderm of the second, third and fourth branchial clefts is drawn out to produce branchial and cervical ducts and the branchial vesicle. These are fused at the closing plates with the entoderm of the second, third and fourth pharyngeal pouches. Branchial duct 2 Epithelial body of 3& pouch Branchial cleft I Cervical sinus Cervical dud Thymus anlaqe '' Epithelial body of ¥h pouch Trachea Pharyngeal pouch 1 Pharyngeal pouch Z Pharynqeal pouch 3 Pharyngeal pouch 4- Siomach Dorsal 'pancreas Pharyngeal pouch 6 Esophagus Apical bud of right Lung Gall bladder Duodenum Fig. 162. — A reconstruction of the pharynx and fore-gut of an 1 1.8 mm. embryo seen in dorsal view (after Hammar). The ectodermal structures are stippled. The first and second pouches soon differ from the others in form and give rise to an entirely different type of permanent structures. With the broadening of the pharynx the first two pouches acquire a common opening into it, the primary tympanic cavity. The first pouch later differentiates into the tympanic cavity of the middle ear and into the Eustachian tube. By the growth and lateral expansion of the pharynx the second pouch is taken up into the pharyngeal wall, its dorsal angle alone persisting to be later transformed into the palatine tonsil. TILE THYMUS 171 According to I laminar (Arch, f. mikr. Anat., Bd. 61, 1903), the lateral pharyngeal recess (of Rosenmueller) is not a persistent portion of the second pouch as His asserted. Lymphocytes appear in the Lymphoid tissue of the tonsils in embryos of 140mm. They take their origin in the mesoderm (Harnmar, Maximo w). Foramen caecum Palatine tonsil Epithelial bodies Tbyreo qloaal duct Thymus 'aniages Post-branchial body Thyreoid antaoe THE THYMUS The third, fourth and fifth pharyngeal pouches give rise to a series of ductless glands, of which the thymus is the most important. The thymus anlage appears in 10 mm. embryos as a ventral and medial prolongation of the third pair of pouches (Figs. 162 and 163). The ducts con- necting the diverticula with the pharynx soon disappear so that the thymus anlages are set free. At first hollow tubes, they soon lose their cavities, their lower ends enlarge and migrate caudally into the thorax passing usually ven- Fig. 163. — Diagram in ventral view of the pharynx and pharyngeal pouches, showing the origin of the thymus and thyreoid glands and of the epithelial bodies (modified after Groschuff and Kohn). I, II, III, IV, and V, first, second, third, fourth and fifth pharyngeal pouches. Fig. 164. — Two reconstructions of the thymus and thyreoid glands. A. in a human embryo of 26 mm.; B, in one of 24 mm. (after Tourneux and Verdun). In A the thymus lies in front, in B, behind the left innominate vein, thyr., thyreoid; thym., thymus; par. TV., parathyreoid of fourth pouch; par. III., parathyreoid of third pouch; pyr., pyramidal lobe of thyreoid; thyg., thyreoglossal duct; c.a.; carotid artery; j.v., jugular vein. tral to the left vena anonyma. Their upper ends become attentuate and atrophy or may persist as an accessory thymus lobe (Kohn) . The enlarged lower ends of the anlages form the body of the gland, which is thus a paired structure (Fig. 164). 172 THE ENTODERMAL CANAL AND ITS DERIVATIVES At 50 mm. the thymus still contains solid cords and small closed vesicles of ento- dermal cells. From this stage on, in development, the gland becomes more and more lymphoid in character. Its final position is in the thorax dorsal to the cranial end of the sternum. It grows under normal conditions until puberty, after which its degeneration begins. This process proceeds slowly in healthy individuals, rapidly in case of disease. The thymus may function normally until after the fortieth year. It is now generally believed that the entodermal epithelium of the thymus is converted into reticular tissue and thymic corpuscles. The "lymphoid" cells are regarded by Hammar and Maximow as immigrant lymphocytes derived from the mesoderm. According to Stohr, they are not true lymphocytes but are derived from the thymic epithelium. Weill (Arch. f. mikr. Anat., Bd. 83, pp. 305-360) has observed the development of granular leucocytes in the human thymus gland. THE EPITHELIAL BODIES OR PARATHYREOIDS The dorsal diverticula of the third and fourth pharyngeal pouches each give rise to a small mass of epithelial cells termed an epithelial body (Fig. 163). Two pairs of these bodies are thus formed and, with the atrophy of the ducts of the pharyngeal pouches, they are set free and migrate caudalward. They eventually lodge in the dorsal surface of the thyreoid gland, the pair from the third pouch lying one on each side at the caudal border of the thyreoid in line with the thymus anlages (Fig. 164). The pair of epithelial bodies derived from the fourth pouches are located on each side near the cranial border of the thyreoid. From their ultimate relation to the thyreoid tissue the epithelial bodies are often termed parathyreoid glands. The solid body is broken up into masses and cords of poly- gonal entodermal cells intermingled with blood-vessels. In post-fetal life, lumina may appear in the cell masses and fill with a colloid-like secretion. The ventral diverticulum of the fourth pouch is a rudimentary thymic an- lage. It soon atrophies. The ultimobranchial body is the derivative of the fifth pharyngeal pouch (Fig. 163). With the atrophy of the duct of the fourth pouch it is set free and migrates caudad with the parathyreoids. It forms a hollow vesicle which has been termed the lateral thyreoid. According to Grosser (Keibel and Mall, vol. 2. p. 467) and Verdun, it takes no part in forming thyreoid tissue but atrophies. The term lateral thyreoid when applied to it is therefore a misnomer. THE THYREOID GLAND The thyreoid anlage (Fig. 163) is bilobed before the thyreoglossal duct dis- appears. It soon loses its lumen and breaks up into irregular solid cords of tissue LARYNX, TRACHEA AND LUNGS 173 as it migrates caudad. It takes up a transverse position with a lobe on each side of the trachea and larynx (Fig. 164). In embryos of 50 mm., lumina appear in the more peripheral cords which break up into hollow or solid groups of cells, the primitive thyreoid follicles (Grosser). LARYNX, TRACHEA AND LUNGS In embryos of 23 segments, the anlage of the respiratory organs appears as a groove in the floor of the entodermal tube just caudal to the pharyngeal pouches. This groove produces an external ridge on the ventral wall of the tube, a ridge Trachea, PespiraTory anlage Esophagus D Trachea. Apical bud Primary bronchus Esophagus Trachea, Bronchus Ventral bud Fig. 165. — Diagrams of stages in the early development of the trachea and lungs of human embryos (based on reconstructions by Bremer, Broman, Grosser, and Xaroth). A, 2.$ mm.; B, 4 mm.; C, B in side view; D, 5 mm.; E, 7 mm. which becomes larger and rounded at its caudal end (Fig. 165V The laryngo- tracheal groove and the ridge are the anlages of the larynx and trachea. The rounded end of the ridge is the unpaired anlage of the lungs. Externally two lateral longitudinal grooves mark off the dorsal esophagus from the ventral respiratory anlages. The lung anlage rapidly increases in size and becomes bilobed in embryos of 4 to 5 mm. The lateral furrows become deeper caudad and a septum is formed which grows cephalad, separating first the lung anlages and then the tracheal tube from the esophagus. At the same time the laryngeal portion of the groove and ridge is developed cranially until it lies be- tween the third and fourth branchial arches (Fig. 82). At 5 mm. the respiratory 174 THE ENTODERMAL CANAL AND ITS DERIVATIVES apparatus consists of the laryngeal groove and ridge, the tubular trachea and the two lung buds. The Larynx. — In embryos of 5 to 6 mm. the oval end of the laryngeal groove is bounded on either side by two rounded prominences, the arytenoid swellings which, continuous orally with a transverse ridge, form the furcula of His (Fig. 152 B). The transverse ridge becomes the epiglottis and, as we saw in connec- tion with the development of the tongue, it is derived from the third and fourth branchial arches. In embryos of 15 mm. the arytenoid swellings are bent near the middle toward the median line. Their caudal portions become parallel, while their cephalic r.pli. portions diverge nearly at right angles (Fig. 166). The opening into the larynx thus becomes T-shaped and o.l _. § — r.a.e. t-c. .. f.i.a.. Fig. 166. — Entrance to larynx in a forty- to forty-two-day human embryo (from Kallius): t, tuberculum impar; p, pharyngo-epiglottic fold; e, epiglottic fold; l.e, lateral part of epi- glottis; at, cuneiform tubercle; com, cornicular tubercle. Fig. 167. — The larynx of 16 to 23 cm. human embryos (Soulie and Bardier). From a dissection. b.l., base of tongue; e, epiglottis; f.i.a., interary- tenoid fissure; r.a.e., plica ary-epiglottica; r.ph.e., plica pharyngo-epiglottica; o.l., orifice of larynx; t.c, tuberculum cuneiformis; t. S., tuberculum corni- culatum. ends blindly, as the laryngeal epithelium has fused. In 40 mm. embryos this fusion is dissolved, the arytenoid swellings are withdrawn from contact with the epiglottis and the entrance to the larynx becomes oval in form (Fig. 167). At 27 mm. the ventricles of the larynx appear and at 37 mm. their margins indicate the position of the vocal cords. The epithelium of the vocal cords is without cilia. The elastic and muscle fibers of the cords are developed by the fifth month. At the eighth week the cartilaginous skeleton of the larynx is indicated by a surrounding condensation of mesenchyme. The cartilage of the epiglottis appears relatively late. The LARYNX. TRACHEA AND LUNGS 175 thyreoid cartilage is formed as two lateral plates, each <>!' which has two centers of chondrili- cation. These plates grow ventrad and fuse in the median line. The anlages of the cricoid and arytenoid cartilages are at first continuous. Later, sepa- rate cartilage centers develop for the arytenoids. The cricoid is at first incomplete <: but eventually forms a complete ring. The cricoid may therefore be regarded as a modified tracheal ring. The comiculate cartilages are portions of the arytenoid cartilages and separate from them. The cuneiform cartilages are derived from the cartilage of the epiglottis. The Tracheal Tube. — This gradually elongates during development and its columnar epithelium becomes ciliated. Muscle fibers and the anlages of the cartilaginous rings appear at 17 mm. The glands develop as ingrowths of the epithelium during the last five months of pregnancy. The Lungs. — Soon after the lung anlages or stem buds are formed (5 mm. embryos), the right bronchial bud becomes larger and is directed more caudally FiG. 168. — Dorsal and ventral views of the lungs from a human embryo of five weeks (Merkel). Ap, apical bronchus; Di, D2, etc., dorsal; Vi, V2, etc., ventral bronchi; Jc, infracardial bronchus. (Fig. 165). At 7 mm. the stem bronchi give rise to two bronchial buds on the right side, to one on the left. The smaller bronchial bud on the right side is the apical bud. The other buds, right and left, are known as ventral bronchi. There are thus formed three bronchial rami on the right side, two on the left, and these correspond to the primitive lobes of the lungs (Fig. 168). On the left side, an apical bud is interpreted as being derived from the first ventral bron- chus. It develops later and remains small so that a lobe corresponding to the upper lobe of the right lung is not developed in the left lung (Xaroth). The upper lobe of the left lung thus would correspond to the upper and middle lobes of the right lung. The bronchial anlages continue to branch in such a way that the stem bud becomes the main bronchial stem (Fig. 168). That is. the branching is mono- podial, not dichotomous. lateral buds being given off from the stem bud. Only in the later stages of development has dichotomous branching of the bronchi and r76 THE ENTODERMAL CANAL AND ITS DERIVATIVES Parietal pleura the formation of two equal buds been described. Such buds formed dichoto- mously do not remain of equal size (Flint, Amer. Jour. Anat., vol. 6, 1906-1907). The entodermal anlages of the lungs and trachea are developed in a median mass of mesenchyma dorsal and cranial to the peritoneal cavity. This tissue forms a broad mesentery termed the mediastinum (Fig. 169). The right and left stem buds of the lungs grow out laterad, carrying with them folds of the mesoderm. The branch- ing of the bronchial buds takes place within this tissue which is covered by the mesothelium which lines the body cavity. The terminal branches of the bronchi are lined with entodermal cells which flatten out and form the respiratory epithelium of the adult lungs. The surrounding mesenchyma differentiates into the muscle, Ductus Venosui Wall of umbilical card Fig. 169. — Transverse section through the lungs and pleural cavities of a 10 mm. human embryo. X 23. Fig. 170. — The lungs of a 10.5 mm. embryo showing the pulmonary arteries and veins (His from McMurrich's "Human Body"). Ap., pulmonary artery; Ep, apical bronchus; Vp, pulmonary vein; /. //, primary bronchi. connective tissue and cartilage plates of the lungs, trachea and bronchial walls. Into it grow blood-vessels and nerve fibers. When the pleural cavities are ESOPHAGUS, STOMACH AND INTESTINE 1 77 separated from the pericardial and peritoneal cavities, the mesothelium covering the lungs with the connective tissue underlying it becomes the visceral pleura. The corresponding layers lining the thoracic wall form the parietal pleura. These layers are derived respectively from the visceral (splanchnic) and parietal (soma- tic) mesoderm of the embryo. In 11 mm, embryos the two pulmonary arteries, from the sixth pair of aortic arches, course lateral then dorsal to the stem bronchi (Fig. 170). The right pulmonary artery passes ventral to the apical bronchus of the right lung. The single pulmonary vein receives two branches from each lung, two larger veins from each lower lobe, two smaller veins from each upper lobe and the middle lobe of the right side. These four pulmonary branches course ventrad and drain into the pulmonary trunk. When this common stem is taken up into the wall of the left atrium, the four pulmonary veins open directly into the latter. According to Kolliker, the air cells of the lungs begin to form at the sixth month and their development is completed during pregnancy. Elastic tissue may be recognized at the third month in the walls of the vessels and during the fourth month it appears in the largest bronchi. The abundant connective tissue found between the bronchial branches in early fetal life becomes reduced in its relative amount as the alveoli of the lungs are developed. Before birth the lungs are relatively small, compact and possess sharp margins. They lie in the dorsal portion of the pleural cavities. After birth they normally fill with air, expanding and completely filling the pleural cavities. Their margins become rounded and the compact fetal lung tissue which resembles that of a gland in structure becomes light and spongy, owing to the enormous increase in the size of the alveoli and blood-vessels. Because of the greater amount of blood admitted to the lungs after birth their weight is suddenly increased. In the most common anomaly involving the esophagus and trachea the former is divided transversely, the trachea opening into the lower portion of the esophagus, while the upper portion of the esophagus ends blindly. According to Lewis (in Keibel and Mall, vol. 2, p. 367), the anomaly may be produced by the abnormal development of lateral esophageal grooves which occlude the lumen of the esophagus. These grooves, though small, were found present in 4 mm. human embryos. ESOPHAGUS, STOMACH AND INTESTINE Esophagus. — The esophagus in 4 to 5 mm. embryos is a short tube, flattened laterally, and extending from the pharynx to the stomach. Its epithelium is composed of two layers of columnar cells. The esophagus grows rapidly in length and in 7.5 mm. embryos its diameter decreases both relatively and absolutely (Forssner). In embryos of from 8 to 16 mm. its laryngeal end is crescent-shaped and concave toward the trachea. Its middle portion is round or oval and opposite the bifurcation of the trachea it begins to enlarge and is flattened laterally. Its lumen is open throughout and shows from i78 THE ENTODERMAL CANAL AND ITS DERIVATIVES two to four rows of nuclei. In 20 mm. embryos, vacuoles appearing in the epithelium give the esophagus the appearance of having several lumina. The result of vacuole formation is to increase the size of the lumen. In later stages the wall of the esophagus is folded and ciliated epithelial cells appear in 44 mm. embryos. The number of cell layers in the epithelium increases until at birth they number nine or ten. Glands are developed as ingrowths from the epithelium. The circular muscle layer is indicated at 10 mm. by a circular layer of myoblasts, but the longitudinal muscle layer does not form a definite layer until 55 mm. (F. T. Lewis in Keibel and Mall, vol. 2). Stomach. — The stomach appears in embryos of 4 to 5 mm. as a laterally flattened fusiform enlargement of the fore-gut caudal to the lung anlages (Fig. 171). Its epithelium is early thicker than that of the esophagus and is sur- rounded by a thick layer of Pharynx Root of tongue Thyreoid Tip of tongue splanchnic mesoderm. It is attached dorsally to the body wall by it's mesentery, the greater omentum, and ventrally to the liver by the lesser omen- tum. The dorsal border of the stomach enlarges to form the fundus and greater curvature,. The dorsal wall grows more rapidly than the ventral wall and thus produces the convex greater curvature. The whole stomach becomes curved and its caudal end is carried ventrad and to the right (Fig. 162). This forms a ventral concavity, the lesser curvature, and produces the first flexure of the duodenum. The rapid growth of the gastric wall along its greater curvature also causes the stomach to rotate upon its long axis until its greater curvature or primitive dorsal wall lies to the left, its ventral wall, the lesser curvature, to the right (Fig. 162). The original right side is now dorsal, the left side ventral in position, and the caudal or pyloric end of the stomach is ventral and to the right of its cardiac or cephalic end. The whole organ extends obliquely across the peritoneal cavity from left to right. The change in position progresses rapidly and is already completed in embryos of 12 to 15 mm. (beginning of the second month). The rotation of the stomach explains the asymmetrical position of the Rathke's pocket Trachea Stomach Liver Dorsal pancreas Hepatic diverti- culum Yolk-stalk Allantois Mesonephric duct Cloaca Hind-gut Fig. 171 A. — Median sagittal section of a 5 mm. embryo to show the digestive canal (modified after Ingalls). KSOI'IIAC.IS, STOMACH WD INTKSTIXK 179 vagus nerves of the adult organ, the left nerve supplying t he ventral wall of the stomach, originally the left wall, while the right vagus supplies the dorsal wall, originally the right. Gastric pits are indicated in 16 mm. embryos and at 100 mm. gland cells of the gastric glands are differentiated. These undoubtedly arise from the gastric epithelium, according to Tongui Rathke's pouch Yolk-stalk Allantoic stalk Cloaca Metan- ephros Mcson- ephric duct Laryngo- tracheal groove L. lung Stomach Dorsal pancreas Allanlois Fig. 171 B. — Reconstruction of a 5 mm. human embryo showing the entodermal canal and its derivatives (His in Kollmann's Handatlas). Lewis. The cardiac glands are developed early (01 mm. embryos) and, according to Lewis, there is no "evidence in favor of Bensley's conclusion that the cardiac glands are decadent . . . fundus glands." At 10 mm. the stomach wall is composed of three layers, entodermal epithelium, a thick mesenchymal layer and the peritoneal mesothelium. At 16 mm. the circular muscle layer is indicated by condensed mesenchyma. The tunica propria forms a dense layer at 55 mm. i So THE ENTODERMAL CANAL AND ITS DERIVATIVES At 91 mm. the cardiac region shows a few longitudinal muscle fibers, which become distinct in the pyloric region at 240 mm. The Intestine. — In 5 mm. embryos (Fig. 171 A), the intestine, beginning at the stomach, consists of the duodenum (from which are given off the hepatic diverticulum and ventral pancreas), and the cephalic and caudal limbs of the intestinal loop, which bends ventrad and connects with the yolk-stalk. Caudally Rathke's pocket Hypophysis Thyreoid Pericardium Allantois Cloacal membrane Notochord Dorsal pancreas Ventral pancreas Ccecum Urogenital sinus Peritoneal cavity Tail-gut \ Mcsonephric duct Rectum Fig. 172. — Diagram in median sagittal section showing the digestive canal of a 9 mm. human embryo (adapted from Mall). the intestinal tube expands to form the cloaca. It is supported from the dorsal body wall by the mesentery (Fig. 171 B). From 5 to 9 mm. the ventral bend of the intestinal loop becomes more marked and the attachment of the yolk-stalk to it normally disappears (Fig. 172). The attachment of the yolk-stalk may persist in later stages (12 to 14 mm. embryos, according to Keibel, Elze and Thyng). Also in 20 per cent, of adult intestines a pouch 3 to 9 cm. long is found where the yolk-stalk was formerly attached. This pouch, the diverticulum of the ileum or Meckel' 's diverticulum, is of clinical importance as many cases of intestinal occlu- sion in infancy are due to its presence. ESOPHAGUS, STOMACH AND INTESTINE 181 At the stage shown in Fig. 172, the dorsal pancreatic anlage has been de- veloped from the duodenum and, in the caudal limb of the intestinal loop, there is formed an enlargement due to a ventral bulging of the gut wall which marks the anlage of the cacum and the boundary line between the large and small in- testine. The caccal anlage later differentiates into the large ccecum and distal vermiform process of the adult. Succeeding changes in the intestine consist (1) in its torsion and coiling due Brain Tip of tongue Thyreoid gland Pericardium Gall-bladder Small intestine Ccecum ^ Hypophysis Foramen ccecum {-Root of tongue Esophagus -Trachea .Notochord -Spinal cord Urogenital sinus Anal membrane Rectum Fig. 173. — Diagrammatic median sagittal section of a 17 mm. human embryo showing the digestive canal (modified after Mall). to its rapid elongation and (2) in the differentiation of its different regions. As the gut elongates in 9 to 10 mm. embryos the intestinal loop rotates. As a result, its caudal limb lies at the left and cranial to its cephalic limb (Fig. 172). At this stage the intestinal loop enters the ccelom of the umbilical cord. The small intestine soon lengthens rapidly and at 17 mm. (Fig. 173) forms loops in the umbilical cord. Six primary loops occur and may be recognized in the arrangement of the adult intestine (Mall, Bull. Johns Hopkins Hosp., vol. 9, 1898). In embryos of 42 mm. the intestine has returned from the umbilical 182 THE ENTODERMAL CANAL AND ITS DERIVATIVES cord into the abdominal cavity through a rather small aperture and the ccelom of the cord is soon after obliterated. In embryos between 10 and 30 mm. vacuoles appear in the wall of the duodenum and epi- thelial septa completely block the lumen. The remainder of the small intestine remains open, although vacuoles form in its epithelium. Villi appear as rounded elevations of the epithelium at 22.S mm. (Johnson). They begin to form at the cephalic end of the jejunum, and at 130 mm. they are found throughout the small intestine (Berry). Intestinal glands appear as ingrowths of the epithelium about the bases of the villi. They develop first in the duodenum at 91 mm. and in the jejunum at 130 mm. The duodenal glands (of Brunner) are said to appear during the fourth month (Brand). From 10 to 12.5 mm. the circular muscle layer is formed. The longitudinal muscle layer is not distinct until 75 mm. The impervious duodenum of the embryo may persist as a congenital anomaly, and we have already alluded to the persistence of the yolk-stalk as Meckel's diverticulum. Fig. 174. — Three successive stages showing the development of the digestive tube and the mesen- teries in the human fetus (modified from Tourneux) : 1, stomach; 2, duodenum; 3, small intestine; 4, colon; 5, vitelline duct; 6, caecum; 7, great omentum; 8, mesoduodenum; 9, mesentery; 10, mesocolon. The arrow points to the orifice of the omental bursa. The ventral mesentery is not shown (Heisler). The large intestine, as we have seen in 9 mm. embryos, forms a tube ex- tending from the coecum to the cloaca. It does not lengthen so rapidly as the small intestine and, when the intestine is withdrawn from the umbilical cord (at 42 mm.), its cranial or caecal end lies on the right side and dorsal to the small intestine (Fig. 174). It extends transversely to the left side as the transverse colon, then bending abruptly caudad as the descending colon, returns by its iliac flexure to the median plane and forms the rectum. The caecum (Fig. 175) is differentiated from the vermiform process at 65 mm. (Tarenetzky). The caecum and vermiform process -make a U-shaped bend with the colon at 42 mm., and this flexure gives rise to the ileo-ccecal valve (Toldt). In stages between 100 and 220 mm. the lengthening of the colon causes the caecum THE LIVER 183 and cephalic end of the colon to descend toward the pelvis (Fig. 174). The ascending colon is thus formed and the vermiform appendix takes the position which it occupies in the adult. The development of the mucous membrane of the intestinal tube has been described by Johnson (American Journal of Anatomy, vols. 10, 14 and 16, pp. 521-561; 187-233; 1-49). Ascending mesocolon Ascending colon Car urn •ding colon Caecum Processus icrmiformis Processus vermiforr.iis Fig. 175. — The caecum of a human embryo of 5 cm. (Kollmann). A, from the ventral side; B, from the dorsal side. THE LIVER In embryos of 2.5 mm. the liver anlage is present as a median ventral out- growth from the entoderm of the fore-gut just cranial to the yolk-stalk (Fig. 161 B). Its thick walls enclose a cavity which is continuous with that of the gut. The liver anlage is embedded in the ventral mesentery which lies in the median line between the fore-gut, the ventral body wall, and the septum transversum (Fig. 1 7 1 A) . Thus, from the first the liver is in close relation to the septum trans- versum and later when the septum becomes a part of the diaphragm the liver remains attached to it. In embryos 4 to 5 mm. long, solid cords of cells proliferate from the ventral and cranial portion of the liver anlage. These cords anastomose and form a crescentic mass with wings extending lateral and dorsal to the gut (Fig. 171 A). 184 THE ENTODERMAL CANAL AND ITS DERIVATIVES Pa-.- This mass, a network of solid trabecular, is the glandular portion of the liver. The primitive, hollow, hepatic diver- ticulum later differentiates into the gall-bladder and the large biliary- ducts. Referring to Figs. 83 and 176, it will be seen that the liver anlage lies between the vitelline veins and is in close proximity to them laterally. The veins send anastomosing branches into the ventral mesen- tery. The trabeculae of the expand- ing liver grow between and about these venous plexuses, and the plex- uses in turn make their way between and around the liver cords. The vitelline veins on their way to the heart are thus surrounded by the liver and largely subdivided into a network of vessels termed sinusoids. The endothelium of the sinusoids is closely applied to the cords of liver cells which, in the early stages, con- tain no bile capillaries (Fig. 177). For the transformation of the vitelline veins into the portal vein and for the relations of the umbilical veins to the liver see Chapter IX. The glandular portion of the liver grows rapidly and in embryos of 7 to 8 mm. is con- nected with the primitive hepa- tic diverticulum only by a single cord of cells, the hepatic duct (Fig. 178 A). That portion of the hepatic diverticulum distal to the hepatic duct is now differentiated into the terminal solid gall-bladder and its cystic duct. Its proximal portion forms the Fig. 176. — The liver anlage of a 4 mm. human embryo (Bremer). In., intestine; Pa., pancreas; V.V., veins in contact with liver trabeculae. Fig. 177. — The trabeculae and sinusoids of the liver in section, h.c, trabecular of liver cells; Si., sinusoids (after Minot). X 300. THE LIVER 18: ductus choledochns . In embryos of 10 mm. (Fig. 178 B) the gall-bladder and ducts have become longer and more slender. The hepatic duct receives a right and left branch from the corresponding lobes of the liver. The gall-bladder is without a lumen up to the 15 mm. stage. Later its cavity appears surrounded by a wall of high columnar epithelium. The glandular portion of the liver develops fast and is largest relatively at 31 mm. (Jackson, Anat. Record, vol. 3, pp. 361-396, 1909). The liver tissue degenerates, especially in the peripheral portion of the left lobe. In embryos of two months the liver weighs 2 gm.; at birth 75 gm.; in the adult 1500 gm. During the development of the liver the endothelial cells of the sinusoids become stellate in outline, and thus form an incomplete layer. From the second month of fetal life to some Hepaii c ducr ]ucfuS choledochus pancreas brsal pancreas Ductus chaledoc Ventral pancreas Duodenum Gall blade Cystic duct Duct of dorsal pancreas Head of dorsal pancreas Duodenum Tail of dorsa,f pancreas Fig. 178. — Reconstructions showing the development of the hepatic diverticulum and pancreatic anlages. A, 7.5 mm. embryo, X 36 (after Thyng); B, 10 mm. embryo, X 33 (specimen loaned by Dr. H. C. Tracy). time after birth, blood-cells are actively developed between the hepatic cells and the endothelium of the sinusoids. The hepatic trabecular are mostly solid in 10 mm. embryos. At 22 mm. hollow periportal ducts develop, spreading inward from the hepatic duct along the larger branches of the portal vein. These ducts form a plexus, as has been proved by injections. Lumina bounded by five or six cells may be observed in some of the liver trabecular of 10 mm. embryos (Lewis). In 44 mm. embryos, bile capillaries with cuticular borders are present, most numerous near the periportal ducts with which some of them connect. At birth, or shortly after, the number of liver cells surrounding a bile capillary is reduced to two, three or four. The lobules, or vascular units of the liver, are formed, according to Mall, by the peculiar and regular manner in which the veins of the liver branch. The primary branches of the portal vein extend along the periphery of each primitive lobule, parallel to similar branches of the hepatic veins which drain the blood from the center of each lobule (Fig. 179). As development proceeds, each primary branch becomes a stem, giving off on either side secondary branches i86 THE ECTODERMAL CANAL AND ITS DERIVATIVES which bear the same relation to each other and to new lobules as did the primary branches to the first lobule. This process is repeated until thousands of liver lobules are developed. Until the 20 mm. stage the portal vein alone supplies the liver. The hepatic artery from the cceliac axis comes into relation first with the hepatic duct and gall-bladder. Later, it grows into the connective tissue about the larger bile ducts and branches of the portal vein, and also supplies the capsule of the liver. The development of the ligaments of the liver is described on p. 200. Anomalies of the liver occur chiefly in connection with the gall-bladder and ducts. The gall-bladder may be absent or two may be present. Duplications and absence of the hepatic duct has been observed, also duplication of the cystic duct. In some animals (horse, elephant) the gall-bladder is normally absent. fs A B Fig. 179. — Diagrams of three successive stages of the portal and hepatic veins in a growing liver. a, Hepatic side; d, portal side; b and c, successive stage of the hepatic vein; e and/, successive stages of the portal vein (Mall). ^THE PANCREAS Two pancreatic anlages are developed almost simultaneously in embryos of 3 to 4 mm. The dorsal pancreas arises as a hollow outpocketing of the dorsal duodenal wall slightly cranial to the hepatic diverticulum. At 7.5 'mm. it is separated from the duodenum by a slight constriction (Fig. 178 A). The ventral pancreas develops in the inferior angle between the hepatic diverticulum and the gut (Lewis) and its wall is continuous with both. With the elongation of the ductus choledochus it is gradually separated from the intestine. The ventral pancreas may arise directly from the intestinal wall. In cases observed by Debeyre, Helly and Kollmann, the anlage was paired and in other embryos a paired structure is indicated. THE PANCREAS l87 Of the two pancreatic anlages, the dorsal grows more rapidly and in 10 mm. embryos forms an elongated structure with irregular nodules upon its surface. Its distal portion is constricted to form a short duct. It lies in the greater omen- tum between the duodenum and the stomach. The ventral pancreas is smaller and develops a short slender duct which opens into the ductus choledochus. As the latter elongates it bends dorsad and to the right of the intestine, while at the same time the stomach and intestine rotate to the right. This shifts the duct of the ventral pancreas so that it opens dorsally and somewhat to the left into the bile duct. At the same time, the ventral pancreas is brought into close proximity to the dorsal pancreas, and the duct of the latter is shifted to the left side of the intestine (Figs. 178 and 180). It is also carried further cephalad during the course Accessory pancreatic duct Dorsal pancreas Ventral pancreas Pancreatic duct Bile duct Accessory pancreatic duct Dorsal pancreas I I Ventral pancreas Bile duct Fancreatic duct Fig. 180. — Two stages showing the development of the pancreas. A, at live weeks; /->, at seven weeks (after Kollman). of development so that in the adult the interval between the ducts is from 10 to 35 mm- In embryos of 20 mm. the tubules of the dorsal and ventral pancreatic an- lages interlock (Fig. 180 B). Eventually, anastomosis takes place between the two ducts and the duct of the ventral pancreas persists as the functional pan- creatic duct of the adult. The proximal portion of the dorsal pancreatic duct forms the accessory duct which remains pervious, but becomes a tributary of the ventral pancreatic duct. The ventral pancreas forms part of the head and uncinate process of the adult gland. The dorsal pancreas takes part in forming the head and uncinate process and comprises the whole of the body and tail. In 10 mm. embryos the portal vein separates the two pancreatic anlages and later they partially surround the vein. The chief branch of the portal in the adult, the superior mesenteric vein, thus passes through the pancreas, receiving the splenic vein which courses along and drains i88 THE ENTODERMAL CANAL AND ITS DERIVATIVES the dorsal surface of the tail. The alveoli of the gland are developed as darkly staining cellular buds in embryos of 40 to 55 mm. The islands characteristic of the pancreas appear first in the tail at 55 mm. Owing to the shift in the position of the stomach and duodenum during development the pancreas takes up a transverse position, its tail extending to the left. To its ventral sur- face is attached the transverse mesocolon. BODY CAVITIES, DIAPHRAGM AND MESENTERIES The Primitive Ccelom and Mesenteries. — In the Peters embryo the primary mesoderm has already split to form the extra-embryonic ccelom (Fig. 232). When the intra-embryonic mesoderm differentiates, numerous clefts appear on either side between the somatic and splanchnic layers of mesoderm. These clefts coalesce in the cardiac region and form two elongated cavi- ties lateral to the paired tubular heart. Simi- larly, right and left pleuro-peritoneal cavities are formed between the mesoderm layers caudal to the heart. The paired pericardial cavities ex- tend toward the midline cranially and com- municate with each other (Fig. 181). They also are prolonged caudally until they open into the pleuro-peritoneal cavities. These in turn communicate laterally with the extra- embryonic ccelom. In an embryo of 1.5 mm. the ccelom thus consists of a U-shaped peri- cardial cavity, the right and left limbs of which are continued caudally into the paired pleuro-peritoneal cavities ; these extend out into the extraembryonic ccelom. When the head-fold and fore-gut of the embryo are developed, the layers of splanchnic mesoderm containing the heart tubes are folded together ventral to the fore-gut and form the ventral mesentery between the gut and the ventral body wall (Fig. 182). Owing to the position of the yolk-sac, the caudal extent of the ventral mesentery is limited. At the level on each side, where the vitello- umbilical trunk courses to the heart, the splanchnic mesoderm and the somatic mesoderm are united (Fig. 182). Thus is formed the septum transversum, which separates the ventral mesentery into a cranial and caudal portion. Cranial to the septum, the heart is suspended in the ventral mesentery which forms the dorsal and ventral mesocardia (Fig. 183 A). Into the ventral mesentery caudal Pericardial Cauitu Surface of fore-gut Pleuro- pericardia I Canal Entoderm of out Peri tonea I Cavity xtra-embryonic coelom -Wall of yolk-sac Fig. 181. — Diagrammatic dor- sal view of the coelom in an early human embryo (modified after Rob- inson). BODY CAVITIES, DIAPHRAGM AND MESENTERIES 189 to the septum grows the liver. This portion of the ventral mesentery gives rise dorsally to the lesser omentum of the stomach and, with the septum transversum, Esophagus Spinal CoroL Pericardial Cavity Ventricle of heart Ventral mesocardium Liver ventral mesentery (falciform I^LVS^U Dorsal me so card- i u m Septum transversum Stomach Ventral mesentery (lesser omentum) Dorsal mesogastrium Dorsal pancreas tei Mesocolon ' Mesorectum FlG. 182. — Diagram showing the primitive mesenteries of an early human embryo in median sagittal sec- tion. The broken lines A, B, and C indicate the level of sections A, B, and C in Fig. 183. neural tube Neural iube Notochord Aorta. A/otochord Post cardinal Aorta. Vein Dorsal mesentery JJuodenum Lesser Om.».u», Guf Liver Peritoneal Cavity Talciform liqament Fig. 183. — Diagrammatic transverse sections. A , through the heart and pericardial cavities of an early human embryo; B, through the stomach and liver; C, through the intestine and peritoneal cavity. it forms the ligaments of the liver. Ventrally it persists as the falciform ligament (Fig. 183 B). 190 THE ENTODERMAL CANAL AND ITS DERIVATIVES Dorsal to the gut the splanchnic mesoderm of ea*ch side is folded together in the median sagittal plane and constitutes the dorsal mesentery which extends to the caudal end of the digestive canal (Figs. 182 and 183 C). This suspends the stomach and intestine from the dorsal body wall and is divided into the dorsal mesogastrium or greater omentum of the stomach ; the mesoduodenum, the mesen- tery proper of the small intestine, the mesocolon and the mesorectum. The covering layers of the viscera, of the mesenteries and of the body wall, are continuous with each other and consist of a mesothelium overlying connective tissue. They are derived from the somatic and splanchnic layers of mesoderm. Bulbus cordis Dorsal mesocardium Pericardial cavity Somatopleure Septum transversum Liver trabecules He pic diverticulum Yolk-stalk Sinus venosus Lateral mesocardium Common cardinal vein Umbilical veiiir Vitelline vein overlying stomach Pleuro-peritoneal canal Peritoneal cavity Fig. 184. — Reconstruction near median sagittal plane of a 3 mm. human embryo, showing the body cavities and septum transversum (Kollmann's Handatlas). The primitive ccelom lies approximately in one plane, as in Fig. 181. With the development of the head-fold and the ventral flexion and fusion of the heart tubes, the pericardial cavity is bent ventrad and enlarged. The ventral meso- cardium attaching the heart to the ventral body wall disappears and the right and left limbs of the U-shaped cavity become confluent ventral to the heart. The result is a single large pericardial chamber, the long axis of which now lies in a dorso-ventral plane nearly at right angles to the plane of the pleuro-peri- toneal cavities, and connected with them dorsally by the right and left pleuro- peritoneal canals (Fig. 184). The division of the primitive cazlom into separate cavities is accomplished by BODY CAVITIES, DIAPHRAGM AND MESENTERIES I9I the development of three membranes: (1) the septum transversum, which sepa- rates incompletely the pericardial and pleural cavities from the peritoneal cavi- ties; (2) the pleuro- pericardial membrane which completes the division between pericardium and pleural cavity; (3) the pleuro-peritonenl membrane which com- pletes the partition between each pleural cavity containing the lung and the peritoneal cavity which contains the abdominal viscera. The Septum Transversum. — In em- bryos of 2 to 3 mm. (Fig. 184) the splanchnic mesoderm of the yolk-sac and that of the heart are continuous where the vitelline veins cross from one layer to the other; also where the um- bilical veins course from the body wall to the heart the somatic and splanchnic layers of mesoderm are continuous. Thus, there is formed caudal to the heart a transverse partition filling the space between the sinus venosus of the heart, the gut and the ventral body wall and separating the pericardial and peri- toneal cavities from each other ventral to the gut. This mesodermal partition was termed by His the septum trans- vcrsum. It is the anlage of a large part of the diaphragm. At first it does not extend dorsal to the gut, but leaves on either side a pleuro-peritoneal canal through which the pericardial and pleuro-peritoneal cavities communicate (Fig. 183). In embryos of 4 to 5 mm. the lungs develop in the median walls of these canals and bulge laterally into them. Thus the canals become the pleural cavities and will be so termed hereafter. On account of the more rapid growth of the embryo, there is an apparent constriction at the yolk-stalk and, with the development of the umbilical cord, the peritoneal cavity is finally separated from the extra-embryonic ccelom. 5 4 3 L Fig. 185. — Diagram showing the change in position of the septum transversum in stages from 2 to 24 mm. (modified after Mall). The septum is indicated at different stages by the numerals to the left, the numbers correspond- ing to the length of the embryo at each stage. The letters and numbers at the right represent the segments of the occipital, cervical, thoracic and lumbar regions. 192 THE ENTODERMAL CANAL AND ITS DERIVATIVES Dorsally the pleural and peritoneal cavities are permanently partitioned length- wise by the dorsal mesentery. The septum transversum in 2 mm. embryos occupies a transverse position in the middle cervical region (Fig. 185, 2). According to Mall, it migrates caudally, its ventral portion at first moving more rapidly so that its position becomes oblique. In 5 mm. embryos (Fig. 185, 5) it is opposite the fifth cervical segment, at which level it receives the phrenic nerve. In stages later than 7 mm. the sep- Pen 'cardial ccu/ity Com. cardinal vein Pleuro -pencardia.L membrane. Vleuro- peritoneal membrane Ye in to limb bud Stomach Mesonephros Fig. 186. — Reconstruction of a 7 mm. embryo showing from the left side the pleuro-pericardial mem- brane, the pleuro-peritoneal membrane and the septum transversum (after Mall). turn migrates caudad, until at 24 mm. it is opposite the first lumbar segment. During this second period of migration its dorsal attachment travels faster than its ventral portion. Therefore, it rotates to a position nearly at right angles to its plane in 7 mm. embryos and its original dorsal surface becomes its ventral surface. In connection with the septum transversum two other membranes develop. The Pleuro-pericardial and Pleuro-peritoneal Membranes. — The common BODY CAVITIES, DIAPHRAGM AND MESENTERIES 193 cardinal veins (ducts of Cuvier) on their way to the heart curve around the pleural cavities laterally in the body wall (Figs. 184 and 186). In embryos of 7 mm. each vein forms a ridge which projects from the body wall mesially into the pleu- ral canals. This ridge, the pulmonary ridge of Mall, later broadens and thickens cranio-caudally (Fig. 186). Its cranial and caudal margins form two sides of a spherical triangle, the third side or base of which is the line of attachment of the dorsal mesentery to the body wall (Fig. 187). At its ventral angle the sides of Phuropericard.ia.1 membra-ne. Phrenic nerve Pericardial cavity iiusn Iratisversum Pleuro- peritonea.1 Membrane Mesonep/iros fomach Fig. 187. — Reconstruction of an 11 mm. embryo to show the same structures as in Fig. 186 (after Mall). this triangle are continuous with the septum transversum. Its cranial side forms the pleuro-pericardial membrane and in 9 to 10 mm. embryos reduces the opening between the pleural and pericardial cavities to a mere slit. Its caudal side be- comes the pleuro-peritoneal membrane, which eventually separates dorsally the pleural from the peritoneal cavity. The membranes at first lie nearly in the sagit- tal plane and a portion of the lung is caudal to the pleuro-peritoneal membranes (Fig. 186). Between the stages of 7 and n mm. the dorsal attachment of the 13 194 THE ENTODEKMAL CANAL AND ITS DERIVATIVES septum transversum is carried caudally more rapidly than its ventral portion and its ventral surface becomes its dorsal side (Figs. 186 and 187). The pleuro- peritoneal membrane is carried caudad with the septum transversum until the lung lies in the angle between the pleuro-peritoneal and pleuro-pericardial mem- branes and is included within the spherical triangle which has been described above (Fig. 187). The dorsal end of the pleuro-pericardial membrane lags behind and so takes up a position in a coronal plane nearly at right angles to the septum transversum (Figs. 187 and 188). In n mm. embryos the pleuro-pericardial membranes have fused completely on each side with the median walls of the pleural Pleural cavity Mesoderm of left luntjlucL Pericardial cavity PI euro -peritoneal membrane Phrenic nerve Wall of heart .iver falciform lioament Septum transversum Fig. 188. — Transverse section through a 10 mm. human embryo showing the pleuro-pericardial mem- brane separating the pericardium from the pleural cavities. X 33. canals and thus separate the pericardium from the paired pleural cavities. By way of the pleuro-pericardial membranes the phrenic nerves course to the septum transversum (Fig. 187). The pleuro-peritoneal membranes are continued dorsally and caudally along the mesonephric folds; ventrally and caudally they become on the liver the dorsal pillars of the diaphragm or coronary appendages (Lewis) (Fig. 189). Be- tween the free margins of the membranes and the mesentery an opening is left on each side, through which the pleural and peritoneal cavities communicate (Figs. 187 and 193). BODY CAVITIES, DIAPHRAGM AND MESENTERIES 195 Owing to the caudal migration of the septum transversum and the growth of the lungs and liver, the pleuro-peritoneal membrane, at first lying in a nearly sagittal plane, is shifted to a horizontal position and gradually its free margin unites with the dorsal pillars of the diaphragm and with the dorsal mesentery. The opening between the pleural and peritoneal cavities is thus narrowed and finally closed in embryos of 19 to 20 mm. The Diaphragm and Pericardial Membrane. — The lungs grow and expand, not only cranially and caudally, but also laterally and ventrally (Fig. 190 A, B). Room is made for them by the obliteration of the very loose, spongy mesenchyme Coronary appendaq of liver Venacaya inferior Pleural Cavity Pleuro-peritoneal membrane ^fffi — Phrenic nerve in septum transversum Fig. 189. — Transverse section through a 10 mm. embryo showing the pleuro-peritoneal membranes, X 16 (from an embryo loaned by Dr. H. C. Tracy). of the body wall (Fig. 189). As the lungs grow laterally and ventrally in the body wall around the pericardial cavity, they split off from the body wrall the pericardial membrane and more and more the heart comes to lies in a mesial position between the lungs (Fig. 190 B). The pleural cavities thus increase rapidly in size. At the same time, the liver grows enormously and on either side a portion of the body wall is taken up into the septum transversum and pleuro- peritoneal membranes. The diaphragm, according to Broman, is thus derived from four sources (Fig. 191): (1) its ventral pericardial portion from the septum transversum; its lateral portions from (2) the pleuro-peritoneal membranes plus (3) derivatives from the body wall; lastly, a median dorsal portion is formed 196 THE ENTODERMAL CANAL AND ITS DERIVATIVES from (4) the dorsal mesentery. In addition to these, the striated muscle of the diaphragm, according to Bardeen, takes its origin from a pair of pre-muscle masses which in 9 mm. embryos lie one on each side opposite the fifth cervical segment. This is the level at which the phrenic nerve enters the septum trans- A Esophagus Septum trans v ers i Pleura- pericardial canal , Luna Peri ea.rdt.al cavity 'Pleura-peritoneal membrane Pleural cavity Pericardial Heart membrane Fig. 190. — Diagrams showing the development of the lungs and the formation of the pericardial mem- brane (modified after Robinson). A, coronal section; B, transverse section. Fig. 191. — Diagram showing the origin of the diaphragm (after Broman). 1, septum transver- sum; 2, 3, derivatives of mesentery; 4, 4, derivatives of pleuro-peritoneal membrane; 5, 5, parts de- rived from the body walls. versum. The exact origin of these muscle masses is in doubt but they probably represent portions of the cervical myotomes of this region. The muscle masses migrate caudally with the septum transversum and develop chiefly in the dorsal portion of the diaphragm, according to Bardeen. Keith derives the muscle of the diaphragm also from the rectus and transversalis muscles of the abdominal wall. The cavities of the mesodermic segments are regarded as portions of the ccclom but in BODY CAVITIES, DIAPHRAGM AXD MESENTERIES 197 man they disappear early. The development of the vaginal sacs which grow out from the inguinal region of the peritoneal cavity into the scrotum will be described in Chapter VIII. The Omental Bursa or Lesser Peritoneal Sac. — According to Broman, the lesser peritoneal sue is represented in 3 mm. embryos by a peritoneal pocket which extends craniallv into the dorsal mesentery to the right of the esophagus. A similar pocket present on the left side has disappeared in 4 mm. embryos. Lateral to the opening-of the primitive peritoneal sac, a lip-like fold of the mesentery is continued caudally along the dorsal body wall into the mesonephric fold as the Fight 'umbilical vein Ventral mesentery flight lobe I > of Liver Lesser peritoneal Left umbilical Vein Ectoderm of body wall Left, lobe of Liver Ventral mesentery Duodenum Dorsal mesentery Left post , Cardinal vein Notochord Fig. 192. — Diagrammatic view of an embryo of 7 to 9 mm. showing the position of the lesser peri- toneal sac. The cranial portion of the embryo is represented as sectioned transversely, caudal to the liver, so that one looks at the caudal surface of the section and of the liver and craniallv into the body cavities. plica vena: cava;, in which later the inferior vena cava develops (Fig. 192). The liver, it will be remembered, grows out into the ventral mesentery from the fore- gut and, expanding laterally and ventrally, takes the form of a crescent. Its right lobe comes into relation with the plica venae cavae and, growing rapidly caudad, forms with the plica a partition between the lesser sac and the peritoneal cavity. Thus the cavity of the lesser peritoneal sac is extended caudally from a point opposite the bifurcation of the lungs to the level of the pyloric end of the stomach. In 5 to 10 mm. embryos it is crescent-shaped in cross-section (Fig. 132) and is bounded mesially by the greater omentum (dorsal mesentery) and the right wall of the stomach, laterally by the liver and plica venae cavae and ven- 198 THE ENTODERMAL CANAL AND ITS DERIVATIVES trally by the lesser omentum (ventral mesentery). It communicates to the right with the peritoneal cavity through an opening between the liver ventrally and the plica venae cavae dorsally (Fig. 194). This opening is the epiploic fora- men (of Winslow). When the dorsal wall of the stomach rotates to the left the greater omentum is carried with it to the left of its dorsal attachment. Its tissue Body wall Inf. Vena cava. Sup. recess of lesser peritonea] ^ac E Pleuro -peritoneal' tnetnbrane Inf.vena. cava. ~Phca venae Cai/ae Mesonephric fold Genital fold ra/fciform ligament Coronary attachment of Liver to diaphragm ~Pleu.ro - peritoneal Pleuro- peritoneal membrane Lesser Omentum Greater omentum Spleen Stomach Lesser peritoneal 3ac orTa. Fig. 193. — A diagrammatic ventral view of the middle third of an embryo 12 to 15 mm. long. The figure shows the caudal surface of a section through the stomach and spleen; a ventral view of the stom- ach, the liver having been cut away to leave the sectioned edges of the lesser omentum and plica venae cava;; and the caudal surface of the septum transversum and pleuro-peritoneal membrane. Upon the surface of the septum is indicated diagrammatically the attachment of the liver (based on figures of Mall and F. T. Lewis and model by H. C. Tracy). grows actively to the left and caudally and gives the omentum an appearance of being folded on itself between the stomach and the dorsal body wall (Fig. 193). The cavity of the lesser peritoneal sac is carried out between the folds of the greater omentum as the inferior recess of the omental bursa. From the cranial end of the sac there is constricted off a small closed cavity which is frequently persistent in the adult. This is the bursa infracardiaca and may be regarded as a BODY CAVITIES, DIAI'IIKACM AM) MESENTERIES IQ9 third pleural cavity. It lies to the right of the esophagus in the mediastinum and its average diameter in the adult is 10 mm. When the stomach changes its position and form so that its mid-ventral line becomes the lesser curvature and lies to the right, the position of the lesser omentum is also shifted. From its primitive location in a median sagittal plane with its free edge directed caudally it is rotated through 900 until it lies in a cor- onal plane with its free margin facing to the right. The epiploic foramen now Mesonephros Greater omentum peritoneal sac Duo- Vitilline vein Intestinal- loop Stomach Left umbilical Vein Fig. 194. — An obliquely transverse section through a 10 mm. embryo at the level of the epiploic foramen (of Winslow). X S3- forms a slit-like opening leading from the peritoneal cavity into the vestibule of the omental bursa. The foramen is bounded ventrally by the edge of the lesser omentum, dorsally by the inferior vena cava, cranially by the caudate pro- cess of the liver and caudally by the wall of the duodenum. During fetal life the greater omentum grows rapidly to the left and caudad in the form of a sac flattened dorso-ventrally. It overlies the intestines ven- trally and contains the inferior recess of the omental bursa (Fig. 195). The dor- sal wall of the sac during the third and fourth months usually fuses with the trans- verse colon where it overlies the latter. Caudal to this attachment, the walls 200 THE ENTODERMAL CANAL AND ITS DERIVATIVES of the greater omentum may be fused and its cavity is then obliterated. The inferior recess of the omental bursa thus may be limited in the adult chiefly to a space between the stomach and the dorsal fold of the greater omentum, which latter is largely fused to the peritoneum of the dorsal body wall. The spleen develops in the cranial portion of the greater omentum and that portion of the omentum which extends between the stomach and spleen is known as the gastro- lienic ligament. The dorsal wall of the omentum between the spleen and kidney is the lieno-renal ligament. Further Differentiation of the Mesenteries: Ligaments of the Liver. — We have seen (p. 188) that the cranial portion of the ventral mesentery forms the mesocardium of the heart. In the ventral mesentery caudal to the septum trans- A B c Fig. 195. — Diagrams showing the development of the mesenteries (Hertwig). A, illustrates the beginning of the great omentum and its independence of the transverse mesocolon; in B the two come into contact; in C they have fused; A, stomach; B, transverse colon; C, small intestine; D, duode- num; E, pancreas; F, greater omentum. versum develops the liver. From the first, it is enveloped in folds of the splanch- nic mesoderm which give rise to its capsule and ligaments as the liver increases in size (Fig. 183 B). Wherever the liver is unattached, the mesodermal layers of the ventral mesentery form its capsule (of Glisson), a fibrous layer covered by mesothelium continuous with that of the peritoneum (Fig. 183 B). Along its mid-dorsal and mid- ventral line the liver remains attached to the ventral mesentery. The dorsal attachment between the liver, stomach and duodenum is the lesser omentum. This in the adult is differentiated into the duodeno-hepatic and gastro-hepatic ligaments. The attachment of the liver to the ventral body wall extends caudally to the umbilicus and forms the falciform ligament. In its early development the liver abuts upon the septum transversum, and in 4 to 5 mm. embryos is attached to it along its cephalic and ventral surfaces. BODY CAVITIES, DIAPHK.U.M AND MESENTERIES 201 Soon dorsal prolongations of the lateral liver lobes, the coronary appendages come into relation with the septum dorsally and laterally. The attachment of the liver to the septum transversum now has the form of a crescent, the dorsal horns of which arc the coronary appendages (Fig. 193). This attachment be- comes the coronary ligament of the adult liver. The dorso- ventral extent of the coronary ligament is reduced during development and its lateral extensions upon the diaphragm give rise to the triangular ligaments of each side. The right lobe of the liver, as we have seen, comes into relation along its dorsal surface with the plica voice cava: in 9 mm. embryos (Figs. 192 and 193). This attachment extends the coronary ligament caudally on the right side and makes possible the connection between the veins of the liver and mesonephros through which the inferior vena cava is in part developed. The portion of the liver included between the plica vena cavce and the lesser omentum is the cau- date lobe (of Spigelius). In a fetus of five months the triangular ligaments mark the position of the lateral coronary appendages. The umbilical vein courses in a deep groove along the ventral surface of the liver and with the vena porta and gall-bladder bounds the quadrate lobe. Changes in tlie Dorsal Mesentery. — That part of the digestive canal which lies within the peritoneal cavity is suspended by the dorsal mesentery which at first forms a simple attachment extending in the median sagittal plane between body wall and primitive gut. That portion of it connected with the stomach forms the greater omentum, the differentiation of which has been described (p. 199). The mesentery of the intestine is carried out into the umbilical cord between the limbs of the intestinal loop. When the intestine elongates and its loop rotates, the caecal end of the large intestine comes to lie cranially and to the left, the small intestine caudally and to the right, the future duodenum and colon crossing in close proximity to each other (Fig. 196 A). On the return of the intestinal loop into the abdomen from the umbilical cord the caecal end of the colon lies to the right and the transverse colon crosses the duodenum ventrally and cranially. The primary loops of the small intestine lie caudal and to the left of the ascending colon (Fig. 196 B). There has thus been a torsion of the mesentery about the base of the superior mesenteric artery as an axis. From this focal point the mesen- tery of the small intestine and colon spreads out fan-like. The mesoduodenum is pressed against the dorsal body wall, fuses with its peritoneal layer and is obliter- ated (Fig. 195). Where the mesentery of the transverse colon crosses the duo- denum it fuses at its base with the surface of the latter and of the pancreas. Its 202 THE ENTODERMAL CANAL AND ITS DERIVATIVES fixed position now being transverse instead of sagittal, the mesentery is known as the transverse mesocolon. The mesentery of the ascending colon is flattened against the dorsal body wall on the right and fuses with the peritoneum. Simi- larly, the descending mesocolon fuses to the body wall of the left side (Fig. 196 A, B). There are thus left free (1) the transverse mesocolon; (2) the mesentery proper of the jejunum and ileum with numerous folds corresponding to the loops of the intestine; (3) the iliac mesocolon; (4) the mesorectum, which retains its primitive relations. Anomalies of the diaphragm and mesenteries are not uncommon. The per- Lesser omentum Dorsal . mesogastrLLLm Stomach Greater omentum Transverse mesocolon ecum Mesentery Mesorectum Iliac mesocolon A B Fig. 196. — Diagrams showing the development of the mesenteries in ventral view (modified from Tourneux in Heisler). * Cut edge of greater omentum; a, area of ascending mesocolon fused to dorsal body wall; b, area of descending mesocolon fused to dorsal body wall. sistence of a dorsal opening in the diaphragm, more commonly on the left side, may be explained as due to the defective development of the pleuro-peritoneal membrane. Such a defect may lead to diaphragmatic hernia, the abdominal viscera projecting to a greater or less extent into the pleural cavity. The mesenteries also may show malformations due to the persistence of the simpler embryonic conditions, usually correlated with the defective development of the intestinal canal. The ascending and descending mesocolon may be free, having failed to fuse with the dorsal peritoneum. The primary folds of the greater omentum may fail also to unite so that the inferior recess extends to the caudal end of the greater omentum. CHAPTER VIII UROGENITAL SYSTEM The urogenital system is composed of distinct urinary and genital glands which, however, possess common ducts and have a common origin from the meso- derm. The excretory glands are the pronephros, mesoncphros, and metanepkros, organs which develop in this order. The first two named are the temporary kid- neys of the mammalian embryo, but are functional in adult fishes and amphibia. The metanephros is the permanent kidney of reptiles, birds and mammals. THE PRONEPHROS The pronephros, when functional, consists of paired segmentally arranged tubules, one end of each tubule opening into the ccelom, the other into a longitu- *—*■ Antaqes ef prgnephrie. duct ^ Notochord Fig. 197. — Diagrams showing the development of the pronephric duct and pronephric tubules (modi- fied from Felix). A shows a later stage than B. dinal pronephric duct which drains into the cloaca (Fig. 197 A). Near the nephrostome^the opening into the ccelom) knots of arteries project into the tubules, 203 >o4 UROGENITAL SYSTEM forming glomeruli. Fluid from the ccelom and glomeruli and excreta from the cells of the tubules are carried by ciliary movement into the pronephric ducts. The human pronephros is vestigial. It consists of seven pairs of rudimentary pronephric tubules derived from the mesoderm of the nephrotomes (Fig. 198), which are segmented portions of the cell mass intermediate between the primitive segments and the mesodermal layers (somatic and splanchnic). Anlages of pronephric tubules are formed as dorsal nodules in each segment from the seventh to the fourteenth. The nodules hollow out and open into the ccelom. Dorsally and laterally, the tubules of each side unite to form a longitudinal collecting duct. Neural tube Notochord Cavity of gut Splanchnic mesoderm Mesoderm of yolk-sac « Mesodermal segment Cavity of segment Intermediate cell mass Anlage of extremity Ccelom Y3 -yfliy.-Q.- _ Somatic mesoderm Umbilical vein Fig. 198. — Transverse section of a 2.4 mm. human embryo showing the intermediate cell mass or nephrotome (Kollmann's Atlas). The tubules first formed in the seventh segment begin to degenerate before those of the fourteenth segment have developed. Caudal to the fourteenth segment no pronephric tubules are developed, but the free end of the collecting duct ap- parently grows caudad, beneath the ectoderm and lateral to the nephrogemc cord, until it reaches, and opens through, the lateral wall of the cloaca. Thus are formed the paired primary excretory {pronephric) ducts. The pronephric tubules begin to appear in embryos of 1.7 mm. (Felix in Keibel and Mall, vol. 2); in 2.5 mm. embryos all the tubules have developed and the primary excretory duct is nearly complete. In 4.25 mm. embryos the duct has reached the wall of the Tin: mi.sum.i-iiros 205 cloaca and soon after fuses with it. The pronephric tubules soon degenerate, but the primary excretory ducts persist and become the ducts of the mesoncphroi, or mid-kidneys. THE MESONEPHROS The mesonephros, like the pronephros, consists essentially of a series of tubules, each of which at one end is related to a knot of blood-vessels and forms a. capsule surrounding a glomerulus, at the other end opens into the primary ex-i)C*4< cretory duct. They differ from the pronephric tubules in that they do not open , into the ccelom, and as many as four may develop in a single segment. They ~~ arise from the mesoderm intermediate between the primitive segments and the'-' lateral mesodermal layers, mesoderm which, in human embryos, is not segmented into nephrotomes caudal to the tenth pair of segments, but constitutes the un- segmented nephrogenic cord on either side. This may extend caudally as far as the twenty-eighth segment. The primary excretory ducts lie lateral to the neph- rogenic cords. When the mesonephric tubules begin to develop and expand there is not room for them in the dorsal body wall and as a result this bulges ventrally into the ccelom. Thus there is produced on either side of the dorsal mesentery a longitudinal urogenital fold, which may extend from the sixth cervi- cal to the third lumbar segment (Fig. 213). Later, this ridge is divided into a lateral mesonephric fold and into a median genital fold, the anlage of the genital gland. Differentiation of the Tubules. — The nephrogenic cord in 2.5 mm. embryos first divides into spherical masses of cells, the anlages of the mesonephric tubules. Four of these may be formed in a single segment. Appearing first in the 13th, 14th and 15th segments, the anlages of the tubules differentiate both cranially and caudally. In 5.3 mm. embryos the cephalic limit is reached in the sixth cervical segment, and thereafter degeneration begins at the cephalic end. In 7 mm. embryos the caudal limit is reached in the third lumbar segment and in later stages the caudal end of the mesonephros undergoes degeneration. The spherical anlages of the tubules differentiate in a cranio-caudal direction (Fig. 199). First, vesicles with lumina are formed (2.5 mm.). Next the vesicles elongate laterally, unite with the primary excretory ducts and become S-shaped (4.9 mm.). The free vesicular end of the tubule enlarges, becomes thin-walled and into this wall grows a knot of arteries to form the glomerulus (embryos of 5 to 7 mm.). The ^ubule, at'first solid, hollows out and is lined with a low colum- nar epithelium-*. The outer wall of the vesicle about the glomerulus is Bowman's 2o6 UROGENITAL SYSTEM capsule, the two constituting a renal corpuscle of the mesonephros (Fig. 199 D). In the human embryo, the tubules do not branch or coil as in pig embryos, con- sequently the mesonephros is relatively smaller. At 10 mm. 32 to 34 tubules are present in each mesonephros and the glomeruli are conspicuous (Fig. 200). Each tubule shows a distal excretory and a proximal collecting portion which connects with the duct (Fig. 201). The glomeruli form a single median column, the tubules are dorsal and the Meion*/jiriz duct Anlaqe of rnesonephric tubule Tubule Bowmans capsule Fig. 199. — Diagrams showing the differentia- tion of the rnesonephric tubules (modified after Felix j . L, lateral; M, median. Degenerating rnesonephric corpuscieJ Degenerating Corpuscle 4 tubules '/refer ^etaner*ns Fig. 200. — Diagram showing the anlages of the urinary organs from the left side (based on reconstructions by Keibel and Felix). Ventro-lateral branches from the aorta supply the glomeruli, while the posterior cardinal veins, dorsal in position, break up into a network of sinusoids about the tubules (see Chapter IX). The primary excretory duct or rnesonephric duct is solid in 4.25 mm. em- bryos. A lumen is formed at 7 mm. wider opposite the openings of the tubules. The duct is important as from it grows out the ureteric anlage of the permanent kidney, while the duct itself is transformed into the genital duct of the male., and TIIK MF.SOXKPIIROS 207 its derivatives. The mesonephros is probably not a functional excretory organ in human embryos for its tubules degenerate before the metanephros becomes functional. It may have some other function and produce an internal secretion. Degeneration proceeds rapidly in embryos between 10 and 20 mm. long, begin- ning cranially. New tubules are formed at the same time caudally. In all, 83 pairs of tubules arise, of which only 26 pairs persist at 21 mm., and these are in- terrupted at the connecting points between the collecting and secretory regions. They are divided into an upper group and a lower group. The upper group, numbering 5 to 12, unites with the rete tubules of the testis or ovary. In the male they form the efferent ductuli of the epididymis. In the female they con- Supra renal gland. Dowman's Capsule Collecting tubule Secretory- tubule Mesonephric, duct r Muel/erian duct -/inlage genital gland. Fig. 201. — Reconstruction of a mesonephric tubule, glomerulus and mesonephric duct from a 12 mm. human embryo as seen in transverse section. X 95. Post.card. v., posterior cardinal vein. stitute part of the e poop hor on. Of the lower group a few tubules persist in the male, as the paradidymis with its canaliculus aberrans. In the female they form the paroophoron. THE METANEPHROS The essential parts of the permanent kidney are the renal corpuscles (glom- erulus with Bowman's capsule), secretory tubules and collecting tubules. The collecting tubules open into expansions of the duct, the pelvis and calyces. The duct itself is the ureter, which opens into the bladder. Like the mesonephros, the metanephros is of double origin. The ureter, pelvis, calyces and collecting tubules are outgrowths of the mesonephric duct.- The secretory tubules and the capsules of the renal corpuscles' are differentiated from the caudal end of the nephrogenic cord and thus have the same origin as the mesonephric tubules. 2o8 UROGENITAL SYSTEM Wolffian duel hind-gut outer zone inner zone .pelvis of kidney In embryos of 4.5 to 5.5 mm. the mesonephric duct makes a sharp bend just before it joins the cloaca and it is at the angle of this bend that the ureteric anlage of the metanephros appears, dorsal and somewhat median in position (Fig. 209 B, C). The bud grows at first dorsally, then cranially. Its distal end expands and forms the primitive pelvis. Its proximal elongated portion is the ureter. The anlage grows into the lower end of the nephrogenic cord which, in 4.6 mm. em- bryos, is separated from the cranial end of the cord at the twenty-seventh seg- ment. The nephrogenic tissue forms a cap about the primitive pelvis and, as the pelvis grows cranially, is carried along with it (Fig. 202). In embryos of 9 to 13 mm. the pelvis has reached a position in the retroperitoneal tissue dorsal to the mesonephros and opposite the second lumbar segment. Thereafter, the kidney grows both cranially and caudally without shifting its position. The ureter lengthens as the embryo grows in length. The cranial growth of the kidney takes place dorsal to the suprarenal gland (Fig. 225). Primary renal tubules grow out from the primitive pelvis in 10 mm. embryos. Of the first two, one is cranial, the other caudal in position, and between these are two to four others (Fig. 203 B, C). From an enlargement, the ampulla, at the end of each primary tubule grow out two, three or four secondary tubules. These in turn give rise to tertiary tubules (Fig. 203 D) and the process is repeated until the fifth month of fetal life, when it is estimated that twelve generations of tubules have been developed. The pelvis and both primary and secondary tubules enlarge during development. The first two primary tubules become the major calyces, and the secondary tubules opening into them form the minor calyces (Fig. 204). The tubules of the third and fourth orders are taken up into the walls of the enlarged second- ary tubules so that the tubules of the fifth order, 20 to 30 in number, open into the calyces minor as papillary ducts. The remaining orders of tubules cloaeal membrane Fig. 202. — Reconstruction of the anlages of the meta- nephros ( after Schreiner). The layers lettered inner and outer zones constitute the nephrogenic tissue of the metanephros. THE METANEPHROS 209 *5e Conakry Col/ecTiny tubules constitute the collecting tubules which form the greater part of the medulla of the adult kidney. When the four to six primary tubules develop, the nephrogenic cap about the primitive pelvis is subdivided and its four to six parts cover the end of each pri- D Cranial pole Zubute VzntraL central tubule Cranial pole tubule Caudal pole rubule Ureter Tertiary col- lecting tubule Fig. 203. — Diagrams showing the development of the primitive pelvis, calyces and collecting tubules of the metanephros (based on reconstructions by Schreiner and Felix). mary tubule. As new orders of tubules arise, each mass of nephrogenic tissue increases in amount and is again sub- divided until finally it forms a peripheral layer about the ends of the branches tributary to a primary tubule. The converging branches of such a tubular "tree" constitute a primary renal unit, or pyramid, with its base at the periphery of the kidney and its apex projecting into the pelvis. The apices of the pyramids are termed renal paj)ill(e_ and through them the larger collecting ducts open. The nephrogenic tissue forms the cortex of the kidney, and each sub-division of it, covering the tubules of a pyramid peripherally, is marked off on the surface of the organ by grooves or depressions. The fetal kidney is thus distinctly lobated, the lobations persist- ing until after birth. The primary pyramids are subdivided into several secondary and tertiary pyramids. Between the pyramids the cortex of nephrogenic tissue dips down to the pelvis, forming the renal r.nl.umtm fpf "RprtiniV The collect- 14 Fig. 204. — The pelvis, calyces, and their branches and a portion of the ure- ter, from the meta- nephros of a 16 mm. embryo (Huber). 2IO UROGENITAL SYSTEM ing tubules, on the other hand, extend out into the cortex as the cortical rays or pars radiata of the cortex. In these rays and in the medulla of the kidney the collecting tubules run parallel and converge to the papilke, DIFFERENTIATION OF THE NEPHROGENIC TISSUE In stages from 13 to 19 mm. the nephrogenic tissue about the ends of the collecting tubules condenses into spherical masses which lie in the angles between the buds of new collecting tubules and their parent stems (Fig. 205). One such Fig. 205. — Semidiagrammatic figures of the anlage and differentiation of renal vesicles and early developmental stages of uriniferous tubules of mammals, i and 2, anlage and successive stages in the differentiation of renal vesicles, as seen in sagittal sections; 3, section and outer form of tubular anlage before union with collecting tubule at the beginning of S-shaped stage; 4 and 5, successive stages in the development of the tubules, Bowman's capsule and glomerulus beginning with a tubular anlage showing a well developed S-shape (Huber). S\f ymetancphric sphere is formed for each new tubule. The spheres are converted ^^ into vesicles with eccentrically placed lumina. The vesicle elongates, its thicker ? outer wall forming an S-shaped tubule which unites with a collecting tubule, DIFFERENTIATION OF THE NEPHROGENIC TISSUE 211 Proximal convoluted tubule Distal convoluted tubule. Renal corpuscle Connecting piec e Ascending limb of Henle's loop Descend/tig limb of Henle's loop Large collecting tubule Arch of collecting tubule Arch of Col/ecfinq tubule 7 Dteta) convoluted tubule Stoerck 3 loop Proximal conuolut- tubule Connecting piece Glomerulus Bowman's capsule Arch of collecting tubule y Proximal convoluted tubule Distal Convoluted tubule , Connecting piece Glomerulus Bowman's capsule Stoercfc's loop Fig. 206. — Diagrams showing the differentiation of the various parts of the uriniferous tubules of the metanephros (based on the reconstructions of Huber and Stoerck). A. from an adult human kid- ney; B, C, from human embryos. its thin inner wall becoming the capsule (Bowman's) of a renal corpuscle. The uriniferous tubules of the adult ' kidney have a definite and peculiar structure and arrange- ment (Fig. 207 A). Beginning with a renal corpuscle, each tubule forms a proximal con- voluted portion, a U-shaped loop (of Henle) withi descending and v ascending limbs,* a connecting piece, which lies close to the renal corpuscle, and a distal convoluted portion continuous with the collecting tubule. These parts are derived from the S-shaped anlage, which is composed of a lower, middle and nr> Fig. 207. — Diagram showing the relation of Bow- man's capsule and the uriniferous tubules to the collecting tubules of the metanephros (Huber). c , collecting tubules; c, end branches of collecting tubules; m, renal corpuscles; 11, neck; pc, proximal convoluted tubule; pi, descending limb of Henle's loop; al, ascending limb of Henle's loop; dc, distal convoluted tubule; /, junctional tubule. 212 UROGENITAL SYSTEM upper limb. The middle limb, somewhat U-shaped, bulges into the concavity of Bowman's capsule (Fig. 206 B). By differentiation the lower portion of the lower limb becomes Bowman's capsule, ingrowing arteries forming the glomerulus. The upper part of the same limb by enlargement, elongation and coiling becomes Fig. 208.— Several stages in the development of the uriniferous tubules and glomeruli of the human metanephros (reconstructions by Huber). the proximal convoluted tubule. The neighboring portion of the middle limb forms the primitive loop (of Stoerck) ; the base of the middle limb gives rise to the connecting piece, and the rest of it, with the upper limb of the S, forms the distal convoluted tubule (intermediate piece of Felix). The primitivejoop of Stoerck CLOACA, BLADDER, URETHRA AND UROGENITAL SIMS 213 includes both the ascending and descending limbs of Henle's loop and a portion of the proximal convoluted tubule. Henle's loop is differentiated during the fourth fetal month (Toldt) and extends from the pars radiata of the cortex into the medulla (Fig. 207). The concavity of Bowman's capsule, into which grow the arterial loops of the glomerulus, is at first shallow. Eventually the walls of the capsule grow about and enclose the vascular knot, except at the point where the arteries enter and emerge (Fig. 205, 4 and 5). Renal corpuscles are first fully formed in 28 to 30 mm. embryos. The new corpuscles are formed peripher- ally from persisting nephrogenic tissue, hence in the adult the oldest corpuscles are those next the medulla. Reconstructions of the various stages in the de- velopment of the uriniferous tubules are shown in Fig. 208. Renal Arteries. — One or more of the mesonephnc arteries is transformed into the renal artery of the metanephros. As any one of the mesonephric arteries may thus form the renal artery, and as they anastomose, the variation of the renal vessels both as to position and number is accounted for. Anomalies. — If the uriniferous tubules fail to unite with the collecting tubules, cystic degeneration may take place. The cystic kidneys of pathology may thus be produced. The nephrogenic tissue of the paired kidney anlages may fuse, resulting in the union of their cortex. Double or triple ureters are sometimes present. DIFFERENTIATION OF CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS In embryos of 3 to 4 mm. the cloaca, a caudal expansion of the hind-gut, is in contact ventrally with the ectoderm, and ectoderm and entoderm together form the cloacal membrane (Fig. 209 A). Ventro-cranially the cloaca gives off the allantoic stalk. At a somewhat later stage, the cloaca receives laterally the mesonephric ducts and caudally is prolonged as the tail-gut (Fig. 209 B). In embryos of 5 mm. the ureteric anlages of the metanephroi are present as buds of the mesonephric ducts (Fig. 209 C, D). Next, the saddle-like partition wall between the intestine and allantois grows caudally. dividing the cloaca into a dorsal rectum and ventral primitive urogenital sinus. The division is complete in embryos of 11 to 15 mm. and at the same time the partition, fusing with the cloacal membrane, divides it into the anal membrane of the gut and the urogenital membrane. At 11 mm., according to Felix, the primitive urogenital sinus by elongation and constriction is differentiated into two regions: (1) a dorsalyesico-urethral anlage which receives the allantois and mesonephric duct, and is connected by the constricted portion with (2) the phallic portion of the urogenital sinus (Figs. 210 and 211). The latter extends into- the phallus of both sexes and forms a greater part of the urethra (Fig. 212). The vesica- 214 UROGENITAL SYSTEM Hind-cut Mesonephric duct Allantois Cloaca Ctoacal membrane Cloacal membrane Mesonephric duct Mesonephric duct Allantois Hind-cut Cloaca Metanephros 'loacat membrane yS Cloaca Tail-out Cloaca! , membrane Tail-qut Fig. 209. — Four stages showing the differentiation of the cloaca into the rectum, urethra and blad- der. A, from an embryo of 3.5 mm.; B, a somewhat later stage; C, from a 5 mm. embryo; D, from an embryo of 7 mm. (after reconstructions by Pohlman). About 100 diameters. Mesonephric duct Metanephros Inlesline Anlage of bladder Cloacal membrane Urogenital sinus flee turn Fig. 210. — Reconstructions from a 12 mm. embryo showing the partial subdivision of the cloaca into rectum and urogenital sinus (after Pohlman). CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS 215 urethral anlage enlarges and forms the bladder and a portion of the urethra. In 7 mm. embryos the proximal ends of the mesonephric ducts are funnel-shaped, and at 10 mm., with the enlargement of the bladder, these ends are taken up into /llJantois Rectum Mesonephric duels. Vesica- urethral anlage Phallic portion of urogenital sinus Metanephros Ureter Fig. 211. — Reconstruction of the caudal portion of an 11. 5 mm. embryo showing the differentiation of the rectum, bladder and urethra (Keibel). IVIesonephric To, /Inlaye of bladder (Iterovag, Mesonephros Mesonephric duct Ureter Muellenan duct Mesonephric duct" Rectum Spinal cord FlG. 212. — Reconstruction of the caudal end of a 25 mm. embryo showing the complete separation of the rectum and urogenital sinus and the relations of the urogenital ducts (after Keibel's model). its wall until the ureters and mesonephric ducts acquire separate openings. The ureters, having previously shifted their openings into the mesonephric ducts from a dorsal to lateral position, now open into the vesico-urethral anlage lateral to the mesonephric ducts. The lateral walls of the bladder anlage grow more rap- tyaWo^ 2l6 UROGENITAL SYSTEM idly than its dorso-median urethral wall, hence the ureters are carried cranially and laterally upon the wall of the bladder, while the mesonephric ducts, now the male ducts, open close together into the dorsal wall of the urethra (Fig. 212). The fate of the phallic portion of the urogenital sinus is described on p. 234 in connection with the external genitalia. The allantois between the bladder and the umbilicus is known as the urachus. Usually the epithelium of the urachus degenerates, but portions may persist and produce cysts. In some cases it forms after birth a patent tube opening at the umbilicus. Its connective tissue layers always persist as the fibrous lig. vesico-umbilicale medium. The transitional epithelium of the bladder appears at 60 mm. The outer longitudinal layer of small muscle develops in 22 mm. embryos, and in 26 mm. embryos the circular muscle appears. The inner longitudinal muscle layer is found at 55 mm. and the sphincter vesicae in embryos of 90 mm. Mesencephalon Prosencephalon Rhombencephalon R. lung Mesonephric fold Lower extremity Gen Hal eminence Fig. 21.3. — Ventral view of the urogenital folds in a human embryo of five weeks (Kollmann's Atlas). THE GENITAL GLANDS AND DUCTS— INDIFFERENT STAGE As to origin and early development, the ovary and testis are alike. The urogenital fold (see p. 205) is the anlage of both the mesonephros and the genital gland (Fig. 213). At first two-layered, its epithelium in embryos of 5 mm. thick- THE GENITAL GLANDS AND DUCTS— INDIFFERENT STAGE 217 ens over the ventro-median surface of the fold, becomes many-layered and bulges into the ccelom ventrally, producing the longitudinal genital fold. The genital fold thus lies mesial and parallel to the mesonephric fold. At 10 to 12 mm. the genital epithelium shows no sexual differentiation (Fig. 214). There is a super- ficial epithelial layer and an inner epithelial mass of somewhat open structure. Owing to the great development of the suprarenal glands and metanephroi, the cranial portions of the urogenital folds, at first parallel and close together, are separated. This produces a double bend in the fold and in 20 to 25 mm. embryos the fold shows a cranial longitudinal portion, a transverse middle portion between the bends, and a longitudinal caudal portion. In the last named, the mesonephric Lat. body wall Post, cardinal Suprarenal gland Glomerulus Mesonephric duel- Inner epithet, mass ofqemtaL gland. Epithelium of" genital gland Mesentery Fig. 214. — Transverse section through the mesonephros, genital gland and suprarenal gland of the right side; from a 12 mm. human embryo. X 165. ducts course to the cloaca and here the right and left folds fuse, producing the genital cord (Fig. 225). As the genital glands increase in size they become con- stricted from the mesonephric fold by lateral and ventral grooves until the origi- nally broad base of the genital fold is converted into a stalk (Fig. 220). This stalk-like attachment extends lengthwise and forms in the male the mesorchium. in the female the mesovarium. The urogenital fold is, at the same time, constricted from the dorsal body wall until it is attached only by a narrow mesentery which eventually forms either the lig. testis or lig. ovarii. The Indifferent Stage of the Genital Ducts.— The mesonephric ducts, with the degeneration of the mesonephroi, become the male genital ducts. In both 2l8 UROGENITAL SYSTEM sexes there also develop a pair of female ducts (of Mueller) . In embryos of 10 to ii mm. the Muellerian ducts develop as ventro-lateral thickenings of the uro- genital epithelium at the level of the third thoracic segment and near the cranial ends of the mesonephroi. Next, a ventro-lateral groove appears in the epi- thelium of the mesonephric fold (Fig. 215 A) . Caudally, the dorsal and ventral lips of the groove close and form a tube which separates from and lies beneath the epithelium (Fig. 215 B). Cranially, the tube remains open as the funnel-shaped ostium abdominale of the Muellerian duct. The solid end of the tube grows caudalward beneath the epithelium, lateral to the mesonephric or male ducts Lateral body wall — -g. Muellerian groove W- Mesentery Mesonephric tubule Genital aland -Anlac/e of Muellerian duct Fig. 215. — Transverse sections through the anlage of the right Muellerian duct from a 10 mm. embryo. A, showing the groove in the urogenital epithelium; B, three sections caudad showing the tubular anlage of the duct. X 250. (Figs. 216 and 218). Eventually, by way of the genital cord, the Muellerian ducts reach the median dorsal wall of the urogenital sinus and open into it. Their further development into uterine tubes, uterus and vagina is described on page 226. Embryos not longer than 12 mm. are thus characterized by the pos- session of indifferent genital glands, and of both male and female genital ducts. There is as yet no sexual differentiation. The development and position of the Muellerian ducts is well shown in ventral dissections of pig embryos (Figs. 216 and 217). Note the enormous size of the mesonephroi. Differentiation of the Testis. — In male embryos of 13 to 15 mm. the genital TILE GENITAL GLANDS AND DUCTS— TESTIS 219 6enital aland. Colon Allantois Umbi/icaL artery Fig. 2 16.— Ventral dissection of an iS mm. pig embryo to show the anlages of the Muellerian ducts. X 7. Tracht / Luna Esophajui Genital qLand ' Mesonephnc duct Allanto, (Bladder) Mesonephm Muellerian Umbilical artery FlG. 217. — Ventral dissection of a 24 mm. embryo showing the anlages of the Muellerian ducts at a later stage of development than in Fig. 216. X 6. 2 20 UROGENITAL SYSTEM glands show two characters which mark them as testes: (i) The occurrence of branched anastomosing cords of cells, the testis cords; (2) the occurrence between epithelium and testis cords of a layer of tissue, the anlage of the tunica albuginea (Fig. 218). According to Felix, the testis cords are developed suddenly from the loose inner epithelial mass by a condensation of its cells. The cords converge and grow smaller towards the mesorchium, where there is formed the undivided epi- thelial anlage of the rete testis. Two or three layers of loosely arranged cells between the testis cords and the epithelium constitute the anlage of the tunica albuginea. According to Allen (Amer. Jour. Anat., vol. 3, 1904), the testis cords are formed as active ingrowths of cords of cells from the epithelium. Mesentery Mesonephric tubule Mesorchium Intermediate cord is cord thelium albuginea. Fig. 218.— Transverse section through the left testis and mesonephros of a 20 mm. embryo. X 250. The testis cords soon become rounded and are marked off by connective tissue sheaths from the intermediate cords, columns of undifferentiated tissue which lie between them (Fig. 219). Toward the rete testis the sheaths of the testis cords unite to form the anlage of the mediastinum testis. The testis cords are composed chiefly of indifferent cells with a few larger genital cells. The cells gradually arrange themselves radially about the inside of the connective tissue sheath as a many-layered epithelium in which during the seventh month a lumen appears. The lumina appear in the peripheral ends of the testis cords and ex- tending toward the rete testis meet lumina which have formed there. Thus the solid cords of both are converted into tubules. The distal portions of the testis THE GENITAL GLANDS AND DUCTS — OVARY 221 tubules anastomose and form the tubuli contorti. Their proximal portions remain straight as the tubuli recti. The rete testis becomes a network of small tubules which finally unite with the collecting tubules of the mesonephros (see p. 225). The primitive genital cells of the testis cords form the spermatogonia of the spermatic tubules and from these at puberty are developed the spermatogonia (p. 24). The indifferent cells of the tubules become the sustentacular cells (of Sertoli) of the adult testis. Primitive genital cells of the intermediate cords are transformed into large pale cells which, after puberty, are numerous in the inter- stitial connective tissue and hence are called interstitial cells. The intermediate cords themselves are resorbed, but the connective tissue sheaths of the tubules unite to form septula which extend from the mediastinum testis to the tunica Mesorchium Ductus deferens Epithelium Rektest, Fig. 219. — Section through the testis of a 100 mm. fetus. X 44. albuginea. The latter becomes a relatively thick layer in the adult testis and is so called because of its whitish appearance. Anomalies. — The testis may be congenitally absent, the glands may be fused or they may fail to descend into the scrotum (cryptorchism). Duplications of the testis are of rare occur- rence. The Differentiation of the Ovary. — The primitive ovary, like the testis, consists of an inner epithelial mass and an outer epithelial layer. Much more slowly than in the testis the ovarian characters appear. In embryos of 50 to 80 mm. the inner epithelial mass composed of indifferent and primitive genital cells becomes less dense centrally and bulges into the mesovarian (Fig. 220). There may be distinguished a dense outer cortex beneath the epithelium, a clearer medullary zone containing large genital cells, and a dense cellular anlage in the mesovarium, the primitive rete ovarii, which is the homologue of the rete testis. 222 UROGENITAL SYSTEM No epithelial cords and no tunica albuginea are developed at this stage, as in the testis. Later, three important changes take place: (i) There is an ingrowth of connec- tive tissue and blood-vessels from the hilus, resulting in the formation of a media- stinum and of septula. (2) Most of the cells derived from the inner epithelial Tubules of mesone.phros (Paroophoron) Uterine tube Epi'i f he Hum Cortex - Genital cells —Medulla. Fig. 220. — Section of an ovary from a 65 mm. embryo. X 44 Primordial egg Primordial ovum Blood-vessel Germinal epithelium Tunica albuginea Pfliiger's egg-tubes Primordial ova \jft» fc *V'.?5 V "■*' #* **»* ja. £* Q4.flvuPf:n m w J FlG. 221.— Ovary of five-months' fetus, showing egg-tubes and primordial follicles (De Lee). THE GENITAL GLANDS AND DUCTS — OVARY 223 mass are transformed into young ova, the process extending from the rete ovarii peripherally (Fig. 221). (3) In embryos of from 80 to 180 mm. length the ovary grows rapidly, owing to the formation of a new peripheral zone of cells, derived in part from the epithelium. At the end of this period the epithelial cells beneath the epithelium are gradually replaced by a fibrous stroma, the anlage of the tunica albuginea. Hereafter, although folds of the epithelium are formed, these do not penetrate beyond the tunica albuginea, and all cells derived from this source subsequently degenerate. In late fetal life, according to Felix, the so-called "germinal epithelium" does not give rise to primitive ova. §■ fa mwr T *-" ' ,'ijB -lifAl-i'-. Membrana ■«*3f/ J/i[ - ' granulosa imordial Vitell follicle men brane I* 0 \% v v^ Fig. 222. — Primordial ova and early stages in the development of the Graafian follicle (De Lee's Obstetrics). Coincident with the origin of a new zone of cells at the periphery of the ovary goes the degeneration of young ova in the medulla. By the ingrowth into this region of connective tissue septa, the ova are separated into clusters or cords, the genital cells of which all degenerate, leaving in the medulla only a stroma of con- nective tissue. Late in fetal life the indifferent cells, by surrounding young ova, produce primordial follicles (Fig. 222 A). During the first year after birth the primitive follicles are transformed into the vesicular Graafian follicles. By cell division the follicle cells form a zone many layers deep about the young ovum (Fig. 222 B). Next a cavity appears in the sphere of follicle cells, enlarges and produces a vesicle filled with fluid (Fig. 223). The ovum is now eccentrically 224 UROGENITAL SYSTEM located and the follicle cells directly surrounding it constitute the cumulus oophorus (egg-bearing hillock). About the stratum granulosum formed by the original follicle cells there is differentiated from the stroma of the ovary the theca joUicidi. This is composed of an inner vascular tunica interna and of an outer fibrous tunica externa. Fully formed Graafian follicles are found in the ovaries during the second year and they may even be present before birth. Ovulation may occur at this time but usually these precociously formed follicles degenerate with their con- ■Tunica, inh •SiraJtu-ttf granulosum Fig. 223. — Graafian follicle and ovum from the ovary of a fifteen-year-old girl. X 30. tained ova. Thus, although thousands of ova are produced in the ovary, only a comparatively few are set free ready for fertilization during the sexually active life of the female, from puberty to the climacteric period or menopause. The relation of ovulation to menstruation has been discussed on p. 96. The Corpus Lulcum. — After ovulation a blood clot, the corpus hemorrhagicum, forms within the empty follicle. The follicle cells of the stratum granulosum proliferate, enlarge and produce a yellow pigment. The whole structure, composed of lutein cells and connective tissue strands, is termed the corpus lutcum or yellow body. The blood clot is resorbed and replaced by fibrous scar tissue white in color and known as the corpus albicans. If pregnancy does not intervene the corpus luteum spurium reaches its greatest development within two weeks and then degenerates. In cases of pregnancy the true corpus lutcum continues its growth until THE GENITAL (.LANDS I'NION WITH MKSONEPHROS 225 at the fifth or sixth month it reaches a maximal diameter of 15 to 30 mm. At birth it is Mill a prominent structure in the ovary and it is believed to produce an internal secretion; for if the corpus luteum is removed the ovum fails to attach itself to the wall of the uterus. The Rete Ovarii. — The cells of the rete ovarii remain compact, distinct and continuous only with the stroma of the medulla, the medullary cords. The anlage is differentiated into a network of solid cords in 60 mm. embryos (head-foot length) and these connect with the collecting tubules of the mesonephros. Some time before birth lumina appear in the cords transforming them into tubules homologous with those of the rete testis. Anomalies. — The ovaries vary greatly in form and position. Congenital absence of one or both glands is rare. Cases of supernumerary and bilobed ovar- ies have been observed. Comparing the testis and ovary in development, it is clear that the superficial epithelium after forming the inner epithelial mass takes no further part in the dif- ferentiation of the testis and only a small part in that of the ovary. The testis cords, rete testis and tunica albuginea are formed early from the inner epithelial mass, which determines their form. The inner epithelial mass of the ovary de- velops slowly and its passive cells are separated and surrounded by actively ingrowing connective tissue. The primordial follicles when developed are not the homologues of the testis cords and the tunica albuginea appears late. The rete ovarii is the homologue of the rete testis but remains a rudimentary structure. The Union of the Genital Glands and Mesonephric Tubules. — In both male and female embryos of 21 mm. the mesonephros has degenerated until, according to Felix, only twenty-six tubules persist separated into a cranial and a caudal group. In the cranial group of 5 to 12 tubules the collecting portions have sepa- rated from the secretory portions. The free ends of these collecting tubules project against that part of the inner epithelial mass which gives rise to the rete tubules of either testis or ovary (Fig. 220). The cords of the rete develop in contact with the collecting tubules of the mesonephros and unite with them. In the male this union was observed by Felix in embryos of 60 mm. head-foot length. The lumina of rete and collecting tubules become continuous and the latter are transformed into the ductuli eferentes of the epididymis. They convey the sperms from the testis tubules into the mesonephric duct, which thus becomes the male genital duet. During the fifth month of pregnancy the ductuli efferentes coil at their proximal ends and when surrounded by connective tissue they are known as coni vaseulosi. The cranial portion of the male genital duct also coils and forms the canalis epididymis. Its blind cranial end persists as the appendix epididymis. 15 226 UROGENITAL SYSTEM The caudal portion of the male duct remains straight and as the ductus def- erens extends from the epididymis to the urethra. Near its opening into the latter it dilates to form the ampulla and from its wall is evaginated the sacculated seminal vesicle in embryos of 60 mm. The epithelium of the genital duct is at first a single layer of cubical cells. At 70 mm. the cells become columnar with non-motile cilia at their free ends. Quite late in development the surrounding mesenchyma gives rise to the muscular layers. In the male, the rete testis, cranial group of mesonephric collecting tubules and mesonephric duct thus form functional structures (Fig. 231 C). The lower group of collecting tubules persist as the vestigial paradidymis. The Muellerian ducts of male embryos begin to retrograde at 30 mm. The middle portion of each degenerates but its cranial end persists as the appendix testis; its caudal end united with its fellow forms a pouch in the median dorsal wall of the urethra. This is the homologue of the vagina of the female and is called the vagina masculina. In the female, the rete ovarii is always a rudimentary structure, yet some time before birth it unites with the cranially persisting group of mesonephric tubules and forms a rudimentary structure, the epoophoron (Fig. 231 B). In its cords lumina appear, the epithelial cells become ciliated and smooth muscle tissue is developed corresponding to that of the epididymis. Usually the greater part of the male genital ducts atrophy in the female, the process beginning at 30 mm. Thus the tubules of the epoophoron are without an outlet. The caudal portions of the male genital ducts persist as Gartner's canals. These may extend as vestigial structures from the epoophoron to the lateral walls of the vagina, passing through the broad ligament and the wall of the uterus. They open into the vagina close to the free border of the hymen (R. Meyer). The canals are rarely present through- out their entire length and are absent in two-thirds to three-quarters of the cases examined. It is an interesting fact that in male and female embryos the ducts of the opposite sex begin to degenerate at the same stage, 30 mm. The Uterine Tubes, Uterus and Vagina. — The Muellerian, or female ducts, after taking their origin as described on p. 218, grow caudally, following the course of the mesonephric ducts (Fig. 217). At first lateral in position, the Muellerian ducts cross the mesonephric ducts and enter the genital cord median to them. In embryos of 20 to 30 mm. their caudal ends are dorsal to the urogenital sinus, extending as far as the Muellerian tubercle, a projection into the median dorsal wall of the vesico-urethral anlage (Fig. 212). This tubercle marks also the posi- tion of the future hymen. In embryos of 70 mm. the Muellerian ducts break through the wall of the urethra and open into its cavity. Before this takes place THE GENITAL GLANDS AND DUCTS — UTERUS 227 the caudal ends of the Muellerian ducts, which are pressed close together between the mesonephric ducts in the genital cord, fuse, and in both male and female em- bryos of 20 to 30 mm. give rise to the unpaired anlage of the uterus and vagina (Fig. 224 A). The utero-vaginal anlage of the male remains rudimentary. The uterine portion of the anlage degenerates with the paired portions of the Muel- lerian ducts. The vaginal portion remains as the vagina masculina, and the extreme cranial end of each Muellerian duct persists as the appendix testis. As pointed out by Felix, the term "uterus masculinus" as applied to the remains of the utero vaginal anlage is a misnomer, for the vaginal portion of the anlage persists and its uterine portion degenerates. Uterus and Vagina. — Since the Muellerian ducts develop in the urogenital folds, they make two bends in their course corresponding to those of the folds Cervix uteri Uterine tube Fundus of Uterus Found- Ligament Horizontal, portion of uterine tube Mesenchyn Fig. 224. — Diagrams showing the development of the uterus and vagina (modified after Felix). (Fig. 224 A). Each consists of a cranial longitudinal portion, a middle transverse portion and a caudal longitudinal portion, which is fused with its fellow to form the utero-vaginal anlage. At the angle between the cranial and middle portions is attached the inguinal fold, the future round ligament of the uterus (Figs. 225 and 226). The mesenchyma condenses about the utero-vaginal anlage and the middle transverse portion of the Muellerian ducts, forming a thick, sharply de- fined layer, from which is differentiated the muscle and connective tissue of the uterus and vagina (Fig. 224 B). As development proceeds the cranial wall be- tween the transverse portions of the Muellerian ducts bulges outward so that its original cranial concavity becomes convex (Fig. 224 B). The middle transverse portions of the ducts are thus taken up into the wall of the uterus forming its 228 UROGENITAL SYSTEM fundus, while the narrow cervix of the uterus and the vagina arise from the utero- vaginal anlage. Through the differentiation of its mesenchymatous wall, the uterus is first brought into relation with the round ligament. At 50 mm. the mesenchyma begins to differentiate a connective layer tissue. At 80 mm. the mucosa and muscularis may be distinguished. The first circular muscle fibers appear in 1 So mm. embryos, the other muscle layers develop later. The epithelium of the uterine tubes and the tubular portion of the uterus (fundus) remains simple with cylindrical or cuboidal cells. The tubular fundus glands of the uterus may not appear until near puberty. At 38 mm. the epithelium of the cervix and vagina becomes stratified. The vagina is at first without a lumen. From the third to the sixth months of fetal life dorsal and ventral outgrowths of the epithelium form the fornices of the vagina. The vaginal lumen appears in embryos of 150 to 200 mm., arising from the degeneration of the central epithelial cells. The fornices hollow out and form the boundary line between the cervix uteri and the vagina. The epithelial cells of the former become stratified and cylindrical, those of the vagina are of the stratified squamous type. The paired cranial portions of the Muellerian ducts become the uterine tubes. The epithelial anlages of the Muellerian ducts form the epithelial layers of the uterine tubes uterus and vagina. The Hymen. — At the point where the utero-vaginal anlage breaks through the wall of the urogenital sinus there is present the tubercle of Mueller, which marks the lower limits of the vagina. The tubercle is compressed into a disk lined internally by the vaginal epithelium, externally by the epithelium of the urogenital sinus. These layers with the mesenchyma between them constitute the hymen, which thus guards the opening into the vagina. A circular aperture in the hymen is for a time closed by a knob of epithelial cells, but later when the hymen becomes funnel-shaped the opening is compressed laterally to form a sagittal slit, the ostium vagina. The Growth of the Uterus. — The uterus grows but slowly until near puberty, being about the same length (27 mm.) at birth as in a girl of nine years. Just before and after puberty growth is more rapid, a length of 72 mm. being attained at 18 years. This is nearly the maximal length of the virginal uterus. Anomalies. — Owing to the complicated processes leading to their formation, many cases of abnormal uterus and vagina occur. A complete classification of these cases is given by Felix (Keibel and Mall, vol. 2, p. 930). The more common anomalies are (1) complete duplication of the uterus and vagina due to the failure of the Muellerian ducts to fuse; (2) uterus bicornis, due to the incomplete fusion of the ducts. Combined with these defects the lumen of the uterus and vagina may fail, partly or completely, to develop and the vaginal canal may not open to the exterior. (3) The body of the uterus may remain flat (uterus planifundis) or may fail to grow to normal size (uterus fetalis and infantilis). (4) Congenital absence of one or both uterine tubes, uterus, and vagina rarely occurs, but may be associated with hermaphroditism of the external genitalia. The Ligaments of the Internal Genitalia. — Female. — The loose mesenchyma of the genital cord gives rise laterally to the broad ligaments of the uterus in THE GENITAL GLANDS AND DUCTS — LIGAMENTS 229 females. In the genital fold, extending from the caudal end of the ovary to the genital cord, connective tissue and smooth muscle fibers developing form the proper ligament of the ovary. The uterus develops in the genital cord so the liga- ment of the ovary extends to the posterior surface of the uterine wall. In the male the homologue of the proper ligament of the ovary is the ligament of the testis. In both sexes the inguinal fold extends from the urogenital fold to the crista inguinalis, located on the inside of the ventral abdominal wall, a point which marks the future entrance of the inguinal canal. The inguinal fold thus forms a bridge between the urogenital fold (in the middle portion of which the uterus develops in the female) and the abdominal wall at the entrance of the inguinal Diaphragmatic by. of mesonephros ' Afetanephros Ureter Chorda, . qubernaculb GLcws of final I us Suprarenal gland Muellerian duct, in mesonephric fold Genital gland. f\ectum Genital cord. Genital SWeltinQ Fig. 225. — Ventral dissection of a human embryo of 23 mm. showing the urogenital organs. The right suprarenal gland has been removed to show the metanephros. canal (Fig. 225). In the inguinal crest is differentiated the conical anlage of the chorda gubemaculi, which later becomes a fibrous cord. The abdominal muscles develop around it and the external oblique muscle leaves a foramen, through which it connects with a second cord termed in the male the lig. scroti, in the female the lig. labiale (Fig. 226). The chorda gubemaculi and the lig. labiale together constitute the round ligament of the uterus, as they form a continuous cord extending from the urogenital fold to the base of the genital tubercle. With the development of the uterus in the urogenital fold, the round ligament becomes attached to its ventral surface. Male. — The ligamcntum testis, like the lig. ovarii, develops in the genital fold and extends from the caudal end of the testis to the mesonephric fold at a 230 UROGENITAL SYSTEM point opposite the attachment of the inguinal fold. The inguina. fold, as we have seen, is continuous with the inguinal crest and the chorda gubernaculi. A cord develops in the mesonephric fold and connects the ligamentum testis with the chorda gubernaculi, for in the male the uterus does not intervene between these two. The chorda gubernaculi is continued to the integument of the scrotum by way of the ligamentum scroti. Thus there is formed a continuous cord, the gubernacnlnm testis, extending from the caudal end of the testis through the in- guinal canal to the scrotal integument. The gubernaculum is composed of the ligamentum testis, of a mesonephric cord, of the chorda gubernaculi, and of the lig. Suprarenal gland Diaphragmatic ligament Ureter Ovary Llq. ovarii Bound liq, of Uterus Phall — fvletanephros Pelvis of metanephros Uterine tube - Rectum Utero-vaqinaL anlaae J 3ladder — Genital swelling Glans clitoris Fig. 226. — Ventral dissection of a female human embryo of 34 mm. The urogenital organs are dissected out and the left suprarenal gland has been removed. scroti, and is the homologue of the ovarian ligament plus the round ligament of the uterus. The Descent of the Testis and Ovary. — The original position of the testis and ovary is changed during the later stages of development. At first they are elongate structures, extending in the abdominal cavity from the diaphragm cau- dally towards the pelvis (Fig. 213). As development proceeds, their caudal ends enlarge and their cranial portions atrophy so that there is a progressive movement of the glands caudad. When the process of growth and degeneration is completed the caudal ends of the testis lie at the boundary line between the abdomen and THE GENITAL GLANDS — DESCENT OF TESTIS 231 pelvis while the ovaries are located in the pelvis itself, a position which they re- tain. Owing to the rotation of the ovary about its middle point as an axis it takes up a transverse position. It also rotates nearly 1800 about the Muellerian duct as an axis and thus comes to lie caudal to the uterine tube. The testis normally leaves the abdominal cavity, descending into the scro- tum. As described above, there is early developed between the testis and the integument of, the scrotum a fibrous cord, the gubemaculum testis. Owing to changes in the position of the ventral abdominal wall and umbilical arteries, changes connected with the return of the intestinal coils into the ccelom, there are formed in each side of the abdominal wall sac-like pockets, the anlages of the vaginal sacs. Close to each saccus vaginalis lies the caudal end of a testis, while extending into the scrotum beneath the peritoneum is the gubemaculum testis. The saccus vaginalis later invaginates into the scrotum over the pubic bone, carry- ing with it also representatives of the muscular layers of the abdominal wall. Whether due to the active shortening or to the unequal growth of the guber- naculum testis, the descent of the testis into the vaginal sac begins during the seventh month of fetal life and by the eighth month or at least before birth the testis is usually located in the scro- tum (Fig. 227). It must be remem- bered that the testis and gubemaculum are covered by the peritoneum before the descent begins, consequently the testis follows the gubemaculum along the inguinal canal dorsal to the peritoneum and, when it reaches the scrotum, is invaginated into the saccus vaginalis. The gubemaculum is said to degenerate during the descent of the testis or immediately after. Abnormally, the testis may remain in the abdomen, a condition known as cryptorchism (concealed testis) and associated with sterility in~man. In some mammals (bat and elephant) it is the normal condition. Shortly after birth the inguinal canal connecting the saccus vaginalis with the abdominal cavity becomes solid and its epithelium is resorbed. The now isolated vaginal sac becomes the tunica vaginalis of the testis. Its visceral layer is closely applied to the testis and its parietal layer forms the lining of the scrotal sac. The ductus deferens and spermatic vessels are of course carried down into the scrotum with the testis and epididymis. They are surrounded by connective Fig. 227. — Descent of the testis (Cunning- ham), ac, abdominal cavity; pv, processus vaginalis; /, testis; 5, scrotum; tv, tunica vagin- alis; .v, rudiment of processus vaginalis. 232 UROGENITAL SYSTEM tissue and, with the spermatic vessels, constitute the spermatic cord. Owing to the descent of the testis, the ductus deferens is looped over the ureter in the abdomen (Fig. 23 1 C) . In some cases the inguinal canals remain open so that the testis may slip back into the abdominal cavity. Such conditions may lead to inguinal hernia of the intestine. Open inguinal canals occur normally in the rabbit. THE EXTERNAL GENITALIA Indifferent Stage. — In both sexes there develops early in the midline of the ventral body wall, between the tail and umbilical cord, the cloacal tubercle (Fig. 228). Upon this appears a knob-like structure, the phallus, and the two together Fig. 228. — Four stages in the development of the external genitalia in embryos of 24 to 34 mm. Indifferent stage: i, phallus; 2, glans; 3, primitive urogenital opening; 4, genital tubercle or swelling; 5, anus; 6, coccyx (Tourneux in Heisler's Embryology). constitute the genital eminence. Cranially about the phallus the cloacal tu- bercle forms a crescent-shaped genital tubercle, which later gives rise to the right and left genital swellings. The phallus grows rapidly and into it extends the phallic portion of the urogenital sinus. At the end of the phallus the epithelium of the sinus forms a solid urethral plate (Fig. 212). Along the anal surface of the phallus in the midline, the wall of the urogenital sinus breaks through to the ex- terior and forms the slit-like primitive urogenital aperture. In embryos of 21 to 28 mm., at the end of the phallus, the glans is marked off from the base by a cir- cular groove, the coronary sulcus (Figs. 225 and 228 B). THE EXTERNAL GENITALIA 233 Female. — A deep groove appears about the base of the phallus separating it from the genital tubercle, which becomes a circular swelling (Fig. 229). From the swelling differentiates (1) cranially, the mons veneris; (2) laterally, the right and left labia major a; (3) caudally, the posterior commissure of the labia majora. The glans of the phallus forms the glans clitoris of the female. On the anal sur- face of the phallus beginning at the coronary sulcus the primitive urogenital open- ing closes distally, forming the urethral groove. Proximally it remains open, as FlG. 229. — Four stages in the development of the female external genitalia (Tourncux in Heisler). 1, clitoris; 2, glans clitoris; 3, urogenital aperture on each side of which are the labia minora (7); 4, labia majora; 5, anus; 5, coccygeal eminence; 7, labia minora. the definitive urogenital opening near the base of the phallus. The lips of this groove and opening enlarge and become the labia minora. The cranial surface of the phallus forms a fold, the prcpucium, which, however, is not the homologue of the male fore-skin. This in the female is represented by a ring-like rudiment at the base of the glans clitoris. Male. — The phallus grows rapidly at its base so that the glans and primitive urogenital opening are carried some distance from the anus (Fig. 230). A cylin- 234 UROGENITAL SYSTEM Br* B drical fold of the epithelium, incomplete on the anal side, grows down into the end of the glans, which becomes the glans penis. By the disappearance of the central cells of the epithelial downgrowth an outer cylindrical mantle, the pre- puciam or fore-skin, is formed about the spherical glans. Where the epithelial downgrowth is incomplete the glans and foreskin remain connected, the persist- ing connection being the frenulum prepucii. The urogenital sinus, as we have seen, extends out into the phallus and in the glans becomes the solid urethral plate. With the great elongation of the male phallus, the open portion of the urogenital sinus also is lengthened and forms the greater part of the penile urethra. In embryos of 70 mm. the groove- like primitive urogenital opening, located in the male near the glans and distant from the anus, closes and thus is formed a further por- tion of the urethra. The failure of this opening to close gives rise to an anomaly known as hypospadias. The lips of the urogenital opening, it will be remembered, correspond to the labia minora or nymphce of the female. Finally at 100 mm. the solid urethral plate of the glans splits, forms a groove to the tip of the glans and this groov«4n turn is closed, continuing the urethra to the definitive opening at the tip of the glans. Owing to the rapid growth in length of the penis, there is formed between its base and the anus an unpaired area, termed by Felix the scrotal area as'* it is the anlage of the scrotum. At 60 mm. this forms a median scrotal swelling continuous with the paired genital swellings. When the scrotal sac develops in the scrotal area, the dense tissue in the median line is compressed and forms the septum scroti. The attachment of this septum forms an external median depression. The testes I . v c Fig. 230.— Four stages in the development of the male external genitals (Tourneux in Heisler). 1, penis; 2, glans; 3, urogenital groove; 4, urogenital swellings corresponding to labia majora of female; 5, anus; 6, coccygeal eminence; 7, scrotal area with perineo-scrotal raphe. THE EXTERNAL GENITALIA 235 descend into the vaginal sacs of the scrotum through the paired genital swellings, as described on p. 231, but the scrotum itself is an unpaired structure derived from the scrotal area. After the descent of the testes the genital swellings dis- appear. Comparing the male and female external genitalia, it is plain that the glans penis and glans clitoris are homologous. The labia minora correspond to the phallic folds which close about the primitive urogenital opening and the anal surface of the penis. The greater part of the stem of the male phallus does not develop in the female. On the other hand, the genital swellings enlarge and be- come the labia majora of the female, while in the male they are only temporary structures. The scrotum does not develop in the female, being represented only by the posterior commissure of the labia majora. The Prostate Gland. — This is developed in both sexes as several outgrowths above and below the entrance of the male ducts into the urogenital sinus. The tu- bules arise in five distinct groups and, according to Lowsley (Amer. Jour. Anat., vol. 13, pp. 299-350), number from 53 to 74, the average being 63. In the male the surrounding mesenchyme differentiates both white fibrous connective tissue and smooth muscle fibers into which the anlages of the prostate grow. In the female the tubules remain isolated. The prostatic anlages appear in male em- bryos of 50 mm. (12th week), chiefly as dorsal and lateral outgrowths. Two- thirds of the tubules are caudal to the openings of the male ducts. In the female the gland is rudimentary, the maximal number of outgrowths being three. The bulbo-urethral glands (of Cowper) arise in male embryos of 48 mm. as solid paired epithelial buds from the entoderm of the pelvic urogenital sinus. The glands grow into the mesenchyme which forms the corpus cavernosum urethra, about which they enlarge. The glands branch and, at 120 mm., the epithelium becomes glandular. The vestibular glands (of Bartholin) are the homologues in the female of the bulbo-urethral glands. They appear in embryos of 36 mm., grow until after puberty, and degenerate after the climacterium. Male and Female Genitalia Homologized. — From the standpoint of embry- ology the genital glands are homologous structures. In the indifferent stage (Fig. 231 A), there are in both male and female a pair of genital glands, a pair of mesonephric or male ducts, a pair of Muellerian or female ducts, and a genital tubercle bearing the phallus. The genital ducts open into the urogenital sinus, a part of which forms the bladder. Male (Fig. 231 C). — In the male the Muellerian ducts degenerate except for small portions cranially and caudally, which persist respectively as the appendix 236 UROGENITAL SYSTEM Fig. 231. Fig. 231. — Diagrams to show the devel- opment of male and female generating organs from a common type (Allen Thomson) . A. Diagram of the primitive urogeni- tal organs in the embryo previous to sexual distinction: 3, ureter; 4, urinary bladder; 5, urachus; ot, the genital ridge from which either the ovary or testicle is formed; w, left mesonephros; w, w, right and left mesonephric ducts; m, m, right and left Miillerian ducts uniting together and running with the mesonephric ducts in g.c, the geni- tal cord; tig, sinus urogenitalis; i, lower part of the intestine; cl, cloaca; cp, phallus which becomes clitoris or penis; Is, fold of integument from which the labia majora are formed. B. Diagram of the female type of sex- ual organs: 0, the left ovary; po, epoo- phoron; w, scattered remains of mesoneph- ric tubules near it (paroophoron); d G, remains of the left mesonephric duct (canal of Gartner) represented by dotted lines; that of the right side is marked w; f, the abdominal opening of the left uterine tube; u, uterus; the uterine tube of the right side is marked m; g, round liga- ment, corresponding to gubernaculum; i, lower part of the intestine; va, vagina; h, situation of the hymen; C, gland of Bar- tholin (Cowper's gland), and immediately above it the urethra; cc, corpus cavernosum clitoridis; sc, vascular bulb or corpus spongi- osum; n, nympha; I, labium; v, vulva. C. Diagram of the male type of sexual organs: t, testicle in the place of its original formation; e, caput epididymis; vd, ductus deferens; W, scattered remains of the meso- nephros, constituting the organ of Giralde's, or the paradidymis; vh, vas aberrans; m, Miillerian duct, the upper part of which remains as the appendix testis, the lower part, represented by a dotted line descend- ing to the prostatic vesicle, constitutes the oc- casionally existing cornu and tube of the va- gina masculina; g, the gubernaculum; vs, the vesicula seminalis; pr, the prostate gland; c, Bulbo-urethral gland of one side; cp, cor- pora cavernosa penis cut short; sp, corpus cavernosum urethrae; s, scrotum; /', together with the dotted lines above, indicates the di- rection in which testicles and epididymis de- scend from the abdomen into the scrotum. THE EXTERNAL GENITALIA 237 testis and the vagina masculina. The mesonephric duct is functional, its deriva- tives being the ductus epididymis, the ductus deferens, the ampulla and seminal vesicle. The collecting tubules of the mesonephros form the ductuli efferentia of the epididymis and the vestigial paradidymis. The phallus enlarges and be- comes the penis, into which extends a portion of the urogenital sinus as the ure- thra. The genital tubercle disappears and the scrotum is developed as a new structure, into the vaginal sacs of which the testes descend. Female (Fig. 231 B). — The genital gland becomes the ovary. The meso- nephric duct degenerates except for the vestigial Gartner's canal. Two groups of mesonephric tubules persist, a cranial group united with the rete ovarii constitut- ing the rudimentary cpobphoron, the homologue of the epididymis, a caudal group forming the paroophoron, comparable to the paradidymis. The Muellerian ducts become the functional female ducts. Their lower ends fuse and with their middle portions form the vagina and uterus. Their upper portions persist as the paired uterine tubes. The phallus remains small and becomes the clitoris, the open lips of the urethral groove form the labia minora, and the genital tubercle constitutes the mons veneris and the paired labia majora of the vulva. The ovar- ies descend only into the true pelvis but the lig. ovarii and the round ligament of the uterus are the homologues of the gubernaculum testis. Hermaphroditism. — True hermaphroditism consists in the development and persistence of both testes and ovaries in the same individual. It is of rare oc- currence in man, is not uncommon in the lower vertebrates, and is the normal condition in some invertebrates (earth worms, snails, etc.). In cases of human hermaphroditism of this type the secondary sexual characters are usually inter- mediate between the male and female, tending now one way now the other. The external genitalia show a small penis with hypospadias, cryptorchism, or small vaginal opening. False hermaphroditism is characterized by the presence of genital glands of one sex in an individual which exhibits more or less marked secondary characters and external genitalia of the opposite sex. In masculine hermaphroditism an in- dividual possesses testicles, but the external genitals and secondary sexual char- acters are like those of the female. In feminine hermaphroditism ovaries are present, but the other sexual characters are male. The cause of hermaphroditism is unknown. 238 UROGENITAL SYSTEM THE UTERUS DURING MENSTRUATION AND PREGNANCY: PLACENTA AND DECIDUAL MEMBRANES Two sets of important changes take place normally in the wall of the uterus. One of these is periodic and is the cause of menstruation (monthly flow) . These periodic changes may also be regarded as preparatory to the second set of changes which take place if pregnancy occurs and give rise to the decidual membranes and placenta. Menstruation. — The periodic changes which accompany the phenomenon of menstruation form a cycle which occupies 28 days. This period is divided into (1) a phase of uterine congestion lasting six or seven days; (2) a phase of hem- orrhage and epithelial desquamation, duration three to five days; (3) a phase of regeneration of the uterine mucosa lasting four to six days; (4) finally an interval of rest or slight regeneration, varying from twelve to sixteen days duration. During the first phase, the uterine mucosa is thickened to two or three times its normal condition, both because of vascular congestion and on account of the actual increase in amount of reticular tissue. The uterine glands become longer and their deeper portions especially are dilated and more convoluted because they are filled with secretion. From the enlarged veins and capillaries blood escapes into the reticular tissue beneath the epithelium and forms haematomata. At the end of this phase the uterine mucosa shows a deep spongy layer and a superficial compact layer, these corresponding to similar layers in the decidual membranes of pregnancy. During the second phase, which is menstruation proper, blood escapes into the uterine cavity between the epithelial cells of the mucosa and there is an active discharge of secretion from the uterine glands. The surface epithelium and a portion of the underlying tissue may or may not be desquamated. In some normal cases the surface epithelium and most of the compact layer may be ex- pelled, aided by painful contractions of the uterus. In the third stage, the mucosa has become thin with straight narrow glands between which are fusiform, closely packed stroma cells. Any surface epithelium which has been desquamated is regenerated from the epithelium of the glands and gradually the mucosa returns to a resting condition during which, however, there is a slow process of cell proliferation. The premenstrual changes of the first phase are regarded as the most impor- tant part of the whole process, the uterine mucosa being prepared for the reception of a fertilized ovum and for the development of the decidual membranes. Men- THE UTERUS DURING MENSTRUATION AND PREGNANCY 239 struation proper, as seen in the second phase, is the result of an over-ripe condi- tion of the mucosa and has been regarded as the abortion of an unfertilized ovum. The Decidual Membranes: Placenta The Implantation of the Ovum. — Our knowledge concerning the implantation of the ovum is fragmentary, but certain facts have been deduced from observations on mammals (hedge-hog and guinea-pig), and from the careful study of early human embryos by Teacher, Bryce, Herzog and Peters. The embryo described by Teacher and Bryce, while it is the youngest yet observed, is perhaps not normal. After ovulation, the ripe ovum is set free within the abdominal cavity, from whence by the beating cilia on the fimbrice of the uterine tube it is carried into the ampulla of the latter. There it may be fertilized and is swept into the uterus by the cilia of the tubar epithelium. During this period of migration, which is esti- mated as occupying from five to eight days, the ovum loses its surrounding fol- licle cells and its membrane and begins its development. Thus when it reaches the uterus, and is ready for implantation, it is an embryo with trophectoderm developed but still not more than 0.2 mm. in diameter (Graf Spee). If ovulation precedes menstruation proper by ten or twelve days as Ancel and Villemin maintain, then the embryo would reach the uterus during the premenstrual period. The con- gestion and loosening of the uterine tissue at this time would favor the implantation of the embryo and the glandular secretion would afford nutriment for its growth until implantation occurs. The first phase of menstruation according to this view, that of Grosser, prepares the uterine mucosa for the reception of the embryo. If pregnancy supervenes, it soon inhibits any further premenstrual changes so that menstruation does not occur. The embryo penetrates the uterine mucosa as would a parasite, the trophec- toderm supposedly producing a ferment which digests away the maternal tissues until the embryo is entirely embedded (Fig. 232). During implantation, the trophectoderm also probably absorbs nutriment from the uterine mucosa for the use of the embryo. The process of implantation is supposed to occupy one day. At the point where the embryo enters the mucosa a fibrin clot soon appears and eventually the opening is completely closed. The Decidual Membranes (Figs. 233 and 234). — With the increase in size of the embryo and chorionic vesicle, the superficial layers of the maternal mucosa bulge into the cavity of the uterus and form the decidua capsularis fold term , de- cidua reflexa) . The deep layer of the mucosa on the side of the embryo away from the uterine cavity forms the anlage of the future maternal placenta and is the decidua basalis (d. serotina). The mucosa lining the rest of the uterus is differ- entiated into the decidua vera ( parietal is of Bonnet). Differentiation of the Trophectoderm. — The chorion is at first composed of 240 UROGENITAL SYSTEM an inner mesodermal layer and an outer epithelial layer, the trophectoderm (Fig. 70) . From the trophectoderm there is developed an outer syncytial layer which we call the trophoderm (Fig. 23 2) . This invades and destroys the maternal tissues. In it large vacuoles are formed either directly by the syncytial tissue (Teacher and Bryce) or by the blood escaping from the ruptured vessels under pressure (Peters), and thus blood lacuna are produced. The trophoderm thickens at intervals and forms on the surface of the chorion solid cords of cells, the primary villi (Fig. 232) . The chorionic mesoderm grows out into these cords, the cords branch Fig. 232. — Section through an embryo of i mm. embedded in the uterine mucosa (semidiagrammatic after Peters). Am., amniotic cavity; b.c, blood-clot; b.s., body-stalk; ect., embryonic ectoderm; era/., entoderm; mes., mesoderm; m.v., maternal vessels; tr., trophoderm; u.e., uterine epithelium; u.g., uterine glands; y.s., yolk-sac. profusely and become secondary, or true villi (Fig. 235). During the development of the villi, the blood lacunae in the trophoderm around the villi expand, run together, and produce intervillous blood spaces which surround the villi and bathe the epithelium with blood. The syncytial trophoderm, from being a spongy net- work, is now reduced to a continuous layer covering the outer surfaces of the villi and chorion. Branches of the umbilical vessels develop in the mesoderm of the chorion and villi. The mesodermal core of each villus and its branches is now covered by a two-layered epithelium, an inner ectodermal layer with distinctly THE UTERUS DURING MENSTRUATION AND PREGNANCY 241 outlined cubical cells, and an outer syncytial trophoderm layer (Fig. 235). The epithelium also forms solid columns of cells which anchor the ends of certain villi to the maternal tissue. Islands, or nodes of epithelial cells, are attached to the Posterior lip of os uteri Border cleft Bonier zone Decidua basalis Fades vesicalis uteri Apex of decidua Cervical canal Fig. 233.— Section through a gravid uterus of twelve to fourteen days (Kollmann's Atlas). villi or lie free in the decidua basalis and represent portions of the primitive troph- ectoderm. In the vessels of the chorionic villi the chorionic circulation of the embryo is established. The blood-vessels of the uterus open into the intervillous 16 242 UROGENITAL SYSTEM Fig. 234. — Diagrammatic section through a pregnant uterus at the seventh or eighth week (after Allen Thomson), c, c, openings of uterine tubes; c , cervix with mucous plug; dv, decidua vera or pari- etalis; dr, decidua capsularis; ds, decidua basalis (serotina); ch, chorion with villi; the villi extending into the decidua basalis are from the chorion frondosum; u, umbilical cord; al, allantois. r^-'-Jz — mcs Fig. 235. — Diagram illustrating the second phase in the development of the chorionic villi and placenta (after Peters), mes, mesoderm; core, core of villus about which is the trophectoderm layer; sy, syncytium of trophoderm; mep, endothelium of maternal capillary; vs, intervillous space. THE UTERUS DURING MENSTRUATION AND PREGNANCY 243 blood spaces and here the maternal blood circulates. The syncytial trophoderm covering the villi is bathed in the maternal blood. Its functions are three-fold: (1) like endothelium it prevents the coagulation of the maternal blood; (2) it allows of transfusion between the blood of fetus and mother; and (3) it assimi- lates substances from the maternal blood and transfers them to that of the embryo. Chorion Laeve and Frondosum. — The villi at first cover the entire surface of the chorion. As the embryo grows more and more out into the uterine cavity the decidua capsularis and that portion of the chorion attached to it are com- pressed, and the circulation in the intervillous spaces of these structures is cut off (Figs. 234 and 236). Thus, beginning at the pole of the decidua capsularis, the Fig. 236. — Human ova: A, of three weeks; B, of six weeks, showing formation of chorion laeve bj- degeneration of the chorionic villi (De Lee's Obstetrics). villi in this portion of the chorion degenerate during the fourth week and form the chorion lave. The villi on that part of the chorion which is attached to the decidua basalis continue their development and persisting form the chorion fron- dosum. This, with the decidua basalis of the uterus, constitutes the placenta. The embryo is attached first to the chorion frondosum by the body-stalk, later by the umbilical cord (Fig. 234). Through the umbilical vein and arteries in the latter the placental circulation of the embryo takes place. The Decidua Vera. — During the first phase of menstruation the uterine mucosa begins to differentiate into a broad superficial compact layer and into a narrower deep spongy layer in which are found the dilated ends of the uterine glands. After pregnancy these two layers are still further differentiated in the 244 UROGENITAL SYSTEM wall of the decidua vera and d. basalis. The compact layer is much thicker than the spongy layer and in it are found numerous stroma cells, enlarged blood-vessels and decidual cells (Fig. 237). The decidual cells are derived from the stroma cells of the mucosa. They are large, being 50 /x in diameter, with clear cytoplasm and vesicular nuclei. Their function is in doubt. Glycogen has been found in them but during the later months of pregnancy many of them degenerate. In the spongy layer of the mucosa occur the enlarged and tortuous uterine glands of pregnancy (Fig. 237). During the first two months of pregnancy the long axes of the glands are perpendicular to the surface of the mucosa. Later, as Amnion ^^-^t^*^:^^--1' > • Chorion aS^g^saf. ^*s$S*r ^•'T5^7>v~ Cavernous layer s*=r#i Muscularis S^&s^ Fig. 237. — Vertical section through the wall of uterus about seven months pregnant with the membranes in situ (Schaper in Lewis-Stohr). X 30. the decidua is stretched and compressed owing to the growth of the fetus, the glands are broadened and shortened and the cavities of the glands become elongated clefts parallel to each other and to the surface of the decidua. The gland cells become stretched and flattened until they resemble endothelial cells. At birth, or in case of late abortion, the plane of separation is in the spongy layer. Only the deep portions of the glands remain attached to the uterine wall and, by the division of their cells, regenerate the epithelium of the uterus. The Decidua Capsularis. — The capsularis, as we have seen, becomes com- pressed as the embryo grows (Fig. 234). To it is attached the chorion Iceve, the THE UTERIS DlklNC MENSTRUATION AND PREGNANCY 245 villi of which degenerate. With the increased size of the fetus, the capsularis comes into contact with the decidua vera and fuses with it. Eventually it largely degenerates, completely so opposite the internal os uteri, where the chorionic villi are obliterated also. During pregnancy, the lumen of the cervix is closed by a plug formed by the secretion of the glands opening into the cervix uteri. The Placenta. — The placenta is composed of the decidua basalis, constituting the maternal placenta, and of the chorion frondosum, the placenta jxtalis. The area throughout which the villi of the chorion frondosum remain attached to the decidua basalis is somewhat circular in form, so that at term the placenta is disc-shaped, about seven inches in diameter and one inch thick (Fig. 238). Near *S ^ ■• it* 0* *m J I'll.. 238. — Mature placenta: a, entire organ, showing fetal surface with membranes attached to the periphery; b, a portion of attached surface (Heisler). the middle of its fetal surface is attached the umbilical cord, and this surface is formed by the amnion, the mesoderm of which is closely applied to and fused with that of the chorion frondosum (Fig. 239). Chorion Frondosum. — The villi of this portion of the chorion form profusely branched tree-like structures which lie in the intervillous spaces (Fig. 240). The ends of some of the villi are attached to the wall of the decidua basalis and are known as the anchoring villi. In the connective tissue core of each villus are usually two arteries and two veins, branches of the umbilical vessels, cells like lymphocytes and special cells of Hofbauer, the significance of which is not known. Lymphatics are also present. The epithelium of the villi, as we have seen, is at 246 UROGENITAL SYSTEM first composed of a layer of trophectoderm (of Langhans) with the outlines of its cuboidal cells sharply defined (Fig. 241 A). This layer forms and is covered by a syncytium, the trophoderm. In the later months of pregnancy as the villi Wimm ^^^^^w^^-^^ Fig. 239. — Section through a normal placenta of seven months in situ (Minot): Am, amnion; Cho, chorion; Vi, root of villus; vi, sections of small villi ramifying in the intervillous blood spaces; D, deep spongy layer showing remnants of large flattened glands; Ve, uterine vessel opening into intervillous spaces; Mc, muscular wall of uterus. THE UTERUS DURING MENSTRUATION AND PREGNANCY 247 grow, the trophectoderm is used up in forming the syncytium, so that at term the trophoderm is the only continuous epithelial layer of the villi (Fig. 241 B). About the margin of the placenta the trophectoderm persists as the closing ring, which is continuous with the epithelium of the chorion lasve. Syncytial giant cells found in the decidua basalis are said to be derived from the trophoderm of the villi, Muscular is Uterine artery Uterine vein Uterine. artery in septum Decidua basalis Uterine artery in decidual septum ^Intervillous space Syncytium Fig. 240. — Scheme of placental circulation (Kollmann's Handatlas). Arrows indicate supply and exhaust of blood in the intervillous spaces. also a portion of the canalized fibrin found in the decidua basalis of the placenta near term. Decidua Basalis. — This, the maternal placenta, like the decidua vera is dif- ferentiated into a compact layer or basal plate which forms the floor of the inter- villous spaces, and into a deep spongy layer (Figs. 239 and 240). The first is the remains of the compact layer of the uterine mucosa formed during the premenstrual phase and partially destroyed by the implantation of the ovum. The second is the modified spongy layer of the premenstrual period and. though thinner, shows the same differentiation as does this same layer in the decidua vera. The glandular spaces are less numerous in the spongy layer of the decidua basalis and between 248 UROGENITAL SYSTEM the spaces occur the syncytial giant cells mentioned above. It is in the plane of this layer that the separation of the placenta takes place at birth. The basal plate, or compact layer of the decidua basalis, is composed of a con- Syncytium Cuboidal cells of the basal layer Connective tissue Blood-vessel containing nucleated red corpuscles Epithelium ■ Epithelial nucleus- Capillaries ■*—- Syncytial knot Small artery — Oblique section of the epithelium Syncytial knot Epithelium Small vein Capillary Syncytial knot — Fh,. 241. — Transverse sections of chorionic villi; A , at the fourth week; B, C, at the end of pregnancy (Schaper in Stohr-Lewis). nective tissue stroma containing decidual cells, canalized fibrin and persisting portions of the epithelium of the villi. The canalized fibrin is believed to be formed both from the syncytial trophoderm of the villi and from the modified I ferine muscle Remaiiu bilical visit. le Fetal villi of chorion Maternal Hood sinus. Decidua it septum Peripheral vein Fused decidua vera and Fig. 242. — Semidiagrammatic section of uterus, showing relations of fetal and maternal placenta 1 Ahlfeld). Decidua serolina, decidua basalis; (/. reflexa, old terminology for i, Cartilage showing calcification and resorption; eK, endochondral bone; .1/, marrow cavity; pK. periosteal bone. BONE 2Q7 Endochondral Bone Formation. — The cartilage cells enlarge, become ar- ranged in characteristic rows and resorb the cartilage matrix (Fig. 287). The perichondrium becomes the periosteum. From its inner or osteogenic layer, which is densely cellular, ingrowths invade the cartilage as it is resorbed and fill the primary cavities. The invading osteogenic tissue gives rise to osteoblasts and bone marrow. By the osteoblasts bone is differentiated directly upon persisting portions of the cartilage. As new bone is developed peripherally, it is resorbed centrally to form large marrow spaces. Eventually, all of the cartilage matrix is destroyed. The fate C D Cartildiyt Cartilage Bone paraphysn one marrow \piphys — Paraphys of the cartilage cells is un- known. Perichondral Ossification. — Compact bone is developed after birth by the osteogenic layer of the periosteum and thus are produced the periosteal la- mella. In the ribs this is said to be the only method of ossi- fication. The bone lamellae de- posited about a blood-vessel are concentrically arranged and form the concentric lamella of a Haversian system. The Haver- sian canal of adult bone is merely the space occupied by a blood-vessel. Growth of Cartilage Bones. — In cartilage bones there is no interstitial growth as in cartilage. Most of the cartilage bones have more than one center of ossification and growth is due to the expansion of the inten-ening cartil- age. Flat bones grow at the periphery, ring-like bones, such as the vertebrae, have three primary centers of ossification, between which the cartilage continues to grow (Fig. 288 A). In the case of the numerous long bones of the skeleton, the primi- tive ossification center forms the shaft or diaphysis (Fig. 288 C-F). The cartilage at either end of the diaphysis grows rapidly and thus the bone increases in length. Eventually, osteogenic tissue invades these cartilages and new ossification centers, the epiphyses, are formed, one at either end. When the growth of the bone in -Epiphysis FlG. 288. — Diagrams to show the method of growth of .1, a vertebra; B, of sacrum; C-F, of a long bone (the tibia). 298 HISTOGENESIS length is completed, the epiphyses, by the ossification of the intervening cartilage, are united to the diaphysis. The shaft of the long bones grows in diameter by the peripheral deposition of bone lamellae and the central resorption of the bone. In the larger long bones spongy, or cancellated bone tissue persists at the ends, but in the middle portion a large medullary, or marrow cavity, is developed. This is filled chiefly with fat cells and constitutes the yellow bone marrow. Regeneration of Bone. — If bone is injured or fractured, new bone is developed by osteoblasts derived either from the periosteum or from the bone marrow. The repair of a fracture is usually preceded by the formation of cartilage which unites the ends of the bones and is later changed to bone. In adults, the periosteum is especially important in the regener- ation of bone tissue. The special development of the various bones of the skeleton is beyond the scope of this book. The student is referred to the various text-books of anatomy, to Kollmann's Handatlas of Embryology, vol. i, and to Bardeen's chapter in Keibel and Mall (vol. i, p. 316 ff). Joints. — In joints of the synarthrosis type in which little movement is allowed the mesenchyma between the ends of the bones differentiates into connective tissue or cartilage. This persists in the adult. In joints of the diarthrosis type the bones are freely movable. The mesen- chyma between the bones develops into an open connective tissue in which a cleft appears, the joint cavity. The cells lining this cavity flatten out and form a more or less continuous layer of epithelium, the synovial membrane. From the con- nective tissue surrounding the joint cavity are developed the various fibrous ligaments typical 0f the different joints. , THE HISTOGENESIS OF MUSCLE The muscular system is composed of muscle fibers which form a tissue in which contractility has become the predominating function. The fibers are of three types : (1) smooth muscle cells found principally in the walls of the viscera and blood-vessels; (2) striated cardiac muscle, forming the myocardium of the heart; (3) striated voluntary muscle, chiefly attached to the elements of the skeleton and producing voluntary movements. All three types are derived from the meso- derm. The only exceptions are the smooth muscle of the iris, and the smooth muscle of the sweat glands, which are derived from the ectoderm. Smooth Muscle in general may be said to arise from the mesenchyme, or from embryonal connective tissue. Its development has been studied by McGill (Internat. Monatschr. f. Anat. u. Physiol., vol. 24, pp. 209-245, 1907) in the esophagus of pig embryos. The stellate cells of the mesenchyma enlarge, elongate THE HISTOGENESIS OF MUSCLE 299 and their cytoplasm becomes more abundant. The resulting spindle-shaped cells (Fig. 289 A) remain attached to each other by cytoplasmic bridges and develop in the superficial layer of their cytoplasm course myoglia fibers (Fig. 289 B) similar to the primitive fibrilke of connective tissue. The myoglia fibers may extend from cell to cell, thus connecting them. These fibers are the products of coalesced granules found within the cytoplasm of the myoblasts. In embryos of 30 mm. fine myofibrillae are differentiated in the cytoplasm of the myoblasts and give it a longitudinally striated appearance. The cytoplasmic processes of the muscle cells, the cytoplasmic bridges, later give rise to white connective tissue fibers which envelop the muscle fibers and bind them together. Smooth muscle increases in amount: (1) by the formation of new fibers from the mesenchyme of the embryo; (2) by the transformation into muscle fibers of interstitial cells; (3) by the multiplication of their nuclei by mitosis in the more advanced fetal stages. Striated Cardiac Muscle. — This is developed from the splanchnic mesoderm which forms both the epicardium and the myocardium. The cells of the myo- cardium at first form a syncytium in which myofibrillae develop from chondri- oconta or cytoplasmic granules. The myofibrillae are developed at the periphery of the syncytial strands of cytoplasm and extend long distances in the syncytium. They multiply rapidly in number and become differentiated each into alternating dark and light bands, due to a difference in density. The syncytial character of cardiac muscle persists in the adult and the nuclei remain central in position. The intercalated discs typical of adult cardiac muscle appear relatively late, just before birth in the guinea-pig, according to Jordan and Steele. Striated Voluntary Muscle. — All striated voluntary muscle is derived from the mesoderm, either from the myotomes of the segments (muscles of the trunk) or from the mesenchyma (muscles of the head). According to Bardeen (in Keibel and Mall, vol. 1), after the formation of the sclerotome (Fig. 282 A), which gives rise to skeletal tissue, the remaining portion of the primitive segment con- stitutes the myotome. All the cells of the myotome give rise to myoblasts. Wil- liams (Amer. Jour. Anat., vol. 88), working on the mesodermal segments of the chick, finds that only the dorsal and mesial cells are myoblasts. By multi- plication they form a mesial myotome, while the lateral cells of the original mesodermal segment persist as a dermatome and give rise only to the connective tissue of the dermis (Fig. 291). The dermatome lies lateral to the myotome and the two together constitute the dcrmo-myotomc, according to Williams (Fig. 45). " ;oo HISTOGENESIS A m. m. ^^^/^Z' vies. vsBJr S .^^^^ Fig. 289.— Two stages in the development of smooth muscle fibers; A , from the esophagus of a 13 mm. pig (McGill); 5, a longitudinal section of the esophagus of a 27 mm. pig (after McGill in Lewis-Stohr). b, b.m., basement membrane; e, epi, epithelium; f.c, myoglia fibrils; g.s., granules coa- lescing; h.s., homogeneous fibers; vies., mesenchyma; mm., m'uscularis mucosae; mu., muscle cell; »., nerve cells; s.c, circular smooth muscle cut across; s.l., longitudinal smooth muscle cut lengthwise. THE HISTOGENESIS OF MUSCLE 3OI As to the origin of the striated voluntary muscle fibers, there is also a differ- ence of opinion. It is generally believed that the myoblasts elongate and, by the repeated mitotic division of their nuclei, become multinucleated. Godlewski holds that several myoblasts unite to form a single muscle fiber. The nuclei lie at first centrally, surrounded by the granular sarcoplasm in which myofibrils differentiate peripherally. The myofibrils become striated like those of cardiac muscle. During development the muscle libers increase enormously in size, the nuclei migrate to the surface and the myofibrillar are arranged in bundles or muscle columns (sarcostyles) . This arrangement of the fibrillar may, however, be due to shrinkage in the preparation of the sections observed. According to Baldwin (Zeitschr. f. allg. Physiol., vol. 14, 191 2), the nucleus and perinu- clear sarcoplasm is separated from the rest of the muscle fiber by the sarcolemma. With Apathy, he would therefore regard the myofibrillar as a differentiated product of the muscle cells and to be homologized with connective tissue fibers. The extrusion of the muscle cell from the muscle fiber may be compared to the extrusion of cartilage cells from the precartilage matrix, as de- scribed by Mall (see p. 294). During the later stages in the development of striated voluntary muscle, there is, according to many observers, an active degeneration of the muscle fibers. While smooth muscle fibers form a syncytium and the enveloping connective tissue is developed directly from the muscle cells, in the case of striated voluntary muscle each fiber is a multinucleated entity which is bound together with others by connective tissue of independent origin. Morphogenesis of the Muscles. — The development of the individual muscles of the human body has been described in detail by W. Lewis (in Keibel and Mall, vol. 1, p. 473) and to this work the student is referred. We may state briefly here the origin of the muscles of the trunk, limbs and head. The muscles of the trunk. — The deep muscles are derived from the myo- tomes which extend ventrally and fuse with one another (Fig. 290). This fusion is well advanced superficially in embryos of 9 to 10 mm. The deep portions of the myotomes do not fuse but give rise to the intervertebral muscles, which thus retain their primitive segmental arrangement. The various long muscles of the back arise by longitudinal and tangential splitting. The thoraco-abdominal muscles arise as ventral extensions of the thoracic myotomes into the somatopleure, growing in along with the ribs. The musculature of the extremities. — It has been generally believed that the muscles of the extremities were developed from buds of the myotomes which grew into the anlages of the limbs. According to Lewis, "there are no observations of distinct myotome buds extending into the limbs." A diffuse migration of 302 HISTOGENESIS cells from the ventral portion of the myotomes has been recorded by various observers, recently by Ingalls. These cells soon lose their epithelial character and blend with the undifferentiated mesenchyma of the limb buds (Fig. 291). From this diffuse tissue then the limb muscles are differentiated, the proximal muscles being the first to appear. Fig. 290. — Reconstruction of a 9 mm. embryo to show the myotomes (Bardeen and Lewis). The musculature of the head is derived from the pre-otic mesenchymal tissue which condenses to form premuscle masses. No myotomes are developed in this region. The pharyngeal muscles probably arise from the mesenchymal tissue of the third branchial arch. THE HISTOGENESIS OF THE ECTODERMAL DERTVATTVES 303 The intrinsic muscles of the larynx arc differentiated from the mesenchyme at the ventral ends of the third and fourth branchial arches. The muscles of the tongue arc supplied by the n. hypoglossus and therefore Spinal ganglion Permatome Ventral root Myotome Spinal nerve Arm bud Proliferating cells of myotome Mesonephric duet M esonephrie tubule and glomerulus Civlom Somatic mesodt rm Fig. 291. — Transverse section of a 10.3 mm. monkey embryo showing the myotome and the mesenchyma of the arm bud (Kollmann's Handatlas). A, aorta; *, sclerotome. it has been assumed that they are derived from myotomes of the occipital region. According to W. Lewis, ''there is no evidence whatever for this statement, and we are inclined to believe from our studies that the tongue musculature is derived from the mesoderm of the floor of the mouth." The Histogenesis of the Ectodermal Derivatives Besides forming the enamel of the teeth and salivary glands (see p. 161), the ectoderm gives rise: (1) to the epidermis and its derivatives (subcutaneous glands, nails, hair, and lens and cornea of the eye) ; ( 2) to the nervous system and sensory epithelia ; (3) to parts of certain glands producing internal secretions such as the pituitary body, adrenal glands, and chromaffin bodies. We shall describe 3°4 HISTOGENESIS here the histogenesis of the epidermis and the development of its derivatives and the histogenesis of the nervous tissues, reserving for final chapters the develop- ment of the nervous organs and the glands formed in part from them. THE EPIDERMIS The single-layered ectoderm of the early embryo by the division of its cells becomes differentiated into a two-layered epidermis composed of an inner layer of cuboidal or columnar cells, the stratum germinativum, and an outer layer of flattened cells, the epitrichium or periderm (Fig. 292 A). Epitrichhim Stratum germinativum Con am Ep'itrichi Intermed- iate I aye T ""^T Stratum . Xa^Kflurrn'e Mill Fig. 292. — Sections of the integument from a 65 mm. embryo. A, section through the integument of the neck showing a two-layered epidermis and the beginning of a third intermediate layer; B, section from the integument of the chin in which three layers are well developed in the epidermis. X 440. The stratum germinativum is the reproducing layer of the epidermis. As development proceeds, its cells by division gradually give rise to new layers above it until the epidermis becomes a many layered or stratified epithelium. The periderm is always the outermost layer of the epidermis. In embryos of 30 to 88 mm. the epidermis is typically three-layered, the outer flattened layer forming the periderm, a middle layer of polygonal cells, the intermediate layer and the inner columnar layer being the stratum germinativum (Fig. 292 B). This con- dition may persist until the end of the fourth month. After the fourth month the epidermis becomes many layered. The inner layers of cells now form the stratum germinativum and are actively dividing cells united with each other by THE HAIR 305 cytoplasmic bridges. The outer layers of cells become cornified, the cornilkation of the cells proceeding from the stratum germinativum toward the surface. Thus, next the germinal layer are cells containing keratohyalin, which constitute the stratum granulosum, a single layer of cells. A thicker layer above the stratum granulosum shows cells in which drops of a substance called eleidin are formed. These droplets, which are supposed to represent softened keratohyalin, give these cells a clear appearance when examined unstained. Hence the layer is termed the stratum lucidum. In the outer layers of the epidermis the thickened walls of the cells become cornified and in the cells themselves a fatty substance collects. These layers of cells constitute the stratum corneum. The cells of this layer are also greatly flattened, especially at the surface. When the hairs develop they do not penetrate the outer periderm layer of the epidermis but, as they grow out, lift it off. Hence this layer is known also as the epitrichium (layer upon the hair). Pigment granules appear soon after birth in the cells of the stratum granulosum. These granules are probably formed in situ. Negro children are quite light in color at birth but within six weeks their integument has reached the normal degree of pigmentation. The dermis or corium of the integument is developed from mesenchyme or from the dermatomes of the mesodermal segments. For special points concern- ing its development see Keibel and Mall's "Human Embryology," vol. 1, p. 254. THE HAIR Hairs are derived from thickenings of the epidermis and begin to develop at the end of the second month on the eyebrows, upper lip and chin. The hair of the general body integument appears at the beginning of the fourth month. The first evidence of a hair anlage is the elongation of a cluster of epidermal cells in the inner germinal layer (Fig. 293 A). The bases of these cells project into the dermis and, above them, cells of the epidermis are arranged parallel to the surface. The elongated cells continue to grow downward until a cylindrical hair anlage is produced (Fig. 293 B, C). This consists of an outer wall formed of a single layer of columnar cells, continuous with the basal layer of the epidermis. This wall bounds a central mass of irregularly polygonal epidermal cells. About the hair anlage the mesenchyma forms a sheath, and at its base a condensation of mesenchyme produces the anlage of the hair papilla, which projects into the enlarged base of the hair anlage. As development proceeds, the hair anlage grows deeper into the corium and its base enlarges to form the hair bulb (Fig. 293 C). 306 HISTOGENESIS The hair is differentiated from the basal epidermal cells surrounding the hair papilla. These cells elongate and grow centrally toward the surface distinct Epitrichium 'dermaV. ye of hair's Anlage of hair papilla. Hair bulb Hair pap'ilh Fig. 293. — Section through the integument of the face of a 65 mm. embryo showing three stages in the early development of the hair. X 330. Inner hair Sheath from the peripheral cells which form the outer sheath of the hair (Fig. 294). The central core of cells gives rise to the inner hair sheath and to the shaft of the hair. At the sides of the outer hair sheath two swellings appear on the lower side of the obliquely directed hair anlage. The more superficial of these is the anlage of the sebaceous gland (Fig. 294) . The deeper swelling is the "epi- thelial bed, " a region where the cells by rapid division con- tribute to the growth of the hair follicle. Superficial to the bulb, the cells of the hair shaft become cornified and differentiated into outer cuticle, middle cortex and central medulla. The inner hair sheath extends from the Outer hair Sheath Epidermis A rrector pili muscle fibers Oebaceous gland. Mesenchymal sheath Epithelial bed Root of hair Hair bulb Hair papilla Fig. 294. — Longitudinal section through a developing hair from a five and one-half months' fetus (Stohr). SWEAT GLANDS — MAMMARY GLANDS 307 bulb to the level of the sebaceous glands, where it disappears. The hair grows at the base and is pushed out through the central cavity of the anlage, the « ells of which degenerate. When the hair projects above the surface of the epidermis it carries with it, and breaks up, the cpitridiial layer. The mesenchymal tissue which surrounds the hair follicle in the neighborhood of the epithelial bed gives rise to the smooth fibers of the corrector pili muscles. Pigment granules develop in the basal cells of the hair and give it its characteristic color. The first generation of hairs are short-lived and begin to degenerate before birth; usually the hair of the head is shed during the first and second years after birth, and new hairs develop as buds from the old hair follicles. SWEAT GLANDS The sweat or sudoriparous glands begin to develop in the fourth month from the epidermis of the finger-tips, of the palms of the hands and soles of the feet: They are formed as solid downgrowths from the epidermis, but differ from hair anlages in having no mesenchymal papillae at their bases. During the sixth month the tubular anlages of the gland begin to coil and in the seventh month their lumina appear. The inner layer of cells forms the gland cells while the outer cells become transformed into smooth muscle fibers which here arise from the ectoderm. In the axillary region sweat glands occur which are large and branched. MAMMARY GLANDS The tubular mammary glands peculiar to mammals are regarded as modified sweat glands. In early embryos an ectodermal thickening extends ventro- laterally between the bases of the limb buds on either side. This linear epidermal thickening is the milk line. In the future pectoral region of this line by the thickening and downgrowth of the epidermis there is formed the papilla- like anlage of the mammary gland (Fig. 295 A). From this epithelial anlage buds appear (B) which elongate and form solid cords 15 to 20 in number, the anlages of the milk ducts (Fig. 295 C). These branch in the mesenchymal tissue of the corium and eventually produce the alveolar end-pieces of the mammary glands. In the region where the milk ducts open on the surface the epidermis is evagi- nated to form the nipple. The glands enlarge at birth, at puberty and after par- turition when they become functionally active. The mammary glands are homologised with sweat glands because their development is similar, and because in the lower mammals their structure is the same. In many mammals ;oS HISTOGENESIS numerous pairs of mammary glands are developed along the milk line (pig, dog, etc.); in some a pair of glands is developed in the pectoral region (primates, elephants); in others only in the inguinal region (sheep, cow, horse). In man supernumerary mammary glands developed along the milk line are of not infrequent occurrence. Fig. 295. — Sections representing three successive "stages of development of the human mammary gland (Tourneux): A, fetus of 32.40 mm. (1.3 in.); B, of 10.16 cm. (4 in.); C, of 24.35 cm- (9-6 in.); a, epidermis; b, aggregation of epidermal cells forming anlage of gland; c, galactophorous ducts; d, groove limiting glandular area; e, great pectoral muscle; /, unstriated muscular tissue of areola; g, subcutaneous adipose tissue. THE NAILS The anlages of the nails proper are derived from the epidermis and may be recognized in embryos of 45 mm. A nail anlage forms on the dorsum of each digit extending from the tip of the digit almost to the articulation of the terminal phalanx. At the base of the anlage, that is, proximally, the epidermis is folded inward to form the proximal nail fold (posterior nail fold of the adult) . This is curved, convex proximally, and extends transversely to the dorsum of the digit (Fig. 296 A, C). The nail fold also extends laterally on either side of the nail anlage and forms the lateral nail fold of the adult (A, B) . The matrix of the nail is developed in the proximal nail fold (C) . In a layer of epidermal cells, lying parallel to the dorsum of the digit, there are developed keratin or horn fibrils during the fifth month of fetal life. These appear without the previous formation of keratohyalin granules as is the case in the cornification of the stratum corneum. The cells flatten and form the plate-like structure of which the solid substance of the nail is composed. Thus the nail substance is formed in the proximal nail fold. Over the area termed the lunula (the whitish crescent at the base of the adult nail) the nail is pushed toward the tip of the digit by the development of new nail substance in the region of the nail fold. The nail matrix, according to Bowen, represents a modified stratum lucidum of the epider- mis. The stratum corneum of the epidermis for a time completely covers the nail THE HISTOGENESIS OF THE NERVOUS TISSUES 309 matrix and is termed the cponychium (Fig. 296 C). Later, this is thrown off but a portion persists during life as the curved fold of the epidermis which adheres to. the lunula of the adult nail. During life the nail constantly grows at its base ( I Maximally), is shifted distally over the bed of the corium, and projects at the tip of the digit. The corium distal to the lunula takes no part in the development Sole plate Eponychium "Nail matrix Nail fold ■Nail bed Fig. 296. — Figures showing the development of the nail. A and B, in surface view; A in a 4 cm., B, in a 10 cm. fetus; C, longitudinal section through the nail anlage of a 10 cm. fetus. X 24 (Kolimann's Handatlas). of the nail substance and its surface of contact with the nail is thrown into parallel longitudinal folds. These folds produce the longitudinal ridges of the nails. The nails of man are the homologues of the claws and hoofs of other mammals. During the third month thickenings of the integument over the distal ends of the metacarpals and metatarsals become prominent. These correspond to the touch-pads on the feet of clawed mammals. Similar pads are developed on the under sides of the distal phalanges. THE HISTOGENESIS OF THE NERVOUS TISSUES The primitive anlage of the nervous system consists of the thickened layer of ectoderm along the mid-dorsal line of the embryo. This is the neural plate (Fig. 297 A, B) which is invaginated to form the neural groove. The edges of the neural plate come together and form the neural tube (Fig. 297 C, D). The cranial portion of this tube enlarges and is constricted into the three primary vesicles of the brain (Fig. 306). Its caudal portion remains tubular and con- stitutes the spinal cord. From the cells of this tube and the ganglion crest con- 3io HISTOGENESIS nected with it are differentiated the nervous tissues, the single exception being the nerve cells and fibers of the olfactory epithelium. The Differentiation of the Neural Tube. — The cells of the neural tube dif- ferentiate along two lines: There are formed: (i) nerve cells and fibers, in which irritability and conductivity have become the predominant functions; (2) neu- roglia cells and fibers which form the supporting or skeletal tissue peculiar to the nervous system. The differentiation of these tissues has been studied by Hardesty in pig embryos (Amer. Jour. Anat., vol. 3, 1904). The wall of the neural tube, consisting at first of a single layer of columnar cells, becomes many Neural groove Neural pi ah Neural qroove Neural plate A Ectoderm. Neural groove -Neural tube Neural tube I X Neural cavity C B Fig. 297. — Four sections showing the development of the neural tube in human embryos: A, of an early embryo (Keibel); B, through head of a 2 mm. embryo (Graf Spee); C, neural tube of 2 mm. em- bryo (Mall); D, neural tube of 2.69 mm. embryo (Kollmann). layered and finally three zones are differentiated (Fig. 298 A-D.) When the wall becomes many layered the cells lose their sharp outlines and form a compact cellular syncytium (Fig. 298 B). On its outer and inner surfaces there is differ- entiated from the cytoplasm an external and internal limiting membrane. In a 10 mm. embryo the cellular strands of the syncytium are radially arranged and directed nearly parallel to each other (Fig. 298 D). The nuclei are now so grouped that there may be distinguished three layers: (1) an inner ependymal zone with cells abutting on the internal limiting membrane, their processes extending peripherally; (2) a middle mantle or nuclear zone, and (3) an outer or marginal THE HISTOGENESIS OF THE NERVOUS TISSUES 311 zone, non-cellular, into which nerve fibers grow. The ependymal zone contributes cells for the development of the mantle layer (Fig. 298 D.) The cellular mantle layer forms the gray substance of the central nervous system, while the fibrous marginal layer constitutes the white substance of the spinal cord. mii mle mli D Fig. 298. — Three stages in the early development of the neural tube showing the origin of the syn- cytial framework: .1 , from rabbit before the closure of neural tube; B, from 5 mm. pig after closure of tube; C, from a 7 mm. embryo; D, from a 10 mm. pig embryo, a, ependymal layer; b, boundary between nuclear layer and marginal layer; g, germinal cell; m, marginal layer; mlc, mli, external and internal limiting membranes; r, mantle or nuclear layer; />, mesoderm. The primitive germinal cells of the neural tube divide by mitosis and give rise to the ependymal cells of the ependymal zone and to indifferent cells of the mantle layer. From these arise spongioblasts and neuroblasts (Fig. 299 ). The spongioblasts are transformed into neuroglia cells and fibers, which form the sup- 312 HISTOGENESIS porting tissue of the central nervous system ; the neuroblasts are primitive nerve cells and by developing cell processes are converted into neurones. The neurones are the structural units of the nervous tissue. Cells rent Cells ndi'fferent NeurogliaCells Neurobfasts Fig. 299. — Diagrams showing the differentiation of the cells in the wall of the neural tube and the theo- retical derivation of the ependymal cells, neuroglia cells and neuroblasts (after Schaper). ) FlG. 300. — A , Transverse section through the spinal cord of a chick embryo of the third day showing neuraxons developing from neuroblasts of the neural tube F and from the bipolar ganglion cells, d. (Cajalj; B, Neuroblasts from the spinal cord of a seventy-two-hour chick. The three to the right show neurofibrils; C, incremental cone. THE HISTOGENESIS OF THE NERVOUS TISSUES 313 The Differentiation of the Neuroblasts into Neurones. — The nerve fibers are developed as outgrowths from the neuroblasts, and a nerve cell with all its pro- cesses constitutes a neurone or cellular unit of the nervous system. The origin of the nerve libers as processes of the neuroblasts is best seen in the development of the root libers of the spinal nerves. The Efferent or Ventral Root Fibers of the Spinal Nerves. — At the end of the first month clusters of neuroblasts separate themselves from the syncytium in the mantle layer of the neural tube. The outline of the neuroblasts becomes pyriform and from the small end of the cell a slender primary process grows out (Figs. 300 and 301). The process becomes the axis cylinder of a nerve fiber. The primary processes may course in the marginal layer of the neural tube, or, converging, may penetrate the marginal layer ventro- laterally and form the ventral roots of the spinal nerves. Similarly, the efferent fibers of the cerebral nerves grow out from neuroblasts of the brain wall. Within the cytoplasm of the nerve cells and their primary processes strands of fine fibrils early are differen- tiated. These are the neurofibrillce and are the conducting elements of the neurones. The cell bodies of the efferent neurones soon become multi- polar by the development of branched Fig. 301. — Transverse section of the spinal cord from an embryo of the fourth week showing pear-shaped neuroblasts giving rise to ventral root fibers (His in Marshall's Embryology). XC, cen- tral canal of spinal cord; XD. dorsal root of spinal nerve; XI, nuclei of spongioblasts; XV, ventral motor root fibers; VII", ventral funiculi; XZ, neuroblasts. secondary processes, the dendrites. The Development of the Spinal Ganglia and Afferent Neurones of the Spinal Cord. — The ganglion crest. — After the formation of the neural plate and groove a longitudinal ridge of cells is differentiated on each side where the ecto- derm and neural plate are continuous (Fig. 302 A). This ridge of ectodermal cells is the neural or ganglion crest. When the neural tube is formed and the ectoderm separates from it, the cells of the ganglion crest overlie the neural tube dorso-laterally (Fig. 302 C). As development continues they separate into right and left linear crests distinct from the neural tube, and migrate ventro-laterally ,14 HISTOGENESIS to a position between the neural tube and myotomes. In this position the ganglion crest forms a band of cells extending the whole length of the spinal cord and as far cranially as the otic vesicles. At regular intervals in its course along the spinal cord the proliferating cells of the crest give rise to enlargements, the spinal ganglia (Fig. 340). The spinal ganglia are segmentally arranged and con- nected at first by bridges of cells which later disappear. In the hind-brain region certain ganglia of the cerebral nerves develop from the crest but are not seg- mentally arranged. tab * C Fig. 302. — Three stages in the development of the ganglion crest in human embryos (after von Lenhossek in Cajal). a, ectoderm; b, neural tube; c, mesodermal segment; G, ganglioblasts. The Differentiation of the Afferent Neurones. — The cells of the spinal ganglia differentiate into (1) ganglion cells and (2) supporting cells, groups which are 1 om parable to the neuroblasts and spongioblasts of the neural tube. The neuro- blasts of the ganglia become fusiform, develop a primary process at either pole and thus these neurones are of the bipolar type. The centrally directed processes of the ganglion cells converge and by elongation form the dorsal roots. They penetrate the dorso-lateral wall of the neural tube, bifurcate and course cranially and caudally in the marginal layer of the spinal cord (Fig. 300, d). By means of branched processes they anastomose with the neurones of the mantle layer. The peripheral processes of the ganglion cells as the dorsal spinal roots join the ventral THE HISTOGENESIS <>F THE NERVOUS IISSl ES 315 roots and, together with them, constitute the trunks of the spinal nerves (Fig. 307)- The Differentiation of the Unipolar Ganglion Cells. At first bipolar, the majority of the ganglion cells become unipolar either by the unilateral growth of the cell body or by the bifurcation of a single primary process. In the first case, if the cytoplasm and nucleus take up an eccentric position, the two processes unite in a single slender connection with the cell body (Fig. 303). The ganglion cell, having one process, is now unipolar and its process is T-shaped. Many of the bipolar ganglion cells persist in the adult, and others develop several secondary- processes and thus become multipolar in form. In addition to forming the spinal ganglion cells, neuroblasts of the ganglion crest are believed to migrate ventrally and form the sym- pathetic ganglia (Fig. 307). C- Tfie Neurone Theory. — The above ac- count of the development of the nerve fibers is the one generally accepted at the present time. It assumes that the axis cylinders of all nerve fibers are formed as outgrowths, each from a single cell, an hypothesis first promulgated by His. The embryological evidence is supported by experiment. It has long been known from the work of Waller that if nerves are severed, the fibers distal to the point of section, and thus iso- lated from their nerve cells, will degenerate; also, that regeneration will take place from the central stumps of cut nerves, the fibers of which are still connected with their cells. More recently Harrison (Amer. Jour. Anat., vol. 5, 1906) experimenting on amphibian lame has shown (1) that no peripheral nerves develop if the neural tube and crest are removed; (2) that isolated ganglion cells growing in clotted lymph will give rise to long axis cylinder processes in the course of four or five hours. A second theory, supported by Schwann, Balfour, Dohrn and Bethe, assumes that the nerve fibers are in part differentiated from a chain of cells, so that the neurone would represent a multicellular, not a unicellular structure. Apathy and O. Schulze modified this cell-chain tlieory by assuming that the nerve fibers differentiate in a syncytium which intervenes between the neural tube and the peripheral end organs. Held further modified this theory by assuming that the proximal portions of the nerve fibers are derived from the neuroblasts and ganglion cells and that these grow into a syncytium which by differentiation gives rise to the peripheral portion of the fiber. This theory accords with the experiments of Bethe who found that in the peripheral portions of severed nerves, functional nerve fibers were regenerated in young animals. Fig. 303. — A portion of a spinal ganglion from a human embryo of 44 mm. Golgi method (Cajal). 316 HISTOGENESIS The Differentiation of the Supporting Cells of the Ganglia and Neural Tube. — The supporting cells of the spinal ganglia at first form a syncytium in the meshes of which are found the neuroblasts. They differentiate (i) into flattened capsule cells which form capsules about the ganglion cells and (2) into sheath cells which ensheath the axis cylinder processes and are continuous with the capsules of the ganglion. It is probable that many of the sheath cells migrate peripherally along with the developing nerve fibers (Harrison). They are at first spindle- shaped and, as primary sheaths, enclose bundles of nerve fibers. Later, by the proliferation of the sheath cells the bundles are separated into single fibers, each with its sheath (of Schwann), or neurilemma. Each sheath cell forms a segment of the neurilemma, the limits of adjacent sheath cells being indicated by constric- tions, the nodes of Ranvier. The Myelin or Medullary Sheath. — During the fourth month an inner myelin sheath appears about many nerve fibers. This consists of a spongy framework of neurokeratin in the interstices of which a fatty substance, myelin, is deposited. The origin of the myelin sheath is in doubt. By some it is believed to be a differ- entiation of the neurilemma, the myelin being deposited in the substance of the nucleated sheath cell. By others the myelin is regarded as a product of the axis cylinder. Its integrity is dependent at least upon the nerve cell and axis cylinder, for, when a nerve is cut, the myelin very soon shows degenerative changes. In the central nervous system there is no distinct neurilemma sheath investing the fibers. Sheath cells are said to be present and most numerous during the period when myelin is developed. Hardesty derives the sheath cells in the central nervous system of the pig from a portion of the supporting cells, or spongioblasts, of the neural tube, and finds that these cells give rise to the myelin of the fibers. Those fibers which are first functional receive their myelin sheaths first. The development of myelin is only completed between the fifteenth and twentieth year (Westphal). Many of the peripheral fibers, especially those of the sympa- thetic system, remain non-medullated and supplied only with a neurilemma sheath. The medullated fibers, those with a myelin sheath, have a glistening white ap- pearance and give the characteristic color to the white substance of the central nervous system and to the peripheral nerves. Ranson (Amer. Jour. Anat., vol. 12, p. 67) has shown that large numbers of non-medullated fibers occur in the peripheral nerves and spinal cord of adult mammals and man. Those found in the spinal nerves arise from the small cells of the spinal ganglia. THE HISTOGENESIS OF THE NERVOUS TISSIKS 317 The Development of the Neuroglia Cells and Fibers. — The spongioblasts of the neural tube (seep. 311) differentiate into the supporting tissue of the central nervous system. This includes the cpendymal cells, which line the neural cavity, forming one of the primary layers of the neural tube, neuroglia cells and their fibers. We have described how the strands of the syncytium formed by the spongio- blasts become arranged radially in the neural tube of early embryos (Fig. 298 D). As the wall of the neural tube thickens, the strands elongate pari passu and form a Fig. 304. — Ependvmal cells from the neural tube of chick embryos: A, of first day; B, of third day. Golgi method (Cajal). radiating branched framework (Fig. 304 A, B). The group of spongioblasts which line the neural cavity constitutes the ependvmal layer. Processes from these cells radiate and extend through the whole thickness of the neural tube to its periphery. The cell bodies are columnar and persist as the lining of the central canal and ventricles of the spinal cord and brain (Fig. 305). Near the median line of the spinal cord, both dorsally and ventrally, the supporting tissue retains its primitive ependvmal structure in the adult. Elsewhere the supporting framework is differentiated into neuroglia cells and fibers. The neuroglia cells form part of the spongioblasts syncytium and are 3i8 HISTOGENESIS scattered through the mantle and marginal layers of the neural tube. By pro- liferation they increase in numbers and their form depends upon the pressure of the nerve cells and fibers which develop around them. Neuroglia fibers are differentiated from the cytoplasm and cytoplasmic Fig. 305. — Ependymal cells of the lumbar cord from a human embryo of 44 mm. Golgi method (Cajalj. A, floor plate; B, central canal; C, line of future fusion of walls of neural cavity; D, neuroglia cells and fibers. processes of the neuroglia cells, and as the latter primarily form a syncytium, the neuroglia fibers may extend from cell to cell. The neuroglia fibers develop late in fetal life and undergo a chemical transformation into neurokeratin, the same substance which is found in the sheaths of medullated fibers. CHAPTER XI THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM In discussing the histogenesis of the nervous tissue we have described the early development of the neural tube, as an infolding of the neural plate and a closure of the neural groove (Fig. 297). The groove begins to close along the mid- dorsal line near the middle of the body in embryos of 2 mm. and the closure Mesencephalon Rhombencephalon Mycloicephalon Amnion (cut) Mesodermal segment 14 Neural tube (not closed) Prosencephalon Stomodaum Amnion (cut) Yolk-sac Body-stalk Fig. 306. — Human embryo of 2.4 mm. showing neural tube closing and the brain vesicles (after Kollmann). extends both cranially and caudally (Fig. 306) . Until the end of the third week there still persists an opening at either end of the neural tube, somewhat dorsad. These openings are the neuroporcs. Before the closure of the neuropores, in embryos of 2 to 2.5 mm. the cranial end of the neural tube has enlarged and is 319 120 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM constricted at two points to form the three primary brain vesicles. The caudal two-thirds of the neural tube, which remains of smaller diameter, constitutes the uiil age of the spinal cord. THE SPINAL CORD The spinal portion of the neural tube is at first nearly straight, but is bent with the flexure of the embryo into a curve which is convex dorsally. Its wall gradually thickens during the first month and the diameter of its cavity is di- Neura.1 cavity Marginal layer Dorsal root Ependymal layer Spinal ganglion Mantle layer Dorsal ramus Ventral root Nerve trunK Sympathetic ganglion Fig. 307. — Transverse section through a 10 mm. embryo at the level of the arm buds showing the spinal cord and a spinal nerve of right side. X 44. minished from side to side. By the end of the first month three layers have been developed in its wall as described in Chapter X, p. 310 (Fig. 307). These layers are the inner ependymal layer which forms a narrow zone about the neural cavity; the middle mantle layer \ cellular, and the outer marginal layer, fibrous. The Ependymal Layer is differentiated into a dorsal roof plate and a ventral floor plate (Fig. 308). Laterally, its proliferating cells contribute neuroblasts and neuroglia cells to the mantle layer. The proliferation of cells ceases first in the THE SPINAL CORD 321 Centra] portion of the layer, which is thus narrower than the dorsal portion in 10 to 20 mm. embryos (Figs. 307 and 308). Consequently, the ventral portion of the mantle layer is differentiated first. The neural cavity is at first somewhat rhomboidal in transverse section, wider dorsally than ventrally. Its lateral angle forms the sulcus litnitans which marks the subdivision of the lateral walls of the neural tube into the dorsal alar plate and ventral basal plate. When the ependy- mal layer ceases to contribute new cells to the mantle laser its walls are approxi- mated dorsally. As a result, in 20 mm. embryos the neural cavity is wider ven- trally (Fig. 208). In the next stage, 34 mm., these walls fuse and the dorsal portion of the neural cavity is obliterated (Fig. 309). In a 65 mm. embryo the Hoof plate Post .funiculus Neural cavity Marginal layer EpendyHial /-aye, Ant. median fissurt Floor plate Fig. 308. — Transverse section of the spinal cord from a 20 mm. embryo. X 44. persisting cavity is becoming rounded (Fig. 310). It forms the central canal of the adult spinal cord. The cells lining the central canal are ependymal cells proper. Those in the floor of the canal form the persistent floor plate. Their fibers extend ventrally to the surface of the cord in the depression of the ventral median fissure. When the right and left walls of the ependymal layer fuse, the ependymal cells of the roof plate no longer radiate, but form a medium septum (Fig. 309). Later, as the marginal layers of either side thicken and are approximated the median septum is extended dorsally. Thus the roof plate is converted into part of the dorsal median septum of the adult spinal cord (Fig. 310). The Mantle Layer, as we have seen, is contributed to by the proliferating 32: THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM cells of the ependymal layer. A ventro-lateral thickening first becomes promi- nent in embryos of 10 to 15 mm. (Fig. 307). This is the ventral gray column, or horn, which in later stages is subdivided, forming also a lateral gray column (Fig. Dorsal funiculus Post-median Septum Lat.funiculus Central canal Ant.column Dura mater [£$, Spinal *j||r gang ft'on Ventral funiculus Fig. 309. — Transverse section of the spinal cord from a 34 mm. embryo, showing also the spinal ganglion and dura mater on the left side. X 44. Post medium s ep tu m Post root Post, col urn Lat, fun ic Ant. column Fascic. gracilis Fascic. bstantla gelatmosa c ..tral , canal Lat. Column Ant. funiculus -J ^Ant.median fissure Fig. 310. — Transverse section of the spinal cord from a 65 mm. embryo. X 44. 310). It is a derivative of the basal plate. In embryos of 20 mm. a dorso-lateral thickening of the mantle layer is seen, the cells of which constitute the dorsal gray column or horn (Figs. 208 and 209) ; about these cells end the collaterals of I 111 SPINAL (OKI) 323 the dorsal roof libers. The cells of the dorsal gray column thus form terminal nuclei for the afferent spinal nerve fibers and they are derivatives of the alar plate of the cord. Dorsal and ventral to the central canal the marginal layer forms the dorsal and ventral gray commissures. In the ventral floor plate nerve fibers cross from both sides of the cord and form the anterior white commissure. The Marginal Layer is composed primarily of a framework of neuroglia and ependymal ceH processes. Into this framework grow the axis cylinder processes of nerve cells, so that the thickening of this layer is due to the increasing number of nerve fibers contributed to it by ganglion cells and neuroblasts located outside of it. When their myelin develops, these fibers form the white substance of the spinal cord. The fibers have three sources (Fig. 342) : (1) they may arise from the spinal ganglion cells, entering as dorsal root fibers and coursing cranially and caudally in the marginal layer; (2) they may arise from neuroblasts in the mantle layer of the spinal cord (a) as fibers which connect adjacent nuclei of the cord (fasciculi proprii or ground bundles) ; (b) as fibers which extend cranially to the brain; (3) they may arise from neuroblasts of the brain (a) as long descending cerebrospinal tracts from the cortex of the cerebrum; (b) as descending tracts from the brain stem. Of these fiber tracts (1) and (2 a) appear during the first month; (2 b) and (3 b) during the third month; (3 a) at the end of the fifth month. The dorsal root fibers from the spinal ganglion cells entering the cord dorso- laterally subdivide the white substance of the marginal layer into a dorsal funi- culus and lateral funiculus. The lateral funiculus is marked off by the ventral root fibers from the ventral funiculus (Fig. 309). The ventral root fibers, as we have seen, take their origin from the neuroblasts of the ventral gray column in the mantle layer. They are thus derivatives of the basal plate. The dorsal funiculus is formed chiefly by the dorsal root fibers of the ganglion cells and is subdivided into two distinct bundles, the fasciculus gracilis, median, and the fasciculus cuneatus, lateral in position. The dorsal funiculi are separated only by the dorsal median septum (Fig. 310). The lateral and ventral funiculi are composed of fasciculi proprii or ground bundles, originating in the spinal cord, of ascending tracts from the cord to the brain, and of the descending fiber tracts from the brain. The fibers of these fasciculi intermingle and the fasciculi are thus without sharp boundaries. The floor plate of ependymal cells lags behind in its development, and as it is interposed between the thickening right and left walls of the ventral funiculi, these do not meet and the ventral median fissure is produced (compare Figs. 307 and 310). 324 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM Cervical enlargement Lumbar enlargement The development of myelin in the nerve fibers of the cord begins in the fifth month of fetal life and is completed between the fifteenth and twentieth years (Flechsig, Bechterew). Mvelin appears first in the root fibers of the spinal nerves and in those of the ventral commissure, next in the ground bundles, and dorsal funiculi. The cerebrospinal (pyramidal) fasciculi are the last in which myelin is developed; they are myelinated during the first and second years. As myelin appears in the various fiber tracts at different periods, this fact has been utilized in tracing the extent and origin of the various fasciculi in the central nervous system. The Cervical and Lumbar Enlargements. — At the levels of the two nerve plexuses supplying the upper and lower extremities the size of the spinal cord is increased. As the fibers to the mus- cles of the extremities arise from nerve cells in the ventral gray column, the number of these cells and the mass of the gray substance is increased; also larger numbers of fibers enter the cord from the integument of the limbs, so that there are larger numbers of cells about which sensory fibers terminate. There is formed consequently at the level of the origin of the nerves of the brachial plexus the cervical enlarge- ment, opposite the origins of the nerves of the lumbo-sacral plexus the lumbar enlargement (Fig. 311). At the caudal end of the neural tube in an 11 cm. fetus an epithelial sac is formed which is adherent to the integument. Cranial to the sac the central canal is obliterated and this part of the neural tube forms the filum terminate. The caudal end of the central canal is irregularly expanded and is known as the terminal ventricle. The vertebral column during and after the third month grows faster than the spinal cord. As the cord is fixed to the brain it is carried cranially with reference to the vertebrae, and with it shift the roots and ganglia of the spinal nerves. In the adult the origin of the coccygeal nerves is opposite the first lumbar vertebra and the nerves course obliquely downward nearly parallel to the spinal cord. As the neural tube is drawn cranially and its caudal tip is attached to the coccyx, its caudal portion is stretched into the slender solid cord known as the filum terminate. Fig. 311 . — Dissection of the brain and cord of a three months' fetus, showing the cervical and lumbar enlargements (after Ko Hiker in Marshall). c, cerebellum; h, cerebrum; m, mid-brain. Nat- ural size. THE BRAIN 325 The obliquely coursing spinal nerves with the lilum terminale constitute the Cauda equina. THE BRAIN We have seen that in embryos of 2 to 2.5 mm. the neural tube is nearly straight, but that its cranial end is enlarged to form the anlage of the brain. The appearance of two constrictions in the wall of the anlage subdivides it into the three primary brain vesicles, the fore-brain or prosencephalon, mid-brain or mesencephalon, and hind-brain or rhombencephalon (Fig. 306). Anterior neuropore iPallium of telencephalon Palli Corpus striatum Diencephalon /Interior neuropore Mes en cephalon Isthmus Mese n c ephalon Cephalic ' flexure Optic recess Future pontine Rhombencephalon flexure - Rhombencephalon A D Fig. 312. — Reconstructions of the brain of a 3.2 mm. human embryo. A, lateral surface; B, sectioned in the median sagittal plane (after His). In embryos of 3.2 mm., estimated age three weeks, three important changes have taken place (Fig. 312 A, B) : (1) the end of the neural tube is bent sharply in the mid-brain region so that the axis of the fore-brain now forms a right angle with the axis of the hind-brain. This bend is the cephalic flexure; (2) the fore- brain shows indication dorsally of a fold the margo thalamicus which subdivides it into the telencepalon and the diencephalon; (3) the lateral wall of the fore-brain shows a distinct evagination, the optic vesicle, which projects laterally and caudad. A ventral bulging of the wall of the hind-brain indicates the position of the future pontine flexure. In embryos of 7 mm. (four weeks) the neuropores have closed. The ceph- 326 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM alic flexure, now more marked, forms an acute angle and the pontine flexure, just indicated in the previous stage, is now a prominent ventral bend in the ventro-lateral walls of the hind-brain (Fig. 313 A, B). This flexure forms the boundary line which subdivides the rhombencephalon into a cranial portion, the metencephalon, and into a caudal portion, the myelencephalon. At a third bend the whole brain is flexed ventrally at an angle with the axis of the spinal cord. This bend is the cervical flexure and is the line of demarcation between the brain and spinal cord. The telencephalon and diencephalon are more distinctly sub- divided, and the invaginated optic vesicle forms the optic cup attached to the brain wall by a hollow stalk, which later becomes the optic nerve. The walls of Diencephalon Mesencephalon Pallium Ceph. fie Myelencephalon Metencephalon Corpus striatum Optic recess Hypothalamus Mesencephalon Isthmus Cerebellum Fig. 313. — Reconstructions of the brain of a 7 mm. human embryo. A, lateral view; B, in median sagittal section (His). Ceph. flex., cephalic flexure. the brain show a distinct differentiation in certain regions. This is especially marked in the myelencephalon, which has a thicker ventro-lateral wall and thinner dorsal wall. Embryos of 10.2 mm. show the structure of the brain at the beginning of the second month (Figs. 323 and 326). In Fig. 341 the external form of the brain is seen with the origins of the cerebral nerves. It will be noted that, with the exception of the first four (the olfactory, optic, oculomotor and trochlear), the cerebral nerves take their superficial origin from the myelencephalon. The five brain regions are now sharply differentiated externally but the boundary line between the telencephalon and diencephalon is still indistinct. The telencephalon THE BRAIN 327 consists in paired lateral outgrowths, the anlagcs of the cerebral hemispheres and rhinencephalon. The cephalic flexure forms a very acute angle and, as a result, the long axis of the fore-brain is nearly parallel to that of the hind-brain. The oculomotor nerve takes its origin from the ventral wall of the mesencephalon. Dorsally there is a constriction, the isthmus, between the mesencephalon and meten- cephalon, and here the fibers of the trochlear nerve take their superficial origin. The dorsal wall of the myelencephalon is an exceedingly thin ependymal layer, Cerebral peduncles Hypothalamus EpUhalamus Thalamus Diencephalon (I iiter-brain) Cerebral aqueduct '■ Mesencephalon {Mid-brain) Rhombencephalic isthmus - .Lamina Corpi.s Rhinencephalon ''terminalis striatum (Olfactory-brain) Fig. 314. — Brain of a 13.6 mm. human embryo in median sagittal section (after His from Sobotta's Atlas of Anatomy). 1, optic recess; 2, ridge formed by optic chiasma, 3; 4, infundibular recess. the tela chorioidca. The ventro-lateral walls of this same region on the other hand are very thick. A median sagittal section of a brain at a somewhat later stage shows the cervical, pontine and cephalic flexures well marked (Fig. 314). The thin dorso- lateral roof of the myelencephalon has been removed. The telencephalon is a paired structure. In the figure its right half projects cranial to the primitive median wall of the fore-brain which persists as the lamina terminalis (see Fig. 324). The floor of the telencephalon is greatly thickened caudally as the anlage of the corpus striatum. A slight evagination of the ventral wall of the telencephalon just cranial to the corpus striatum marks the anlage of the rhinencephalon. The remaining portion of the telencephalon forms the pallium or cortex of the cerebral hemispheres. The paired cavities of the telencephalon are the lateral (second) 328 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM ventricles and these communicate through the interventricular foramina (Monroi) with the cavity of the diencephalon, the third ventricle. The cavities of the ol- factory lobes communicate during fetal life with the lateral ventricles and were formerly called the first ventricles. The crossing of a portion of the optic nerve fibers in the floor of the brain forms the optic chiasma and this, with the transverse ridge produced by it inter- nally, is taken as the ventral boundary line between the telencephalon and dien- cephalon (Fig. 314). A dorsal depression separates the latter from the mesenceph- alon. The lateral wall of the diencephalon is thickened to form the thalamus, the caudal and lateral portion of which constitutes the metathalamns . From the metathalamus are derived the geniculate bodies. In the median dorsal wall, near the caudal boundary line of the diencephalon, an outpocketing begins to appear in embryos of five weeks (Fig. 314). This is the epithalamus which later gives rise to the pineal body, or epiphysis. The thalamus is marked of from the more ventral portion of the diencephalic wall, termed the hypothalamus by the obliquely directed sulcus hypothalamicus. Cranial to the optic chiasma is the optic recess, regarded as belonging to the telencephalon. Caudal to it is the pouch-like infundibulum, an extension from which during the fourth week forms the posterior lobe of the hypophysis. Caudal to the infundibulum the floor of the diencephalon forms the tuber cinereum and the mammillary recess; the walls of the latter thicken later and give rise to the mammillary bodies. An oblique transverse section through the telencephalon and hypothalamic portion of the diencephalon (Fig. 325), shows the relation of the optic recess to the optic stalk, the infundibulum and Rathke's pocket, and the extension of the third ventricle, the proper cavity of diencephalon, into the telen- cephalon between the corpora striata. The mesencephalon in 13.6 mm. embryos (Fig. 314) is distinctly marked off from the metencephalon by the constriction which is termed the isthmus. Dorso- laterally thickenings form the corpora quadrigemina. Ventrally, the mesenceph- alic wall is thickened to form the tegmentum amd crura cerebri. In the tegmen- tum are located the nuclei of origin for the oculomotor and trochlear nerves. The former, as we have seen, takes its superficial origin ventrally, while the trochlear nerve fibers bend dorsad, cross at the isthmus and emerge on the opposite side. As the walls of the mesencephalon thicken, its cavity later is narrowed to a canal, the cerebral aqueduct (of Sylvius). The walls of the metencephalon are thickened dorsally and laterally to form THE BRAIN 329 the anlage of the cerebellum. Its thickened ventral wall becomes the pons (Varolii). Its cavity constitutes the cranial portion of the fourth ventricle. The caudal border of the pons is taken as the ventral boundary line between the metencephalon and myclcnccphalon. The myelencephalon forms the medulla oblongata. Its dorsal wall is a thin non-nervous ependymal layer, which later becomes the posterior medullary velum. From its thickened ventro-lateral ib mb is hb IVv ~ab in m pf ol 0 pf 0 mb hb Fig. 315. — Brains of human embryos, from reconstructions by His: A, brain from fifteen-day em- bryo; B, from three-and-a-half-week, embryo; C, from seven-and-a-half-week fetus; fb, ib, mb, hb, ab, fore-, inter-, mid-, hind-, and after-brain vesicles; 0, optic vesicle; ov, otic vesicle; in, infundibulum; m, mammillary body; pf, pontine flexure; IVv, fourth ventricle; nk, cervical flexure; ol, olfactory lobe; b, basilar artery; p, pituitary recess (American Text-Book of Obstetrics). walls the last eight cerebral nerves take their origin. Its cavity forms the greater part of the fourth ventricle which opens caudally into the central canal of the spinal cord, cranially into the cerebral aqueduct. The increase in the flexures of the brain and the relative growth of its different regions may be seen by comparing the brains of embryos of the third, fourth, and eighth weeks (Fig. 315). In the following table are given the primitive subdivisions of the neural tube and the parts derived from them: 330 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM THE DERIVATIVES OF THE NEURAL TUBE Primary Vesicles Subdivisions Derivatives Cavities Telencephalon Cerebral cortex Corpora striata Rhinencephalon Lateral ventricles Cranial portion of third ventricles Prosencephalon Diencephalon Epithalamus (pineal body) Thalamus Optic tract Hypothalamus hypophysis tuber cinereum mammillare bodies Third ventricle Mesencephalon Mesencephalon Corpora quadrigemina Tegmentum Crura cerebri Aquaeductus cerebri Rhombencephalon Spinal cord Metencephalon Cerebellum Pons Fourth ventricle Myelencephalon Medulla oblongata Spinal cord Central canal The Later Differentiation of the Subdivisions of the Brain Myelencephalon. — We have seen that .the wall of the spinal cord differen- tiates dorsally and ventrally into roof plate and floor plate, laterally into the basal plate and alar plate. The boundary line between the basal and alar plates is the Hoof plate Mantle Hoof plate Sulcus limi Ependymal layer Alar plate S. limitans Basal plate •Spinal ganglion /'; ■ Ventral spinal root Fig. 316. — Transverse sections. A, through the upper cervical region of the spinal cord in a 10 mm. human embryo; B, through the caudal end of the myelencephalon of the same. X 44. sulcus limitans (Fig. 316 A.). The same subdivisions may be recognized in the myelencephalon. It differs from the spinal cord, however, in that the roof plate is broad, thin and flattened to form the ependymal layer (Fig. 316 B.). In the alar and basal plates of the myelencephalon the marginal, mantle and ependymal zones are differentiated as in the spinal cord (Fig. 317 A, B). Owing to the for- THE BRAIN 331 mation of the pontine flexure at the beginning of the second month, the roof plate is broadened, especially in the cranial portion of the myelencephalon, and the alar plates bulge laterally (Figs. 318 and 319 A). The cavity of the myelen- Alar plate Sulcus Jim 1 tans Basal plate Garry/ion juaulare '.Hypoqlossus \N. accessor! 'us N .vagus ' MHypog/ossusI Fig. 317. — Transverse sections through the myelencephalon of a 10.6 mm. embryo (His). A, through the nuclei of origin of the spinal accessory and hypoglossal nerves; B, through the vagus and hypoglossal nerves (after His). Inner layer Roof plaie rormaiio reticularis grisea Form at 10 reticularis alba Tractus . solilarivs Rhombic lip Restiform body Z Spinal V. xivr^^ Neuroblasts from alar plate. — Marginal layer N. XII Septum medullae Neuroblasts from alar plate r QRudimenl or accessory olive) Fig. 318. — Transverse section through the myelencephalon of an eight weeks' human embryo (His) cephalon is thus widened from side to side and flattened dorso-ventrally. This is most marked cranially where, between the alar plates of the myelencephalon and metencephalon, are formed the lateral recesses of the fourth ventricle (Fig. 332 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 319 A). Into the ependymal roof of the myelencephalon blood-vessels grow and, invading the lateral recesses, form there the chorioid plexus of the fourth ventricle. The plexus consists of small finger-like folds of the ependymal layer and its cover- ing mesenchymal layer. The line of attachment of the ependymal layer to the alar plate is known as the rhombic lip and later becomes the tcenia and obex of the fourth ventricle (Fig. 319 B). The further growth of the myelencephalon is due (1) to the rapid formation of neuroblasts, derived from the ependymal and mantle layers; (2) to the de- velopment of nerve fibers from these neuroblasts; (3) to the development and growth into it of fibers from neuroblasts in the spinal cord and in other parts of the brain. The neuroblasts of the basal plates early give rise chiefly to the efferent fibers of the cerebral nerves (Fig. 317). They thus constitute motor nuclei of origin of the trigeminal, abducens, facial, glossopharyngeal, vagus complex and hypo- glossal nerves, nuclei corresponding to the ventral and lateral gray columns of the spinal cord. The basal plate also produces part of the reticular formation which is derived in part also from the neuroblasts of the alar plate (Fig. 318). The axons partly cross as external and internal arcuate fibers and form a portion of the median longitudinal bundle, a fasciculus corresponding to the ventral ground bundles of the spinal cord. Other axons grow into the marginal zone of the same side and form intersegmental fiber tracts. The reticular formation is thus differ- entiated into a gray portion situated in the mantle zone and into a white portion located in the marginal zone (Fig. 318). The marginal zone is further added to by the ascending fiber tracts from the spinal cord and the descending pyramidal tracts from the brain. As in the cord, the marginal layers of each side remain distinct, being separated by the cells of the floor plate. The alar plates differ- entiate later than the basal plates. The afferent fibers of the cerebral nerves first enter the mantle layer of the alar plates and coursing upward and downward form definite tracts (tractus solitarius, descending tract of fifth nerve). To these are added tracts from the spinal cord so that an inner gray and an outer white substance is formed. Soon, however, the cells of the mantle layer proliferate, migrate into the marginal zone and surround the tracts. These neuroblasts of the alar plate form groups of cells along the terminal tracts of the afferent cerebral nerves (which correspond to the dorsal root fibers of the spinal nerves) and con- stitute the receptive or terminal nuclei of the fifth, seventh, eighth, ninth and tenth cerebral nerves. Caudally, the nucleus gracilis and nucleus cuneatus are developed from the alar plates as the terminal nuclei for the afferent fibers which ascend from THE BRAIN 333 the dorsal funiculi of the spinal cord. The axons of the neuroblasts forming these receptive nuclei decussate through the reticular formation chiefly as internal arcuate fibers and ascend to the thalamus as the median lemniscus. There are developed from neuroblasts of the alar plate other nuclei the axons of which connect the brain stem, cerebellum and fore-brain. Of these the most conspicuous is the inferior olivary nucleus. The characteristic form of the adult myelencephalon is determined by the further growth of the above-mentioned structures. The nuclei of origin of the Mid-brain Cerebellum Lobule} of vermis f/occu/i ■Medulla oblongata. L ateral lobe of Cereb ellum Rhomb, lip Flocculus /\l/n/la. Nodulus Fig. 319. — Dorsal views of four stages in the development of the cerebellum. A, of a 13.6 mm. em- bryo (His); B, of a 24 mm. embryo; C, of a no mm. embryo; D, of a 150 mm. embryo. cerebral nerves, derived from the basal plate, produce swellings in the floor of the fourth ventricle which are bounded laterally by the sulcus limitans. The terminal nuclei of the mixed and sensory cerebral nerves lie lateral to this sulcus. The enlarged cuneate and gracile nuclei bound the ventricle caudally and laterally as the cuneus and clava. The inferior olivary nuclei produce lateral rounded prominences and ventral to these are the large cerebrospinal tracts or pyramids. The Metencephalon. — Cranial to the lateral recesses of the fourth ventricle the cells of the alar plate proliferate ventrally and form the numerous and rela- tively large nuclei of the pons. The axons from the cells of these nuclei mostly 334 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM cross to the opposite side and form the brachium pontis of the cerebellum. Cere- bral fibers from the cerebral peduncles end about the cells of the pontine nuclei. Others pass through the pons as fascicles of the pyramidal tracts. Cerebellum. — When the alar plates of the cranial end of the myelencephalon are bent out laterally the caudal portions of their continuations into the meten- cephalic region are carried laterally also. As a result, the alar plate of the meten- cephalon takes up a transverse Mesencephalon Cerebellum Ependymal layer B Mesencephalon Cerebellum position and forms the anlages of the cerebellum (Fig. 319 A). During the second month the paired cerebellar plates thicken and bulge into the ventricle (Fig. 320 A). Near the mid- line a thickening indicates the anlage of the vermis, while the remainder of the alar plates form the anlages of the lateral lobes or cerebellar hemispheres. The cerebellar anlages grow rapidly laterally and also in length so that their surfaces are folded transversely. During the third month their walls bulge outward and form on either side a convex lateral lobe connected with the pons by the brachium pontis (Fig. 319 C). In the meantime, the anlages of the vermis have fused in the mid- line producing a single structure marked by transverse fissures. The rhombic lip gives rise to the flocculus and nodulus. Between the third and fifth months the cortex cerebelli grows more rapidly than the deeper layers of the cerebellum and its principal lobes, folds and fissures are formed (Fig. 319 C, D). The hemispheres derived from the lateral lobes are the last to be differentiated. Their fissures do not appear until the fifth month. Cranial to the cerebellum the wall of the neural tube remains thin dorsally Post. med. velum Ani)med. velum Fig. 320. — Median sagittal section of the cerebellum and part of mid-brain. A, from a 24 mm. embryo; B, from a 150 mm. embryo. Ant. med. velum, anterior medullary velum; Post. med. velum, posterior medullary velum. THE BRAIN 335 and constitutes the anterior medullary velum of the adult. Caudally, the ependy- mal root" of the fourth ventricle becomes the posterior medullary velum. The points of attachment of the vela remain approximately fixed, while the cerebellar cortex grows enormously. As a result, the vela are folded in under the expanding cerebellum (Fig. 320). The anlages of the cerebellum show at first differentiation into the same three layers which are typical for the neural tube. During the second and third months cells from the ependymal, and perhaps from the mantle layer, of the rhombic lip, migrate to the surface of the cerebellar cortex and give rise to the molecular and granular layers which are character- istic of the adult cerebellar cortex (Schafer). The later differentiation of the cortex is only completed at or after birth. The cells of the granular layer become unipolar by a process of unilateral growth. The Purkinje cells differentiate later. Their axons and those of entering afferent fibers form the deep medullary layer of the cerebellum. D. iv -*• Alar plate / / ^i!k / / \k A -•Marginal layer -. -Nucleus N. Ill ... Root fibers N. Ill Nu. IV. Fig. 321. — Transverse sections through the mesencephalon of a 10.6 mm. embryo. A, through the isthmus and origin of the trochlear nerve; B, through the nucleus of origin of the oculomotor nerve (His). D. IV, decussation of oculomotor nerve; MJ., mantle layer; Nu. IV. nucleus of oculomotor nerve. The cells of the mantle layer may take little part in the development of the cerebellar cor- tex, but give rise to neuroglia cells and fibers and to the internal nuclei. Of these the dentate nucleus may be seen at the end of the third month; later, its cellular layer becomes folded, producing its characteristic convolutions. The fibers arising from its cells form the greater part of the brack ium conjunctivum. (For a detailed account of the development of the cere- bellum see Streeter, in Keibel and Mall, vol. 2, p. 67.) Mesencephalon. — The basal and alar plates can be recognized in this sub- division of the brain and each differentiates into the three primitive layers (Fig. 32 1 V In the basal plate the neuroblasts give rise to the axons of motor nerves, the oculomotor cranial, the trochlear caudal in position (Fig. 321 B). In ad- dition to these nuclei of origin, the nucleus ruber (red nucleus) is developed in 336 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM the basal plates ventral and somewhat cranial to the nucleus of the oculomotor nerve. The origin of the cells forming the red nucleus is not definitely known. The alar plates form the paired superior and inferior colliculi which together constitute the corpora quadrigemina (Fig. 331). The plates thicken and neuro- blasts migrate to their surfaces, forming stratified ganglionic layers comparable to the cortical layers of the cerebellum and the cerebellar nuclei. With the development of the superior and inferior colliculi the cavity of the mesencephalic region decreases in size and becomes the cerebral aqueduct. The mantle layer of the basal plate region is enclosed ventrally and laterally by the fiber tracts which develop in the marginal zone. Ventro-laterally appear the median and lateral lemnisci and ventrally develop later the descending tracts from the cerebral cortex, which together constitute the peduncles of the cerebrum. Poof plate (with chorioid plexus) Alar plate orThalamus Sulcus limitans or ".; -'' ' I S. hypothalamicus Basal plate or Hypothalamus ' Mammillary recess Fig. 322. — Transverse section through the diencephalon of a five weeks' human embryo (His). The Diencephalon. — In the wall of the diencephalon we may recognize laterally the alar and basal plates, dorsally the roof plate and ventrally the floor plate (Fig. 322). The roof plate expands, folds as seen in the figure, and into the folds extend blood capillaries. The roof plate thus forms the ependymal fining of the tela chorioidea of the third ventricle. The vessels and ingrowing mesenchy- mal tissue form the chorioid plexus. Cranially, the tela chorioidea roofs over the median portion of the telencephalon and is folded laterally into the hemispheres as the chorioid plexus of the lateral ventricles. Laterally, the roof plate is attached to the alar plates and at their point of union are developed the ganglia habenulcE. The pineal body or epiphysis is developed caudally as an evagination of the roof plate. Tt appears at the fifth week (Fig. 327) and is well developed by the third month (Fig. 324). Into the thickened wall of the anlage is incorporated a certain amount of mesenchymal tissue and thus the pineal body proper is formed. The THE BRAIN 337 alar plate is greatly thickened and becomes the anlage of the thalamus and metathalamus. The latter, really a part of the thalamus, gives rise to the lateral and median geniculate bodies. The sulcus liypothalamicus (Fig. 323) forms the boundary line between the thalamus (alar plate) and the hypothalamus (basal plate plus the floor plate). This sulcus thus corresponds to the sulcus limitans of the spinal cord and brain stem. The basal plate is comparatively unimportant in the dien- cephalic region as no nuclei of origin for motor nerves are developed here. Sulcus hypothalami Hypothalamus Pallium Mammillary recess Corpus striatum I Injundibulum Optic ridge Fig. 323. — Median sagittal section of the fore- and mid-brain regions of a brain from a 10.2 mm. embryo (after His). In the floor plate the ridge formed by the optic chiasma constitutes the pars optica Jiypot/ialamica. The Hypophysis. — The injundibulum develops as a recess caudal to the pars optica hypothalamica (Figs. 324 and 325). At its extremity is the sac-like anlage of the posterior lobe of the hypophysis or pituitary body. During the fourth week the infundibular anlage comes into contact with Rathke's pouch, the epi- thelial anlage of the anterior lobe of the hypophysis (Fig. 325). The epithelial anlage is at first flattened and soon is detached from its epithelial stalk. Later, it grows laterally and caudally about the anlage of the posterior lobe and during the second month its wall is differentiated into convoluted tubules which obliterate its cavity. The tubules become closed glandular follicles surrounded by a rich 333 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM network of blood-vessels and produce an important internal secretion. Pari passu with the differentiation of the anterior lobe the infundibular anlage of the Pined body epithala, Cerebral peduncle Cerebral aqueduct Mesencephalon "\ {Mid-brain) Spinal cord Central canal Fig. 324. — Median sagittal section of the brain from a fetus of the third month (His from Sobotta's Atlas). Foramen Monro Third ventricle Optic vesicle Lens vesicle Infundibulum HathKe's pocfat- Fig. 325. — Oblique transverse section through the diencephalon and telencephalon of a 10 mm. embryo. X 61. posterior lobe loses its cavity, but the walls of the infundibulum persist as its solid permanent stalk. The lobe enlarges and its cells are differentiated into a THE BRAIN 339 diffuse tissue resembling neuroglia. About the two lobes of the hypophysis the surrounding mesenchyme develops a connective tissue capsule. Caudal to the infundibulum in the floor plate are developed in order the tuber cinereum and the mammillary recess (Figs. 323 and 324). The lateral walls of the latter thicken and give rise to the paired mammillary bodies. The third ventricle lies largely in the dienccphalon and is at first relatively broad. Owing to the thickening of its lateral walls it is compressed until it forms a narrow vertical cleft. In a majority of adults the thalami are approximated, fuse and form the massa intermedia or commissura mollis, which is encircled by the cavity of the ventricle. Mesencephalon Dienccphalon Pallium Mammillary body Hypophysis Optic stalk Lobus olfactorius Fig. 326. — Lateral view of the fore- and mid-brains of a 10.2 mm. embryo (His). The Telencephalon. — This is the most highly differentiated division of the brain (Fig. 326). The primitive structures of the neural tube can no longer be recognized but the telencephalon is regarded as representing greatly expanded alar plates and is therefore essentially a paired structure. Each of the paired out- growths expands cranially, dorsally, and caudally, and eventually overlies the rest of the brain (Figs. 326, 327 and 328). The telencephalon is differentiated into the corpus striatum, rhinencephalon, and pallium (primitive cortex of cerebral hemisphere). The median lamina between the hemispheres lags behind in its development and thus is formed the great longitudinal fissure between the hemi- spheres. The lamina is continuous caudally with the roof plate of the dien- 340 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM cephalon, cranially it becomes the lamina terminalis, the cranial boundary of the third ventricle. Chorioid Plexus of the Lateral Ventricles. — It will be remembered that in the folds of the roof plate of the diencephalon develops the chorioid plexus of the third ventricle. Similarly the thin median wall of the pallium at its junction with the wall of the diencephalon is folded into the lateral ventricle. Into this fold grows a vascular plexus continuous with that of the third ventricle and pro- jects into the lateral ventricle of either side (Figs. 327 and 329). The fold of the pallial wall forms the chorioidal fissure and the vascular plexus is the chorioid Fissura prima Chorioid plexus of lat. ventricle Pallium Pineal body Sup. collicidus Corpus striatum Hippocampus Roof plate Mesencephalon Fig. 327. — The fore-brain and mid-brain of an embryo 13.6 mm. long seen from the dorsal surface. The pallium of the telencephalon is cut away exposing the lateral ventricle (His). plexus of the lateral ventricle. This is a paired structure and with the plexus of the third ventricle forms a T-shaped figure, the stem of the T overlying the third ventricle, its curved arms projecting into the lateral ventricles just caudal to the interventricular foramen. Later, as the pallium extends, the chorioid plexus of the lateral ventricles and the chorioidal fissures are extensively elongated into the temporal lobe and inferior horn of the lateral ventricle (Fig. 330). The interventricular foramen (of Monro) is at first a wide opening (Fig. 325) but is later narrowed to a slit, not by constriction but because its boundaries grow more slowly than the rest of the telencephalon (Fig. 329). THE BRAIN 341 The third ventricle extends some distance into the caudal end of the telen- cephalon and laterally in this region develop the optic vesicles. Into each optic stalk extends the optic recess (Fig. 325). DiencephaAon Mesencephalon 1' all inn: Corpus mammillarc Pars ant. olf. lobe Pars post. olf. lobe Tuber cinereutn Tnfundibulum Optic stalk FlG. 328. — Lateral view of the fore-brain and mid-brain of a 13.6 mm. embryo (His). Lateral ventricle Chor ioid plexus of lateral j ventricle (,] Thalamus — if M Corpus striatum Third ventricle FlG. 329. — Transverse section through the fore-brain of a 16 mm. embryo showing the early develop- ment of the chorioid plexus and fissure (His). The corpus striatum is developed as a thickening in the floor of each cerebral hemisphere. It is already prominent in embryos of five weeks (13.6 mm.) bulg- ing into the lateral ventricle (Figs. 327 and 329). It is in line caudally with the 342 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM falx Fig. 330. — A transverse section through the telencephalon of an 83 mm. embryo (after His). Th, thalamus; cs, corpus striatum; hf, hippocampal fissure; fa, marginal gray seam; fi, edge of white sub- stance. Chorioid fissure Sup. colliculus Inf. colliculus Cerebellum Nucleus caudatus Internal capsule Olfactory lobe Fig. 331. — Lateral view of the brain of a 53 mm. fetus. The greater part of the pallium of the right cerebral hemisphere has been removed, leaving only that covering the lenticular nucleus, and exposing the internal capsule, caudate nucleus and hippocampus (His). THE BRAIN 343 thalamus of the diencephalon and in development is closely connected with it, although the thalamus forms always a separate structure. The corpus striatum elongates as the cerebral hemisphere lengthens, its caudal portion curving around to the tip of the inferior horn of the lateral ventricle and forming the slender tail of the caudate nucleus (Fig. 331). The thickening of the corpus striatum is due to the active proliferation of cells in the ependymal layer which form a prominent mass of mantle layer cells. Nerve fibers to and from the thalamus to the cere- bral cortex course through the corpus striatum as laminae which are arranged in. Anterior horn Nucleus caudatus Interventricular foramen- Third ventricle Chorioid plexus of lat. ventricle Posterior horn Lenticular nucleus Ant. columns of fornix Internal capsule Thalamus Hippocampus Fig. 332. — Horizontal (coronal) section through the fore-brain of a 16 cm. fetus (His). the form of a wide V, open laterally, when seen in horizontal sections. This V- shaped tract of white fibers is the internal capsule, the cranial limb of which partly separates the corpus striatum into the caudate and lenticular nuclei (Fig. 33 2) . The caudal limb of the capsule extends between the lenticular nucleus and the thalamus. The thalamus and corpus striatum are separated by a deep groove until the end of the third month (Fig. 329). As the structures enlarge, the groove between them disappears and they form one continuous mass (Fig. 332). According to some investigators, there is direct fusion between the two. 344 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM The Rhinencephalon or Olfactory Apparatus. — This is divided into a basal portion and a pallial portion. The basal portion consists (i) in a ventral and cranial evagination (pars anterior) formed mesial to the corpus striatum, which is the anlage of the olfactory lobe and stalk (Fig. 328). This receives the olfactory fibers and its cells give rise to olfactory tracts. The tubular stalk connecting the olfactory lobe with the cerebrum loses its lumen. (2) Caudal to the anlage of the olfactory lobe a thickening of the brain wall develops (pars posterior) which ex- tends mesially along the lamina terminalis and laterally becomes continuous with the tip of the temporal lobe (Figs. 323 and 328). This thickening consti- tutes the anterior perforated space and the parolfactory area of the adult brain. The pallial portion of the rhinencephalon is termed the archipallium because it forms the primitive wall of the cerebrum. It forms a median strip of the pallial wall curving along the dorsal edge of the chorioidal fissure from the anterior perforated space around to the tip of the temporal lobe, where it is again con- nected with the basal portion of the rhinencephalon. The archipallium differ- entiates into the hippocampus, a portion of the gyrus hippocampi and into the gyrus dentatus. It resembles the rest of the cerebral cortex in the arrangement of its cells. The infolding of the hippocampus produces the hippocampal fissure. The Commissures of the Telencephalon. — The important commissures are the corpus callosum, fornix and anterior commissure. The first is the great trans- verse commissure of the neopallium or cerebral cortex, while the fornix and an- terior commissure are connected with the archipallium of the rhinencephalon. The commissures develop in relation to the lamina terminalis, crossing partly in its wall and partly in fused adjacent portions of the median pallial walls. Owing to the fusion of the pallial walls dorsal and cranial to it, the lamina terminalis thickens rapidly in stages between 80 and 150 mm. (Streeterin Keibeland Mall, vol. 2). "It [the lamina terminalis] is distended dorsal ward and antero-lateral- ward through the growth of the corpus callosum, the shape of which is determined by the expanding pallium." Between the curve of the corpus callosum and the fornix a space is formed, the fifth ventricle, or space of the septum pellucidum (Fig. 333 A, B). This space is bounded laterally by a portion of the median pallial wall which remains thin and membranous, and constitutes the septum pellucidum of the adult. The fornix takes its origin early, chiefly from cells in the hippocampus. The fibers course along the chorioidal side of the hippocampus cranially, pass- ing dorsal to the foramen of Monro (Fig. 333 A). In the cranial portion of the lamina terminalis fibers are given off and received from the basal portion of the THE BRAIN 345 rhinencephalon. In this region libers crossing the midline form the hippocampal commissure. Other libers, as the anterior pillars of the fornix, curve ventrally and end in the mammillary body of the hypothalamus. The commissure of the hippocampus, originally cranial in position, is carried caudal ward with the caudal extension of the corpus callosum (Fig. 333 B). Corpus callosum Hippocampal commissure Anterior commissure Arit. pillars of form* Body oF fornix Chorioid fissure Thalamus Body of fornix Jeptum pellucidum Hippocampal commissure Corpus callosum Ant. commissure ' ■Thalamus Yirit. pillar of fornix Fig. 333. — Two stages in the development of the cerebral commissure. A, Median view of the right hemisphere of an 83 mm. embryo; B, the same of a 120 mm. embryo. (Based on reconstructions by His and Streeter). The fibers of the anterior commissure cross in the lamina terminalis ventral to the hippocampal commissure. They arise as a cranial and a caudal division. The fibers of the former take their origin from the olfactory stalk and the adjacent cortex. The fibers of the caudal division pass ventrally about the corpus striatum between it and the cortex, and may be derived from one or both of these regions. The corpus callosum appears cranial and dorsal to the hippocampal com- 346 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM missure in the roof of the thickened lamina terminalis (Fig. 333 A). Its fibers arise from neuroblasts in the wall of the neopallium (cerebral cortex) and by them nearly all regions of one hemisphere are associated with corresponding regions of the other. With the expansion of the pallium the corpus callosum is extended cranially and caudally by the development of interstitial fibers. The fibers first found in the corpus callosum arise in the median wall of the hemispheres. In Lateral fissure Lobus frontalis Lobus temporalis Pons Lobus parietalis £ T.nhiis occipitalis Cerebellum w ■ ■' Myelenceph . alon Spinal cord Fig. 334. — Lateral view of the brain of a 90 mm. embryo (His). fetuses of 150 mm. (five months) this great commissure is a conspicuous structure and shows the form which is characteristic of the adult (Fig. 333 B). The Form of the Cerebral Hemispheres. — When the telencephalon expands cranially, caudally and at the same time ventrally. four lobes may be distin- guished (1) a cranial frontal lobe; (2) a dorsal parietal lobe; (3) a caudal occipi- tal lobe, and (4) a ventro-lateral temporal lobe (Fig. 334). The ventricle extends into these regions and in each forms respectively the anterior horn, the body, the posterior horn and the inferior horn of the lateral ventricle. The surface extent THE HRAIN 347 ■of the cerebral wall, the thin gray cortex, increases more rapidly than the un- derlying white medullary layer. As a result the cortex is folded, producing i on- volutions between which are depressions, the sulci and fissures. The i borioidal fissure is formed, as we have seen, by the ingrowth of the chorioid plexus. During the third month the hippocampal fissure develops as a curved infolding along the median wall of the temporal lobe. The infolded cortex forms the hippocampus. The lateral fissure (of Sylvius) makes its appearance also in the third month, but its development is not completed until after birth. The cortex overlying the corpus striatum laterally develops more slowly than the surrounding Occipital lobe of cerebrum Corpora quadrigemina Impression of thalamus Hemisphere of \___^EL- ^\ I I ^ Temporal lobe cerebellum Lateral recess of Vermis cerebelli — — "" ^'^^ V j^f^F" ventricle 4 Fasciculus gracilis Medulla oblongata J' u;- 335- — Posterior view of the brain from a 100 mm. embryo (Kollmann's Handatlas). areas and is thus gradually overgrown by folds of the parietal and frontal lobes (fronto-parietal operculum) and of the temporal lobe (temporal operculum). The area thus overgrown is the i>isula (island of Reil) and the depression so formed is the lateral fissure (of Sylvius). Later, frontal and orbital opercula are developed ventro-laterally from the frontal lobe (Fig. 337). These are not approximated over the insula until after birth. The frontal operculum is included between the anterior limbs of the Sylvian fissure and the extent of its development, which is variable, determines the form of these limbs. In fetuses of six to seven months four other depressions appear which later form important landmarks in the cerebral topography. These are: (1) the 548 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM central sulcus, or fissure of Rolando, which forms the dorso-lateral boundary fine between the frontal and parietal lobes (Fig. 337); (2) the parieto-occipital fissure, which, on the median wall of the cerebrum, is the fine of separation between the Median olfactory gyrus Middle olfactory gyrus Diagonal gyrus Cerebellum Insula Lat. olfactory gyrus Gyrus ambiens Gyrus semilunaris Oliva Fig. 336. — Ventral view of the brain of a 100 mm. embryo showing development of the rhinencephalon (Kollmann). Sulcus postcentralis Sulcus centralis Lobus parielalis superior Supra- marginal \ and an- gular gyri Post. ramus of lateral fissure Middle temporal sulcus Occipital pole Inferior frontal sulcus Ascend- Temporal lobe Superior temporal gyrus Middle temporal gyrus FlG. 337.-4-Lateral view of the right cerebral hemisphere, from a seven months' fetus (Kollmann). THE BRAIN 349 occipital and parietal lobes (Fig. 338); (3) the odcarinc fissure which includes between it and the parieto-occipital fissure the cuneus and marks the position oi the visual area of the cerebrum; (4) the collateral fissure on the ventral surface of the temporal lobe, which produces the inward bulging on the floor of the posterior horn of the ventricle known as the collateral eminence. The calcarine fissure also affects the internal wall of the ventricle, causing the convexity termed the calcar avis. Simultaneously with the development of the collateral fissure appear other shallower depressions known as sulci. These have a definite arrangement and Sparc of septum pellu- cidum Rostral lamina Parol- factory area Corp. callosum singuli Side. corp. callosi Splenium ial fissure Cuneus Olfactory lobe Fissura rliinica Optic nerve Temporal lobe Fig. 338. — Median surface of the right cerebral hemisphere from a seven months' fetus (Kollmann). with the fissures mark off from each other the various functional areas of the cere- brum. The surface convolutions between the depressions constitute the gyri and lobules of the adult cerebrum. Histogenesis of the Cerebral Cortex. — The three primitive zones typical of the neural tube are differentiated in the wall of the pallium: the ependymal, mantle and marginal layers. During the first two months the cortex remains thin and differentiation is slow. At eight weeks neuroblasts migrate from the ependymal and mantle zones into the marginal zone and give rise to layers of pyramidal cells typical of the cerebrum. The differentiation of these layers is most active during the third and fourth months. From the fourth month on the 350 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM cerebral wall thickens rapidly owing to the development of (i) the fibers from the thalamus and corpus striatum; (2) of endogenous fibers from the neuroblasts of the cortex. The fibers form a white inner medullary layer surrounded by the gray cortex. As the cerebral wall increases in thickness the size of the lateral ventricle becomes relatively less, its lateral diameter especially being decreased. For the special differentiation of the cerebral cortex in different regions the student is- referred to text-books on neurology. CHAPTER XII THE PERIPHERAL NERVOUS SYSTEM The nerves, ganglia and sense organs constitute the peripheral nervous system. The peripheral nerves consist of bundles of medullated and non-medul- lated nerve libers and aggregations of nerve cells, the ganglia. The fibers are of two types: Afferent fibers which carry sensory impulses to the central nervous system, and efferent fibers, which carry effective impulses away from the nervous centers. The peripheral efferent fibers of both brain and spinal cord take their origin from neuroblasts of the basal plate. Typically they emerge ventro-later- ally from the neural tube. Those arising from the spinal cord take origin in the mantle layer, converge and form the ventral roots of the spinal neroes. The efferent fibers of the brain take origin from more definite nuclei and constitute the motor or effector portions of the cerebral nerves. The peripheral afferent fibers take origin from nerve cells which lie outside the neural tube. Those sensory nerve cells related to the spinal cord and to the brain stem caudal to the otic vesicle are derived from the ganglion crest, the origin of which has been described (Chapter X,p. 314). K-Xigang. crest. xi fibres. bridge. Fig. 339. — Reconstruction of an embryo of 4 mm. showing the development of the cerebrospinal nerves. Ci., 2., etc., cervical spinal nerves (Streeter). A. SPINAL NERVES The spinal nerves are segment- ally arranged and each consists of dorsal and ventral roots, spinal ganglion and nerve trunks. In embryos of 4 mm. the ventral roots are already developing as outgrowths of neuroblasts in the mantle layer of the spinal cord (Fig. 339). The spinal ganglia are represented as enlargements along the ganglion crest and are connected by a bridge of cells. In 7 mm. embryos (four weeks old) the cells of the spinal ganglia begin to develop centrally directed processes which enter the marginal zone of the cord as the dorsal root fibers (Fig. 340). These fibers course in, and eventually form the 351 352 THE PERIPHERAL NERVOUS SYSTEM greater part of, the dorsal funiculi. Peripheral processes of the ganglion cells join the ventral root fibers in the trunk of the nerve (Fig. 342). At 10 mm. (Fig. 341) the dorsal root fibers have elongated and the cellular bridges of the ganglion crest between the spinal ganglion have begun to disappear. In transverse sec- tions at this stage (Figs. 307 and 342) the different parts of a spinal nerve may be seen. The trunk of the nerve just ventral to the union of the dorsal and ventral Ophthal. dio. Sup. max. div, N.masticatorius Inf. max. div. xi gang, crest. Fig. 340. — Reconstruction of a 6.9 mm. embryo showing the development of the dorsal root fibers from the spinal and cerebral ganglia (Streeter). X 16.7. roots gives off laterally the dorsal, or posterior ramus, the fibers of which supply the dorsal muscles. The ventral ramus continuing gives off mesially the ramus communicans to the sympathetic ganglion and divides into the lateral and ventral (anterior) terminal rami. The efferent fibers of these rami supply the muscles of the lateral and ventral body wall and the afferent fibers end in the integument of the same regions. SPINAL NERVES 353 At the points where the anterior and lateral terminal rami arise, connecting loops may extend from one spinal nerve to another. Thus in the cervical region superficial and deep nerve plexuses are formed. The deep cervical plexus forms the ansa hypoglossi and the phrenic nerve. Vesicula auditiva Gang, acusticum Cans, semilunare n.V Cerebellum Gang, radicls n IX :Gang. petrosuni Gang, radicls n.X N. frontalis " Gang. Frorlep N. hypoglossus I.C " -*-N.XI. ~ Gang, nodos. — N. desc. cerv. --■Kami hyoid. (Ansa hypoglossi) -— N. Musculocutan. ;N. axillaris N. phrenicus ■~N. medianus -- N. radialis ™N. ulnaris ITh- I Co N. tibialis N. peroneus Tubus digest. N. femoral j N. obturator R. posterior It. terminalis lateralis R. terminalis anterior Mesonephros Nn. ilioing. et hypogaatr. Fig. 341. — Reconstruction of the nervous system of a 10 mm. embryo (Streeter). X 12. The Brachial and Lumbo-sacral Plexuses.- -The nerves supplying the arm and leg also unite to form plexuses. In embryos of 10 mm. (Fig. 341) the trunks of the last four cervical nerves and of the first thoracic are united to form a flat- tened plate, the anlage of the bracliial plexus. From this plate nervous cords 23 354 THE PERIPHERAL NERVOUS SYSTEM extend into the intermuscular spaces and end in the premuscle masses. The developing skeleton of the shoulder splits the brachial plexus into dorsal and ventral laminae from which the various nerves to the arm and shoulder arise. In 10 mm. embryos the lumbar and sacral nerves which supply the leg unite in a plate-like structure, the anlage of the lumbosacral plexus (Fig. 341). The plate is divided by the skeletal elements of the pelvis and femur into two lateral and two median trunks. Of the cranial pair the lateral becomes the femoral nerve; the median, the obturator nerve. The caudal pair constitute the sciatic nerve; the lateral trunk is the peroneal nerve, and the median trunk is the tibial. Save for the neurones from the special sense organs (nose, eye and ear) which Dorsal not Somah neurone Visceral sensory neurone. Marginal layer Ependymal laye. Mantle layer Lot. Term-. a 1 vi 5/ on Ventral terminal division of spinal nerve Ramus commumcans Sympathetic ganglion Fig. 342. — Transverse section of a 10 mm. embryo showing the spinal cord, spinal nerves and their function nervous components. Diagrammatic. form a special sensory group, the neurones of the peripheral nerves, both spinal and cerebral, fall into four function groups (Fig. 342). (1) Somatic afferent, or general sensory, with fibers ending in the integument of the body wall. (2) Visceral afferent, or sensory, with fibers ending in the walls of the viscera. (3) Somatic efferent, or motor, with fibers ending on voluntary muscle fibers. (4) Visceral efferent, or motor, fa) with fibers ending about sympathetic ganglion cells, which in turn control the smooth muscle fibers of the viscera and blood-vessels fspinal nerves) ; or fb) with fibers ending directly on visceral muscle THE CEREBRAL NERVES 355 fibers (mixed cerebral nerves). The relation of the sympathetic system to the central nervous system is described on page 364. B. THE CEREBRAL NERVES The cerebral nerves of the human brain are twelve in number. They differ from the spinal nerves: (1) in that they are not segmentally arranged, and (2) in that they do not all contain the same types of nervous components. Classed according to the functions of their neurones they fall into three groups. Special Somatic Somatic Motor Visceral Sensory Sknsory or Effector and Motor I. Olfactory. III. Oculomotor. V. Trigeminal. II. Optic. IV. Trochlear. VII. Facial. VIII. Acoustic. VI. Abducens. IX. Glossopharyngeal. XII. Hypoglossal. X. Vagus complex including XI. Spinal Accessory. It will be seen (1) that the nerves of the first group are purely sensory, corresponding to the general somatic afferent neurones of the spinal nerves; (2) that the nerves of the somatic motor group are purely motor and correspond to the somatic efferent or motor neurones of the spinal nerves; (3) that the nerves of the third group are of mixed function and correspond to the visceral components of the spinal nerves. I. The Special Somatic Sensory Nerves 1. The Olfactory Nerve though purely sensory has no ganglion. Its nerve cells lie at first in the olfactory epithelium of the nose and are of the bipolar type. From these cells peripheral processes develop and end directly at the surface of the olfactory epithelium. Central processes grow into contact with the olfactory lobe and form the strands of the olfactory nerve. They end in the olfactory bulb about the peculiar mitral cells. Some of the olfactory cells migrate from epithelium along with the developing nerve fibers, and may be found as bipolar cells along the course of the nerve. The olfactory nerve fibers are peculiar in that they remain non-medullated. When the ethmoidal bone of the cranium is developed its cartilage is formed around the strands of the olfactory nerve, which thus in the adult penetrate the cribriform plate of the ethmoid. Nerve fibers which pass from the epithelium of the organ of Jacobson also end in the olfactory bulb. The organ of Jacobson is a vestigial sense organ and its nerve is rudimentary. For the development of the olfactory organ see p. 369. 356 THE PERIPHERAL NERVOUS SYSTEM 2. The Optic Nerve is formed by fibers which take their origin from neuro- blasts in the nervous layer of the retina. The retina is differentiated from the wall of the fore-brain and remains attached to it by the optic stalk (Fig. 325). The neuroblasts from which the optic nerve fibers develop constitute the gang- lion cell layer of the retina (Fig. 362). During the sixth and seventh weeks these cells give rise to central processes which form a nerve fiber layer on the inner side of the retina. The optic fibers converge to the optic stalk and grow through its wall back to the brain. The cells of the optic stalk are converted into a neu- roglia framework and the cavity .is obliterated. In the floor of the fore-brain at a point which forms the boundary line between telencephalon and dienceph- alon, the fibers from the median half of each retina cross to the opposite side (decussate), and this crossing constitutes the optic chiasma (from Greek letter X or "chi"). The decussation of the optic fibers takes place about the end of the sec- ond month. The crossed and uncrossed fibers constitute the optic tract which rounds the cerebral peduncles laterally and dorsally (Fig. 336). Eventually, the optic fibers end in the lateral geniculate body, thalamus and superior colliculus. 8. The Auditory Nerve, or N. Acusticus, is formed by fibers which originate from the cells of the acoustic ganglion. The origin of these cells is unknown, though they appear in 4 mm. embryos just cranial to the otic vesicle (Fig. 340). The cells become bipolar, central processes uniting the ganglion to the tuberculum acusticum of the myelencephalon and peripheral fibers connecting it with the wall of the otocyst. The acoustic ganglion is differentiated into the vestibular and spiral ganglia (Fig. 343). Its development has been studied by Streeter (Amer. Jour. Anat., vol. 6). The ganglion elongates and is subdivided into superior and inferior portions in 7 mm. embryos. The superior part supplies nerves to the utriculus and to the anterior and lateral semicircular canals. It forms part of the vestibular ganglion of the adult. The inferior portion supplies nerves to the sacculus and to the ampulla of the posterior semicircular canal and this portion of it with the pars superior constitutes the vestibular ganglion. The greater part of the pars inferior is, however, differentiated into the spiral ganglion, the peripheral fibers of which innervate the hair cells of the spiral organ (of Corti) in the cochlea. The spiral ganglion appears in 9 mm. embryos and conforms to the spiral turns of the cochlea, hence its name. Its central nerve fibers form the cochlear division of the acoustic nerve. This is distinctly separated from the central fibers of the vestibular ganglion which constitute the vestibular division of the acoustic nerve, the fibers of which are not auditory in function. The pars inferior of the vestibular ganglion becomes closely connected with the n. coch- THE CEREBRAL NERVES 357 -n.vestib.--- 4mm. /mm. re -pars sup. — pars inf: pars sup. ,pars sup. nar^.'nf ^ ,X^rn-v Pigment lay' of retina Nervous layer of retina Fig. 357. — A transverse section through the optic cup, stalk and lens of a 10 mm. human embryo. X 100. gives rise to the sclerotic layer, to the chorioid layer and to the anterior layers of the ciliary body and iris. 376 THE PERIPHERAL NERVOUS SYSTEM Both the inner and outer layers of the optic cup are continued into the optic stalk, as seen in Fig. 357. This is due to the invagination of the ventral wall of the optic stalk and the formation in it of the chorioid fissure when the optic vesicle is transformed into the optic cup (Fig. 356). Into the chorioid fissure grows the central artery of the retina, and carries with it into the posterior cavity of the eye a small amount of mesenchyme, as seen in the eye of a 12 mm. embryo (Fig. 358). Branches from this vessel extend to the posterior surface of the lens and supply it with nutriment for its growth. At a later stage the chorioid fissure closes, so that the distal rim of the optic cup forms a complete circle. 1 Ectoderm I Epithet 7 at laiyeroflens ^lament Layer of the retina. Nervous layer of retina. Chorioid fissure Central artery Vitreous body Layer of lens fibers Mesenchyme Fig. 358. — Transverse section passing through the optic cup at the level of the chorioid fissure. The central artery of the retina is seen entering the fissure and sending a branch to the proximal surface of the lens; from a 12.5 mm. embryo. X 105. If the chorioid fissure fails to close, the optic cup remains open at one point and this results in the defective development of the iris, ciliary body and chorioid layer. Such defects are known as coloboma. It was formerly supposed that the development of the lens vesicle caused the formation of the optic cup by pushing in its distal wall. It has been shown by W. H. Lewis that this is not the case, for if an anlage of the optic vesicle from an amphibian embryo is transplanted to some other part of the embryo, it will not only develop into an optic cup, but the ectoderm over it will differentiate a lens vesicle. The lens vesicle in its early development from the ectoderm has been de- scribed. Its proximal wall is much thickened in 10 mm. embryos and these cells form the lens fibers (Fig. 357). A few cells early separated off from the wall of DEVELOPMENT OF THE SENSE ORGANS 377 the lens pit are enclosed in the vesicle and have degenerated in 12.5 mm. embryos (Fig. 358). At this stage the lens fibers of the proximal wall are longer and this layer will soon obliterate the cavity of the vesicle, as in embryos of 15 to 17 mm. (Fig. 359). The cells of the distal layer remain of a low columnar type and con- stitute the epithelial layer of the lens. When the lens fibers attain a length of 0.18 mm. they cease forming new fibers by cell division. New fibers thereafter arise from the cells of the epithelial layer at its line of union with the lens fibers. The nuclei are arranged in a layer convex toward the outer surface of the eye and later degenerate, the degencra- .EpitheliaL layer Capsule Vascular •^ membrane Lens fibers tion beginning centrally. The structureless capsule of the lens is probably derived from the lens cells. Proximal and distal lens sutures are formed when the longer peripheral fibers over- lap the ends of the shorter cen- tral fibers. These are later trans- formed into "lens-stars" (Fig. 360). The lens, at first some- what triangular in cross section, becomes nearly spherical at three months (Fig. 360). The origin of the vitreous body has been in doubt, one view deriving it from the mesen- chyma which enters the optic cup through the chorioid fissures and about the edge of lens, another view hold- ing that it arises from cytoplasmic processes of cells in the retinal layer. It is certain that the vitreous tissue is formed before mesenchyma is present in the cavity of the optic cup. Szily regards this primitive vitreous body as a derivative of both retinal and lens cells, it forming a non-cellular network of cytoplasmic processes which are continuous with the cells of the lens and retina. With the ingrowth of the central artery of the retina, from which the artery of the lens passes to and branches on the proximal surface of the lens, a certain amount of mesenchymal tissue invades the optic cup and this tissue probably con- tributes to the development of the vitreous body (Fig. 358). The vitreous body may therefore be regarded as a derivative both of the ectoderm and of the mesoderm. ' Ectoderm Fig. 359. — Section through the lens and corneal ecto- derm of a 16 mm. pig embryo. X 140. The mesenchyma accompanying the vessels to the proximal surface of the 378 THE PERIPHERAL NERVOUS SYSTEM lens, and that on its distal surface, give rise to the vascular capsule of the lens (Fig. 358). On the distal surface of the lens this is supplied by branches of the anterior ciliary arteries and is known as the pupillary membrane. The vessels in this disappear and it degenerates just before birth. The artery of the lens also degenerates, its wall persisting as the transparent hyaloid canal. Fibrillar ex- tending in the vitreous humor from the pars ciliata of the retinal layer to the cap- sule of the lens persist as the zonula ciliata or suspensory ligament of the lens. Anterior epithelium of cornea \ ■■r/f*1: ■'./.■'■<■: Raphe between palpebral Posterior epithelium of cornea \\ Epithelium of lens Pars iridica retina Pigment layer of retina Pars optica retina Lens fibers Lens capsule Vitreous body Xarw.ne.H;l\. Fig. 360. — Section through the distal half of the eyeball and through the eyelids of a 65 mm. embryo. X35- Differentiation of the Optic Cup. — We have seen that of the two layers of the optic cup the outer becomes the pigment layer of the retina. Pigment granules appear in its cells in embryos of 7 to 9 mm. and the pigmentation of this layer is marked in 12 mm. embryos (Fig. 358). The inner layer of the optic cup is the retinal layer and is subdivided into a distal zone, the pars cceca, which is non-nervous, and into the pars optica, or the DEVELOPMENT OF THE SENSE ORGANS 379 nervous retina proper. The line of demarcation between the pars optica and the pars ca?ca is a serrated circle, the ora scrrala. The Mind portion of the retinal layer, the pars caeca, with the development of the ciliary bodies is differentiated into a pars ciliaris and pars iridis retina. The former, with a corresponding zone of the pigment layer, covers the ciliary bodies. The pars iridis forms the proximal layer of the iris and blends intimately with the pigment layer in this region, its cells also becoming heavily pigmented (Fig. 360). The pars 0 plica, or nervous portion of the retina, begins to differentiate prox- imally, the differentiation extending distally. An outer cellular layer and an inner fibrous layer may be distinguished in 12 mm. embryos (Fig. 358). These correspond to the cellular layer (ependymal and mantle zones) and marginal Cone cell Hod cell Hod cell Fiber o f Mueller. Arnaerin cell Ganglion cell Optic - fibers Fig. 361. — Section of the nervous layer of the retina from a 65 mm. embryo figure shows diagrammatically the cellular elements of the retina according to Cajal External limit- ing membrane Layer of rod and cone cells Ganglionic layer rous layer Internal limit- ing membrane The left portion of the X 440. layer of the neural tube. In embryos of 65 mm. the retina shows three layers, large ganglion cells having migrated in from the outer cellular layer of rods and cones (Fig. 361). In a fetus of the seventh month all the layers of the adult retina may be recognized (Fig. 362). As in the wall of the neural tube, there are differentiated in the retina supporting tissue and nervous tissue. The supporting elements, or fibers of Mueller, resemble ependymal cells and are radially arranged (Figs. 361 and 362). Their terminations form internal and external limiting membranes. The neuroblasts of the retina differentiate into an outer layer of rod and cone cells, the visual cells of the retina (Fig. 362). Internal to this layer are layers of bipolar and multipolar cells. The inner layer of multipolar cells constitutes the ganglion cell layer. Axons from these cells form the inner nerve fiber layer 38o THE PERIPHERAL NERVOUS SYSTEM of optic fibers. These converge to the optic stalk and grow back in its wall to the brain. The cells of the optic stalk are converted into neuroglia supporting tissue and the cavity of the stalk is gradually obliterated. The optic stalk is thus transformed into the optic nerve (see p. 356). The Sclerotic and Chorioid Layers, and their Derivatives. — After the mes- enchyme grows in between the ectoderm and the lens (Fig. 358) the lens and op- tic cup are surrounded by a condensed layer of mesenchymal tissue, which gives rise to the supporting and vascular layers of the eyeball. By condensation and differentiation of its outer layers, a mmm&sSM mm dense layer of white fibrous tissue is developed, which forms the sclerotic layer. This corresponds to the dura mater of the brain. In the mesen- chyme of 25 mm. embryos a cavity appears distally, which separates the condensed layer of mesenchyme continuous with the sclerotic from the vascular capsule of the lens (Fig. 360). This cavity is the anterior chamber of the eye and separates the anlage of the cornea from the lens capsule. An inner layer of mesenchyme, between the anlage of the sclerotic and the pigment layer of the retina, becomes highly vascular during the sixth month. Its cells become stellate in form and pigmented so that the tissue is loose and reticulate. This vascular tissue constitutes the chorioid layer in which course the chief vessels of the eye. The chorioid layer corresponds to the pia mater of the brain. Distal to the ora serrata of the retinal layer the chorioid is differen- tiated: (1) Into the vascular folds of the ciliary bodies; (2) into the smooth fibers of the ciliary muscle; (3) into the stroma of the iris. The proximal pig- mented layers of the iris are derived from the pars iridis retina; and from a cor- responding zone of the pigment layer. Of these the pigment layer cells give rise to the sphincter and dilator muscles of the iris. These smooth muscle fibers are thus of ectodermal origin. Pigment layer Bods and Cones Outer nuclear layer Outer reticular I aver Inner nuclear layer ntier reticular layer Ganglion cell layer I — Nerve fiber layer Tibers of Mueller Infernal limiting membrane Fig. 362. — Section through the pars optica of the retina from a seven months' fetus. X 440. DEVELOPMENT OF THE SENSE ORGANS 381 The Eyelids appear as folds of the integument in 20 mm. embryos. The lids come together and the epidermis at their edges is fused in 33 mm. embryos (Fig. 360). Later, when the epidermal cells are cornified separation of the eyelids takes place. The epidermis of the eyelids forms a continuous layer on their inner surfaces as the conjunctiva, which in turn is continuous with the anterior epithelium of the cornea. The Eyelashes, or cilia, develop like ordinary hairs and are provided with small sebaceous glands. In the tarsus, or dense connective tissue layer of the eyelids, which lies close to the conjunctival epithelium, there are developed about 30 tarsal (Meibomian) glands. These arise as ingrowths of the epithelium at the edges of the eyelids, while the latter are still fused. The Lachrymal Glands appear in embryos of 22 to 26 mm., according to Keibel and Elze. They arise as five or six ingrowths of the conjunctiva, dorsally and near the external angle of the eye. The anlages are at first knob-like and rapidly lengthen into solid epithelial cords. They begin to branch in 30 mm. embryos. At stages between 40 and 60 mm. additional anlages appear which also branch. In 3S mm. embryos a septum begins to divide the gland into orbital and palpebral por- tions. This septum is complete at 60 mm., the five or six anlages first developed constituting the orbital part. Lumina appear in the glandular cords in embryos of 50 mm. by the degener- ation of the central cells. Accessory lachrymal glands appear in 30 cm. fetuses. The lachrymal gland is not fully developed at birth, being only one-third the size of the adult gland. In old age marked degeneration occurs. The Naso-lachrymal Duct is formed as a solid epithelial outgrowth from the conjunctiva of the lachrymo-nasal groove at the internal angle of the eye. The anlage grows down through the mesenchyme to the nasal cavity. The lachrymal canals are budded out from the solid anlage of the lachrymal duct and become connected secondarily with the inner margins of the palpebral. The primitive connection of the lachrymal duct with the conjunctiva is lost. The anlage of the duct appears in 10 mm. embryos and in 25 mm. embryos has not yet reached the nasal cavity. A lumen appears in the duct during the third month. The Development of the Ear The human ear consists of a sound-conducting apparatus and of a receptive organ. The conveyance of sound is the function of the external and middle cars. The end organ proper is the inner car with the auditory apparatus residing in the cochlear duct. Besides this acoustic function the labyrinthine portion of the inner ear acts as an organ of equilibrium. ;82 THE PERIPHERAL NERVOUS SYSTEM The Inner Ear. — The epithelium of the internal ear is derived from the ecto- derm. Its first anlage appears in embryos of 2 mm. as thickened ectodermal plates, the auditory placodes (Fig. 363 A). These are developed dorsal to the second branchial grooves at the sides of the hind-brain opposite the fifth neuro- meres (Fig. 364) . The placodes are invaginated to form hollow vesicles which close Otic vesicle Auditory ganglion Auditory placode Hind-brain Otic vesicle Fig. 363. — Two stages in the early development of the internal ear. A, section through the head of a 2 mm. embryo showing the auditory placode and otic vesicles; B, section through the hind-brain and otic vesicles of an early human embryo (Keibel and Elze). Ectoderm Wall of hind bra-m Near, t Neur.5 Fig. 364. — Four sections through the right otic vesicle of an early human embryo, r. e., endo- lymphatic recess, the anlage of the endolymph duct and sac; 0. v., otic vesicle; Neur. 4, Near. 5, neuro- meres four and five of the myelencephalon (Keibel and Elze). About 30 diameter. in embryos of 2.5 to 3 mm., but remain attached to the ectoderm for some time (Fig. 363 B). The auditory vesicle or otocyst when closed and detached is nearly spherical, but at the point where it was attached to the ectoderm a recess is formed. The point of origin of this recess is shifted later from a dorsal to a mesial position and it constitutes the ductus endolymphaticus (Figs. 365 and 366 a). The endolymph DEVELOPMENT OF THE SENSE ORGANS 383 Wa.ll of muelnncepha/on Cndolymph Juct \/estibult far an Icy e duct corresponds to that of selachian fishes, which remains open to the exterior. In man, its dorsal extremity is closed and dilated to form the endolymphatic sac (Fig. 366 e). The differentiation of the auditory vesicle has been described by His, Jr. and more recently by Streeter (Amer. Jour. Anat., vol. 6, 1906). In an em- bryo of about 7 mm. the vesicle has elongated, its narrower ventral process con- stituting the anlage of the cochlear duel (Fig. 366 a). The wider dorsal portion of the otocyst is the vestibular anlage and it shows indications dorsally of the de- veloping semicircular canals. These are formed in 11 mm. embryos as two pouches, the anterior and posterior canals from a single pouch at the dorsal border of the otocyst, the external canal later from a lateral outpocketing (Fig. 366 d). The mar- gins of these pouches are thickened, but elsewhere their walls are flattened together and fused to form an epi- thelial plate. Three such epithelial plates are produced and internally about the periphery of each plate canals are left communicating with the cavity of the vestibule. Soon the epithelial plates are resorbed, leaving spaces between the semicircular epi- thelial canals and the vestibule (Fig. 366 c). Dorsally a notch separates the anterior and posterior canals. Of these the anterior is completed first, next the posterior canal. The external canal is the last to develop. In a 20 mm. embryo (Fig. 366 e) the three canals are present and the coch- lear duct has begun to coil like a snail shell. It will be seen that the anterior and posterior canals have a common opening dorsally into the vestibule, while their opposite ends and the cranial end of the external canal are dilated to form ampulla. In each ampulla is located an end organ, the crista acustica, which will be referred to later. By a constriction of its wall the vestibule is differentiated into a dorsal portion, the utriculus, to which are attached the semicircular canals, and a ventral portion, the sacculus, which is connected with the cochlear duct (Fig. 366 e,f). At 30 mm. the adult condition is more nearly attained. The Cochlear anlage ■'•V V; *'' . • •* • *• • ■<*l«\*v« Fig. 365. — Transverse section through the right half of the hind-brain and through the right otic vesicle showing the position of the endolym- phatic duct. From an embryo 6.9 mm. long (His). 384 THE PERIPHERAL NERVOUS SYSTEM sacculus and utriculus are more completely separated, the canals are relatively longer, their ampullae more prominent and the cochlear duct is coiled about two and a half turns (Fig. 366/). In the adult, the sacculus and utriculus become completely separated from each other, but each remains attached to the endo- lvmph duct by a slender canal which represents the prolongation of their re- spective walls. Similarly, the cochlear duct is constricted from the sacculus, the basal end of the former becomes a blind process and a canal, the ductus reuniens, connects the cochlear duct with the sacculus. The epithelium of the labyrinth at first is composed of a single layer of low columnar cells. At an early stage, fibers from the acoustic nerve grow between the epithelial cells in certain regions and it becomes modified to produce special sense organs. These end organs are the crista, acusticce in the ampullae of the semicircular canals; the macula acusticce in the utriculus and sacculus, and the spiral organ (of Corti) in the cochlear duct. The cristas and maculae are static organs, or sense organs for equilibrium. In each ampulla transverse to the long axis of the canal the epithelium and under- lying tissue form a curved ridge, the crista. The cells of the epithelium are differentiated: (1) Into sense cells with bristle-like hairs at their ends, and (2) into supporting cells. About the bases of the sensory cells branch nerve fibers from the vestibular division of the acoustic nerve. The maculae resemble the cristae in their development save that larger areas of the epithelium are differ- entiated into cushion-like end organs. Over the maculae concretions of lime salts may form otoconia which remain attached to the sensory bristles. The true or- gan of hearing, the spiral organ, is developed in the basal epithelium of the coch- lear duct, basal having reference here to the base of the cochlea. The develop- ment of the spiral organ has been studied carefully only in the lower mammals, in the pig by Shambaugh, Hardesty and Prentiss. In pig embryos of 5 cm. the basal epithelium is thickened, the cells becoming highly columnar and the nuclei forming several layers. In later stages, 7 to 9 cm., inner and outer epithelial thickenings are differentiated, the boundary line between them being the future spiral tunnel (Fig. 367 A). At the free ends of the cells of the epithelial swellings there is differentiated a cuticular structure, the membrana tectoria, which appears first in embryos of 4 to 5 cm. The cells of the inner (axial) thickening give rise to the epithelium of the spiral limbus, to the cells lining the internal spiral sulcus and to the supporting cells and inner hair cells of the spiral organ (Fig. 367 B, C). The outer epithelial thickening forms the pillars of Corti, the outer hair cells and sup- porting cells of the spiral organ. Differentiation begins in the basal turn of the endolymph vest i b. pouch ' fat qroove vesUb.p. CSCSUP ^^^■..coch. ^^^ pouch c.sc.post. absorpt foci lat groove C.Sc.lat. a. 6.6 mm lateral cochlea D 9 mm. lateral. C . unim lateral e . 20mm .ateral. f. 30Tnm. lateral. Fig. 366. — Six stages in the development of the interna] ear. The figures show lateral views of models of the membranous labyrinth— a at 6.6 mm.; 6, at 9 mm.; c at 13 mm.; d at 11 mm.; c at 20 mm., and/at 30 mm. (Streeter) (X 25). The colors yellow and red arc used to indicate respectively the cochlear and vestibular divisions of the acoustic nerve and its ganglia, absorp. focus, area of wall where absorption is complete; cms, crus commune; c.sc.lal., ductus semicircularis lateralis; c. sc. post., ductus semicircularis posterior; c.sc.sitp., ductus semicircularis superior; cochlea, ductus cochlearis; coch. pouch, cochlear anlage; endolymph., appendix endolymphaticus; sacc, sacculus; sac. cndoL, saccus en- dnlymphaticus; sinus tttrc. la!., sinus utriculi lateralis; utric, utriculus. DEVELOPMENT OF THE SENSE ORGANS 385 n.coc n.cocW Fig. 367. — Three stages in the differentiation of the basal epithelium of the cochlear duct to form the spiral organ (of Corti), internal spiral sulcus and labium vestibularc. A , section through the cochlear duct of an 8.5 cm. pig fetus; B, the same from a 20 cm. fetus; C, from a 30 cm. fetus (near term). cp. s. s p., epithelium of spiral sulcus; h.c., hair cells; i.ep.c, inner epithelial thickening; i.h.c, inner hair cells; i.pil., inner pillar of Corti; lab. vest., labium vestibulare; limb, sp., limbus spiralis; m. bas., basilar membrane; m. ted., membrana tectoria; m. vest., vestibular membrane; n. cock., cochlear division of acoustic nerve; o.ep.c., outer epithelial thickening; o.h.c, outer hair cells; s.sp., sulcus spiralis; sc.tymp., scala tympani; st.H., stripe of Hensen; t.sp., spiral tunnel. 25 386 THE PERIPHERAL NERVOUS SYSTEM cochlea and proceeds toward the apex. The internal spiral sulcus is formed by the degeneration and metamorphosis of the cells of the inner epithelial thicken- ing which lie between the labium vestibulare and the spiral organ (Fig. 367 B, C). These cells become cuboidal, or flat, and line the spiral sulcus, while the membrana tectoria loses its attachment to them. The membrana tectoria becomes thickest over the spiral organ and in full term fetuses is still attached to its outer cells (Fig. 367 C). According to Hardesty (Amer. Jour. Anat., vol. 8) the membrana tectoria is not devel- oped from the cells of the spiral organ and therefore is not attached to it at any time. From what is known of the development of the spiral organ in human embryos, it follows the same lines of development as described for the pig. It must develop relatively late, however, for in the cochlear duct of a new-born child figured by Krause the spiral sulcus and the spiral tunnel are not yet present. The mesenchyme surrounding the labyrinth is differentiated into a fibrous membrane directly surrounding the epithelium, and into the perichondrium of the cartilage which develops about the whole internal ear. Between these two is a more open mucous tissue which largely disappears, leaving the perilymph space. The membranous labyrinth is thus suspended in the fluid of the peri- lymph space. The bony labyrinth is produced by the conversion of the cartilage capsule into bone. In the case of the cochlea, large perilymph spaces form above and below the cochlear duct. The duct becomes triangular in section as its lateral wall remains attached to the bony labyrinth, while its inner angle is ad- herent to the modiolus. The upper perilymph space is formed first and is the scala vestibuli, the lower space is the scala tympani. The thin wall separating the cavity of the cochlear duct from that of the scala vestibuli is the vestibular membrane (of Reissner). Beneath the basal epithelium of the cochlear duct a fibrous structure, the basilar membrane, is differentiated by the mesenchyme. The modiolus is not preformed as cartilage, but is developed directly from the mesenchyme as a membrane bone. The development of the acoustic nerve has been described on page 356 with the other cerebral nerves. The Middle Ear. — The middle ear cavity is differentiated from the first pharyngeal pouch which appears in embryos of 3 mm. The pouch enlarges rap- idly up to the seventh week, is flattened horizontally and is in contact with the ectoderm. During the latter part of the second month, in embryos of 24 mm., the wall of the tympanic cavity is constricted to form the tubo-tympanic (Eus- tachian) canal. This tube lengthens and its lumen becomes slit-like during the fourth month. The tympanic cavity is surrounded by loose areolar connec- DEVELOPMENT OP Till. SENSE ORGANS 387 tive tissue in which the auditory ossi< Irs arc developed and for a time embedded. The pneumatic cells arc formed at the close of fetal life. The development of the auditory ossicles has been described by Broman (Verh. Anat. Gesellsch., Kiel, Anat. An/. Suppl., vol. 14, 1898). According to his account, the condensed mesenchyma of the first and second branchial arches gives rise to the ear ossicles. This tissue is divided in the proximal part of the arches into lateral and median masses. The malleus is formed from the distal portion of the median mesenchymal mass of the first arch, along with Meckel's cartilage of the mandible. The cartilaginous anlage of the malleus is continuous with Meckel's cartilage. Be- tween it and the incus is an intermediate disk of tissue, which later forms an articulation. When the malleus begins to ossify it separates from Meckel's cartilage. The incus is derived from the proximal portion of the lateral mesenchymal mass of the first branchial arch. The anlage of the incus unites with that of the capsule of the labyrinth and separates from it only when its cartilage develops. It is early connected with the anlage of the stapes, and the connected portion be- comes the cms longum. Between this and the stapes an articulation develops. The stapes and Reichert's cartilage are derived from the median mesenchy- mal mass of the second branchial arch. The mesenchymal anlage of the stapes is perforated by the stapedial artery, and its cartilaginous anlage is ring-shaped. This form persists until the middle of the third month when it assumes its adult structure and the stapedial artery disappears. Fuchs, after studying the development of the ear ossicles in rabbit embryos, concludes: (1) that the stapes is derived from the capsule of the labyrinth; (2) that the malleus and incus arise independent of the first branchial arch. The External Ear. — The external ear is developed from and about the first branchial groove. The auricle arises from sLx elevations which appear three on the mandibular and three on the hyoid arch (Fig. 368). These anlages were first described by His. They are numbered ventro-dorsally on the mandible and in the reverse direction on the hyoid arch. Caudal to the hyoid anlages a fold of the integument is formed, the hyoid helix or auricular fold. A similar fold forms later dorsal to the first branchial groove and unites with the auricular fold, to form with it the free margin of the auricle. The point of fusion of these two folds marks the position of the satyr tubercle, according to Schwalbe. Schwalbe derives the tragus from mandibular hillock 1; the helix from mandibular hillocks 2 and 3; the antihelix from hyoid hillocks 4 and 5; the antitragus from hyoid hillock 6. Darwin's tubercle appears at 388 THE PERIPHERAL NERVOUS SYSTEM about the middle of the margin of the free auricular fold, and corresponds to the tip of the mam- malian auricle. The external auditory meatus is formed as an ingrowth of the first branchial groove. In embryos of 12 to 15 mm. the wall of this groove is in contact dorsally with the entoderm of the first pharyngeal pouch. Later, however, this contact is lost, and during the latter part of the second month, according to Hammar, Fig. 368. — Six stages in the development of the external ear. 1, 2, 3, elevations on the mandibular arch; 4, 5, 6, elevations on the hyoid arch. 1, tragus; 2, 3, helix; 4, 5, antihelix; 6, antitragus. c, hyoid helix or auricular fold (His from McMurrich's "Human Body"). A, n mm.; B, 13.6 mm.; C, 15 mm.; D, beginning of third month; E, fetus of 85 mm.; F, fetus at term. an ingrowth takes place from the ventral portion of the groove, to form a funnel- shaped canal. The lumen of this tube is temporarily closed during the fourth and fifth months, but later re-opens. During the third month a plate of cells at the extremity of the primary auditory meatus grows in and reaches the lower wall of the tympanic cavity. During the seventh month a space is formed by the splitting of this plate, and the secondary portion of the meatus is thus developed. The tympanic membrane is formed by a thinning out of the tissue in the region where the wall of the external auditory meatus abuts upon the wall of the tym- panic cavity. NDEX Abdominal pregnancy, 30 Abortion, So Acid carmine, 48 haematoxylin, 48 Adipose tissue, 294 Alee nasi, 154 Allantoic stalk, 76, 79, 152 vessels, 76, 77 Allantois, 67, 76, 77, 83, 92, 105, 119 derivation of, 79, 83 Amboceptor, 30 Ameloblasts, 163 Amitosis, 22 Amnion, origin of, 74, 83 bat, 83 chick, 65, 75 human, 80, 83 Pig, 77 Amniota, 74 Amphiaster, 22 Amphioxus, ^^ Ampulla of ductus deferens, 226 Anal membrane, 169 Angioblast, 48 chick, 48 human, 251 Anlage defined, 13, 48 Annelids, ^^ Annulus, 22 Anomalies, 167, 177, 182, 186, 202, 213, 225 Ansa hypoglossi, 353, 358 Anus, 105, 152, 169 Aorta, origin of, 255 chick, 52 descending, 52, 92, 106, 137, 267 dorsal, 56, 106, 107 pig, 106, 107, 137 ventral, 56, 267 Aortic arches, 92, 129, 137, 276 chick, 67 human, 92 pig, 107, 129, 137 Appendix epididymis, 225 testis, 226, 236 vermiformis, 182 Aqueduct, cerebral, 336 Archenteron, 38 Archipallium, 344 Arcuate fibers, $^^ Arcus pharyngo-palatini, 158 Area, germinal, 36 opaca, 43, 48 pellucida, 43, 54 vasculosa, 57 Arteries, allantoic, 76, 77 basilar, 135, 272 carotid, 107, 129, 270 central, of retina, 376 changes at birth, 286 cceliac, 107, 129, 274 epigastric, 274 hepatic, 186 hypogastric, 275 iliac, 129, 275 intercostal, 274 internal mammary, 274 intersegmental, 107, 129, 268, 274 mesenteric, 129 inferior, 129, 269 superior, 129, 269, 274 of extremities, 275 of pig, 106, 129 ovarian, 274 phrenic, 274 pulmonary, 107, 129, 177, 265, 271 renal, 213, 274 spermatic, 274 subclavian, 107, 129, 270, 274 suprarenal, 274 umbilical, 92, 107, 118, 144 ventro-lateral, 107, 129 vertebral, 129, 272 vitelline, 57, 68, 92, 107, 118, 129, 268, 269 Arytenoid swellings, 101, 124, 160, 174 Ascaris megalocephala, 27 Atrium, 67, 92, 258 Auricle of ear, 387 of heart. See Atrium Autonomic system, 364 Axial filament, 22 389 39° INDEX Basophile (mast leucocyte), 254 Biogenesis, law of, 14 Bladder, 85, 152, 213, 215 Blastoccel, 33 Blastoderm, 36 Blastodermic vesicle, 36 Blastopore, 38 Blastula, 33, 37 Blood, 53 cells, 251 islands, 4S, 251 plastids, 253 plates, 254 Blood-vessels, changes at birth, 286 chick, 53, 56, 67 human, 92, 251, 255 pig, 105, 128 primitive, 267 Body cavities, 188 Body-stalk, 80, 85 Bone, endochondral, 296 growth of, 297 histogenesis of, 295 membrane, 295 palatine, 158 Brachial plexus, 353 Brachium conjunctivum, 335 pontis, 334 Brain, human, 325 01 pig, i23 Branchial arches, chick, 67 human, 89 Pig, 97, 112 clefts, 65, 97 ducts, 170 vesicles, 170 Bulbar swelling, 128 Bulbo-urethral glands, 235 Bulbus cordis, 67, 92 Bursa infracardiaca, 199 omentalis, 197 Cecum, 152, 182 Calcar avis, 349 Canal, atrio-ventricular, 106, 259 Gartner's, 226, 237 hyaloid, 378 incisive (of Stenson), 157 inguinal, 231, 232 neurenteric, 41 notochordal, 41 pleuro-peritoneal, 191 semicircular, 383 tubo-tympanic, 387 Canalis epididymis, 225 Capsule of Bowman, 205 internal, 343 of liver, 200 Cartilage, arytenoid, 175 corniculate, 175 cricoid, 175 cuneiform, 175 histogenesis of, 294 Meckel's, 295 of epiglottis, 174 Cauda equina, 325 Caudal flexure, 89 Caudate lobe of liver, 201 nucleus, 343 Caul, 83 Cavity, body, 188 oral, 151 pericardial, 63, 188, 190 peritoneal, 63, 194, 195 pleural, 63, 194, 195 pleuro-pericardial, 50, 190 pleuro-peritoneal, 188, 190 tympanic, 170, 386 Cells, germ, 17 giant, 254 lutein, 224 sustentacular, 221 taste, 369 Centra of vertebrae, 151 Central nervous system, 319 chick, 49, 54, 66 human, 89, 319 pig, 122 Centriole, 22 Centrosome, 17 Cephalic flexure, 65, 87 Cerebellum, 150, 334 Cerebral cortex, 349 hemispheres, 147, 327, 346 nerves. See Nerves, 123 Chick embryos, 43, 48 preservation of, 48 study of, 48, 54, 65 Chin, 154 Choanas, 371 Chorda dorsalis, 49. See Notochord gubernaculi, 230 Chorioid plexus, 150, 332, 336, 340 Chorion, origin of, 74 chick, 74, 75 frondosum, 243 human, 81 keve, 243 Pig, 77 INDKX 391 Chorion, villi of, cSo, 240 Chromaffin bodies, 300 aortic, 366 Chromatin, 17, 24 Chromosomes, 23 eu cessory, 24, 32 number of, 24 Circulation, fetal, 284 Circulatory system, 92, 251 Clava, ^3 Cleavage, ss Clitoris, 152, 233, 237 Cloaca, oi, 104, 105, 119, 213 Cloaca] membrane, 105, 169, 213 tubercle, 2^2 Closing plates, 103, 169 Cochlea, 384 Coelom, 45. 50, 190 chick, 63 extraembryonic, 188 human, 188, 190 umbilical, 182 Collateral eminence, 349 Colliculus, facial, 361 inferior, 336 superior, 336 Coloboma, 376 Colon, 142, 182, 183 Column, gray, 322 Columna nasi, 154 Columns, renal, 209 Commissure, anterior, 344, 345 ganglionic, 364 gray, 322 hippocampal, 345 posterior, of labia majora, 233 white, 322 Conchae, 373 Concrescence theory, 41 Coni vasculosi. 2:5 Conjunctiva, 381 Copula, 102, 124, 160 Cord, spinal, 320 nephrogenic, 205, 208 umbilical, 79 Cornea, 380 Corona radiata, 20 Coronary appendage, 194, 201 sinus of heart, 279 sulcus of phallus. 23a Corpora quadrigemina, 328, 336 Corpus albicans, 224 callosum, 344, 345 haemorrhagicum, 224 luteum, 224 Corpus striatum, 327, 339, 341 Cortex cerebri, 339, 349 t !re» entic groove, 40 Crista acustica, 383 Crura cerebri, 328 Cryptorchism, 221, 231 Cumulus oophorus, 224 Cuneus, m, 349 Cutis, 305 plate, 119 Decidua basalis (serotina), 239 capsularis (reflexa), 239, 244 vera (parietalis), 239, 243 Decidual cells, 244 membranes, 239 separation of, 249 Dentition, 167 Dermatome, 299 Dermis (corium), 305 Dermo-muscular plate, 62 Descending spinal tract of trigeminal nerve, 359 Diaphragm, 195 dorsal pillars of, 194 origin of, 195 Diaster, 23 Diencephalon, 66, 89, 100, 336 Differentiation of tissues, 13 Digestive canal, chick, 55, 67 human, 89 pig, 6 mm., 101 10 mm., 124 glands, 89 Dissecting instruments, 145 Dissections, face, 153 lateral, 99, 146 median, 148 palate, 155 pig embryos, 145 tongue, 158 ventral, 153 Diverticulum, cardiac, 127 hepatic, 104 Meckel's, 182 of pharyngeal pouches, 103 Ducts, bile, 127 branchial, 170 cervical, 1 70 cochlear, 383 cystic, 184 hepatic, 127, 184 mesonephric, 92 Muellerian, 218 naso-lachrymal, 381 392 INDEX Ducts, papillary, 208 periportal, 185 pronephric, 204 thoracic, 287 Ductuli efferentes, 207, 225 Ductus arteriosus, 271, 286 choledochus, 127, 1S5 deferens, 226 endolymphaticus, 382 venosus, 108, 131, 278, 286 Duodenum, 182 Dyads, 27 Ear, external, 155, 3S7 internal, 55, 3S2 middle, 386 Ectoderm, 37 derivatives of, 34 formation of, 37 Elastic tissue, 293 Embryos, chick, 43, 48, 54, 65 human, 80. See Human embryos pig, 6 mm., 97 10 mm., 120 dissection of, 145 Enamel organ, 162 Encephalon, 89 Endocardial cushion, 138, 259 Endocardium, 139, 256 Endothelium, 64 End-piece, 22 Enlargement, cervical, 324 lumbar, 324 Entoderm, 37 derivatives of, 64 histogenesis of, 290 origin of, 38 Entodermal canal, 168 Eosinophils, 254 Ependymal cells, 317 layer, 133, 31' Epicardium, 52, 106, 139, 256 Epidermis, 304 Epididymis, 226 Epigenesis, 12 Epiglottis, 101, 124, 151, 160, 174 Epiphysis (pineal body), 150, 336 Epithalamus, 328 Epithelial bodies, 90, 172 Epithelium, 64 Epitrichium, 305 Eponychium, 309 Epoophoron, 226, 23 Erythroblasts, 252 Erythrocytes, 252, 253 Esophagus, 89, 103, 127, 151, 177 Ethmoidal cells, 373 Eustachian tube, 90, 170 External auditory meatus, 98, 120, 155 Extraembryonic mesoderm, 46, 80 Extremity, 89, 98, 121 Eye, chick, 54, 66 human, 89, 373 anterior chamber of, 375 pig, 97, 122 Eyelashes, 381 Eyelids, 381 Face, development of, 153 Facial colliculus, 361 nerve, 100, 123 Falciform ligament, 142 Fasciculi of spinal cord, 323 Fasciculus, median longitudinal, 332 Fertilization, 30 Fetal membranes, human, 80 pig, 77 Fetus, 93 Fibrin, canalized, 248 Filum terminale, 324 Fissure, calcarine, 349 chorioid, of cerebrum, 340 of eye, 374, 376 collateral, 349 great longitudinal, 340 hippocampal, 347 lateral (Sylvian), 347 parietooccipital, 348 Fixation of pig embryos, 145 Flagellum of spermatozoon, 21 Flexures, caudal, 89 cephalic, 65, 89, 325 cervical, 147, 326 dorsal, 147 pontine, 147, 326 Flocculus, 334 Floor plate, 321, 330 Foramen, epiploic (of Winslow), 142, 198, 199 interatrial, 106, 260 interventricular, of heart, 265, 266 Monroi, 328, 340 ovale, 106, 138, 260, 263, 286 Fore-brain, chick, 49, 53 human, 319 Fore-gut, chick, 51, 67 human, 168 Fornix, 344 Fossa, incisive, 157 INDEX 595 Fossa ovalis, 264 Fovea cardiaca, 48, 51 Frenulum «>f prepuce, 234 Funiculi of spinal cord, 32$ Gall bladder, 184 Ganglion, accessory, 362 cervical, 365 ciliary, 364 Froriep's, 101, 124, 134, 35S geniculate, 100, 124, 360 jugular, 101, 124, 362 nodosum, 101, 124, 362 otic, 365 petrosal, 101, 124, 362 prevertebral, 365 semilunar, 100, 123, 359 sphenopalatine, 365 spinal, 123, 313 spiral, 357 submaxillar}-, 365 superior, 101, 124, 362 sympathetic, 364 vestibular, 356 visceral, 365 Ganglion cells, 315 crest, 100, 313 Gartner's canal, 226, 237 Gastrula, 38 Gastrulation, 38 in mammals, 41 Geniculate bodies, 328 Genital ducts, 216, 218 folds, 105, 128, 152, 205, 217 glands, 105, 152, 216 swelling, 232 tubercle, 232 Genitalia, external, 232 Germ cells, 17 layers, 12, 13, 37, 64 derivatives of, 64 Germinal area, 36 Glands, bulbo-urethral, 235 cardiac, 179 carotid, 366 Fbner's, 161 gastric, 179 lachrymal, 381 lingual, 162 mammary. 307 of pregnancy, 244 parotid, 161 prostate, 235 salivary, 161 Glands, sebaceous, 306 sublingual, 161 submaxillary, 161 sudoriparous (sweat), 307 suprarenal, 152, 367 vestibular, 235 Glans clitoris, 233 penis, 234 Glomerulus, 141, 205 Glottis, 124, 160 Graafian follicle, 20, 224 Gray column of spinal cord, 322 Groove, laryngotracheal, 173 neural, 309 primitive, 41 urethral, 233 Growth, law of, 13 Gubernaculum testis, 230 Gyrus dentatus, 344 hippocampus, 344 H-Emolymph glands, 288 Hair, 305 Head-fold, 45 Head-process, 44 Heart, chick, 52, 56 human, 92, 255, 256 pig, 105, 114, 128, 137, 152 Hemispheres, cerebellar, 334 cerebral, 327, 346 Henle's loop, 211 Hensen's node (knot), 44, 61 Hepatic diverticulum, 104, 116, 142 Heredity, theory of, 31 Hermaphroditism, 237 Hernia, diaphragmatic, 202 inguinal, 232 umbilical, 79 Hind-brain, chick, 54 human, 325 Hind-gut, 67, 91, 168 Histogenesis, defined, 13 of ectodermal derivatives, 303 of entodermal derivatives, 290 of mesodermal derivatives, 291 Human embryos, So estimated age, 95 of Coste, 86 of Dandy, 85 of Eternod, 86 of His, 2.6 mm., 87 4.2 mm., 88 Normentafel, 94, 95 of Kollmann, 319 394 INDEX Human embryos of Mall, 85, 98 of Peters, 85 of Spee, 85 of Thompson, 85 Hydramnios, 83 Hymen, 228 Hyoid arch, 98 Hyomandibular cleft, 98 Hypophysis, 67, 89, 328, 337 Hypospadias, 234 Hypothalamus, 328, 330, 337 Ileocecal valve, 182 Implantation of ovum, 239 Incisive fossa, 157 Incus, 387 Infundibulum, 328, 337 Inguinal canal, 229 fold, 227, 229 Inner cell mass, 36, 80 epithelial mass, 217 Insula, 347 Intermediate cell mass, 50, 62 Internal capsule, 343 ear, 55 Interstitial cells of testis, 221 Intestinal glands, 182 loop, 180 Intestine, human, 89, 180 pig, 104, 128 Introduction, 11 Iris, muscles of, 380 Isthmus, 327, 328 Jacobson's organ, 355, 372 Joints, 298 Kidney, human, 207 Knot, primitive, 44, 61 Labia majora, 233. 237 minora, 233, 234 Lachrymal gland, 381 groove, 98 Lamellated corpuscle, 368 Lamina terminalis, 327, 340 Laryngotracheal groove, 173 Larynx, 173, 174 Layer, chorioid, of eye, 380 compact, 244 ependymal, 133, 330 Layer, ganglion cell, 379 mantle, 133, 310, 321 marginal, 133, 310, 323 nerve-fiber, 379 sclerotic, of eye, 380 spongy, 244 Lecithin, 17 Lemniscus, 333, 336 Lens of eye, chick, 55 human, 374 capsule of, 377 fibers of, 376 pupillary membrane of, 378 suspensory ligament of, 378 vascular capsule of, 375 Pig, 97 Lesser peritoneal sac, 141, 197 inferior recess of, 198 superior recess of, 198 Leucocytes, 253 Ligament, broad, 228 coronary, 201 duodeno-hepatic, 200 falciform, 200 gastro-hepatic, 200 gastro-lienic, 200 labial, 229 lieno-renal, 200 round, 227, 230 triangular, 201 vesico-umbilical, 216 Ligamentum labiale, 229 ovarii, 229, 230 scroti, 230 teres, 286 testis, 229 venosum, 278 Limbus ovalis, 263 Lip, anlage of, 154 hare, 154 Liver, anlage of, 67, 183 bile capillaries, 185 chick, 67 human, 183 lobules of, 186 pig, 104, 127, 139 quadrate lobe, 201 weight of, 185 Lizard, germ layers of, 39, 40 Lobes of cerebrum, 346 of liver, 201 Lumbo-sacral plexus, 354 Lungs, human, 89, 175 apical bud, 175 changes at birth, 177 INDEX 395 Lungs, human, stem bud, 175 pig, 126, 141 Lunula, 308 Lutein cells, 224 Lymph glands, 287 sacs, 286 Lymphocytes, 253 Macula acustica, 384 Malleus, 387 Mammary glands, .^07 Mammillary body, 328, 339 recess, 328 Mandibular arch, 98 process, 87, 89, 98 Mantle layer, 133 Marginal layer. 133 Margo thalamicus, 325 Marrow of bone, 251 Massa intermedia, 339 Maturation, 24 of mouse ovum, 28 Maxillary process, 87, 89, 97 Maxillo-turbinal anlage, 373 Meatus, external auditory, 98, 120, 155, 388 Meckel's cartilage, 295 diverticulum, 88 Median thyreoid, 102 Mediastinum, 176 Medulla oblongata, 329 of kidney, 209 Medullary cords, 225 velum, 335 Membrana tectoria, 385 Membrane, anal, 169, 213 cloacal, 105, 169, 213 pericardial, 195 pharyngeal, 53, 168 pleuro-pericardial, 192 pleuro-peritoneal, 192 tympanic, 388 urogenital, 169, 213 Mendel's law, 31 Menstruation, 20, 238 Mesamoeboid. 251 Mesencephalon, chick, 54 human, 89, 325, 328, 335 pig, 100 Mesenchyma, chick, 51, 63 human, 291 Mesentery, 104, 141, 188, 190, 201 dorsal, 80, iqo. 201 Mesocardium, 52, 188, 256 Mesocolon, iqo, 201, 202 Mesoderm, amphibian, 43 Amphiozu bird, 43 mammal, 45. 4'» somatic, 50, 63, 291 splanchnic, 50, 63, 291 Mesodermal segments, 45, 50, 61 Me ^ xluodenum, 190, 201 Mesogastrium, 190 Mesonephric duct, 92, 105 fold, 205, 206 pig, 105, 128, 141, 144 tubules, 205 Mesonephros, 62, 92, 128, 152, 205 Mesorchium, 217 Mesorectum, 190, 202 Mesothelium, 64, 291 Mesovarium, 217 Metamerism, 12 Metanephros, 105, 120, 128, 144, 152, 207, 208 calyces of, 207 collecting tubules, 128, 207 cortex, 209 medulla, 209 pelvis, 128, 207 tubules, 208 ureter, 128, 207 Metathalamus, 328 Metencephalon, S9, 100, 329, 333 Methods of dissection, 145 of study, 14 Mid-brain, 54, 325, 328, 335 Mid-gut, 67, 168 Milk line, 307 Mitochondria sheath, 22 Mitosis, 22 significance of, 31 Modiolus, 386 Monaster, 23 Monkey, ovum of, 20 Mons veneris, 233, 237 Morula, t,^ Mouse, ovum of, 28, 30 Mucous tissue, 79 Muellerian ducts, 218 tubercle, 227 Muscle, cardiac, 299 histogenesis of, 298 plate, 119 smooth, 298 striated voluntary, 299 Muscles, morphogenesis of, 301 of diaphragm. 195 Myelencephalon, 89, 100, 112, 320, 330 Myelocytes, 253 396 INDEX 373 See Cervical flexure Myocardium, 52, 106 Myotome, chick, 62 human, 299 Pig, 119 Nail, human, 308 Naris, 370 Nasal passages, 157 pits, human, 370 pig, 97, 120 processes, 153, 370 Naso-turbinal anlage, Neck-bend, 147, 346. Nephrogenic cord, 205, 208 tissue, 128, 210 Nephrostome, 203 Nephrotome, chick, 62 human, 204 Nerve fibers, histogenesis of, 310 Nerves, abducens, 123, 359 acoustic, 100, 124, 356 cerebral, 355 chorda tympani, 124, 361 facial, 100, 123, 360 femoral, 354 glossopharyngeal, 101, 124, 361 hypoglossal, 101, 124, 358 mandibular, 123, 359 maxillary, 123, 359 oculomotor, 100, 123, 358 olfactory, 123, 355 ophthalmic, 123, 359 optic, 123, 356 phrenic, 192, 353 sciatic, 354 spinal, 124, 351 accessory, 101, 124, 362 superficial petrosal, 361 sympathetic, 364 trigeminal, 100, 123, 359 trochlear, 123, 359 vagus, 101, 124, 362 Nervous system, 54, 99, 122 Neural crest, 100, 313 folds, 45, 309 groove, 45, 309 tube, 45, 330 Neurenteric canal, 41 Neuroblasts, 311, 313 Neuroglia, origin of, 310, 317 Neuromeres, 54, 112 Neurone theory, 315 Neurones, 312, 313 Neuropore, anterior, 54, 319 Neutrophiles, 254 Node of Ranvier, 316 primitive, 44, 61 Nodulus cerebelli, 334 Normoblasts, 252 Notochord, origin of, 42, 46 chick, 49 human, 46 Notochordal canal, 41 plate, 40 Nuclei of origin, 332 terminal, 332 Nucleolus, 17 Nucleus ambiguus, 362 caudatus, 343 cuneatus, 332 dentatus, 335 gracilis, 332 lenticular, 343 of ovum, 17 of pons, 333 olivary, 333 ruber, 335 Obex, 332 (Esophagus, 151, 177. See Esophagus Olfactory bulb (lobe), 344 fossa, 369 nerves, 123, 335 organ, 369 placode, 369 tracts, 344 Olivary body, inferior, 333 Omental bursa, 197 inferior recess of, 198 Omentum, 141, 178, 199 Oocyte, 28 Oogonia, 28 Operculum, 347 Optic chiasma, 328 cup, 67, 374 nerve, 123, 356 recess, 341 stalk, 374 vesicle, 49, 54, 328 Ora serrata, 379 Organ of Jacobson, 355, 372 spiral, 384 Ostium abdominale, 218 vaginae, 228 Otic vesicle, 55, 382 Otoconia, 384 Otocyst, 55, 67, 89, 100, 133, 382 Ova, primordial, 223 i.\i)i;\ 397 Ovary, 221 compared with testis, 225 descent of, 230 Ovulation, 20, 239 Ovum, human, 17 implantation of, 239 maturation of, 28 structure of, amphibian, 17 bird, 17 frog, 35 monkey, 21 rabbit, 36 Palate, cleft, 158 hard, 155, 157 premaxillarv, 371 Pallium of cerebrum, 327 Pancreas, human, 186 accessory duct of, 1S7 islands of, 188 pig, 104, 117, 127, 142 Papilla; of tongue, 160 renal, 209 vallate, 161 Paradidymis, 226, 237 Parathyrcoid gland, 90, 172 Parolfactory area, 344 Paroophoron, 237 Pars optica hypothalamica, 337 Penis, 152, 237 Perforated space, 344 Perforatorium, 21 Pericardial cavity, chick, 50 human, 188, 190 Perichondrium, 294 Periosteum, 297 Peritoneum, 141, 291 Phallus, 152, 232, 237 Pharyngeal membrane, 53, S9 pouches, 90, 103, 125, 134, 169 Tharynx, human, 89, 169 pig, 124 Pia mater, 133 Pig embryos, 41, 97, 120, 146, 148, 151 dissection of, 145 sections of 6 mm., in 10 mm., 132 Pigment layer of retina, 379 Pillars of Corti, 384 Pineal gland, 328 Pituitary body, 337 Placenta, human, 82, 245 cotyledons of, 249 intervillous spaces of, 249 Placenta, human, position of, 250 relation of fetus to, 249 vessels of, 249 pig, 78 Placodes, auditory, 55, 382 olfactory, 369 optic, 55, 374 Plate, alar, 321, 330 basal, 321, 330 closing, 169 cutis, 119 floor, 321, 330 muscle, 119 neural, 309 notochordal, 40 roof, 321, 330 urethral, 232 Pleura, 177 Pleural cavity, 177 Pleuro-pericardial cavity, 50, 190 Pleuro-pcritoneal cavity, 5c, 188, 190 Plexus, brachial, 353 cardiac, 365 chorioid, 150, 332, 336, 340 cceliac, 365 lumbo-sacral, 354 Plica venae cavae, 142, 201, 282 Polar bodies, 28 Polocytes, 28 Polydactylism, 84 Polyspermy, 30 Pons, 147, 329 Pontine flexure, 147, 326 Preformation theory, n Pregnancy, abdominal, 30 tubular, 30 Premyelocytes, 252 Prepuce of clitoris, 233 of penis, 234 Primary excretory ducts, 62, 204 Primates, 77 Primitive choanal, 371 groove, 41, 61 knot, 44, 61 node, 44, 61 streak, 40, 61 Proamniotic area, 44, 48 Process, lateral nasal, 153, 370 mandibular, S7, 89, 98, 370 maxillary, 87, 89, 97, 370 median frontal, 370 nasal, 153,370 palatine, 155, 373 Pronephros, 203 Pronuclei, union of, 30 / 393 INDEX Pronucleus, 28, 29 Prosencephalon, 54, 325 Prostate gland, 235 Pyramidal cells, 349 Pyramids, 2>2>2> of kidney, 209 Quadrate lobe of liver, 201 Ramus communicans, 352, 364 Rathke's pocket, 67, 169 Receptaculum chyli, 287 Recess, lateral, 331 Rectum, 127, 143, 152 Reduction of chromosomes, 27 Reference, titles for, 15 Regression, 14 Renal anomalies, 213 arteries, 213 columns, 209 corpuscles, 206, 207 cortex, 209 papillae, 209 pelvis, 207 pyramids, 209 tubules, 208 Respirator}- epithelium, 291 Rete ovarii, 221, 225 testis, 220 Reticular formation, 332 tissue, 292 Retina, 378 Rhinencephalon, 147, 327, 339, 344 Rhombencephalon, 54, 89, 325 Rhombic lip, 332 Rhomboidal sinus, 49, 54, 67 Ridge, pulmonary, 193 Roots of spinal nerves, 139 Saccules, 383 Saccus vaginalis, 231 Scake, 386 Sclerotome, 291 Scrotum, 234 ligament of, 230 Sections, chick, fifty-hours, 69 thirty-six-hours, 57 twenty-five-hours, 50 pig, 6 mm., in 10 mm., 132 Seessel's pocket, 89, 101, 169 Segmental zone, 73 Segmentation of ovum, 33 Amphioxus, 33 frog, 34 mammals, 36 reptiles and birds, 35 Segments, primitive, 119. See Mesodermal seg- ments Seminal vesicle, 226 Sense organs, chick, 54, 66 human, 368 pig, 122 Septum, dorsal median, 321 interatrial, 259 interventricular, 264 membranaceum, 267 nasal, 372 pellucidum, 344 primum, 106, 260 secundum, 260 spurium, 128, 262 transversum, 91, 114, 140, 152, 188, 191 Sex, determination of, 31 Sheath, medullary, 316 mitochondria, 22 myelin, 316 neurilemma, 316 Sinus, cavernous, 133, 279 cervical, 97 coronary, 279 frontal, 373 lateral, 279 maxillary, 373 petrosal, 279 rhomboidal, 49, 55 sphenoidal, 373 superior sagittal, 279 urogenital, 85, 127, 142, 213, 218, 234 venosus, 67, 137, 257, 261 Sinusoids of liver, 67, 184, 276 Somatopleure, 42, 63 Somites. See Segments Spermatic cord, 232 Spermatid, 26 Spermatocyte, 24 Spermatogenesis, 24 Spermatogonia, 24, 221 Spermatozoon, 21 Spinal cord, 320 Spireme, 23 Splanchnopleure, 42, 63 Spleen, 200, 288 Spongioblasts, 311 Stapes, 387 Stoerck's loop, 212 Stomodseum, 67 INDEX 399 Stomach, 89, 103, 115, 127, 141, 177 Stratum corneum, 305 germinativum, 304 granulosum, 224, 305 lucidum, 305 Streak, primitive, 40 Stroma of ovary, 223 Sulci of cerebrum, 349 Sulcus centrale, 348 coronary, 232 hypotnalamicus, 328, 337 limitans, 321, 330, 337 spirale, 384 terminalis, 158 Suprarenal glands, 152 Sweat glands, 307 Sympathetic system, 364 Tactile corpuscles, 368 Taenia, 332 Tail-fold, 66 Tail-gut, 104 Tarsius, 39 Taste-buds, 368 Teeth, anlages of, 162 cement of, 165 dental lamina of, 162 papilla, 162 pulp, 165 sac, 165 dentine, 165 enamel, 163 odontoblasts, 165 Tegmentum, 328 Tela chorioidea, 327 Telencephalon, chick, 66 human, 89, 339 pig, 100 Telolecithal ova, 33 Tendon, 293 Terminal nuclei, 332 Testis, 218 anomalies of, 221 cords, 220 descent of, 230 gubernaculum of, 230, 231 intermediate cords of, 220 interstitial cells of, 221 mediastinum, 220 rete, 220 tubuli contorti, 221 recti, 221 septula, 221 Tetrads, 27 Thalamus, 150, 328, 343 Theca folliculi. 224 Thomes' fibers, 165 Thj mic corpuscles, 172 Thymus, 171 Thy Hyoglossal duct, 172 Thyreoid gland, 172 human, 91, 169, 172 pig, 102, 126 Tissue, adipose, 294 bone, 295 cartilage, 294 1 lassification of, 64 connective, 292 differentiation of, 13 elastic, 293 muscle, 298 nervous, 309 origin of, 64 reticular, 292 supporting, 292 white fibrous, 292 Titles for reference, 15 Tongue of pig, 124, 151, 158 Tonsil, human, 159 origin of, 171 palatine, 90, 170 Trabecular of liver, 184 Trachea, human, 173, 174 pig, 103, 126, 151 Tractus solitarius, 362 Triangular ligament of liver, 201 Trigeminal nerve, 100, 123, 359 Trophectoderm, 36, 80, 239 Trophoderm, 81, 239, 243 Tuber cinereum, 328, 339 Tuberculum impar, 91, 102, 124, 159 Tubules of kidney, 211 Tubuli contorti, 221 recti, 22 Tunica albuginea, 220, 223 vaginalis, 231 vasculosa lentis, 375 Turbinal anlages, 151 Tympanum, 386 Ultimobraxchial body, 172 Umbilical cord, human, 79 of pig, 79 Umbilicus, 79, 216 Unguiculates, 77 Ungulates, 36, 77, 82 Urachus, 85, 216 Ureter, 152, 207, 215 Urethra, 152, 213, 215, 234, 237 Urethral groove, 233 Uriniferous tubules, 211 Urogenital ducts, 153 4oo INDEX Urogeital folds, 216 "organs, 92, 152, 203 sinus, 85, 127, 142, 213, 218, 234 system, 127, 203 Uterine tube, 226 Uterus, 226, 227, 238 anomalies of, 228 fundus of, 228 growth of, 228 ligaments of, 228, 229 masculinus, 227 menstrual changes of, 238 Utriculus, 384 Uvula, 158 Vagina, 226, 227 fornices of, 228 masculina, 226 237 Valves, bicuspid, 128, 267 Eustachian, 262 ileo-csecal, 182 of coronary sinus, 263 of sinus venosus, 128, 258, 261, 262 semilunar, 266 Thebesian, 263 tricuspid, 128, 267 Veins, allantoic, of chick, 76, 77 anonymous, 171, 279 anterior cardinal, 68, 93, 109, 130, 268, 279 axillary, 283 azygos, 283 basilic, 283 brachial, 283 cephalic, 283 cerebral, 279 changes at birth, 286 common cardinal, 68, 93, 109, 269, 279 femoral, 284 gluteal, 284 hepatic, 278, 282 iliac, 281, 283 ischiadic, 284 jugular, 130, 279 linguo-facial, 109 lumbar, 283 mesenteric, superior, 278 of extremities, 283 of head, 279 of pig, 106, 107, 130 ophthalmic, 279 ovarian, 283 portal, 107, 132, 276 posterior cardinal, 68, 93, 109, 130, 269, 280 pulmonary, 177, 264 renal, 283 subcardinal, 109, 130, 281 Veins, subclavian, 283 supracardinal, 283 suprarenal, 283 umbilical, human, 93, 276 pig, 107, 130 vitelline, 49, 93, 107, 132, 268, 276 vitello-umbilical, 93, 268 Velum, medullary, 335 Vena capitis lateralis, 279 medialis, 279 cava inferior, in, 130, 282, 283 superior, 279 porta, 276, 278 Venous system, 276 Ventral roots, 313 Ventricle, fifth, 344 first, 328 fourth, 329 lateral, 327 of heart, 67, 259, 264 terminal, 324 third, 328 Vermiform process, 182 Vermis cerebelli, 334 Vernix caseosa, 83 Vertebrae, human, 297 pig, 119, 149 centra of, 151 Vesicle, brain, 320 branchial, 170 cerebral, 100 lens, 97 optic, 49, 54, 328 seminal, 226 Vesico-urethral anlage, 213 Vestibular glands, 235 Vestibule, 383 Villi, origin of, 82, 240 of chorion, 80, 240, 246 of intestine, 182 vessels of, 240 Vitelline membrane, 48 Vitreous body of eye, 377 Vocal cords, 174 Whole embryos for study, 146 Yolk, 17 Yolk-cavity, 37 Yolk-sac, 75, 77, 85, 86 Yolk-stalk, 75, 79, 86, 87, 88, 168, 180 Zona pellucida, 17 Zone, segmental, 73 Zonula ciliata (of Zinn), 378 ■