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Gould wii Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924002969255 TEXT-BOOK OF KMBRYOLOGY BY FREDERICK RANDOLPH BAILEY, A. M., M. D. FORMERLY ADJUNCT PROFESSOR OF HISTOLOGY AND EMBRYOLOGY, COLLEGE OF PHYSICIANS AND SURGEONS (MEDICAL DEPARTMENT OF COLUMBIA UNIVERSITY) AND ADAM MARION MILLER, A. M. INSTRUCTOR IN ANATOMY, COLLEGE OF PHYSICIANS AND SURGEONS (MEDICAL DEPARTMENT OF COLUMBIA UNIVERSITY) Second Lodition WITH FIVE HUNDRED AND FIFTEEN ILLUSTRATIONS NEW YORK WILLIAM WOOD AND COMPANY MDCCCCXI be SUS CopyRIcHT, IQII, By WILLIAM WOOD & COMPANY. Printed by The Maple Press York, Pa. PREFACE TO THE SECOND EDITION The generally favorable criticisms with which the first edition of the text- book has been received and the necessity for a second edition in two years is gratifying. If its reception may serve as a criterion of its usefulness as a book intended primarily for the student of medicine, it is fulfilling the main pur- pose of,its authors. In the present edition some errors have been corrected and some para- graphs have been rewritten, owing to the fact that important advances have been made in the science even in the brief time elapsing between the two editions. There has been however no change in the general plan and scope of the work, as outlined in the preface to the first edition. Tue AUTHORS. AUGUST 15, IQII.° iii PREFACE TO THE FIRST EDITION The Text-book, as originally planned, is an outgrowth of the course in ‘Embryology given at the Medical Department of Columbia University. It was intended primarily to present to the student of medicine the most important facts of development, at the same time emphasizing those features which bear directly upon other branches of medicine. As the work took form, it seemed best to broaden its scope and make it of greater value to the general student of embryology and allied sciences. With the opinion that illustrations convey a much clearer conception of structural features than verbal description alone, the writers have made free use of figures. The plan of adding brief “Practical Suggestions” at the end of each chapter has been so thoroughly satisfactory in the Text-book of Histology, especially in connection with laboratory work, that it has been adopted here. ‘These “suggestions” are not intended to be complete descriptions of embryological technic, but are for the purpose of furnishing the laboratory worker with cer- tain of the more essential practical hints for studying the structures described in the chapter. To avoid frequent repetition, some of the best methods of procuring, handling, and preparing embryological material, and some of the more important formule are given in the Appendix, which is intended to be used mainly for the carrying out of the “Practical Suggestions.” The development of the Germ Layers has been treated rather elaborately from a comparative standpoint, because this has been found the most satisfac- tory method of teaching the subject. In the chapter on the Nervous System the aim has been to give a general conception of the subject, which, if once mastered by the student, will give him an insight into the structure and significance of the nervous system that will bring this difficult subject more fully within his grasp. In Part II (Organogenesis), at the end of each chapter there is given a brief description of certain developmental anomalies which may occur in connection Vv vi PREFACE, with the organs described in the chapter. In Chapter XIX (Teratogenesis) the nature and origin of the more complex anomalies and monsters are dis- cussed, and also the causes underlying the origin of malformations. The writers wish to thank Dr. Oliver S. Strong for his painstaking work on the chapter on the Nervous System. Dr. Strong in turn wishes to acknowledge his indebtedness to Dr. Adolf Meyer for important ideas underlying the treat- ment of his subject, and also for many valuable details. He expresses his thanks also to Professors C. J. Herrick, H. von W. Schulte and G. L. Streeter for helpful criticisms and suggestions. The writers would also express their thanks to Dr..H. McE. Knower for helpful criticisms on Part I and the chapter on Teratogenesis; to Dr, Edward Leaming for making the photo- graphs reproduced in the text; to the American Journal of Anatomy for the loan of plates; and to Messrs. William Wood & Company for their uniform courtesy and kindness. FREDERICK RANDOLPH BAILEY. APRIL I, 1909. ApAm Marion MILLER. CONTENTS. PART I.—GENERAL DEVELOPMENT. CHAPTER I. THE CELL AND CELL PROLIFERATION . . The Cell bvta dee Cell Division ......, Amitosis . Mitosis Practical Suggestions . ..... See ian an he REI Abs chemtaha cle References for Further Study ... CHAPTER II. THE SEXUAL ELEMENTS—OVUM AND SPERMATOZOON The Ovum : The Spermatozodn .. ..... Practical Suggestions . ... . References for Further Study CHAPTER III. MATURATION... . 05 4 @ SW a RSE Ee we Re Ge a es eS Maturation of the Ovum 2). ew Spermatogenesis: a. a0 o Hdd Cathe we Ge ae odo Oe eo dd @ Bobo Theoretical Aspects of Reduction. ........... Ovulation and Menstruation... ..... Practical Suggestions ; References for Further Study sy eA CHAPTER IV. ERRTIDIZATION 49 @ #8 34 Fe a Ek RE SR Re Se Se eB es Significance of Fertilization ©... 2 1. eee ee ee es Practical Suggestions . 1 1 1 1 1 ee ee eh we hw ee we ee References for Further Study .......4.-. Bilas Senha ras vii viii CONTENTS. CHAPTER V. CLEAVAGE (SEGMENTATION) Forms of Cleavage Holoblastic Cleavage Meroblastic Cleavage Some General Features of ClearegeCleavagei in Wénanals . Practical Suggestions. Li cht aR Glo Grune ot «8 References for Further Study CHAPTER VI. GERM LAYERS 2.6.00 ee es The Two Primary Germ ‘hayers—Fonnation of the Gastrula. . Gastrulation in Amphioxus ... . . 0 1 ee eee Gastrulation in Amphibians Gastrulation in Reptiles and Birds Gastrulation in Mammals Formation of the Middle Germ Layer Mesedern Mesoderm Formation in Amphioxus Mesoderm Formation in Amphibians Mesoderm Formation in Reptiles and Birds Mesoderm Formation in Mammals The Germ Layers in Man... . Practical Suggestions References for Farther Study CHAPTER VII. FatTaL MEMBRANES Foetal Membranes in Binds and Reptiles: The Amnion. . Sh oo ‘ ; The Yolk Sac . . : The Allantois : The Chorion or Serosa Foetal Membranes in Mammals Amnion, Chorion, Yolk Sac, Allantois, Umbilical Cord. Further Development of the Chorion The Foetal Membranes in Man The Amnion The Yolk Sac The Allantois Gide Se GS at et ele et eee: ey Histogenesis of the Gastvolntestinal Tract b gied The Development of the Liver . . Histogenesis of the Liver The Development of the Pancreas. Histogenesis of the Pancreas Anomalies... . Practical Slipaedtione: References for Further Study a CHAPTER XIII. THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. . The Larynx The Trachea . The Lungs ‘ Changes in the Tings at Binh: Anomalies ............ Practical Siiguestinay References for F orther Study CHAPTER XIV. THE DEVELOPMENT OF THE C@:LOM, THE PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES . The Pericardial Cavity, Pleural Cavities and Diephragmn. The Pericardium and Pleura et ee te The Omentum and Mesentery . The Greater Omentum and Onan Burts The Lesser Omentum The Mesenteries The Peritoneum Anomalies Practical Suggestions References for Further Study CHAPTER XV. THE DEVELOPMENT OF THE UROGENITAL SYSTEM The Pronephros. ee eee The Mesonephros_. Gt Gas eee Se Nee ei He Aes da SEUSS BUD ors The Kidney (Metanephiras) Ii oe A ee ae ame, RR Ae he The Ureter, Renal Pelvis, and ‘Gteaight Renal Tubules. . , The Convoluted Renal Tubules and Glomeruli. ....... The Renal Pyramids and Renal Columns ; Changes in the Position of the Kidneys... ©. +. +. xii CONTENTS. The Urinary Bladder, Urethra, and Urogenital Snus ......- The Genital Glands The Germinal Epithelium and Genteal Rides: Differentiation of the Genital Glands, . The Ovary The Testicle Determination of Sex . .. 1... wee The Ducts of the Genital Glands and the poner a the Meso- : a IY nephroi In the Female Oviduct : Uterus and Vagina . In the Male - 403 . 406 . 406 . 408 . 409 . 414 Changes in the Positions of the Genital Glands dad the Developments of their Ligaments Descent of the Testicles Descent of the Ovaries The External Genital Organs. The Development of the Suprarenal Glands The Cortical Substance The Medullary Substance Anomalies ‘Practical Suggestions References for Further Study CHAPTER XVI. THe DEVELOPMENT OF THE INTEGUMENTARY SYSTEM The Skin The Nails . The Hair : The Glands of the Skin The Mammary Glands Anomalies . Practical Suggestions References for Further Study CHAPTER XVII. ‘Tue Nervous SYSTEM General Considerations : General Plan of the Vertebrate Mewous Systeme Spinal Cord and Nerves ....... The Epichordal Segmental Brain and Nerves The Cerebellum ..........., The Mid-Brain Roof The Prosencephalon. 415 417 418 419 420 421 » 423 - 426 . 427 - 430 - 431 . 431 » 433 439 . 441 444 . 444 446 447 449 449 . 451 + 8 © © © e@ 453 453 454 . 454 457 464 . 466 . 473 . ATA . 474 CONTENTS. xii General Development of the Human Nervous System During the First Month 2.38 644% ear ae ee eee Oe ee 479. Histogenesis of the Nervous System... ..... 0 .....004 mie Epithelial Stage—Cell Proliferation .. 2... 0.0... 486 Early Differentiation of the Nerve Elements ........... 490 Differentiation of the Peripheral Neurones of the Cord and Epichordal Segmental Brain 2. 7 we ee 493 Efferent Peripheral Neurones .... . 6) 148 . . « 4O3 Afferent Peripheral and Sympathetic Newrones oa . 496 Development of the Lower (Intersegmental) Intermediate Neurones . 509 Further Differentiation of the Neural Tube. 513 The Spinal Cord. : 2 elgg The Epichordal Segmental Brain . 519 The Cerebellum ... . ; 532 Corpora Quadrigemina . se 537 The Diencephalon ..... . 538 The Telencephalon (Rhinencephalon, Corpora Striata andl Pallium) . 545 Rhinencephalon ....... ; . 547 Corpora Striata and Pallium . 548 The Archipallium 553 The Neopallium . . 559 Anomalies. ..... 567 Practical Suggestions. . . . ; 568 References for Further Study . gees F 571 CHAPTER XVIII. THE ORGANS OF SPECIAL SENSE... ‘ 573 The Eye .. . ae 573 The Lens . . core : e & S45 The Optic Cup . - $79 The Retina. . : . . . 580 The Chorioid and Sclera + a 585 The Vitreous... A. 41% . 585 The Optic Nerve . 586 The Ciliary Body, Iris, Cornea, Antena ‘Charaber 4 ab 587 The Eyelids . . re: . 588 The Nose : Ql oh: oH . 589 TheHars 292 S05. 40=°3 Rte ge Shes Fee 592 The Inner Ear be Re ey aS eo md Ss f . 592 The Acoustic Nerve ..... ++ sees - 2. §98 The Middle Ear . ..- 1. +e ee eee _ S . 599 The Outer Ear... - ee eet es wa ee tg O60 Anomalies... . . a ig: Gs Se sth Sta . 601 602 Practical Suggestions ..... - ee er ee: Dee sae References for Further Stay arate eagle OR de EE DR mcs See me 603 Xiv TERATOGENESIS Malformations Involving More Than One Individual CONTENTS. CHAPTER XIX. Classification, Description, Origin Symmetrical Duplicity Origin of Symmetrical Duplicity Asymmetrical Duplicity Origin of Asymmetrical (Parasitic) Dupliaey Malformations Involving One Individual Description, Origin Defects in the Region of Neural tube ee | Origin of Malformations in the Region of Neural Tube . Defects in Regions of the Face and Neck, and their Origin Defects in the Thoracic and Abdominal Regions, and their Origin Malformations of the Extremities Causes Underlying the Origin of Monsters . The Production of Duplicate (Polysomatous) ‘Monsters The Production of Monsters in Single Embryos. . The Significance of the Foregoing in Explaining the Production of Human Monsters. . . References for Further Study General Technic APPENDIX. Procuring and Handling Material Fixation Hardening Preservation Embedding Section Cutting Staining Methods of Rooence References . 605 605 . 605 606 . O11 . 612 . . 614 . 616 . . 616 . 616 . 619 . 620 622 622 624 625 626 627 627 . 629 . 629 . 631 633 633 . 633 634 635 . 637 . 638 INTRODUCTION. While Embryology as a science is of comparatively recent date, recorded observations upon the development of the foetus date back as far as 1600 when Fabricius ab Aquapendente published an article entitled “De Formato Feetu.” Four years later the same author added some further observations under the title, ““ De Formatione Foetus.” Harvey (1651), using a simple lens, studied and described the chick embryo of two days’ incubation. Harvey’s idea was that the ovum consisted of fluid in which the embryo appeared by spontaneous generation. Regnier de Graaf (1677) described the ovarian follicle (Graafian follicle), and in the same year was announced the discovery by Von Loewenhoek of the spermatozo6n. These and other embryologists of this period held what is now known as the preformation theory. According to this theory, the adult form exists in miniature in the egg or germ, development being merely an enlarging and unfolding of preformed parts. With the discovery of the spermatozoén the “ preformationists” were divided into two schools, one hold- ing that the ovum was the container of the miniature individual (ovists), the other according this function to the spermatozoén (animalculists). According to the ovists, the ovum needed merely the stimulation of the spermatozoén to cause its contained individual to undergo development, whereas the animalcu- lists looked upon the spermatozoén as the essential embryo-container, the ovum serving merely as a suitable food-supply or growing-place. Nearly a hundred years of almost no further progress in embryological knowledge came to a close with the publication of Wolff’s important article, “Theoria Generationis,” in 1759. Wolff’s theory was theory pure and simple, with very little basis on then known facts, but it was significant as being ap- parently the first clear statement of the doctrine of epigenesis. The two es- sential points in Wolff’s theory were: (1) that the embryo was not preformed; that is, did not exist in miniature in the germ, but developed from a more or less unformed germ substance; (2) that union of male and female substances was necessary to initiate development. The details of Wolff’s theory were wrong in that he looked upon the ovum as a structureless substance and upon the seminal fluid and not upon the spermatozoén as the male fecundative agent. Dollinger and his two pupils, von Baer and Pander, were the next to make important contributions to Embryology. Von Baer’s publication in 1829 was of extreme significance in the development of embryological knowledge, for xv xvi INTRODUCTION. in it we have the first definite description of the primary germ layers as well as the first accurate differentiation between the Graafian follicle and the ovum. It will be remembered that the cell was not as yet recognized as the unit of organic structure. Only comparatively gross Embryology was thus possible. With the recognition of the cell as the basis of animal structure (Schleiden and Schwann, 1839) the entire field of histogenesis was opened to the embryologist ; the ovum became known as a typical cell, while a little later (Kolliker, Reichert and others, about 1840) was established the function of the spermatozoon and the fact that it also was a modified cell structure. From this time we may consider the two fundamental facts of Histology and of Embryology, respectively, as firmly fixed beyond controversy; for Histology, the fact that the body consists wholly of cells and cell derivatives; for Embryology, the fact that all of these cells and cell derivatives develop from a single original cell—the fertilized ovum. The adult body being thus composed of an enormous number of cells, vary- ing in structure and in function, forming the different tissues and organs, and these cells having all developed from the single fertilized germ cell, it is the province of Embryology to trace this development from the union of male and female germ cells to the cessation of developmental life. While Embryology thus properly begins with the fertilized ovum, that is, with the first cell of the new individual, certain preliminary considerations are essential to the proper understanding of this cell and its future development. These are the structure of the ovum and of the spermatozoén and their de- velopment preparatory to union. Also, as it is with cells and cell activities that Embryology has largely to deal, it is necessary to consider the structure of the typical animal cell and the processes by which cells undergo division or proliferation. While the subject of this work is distinctly human Embryology, it is neither possible nor advisable to confine our study wholly to human material. It is not possible, for the reason that material for the study of the earliest stages in the human embryo (first 12 days) is entirely wanting, while human embryos of under 20 days are extremely rare. Again, even later stages in human develop- ment are often best understood by comparison with similar stages in lower forms. For practical study by the student, human material for all even of the later stages is rarely available, so that recourse must frequently be had to material from lower animals. Such study is, however, usually thoroughly satisfactory if the student has sufficient knowledge of comparative anatomy, and the deductions regarding human development, from the study of development in lower forms, are rarely in error. PART I. GENERAL DEVELOPMENT. A TEXT-BOOK OF EMBRYOLOGY CHAPTER I. THE CELL AND CELL PROLIFERATION. THE CELL. The Typical Animal Cell (Fig. 1) is a small definitely restricted mass of protoplasm. It contains or has at some period of its development contained two specially differentiated bodies, the nucleus and the centrosome. It may be limited by a more or less definite cell membrane. Of the ultimate structure of living protoplasm our knowledge is extremely small. It is of an albuminous nature, coagulated by heat and by many chemical reagents, It varies both in structure and in chemical composition in different cells and is probably best considered, not as a definite structure either chemically or morphologically, but as the material basis of life activities. Protoplasm can usually be resolved into a formed part, spongioplasm, which takes the form of a reticulum, a feltwork, or fibrille, and an unformed homogeneous element, hyaloplasm, which fills in the meshes of the reticulum or forms the perifibrillar substance. Various protoplasmic inclusions are frequently found in cells. To these the term metaplasm (paraplasm, deutoplasm) has been applied. Among them may be mentioned plastids, fat droplets, pigment granules and various excretory and secretory substances. The Nucteus is usually separated from the rest of the protoplasm by a nuclear membrane. Within the nucleus the nuclear membrane is continuous with a nuclear reticulum which consists of two parts: a chromatic part—chroma- tin, and an achromatic part—linin. At nodal points of the network there are frequently considerable accumulations of chromatin to form net knots (false nucleoli or karyosomes). Filling the meshes of the nuclear reticulum is a fluid or semifluid substance, the nucleoplasm or karyoplasm. The structure of the nucleus is thus seen to correspond closely to the structure of the surrounding protoplasm. This is especially evident in those cells in which there is no limiting nuclear membrane, the nuclear reticulum and the cytoreticulum being continuous, the nucleoplasm and cytoplasm mingling. This condition, true 2 TEXT-BOOK OF EMBRYOLOGY. only for some resting cells, is always present in cells which are undergoing mitotic division. In addition to the net knots are the érue nucleoli or plasmosomes. ‘These are spheroidal bodies which lie free in the meshes of the nuclear reticulum. They vary in number in different cells and sometimes in the same cell in different conditions of activity. They stain intensely with basic dyes. The function of the nucleolus is not known. It has been regarded by some as material in process of constructive metabolism, by others as a waste product. The nucleus is typically spherical. Its shape may or may not be modified by the shape of the cell body. Nuclei may assume very irregular shapes, as in polymorphonuclear leucocytes, or they may be lobulated, as in some of the en ee Aster (attraction-sphere) Cell membrane "~-—~~--~-~ NN pee Centriole Plastids (metaplasm) Metaplasm} <.7 granules _ Chromatin network Karyosome al aoe net knot ~~" Nuclear membrane Hyaloplasm i “= Nucleolus Spongioplasm ~~~ Linin network Nucleoplasm Vacuole Ant Fic. 1.—Diagram of a typical cell. Bailey. large cells of bone marrow; or a cell may have a number of nuclei. The shape of the nucleus may vary considerably within comparatively short periods of time. Such nuclei have been described as having amceboid movement. The size of the nucleus also appears to be independent of the size of the cell body, some large cells having small nuclei, while some small cells are almost completely filled by their nuclei. The nucleus tends to lie near the center of the cell, yet may be eccentric to any degree and appears to be suspended in the cytoplasm in such a way that its location within the cell may change. In some of the lowest forms no true nuclear structure exists, scattered granules of chromatin consti- tuting the rudimentary nucleus, generally called a diffuse nucleus. As the nucleus is an essential element in all reproduction, it follows that all cells have been nucleated at some time in their developmental history, and that the adult nonnucleated condition of some cells (e.g., respiratory epithelium) is indicative of their having passed beyond the age of reproductive power. If the nucleus be removed from a living cell, the cytoplasm does not necessarily THE CELL AND CELL PROLIFERATION. 3 die, but may live for some time and show active motile powers. Such a de- nucleated cell has, however, lost two of its most important functions: (1) its power of constructive metabolism; that is, of taking up nutritive material from without and building this up into its own peculiar structure—the power of repair; and (2) the power of reproduction. For these reasons the nucleus has been considered as especially presiding over these two cell functions. The CENTROSOME is a structure found in the cytoplasm near the nucleus, less commonly within the nucleus. It consists typically of a minute central granule, the centriole, a relatively clear surrounding area, the centrosphere, and, radiating from this, the delicate rays which constitute the aster or attraction sphere (Fig. 1). On account of the behavior of the centrosome in relation to cell division, it is usually looked upon as the dynamic center of the cell. Tn the simplest forms of animal life a single cell, such as has been described above, constitutes the entire individual, and as such is capable of performing the functions which are recognized as characteristic of living organisms—metab- olism, irritability, motion, reproduction and special functions. The develop- mental history of such an individual is extremely simple. The nucleus under- goes division and this is accompanied or followed by division of the cytoplasm. The single cell thus becomes two cells, similar in all respects to the parent cell. In all higher, that is multicellular animals, however, the different functions are distributed specifically to different cells and these cells are specifically differentiated morphologically for the performance of these different functions. There is, therefore, not the simple division of a parent cell to form two similar daughter cells, each constituting an individual, but a differentiation from the single original germ cell, the fertilized ovum, of many different kinds of cells, and their specialization to form the various tissues and organs which constitute the adult body. CELL DIVISION. In the development of the embryo, cell division of course succeeds fertiliza- tion. A proper understanding, however, of the changes which take place in the ovum and in the spermatozoon previous to fertilization requires the con- sideration of cell division at this point. Two types of cell division are recognized: (r) direct cell division or amitosis and (2) indirect cell division or mitosis. (x) Amitosis (Fig. 2).—In this form of cell division there is no formation of spindle or of chromosomes (see Mitosis, p. 4), the nucleus retaining its reticular structure during division. There is first a constriction of the nucleus, followed by complete division into two daughter nuclei. During the division of the nucleus a constriction appears in the cytoplasm. This increases until the cytoplasm is divided into two separate masses (daughter cells), each containing 4 TEXT-BOOK OF EMBRYOLOGY. anucleus. This form of cell division, which was considered by Remak and his associates (t85s-1858) as the only method by which cells proliferated, is now known to be of rare occurrence. Flemming goes so far as to state that in the. higher animals amitosis never occurs as a normal physiological process in ac- tively dividing cells, but is rather to be considered as a degeneration phenomenon occurring in cells whose reproductive powers are on the wane. It frequently results in nuclear division only, the cytoplasm remaining undivided, thus giving rise to multinuclear cells. It is a common method of cell division in the Protozoa. (2) Mitosis.—In this form of cell division the cell passes through a series of complicated changes. These changes occur as a continuous process, but for clearness of description it is convenient to arbitrarily subdivide the process into a number of phases. These are known as the prophase, the metaphase, the anaphase, and the telophase. Of these the prophase in- cludes the changes preparatory to division of the nucleus; the metaphase, the actual separation of the nuclear elements; the anaphase, their arrangement to form the two daughter nuclei; the telophase, the division of the cytoplasm to form two daughter cells and the reconstruction of the two daughter nuclei. PROPHASE (Fig. 3).—In actively divid- Fic. 2.—Epithelial cells from ovary of ing cells the centrosome, or, more specific- cockroach, showing nuclei dividing ami- ‘ . totically. Wheeler. ally, the centriole, may be double (Fig. 3, A), having undergone division as early, fre- quently, as-the anaphase of the preceding division (p. 6). Each centriole is surrounded by a clear area, the centrosphere, from which radiate the delicate astral rays, the whole being known as the attraction sphere (Fig. 3, B,C, D). Connecting the two centrosomes are other delicate fibrils forming a structure known as the central or achromatic spindle (Fig. 3, B, better developed in C and D). The two centrioles with their surrounding centrospheres, astral rays and connecting spindle, constitute the amphiaster. If the resting cell contains only one centriole, division of the centriole with formation of the amphiaster is usually the first phenomenon of mitosis, the connecting central spindle fibers appearing as the centrioles move apart. During or following the formation of the amphiaster, important changes occur in the nucleus. It increases somewhat in size and the reticulum charac- teristic of the resting nucleus becomes converted into a single long thread THE CELL AND CELL PROLIFERATION. 5 (spireme thread) arranged in a closed skein—closed spireme (Fig. 3, B). This soon becomes more loosely arranged, the thread at the same time becoming shorter and thicker and frequently broken, forming the open spireme. During the formation of the spireme the nucleolus and nuclear membrane usually disappear, the nucleoplasm thus becoming continuous with the cytoplasm. The spireme now lies with the amphiaster in the general cell protoplasm. The morphological change from reticulum to spireme is apparently accom- Fic. 3.—Diagrams of successive stages of mitosis. Wilson, A, Resting cell with reticular nucleus and true nucleus; c, two centrioles—the single preceding one having divided in anticipation of the division of nucleus and cell body. B, Early prophase. Chromatin forming a continuous thread—closed spireme; nucleolus still present; @, centrioles surrounded by astral rays and connected by achromatic spindle. C, Later prophase. Spireme has segmented to form chromosomes; astral rays and achromatic spindle larger and more distinct; nuclear membrane less distinct. D, End of prophase; ep, chromosomes arranged in equatorial plane of spindle. panied by changes of a chemical nature, as the spireme thread stains much more intensely than do the strands of the reticulum. The next step is the transverse division of the spireme thread into a number of segments (Fig. 3, C). These are usually at first rod-shaped, and are known as chromosomes. They may remain rod-shaped or the rods may become bent to form U’s or V’s. Some chromosomes are spheroidal. The most remarkable feature of the breaking up of the spireme thread to form 6 TEXT-BOOK OF EMBRYOLOGY. chromosomes is that the number of segments into which the thread divides, while differing for different species of plants and animals, is fixed and definite for each particular species. For example, in Ascaris megalocephala—a very convenient type for study on account of its simplicity—the number of chro- mosomes is 4, in the mouse 24. In man the number is not known with certainty; by some it is estimated at 16, by others at 24. There are thus at this stage present in the cytoplasm, two distinct though closely related structures—the amphiaster and the chromosomes, These together constitute the mitotic figure. As the chromosomes form they become arranged in the equator of the central spindle, along what is known as the equatorial plane (Fig. 3, D). When, as is frequently the case, the chromosomes are U-shaped, the closed ends of the loops lie toward the center, the open ends radiating. Three sets of fibers can now be distinguished in connection with the centrosomes (Fig. 3,C, D): (1) the fibers of the central spindle connecting the two centrosomes; (2) the polar rays which radiate from the centriole toward the periphery of the cell; (3) the mantle fibers which pass from the centrosomes to the chromosomes. The mitotic figure is at this stage known as the monaster, and its complete formation marks the end of the prophase. METAPHASE.—The essential feature of the metaphase is the longitudinal splitting of each chromosome into exactly similar halves (Fig. 4, E), each half containing an equal amount of the chromatin of the parent chromosome. In the case of U- or V-shaped chromosomes, the splitting begins at the crown and extends to the open ends. The latter often remain united for a time, giving the appearance of rings or loops. The significance of this equal longi- tudinal splitting of the chromosomes is apparent when one considers that through this means an exactly equal part of each chromosome and thus exactly equivalent parts of the chromatin of the parent nucleus are distributed to the nucleus of each daughter cell. ANAPHASE.—Actual division of the chromosomes having taken place, the next step is their separation to form the daughter nuclei. In separating, the daughter chromosomes pass along the fibers of the central spindle (Fig. 4, F), apparently under the guidance of the mantle fibers, each group toward its respective centrosome, around which the chromosomes finally become arranged (Fig. 4, G), thus forming two daughter stars. The mitotic figure is now known as the diaster. In actively dividing cells it is common for the centriole to undergo division at this stage, thus making four centrioles in the cell. (Fig. 4, F, G.) TELOPHASE (Fig. 4, H).—This is marked by division of the cytoplasm, usually in the equatorial plane of the achromatic spindle, and the reconstruction of the two daughter nuclei. Each new cell now contains a nucleus, a centrosome THE CELL AND CELL PROLIFERATION, 7 with its aster (or two centrioles with asters) and one-half the achromatic spindle. The resting nucleus is formed by a reverse of the series of changes described as occurring in the prophase, the chromosomes uniting end to end to form a skein or spireme, lateral buds appearing which anastomose, thus giving rise to the reticulum of the resting nucleus. The nucleolus reappears as mysteriously as it disappeared during the prophase and the nuclear membrane is reformed. Fic. 4.—Diagrams of successive stages of mitosis. Walson. E, Metaphase. Longitudinal splitting of chromosomes to form daughter chromosomes, ep; n, cast-off nucleolus. F, Anaphase. Daughter chromosomes passing along fibers of achromatic spindle toward centro- somes; centrioles again divided; 7f, interzonal fibers of central »pindle. G, Late anaphase. Chromosomes at ends of spindle; spindle fibers less distinct; thickenings of fibers in equatorial plane indicate beginning of cytoplasmic plate; cell body beginning to divide; nucleolus has disappeared. H, Telophase. Cell body divided; chromatic substance in each daughter nucleus as in resting stage; nuclear membrane and nucleolus has reappeared in each daughter cell. It is to be noted that the number of chromosomes which enter into the forma- tion of the chromatic reticulum of the resting nucleus is the same as the number of chromosomes derived from that nuclear reticulum when the cell prepares for mitotic division, It is thus possible that the chromosomes maintain their individuality even during the resting stage. In plant mitosis the central spindle fibers show minute chromatic thicken- 8 TEXT-BOOK OF EMBRYOLOGY. ings along the plane of future division of the cell, forming what is known as the mid-body or cell-plate. This splits into two layers, between which the division of the cell takes place. The formation of a distinct cell-plate in animal mitosis is rare. In place of this there is a modification of the cytoplasm along the line of future division, sometimes called the cytoplasmic plate. As to what may be called the dynamics of mitosis, there has been much controversy, but comparatively little has been definitely settled. It would appear that in most cases the centrosome is the active agent in initiating, and possibly in further controlling the mitotic process. Boveri, for this reason, refers to the centrosome as the “dynamic center” of the cell. The centriole first divides into two, around each of which an astral system of fibers is formed. The origin of these fibers appears to differ in different cells. Thus, in some cases—Infusoria, for example—the centrosome lies within the nucleus and the entire mitotic figure apparently develops from nuclear struc- tures. In some of the higher plants both central spindle fibers and asters are formed from the spongioplasm. In still other cases—for example, the eggs of Echinoderms—part of the figure (the asters) is developed from the cytoplasm, while the fibers of the central spindle are of nuclear origin. It must, however, be admitted that centrosome activity is not absolutely essential to cell division, for there are cases in which division of the chromo- somes occurs without division of the centrosome, while in the higher plants mitosis occurs, although no centrosome can be distinguished at any stage of the process. The behavior of the centrosome before, during and after mitosis varies in different cells. In some cells the centriole is apparently an integral part of the cell, persisting throughout the resting stage. With it may remain more or less of the aster, the whole constituting the already mentioned attraction sphere. In other cells—for example, mature egg cells—the centriole with its fibrils apparently entirely disappears during the resting stage. In regard to the origin of the chromatic portion of the mitotic figure, no difference of opinion exists, so evidently does it arise, as already noted, from the chromatic portion of the nuclear reticulum. Its destination in the nuclear reticulum of the daughter cells is equally well established. The details of the formation of the chromosomes vary. Thus in some cases there is no single spireme thread, the spireme being segmented from its formation, each segment of course corresponding with a future chromosome. In other cases no spireme whatever is formed, the chromosomes taking origin directly from the nuclear reticulum. In still other cases the spireme while yet a single thread splits longitudinally so that there are two threads present, the transverse divisions into chromosomes taking place subsequently. As to the time required for the mitotic process, considerable variation exists THE CELL AND CELL PROLIFERATION. 9 The process usually requires from one-half to three-quarters of an hour, but may extend over irom two to three hours. Mitosis is naturally most active wherever active growth of tissue is taking place—for example, in embryonic tissues, in granulation tissue, in the healing of wounds, in rapidly growing tumors (usually an evidence of malignancy). The earlier generations of cells derived from the fertilized ovum are indifferent cells in the sense that they are capable of development into any type of tissue cells. As differentiation takes place, the cells asstime more definite and fixed types. With differentiation, mitosis becomes less and less active and cells become incapable of producing cells of any type other than their own. Finally, the most highly differentiated (specialized) cells—for example, muscle cells and nerve cells—lose entirely their powers of reproduction, and if destroyed are not replaced by new cells of the same type. What is known as multipolar or pluripolar mitosis occurs in some of the higher plants, less commonly in the rapidly growing connective tissue of healing wounds and in cancer cells. Such atypical mitosis has also been artificially induced in rapidly dividing cells by the injection of chemical substances into the tissues. In multipolar mitosis the centrosome divides into more than two daughter centrosomes and not infrequently results in an unequal distribution of chromatin to the daughter cells. PRACTICAL SUGGESTIONS. The larval salamander and newt are classical subjects for the study of cell division. The small larve are fixed in Flemming’s fluid, cut into thin sections in either celloidin or par- affin and stained with Heidenhain’s hematoxylin (see Appendix). Mitotic figures may be found in almost any of the tissues. Certain vegetable tissues, such as magnolia buds or the root-tips of rapidly growing onions, also afford excellent material for the study of mitosis. The technic is the same as for animal tissues. References for Further Study. Conxitn, E. G.: Karyokinesis and Cytokinesis. Jour. Acad. Nat. Sci. of Philadel- phia, Vol. XII, 1902. Hertwic, O.: Die Zelle und die Gewebe. 1898. Liu, F. R.: A Contribution towards an Experimental Analysis of the Karyokinetic Figure. Science, New Series, Vol. XXVII, 1908. Witson, E. B.: The Cell in Development and Inheritance. 2d Ed., 1900. CHAPTER II. THE SEXUAL ELEMENTS—OVUM AND SPERMATOZOON. In practically the whole animal kingdom and without exception in the entire vertebrate series the development of a new being can take place only when reproductive elements, produced by two sexually different individuals, are brought into union at the proper time as the result of the procreative act. These elements are, on the one hand, the egg or ovum, which is produced by the female, and, on the other hand, the seminal filament or spermatozoén produced by the male. To a student of histology it is a matter of knowledge that the ovum and spermatozo6n are histological units or cells. It is a familiar fact, too, that both are produced in special glandular organs—the ovum in the ovary of the female and the spermatozoén in the testis of the male. Furthermore, it is known that during sexual maturity they detach themselves from the sexual organs at definite times. They then form under suitable conditions, among which the union of the two sex cells is the most important, the starting point of a new organism. As a necessary preliminary to the understanding of the development which follows the union of the two sex cells, a brief consideration of the structural and to a certain extent of the physiological peculiarities of these cells is essential. THE OVUM. The ovum or egg cell is a cell specially organized for the perpetuation of the species. It possesses those extraordinary peculiarities of growth and differ- entiation possessed by no other cell, which enable it to give rise directly to a new organism with all the characteristics of the species. Among Vertebrates, however, the process of development is made possible only by union with the male sexual element—the spermatozoén. It should be stated, moreover, that the ovum as seen in a Graafian follicle in the ovary is not a true sexual element, not having undergone certain processes which eliminate part of its chromatic substance and put it on a par with the spermatozoén as a mature sexual ele- ment (see page 27). The large egg cell in the ovary is properly called a primary odcyte. With the exception of some neurones, the human ovum (Fig. 5) is the largest cell in the body. It is spherical in shape, measuring from 0.15 mm. to 0.2 mm. in diameter, contains a large spherical nucleus and is surrounded by a relatively thick, transparent membrane. As seen in section in the ovary it has 10 THE SEXUAL ELEMENTS—OVUM AND SPERMATOZOON. 11 essentially the structure of a typical cell. Around the ovum and separated from it by a narrow cleft—the perivitelline space—is the zona pellucida, a rather thick, highly refractive membrane which shows radial striations. These striations are probably due to the presence of minute canals which penetrate the zona, It has been suggested that these canals serve for the passage of nutri- ment to the ovum. Immediately outside of the zona pellucida the epithelial cells of the Graafian follicle are arranged radially in one or two layers, These Corona radiata ~| Zona | pellucida Fic. 5.—From a section of the ovary of a 12 year old girl, The primary odcyte lies in a large mature Graafian follicle and is surrounded by the cells of the “‘ germ hill’ (the inner edge of which is shown in the upper left-hand comer of the figure). Photograph. constitute the corona radiata (Fig. 5). Some investigators have described a thin, delicate vitelline membrane between the perivitelline space and the ovum. Others have failed to observe this. The egg protoplasm, originally called the vitellus, differs from the pro- toplasm of most cells in that it appears somewhat more opaque and coarsely granular. This appearance is due to the fact that the ovum stores up within itself food stuffs. ‘These consist of fatty and albuminous substances which are 12 TEXT-BOOK OF EMBRYOLOGY. later utilized in the growth and increase of the embryonic cells. The food granules—deutoplasm—are suspended in the cytoplasm. The distribution, however, of these granules in the human ovum is not uniform; a mass of them being found in the center of the cell surrounding the nucleus, while an almost clear zone of cytoplasm forms the periphery of the cell. The nucleus of the ovum occupies a position near the center within the deutoplasm mass, though in the ovum of a mature Graafian follicle it is almost invariably slightly eccentric. It is large proportionately as the ovum is large. Its structure does not differ essentially from that of any other nucleus. There is a distinct nuclear membrane enclosing the usual nuclear structures—the nuclear liquid, the network of chromatin, the achromatic network and a single nucleolus or germinal spot (p. 2,Fig. 1). In a fresh human ovum amceboid movements have been observed in the nucleolus. The significance of the nucleolus is as little known as in any other cell. A centrosome, though it may be present, has not been observed in the human ovum. The amount and distribution of the yolk \ % granules have an important bearing upon the changes which take place in the egg subse- quent to fertilization and have led to the = — classification of eggs as alecithal, telolecithal Fic. 6.—Semidiagrammatic representa- and centrolecithal. Alecithal eggs (Fig. 5) are tion of ovum of frog (Rana sylvatica). . : : The dark shading represents the cyto- those in which the yolk granules are fairly ca ela terete pee ca bene evenly distributed throughout the cytoplasm diel oe eo ae (Amphioxus, most Mammals, including man). stance (secondary egg membrane). Telolecithal eggs (Figs. 6 and 7) are those in which the yolk isin excess at one pole, the cytoplasm at the opposite pole (Amphibians, Reptiles, Birds). Centrolecithal eggs are those in which a central yolk mass is surrounded by a compara- tively thin layer of cytoplasm (Arthropods). (For further description see Cleavage, page 42.) Telolecithal ova show a condition known as polar differentiation. By polar differentiation is meant the more or less complete separation of cytoplasm and deutoplasm, so that the cytoplasm is present in excess at one pole of the egg and the deutoplasm in excess at the opposite pole. The frog’s egg is a familiar example of this differentiation, the dark side of the egg indicating an excess of cytoplasm. Inasmuch as deutoplasm is generally heavier than cytoplasm, an egg with polar differentiation, if left free to revolve, as in water, will assume a definite position with the protoplasmic or animal pole above and the deuto- THE SEXUAL ELEMENTS—OVUM AND SPERMATOZOON. 18 plasmic or vegetative pole below. An exception to this is found, however, in the pelagic teleost eggs, which float with the deutoplasmic pole upward, In the hen’s egg the cytoplasm and deutoplasm are. distinct and separate with no mingling of the two substances (Fig. 7). While still in the ovary, the egg consists of the yellow yolk in the form of an enormously large cell sur- rounded by the zona pellucida, upon which lies a small white spot, the so- called germinal disk. The disk is 3 or 4 mm. in diameter and consists of finely granular protoplasm with a somewhat flattened nucleus. This disk Germinal disk (cytoplasm) White yolk AQG Shell i Shell membrane (outer layer) _ Albumen (‘white’) + Vitelline membrane By Chalaza ge Shell membrane White yolk (inner layer) Yellow yolk (deutoplasm) Fic. 7.—Diagram of a vertical section through an unfertilized hen’s egg. Bonnet. alone gives rise to the embryo proper. All the rest of the mass consisting of a vast number of spherules united by a small amount of cement substance, is simply nutritive material or deutoplasm which is later utilized for the nourish- ment of the embryo. THE SPERMATOZOON. In marked contrast to the ovum, the spermatozo6n is one of the smallest cells of the body, being only about fifty microns in length. The spermatozoén, as seen in the seminal fluid, in any of the sexual passages, or even in the lumen of a seminiferous tubule, is a true sexual element, since it has passed through certain processes which prepare it for union with the mature ovum. (See Spermatogen- esis, Chap. III.) Like the ovum the spermatozoén is an animal cell of which, however, both cell body and nucleus have undergone important modifications. The flagellate spermatozoén, of which the human spermatozoén is an example (Fig. 8), resembles a tadpole in shape and like the latter swims about by means of the undulatory movements of its long slender flagellum or tail. It consists of (1) a head, (2) a middle-piece or body and (3) a éail. t. THE Hrap.—This in the human spermatozoén is from three to five microns long and about half as broad. On side view it appears oval; when 14 TEXT-BOOK OF EMBRYOLOGY. seen on edge, it is pear-shaped, the small end being directed forward. It consists mainly of nuclear material derived from the nucleus of the parent cell. (See Spermatogenesis.) Acrosome Galea | capitis q oO Py ror os ——eee Anterior end knob Posterior end knob Spiral fibers Sheath of axial thread Main segment gy Axial thread of tail r—— Capsule Terminal filament ~ Fic. 8.—Diagram of a human sperma- tozobn. Meves, Bonnet. A thin layer of cytoplasm, the galea capitis or head- cap, envelops the nuclear material, while in front there is a sharp edge known as the apical body or acrosome. In contrast to the nuclear portion of the head, which of course takes a basic stain, the acrosome stains with acid dyes. In some forms the acrosome is much larger than in man and extends forward from the head-cap as a long spear, sometimes barbed—the perforatorium. This process perhaps assists the spermatozo6n in clinging to or in burrowing its way into the ovum. Many peculiar types of perfora- for example, lance-shaped, awl- shaped, spoon-shaped, corkscrew-shaped, have been described and have given charac- teristic names to the spermatozoa possessing them. 2. THE Bopy in the human sperma- tozoon is cylindrical and about the same length as the head. It consists of a deli- cately fibrillated cord, the axial thread, sur- rounded by a protoplasmic capsule. In some forms (Mammals) a short clear por- tion, the neck, unites the head and body. In the neck there can sometimes be demon- strated an anterior end knob and one or more posterior end knobs to which is attached the axial filament. In man and in some other forms, delicate fibers—spiral fibers— wind spirally around that portion of the axial filament which lies within the body. At the posterior end of the body, the axial filament passes through the end disk or end ring. toria, 3. THE TAIL in the human spermatozo6n is forty to fifty microns in length; is the direct continuation of the axial thread of the body; and consists of a main segment thirty-five to forty-five microns in length, and a short terminal segment. As in the body, the axial filament is delicately fibrillated. Sur- THE SEXUAL ELEMENTS—OVUM AND SPERMATOZOON. 15 rounding the axial filament is a thin cytoplasmic membrane or capsule continuous with that of the body. In the human spermatozoén it is ap- - parently structureless; in other forms it assumes curious shapes as, for example, the so-called membrana undulatoria, or wavy membrane of Amphibia, or the fine membrane of some Insects. The terminal segment consists of the axial fila- ment uncovered by any sheath. The significance of the various parts of the spermatozoén can be best understood by reference to spermatogenesis (p. 21). Comparing the spermatozoén with a cell, the head contains the nucleus while the body contains the centrosome. It is these parts of the spermatozoén which are essential to fertilization. The acrosome and the tail may therefore be considered as accessory structures which serve to bring and attach the spermatozo6dn to the ovum. Within the tubule of the testis the spermatozoa show no evidence of motile power. In the semen, however, which consists mainly of fluid secretions of the accessory sexual glands, they move about freely, as also in the fluids of the female genital tract. Their speed has been estimated at from 1.5 to 3.5 mm. per minute and enables them to swim up through the uterus and oviduct, in spite of the fact that the action of the cilia lining these tracts is against them. The life of the spermatozo6n within the female genital tract is not known. Moving spermatozoa have been found there seven to eight days after coitus. In one case reported of removal of the tubes, living spermatozoa were found three and one-half weeks after coitus. PRACTICAL SUGGESTIONS. The fresh unfertilized eggs of the star-fish or sea-urchin serve for general pictures of alecithal ova. They are carefully removed from the animal and prepared in the following manner. It is best to use slender vessels in order to facilitate changing the fluids. Formalin 5% or Flemming’s fluid, few hours. Water (after Flemming’s fluid only) 2 or 3 changes, few hours. Alcohol 70%, 1 or 2 changes, few hours. Borax carmin, 24 hours. HCl 1% in alcohol 70%, 12 to 24 hours. Alcohol 70%, several changes, few hours. Alcohol 95%, few hours. Alcohol absolute, few hours. Xylol, few hours. Thin balsam, indefinitely. A small amount of the balsam containing some of the ova is taken up in a pipette, placed on a slide and covered with a cover-glass. For the study of the mammalian (alecithal) ovum, the ovary of a dog or cat, or of a girl or young woman may be fixed in Orth’s fluid, or in Bouin’s fluid (see page 631), embedded 2 16 TEXT-BOOK OF EMBRYOLOGY. and cut in either celloidin or paraffin, stained in hematoxylin and eosin (see page 6 36) and mounted in xylol-damar. The frog’s eggs are good examples of telolecithal ova with a definite but not complete polar differentiation. They may be collected in ponds in the early spring and preserved indefinitely in 5 per cent. formalin. For gross study a few of the ova may be separated from the general mass and examined under a hand lens. For internal structure, the ova may be carefully removed from the gelatinous mass, stained im tofo with borax carmin, embedded in celloidin or paraffin, sectioned, and mounted in damar. Ova fixed in any osmic acid mixture will show the yolk granules stained black. The various parts of a hen’s egg (telolecithal ovum with extreme polar differentiation) may be observed by simply removing a part of the shell. To study the egg, however, with- out the secondary membranes, it must be seen in the hen’s ovary. References for Further Study. ConkLin, E. G.: Organ-forming Substances in the Eggs of Ascidians. Biol. Bull., Vol. VIII, 1905. Keser, F. and Matt, F. P.: Manual of Human Embryology, 1910. Vol. I, Chap. I. WaLpeyer, W.: In Hertwig’s Handbuch der vergleichenden «. experimentellen Entwick- elungslehre der Wirbeltiere. Bd. I, Teil I, 1903. Also contains extensive bibliography. Witson, E. B.: The Cell in Development and Inheritance. 2d Ed., rg00. CHAPTER III. MATURATION. In the preceding chapter (p. 10) it was stated that in practically the entire animal kingdom, and without exception in Vertebrates, the condition essential to the production of a new individual was the union of two sexually different cells. Before this can take place, however, the sex cells must pass through certain preliminary and preparatory processes which are known collectively as reduction of chromosomes or maturation. The manner in which this reduction takes place is in most sex cells extremely difficult of demonstration, this being especially true in the higher forms, notably in Mammals. The fact, however, that such reduction occurs in all cases in- vestigated, where there is sexual reproduction, is undisputed. The result also is invariably the same. The number of chromosomes in the mature sex cell is reduced to half the somatic number for the species. MATURATION OF THE OVUM. Because of the difficulties of observing this process in higher forms, it is advisable to describe first the maturation of the ovum in such a simple type as Ascaris. This has become a classic for the study of maturation owing to the fact that it shows the various stages of the process with remarkable clearness. In Ascaris at about the time the spermatozoon enters the ovum, the chromatic elements of the nucleus are found collected into two groups and each group in turn composed of four rod-shaped pieces (Fig.9, A,B,C and D). The groups are known as éetrads, and the number of tetrads is always one-half the usual number of chromosomes. In this particular form the usual number of chromosomes is four. An achromatic spindle next forms as in ordinary mitosis, and two of the chromatin rods from each tetrad pass out into a small mass of cytoplasm which becomes separated from the ovum (Fig. 9, E, F, G and H) as the first polar body. The four chromatin rods which remain in the ovum constitute two dyads; each dyad representing one-half of a tetrad. Closely following the formation of the first polar body and without any return of the chromatin to a resting stage, the second polar body is given off. This is accomplished by two chromatin rods, one from each dyad, passing out from the ovum, surrounded by a small mass of cytoplasm, as in the case of the first polar body. There thus remain in the ovum two rods or two chromosomes, one-half the usual number 17 18 TEXT-BOOK OF EMBRYOLOGY. (Fig. 9, I, J and K). The somatic number of chromosomes has been reduced one-half, and the egg nucleus—jemale pronucleus—is ready for union with the sperm nucleus—male pronucleus. Since the chromatin rods left after the formation of the polar bodies are Fic. 9.—Maturation of the ovum of Ascaris megalocephala (bivalens). Boveri, Wilson. A, The ovum with the spermatozoén just entering at .7; the egg nucleus contains 2 tetrads (one not clearly shown), the somatic number of chromosomes being 4. B, Tetrads in profile. C,Tetrads onend. D, E, First spindle forming. F, Tetrads dividing. G, First polar body formed, containing 2 dyads; 2 dyads left in the ovum. Hy, J, Dyads rotating in preparation for next division. J, Dyads dividing. K, Each dyad divided into 2 single chromosomes, thus completing the reduction. MATURATION. 19 considered as chromosomes, the question arises as to the relation that the rod- like elements composing the tetrads bear to the chromatin of the nucleus before the tetrads are formed. In other words, what is the origin of the tetrads? The answer to this, even in Ascaris, is not quite clear. It is generally agreed, how- Fic. 10.—From section of ovum (primary odcyte) of the mouse, showing first maturation spindle. Note the 12 chromatin segments, the somatic number of chromosomes being 24. The ovum is surrounded by the zona pellucida (z.p.) and the corona radiata. Sobotta. ever, that after the formation of the spireme thread, the latter segments into two chromatin rods instead of four; that is, it segments into one-half the usual number of pieces. Each piece splits longitudinally and then each resulting piece splits longitudinally again. Thus each primary segment gives rise to Fic. 11.—-From sections of ova of the mouse. The figure on the left shows a tangential matu- ration spindle with 12 chromatin segments. The figure on the right shows a tangential maturation spindle in which each of the r2 chromatin segments has divided transversely into two equal parts, thus forming 24 segments. Sobotta. four segments which constitute a tetrad. From this it is evident that the crucial point in reduction is the segmentation of the spireme thread. In Ascaris, as described above, reduction takes place by means of tetrad formation, the tetrads being formed by a double longitudinal splitting of the 20 TEXT-BOOK OF EMBRYOLOGY. primary chromatin rods. Tetrad formation, however, is of comparatively rare occurrence, for in most animals, especially in the higher forms, no distinct tetrads are formed. It is possible, however, that certain chromatin structures in the primary odcyte may be analogous to tetrads. In the mouse, for example, the most thoroughly investigated mammalian form, the spireme thread prob- Fic. 12.—From sections of ova of the mouse, showing three stages in the maturation process, A, Ovum showing prophase of maturation division. /, fat; z.p., zona pellucida, B, Ovum showing maturation spindle with chromatin segments undivided. C, Ovum showing diaster stage of maturation division, formation of rst polar body (.b.), and sperm nucleus (male pronucleus, m.pn.) just after its entrance. Sobotta. ably segments into one-half the usual number of pieces, but the further behavior of these pieces differs from their behavior in Ascaris. Each chromatin seg- ment is possibly equivalent to a tetrad; and the number of chromatin segments is one-half the somatic number of chromosomes, just as the number of tetrads is always one-half the somatic number of chromosomes. When the first Fic. 13.—From sections of ova of the mouse, showing the polar bodies (p.b.y and three stages of the male (m.pn.) and female (f.pn.) pronuclei. Sobotta, maturation spindle forms in the ovum—primary odcyte—of the mouse, twelve chromatin segments are found grouped in the equatorial plane (the somatic number being twenty-four) (Fig. ro). Each chromatin segment next divides transversely into two equal parts (Fig. 11). One part from each segment now passes out into a small globule of cytoplasm which becomes detached from the MATURATION. 21 ovum and forms the first polar body (Fig. 12), thus leaving within the ovum secondary odcyte—twelve parts or pieces. Closely following the formation of the first polar body, the twelve remaining pieces divide again transversely and one-half of each piece passes out into the second polar body (Fig. 13). Within the ovum—now the mature ovum—there still remain twelve pieces of chromatin or twelve chromosomes—one-half the somatic number. As a matter of fact, Sobotta, who studied the process in the mouse, observed in the majority of cases only one polar body (Fig.13, A). Later investigators have suggested that the second polar body also was probably formed but at such a time or in such a plane that Sobotta had failed to see it. If only one polar body is formed, the reduction is atypical; but if, as suggested later, two polar bodies are formed, it would make the process typical. In either case the result is the same so far as the number of chromosomes is concerned, but in the former case the bulk of chromatin is reduced only to one-half, in the latter case it is reduced to one-fourth the original amount. SPERMATOGENESIS. The maturation of the male sex cells in the majority of forms is perhaps more difficult of demonstration than the maturation of the female sex cells, both on account of the extreme minuteness of the former and on account of the fact that it is necessary to consider all the generations of cells from the mature spermatozoa back to the spermatogonia. In some forms the reduction of chromosomes has been demonstrated, and it is reasonable to assume that it occurs in all forms. In Ascaris, for example, the reduction has been clearly shown to occur, as in the case of the ovum, through tetrad formation (Fig. 14). In the higher forms, and especially in Mammals, the process has not been observed with such accuracy; but it is possible in a general way to trace the changes through which the cells pass to arrive at the stage of mature sperma- tozoa. In the mammalian testis (Fig. 15, A) the stratified epithelium of the con- voluted seminiferous tubule consists of two distinct kinds of cells: (1) the so-called supporting cells or cells of Sertoli, and (2) the different generations of male sex cells or spermatogenic cells. In a portion of a tubule where active formation of spermatozoa is not going on, the cells are arranged as follows. Upon the basement membrane is a single layer of small oval or cuboidal cells with nuclei rich in chromatin. These are known as spermatogonia. Internal to these are one or two layers of larger round or oval cells with large, vesicular, densely staining nuclei, the spermatocytes. Between these and the lumen are several layers of small, oval cells whose nuclei contain closely-packed chromatin granules. These are the spermatids. Usually the lumen of the tubule contains a number of mature spermatozoa, which may lie either free in the 22 TEXT-BOOK OF EMBRYOLOGY. lumen or with their heads embedded in the supporting cells. The supporting cells extend from the basement membrane to the Jumen of the tubule where they frequently spread out in a fan-like shape. The developmental cycle through which the spermatogenic cells pass, in- cluding reduction of chromosomes, occurs in a definitely progressive manner Fic. 14.—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 the 2 tetrads (D, in profile, E,onend). F,G, H, First division to form 2 secondary spermatocytes, each receiving 2 dyads. IJ, ’ Secondary spermatocyte. J, K, The same dividing. LZ, Two resulting spermatids, each containing 2 single chromosomes. along length-segments of the seminiferous tubule. Thus a particular cross section may contain only the earlier stages of spermatogenesis, the succeeding serial sections containing the middle and later stages. After the sections con- taining fully developed spermatozoa, the next succeeding sections contain the early stages again. , MATURATION. 23 E spm | Spe | spe “| oo ‘ff i : it! / Mm, pe : “| spz spm spe Spe SPE spe Fic. 15.—From sections of seminiferous tubules of Rat, showing successive stages in spermato- genesis. Lenhossek. A, Typical section showing clumps of spermatozoa (spz) with their heads embedded in Sertoli cells; several layers of spermatids (spm); layer of spermatocytes (spc) with nuclei preparing for division; layer of spermatogonia (spg) next basement membrane. B, Section showing beginning of transformation of spermatids (spm) into spermatozoa; spermato- cytes (spc) with larger cell bodies and spiremes in nuclei. C, Section showing large spermatocytes (spc) in process of division; spermatogonia (spg) pre- paring for division to form new spermatocytes. Spermatids of preceding stage (B) are repre- sented here by spermatozoa (sz). D, Section showing further advance. The primary spermatocytes of the preceding stage (C) have divided to form secondary spermatocytes and the latter to form the spermatids (spm) represented here. During these divisions the reduction of chromosomes has taken place. The spermatogonia of the preceding stage have divided, one daughter cell from each forming a new spermatocyte (spc), the other still remaining as a spermatogonium (sg). 24 TEXT-BOOK OF EMBRYOLOGY. The spermatogonia multiply by ordinary mitosis. During the period of sexual maturity of the individual, the spermatogonia are constantly proliferat- ing, while at the same time, some of the spermatogonia, ceasing to proliferate, enter upon a period of growth in size (Fig. 17). When the limit of this growth is reached, these cells are known as primary spermatocytes. These large cells -have distinct nuclear networks. As the primary spermatocyte prepares for division, the spireme thread probably segments into one-half the usual number of pieces. As already noted, this results in Ascaris in the formation of tetrads. The process is not so clear, however, in higher forms, but in all cases, whether there is tetrad formation or not, the behavior of the chromatin is such that each daughter cell—secondary spermatocyte—receives one-half the usual num- ber of chromatin segments.* Without any return of the nucleus to the resting state, each secondary spermatocyte divides into two cells which are known as spermatids, each of which also receives one-half the usual number of chromatin segments, or chromosomes. Thus each primary spermatocyte gives rise to four spermatids, each of which contains one-half the somatic number of chromosomes. A careful study and comparison of the stages represented in Fig. 15, A, B, Cand D will assist in understanding the processes of spermatogenesis. After the last spermatocyte division and the resulting formation of the spermatid, the nucleus of the latter acquires a membrane and intranuclear net- work, thus passing into the resting condition. Without further division the spermatid now becomes transformed into a spermatozoén. ‘This is accomplished by rearrangement and modification of its component structures (Fig. 16). The centrosome either divides completely, forming two centrosomes, or partially, forming a dumb-bell-shaped body between the nucleus and the surface of the cell. ‘The nucleus passes to one end of the cell and becomes oval in shape. Its chromatin becomes very compact and is finally lost in the homogeneous chromatin mass which forms the greater part of the head of the spermatozoén. Both centrosomes apparently take part in the formation of the middle piece. The one lying nearer the center becomes disk-shaped and attaches itself to the posterior surface of the head. The more peripheral centrosome also becomes disk-shaped and from the side directed away from the head a long delicate thread grows out—the axial filament. The central portion of the outer cen- trosome next becomes detached and in Mammals forms a knob-like thickening— end knob—at the central end of the axial filament. In Amphibians this part of the outer centrosome appears to pass forward and to attach itself to the inner centrosome. In both cases the rest of the outer centrosome in the shape of a ring passes to the posterior limit of the cytoplasm. As the two parts of the posterior centrosome separate, the cytoplasm between them becomes reduced in amount, at the same time giving rise to a delicate spiral thread—the spiral *According to Bonnet, the reduced number of chromosomes first appears in the spermatids, MATURATION. 25 filament—which winds around the axial filament of the middle piece. Mean- while the axial filament has been growing in length and part of it projects be- yond the limits of the cell. The cytoplasm remaining attached to the anterior part of the filament surrounds it as the sheath of the middle piece. In Mam- mals there appears to be more cytoplasm than is needed for the formation of the sheath of the middle piece, and a large part of it degenerates and is cast aside. Head Head Anterior end knob Posterior end knob Anterior end knob L | fax——- Posterior end knob Body ? “~ End ring End ring Tail 2 = Cytoplasm __— Proximal centrosome Distal centrosome Fic. 16.—Transformation of a spermatid into a spermatozoén (human). Schematic. Meves, Bonnet. The sheath which surrounds the main part of the axial filament appears in some cases at any rate to develop from the filament itself. The galea capitis or delicate film of cytoplasm which covers the head is undoubtedly a remnant of the cytoplasm of the spermatid. The developing spermatozoa lie with their heads directed toward the base- ment membrane, and attached, probably for purposes of nutrition, to the free ends of the Sertoli cells (Fig. 15). Their tails often extend out into the lumen of the tubule. When fully developed they become detached from the Sertoli cells and lie free in the lumen of the tubule. 26 TEXT-BOOK OF EMBRYOLOGY. Comparing maturation in the male and female sex cells, it is to be noted that the descendants of the primordial germ cells, the spermatogonia and the oégonia, proliferate by ordinary mitotic division, with the preservation of the somatic number of chromosomes, up to a certain definite period in their life history. They then cease proliferating for a time and enter upon a period of growth in size (Fig. 17). The results of this growth are the primary spermato- cyte and the ovarian egg or primary odcyte. When, however, the primary spermatocyte and the primary oécyte prepare for division, the nuclear reticulum in each case resolves itself into one-half the somatic number of chromosomes, e aes a aac asad e e ra ‘ Spermatogonia ye \ | \ Fraltier: oeee e @ Eades y m e @ Oégonia Pa Pe Proliferation /\ /\ |\ aoe @ e e ‘e -© @60@86088 0 /\ /\ I\ \ Primary \ Growth spermatocyte -@ @@0@6€@086 & Primary Growth / \ odcyte Secondary «-++++2++--+-- zt \ Maturation spermatocyte PA / Spermatid --------+--@ @ @ Secondary Maturation | | | Trans- oocyte wht. GvetAeeos formation Spermatozoon : Mature } { { ovum Fic. 17.—Diagrams representing the histogenesis of (a) the female sex cells and (b) the male sex cells. Modified from Boveri. and this reduced number is given to each of the resulting secondary spermato- cytes and secondary odécytes. There is, however, this marked peculiarity in the division of the primary oécyte, in addition to the reduction in chromosomes, that while the di- vision of the nuclear material (chromosomes) is equal, the division of the cytoplasm is very unequal, most of the latter remaining in one cell, usually designated the secondary oécyte proper. The other cell is of course small, owing to its lack of cytoplasm, and is extruded from the odcyte proper as the first polar body (Figs. 9, 12,13 and 17). In the next division, that of the second- ary odcyte, a similar condition obtains. Each resulting cell contains the re- duced number of chromosomes, but one of the cells is large containing nearly all the cytoplasm and is the mature egg cell, while the other is small owing to its small amount of cytoplasm, and is extruded from the larger cell as the second MATURATION. 27 polar body (Figs. 9, 12, 13 and 17). In some cases the first polar body divides, giving to each resulting cell the reduced number of chromosomes. There have thus resulted from the reduction division in the odcyte, three or four cells (Fig. 17). One of these is the mature ovum containing one-half the somatic number of chromosomes, the other two or three are the cast off and apparently useless polar bodies, each of which contains one-half the somatic number of chromosomes. The primary spermatocyte, on the other hand, gives rise to four cells which are equal in size as well as in their chromatin content (Fig. 17). There thus develop from both spermatocyte and odcyte four structures, each containing one-half the somatic number of chromosomes, only, as already noted, in the case of the spermatocyte, all four of the resulting products of division become functional as spermatozoa, while in the case of the odcyte only one product, the mature egg cell, becomes of functional value. (Compare Fig. 17 @ and 3.) The time of formation of the polar bodies differs for different eggs. In some cases both polar bodies are extruded before the entrance of the spermato- zoén, In other cases one polar body is formed before, the other after the entrance of the spermatozoén. In still other eggs both polar bodies are formed after the entrance of the spermatozoén. From the above description it is evident that the phenomena of maturation are essentially similar, and the process itself is identical in both male and female sex cells. The details of the process vary, but the result—the reduction of the number of chromosomes in the mature sex cell to one-half the number char- acteristic for other cells of the species—is always the same. The exact manner in which reduction takes place has been the subject of much investigation and controversy and has as yet been determined for com- paratively few forms. In the higher animals the point at which the actual reduction in number of chromosomes takes place is usually two generations of cells before the formation of the mature sex cells. The behavior of the chromatic spireme of the odcyte or of the spermatocyte, as it breaks up into one- half the somatic number of chromosomes, varies sufficiently to allow two general types of maturation to be distinguished, known, respectively, as reduc- tion with tetrad formation and reduction without tetrad formation. In reduction with tetrad formation the spireme (primary oécyte or primary spermatocyte) segments into the reduced number of chromatin masses, each one of which divides into four pieces and is consequently known asa tetrad. The two maturation divisions now follow, the second following the first rapidly with- out reconstruction of a nuclear reticulum. In the first of these each tetrad divides equally, giving rise to two dyads, each consisting of two pieces of chromatin and each passing to one of the daughter cells (secondary odcyte, first polar body, spermatocyte) (Figs. 9 and 14). In the second maturation 28 TEXT-BOOK OF EMBRYOLOGY. division each dyad gives one of its pieces to each daughter cell (mature ovum, second polar body, spermatid) (Figs. 9 and 14). In reduction without tetrad formation, the spireme segments into the re- duced number of pieces. Each of these segments, however, does not show an immediate differentiation into four pieces to form a tetrad, as in the case of reduction with tetrad formation; but in the first maturation division each chromatin segment divides equally, giving half its substance to each daughter cell. Then there may be a more or less complete reconstruction of the nucleus. Finally, in the second maturation division, each of the halves of the original chromatin segments divides again, one part passing to each daughter cell. Comparison of the two types of reduction makes evident the fact that there is no essential difference between them, other than the time of division of the reduced number of chromatin masses into their ultimate number for distribu- tion to the four granddaughter cells. In reduction with tetrad formation, the four ultimate divisions are evident from the first. In reduction without tetrad formation no such subdivision is apparent, the division of each chromatin mass into two being accomplished apparently by the first maturation division and into the final four of the mature cells, by the second maturation division. The apparent difference between male and female maturation—the single functional cell in the female as contrasted with the four in the male—loses some of its significance when one notes that in some forms the polar bodies are not so rudimentary as is generally the case. Thus in certain forms one or more of the polar bodies may develop into cells very similar to the mature egg cell, may be penetrated by spermatozoa, and may even become fertilized and proceed a short distance in segmentation, There is thus warrant for considering the polar bodies rudimentary or abortive ova. Of interest in connection with the apparent necessity, in sexual reproduction, that the egg rid itself of the polar bodies before the female pronucleus is ready for union with the male pronucleus, is the fact that in certain parthenogenetic eggs only a single polar body is extruded, while the eggs of the same species which are to be fertilized produce two polar bodies; also that in certain cases of parthenogenesis the second polar body has been shown to form what is ap- parently a pronucleus, so that there are two pronuclei within the egg cytoplasm, both derived from the secondary odcyte. These unite to form the segmenta- tion nucleus, the second polar body thus acting the part of the male pronucleus. A brief discussion of accessory chromosomes will be found under the head “Determination of Sex” in the chapter (XV) on the Urogenital System. Theoretical Aspects of Reduction. When we come to consider why reduction of chromosomes occurs, we are led into a. maze of conflicting hypotheses. The field is purely speculative in character. Some of MATURATION. 29 the hypotheses have received support from observation, others are still hypotheses in the strictest sense of the word. Some of the earlier investigators interpreted reduction merely as a means to prevent the summation of the nuclear mass in succeeding generations, or in other words, as a means to prevent the doubling of the number of chromosomes in each succeeding generation. For if there were no reduction, the egg cell and the sperm cell each would bring to the segmenta- tion nucleus the full amount of chromatin or the somatic number of chromosomes. This would result in the doubling of the chromatin mass at each fertilization. While reduction does, as a matter of fact, prevent this summation, the inadequacy of this explanation is apparent when one considers that in the vast majority of cases the mass reduction is not one-half but three-fourths, and furthermore that mass reduction really means but little because the bulk of the nuclear substance may increase or decrease enormously at different periods in the life history of the cell. Another interesting view first held by Minot, and later adopted by others, was that the ordinary cell is bisexual or hermaphroditic, and that maturation is an effort on the part of the germ cells to rid themselves of the opposite sexual elements, i.e., the ovum rids itself of its male elements to become a true female sex cell; the spermatozoén of its female elements, to become a true male sex cell. This theory also met with a serious objection when it was found that four functional spermatozoa are derived from a single primary spermatocyte. The fact that female qualities are transmitted by the male germ cell and male qualities by the female germ cell is also opposed to this theory. The most modern theory is extremely complex and is closely associated with, even forms a part of, the modern ideas of inheritance. Weismann first, in 1885, considered the deeper meaning of reduction and set forth his views in an article of highly speculative character, but which gave great stimulus to further study of the problem. Weismann’s first assump- tion was that ‘Chromatin is not a uniform and homogeneous substance, but differs quali- tatively in different regions of the nucleus; that the collection of the chromatin into the spireme thread and its accurate division into halves is meaningless unless the chromatin in different regions of the thread represents different qualities which are to be divided and distributed to the daughter cells according to some definite law.” He argued that if the chromatin were the same throughout, mitosis would be superfluous and direct cell division " would be sufficient. The real starting-point of Weismann’s theory is that the chromatin is a colony of minute particles—biophores—each of which has the power of developing some quality. The biophores are grouped together into larger masses or determinants, and the latter are grouped to form ids. The ids are identified with the visible chromatin granules. Each id is assumed to possess potentially the complete architecture of the species, i.e., each id has the power of determining the development of all the qualities characteristic of the species. The ids differ slightly from one another according to individual variations within the species, and are arranged in a linear manner to form the chromosomes. Thus each chromosome is a group of slightly different germ-plasms and differs qualitatively from all others. The interpretation of these hypotheses and their formulation into a theory leads to still greater complexity. If there were no reduction and each germ nucleus brought to the segmentation nucleus the full amount of chromatin, the number of chromosomes would be doubled and consequently the number of ids. And since each id has the power of deter- mining the development of all the qualities of the species, an infinite complexity would arise after a few generations. By reducing the ids both in size and in number in each germ cell, the tendency toward this infinite complexity would be held in check. Thus on the assump- 30 TEXT-BOOK OF EMBRYOLOGY. tion that the ids are arranged in linear series in the chromosomes, Weismann ventured the prediction that two forms of mitosis would be found to occur. One would be a longitudinal splitting so that each daughter nucleus would receive one-half the amount of each id. This form of division was known at that time and is characteristic of ordinary mitosis. The other form would, he suggested, be such as to give each daughter nucleus one-half the number of ids. This might be brought about either by transverse division of the chromosomes or by the elimination of one-half the number of chromosomes. This form of division was not known at that time and the fulfillment of this second part of Weismann’s predictions has been one of the most remarkable discoveries in cytology, for it has been demonstrated for some forms at least that transverse division of chromosomes does actually occur. Weismann’s views form a wonderfully ingenious theory against which there is thus far no structural ground for opposition. Indeed, some known facts are in its favor. Whether, however, these facts possess the significance which Weismann attributed to them is still an open question. Germinal epithelium Stratum granulosum Tunica albuginea Germ hill Theea folhiculi with ovum Fic. 18.—From section of human ovary, showing mature Graafian follicle ready to rupture. Kollmann’s Ailas. OVULATION AND MENSTRUATION. By ovulation is meant the periodic discharge of the ovum from the Graafian follicle and ovary. By menstruation is meant the periodic discharge of blood from the uterus associated with structural changes in the uterine mucosa. The two phenomena are usually associated although either may occur independ- ently of the other. They normally occur every twenty-eight days. That ovulation and menstruation are not necessarily dependent upon each other and that either may occur without the other has been proved by a number of MATURATION. 31 observations; thus the occurrence of fertilization during lactation when the menstrual function is in abeyance; the occurrence of impregnation in young girls before the onset of the menstrual periods and in women a number of years after the menopause. Leopold reports the examination of twenty-nine pairs of ovaries on successive days after menstruation and the finding of Graafian follicles just ruptured or just ready to rupture on the eighth, twelfth, fifteenth, eighteenth, twentieth and thirty-fifth days. He reports also five cases in which there were no evidences of ovulation during menstruation. At the time of ovulation the mature follicle, which has a diameter of 8 to 12 mm., occupies the entire thickness of the ovarian cortex, its theca being in con- Fic. 19.—Showing ovary opened by longitudinal incision. The ovum has escaped through the tear in the surface of the ovary. The cavity of the follicle is filled with a clot of blood (corpus hamor- rhagicum) and irregular projections composed of lutein cells, Kollmann’s Atlas. tact with the tunica albuginea (Fig. 18). Thinning of the follicular wall nearest the surface of the ovary, and increase in the amount of the liquor folliculi, thus causing increased intrafollicular pressure, finally result in rupture of the “follicle through the surface of the ovary and the escape of the ovum together with the liquor folliculi and some of the follicular ceils. The escaped ovum normally passes into the fimbriated end of the Fallopian tube and so to the uterus. In exceptional cases it,may remain in the tube after fertilization and so give rise to a tubal pregnancy, or, falling into the abdomi- nal cavity and becoming there fertilized, to an abdominal pregnancy. Both are known as ectopic gestations. As the ovum escapes from the follicle there is more or less bleeding into the follicle from the torn vessels of the theca. Closure of the opening in the follicle results in a closed cavity containing a blood clot, the corpus (Fig. 19) 3 32 TEXT-BOOK OF EMBRYOLOGY. a hemorrhagicum, which then becomes gradually transformed into the corpus luteum. The transformation of the corpus hemorrhagicum into the corpus luteum (Figs. 20 and 2r) is brought about by ingrowths of strands of connective tissue from the inner layer of the theca and the replacement of the remainder of the clot by large yellowish cells containing pigment (lutein granules) and known as lutein cells, The lutein cells are considered by some as derived from the con- nective tissue cells of the theca, by others as due to proliferation of the cells of the stratum granulosum. Degeneration and absorption of the tissues of the Cavity of follicle Theca folliculi Corpus hemorrhagicum Blood vessel of theca = Ovarian stroma Stratum granulosum Fic. 20.—From section of human ovary, showing early stage in formation of corpus luteum. Kollmann’s Atlas. corpus luteum follows, resulting in shrinkage, and loss of its characteristic yellow color. The whitish body resulting is known as the corpus albicans and is itself in turn either wholly absorbed or represented only by a small scar of fibrous tissue. The rapidity with which the changes, both constructive and destructive, take place in the corpus luteum, appears to be largely dependent upon whether the egg which escaped from the follicle is or is not fertilized. If ovulation is not followed by fertilization the corpus luteum reaches the height of its development in about twelve days, and within a few weeks has almost wholly disappeared. If, on the other hand, pregnancy supervenes, the corpus luteum becomes much larger, does not reach its maximum development until the fifth or sixth month and is still present at the end of pregnancy. The above differences have led to the distinction of the corpus luteum of pregnancy or the ¢rue corpus luteum, and MATURATION. 33 the corpus luteum of menstruation, or the false corpus luteum, although there are no actual microscopic differences between the two. Point of rupture Blood vessel x of theca AN \) \\ Lutein cells f Wo) Connective tissue # / 2 : ¢ EOS) / Sew (012 sian os pe a Connective tissue Hj f itt from theca hf Theca folliculi Remnant of corpus hemorrhagicum owt 22 9JFo NOR of Sala eae o TR Cre 4) oN BRIS = OA 1 O88 a ‘a BONES 2} OPN Blood vessels Done 2 hg of theca RY: igo Eee DS Fo Bee teed gh BE ha Oe. Fic. 21.—From section of human ovary, showing later stage of corpus luteum than Fig. 20. Kolimann’s Allas. PRACTICAL SUGGESTIONS. The reduction of chromosomes is beautifully illustrated in Ascaris megalocephala (bivalens). This species of Nematode is parasitic in the intestine of the horse and can usually be procured by a veterinarian. The oviducts are carefully removed and treated as follows: Gilson’s alcohol-acetic-sublimate mixture, 15 to 30 minutes. 70% alcohol, with little tincture iodine, several changes, 24 hours. 70% alcohol, indefinitely. , Borax-carmin, 24 hours. 1% HCl in 70% alcohol, 24 hours. Absolute alcohol and glycerin, equal parts, 24 hours. Glycerin, indefinitely. Mounting for study is carried out as follows: Cut the oviduct into about six pieces of equal length, being careful to keep the pieces in order. Cut a small bit from one of the pieces, gently shake some of the ova into a small drop of glycerin on a slide and carefully apply a glass. Pressure on the cover-glass will crush the ova. Ova from all the pieces are 34 TEXT-BOOK OF EMBRYOLOGY. treated in the same manner. For permanent mounts the cover-glasses should be cemented. The first (most cranial) piece will show the sperm in the ovum and the nucleus of the latter in the resting or spireme stage. The second piece will show a distinct membrane around the ovum, the sperm head, and the chromatin of the egg nucleus arranged in tetrads. The third piece will show the membrane, the sperm nucleus as a fainter structure, and the first maturation spindle. The fourth piece will show the membrane, the sperm nucleus still fainter, the first polar body, and also the second maturation spindle. The fifth piece will show the membrane, the very faint sperm nucleus, the first polar body some- where around the periphery of the ovum, the second polar body, and often two chromosomes remaining in the ovum. The sixth piece (near the junction of the two oviducts) will often show both polar bodies and the two pronuclei (with no apparent difference between them) in the resting stage. It should be borne in mind that the maturation process goes on pro- gressively from the cranial to the caudal ends of the oviducts, so that it is possible to find other stages between those mentioned. Maturation in a mammalian ovum is probably best seen in the mouse. The ovaries of a mouse are fixed in Flemming’s fluid, cut in either celloidin or paraffin and stained with Heidenhain’s hematoxylin (see Appendix). In some of the sections ova are likely to be found which will show maturation spindles, or polar bodies, or some intermediate stages. Owing to their extreme minuteness, the study of the maturation processes in the sperm cells is much more difficult than in the ova and, furthermore, since each primary spermato- cyte gives rise to four spermatids and each spermatid is transformed directly into a sperma- tozo6n, it is necessary to consider all the generations of cells from the spermatozoa back to the spermatogonia. Instructive specimens may be obtained by fixing small pieces of a fresh human testis in Orth’s fluid, cutting thin sections in celloidin and staining with hematoxylin-eosin (see Appendix). Better results may be obtained from the testis of some small animal. Remove imme- diately the testis of a recently killed mouse or rat, make an incision to insure quick penetration of the fixative and fix in Flemming’s fluid. Cut thin sections in paraffin and stain with Heidenhain’s hematoxylin (see Appendix). Different seminiferous tubules will show dif- ferent stages in the development of the spermatozoa. It is scarcely possible in these prepa- rations to trace the actual process of reduction of chromosomes. References for Further Study. Boveri, T.: Zellstudien. Jena, 1887-1901. CuiLp, C. M.: Studies on the Relation between Amitosis and Mitosis. Biolog. Bull., Vol. XII, Nos. z, 3, 4; Vol. XIII, No. 3, 1907. Conktin, E. G.: The Embryology of Crepidula. Jour. of Morphol., Vol. XIII, 1897. HeErtwic, R.: Eireife, Befruchtung u. Furchungsprozess. In Hertwig’s Handbuch der vergleichenden ut. experimentellen Entwickelungslehre der Wirbeltiere. Bd. I, Teil I, 1903. Also contains extensive bibliography. Sogorta, J.: Die Befruchtung und Furchung des Eies der Maus. Archiv j. mik. Anatomie, Bd. XLV, 1895. Sosotta, J.: Ueber die Bildung des Corpus luteum beim Meerschweinchen. Anat. Flefte, Bd. XXXII, Heft XCVI, 1906. von LeNHossEK, M.: Untersuchungen iiber Spermatogenese. Archiv. }. mik. Anatomie, Bd. LI, 1808. Witttams, J. W.: Text-book of Obstetrics. New York, 1903. Witson, E. B.: The Cell in Development and Inheritance. 2d Ed., 1900, CHAPTER IV. FERTILIZATION. Mitosis, as described in Chapter I, is the process by which cells proliferate to form the various tissues and organs and to take the place of cells worn out in the carrying on of their labors or destroyed by injury. It is constantly going on throughout the life of the individual. Attention has been called to the fact that in all such reproduction there is a constantly maintained somatic number of chromosomes. This has been seen to hold true up to the formation of the mature germ cells—the mature ovum and the spermatozoén, each of which contains one-half the somatic chromosome number. In all sexual reproduction the starting point of the new individual, that is, the formation of the single cell from which all the tissues and organs develop, is the union of two mature germ cells, the spermatozoén and the ovum, one from the male the other from the female. This union is known as fertilization and the resulting cell is the fertilized ovum. The details of the process vary in different animals. Its essence is the entrance of the spermatozodn into the ovum and the union of the nucleus of the spermatozo6n with the nucleus of the ovum. At the time of its entrance into the egg, the sperm head is small and its chromatin extremely condensed (Fig. 22, 2). Soon after entering the ovum, however, the sperm head un- dergoes development into a typical nucleus, the male pronucleus (Figs. 22, 3, and 13, C). This male pronucleus is to all appearances exactly similar in structure to the nucleus of the egg, which latter is now known as the female pronucleus. The chromatin networks in both pronuclei next pass into the spireme stage, the spiremes segmenting into chromosomes of which each pro- nucieus contains one-half the somatic number. The nuclear membranes mean- while disappear and the chromosomes lie free in the cytoplasm. During these ~ changes in the pronuclei, the amphiaster has formed and the male and the female chromosomes mingle in its equatorial plane (Fig. 22,5). At this stage no actual differentiation can be made between male chromosomes and female chromosomes, the differentiation shown in Fig. 22, 5, being schematic. The picture is now that of the end of the prophase of ordinary mitosis, the somatic number of chromosomes being arranged in a plane midway between the two centrosomes. With the mingling of male and female chro- mosomes fertilization proper comes to an end. The further steps are also identical with those of ordinary mitosis. Each chromosome splits longitudinally 30 36 TEXT-BOOK OF EMBRYOLOGY. into two exactly similar parts (Fig. 22, 5), one of which is contributed to each daughter nucleus (Fig. 22, 6), and the cell body divides into two equal parts. (For details of succeeding anaphase and telophase see p. 6.) There thus result from the first division of the fertilized ovum, two cells which are eal Zona pellucida \--~ Nucleus Female 7 pronucleus =" Spermatozoon Head of -- Cytoplasm --spermatozoén with centrosome ~ Female pronucleus ae —- Centrosome Male pronucleus “™* Male pronucleus ~~ Female pronucleus - Chromosome from female pronucleus ._ Chromosome from male pronucleus Chromosomes of 1emale pronucleus ‘ | ~Chromosomes of male pronucleus sa>/-—--Centrosome 6 Fic. 22.—Diagram of fertilization of the ovum. (The somatic number of chromosomes is 4.) Boveri, Bohm and von Davidoff. Sn, Centrosome uw apparently exactly alike and each of which contains exactly the same amount of male and of female chromosome elements (Fig. 22, 6). The amphiaster of the fertilized ovum appears to develop as in ordinary mitosis. As to the origin of the centrosomes, however, much uncertainty still FERTILIZATION. 37 exists. The middle piece of the spermatozoén always enters the ovum with the head. It has already been shown (p. 24) that one or two spermatid centro- somes take part in the formation of the middle piece. Male centrosome elements are therefore undoubtedly carried into the ovum in the middle piece. It is equally well known, for some forms at least, that the centrosome of the ovum disappears just after the extrusion of the second polar body. In a considerable number of forms the development of the centrosome of the fertilized egg from, or in close relation to the middle piece of the spermatozoén has been observed. The details of the process as it occurs in the sea-urchin have been care- fully described by Wilson. In cases of this type the tail of the spermato- zoén remains outside the egg while the head and middle piece, almost im- mediately after entering, turn completely around so that the head points away from the female pronucleus (Fig. 23, @). An aster with its centrosomes next appears, developing from, or in very close relation to the middle piece. The aster and sperm nucleus now approach the female pronucleus, the aster leading and its rays rapidly extending. On or before reaching the female pronucleus the aster divides into two daughter asters (Fig. 23, 6) which separate with the formation of the usual central spindle, while the two pronuclei unite in the equatorial plane and give rise to the chromosomes of the cleavage nucleus (Fig. 23, ¢ and d). In the sea-urchin the polar bodies are extruded before the entrance of the spermatozoén. In cases where the polar bodies are not ex- truded until after the entrance of the spermatozoén (Ascaris, Fig. 9) the amphiaster forms while waiting for their extrusion, the nuclei joining sub- sequently. When the sperm head finds the polar bodies already extruded, union of the two pronuclei may take place first, followed by division of the centrosome and the formation of the amphiaster. The coming together of ovum and spermatozoén is apparently determined largely by a definite attraction on the part of the ovum toward the spermato- zoén. ‘This attraction seems to be chemical in nature and is specific for germ cells of a particular species, that is, ova possess attractive powers toward spermatozoa of the same species only. This has been proved in some of the lower forms by mixing ova and spermatozoa in a suitable medium with the re- sult that the spermatozoa become attached to the membrane of the ova in large numbers. Spermatozoa of other species will not, however, be thus attracted. That this attraction is not dependent upon the integrity of the ovum as an organism is shown by the fact that small pieces of egg cytoplasm free from nuclear elements exert the same attractive force, so that spermatozoa are not only attracted to them but will actually enter them. Of eggs which are enclosed by a distinct membrane, the vitelline membrane, some (e.g., those of Amphibians and of Mammals) are permeable to the spermatozoon at all points; others have a definite point at which the spermat- 38 TEXT-BOOK OF EMBRYOLOGY. ozoén must enter, this being of the nature of a channel through the membrane —the micropyle. In some instances a little cone-shaped projection from the Fic. 23.—Fertilization of the ovum of Thalassema. Griffin. x7, Male pronucleus, .*, female pronucleus. surface of the egg, the attraction cone (Fig. 22, 1), either precedes or imme- diately follows the attachment of the spermatozoén to the egg. Instead of a projection there may be a depression at the point of entrance. . ’ FERTILIZATION. 39 There seems to be no question that but one spermatozoén has to do with the fertilization of a particular ovum. In Mammals only one spermato- zoén normally pierces the vitelline membrane although several may penetrate the zona pellucida (Fig. 22, 1) to the perivitelline space. Should more than one spermatozoén enter such an egg—as, for example, in pathological polyspermy— the result is an irregular formation of asters and polyasters (F ig. 24) and the early death of the egg either before or soon after a few attempts at cleavage. In some Insects, and in Selachians, Reptiles and Birds, a number of sperma- tozoa normally enter an ovum, but only one goes on to form a male pronucleus. The ovum thus not only exerts an attractive influence toward spermatozoa, but it apparently exerts this influence only until the one requisite to its fertiliza- tion has entered, after which it appears able to protect itself against the further entrance of male elements. As to the means by which this is accomplished little is known, although several theories have been advanced. It may be that aa mviZ” iN Fic. 24.—Polyspermy in sea-urchi eggs treated with 0.005 per cent. nicotine solution. O. and R. Hertwig, Wilson. B, Showing ten sperm nuclei, three of which have conjugated with female pronucleus. C, Later stage showing polyasters formed by union of sperm amphiasters. when the single spermatozo6n necessary to accomplish fertilization has entered the ovum, it sets up within the ovum such changes as to destroy the attractive powers of the ovum toward other spermatozoa, or as even to prevent their entrance. In the case of eggs where the spermatozoén enters through a micro- pyle, it has been suggested that the tail of the first spermatozoén remaining in the opening might effectually block the entrance to other spermatozoa; or the passage of the first spermatozoén might set up such mechanical or chemical changes in the canal as would prevent further access. In most cases of eggs which have no vitelline membrane previous to fertilization, such a membrane is formed immediately after the entrance of the first spermatozo6n, a natural inference being that this membrane may prevent the entrance of any more spermatozoa. Biologists, however, are inclined to discredit the view that the fertilization membrane is a protection against polyspermy. 40 TEXT-BOOK OF EMBRYOLOGY. Nothing is known in regard to fertilization of the human ovum. It has been shown that in some of the lower Mammals fertilization regularly takes place in the oviduct, and it is reasonable to assume that it occurs in the oviduct in man, That spermatozoa can pass into and even all the way through the oviduct is proved by cases of tubal, abdominal and, rarely, ovarian pregnan- cies. On the other hand Wyder considers the uterus as the normal site of fertilization, and some other gynecologists say that fertilization may take place in the uterus. Waldeyer also concludes that fertilization may occur in the uterus, Significance of Fertilization.* Mitotic cell division constitutes the wonderful mechanism by which not only the con- tinuity of life but also the maintainance of the species is accomplished. ‘From an a priori point of view there is no reason why, barring accidents, cell division should not follow cell division in endless succession.’’ And it is probable that such is the case among the lower forms of animal and vegetable life, where no true sexual reproduction occurs. A one-celled animal or plant may divide and produce two individuals; each of these two may produce two more, and so on ad infinitum. ‘In the vast majority of forms, however, the series of cell divisions tend to run in cycles in which the energy of division comes to an end and is restored only by an admixture of living matter from another cell.” This admixture of the living matter of two cells is known as fertilization and is the essential feature of sexual reproduction. It is the process which on the one hand restores to the cell the energy necessary to continue its division and on the other hand accomplishes the blending of two independent lines of descent. Certain views regarding the significance of fertilization may be grouped together as the rejuvenescence theory. "The earlier embryologists regarded fertilization as “a stimulus given by the spermatozo6n, by which the ovum is ‘animated’ and made capable of development.” The more modern “dynamic” theorists express practically the same conception. They hold that protoplasm tends to pass gradually into a state of equilidrium in which activity dimin- ishes, and that fertilization restores to it a state of activity tarough the admixture of protoplasm which has been subjected to different conditions. Certain known facts tend in a general way to support these theories. For example, among the Protozoa or one-celled animals, a long series of ccll divisions is followed by con- jugation. In conjugation two individuals come together and fuse permanently, or inter- change substances and separate again. This process results in the restoration of the energy of growth and division and a new life-cycle is begun. Among the higher animals and plants fertilization is always necessary for the initiation of a new life-cycle with its subsequent cell division and growth. In some lower forms, however, parthenogenesis occurs and a new life-cycle is initiated without the union of cells from two sexually different individuals. It must therefore be admitted that whether or not the tendency toward senescence and the need of fertilization are primary attributes of living matter is unknown. Parallel with the rejuvenescence theory are other views which do not necessarily oppose or confirm it. One view in particular is that fertilization is in some way concerned with variations in the individuals of a species. Brooks and Weismann developed the hypothesis that fertilization, the admixture of germ plasms from two individuals, is a source of varia- *The quotations in the following paragraphs are from “The Cell in Devel aaa Inheritance,” by E. B. Wilson. @ paragrap in Development an FERTILIZATION. 41 tion—‘‘a conclusion suggested by the experience of practical breeders of plants and animals.”’ Weismann himself holds that the need of fertilization is a secondary matter, but that the admixture of different germ plasms insures the mingling and renewal of variations. Spencer and Darwin, on the other hand, believe that although crossing among animals or plants may lead to variability within certain limits, it tends in the long run to hold in check any wide digression from a norm and hold the species true to type. PRACTICAL SUGGESTIONS. Fertilization of the ovum may ke observed in some of the Invertebrates and makes a wonderfully interesting process to observe under the microscope. In the sea-urchin, for example, whose ova are relatively small and transparent, the steps may be followed in the living objects. ‘The mature ova are removed from the animal and placed in a drop of sea- water on a slide; then a drop of seminal fluid mixed with sea-water is added. A cover-glass is gently applied, and it is best to place a thin bit of glass under one edge to prevent crushing. The following phenomena may be seen under a fairly high power lens: In less than a minute a vast number of the spermatozoa are clustered around each egg. Under the most favorable conditions one of these spermatozoa may soon be seen with its head attached to the ovum. The head penetrates deeper into the cytoplasm, the tail becomes motionless and finally invisible. The egg nucleus and sperm nucleus then seem to exert a mutual attraction and move toward each other. They finally come in contact and fuse, the product of their fusion being the segmentation nucleus. The whole process of fertilization has not occupied more than ten minutes. In Ascaris the behavior of the sperm nucleus during the maturation of the ovum may be inferred from the different stages (see ‘‘Practical Suggestions,” p. 33). In this particular animal the two pronuclei do not actually fuse but return to the resting stage within the ovum and then, when the first cleavage is about to occur, break up into their respective chromo- somes. References for Further Study. Conxuin, E. G.: The Embryology of Crepidula. Jour. of Aforphol, Vol. XIII, 1897. Harper, E. H.: The Fertilization and Early Development of the Pigeon’s Egg. Am. Jour. of Anat., Vol. ITI, No. 4, 1904. _ Hertwie, R.: Eireife, Befruchtung u. Furchungsprozess. In Hertwig’s Handbuch d. vergleich, u. experiment. Entwickelungslehre der Wirbeltiere, Bd. I, Teil I, 1993. Kine, H. D.: The Maturation and Fertilization of the Egg of Bufo lentiginosus. Jour. of Morphol., Vol. XVII, rgot. Sozsorta, J.: Die Befruchtung u. Furchung des Eiesder Maus. Arch. f. mik. Anat., Bd. XLV, 1895. Wusoy, E. B.: The Ceil in Development and Inheritance. 2d Ed., rgoo. CHAPTER V. CLEAVAGE—(SEGMENTATION). Following fertilization and the commingling of male and female chromo- somes, there occurs the usual longitudinal splitting of these chromosomes as in ordinary mitosis. One-half of each chromosome now passes toward each centrosome. ‘The result is that one-half of each male chromosome and one- half of each female chromosome enter into the formation of each of the two new, daughter nuclei (Fig. 22, 4,5 and6). The phenomena which follow are apparently identical with those of ordinary mitosis and result in two similar daughter cells. Each of the latter next undergoes mitotic division. In this manner are formed four cells, eight cells, sixteen cells, and so on. This early multiplication of cells which follows fertilization is known as cleavage or segmentation of the ovum, the cells themselves are known as blastomeres and the cell mass as the movula, While the object of cleavage and its results—the proliferation of cells from the fertilized ovum and subsequent growth and development of the embryo— also the general features of the process, are essentially similar in all eggs, marked variations in details of cleavage occur in eggs of different forms, apparently dependent largely upon the amount of yolk present and its distribu- tion within the egg. (See page 12.) We distinguish the following : FORMS OF CLEAVAGE. a. Equal—e.g., alecithal eggs of Sponges, Echinoderms, some Annelids, some Crustaceans, some Mol- lusks, Amphioxus, Mammals. Holoblastic (complete or total) b. Unequal—e.g., telolecithal eggs of Cyclostomes, Ganoid Fishes, Amphibians; usual type in Annelids and Mol- | lusks. a. Superficial—e.g., centrolecithal eggs of Arthropods. Meroblastic (incomplete or partial) ; b. Discoidal—e.g., telolecithal eggs of Cephalopods, Bony Fishes, Reptiles, Birds. 42 CLEAVAGE. 43 Holoblastic Cleavage. (a) Equat.—In this form of cleavage the entire egg divides and the cells resulting from the early cell divisions are of approximately the same size. One of the Echinoderms—Synapta—presents a beautiful example of this, the simplest type of cleavage (Fig. 25). The egg of Synapta is alecithal, containing very little yolk, The first cleavage is in a vertical plane at right angles to the long axis of the central spindle and divides the egg into halves. The second plane of cleavage is also vertical but is at right angles to the first cleavage plane and results in four equal cells. The third cleavage plane is horizontal, cutting the four cells result- ing from the second cleavage into eight equal cells. The fourth cleavage is Fic. 25.—Cleavage of the ovum of Synapta (slightly schematized). Selenka, Wilson. A-E, Successive cleavages to the 32~cell stage. F, Blastula of 128 cells. vertical, the fifth horizontal and so on, regular alternation of vertical and hori- zontal cleavage planes being continued through the ninth set of divisions, re- sulting in 5r2 cells. At this point gastrulation begins and the regularity of the cleavage planes is lost. Amphioxus is another classical example of equal holoblastic cleavage, being classed as such, although after the third cleavage the cells are not of exactly the same size. In Amphioxus the first two cleavage planes are vertical and at right angles, asin Synapta. The third cleavage plane is horizontal, as in Synapta, but the cells lying above the third cleavage plane are smaller than those lying below it. The eight-cell stage of Amphioxus thus presents four upper smaller cells and four lower larger cells (Fig. 26). 44 TEXT-BOOK OF EMBRYOLOGY. (ps) Unrquar.—A good example of this form of cleavage is found in the common frog’s egg (Fig. 27). This egg while containing little yolk when com- pared with such eggs as those of the fowl, contains much more yolk than does the egg of Synapta or of Amphioxus. The frog’s egg being a telolecithal egg, the yolk is gathered at one pole, enabling a distinct differentiation to be made between the upper darker protoplasmic or animal pole, and the lower lighter vegetative pole (Fig. 6). The cleavage is complete but the cells which develop at the yolk pole are much larger than those which develop at the protoplasmic pole. The first and second cleavage planes are as in Synapta and Amphioxus, vertical and at right angles to each other. Each of the four cells which result from the second cleavage in the frog consists of a small upper darker proto- plasmic pole and of a larger lower lighter yolk pole (Fig. 27, A). The 3 Micromeres Micromeres Segmentation Q |cavity 7-| Macromeres Fic. 26.—Cleavage of the ovum of Amphioxus. Hatschek, Bonnet, 1-5, Lateral views of segmenting cells; 6, section of blastula, nuclear elements lying, as they always do, within the protoplasmic portion of the cell, determine the next cleavage plane which is horizontal and lies nearer the protoplasmic ends of the cells. The result is that the third cleavage gives rise to eight cells, four of which are small protoplasmic cells lying above the line of cleavage, while the other four are large yolk-containing cells which lie below the line of cleavage (Fig. 27, A). This distinction between protoplasmic cells and yolk cells not only persists but tends to become more and more marked as segmentation proceeds, and it soon becomes evident that the cells unencum- bered by yolk have a tendency to segment more rapidly than do their yolk- laden brethren (Fig. 27,C,D,E, F and G). Thus, while the fourth cleav- age is vertical in both types of cells, giving rise to eight upper protoplasmic cells and the same number of lower yolk cells, this uniformity of number per- CLEAVAGE. 45 sists only up to this point, while beyond this point the protoplasmic cells in- crease in number much more rapidly than do the yolk cells, so that when the protoplasmic cells number 128, there are still but comparatively few yolk cells. There thus result in total unequal cleavage two very different types of cells each confined to its own part of the segmenting cell mass. G os H ] : Fic. 27.—Cleavage of the frog’s egg. Morgan. A, Eight-cell stage; B, beginning of sixteen-cell stage; C, thirty-two-cell stage; D, forty-eight-cell stage (more regular than usual); E, F, G, later stages; H, I, formation of blastopore. Meroblastic Cleavage. (4) SuperFiciat.—This form of cleavage is seen in the centrolecithal eggs of Arthropods. These eggs (see p. 12) consist of a central mass of nutritive yolk surrounded by a comparatively thin layer of protoplasm. The seg- mentation nucleus lies in the middle ofthe nutritive yolk where it undergoes the usual mitotic divisions. The resulting daughter nuclei leave the central yolk mass and pass out into the peripheral: layer of protoplasm where they ap- + a 46 TEXT-BOOK OF EMBRYOLOGY. parently determine segmentation of the protoplasm, the number of protoplasmic segments corresponding to the number of nuclei. There is thus formed a superficial layer of cells (blastomeres) enclosing the central nutritive yolk. (B) Discorpat.—This type of cleavage occurs in eggs which have an excessive amount of yolk and in which the protoplasm is confined to a small superficial germ disk. The telolecithal ova of Birds furnish typical examples of this form of cleavage. The first cleavage plane is vertical and divides the 4 bd a 8 8 = fi Fic. 28.—Cleavage in hen’s egg. Coste. Germinal disk and part of yolk, seen from above. protoplasmic disk into halves. The second cleavage plane is also vertical and at right angles to the first, resulting in four approximately equal cells (Fig. 28, a). The third cleavage plane is also vertical, dividing two of the four cells (Fig. 28, 6). The germ disk at the end of the third cleavage consists of six pyramidal cells lying with their apices together in the center of the germ disk, their bases lying peripherally and toward the yolk mass. They are separated from one another at the surface, but are still continuous below and CLEAVAGE. 47 peripherally with the underlying yolk mass and consequently with each other. The analogy between this condition and that described for the frog’s egg is complete with the one exception that in the latter the cleavage furrows cut completely through the yolk cells or the yolk-containing portions of the cells, while in the bird’s egg the amount of yolk is so great that the cleavage furrow merely passes a short distance into it without completely dividing it into seg- ments. The fourth cleavage plane is tangential, cutting off the apices of the six pyramidal segments. The germ disk after the fourth cleavage thus con- sists of six small superficial central cells and six larger cells which surround the small cells and also separate the latter from the underlying yolk. From this point radial and tangential cleavages follow each other without any sem- blance of regularity. The result is a mass of small cells lying at the center of Fic. 29.—From a vertical section through the germ disk of a fresh-laid hen’s egg. Duval, Hertwig. g.d., Upper layer of germ disk; s.c., segmentation cavity; w.y., white yolk (see Fig. 7); y.s., lower layer of germ disk (yolk cells, merocytes). the disk and surrounded by larger cells (Fig. 28, c, d). The smaller cells are completely separated from the underlying yolk while the larger cells are for a time continuous with it (Fig. 29). Comparing the unequal holoblastic cleavage of the frog’s egg with discoidal meroblastic cleavage as seen in the eggs of Birds, it becomes immediately evident that the differences between them are explainable entirely by reference to the greater quantity of yolk in the bird’s egg. The real activity of segmenta- tion is in both cases confined almost wholly to the protoplasm. In the frog’s egg the amount of yolk present is sufficient to impede segmentation in the larger cells but not to prevent it. In the bird’s egg the amount of yolk is so great that it cannot be made to undergo complete segmentation. Some General Features of Cleavage. Cleavage in Mammals. The two fundamental laws of cleavage as formulated by Sachs are: t. That each cell tends to divide into equal parts. 4 48 TEXT-BOOK OF EMBRYOLOGY. 2. That each division plane tends to intersect the preceding division plane at right angles. The first of these laws is apparently dependent upon the fact that the nucleus tends to occupy the center of the protoplasmic mass which is to undergo division. In the case of a spherical cell the spindle may lie in any diameter. In case one axis of the cell is longer than the other axis, the axis of the spindle corresponds to the long diameter of the cell. When the cell divides the division plane always bisects the spindle perpendicularly to its long axis. Applying these laws to cleavage, the first division plane bisects the long axis of the first division spindle at right angles, the axis coinciding with the greatest diameter of the cytoplasmic mass. The result is two cells, the long axes of which are parallel to the first division plane. The axes of the mitotic spindles in these two daughter cells, coinciding with the long diameters of the cells, are therefore at right angles to the mitotic spindle of the mother cell, and the second division plane is therefore at right angles to the first. The result of the second division is four cells and as the first two division planes were vertical, the long axes of all four of these cells are vertical, as are also their mitotic spindles. The third division plane to be at right angles to these spindles must bisect them in a horizontal plane. The third cleavage plane is therefore horizontal. Such regular cleavage as that seen in Synapta (p. 43) in which after the first vertical cleavage, vertical and horizontal cleavage planes alternate with perfect regularity and the number of cells is exactly doubled at each cleavage through the ninth cleavage or 512 cells, is extremely rare. Thus in many forms the regular doubling of the number of cells occurs only through the first four or five cleavages. This is exemplified in Nereis where regular doubling occurs through the fourth cleavage, giving rise to sixteen cells. The fifth cleavage results in twenty cells, the sixth in twenty-three, the numbers at successive cleavages being twenty-nine, thirty-two, thirty-seven, thirty-eight, forty-one, forty-two. The immediate cause of such irregularity in the number of cells is that some of the cells divide more rapidly than others. Thus in Nereis after the fourth cleavage not all the cells divide at any one time. In some cases the amount of yolk present in the cell appears to influence the rapidity of division, the cells containing the greater quantity of yolk dividing more slowly than those containing less yolk. While this is generally true, exceptions where cells con- taining much yolk divide as rapidly as, or more rapidly than, those containing less, prove the inadequacy of this as a general explanation of numerical ir- regularity in cleavage. It is possible that in many instances, at any rate, the future réle of the cell as regards function may play a large part in determining the rapidity of its segmentation. After the forty-two-cell stage in Nereis the number of cells at each successive cleavage is indeterminate, that is, it varies CLEAVAGE. 49 for different individuals. In other species this variability begins much earlier in the segmentation series. In addition to variations in the number and size of the cells above de- scribed, variations also occur in the direction of the cleavage planes and in the relations of the resulting cells. In all eggs except centrolecithal, the first two cleavage planes bisect each other at right angles through the protoplasmic pole. Subsequent cleavage may follow one of three types, which are distinguished as radial, spiral and bilateral. Radial cleavage is the simple type of cleavage already described as occurring in Synapta and in the frog (Figs. 25 and 27). Spiral or oblique cleavage occurs chiefly in Worms and Mollusks. The differ- ence between radial and spiral cleavage is brought out in the second division in the latter, and may manifest itself in the telophase of the first division. The third cleavage is not a continuous horizontal plane but cuts the cells in such a manner that the four lower cells have a position as if rotated one-half cell, usually to the right. The divisions between the cells of the lower row thus alternate with those of the upper row, like layers of bricks in a wall. The next horizontal cleavage plane is also oblique but is at right angles to the preceding and results also in alternation of all the cells. This regular alternation of spiral cleavage planes may continue for some time, but, as a rule, the cleavage soon becomes very irregular and there is usually much variation in size of the blastomeres. In the bilateral form of cleavage seen in Tunicates and Cephalopods, after the first cleavage, the cells segment symmetrically on each side of the first plane. In some forms the cleavage appears to follow definite rules as regards the number of cells which result from each segmentation. This is known as deter- minate cleavage. In other cases, after a number of divisions, the number of cells resulting from each cleavage is indefinite, that is, it varies for each indi- vidual. This is designated indeterminate cleavage. Reviewing the results of cleavage, it is to be noted that in every case there is _ formed a larger or a smaller group of cells. In the case of equal holoblastic cleavage, these cells are all of the same or of nearly the same size, and constitute what is known as the morula or mulberry mass (Fig. 25,£). A similar condition obtains in unequal holoblastic cleavage with the one exception, that there is a marked difference in the size of the cells constituting the morula (Fig. 27). In superficial meroblastic cleavage the group of cells forms a layer enclosing the central yolk, the latter being unsegmented but containing some nuclei. In discoidal meroblastic cleavage the group of cells spreads itself over a limited superficial area, while beneath it lies the large mass of unsegmented yolk, con- ~ taining, however, some nuclei (Figs. 28 and 29). In holoblastic cleavage the blastomeres in the interior of the mass become 50 TEXT-BOOK OF EMBRYOLOGY. more or less separated during segmentation, a cavity thus being formed within the so-called morula. This cavity increases in size, the cells being pushed centrifugally and, the embryo soon consists of a layer or layers of cells enclosing a cavity, the segmentation cavity. The entire embryo is now known as the blastula. The simplest type of blastula is seen in Amphioxus, where it consists of a nearly spherical segmentation cavity surrounded by a single layer of cells. Some of the cells—those which are more ventral and contain the larger amount of yolk—are slightly larger than others (Fig. 26, 6). In the eggs of the frog, in which the cellsresulting from segmentation show greater inequality in size (due to difference in yolk content), the segmenta- tion cavity is surrounded by several layers of cells. In such a blastula the roof of the cavity is comparatively thin, being composed of small cells containing Micromere eee Fic. 30.—From a sagittal section through blastula of frog. Bonnet. mz., Marginal zone. little yolk, micromeres, while the floor of the cavity is thick, being composed of large yolk cells, macromeres. So thick is this wall of the vegetative pole of the blastula that the large yolk cells extend into the segmentation cavity compress- ing it into a crescentic cleft (Fig. 30). In the frog the roof of the segmentation cavity is sharply defined from the floor, due to the fact that the outer layer of cuboidal roof cells is densely pigmented. The rather sharply defined zone of transition between pigmented micromeres and nonpigmented macromeres is known as the marginal zone. In discoidal segmentation, the segmentation cavity is a mere slit between the superficial protoplasmic cells and the underlying unsegmenting yolk with its yolk nuclei (Fig. 29). Comparing it with unequal holoblastic cleavage, these partially divided yolk cells which form the floor of the segmentation cleft in CLEAVAGE. 51 discoidal cleavage are analogous to the large yolk cells which form the floor of the segmentation cavity in the frog. (Compare Figs. 29 and 30.) In the mammalian ovum, as in the other cases just described, segmentation leads up to the formation of a solid mass of cells—the morula. While cleavage Fic. 31.—Four stages in cleavage of the ovum of the mouse. Sobotia Small cell marked with x is the polar body, here is of the holoblastic equal type, the irregularity is especially marked. In the mouse, for example, the second cleavage is complete in one of the blasto- meres before it has begun in the other, so that a three-celled stage results Fic. 32.—Morula of rabbit. van Beneden. (Fig. 31). Following this is a four-celled stage. From this time on cleavage continues irregularly until a solid mass is formed, as in the lower forms, which is composed of apparently similar cells (Fig. 32). The next step in mammalian development is a differentiation of the super- 52 TEXT-BOOK OF EMBRYOLOGY. ficial layer of the cells of the morula. The result, then, is a single surface layer, the covering layer, surrounding a central mass of polygonal cells (Fig. 33, @). This solid mass of cells is transformed into a vesicle by vacuolization of some of the inner cells (Fig. 33) and the confluence of these vacuoles to form a cavity. The mammalian ovum at this stage thus consists of two groups of cells and a cavity, an outer group or layer of cuboidal cells, the outer cell layer or covering layer (trophoderm), forming the wall of the cavity, and an inner group of Fic. 33.—Four stages in the development of the bat. van Beneden. a, Section of morula; b, section of later stage of morula, showing differentiation of outer layer of cells; ¢, section of still later stage, showing vacuolization of central cells; d, section showing outer layer (trophoderm) and inner cell mass. polygonal or spheroidal cells, the inner cell mass which at one point is attached to the outer layer of cells (Fig. 33, d). The mistake must not, however, be made of considering the mammalian ovum at this stage as a true blastula. The mammalian ovum apparently does not pass through any true blastula stage. Of the parts just described, the inner cell mass alone is comparable to the blastoderm of birds, while the cavity corresponds not to the segmentation cavity but to the yolk mass of meroblastic eggs. The vacuolization of the cells of the inner cell mass would thus represent a late and abortive attempt at yolk formation, the actual nutritive yolk being CLEAVAGE. 53 made unnecessary, since the attachment of the ovum to the walls of the uterus provides for direct parental nutrition. In the separation of the cells of the morula into an inner cell mass and an outer covering layer is seen the earliest differentiation into cells (inner cell mass), which are destined to form the embryo proper, and cells (outer cells—covering layer) which are to engage in the development of certain accessory structures, PRACTICAL SUGGESTIONS. The ova of the star-fish or sea-urchin afford excellent material for the study of total (holoblastic) equal cleavage up to and including the blastula and gastrula stages. During the breeding season for these animals (late in June or early in July in the latitude of New York) the ova are readily obtained and easily fertilized under artificial conditions. The ova are removed and placed in shallow vessels containing sea-water. The seminal fluid is mixed with a small quantity of sea-water and some of the mixture is added to the water containing the ova. Gentle agitation will serve to disseminate the spermatozoa. Fertilization occurs within half an hour and cleavage follows very shortly. A few of the ova are placed on a slide and watched under the microscope until the first cleavage is nearly complete and then any desired quantity may be treated as follows: Remove the ova from the vessel by means of a pipette, care being taken not to take up any more water than is absolutely necessary. Eject them into 5 per cent. formalin contained in a slender, cylindrical bottle. After a few minutes (when the ova have settled to the bottom) it is best to change the formalin. The ova may remain in the formalin indefinitely. The later stages in cleavage may be determined and secured in the same manner. The process goes on rapidly and blastule will appear in a few hours, gastrule in a little longer time. To prepare permanent mounts, proceed as follows: Alcohol, 30%, 50%, 70%, an hour in each grade. Borax-carmin, few hours. Alcohol, 70%, slightly acidulated with HCl, few hours. Alcohol, 70%, several changes, few hours. Alcohol, 95%, one hour. Alcohol, absolute, one hour. Xylol, one hour. Thin xylol-damar, indefinitely. Remove some of the ova in a pipette, place on a slide and apply a cover-glass. In order to prevent crushing, it is best to place a few bits of broken cover-glass in the damar on the slide before applying the cover-glass. Eggs of the common frog are good examples of total unequal cleavage. The ova in various stages of cleavage can be procured in ponds during the spring and preserved indefi- nitely in 5 per cent. formalin. A hand lens or even the naked eye will be sufficient to deter- mine the earlier stages (two, four, eight and sixteen cells). For closer examination a few of the ova may be placed in the formalin or in waterin a watch-glass. ‘The cleavage furrows on the surface are remarkably clear. Permanent preparations of a number of stages for use in a large class may be made by removing the individual ova, each with its gelatinous capsule still intact, from the general mass and placing in formalin in slender test-tubes. The tubes should have a diameter just 54 TEXT-BOOK OF EMBRYOLOGY. small enough not to allow the eggs to slip past one another. In this way the stages may be kept in order, and the tightly-corked tubes may be handled in any way to get the best light on the eggs. References for Further Study. ASSHETON, R.: The Segmentation of the Ovum of the Sheep, with Observations on the Hypothesis of a Hypoblastic Origin for the Trophoblast. Quart. Jour. of Mic. Science, Vol. XLI, 1898. Biount, M.: The Early Development of the Pigeon’s Egg, with Especial Reference to the Supernumerary Sperm Nuclei, the Periblast and the Germ Wall. Biolog. Bull., Vol. NIT, No. 5, 1907. Conk, E, G.: Karyokinesis and Cytokinesis. Jour. Acad. Nat. Sci. of Philadelphia, Vol. XII, 1902. Conktitn, E. G.: The Embryology of Crepidula. Jour. of \Jforphol., Vol. XTII, 1897. EyctEsHyMER, A. C.. The Early Development of Amblystoma, with Observations on Some Other Vertebrates. Jour. of Jlorphol., Vol. X, 1895. Harper, E. H.: The Fertilization and Early Development of the Pigeon’s Egg. Am. Jour, of Anat., Vol. THI, No. 4, 1904. HatTscHEK, B.: Studien iiber Entwickelung des Amphioxus. Arbeiten aus dem zool. Instit. su Wien, Bd. IV, 188r. 5 Hertwic, R.: Eireife, Befruchtung u. Furchungsprozess. In Hertwig’s Handbuch d. vergleich u. experiment. Eentwickelungslehre der Wirbeltiere, Bd. I, Teil I, 1903. Linu, F. R.: The Development of the Chick. New York, 1908. Morean, T. H.: The Development of the Frog’s Egg. New York, 1897. Sozsotta, J.: Die Befruchtung u. Furchung des Eies der Maus. Arch. j. mik, Anai., Bd. XLV, 1895. VAN BENEDEN, E.: Recherches sur les premiers stades du développement du Murin (Vespertilio murinus). Anat. Anz., Bd. XVI, 1899. Witson, E. B.: The Cell in Development and Inheritance. 2d Ed., rgo0. CHAPTER VI. GERM LAYERS.* THE TWO PRIMARY GERM LAYERS—FORMATION OF THE GASTRULA. Gastrulation in Amphioxus. The changes which immediately follow the formation of the blastula can be observed in their simplest form in Amphioxus, where, it will be remembered, the blastula is a hollow sphere the wall of which consists of a single layer of cells which enclose the segmentation cavity (Fig. 26,6). Gastrulation begins by a flattening of the ventral wall of the blastula (Fig. 34, A). This is followed by a folding in or invagination of the yolk cells which form the ventral wall (Fig. 34, B). These cells press upward into the segmentation cavity which they soon completely obliterate, and come to lie immediately beneath and in contact with the smaller cells which had formed the roof of the cavity (Fig. 34, C). The gastrula, as the embryo is now called, thus consists of two layers of cells which lie in close apposition and enclose the new cavity, the archenteron (ccelen- teron—primitive gut) formed by the invagination (Fig. 34,C and D). This cavity remains open externally, the opening being known as the Dlastopore (Fig. 34,C and D). These two layers of cells which form the wall of the gastrula are the primary germ layers. The outer layer is known as the ectoderm or epiblast, the inner layer as the entoderm or hypoblast. As seen by reference to Fig. 34, C and D, the two primary germ layers are directly continuous with each other at the blastopore. The most significant feature of the transformation of the blastula into the gastrula is that whereas in the blastula all the cells are essentially similar, differing if at all only in the amount of yolk contained, in the gastrula two dis- tinct types of cells are recognizable. The cells of the outer layer differ from those of the inner layer both structurally and functionally. Thus in some of the lowest forms the gastrula stage is the adult stage. In such the outer cells are protective, react to external stimuli, develop cilia which determine locomotion, etc. The inner cells, on the other hand, are more especially concerned with nutrition, absorbing food, and giving off waste products. Von Baer’s apprecia- *For many of the ideas contained in this chapter, especially the correlation of gastrulation and the formation of the mesoderm in different forms, the writers are indebted to Bonnet’s excellent de- scription in his ‘“‘Lehrbuch der Entwickelungsgeschichte.”’ : The homologizing of gastrulation in the different forms has been found the most satisfactory method of teaching the subject. At the same time it must be admitted that some of the correlations are not based on actual observations. 55 56 TEXT-BOOK OF EMBRYOLOGY. tion of the significance of this first cell differentiation is evidenced by the fact that he designated the two primary germ layers the ‘‘primitive organs” of the body. It should be noted that with the completion of gastrulation certain important landmarks in adult topography have been established. Thus the animal A B Segmentation cavity Micromeres Segmentation cavity Macromeres Invagination (& D Archenteron Blastopore Archenteron Anterior lip of blastopore Blastopore Sees Post. lip of fr blastopore Ectoderm Entoderm Ectoderm Entoderm Fic. 34.—Gastrulation in Amphioxus. Haischek, Bonnet. (micromere) pole is always the dorsum; the vegetative (macromere) pole always the ventrum; the blastopore, being always caudal, differentiates the tail end from the head end of the embryo. . Gastrulation in Amphibians. This is modified as compared with gastrulation in Amphioxus by the presence of a greater amount of yolk. A clear understanding of the modifica- tions which this increased yolk content causes in the gastrulation of Amphibians, GERM LAYERS. 57 as well as of Reptiles and Birds, is essential toa proper appreciation of the process in Mammals. Recalling the amphibian blastula (p. 50), it will be remembered that its roof was formed of smaller protoplasmic cells (micromeres) while its floor con- sisted of a mass of yolk cells which encroached upon the segmentation cavity Micromeres -. Segment. cavity Marginal zone Fic. 35.—Vertical section through blastula of Triton. Hertwig. (Fig. 30). The zone of union between the two kinds of cells is known as the marginal zone. The simplest type of amphibian gastrulation, and the type thus most easily compared with gastrulation in Amphioxus, is exemplified by the water salamander—Triton teniatus. (Compare Figs. 34 and 35.) Ectoderm Entoderm ._ Anterior lip of blastopore Cy rae Arr . Blastopore 4 ALE LET Posterior lip of blastopore see ; = Yolk ceils (entoderm) Segmenta- tion cavity Fic. 36.—Vertical section through embryo of Triton, showing beginning of gastrulation. Hertwig. In Triton, a slight groove or furrow appearing along a portion of the marginal zone marks the blastopore and the beginning of gastrulation. The upper lip of this groove is formed by the smaller protoplasmic cells, the lower by the large yolk cells (Fig. 36). The groove next deepens, the micromeres growing in at the dorsal lip to form the roof of the archenteron, while the yolk cells are carried 58 TEXT-BOOK OF EMBRYOLOGY. over the ventral lip to form the floor. The invagination cleft which thus be- comes the archenteron is at first small as compared with the segmentation cavity, but rapidly increases in size, until as in Amphioxus, the earlier cavity is finally completely obliterated (Fig. 37). Coincident with the carrying of the yolk cells into the interior of the vesicle and the obliteration of the segmentation cavity, proliferation of the micromeres carries them completely around the yolk cells, so that the entire surface of the gastrula is formed of small cells (Fig. 37). The amphibian gastrula thus consists of a central cavity, the archenteron, communicating with the exterior by means of a small opening, the blastopore, the roof of the cavity being formed by two or more layers of small cells, the floor by the mass of large yolk cells. The outer layer of cells completely sur- rounds the yolk cells except at the blastopore, and constitutes the ectoderm (Fig. 37). The inner layer or entoderm is distinct only in the roof of the cavity. Laterally its cells pass over without any distinct demarcation into the mass of CSTE rr, Ectoderm Sareea aii EB Entoderm (protentoderm) yar Archenteron - Yolk cells (yolk entoderm) Peristomal mesoderm Yolk plug Posterior lip of blastopore Peristomal mesoderm Fic. 37.—Vertical section through gastrula of Triton. Hertwig. yolk cells which form the floor of the cavity. As the ectoderm forms a com- plete outer layer, the only point at which the yolk cells now appear externally is the blastopore, into which they project as the yolk plug (Fig. 37). It is possible in the amphibian gastrula to make the distinction between the entoderm of the roof which has grown in from the surface and is continuous with the surface ectoderm, and the entoderm of the floor which is formed of yolk cells. By those who make this distinction, the former is called the protentoderm, the latter the yolk entoderm (Fig. 37). In the case of the common frog, the eggs of which are so easily obtained that they furnish most satisfactory subjects for study, gastrulation is somewhat less simple than in Triton. As already noted (p. 50) the demarcation between micromeres and macromeres is in the frog very distinct, owing to the dark pig- mentation of the former. This is shown in Fig. 30, as is also the fact that the roof of the segmentation cavity consists of a surface layer of strongly pig- GERM LAYERS. 59 mented cells, and beneath this a layer of less pigmented cells. Fig. 38 shows the beginning of gastrulation, being a slightly earlier stage than the Triton gastrula (Fig. 36). In the frog (also in the toad and salamander) a modification of the comple- tion of gastrulation occurs, which, while apparently unimportant, is considered by some investigators as having significance in the interpretation of gastrulation in higher forms, especially in Mammals. It is illustrated in Fig. 39. The wedge-shaped mass of yolk cells is pushed in front of the invagination cleft and carried around dorsally just beneath the ectoderm (Fig. 39,5). This is met in the medial dorsal plane by yolk cells which have grown up from the floor of the segmentation cavity on the opposite side (Fig. 39, c). What was the segmenta- _ Cells with much pigment — Cells with less pigment Be a sa Invagination (blastopore) Fic. 38.—From sagittal section of blastula of frog, showing beginning of gastrulation. Bonnet. tion cavity thus becomes divided into a cleft beneath the ectoderm and a cavity surrounded by yolk cells. The cavity is designated by Bonnet the “ Erganzungs- hohle” or “completion cavity” (Fig. 39, ¢,d,e). With continued enlargement of the invagination cavity, the cleft-like remains of the segmentation cavity beneath the ectoderm becomes obliterated andthe “completion cavity” becomes pressed ventrally. The wall between the latter and the invagination cavity thins and finally ruptures so that the two cavities become one. It thus happens that at one stage there are three cavities (Fig. 39, d)—(1) the slit-like remains of the segmentation cavity, (2) the invagination cavity and (3) the so-called “completion cavity.” The remains of the segmentation cavity is seen by reference to the figures to lie between the ectoderm externally and the protentoderm and yolk entoderm internally. The invagination cavity 60 TEXT-BOOK OF EMBRYOLOGY. is limited mainly by protentoderm, the “completion cavity” by yolk entoderm. The breaking of the partition between the invagination cavity and the ‘‘com- pletion cavity” results in the formation of the archenteron proper or primitive gut, which is thus lined partly by protentoderm and partly by yolk ento- Ectoderm Segmenta- tion cavity ¥ __ Ant. lip of NS blastopore Blastopore c d «~ Ectoderm> Ectodenm Segment. cay. P-otento- , “Wedge” derm Py Segment tion Comp. plate Z Protento- 4 (‘completion”’) : derm \ “Completion cavity” Yolk entoderm Blastopore Blastopore Peristomal Yolk plug mesoderm Botade: ctoderm Ectoderm — Protentoderm ~ Protentoderm Ant. lip of blastopore Yolk plug Post. lip of blastopore Fic. 39.—Successive stages of gastrulation in the frog, showing especially the formation of the protentoderm, yolk entoderm and “completion cavity.” Schulise, Bonnet. plate.” Com, pl., ‘Completion derm, the two being from now on called simply entoderm. The somewhat thickened area of yolk cells at the junction of the protentoderm and yolk entoderm is designated by Bonnet, the “Ergainzungsplatte” or “completion plate” (Fig. 39, d, e). GERM LAYERS. 61 Gastrulation in Reptiles and Birds. This is further modified by the still greater increase in yolk, yet retains sufficient similarity to the process in Amphibians and Amphioxus to allow of comparison. Fic. 40.—Surface view of blastoderm of snake. Hertwig. Blastopore 1s represented by dark transverse band near lower side of figure. In the types of gastrulation thus far described—in Amphioxus, Triton and the frog—the entire egg is involved in segmentation and gastrulation. Up through these forms there is a progressive increase in the amount of yolk, which Blastoderm Embryonic disk x Anterior lip Blastopore Posterior lip (crescentic groove) of blastopore Fic. 41.—Surface view of embryonic disk of turtle (Emys taurica). Bonnet. : X, The lighter shading represents the opacity due to the growth of the protentoderm (see Fig. 42). in Triton and still more in the frog was seen to modify the gastrulation process. In the reptilian and the avian ovum there is a much greater increase in yolk content, the segmentation being confined to the germ disk and to a small part of 62 TEXT-BOOK OF EMBRYOLOGY. the underlying yolk (p. 46). Just as cleavage in Reptiles and Birds was modified by the presence of the large unsegmenting yolk mass, so, for the same Ectoderm of embryonic disk Blastopore Ectoderm Yolk entoderm Blastopore Ectoderm “Completion Protentoderm Yolk entoderm plate” Blastopore Peristomal mesoderm “Completion plate” Peristomal Ectoderm Blastopore mesoderm “Completion Remnant of plate” protentoderm Peristomal mesoderm “Completion plate’ Yolk entoderm Fic. 42.—From medial vertical sections through embryonic disk of lizard, showing five successive stages in gastrulation. JVenckebach, Bonnet. reason, is gastrulation quite modified, as compared with the simple process seen in Amphioxus. At the same time, however, it is possible to correlate the reptil- ian and avian gastrulation with gastrulation in the lower forms. GERM LAYERS. 63 It will be remembered that in the discoidal cleavage of Birds the blastula consists of a cleft-like segmentation cavity, the roof of which is formed by the proliferating micromeres constituting the germ disk, while the floor is formed by the partially segmenting yolk (Fig. 29). The former corresponds to the micro- meres of the blastula roof in Amphioxus and Amphibians, the latter to the underlying yolk cells. (Compare Figs. 26, 6, 30 and 29.) In Reptiles the beginning of gastrulation is evidenced by the appearance of an opacity just in front of what may now be designated the posterior margin of the disk (Fig. 40). This is due to more rapid proliferation of cells at this point. The opacity soon shows a depression or groove which more or less sharply defines the posterior margin of the disk. It varies in shape in different Rep- , HAS dae—ansiaes view OF bla : erm of unincubated hen’s tiles. It is frequently crescent-shaped and has Hertwig. ga been called the crescentic groove (Fig. 41). This ; a groove is the blastopore, and corresponds to the blastoporic invagina~ tions of Amphioxus, Triton and the frog. Soon after the formation of the crescentic groove, there appears in front of it an oval opacity which extends forward in the medial line (Fig. 41). This opacity is due to growth of cells forward from the blastopore under the surface cells as seen in Fig. 42 which shows the progress of the invagination in the lizard. These figures should be compared with Figs. 34, 36 and 37, showing the stages of gastrulation in Amphioxus and Triton, and especially with Figs. 38 and 39 showing gastrula- tion in the frog. In Fig. 42, 1, the blastopore is seen as a distinct invagination. As in the frog (Fig. 39) the invagination pushes ; raat __ in front of it a wedge-shaped mass of Fic. 44.—From vertical longitudinal section . through germ disk of siskin, showing beginning cells which extends forward under the . ie prsteee re or Hodes arc., archen- outer layer. These cells are the oad teron; ec., ectoderm; en., entoderm; #.b., posterior tentoderm. They form the roof and; ape el By ey aie Pie eo ease oath te underlying yolk entoderm, the floor of the new invagination cavity (Fig. 42, 2). As they extend forward they meet with a thickened part of the yolk entoderm, the “Erganzungsplatte” or ‘completion plate” (Fig. 42, 2,3, 4 and 5; compare Fig. 39). There are thus present at this stage, just as in the frog, three cavities, (x) the slit-like remains of the segmentation cavity, (2) the invagination cavity and (3) the “completion cavity.” Also 5 -—~ Area apaca Area pellucida Blastopore y (erescentic 4 groove) arc.ec.en. =. 4y.C. 4.6 64 TEXT-BOOK OF EMBRYOLOGY. as in the frog (Fig. 39), by a breaking through of the two layers—the pro- tentoderm and the yolk entoderm—which separate the invagination cavity from the “completion cavity” in Fig. 42, 2, the two cavities are united to form the archenteron or primitive gut (Fig. 42,3, 4and5). The single-layered germ disk has thus become transformed into a two-layered disk consisting of an outer (upper) layer—the ectoderm—and an inner (lower) layer—the entoderm (protentoderm). In Birds the gastrula is formed in a manner quite comparable with its forma- tion in Reptiles. Taking the hen’s egg as an example, it will be remembered that the entire segmentation area is confined to the germ disk, and that this con- sists of a superficial layer (roof of segmentation cavity) of small well defined cells (micromeres) beneath which is the cleft-like segmentation cavity, while the floor of this cavity is formed of incompletely segmented yolk (Fig. 29). The beginning of gastrulation is marked by the appearance of a crescentic bar near the posterior margin of the disk. ‘This bar is due to more rapid proliferation of the cells in this region, and in it there appears the crescentic groove or blasto- Fic. 45.—From vertical longitudinal section through two-layered germ disk of nightingale. Hertwig. a.b., anterior lip of blastopore; avc., archenteron; ec., ectoderm; en., entoderm (protentoderm); ¥.c., yolk cells (merocytes.) pore (Fig. 43). Just as described in lower forms, especially Reptiles, the micromeres invaginate or fold under at this point and grow forward as the protentoderm, and roof in the new cavity formed by the invagination (Fig. 44). The single-layered germ disk is thus transformed into a two-layered disk con- sisting of an outer (upper) layer—the ectoderm—and an inner (lower) layer— the entoderm (protentoderm). The protentoderm in a sense replaces the original layer of yolk cells in the area where the invagination occurs; the original outer layer (micromeres) becomes the ectoderm, except that portion which is invaginated to form the protentoderm (Fig. 45). This process is comparable with the disappearance of the yolk entoderm in Reptiles (Fig. 42). At the same time the segmentation cavity is obliterated and the new cavity—invagination cavity—which is in communication with the exterior, appears beneath the protentoderm. (Compare Figs. 42 and 4s.) Under the central portion of the germ disk the yolk becomes liquefied, while at the margin of the disk it continues to segment and give rise to large nucleated cells—the yolk entoderm. ‘This is known as the area of supplemental GERM LAYERS. 65 cleavage and apparently corresponds to the “Erganzungsplatte” or ‘‘com- pletion plate” described in lower forms (p. 60; see also Figs. 39 and 42). The germ disk continues to spread out over the yolk and at the same time the area of liquifying yolk increases. The portion of the disk above the liquified yolk appears translucent on surface view and is known as the area pellucida; the more peripheral part of the disk is less transparent, being more closely attached to the unchanged yolk, and is known as the area opaca. Area opaca 5 Area pellucida ~— ‘Completion plate” Hensen’s node > -— Head process Primitive groove Primitive Bt reak* | Post. lip of | blastopore ’. Fic. 46.—Surface view of embryonic disk of chick. Bonnet. . There next appears in front of the crescentic groove and extending from its middle point forward in the medial line, a linear opacity which is known as the primitive streak (Fig. 46). This ends anteriorly in a knob-like expansion— Hensen’s node. According to Duval, Hertwig, Bonnet and others, the primi- tive streak is formed in the following manner. A notch or indentation appears in the anterior lip of the transverse blastoporic slit (Figs. 43 and 47, A). As y > Area opaca fi _____-\_lArea pellucida Atos pelucids | i - Area opaca { ~ Primitive streak ‘ _ __.____-— |Primitive streak Blastopore X (crescentic groove) A B Fic. 47.—Surface views of blastoderms of Haliplana, showing formation of primitive streak. Schauinsland. the germ disk is constantly spreading in all directions, if the apex of this notch remains fixed, the extension of the disk posteriorly must result in a drawing out of the notch into a longitudinal slit (Fig. 47, B). In other words, the horns of the crescentic slit are pushed together to form a longitudinal slit- And as the two lips of the slit come together they fuse, and the line of fusion is marked by a shallow groove, the primitive groove. At the anterior end of the slit in the region 66 TEXT-BOOK OF EMBRYOLOGY. of Hensen’s node, there is a small area where fusion does not occur, thus leaving a small opening which communicates with the cavity of the primitive gut. Since the primitive groove is formed from the original crescentic slit, and the original crescentic slit is the blastopore, the primitive groove may be considered as a modified blastopore in which the only opening is at Hensen’s node. The primitive groove lies in the medial line of the primitive streak; and since the primitive groove is a modified blastopore, the two primary germ layers are fused Fic, 48.—From transverse section through Hensen’s node—germ disk of chick of 2 to 6 hours’ incu- bation. Duval. For lettering see Fic. 49. at the lips of the primitive groove (Figs. 48 and 49). To this fusion is due the opacity which constitutes the primitive streak as seen from the surface (Fig. 46). After the formation of the primitive groove and streak there is no longer any specially marked definition of the posterior margin of the germ disk, the entire circumference having a uniform demarcation. Very soon after the formation of the primitive streak a new opacity appears which extends forward in the medial line from Hensen’s node (anterior lip of the blastopore). This is known as the head process, or “primitive intestinal en. CC. 2.8. surface view a uniform appearance. The first differentiation noticeable in the disk is an opacity at what now becomes defined as the posterior margin of the disk (Fig. 53). As the em- Entoderm Yolk cavity Fic. 52.—Sections of blastodermic vesicle of bat, showing (a) formation of the entoderm and (6 and c) of the amniotic cavity. van Beneden. bryonic disk increases in size a linear opacity appears extending from the opacity at the posterior margin of the disk forward in the medial line to a point somewhat anterior to the center of the disk. The appearance (Fig. 53) is strikingly similar to that of the chick at the same stage (Fig. 46). The posterior opacity corresponds to the crescentic groove, the linear opacity to the primitive 70 TEXT-BOOK OF EMBRYOLOGY. streak, its anterior club-shaped end to Hensen’s node. If we assume the same transformation of the crescentic groove into the primitive groove, the two to- gether corresponding to the blastopore, the condition is quite analogous to that in the chick (p. 65). At a slightly later stage than shown in Fig. 53, a new opacity appears ex- tending forward in the medial line from Hensen’s node (Fig. 54, a). This is the head process, and may be considered as homologous with the head process in the chick. (Compare Fig. 54, a, with Fig. 50.) The opacity is due toa plate or cord of cells which grows from the region of Hensen’s node forward under the surface layer of cells (ectoderm) (Fig. 55). On the assumption that Hensen’s Embryonic disk — sie: Hensen’s node — Primitive _ streak Peristomal mesoderm Fic. 53.—Embryonic disk of dog. Bonnet. The letters and figures on the right (Sr-S,) indicate planes of sections shown in Fig. 73. node is the anterior lip of the blastopore, this plate of cells may possibly be con- sidered as homologous with the invaginated cells which form the protentoderm in Reptiles and Birds. (Compare Figs. 42, 51 and 55.) Consequently, since the protentoderm in the lower forms was designated the “ primitive intestinal cord” (Urdarmstrang), so in Mammals this invaginated cord of cells may be called the “primitive intestinal cord” (protentoderm) (Fig. 54). In Reptiles it has been seen that as the protentoderm grows forward under the surface layer (ectoderm) the yolk entoderm for some distance disappears, and the protentoderm fuses with the remaining yolk entoderm in an area known as the completion plate (Fig. 42). In the chick also it has been stated that a similar process occurs (p. 66). In Mammals the yolk entoderm, which GERM LAYERS. 71 oe Completion plate Ca ae } Head process Br re Embryonic disk ey 11. Prim. int. cord ' | Hensen's node ——~+-—__.. (protentoderm) Primitive streak |" and groove ———— Embryonic disk \atesoderm Ectoderm os Mesoderm v7 e eae OK Tee LL) aces) ° : ® 90%? OS & x Bi ae) Yolk entoderm Completion plate ) 08 Bate ay : Ectoderm a Xeo. oecsanete « Chorda anlage Entoderm Fic. 64.—Transverse section through dorsal part of embryo of frog (Rana fusca). Zzegler. x, Groove indicating evagination to form mesoderm, of invagination. As a matter of fact, observations do to a certain extent fulfill the expectation, but, on the other hand, it is not possible to trace the earliest steps in its formation with anything like the degree of certainty with which it can be traced in the lower forms. Neural crest Neural canal Primitive segment Notochord } Ectoderm Parietal mesoderm Ccelom 1 Visceral mesoderm Ventral mesoderm < Yolk cells 7 Entoderm Fic. 65.—Transverse section through embryo of frog (Rana fusca). Bonnet. Taking the chick again as an example, the mesoderm appears first in the region of the primitive groove (blastopore). ‘Transverse sections through this region show the mesoderm as several layers of small irregular cells interposed laterally between the ectoderm and entoderm. In the medial line, or line of the 6 80 TEXT-BOOK OF EMBRYOLOGY. primitive groove, all three germ layers are blended into a solid mass of cells (Fig. 66). On the ground that the primitive groove is the blastopore, the meso- derm here is the peristomal mesoderm, the homologue of the peristomal mesoderm which encircles the blastopore in lower forms (Fig. 37). Primitive groove and folds R— Ectoderm Wf — Mesoderm <—er — Entoderm — Ectoderm = — Mesoderm — Entoderm Fic. 66.—Transverse sections of blastoderm of chick (21 hours’ incubation). Hertwig. a, Section through primitive groove, posterior to Hensen’s node. b, Section through Hensen’s node. At a somewhat later stage, after the head process appears, sections through the head process also show all three germ layers. Here the ectoderm is a sepa- rate layer; but the entoderm and mesoderm are fused in the medial line; that Head process Neural plate Ectoderm — Mesoderm eoe* — Entoderm -——— Archenteron ‘olk ce Yolk cell wo a #— Volk Fic. 67.—Transverse section of blastoderm of chick (21 hours’ incubation). Hertwig. Section through head process, anterior to’ Hensen’s node. is, in the line of the ‘primitive intestinal cord.’ Laterally, the layers are all separate, a cleft existing between the mesoderm and the ectoderm and another between the mesoderm and the entoderm (Fig. 67). Since the mesoderm in the region of the head process is in front of the primitive groove (blastopore) GERM LAYERS. 81 and appears in connection with the “‘ primitive intestinal cord,” it is the gastral mesoderm, the homologue of the gastral mesoderm described in lower forms (Fig. 63). Here also, as in the case of the peristomal mesoderm, the mesoderm is primarily a solid plate of cells. Furthermore, immediately in front of the primitive groove the gastral mesoderm is continuous with the peristomal. At a still later stage the gastral mesoderm is found to be separated from the entoderm, so that the “ primitive intestinal cord” (now the notochord) separates the mesoderm of the two sides in the medial line (Fig. 68). Neural plate Notochord — Ectoderm — Mesoderm = — Entoderm #— Archenteron Fic. 68.—Transverse section of blastoderm of chick (40 hours’ incubation), -Hertwig. Section taken short distance anterior to Hensen’s node. Comparing the conditions in sections through the head process in the chick with sections through the body region of the frog (Figs. 63 and 64), a fairly clear homology may be drawn. While in the stages just described in the chick the mesoderm is present and interposed between the ectoderm and entoderm, the crucial point is its actual origin. In the lower forms it originated from the entoderm, that is, from the cells which have been invaginated at the blastopore. In the chick the blasto- pore, which is crescent-shaped, is transformed into a longitudinal structure— Mesoderm Primitive groove Ectoderm Entoderm Yolk Fic. 69.—Transverse section of blastoderm of chick (10 hours’ incubation). ertwig. Section taken through primitive groove and streak. the primitive groove—but still the blastopore. As the crescentic blastopore becomes longitudinal, the two horns come together and fuse (see p. 65), and the line of fusion still represents the area of invagination, where some of the surface cells have grown under the remaining surface cells to form the entoderm (protentoderm). And it is along this area of invagination that the mesoderm first appears. In very early stages there is an especially active cell proliferation in the thickened layer of cells which represents the primitive streak. This activity gives rise to a mass of cells which lie immediately beneath the primitive 82 : TEXT-BOOk OF EMBRYOLOGY. groove and represent the first mesodermal cells (Fig. 69). It is reasonable to assign the origin of these cells to the cells which have been invaginated along the line of the primitive groove (blastopore). These invaginated cells constitute the protentoderm, hence the mesodermal cells may be considered as derivatives of the protentoderm. As proliferation continues, the mesodermal cells spread out between the ectoderm and entoderm (which is here yolk entoderm) (Fig. 70). Finally, the — Eectoderm p.er. Mesoderm Was — Ectoderm i — Entoderm Yolk Fic. 70.—Transverse section of blastoderm of chick (slightly older than that shown in Fig. 69). Hertwig. Section taken through primitive groove (p.gr.) and streak. mesoderm fuses with the yolk entoderm, so that all three germ layers are fused beneath the primitive groove (Fig. 66). The fusion between the mesoderm and yolk entoderm in this region is a secondary matter. That the peristomal-mesoderm is a derivative of the invaginated cells is even more clearly demonstrated in Fig. 71, in which the two lips of the blasto- pore have not yet fused. Primitive fold Primitive groove Fic. 71.—Transverse section through primitive streak and primitive groove of Diomedea. Schauinsland. In front of the primitive groove, that is, in the region of the head process, the gastral mesoderm is at first seen to be continuous with the * primitive intestinal cord” (Fig. 67); later it becomes separated on each side from the “ primitive intestinal cord” (now the notochord). While the actual process has not been observed, it is reasonable to assume that the mesoderm is here also a derivative of the “primitive intestinal cord,” and since the latter is produced by the in- vagination (gastrulation, see p. 66) and consists of protentoderm, the gastral GERM LAYERS. 83 mesoderm is a derivative of the protentoderm or invaginated cells. Also, as the invagination is a continuous process from the first formation of the crescentic Neural Primitive Ectoderm tube segment 1 ‘ 1 ' ( | 1 i Entoderm Notochord Visceral Coelom mesoderm Fic. 72.—Transverse section of chick embryo (2 days incubation). Photograph. The parietal mesoderm (lying above the coslom) is not labeled. The two large vessels under the primitive segments are the primitive aorte. Spaces separating germ layers are due to shrinkage. : groove up through the formation of the “ primitive intestinal cord” (see p. 66), one can readily understand how the mesoderm is first formed in the line of the primitive groove and continues to be formed progressively forward as the invagi- ‘Area pellucida | _|Head fold Area vasculosa Neural groove Primitive segment _| Primitive groove Fic. 73.—Dorsal view of duck embryo, with two pairs of primitive segments. Bonnet. nation pushes farther and farther forward to form the “primitive intestinal cord.” The gastral mesoderm is thus from its beginning continuous with the peristomal mesoderm, the two together forming a single plate of cells. - 84 TEXT-BOOK OF EMBRYOLOGY. As described above, the mesoderm of the chick is at first a solid plate of cells. The cavity in the mesoderm—the coelom—appears as the result of a splitting of the originally solid mesoderm layer into two sublayers—the parietal and the visceral (Fig. 72). At the same time that portion of the mesoderm which lies adjacent to the neural groove on both sides of the medial line becomes differen- tiated into two series of bilaterally symmetrical segments—the primitive seg- ments, which are connected with one another by intermediate thinner parts (Figs. 73,74and 72). The splitting of the mesoderm to form the coelom begins some distance from the medial line and progresses both laterally and medially. Neuropore Fore-brain vesicle Head fold ——[—2 Proamnion —— Mid- and hind- Area pellucida brain vesicles Area vasculosa ee Primitive Area opaca segments Yolk Edge of Neural fold blastoderm Primitive groove Fic. 74.—Dorsal view of chick embryo with ten pairs of primitive segments. Bonnet. The coelom does not, however, reach the primitive segments, for a small solid mass of cells—the infermediale cell mass (Fig. 81)—always intervenes between the ccelom and the segments. Furthermore, the ccelom from the beginning shows no segmentation. The formation of the neural groove and neural tube from the ectoderm and the separation of the chorda anlage from the entoderm are much the same asin the frog. A decided difference is, however, to be noted in the shape of the chick’s blastoderm, Since in this case the yolk plays but a small part in seg- mentation, the germ layers at first lie flat upon the surface of the yolk, the GERM LAYERS. 85 archenteron being a flat cavity between the entoderm and the yolk (Figs. 67, 68 and 69). The tubular form of the intestine is brought about later in connection with the constriction of the embryo from the yolk sac (p. 140; see also forma- tion of primitive gut, p. 317). Mesoderm Formation in Mammals.—In Mammals the same difficulties are met with in determining the origin of the mesoderm as in the chick. At the same time, transverse sections through the developing mammalian blastoderm Ectoderm Waa: FOE Ore dr ¢ é A es em SI G Yt) 3 @ mk. An Ry .: ee Mesoderm aad a ee 3 P — SS Yolk Completion Br entoderm plate Ectoderm a. 8 ~ a Wo ee : - RC] Say he ee . ee @ * ger : a . gar Pe G4 : «oC : : “= Meso- gah is 28 E ‘ R ¢ a derm ve 2 7 ua = Be DY “aN oo” i Pia. es = & ew Yolk entoderm Pr.int.co, Yoik entoderm Prst, Fic. 75.—Transverse sections of embryonic disk of dog. Bonnet. Sections of disk shown in Fig. 53. Letters and numbers at right (S,-S,) indicate plane of sections in Fig. 53. P.gr., Primitive groove; Pr.int.co., primitive intestinal cord; Pr.st., all three germ layers fused in primitive streak. at different stages show conditions which bear much resemblance to those in the chick, and lead toward the conclusion that the processes in the two cases are much alike. Referring back to gastrulation, it will be remembered that on surface view the germ disks of the chick and of the dog were very similar (compare Fig. 46 with Fig. 53, and Fig. 50 with Fig. 54, a). After the formation of the primitive streak in the dog, sections through this region show the mesoderm interposed between the ectoderm and entoderm (here yolk entoderm) and all three germ 86 TEXT-BOOK OF EMBRYOLOGY. layers fused beneath the primitive groove (Fig. 75, 5; and S,; compare with Fig. 66). The origin of the mesoderm is probably, as in the chick, to be at- tributed to the invaginated cells (protentoderm) along the line of the primitive groove. The mesodermal cells first appear as a small mass beneath the primi- tive groove (Fig. 76, a); they then spread out laterally between the ectoderm and (yolk) entoderm (Fig. 76, 0). Beneath the point of origin, that is, along the 3 Se Ror eae § ee Fic. 76.—Transverse sections of embryonic disks of rabbit. (a) Kélliker, (b) Rabl. a, section through primitive streak of embryo of 6 days and 18 hours; b, section through Hensen’s node of embryo of 7 days and 3 hours, line of the primitive groove, they finally fuse with the (yolk) entoderm (Figs. 75,5, and S,; compare Figs. 76, a and 0, and Figs. 75, S, and S, with Figs. 69, 7o and 66). In the region of the head process, as in the chick, sections show at first the entoderm and mesoderm fused in the medial line, and the ectoderm as a sepa- rate layer (Fig. 77 and Fig. 75,5,). The entoderm with which the mesoderm is Mesoderm Notochord Entoderm Fic. 77.—Transverse section of embryonic disk of rabbit. van Beneden. fused represents the invaginated cells, that is, the protentoderm (‘primitive intestinal cord’’); and, as in the chick, it seems reasonable to assume that the mesoderm is derived from the “primitive intestinal cord” (protentoderm) and grows out laterally between the ectoderm and entoderm (compare Fig. 75, S, with Fig. 67). A little later, in the region of the head process, the mesoderm on each side is GERM LAYERS. 87 found to be separated from the parent tissue (‘‘ primitive intestinal cord’’), and the latter now represents the anlage of the notochord (compare Fig. 72 with Fig. 78). On the ground that the primitive groove is the blastopore, the mesoderm arising in that region is the peristomal mesoderm; that arising from the “primitive intestinal cord” in front of the primitive groove is the gastral meso- Mesoderm Ectoderm Neural groove Yolk entoderm Chordal plate Fic. 78.—Transverse section of embryonic disk of dog. Bonnet. Section taken near anterior end of head process. derm. The peristomal and gastral portion together constitute a continuous plate of cells interposed between the ectoderm and entoderm, which has been derived from the invaginated cells of the protentoderm. In a few Mammals (sheep, roe, shrew), mesoderm has been seen to arise some distance from the primitive streak and head process (Fig. 79). This has been called the peripheral mesoderm, but it soon unites with the peristomal and gastral. Embryonic disk —Areaof — Peripheral invagination mesoderm Nuclei of yolk entoderm Ectoderm Fic. 79.—Surface view of embryonic disk of sheep. Bonnet. : ok, Disk is at that stage of development when gastrulation begins (in region marked area of invagination). Primarily, the mesoderm is a solid plate of cells with no indication of a body cavity (ceelom). A little later the mesoderm splits into two layers, the parietal and the visceral, between which lies the coelom (Fig. 81). T he splitting does not effect, however, the mesoderm which lies adjacent to the neural groove on both sides of the medial line, for this portion becomes differentiated into two series of bilaterally symmetrical segments—the primitive segments (Figs. 80 and '88 TEXT-BOOK OF EMBRYOLOGY. & Telencephalon é ; Diencephalon fi Mesencephalon aa mia are Ba ald 4 —~——- —— Metencephaton Anlage _f £5: ‘ ofheart ~ Myelencephalon / | h { i ( j a eal ¥ ee) Peripheral limit i 4 A of celom | G3} ‘ p % § ‘ | : b { Tail fold of amnion Fic. 80.—Dorsal view of dog embryo with ten pairs of primitive segments. Bonnet. Neural Prim. Intermed. groove seg. cell mass Parietal and visceral mesoderm Ectoderm (epidermis) aorta Coelom Entoderm Blood vessels Fic. 81.—Transverse section of dog embryo with ten pairs of primitive segments. Bonnet. y GERM LAYERS. 89 81). The splitting of the mesoderm begins some distance from the medial line and proceeds both laterally and medially, but does not extend quite to the primitive segments. ‘Thus a solid plate of cells still remains between the ccelom and the segments—the intermediate cell mass (Fig. 81). The coelom shows no segmentation. (Compare Fig. 80 with Fig. 74 and Fig. 81 with Fig. 72.) The formation of the neural groove and tube from the ectoderm and the separation of the chorda from the entoderm are processes quite analogous to the development of those same structures in the lower forms. As in the chick, so also in Mammals, the blastoderm is at first spread out flat, forming the roof, so to speak, of the yolk sac. At a later period, in connection with the closure of the gut and the establishment of the external forms of the body, the blastoderm assumes a tubular shape (see p. 140). A comparison of the foregoing description of the formation of the mesoderm in Mammals with the description of the corresponding processes in the chick (p. 79) shows their essential similarity. Part of exoccelom Strand of mesoderm Trophoderm in exoccelom Mesoderm of chorion Ectoderm of amnion Entoderm Amniotic cavity Entoderm Embryonic ectoderm of yolk sac] . ~| Mesoderm Yolk cavity Mesoderm of yolk sac OY sesodenen 95 abi | Fic. 82.—Section through human chorion, amnion, embryonic disk, and yolk sac. Peters. Compare with Fig. 83. - The Germ Layers in Man. Of the actual formation of the germ layers in man, practically nothing is known. ‘There are no observations on the segmentation of the ovum, the first differentiation of cells, or the origin of the embryonic disk and germ layers. A very young human ovum, described by Leopold, does not show any structures which can be interpreted as the embryonic disk or any part of it. Another 90 TEXT-BOOK OF EMBRYOLOGY. young ovum described by Peters shows all three germ layers and the flat embry- onic disk. Bryce and Teacher have recently described an ovum, the youngest on record, in which all three germ layers are formed (see Fig. 106; cf. Fig. 83). A section through the ovum described by Peters (Fig. 82) shows the ectoderm as a flat layer of stratified or pseudostratified cells, the margin of which is re- flected dorsally as the lining of the roof of the amniotic cavity (compare Fig. 52, ¢). Beneath the ectoderm is a layer of cells—the mesoderm—which is continu- Coagulum Trophoderm Uterine epithelium Intervillous spaces, Jassea poolg : a ent & spuely Chorion Embryonic disk slie[nsdeo enploaq twinypeyyide sulsey/) puelg Gland Decidua basalis Blood Fic. 83.—Section through very young human chorionic vesicle embedded in the uterine mucosa. Peters. The vesicle measured 2.4 x 1.8 mm., the embryo .19 mm. Peters reckoned the age as 3 or 4 days, but later studies of other embryos go to show that the age is much greater; Bryce and Teacher estimate it at 14 to 15 days. ous at its margin with the mesoderm of the roof of the amnion, with mesoderm lining the chorionic vesicle, and also with the mesoderm covering the yolk sac Fig. 83). Beneath the mesoderm of the embryonic disk is a layer of entoderm which also extends ventrally to line the yolk sac. There is here no trace of an invaginated entoderm from which the mesoderm might arise. Graf Spee has described an ovum somewhat older than Peters’, in which the embryonic disk shows certain features which are comparable with those in lower Mammals. On surface view (Fig. 84), the primitive groove is especially GERM LAYERS. 91 prominent and the opening at its anterior end, corresponding to Hensen’s node, is usually well marked. The line of the head process is strongly marked by a deep groove—the neural groove (compare Fig. 84 with Fig. 54, a). A longitudinal section in the medial line of this disk (Fig. 85) shows a re- markable similarity to a corresponding section of the bat’s disk (Fig. 55). The ectoderm consists of a single layer of columnar cells interrupted only at the opening of the blastopore (anterior end of the primitive groove). The entoderm (chorda anlage) also consists of a single layer of cells which is continuous at the blastopore with the ectoderm. In the region of the primitive groove the per- Yolk sac See Pi Werte i si 4 i P fi & . | Amnion ‘ Neural groove Oe ‘ \ f ‘ ey ee Neurenteric “Qt. Bog fe canal * Primitive Reet Ve groove Se oo ; : laos Belly stalk me 4 | Sasa if Chorion = ST gD JS & ec Fic. 84.—Dorsal view of human embryo, two millimeters in length, with yolk sac. von Spee, Kollmann. The amnion is opened dorsally. istomal mesoderm is present. The embryonic disk forms the roof, so to speak, of the yolk sac. A transverse section (Fig. 86) through the primitive groove shows all three germ layers fused in the medial line, but separated laterally. In this case there is a striking resemblance to the condition seen in a corresponding section of the rabbit’s disk (Fig. 87). Apart from the embryonic disk, the conditions are very similar to those i in Peters’ ovum (compare Figs. 85 and 82). The unusual feature in both these embryos is the enormous extent of the 92 TEXT-BOOK OF EMBRYOLOGY. mesoderm. In Graf Spee’s ovum both longitudinal and transverse sections would suggest the same origin for the intraembryonic mesoderm as in lower ae Mesoderm Amnion ee of chorion Anlage of iG notochord ( { Belly stalk S 7 —— Primitive streak Heart —— fp Allantois region y Yolk sac Entoderm —— of yollksac A Mesoderm of yolk sac Blood vessel Fic. 85.—Medial section of human embryo shown in Fig. 84. von Spee, Kollmann. Mammals, but the extent of the extraembryonic mesoderm, at this early stage of the embryonic disk, would indicate a departure from the conditions seen in the lower Mammals. In other words, it scarcely seems possible that the Ecto- Mesoderm derm Primitive groové Ectodertr smo Parietal mesoderm Visceral mesoderm Entoderm Fic. 86.—Transverse section through primitive streak of embryo shown in Fig. 84. von Spee. mesoderm which lines the chorionic vesicle and covers the yolk sac has grown out from the mesoderm which arises within the embryonic disk; it seems more + GERM LAYERS. 93 ,,_ Parietal mesoderm Primitive groove Visceral mesoderm | Primitive fold Fic. 88.—Diagrams representing hypothetical stages in the development of the human embryo. > A, Morula; compare with Fig. 33,4. B, Morula with differentiated superficial cells; compare with Fig. 33,8. C, Central cells have become vacuolized to form the yolk cavity, leaving a small group (the inner cell mass) attached to the enveloping layer (trophoderm); compare with Fig. 33, d. D, Cells of the inner cell mass which are adjacent to the yolk cavity have become differentiated and have begun to grow around the cavity, forming the entoderm; compare with Fig. 52, a. 94 TEXT-BOOK OF EMBRYOLOGY. reasonable to suppose that it has arisen outside the embryonic disk and united with the intraembryonic mesoderm secondarily. While neither the origin of the extraembryonic mesoderm, nor its behavior up to the stage in Bryce and Teacher’s ovum, has been observed in man, it is possible to construct hypothetical diagrams which allow of comparison with what actually occurs in the lower Mammals. The morula, the differentiation my RD > EON > EY, ee eare oe "~Enveloping L g A Urrophedsra), B Visceral Mesoderm Parietal Mesoder ” Fic. 89.—Diagrams representing hypothetical stages in the development of the human embryo (to follow Fig. 88). A, Entoderm surrounds the yolk cavity; part of the cells of the inner cell mass have become vacuolated, thus forming the amniotic cavity, while the remainder constitute the embryonic ectoderm; compare with Fig. 52. B, Mesoderm (represented by dotted portion) has appeared between the entoderm and trophoderm, between the entoderm and ectoderm of the embryonic disk, and in the roof of the amnion. C, The mesoderm around the yolk cavity has split into a parietal and a visceral layer, the cleft between being the anlage of the extraembryonic body cavity (exoccelom). of the superficial layer of cells, the formation of the trophoderm and inner cell mass, and the differentiation of the primary entoderm may be represented hypothetically by the diagrams in Fig. 88. These are quite comparable with the corresponding stages of development in the bat (Fig. 33). In Fig. 89, A, the amniotic cavity formed by a vacuolization of a part of the inner cell mass is shown, and also the entoderm lining the entire yolk cavity. This is also com- GERM LAYERS. 95 parable with conditions in the bat (Fig. 52). In the next stage (Fig. 89, B) the mesoderm is present all the way around between the trophoderm and ento- derm, in the roof of the amniotic cavity, and between the ectoderm and entoderm in the embryonic disk. It is possible that the mesoderm arises in situ as a deriv- ative of the entoderm or trophoderm. Since in the lower Mammals it arises from entoderm, a similar origin here seems the more reasonable. Extraew bryonte Body Cavity Fic. 90.—Diagrams representing stages of development of the human embryo (to follow Fig. 89). A, A stage that corresponds approximately to those of Peters’ and Bryce-Teacher’s embryos (Figs. 83 and 107). Owing to the rapid enlargement of the chorionic vesicle, the extraembryonic body cavity has become much larger than in Fig. 89, C._ B, A stage (in longitudinal section) corresponding to that of von Spee’s embryo (Fig. 85). Only a part of the chorion is shown; the embryonic disk is slightly constricted from the yolk sac; note the belly stalk, comparing with A. C, Transverse section, same stage as B. D, Longitudinal section, stage somewhat later than B. Note the greater degree of constriction between the embryo and yolk sac, and the larger amnion. In the majority of the lower Mammals the intraembryonic mesoderm arises from the entoderm and then grows out into the wall of the blastodermic vesicle. In a few, however (sheep, roe, shrew), the peripheral mesoderm (p. 87) arises outside of the embryonic disk and unites with the intraembryonic meso- derm secondarily. It might be suggested that the formation of peripheral Kf 96 TEXT-BOOK OF EMBRYOLOGY. mesoderm outside of the embryonic disk is an intermediate step between the formation of mesoderm entirely within the embryonic disk and its formation around the entire vesicle, as in the hypothetical case. Neither in Peters’ nor in Graf Spee’s ovum is any embryonic body cavity present. But in both cases a very large cavity exists between the mesoderm of the yolk sac and that of the chorion. This cavity—the extraembryonic body cavity (exocelom)—probably arises by a splitting of the extraembryonic meso- derm into two layers, parietal and visceral, just as the embryonic body cavity in other Mammals is the result of a splitting of the intraembryonic mesoderm (p. 87). The splitting would occur as shown in Fig. 89, C. The parietal layer which with the trophoderm becomes the chorion, then grows rapidly and becomes widely separated from the visceral layer, the latter with the entoderm constituting the wall of the yolk sac. Thus a stage is reached which is shown in Fig. 89, C, and which corresponds with Peters’ ovum (Fig. 83). The embry- onic disk with its yolk sac and amniotic cavity occupies but a small space within the chorionic vesicle. Consult also Fig. 106, showing the Bryce-Teacher ovum. The stage corresponding to Graf Spee’s ovum would be produced by a fur- ther splitting of the mesoderm in the roof of the amnion, so that finally the em- bryonic disk and yolk sac remain attached to the chorion only by a band of mesoderm, the belly stalk (Fig. 90; compare with Fig. 85). Even at this stage, no body cavity is present within the embryonic disk (Fig. 86). When it does appear, however, it becomes continuous laterally with the exoccelom (see Chap. XIV), and the parietal and visceral layers of meso- derm within the embryonic body are continuous, respectively, with the parietal and visceral extraembryonic mesoderm. PRACTICAL SUGGESTIONS. For a complete demonstration of the formation of the germ layers, especially of the mesoderm, in any class of animals, sections of many successive stages are necessary. A few specimens, however, suffice to illustrate certain principles of development. The formation of the gastrula in Invertebrates is fairly well illustrated by the star-fish. After the blastule appear (see “‘Practical Suggestions,” p. 53), they may be observed from time to time within the next few hours and the various steps in the invagination process made out quite definitely. Any of these stages may be preserved in the same manner as the earlier stages (p. 53). The formation of a gastrula from a blastula, in which there is a considerable amount of yolk, is seen in the case of the common frog. The ova are taken at a time when a small crescentic depression (the blastopore) appears at the marginal zone, and at intervals from this time until the yolk plug is seen projecting through the blastopore. They are fixed in 5 per cent. formalin, stained im ¢ofo in borax-carmin (p. 53), cut in rather thick sections in celloidin, and mounted in xylol-damar. The most instructive sections are those which pass through the blastopore, in the meridional plane. For means of removing the gelatinous capsules surrounding the eggs, see page 629. GERM LAYERS. 97 Transverse sections cut after the gastrula has begun to elongate to form the embryonic body, prepared by the same technic as above, will show the mesoderm at each side of the primitive gut. Surface views are very instructive in cases of discoidal cleavage. The primitive streak is well shown in the hen’s egg during the second half of the first day of incubation. The blastoderm is removed from the surface of the yolk, fixed in Zenker’s fluid, stained im foto in borax-carmin and mounted 7m toto in xylol-damar (see Appendix). A blastoderm of the same stage (second half of first day), fixed in Zenker’s fluid or in Flemming’s fluid, preferably the latter, cut in paraffin and stained with Heidenhain’s hema- toxylin, will show instructive pictures of the three germ layers before the splitting of the mesoderm; and will also show the fusion of all three germ layers in the line of the primitive streak. Surface views of the chick blastoderm, during the second day of incubation, are also instructive as to general topography, showing the neural groove (or tube if late in the second day) and its relation to the primitive streak, the primitive segments, the area pellucida and the area opaca. The blastoderm is removed from the surface of the yolk, fixed in Zenker’s fluid, stained im toto in borax-carmin, and mounted im toto in xylol-damar (see Appendix). Sections of a blastoderm of the same stage (second day) are especially valuable in showing the early conditions of the germ layers after the primitive segments have appeared and the mesoderm has split into two layers. The blastoderm is fixed in Flemming’s or Zenker’s fluid, cut transversely in paraffin and stained with Heidenhain’s hematoxylin. The ectoderm and neural groove (or tube), the mesoderm (primitive segments, parietal and visceral layers with enclosed ccelom), and the entoderm and notochord are very clearly shown. The visceral mesoderm usually contains a number of developing blood vessels. For methods of procuring mammalian blastodermic vesicles and preparing them for study, see page 630. References for Further Study. AssHETON, R.: The Reinvestigation into the Early Stages of the Development of the Rabbit. Quart. Jour. of Mic. Sci., Vol. XXXVII, 1894. ASsHETON, R.: The Segmentation of the Ovum of the Sheep, with Observations on the Hypothesis of a Hypoblastic Origin for the Trophoblast. Quart. Jour. of Mic. Sct., Vol. XLI, 1898. Van BENEDEN, E.: Recherches sur les premiers stades du développement du Murin (Vespertilio murinus). Anat, Anz., Bd. XVI, 1899. Bonnet, R.: Lehrbuch der Entwicklungsgeschichte. Berlin, 1907. Bonnet, R.. Beitrige zur Embryologie der Wiederkauer gewonnen aus Schafei. Arch. }. Anat. u. Physiol., Anat, Abth., 1884, 1889. BonneET, R.: Beitrige zur Embryologie des Hundes. Anat. Hefte, Bd. IX, 1897; Bd. XVI, r901. Bryce, T. H.,and TEACHER, J. H.: Early Development and Imbedding of the Human Ovum. Glasgow, 1908. Harper, E. H.: The Fertilization and Early Development of the Pigeon’s Egg. Am. Jour. of Anat., Vol. TIT, 1904. HatscHex, B.: Studien tiber Entwicklung des Amphioxus. Arbeiten aus dem z00l. Instit, zu Wien, Bd. IV, 1881. 98 TEXT-BOOK OF EMBRYOLOGY. HEAPE, W.: The Development of the Mole (Talpa europea). Quart. Jour. of Mic. Sci., Vol. XXIII, 1883. Hertwic, O.: Die Lehre von den Keimbladitern. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. I, Teil I, 1903. Husrecut, A. A. W.: Studies on Mammalian Embryology. II: The Development of the Germinal Layers of Sorex vulgaris. Quart. Jour. of Mic. Sci., Vol. XXXI, 1890. Lititz, F. R.: The Development of the Chick. New York, 1908. McMorricu, J. P.: The Development of the Human Body. Philadelphia, 1907. Minor, C. S.: Laboratory Text-book of Embryology. Philadelphia, 1903. Morcan, T. H.: The Development of the Frog’s Egg. New York, 1897. Morean, T. H., and Hazen, A. P.: The Gastrulation of Amphioxus. Jour. of Morphol., Vol. XVI, 1900. Patterson, J. T.: On Gastrulation and the Origin of the Primitive Streak in the Pigeon’s Egg. Biolog. Bull., Vol. XIII, 1907. PEEBLES, F.: The Location of the Chick Embryo upon the Blastoderm. Jour. of Ex- periment. Zool., Vol. I, 1904. Perers, H.: Ueber die Einbettung des menschlichen Eies und das friiheste bisher be- kannte menschliche Placentationstadium. Leipzig u. Wien, 1899. ‘ Von SPEE, GraF: Beobachtungen an einer menschlichen Keimscheibe mit offener Medul- larrinne und Canalis neurentericus, Arch. j. Anat. 4. Physiol., Anat. Abth., 1889. Sozotta, J.: Die Entwickelung des Eies der Maus vom Schluss der Furchungsperiode bis zum Auftreten der Amnionfalte. Arch. }. mik. Anat., Bd. LXI, r1goz2. Witson, E. B.: Amphioxus and the Mosaic Theory of Development. Jour. of Morphol., Vol. VITI, 1893. CHAPTER VII. FETAL MEMBRANES. In all Vertebrates, with the exception of Fishes and Amphibians which lay their eggs in water, there begin to develop at a very early stage certain accessory or extraembryonic structures which may be conveniently called fatal mem- branes. The development of these structures is very closely related to the de- velopment of the embryo itself, and their presence is apparently largely depend- ent upon the very considerable length of embryonic life in these forms, during which it is necessary for the embryo to maintain a definite relation to its food supply and to possess means of discharging waste products. The foetal mem- branes, therefore, have to do with the protection and nutrition of the growing embryo and also are connected with the care of the waste products of foetal metabolism. Under the head of foetal membranes are to be considered (1) the amnion, (2) the allantois, (3) the chorion; also in connection with these, the yolk sac and the umbilical cord. The development of these structures in Mammals and especially in man is extremely complex and can be best understood by comparison with their simpler development in Reptiles and Birds. FETAL MEMBRANES IN BIRDS AND REPTILES. Throughout these two classes there is such uniformity in the formation of the foetal membranes that the chick may be taken as typical. The chief characteristic of these classes, as influencing the form and structure of the foetal membranes, is the very large amount of yolk stored up within the egg for the nutrition of the embryo. This is made necessary by the early separation of the egg from the mother, in contrast to the close nutritional relationship between mother and foetus which obtains in Mammals (excepting Monotremes), where the young are retained within the body of the mother up to a comparatively late developmental stage. The Amnion.—Returning to that point in the development of the blastoderm of the chick where no trace of amnion has as yet appeared, we recall that the blastoderm at this stage consists of three layers, ectoderm, mesoderm and entoderm; that the medial line of the embryo is marked by the neural groove, flanked by the neural folds which are continuous with each other anteriorly; that 99 100 TEXT-BOOK OF EMBRYOLOGY. on each side of the neural groove between ectoderm and entoderm the mesoderm is a solid mass of cells, while more laterally the mesoderm is split, its peripheral layer with the adjacent ectoderm forming the somatopleure, its central layer with the adjacent entoderm forming the splanchnopleure; that between soma- topleure and splanchnopleure is the body cavity. Ventral to the neural groove is the notochord, while ventral to the latter is the primitive gut, the roof of which is formed of entoderm (Fig. 72). The first indication of amnion formation is the appearance of a fold—the head amniotic fold—just in front of the anterior union of the neural folds (Figs. ar, op.! ed. mes. h, am. f. coe. pr. seg. ar. pel. ar. op.? Pp Fic. 91.—Dorsal view of embryo of bird (Phaeton rubricauda) with fifteen pairs of primitive segments. Schawinsland. ar.op.t, Area opaca, portion in which mesoderm is not yet present; ar.op.?, area opaca; ar. pel., area pellucida; cw., bladder-like dilatation of ccelom; ed. mes., edge of mesoderm; h.am. f., head amniotic fold; pr. seg., primitive segments; x, portion of amniotic fold containing no mesoderm. gtand 97,b). Thisoccurs during the second day of incubation. , After the head fold has become well developed and extends back over the embryo like a hood (Fig. 93), similar lateral and tail folds make their appearance (Figs. 92 and 97, a and 6). The folds continue to grow over the dorsum of the embryo and finally meet and fuse in the mid-dorsal line, forming the amniotic suture (Fig. 94). The amniotic folds from the beginning involve the somatopleure, that is, the ectoderm and parietal mesoderm. But since they arise some distance from the developing embryonic body, the extraembryonic portions only are involved. At the same time a portion of the extraembryonic body cavity is also carried dorsally within the folds (Figs. 92 and 95). When the folds unite over the FETAL MEMBRANES, 101 embryo they break through at the line of contact, thus leaving the outer layers of the folds continuous and the inner layers continuous, with the extraembryonic body cavity continuous between the outer and inner layers, mes. b.c. al, a.m. cg. ta. Fic. 92.—Medial section of caudal end of chick embryo (at end of second day of incubation). Duval. al., Beginning of allantoic evagination; a.m., anal membrane; b.c., extraembryonic body cavity; ¢.g., caudal gut; ect., ectoderm; ent., entoderm; mes., mesoderm; mes.t, parietal mesoderm; mes.?, visceral mesoderm; 1. tu., neural tube; pr.g., primitive gut; 4am. j., tail amniotic fold; éa., tail. The result of the development of the amniotic folds is:— 1. That the embryo is completely enclosed dorsally and laterally by a cavity, the amniotic cavity, which is lined by ectoderm continuous with the ectoderm— =| Area ‘| opaca _| Edge of mesoderm x -| Dorsal amniotic suture Primitive streak Fic. 93.— Dorsal view of embryo of albatross, showing amnion covering cephalic end of embrvo. Schauinsland. x, Portion of blastoderm containing no mesoderm. later epidermis—of the embryo, the ectoderm lining the cavity and the overlying parietal mesoderm together constituting the amnion (Fig. 96). 2. That the outer parts of the amniotic folds become completely separated 102 TEXT-BOOK OF EMBRYOLOGY. from the inner—the amnion—to form a second membrane consisting externally of ectoderm, internally of mesoderm and called at first the serosa or false amnion, later the primitive chorion (Fig. 96). 3. That the extraembryonic body cavity unites across the medial line dorsally, thus separating the amnion from the primitive chorion (Fig. 97, a, b and c). During the formation of the amnion the chick embryo is becoming more and more definitely constricted off from the underlying large yolk mass which is liquefying and into which the embryo sinks somewhat. At the same time the Ant. vitelline vein Mesoderm Area opaca Sinus terminalis Extraembryonic body cavity Amnion Amniotic suture Area pellucida Omphalomesenteric (vitelline) vein Amniotic suture jeter amniotic fold Primitive streak [> =-"=at=-- Seah Tail amniotic fold ---| Area opaca Fic. 94.—Dorsal view of embryo of albatross, showing amnion covering greater part of embryo. Schauinsland. amniotic cavity continues to increase in size and extends also ventrally beneath the embryo so that the embryo is everywhere enclosed within the amnion except at its narrow connection with the yolk (Fig. 97, ¢, d). The amniotic cavity is filled with fluid, the léguor amnii, the origin of which is uncertain. In it the embryo floats freely, attached only by its ventral con- nection with the yolk. At about the fifth day of incubation rhythmical con- tractions of the amnion begin. These are apparently due to the development of contractile fibers in its mesodermic tissue and give to the embryo a regular oscillating motion. FETAL MEMBRANES. 103 The Yolk Sac.—The simplest type of yolk sac is found in Amphibians and Fishes. In Amphibians the yolk is enclosed within the embryo, the cells form- lLamd, @s.b.@ ser. s@t. 1 | 1 pe. ep. ht. pe. Fic. 95.—Transverse section of embryo of albatross. Schauinsland. Section taken through region of heart. am., amnion; ao., aorta; u.v.v., anterior vitelline veins; ect., ectoderm; ent., entoderm; ep., epicardium; ex. b. c., extraembryonic body cavity; hf., heart; l.am.}., lateral amniotic fold; pc., pericardium; ph., pharynx; p. mes., parietal mesoderm; ser.. serosa (chorion); v. mes., visceral mesoderm; * point at which extraembryonic body cavity passes over into the intraembryonic (or coclom proper). ing a part of the intestinal wall. The superficial cells are split off to form the yolk eutoderm. Investing the yolk entoderm is the visceral mesoderm which ser. am. sut. am, 1 p. pe. ! ! ht. ph. p. pe. Fic. 96.—Transverse section of embryo of albatross. Schauinsland. ; Section taken through region of heart. amt., Amnion,; am. sut., amniotic suture; u.v.v., anterior vitelline veins; ect., ectoderm; ent., entoderm; ex. b.c., extraembryonic body cavity; /it., heart; p. pe., primitive pericardial cavity; ph., pharynx; p.mes., parietal mesoderm; ser., serosa (chorion); v. mes., visceral mesoderm; * point at which extraembryonic body cavity passes over into intraembryonic (or coelom). is separated from the parietal mesoderm by the body cavity. Outside of the parietal mesoderm is the ectoderm (Fig. 65). In many of the Fishes the germ 104 TEXT-BOOK OF EMBRYOLOGY. disk, as in Reptiles and Birds, is confined to one pole of the egg. Thus in these forms the embryonic body develops on the surface of the large yolk mass. As the embryo develops the germ layers simply grow around the yolk and suspend it from the ventral side of the embryo. At the same time a constriction appears between the embryo and the yolk mass, thus forming the yolk stalk. In this case the yolk is surrounded from within outward, by entoderm, visceral and G60, orate esse ONlaTs h. am. fF. ry e ye K(€ ! ae 3 as 4 [TF ll i Mla Fic. 97.—Diagrams representing stages in the development of the foetal membranes in the chick. Hertwig. a, Transverse section; 5, ¢, d, longitudinal sections; yolk represented by vertical lines. aJ., Allantois; am., amnion; am.c., amniotic cavity; c@., ceelom; diz, vitelline area between two dotted lines which represent the edge of the mesoderm (at s.t.) and entoderm (at z. g.); dg., yolk stalk; ds., yolk sac; d.umb., dermal umbilicus; ect., ectoderm; ent., entoderm; ex. b.c., extraem- bryonic body cavity; gh., area vasculosa; h.am.}., head amniotic fold; m., mouth; p. mes., parietal mesoderm; s./., sinus terminalis; ser., serosa (chorion); ¢. amt. /., tail amniotic fold; umb., umbilicus; t mes., visceral mesoderm; 2. g., dotted line represents edge of entoderm. parietal mesoderm, and ectoderm (Fig. 98). The yolk furnishes nutriment for the embryo. This is conveyed to the tissues by means of blood vessels. Branches of the vitelline artery ramify in the wall of the yolk sac (in the meso- dermal tissue) ; the branches converge to form the vitelline veins which carry the blood back to the embryo. In the chick, while the amnion is forming, the inner germ layer gradually extends farther and farther around the yolk (Fig. 97, a, b, c and d). At the FETAL MEMBRANES. 105 same time, as already noted (p. 102), the growth of the amnion ventrally results in a sharp constriction which separates the embryo from the underlying yolk. This constriction is emphasized by constant lengthwise growth of the embryo. Following the gradual growth of the entoderm around the yolk, the mesoderm also gradually extends around, at the same time splitting into visceral and parietal layers, so that the entoderm is closely invested by visceral mesoderm (Fig. 97, a, 6, c and d). Finally, both entoderm and mesoderm enclose com- pletely the mass of yolk. The yolk thus becomes enclosed in the yolk sac which consists of two layers, entoderm and visceral mesoderm. The constricted connection between the yolk sac and the embryo is the yolk stalk. It is seen by reference to the diagrams (Fig. 97) that the entoderm lining the yolk sac is Fic. 98.—Diagrammatic longitudinal section of selachian embryo. Hertwig. a., Anus; d., yolk sac; dv., intestinal umbilicus; ds., visceral layer of yolk sac; hs., parietal layer of yolk sac; hn., dermal umbilicus; 2h, ccelom; /h?, exoccelom; m., mouth; sf., yolk stalk. directly continuous through the yolk stalk with the entoderm lining the primi- tive gut. The transition line between extra- and intraembryonic entoderm is sometimes referred to as the intestinal umbilicus, in contradistinction to the line of union, on the outside of the yolk stalk, of amniotic and embryonic ecto- derm (the latter becoming later the epidermis) which is known as the dermal umbilicus. As in Fishes and Amphibians, so also in Reptiles and Birds, the yolk furnishes nourishment for the growing embryo, and is conveyed to the embryo by the blood. At a very early stage the mesoderm layer of the yolk sac (visceral mesoderm) becomes extremely vascular. This vascular area is indicated by an irregularly reticulated appearance in the periphery of the blastoderm and is known as the area vasculosa (Fig. 74). The area vasculosa increases in size as the mesoderm grows around the yolk and its vessels become continuous with those in the embryo (Fig. 212). Some of these vessels enlarge as branches of two large vessels which are given off from the primitive aorte, the vitelline or omphalomesenteric arteries. (When the two aorte fuse to form a single vessel, the proximal ends of the vitelline arteries fuse likewise.) The branches of the arteries ramify in the mesoderm over the surface of the yolk and then ‘106 TEXT-BOOK OF EMBRYOLOGY. converge to form other vessels which enter the embryo as the vitelline or omphalo- mesenteric veins (Fig. 213). As the mesoderm extends farther and farther around the yolk, the vessels extend likewise until the entire yolk is surrounded by a dense plexus of blood vessels in the wall of the yolk sac. The Allantois.—While the embryonic intestine is first assuming the form of a tube, there grows out ventrally from near its caudal end, during the third day of incubation, a diverticulum which is the beginning of the allantois (Fig. 99). This increases rapidly in size and pushes out into the extraembryonic body cavity behind the yolk stalk. As it is a diverticulum from the intestine, it consists primarily of entoderm. ‘This pushes in front of it, however, the splanchnic (visceral) mesoderm which becomes the outer layer of the membrane. The connection between the intestine and the allantois is known as the urachus. In the chick the allantois attains a comparatively large size, pushing out dorsally t * . al. mes. ent. a.m. Cg. am, Fic. 99.—Longitudinal section of caudal end of chick embryo (end of third day of incubation). Gasser. al., Allantois; aJ.~., allantois prominence; a.m., anal membrane; am., amnion; am.c., amniotic cavity; ¢.g., caudal gut; cw., coelom; ect., ectoderm; ent., entoderm; ex. b.c., extraembryonic body cavity; mes., mesoderm; pr. g., primitive gut; 4., tail. between the amnion and the primitive chorion and ventrally between the latter and the yolk sac (Fig. 97, 8,c and d), The inner wall of the allantoic sac blends with the amnion about the seventh day of incubation and with the yolk sac con- siderably later, while the outer wall joins the primitive chorion to form the true chorion, or as it is sometimes designated, the allanto-chorion (see p. 111). As the allantois reaches the limit of the yolk, it leaves the latter, and pushing the primitive chorion before it, continues around close under the shell (Fig. 97) until it completely encloses the albumen at the small end of the egg. The allantois of the chick performs three important functions: 1. It serves as a receptacle for the excretions of the primitive kidneys. 2, United with a part of the primitive chorion to form the albumen sac, its vessels take up the albumen as nourishment for the embryo. Because of this function and also because of the fact that little papilla sometimes appear on the FETAL MEMBRANES. 107 inner surface of the albumen sac, evidently for the purpose of increasing its absorptive surface, this albumen sac has been compared by some to a placenta. 3. It blends with the primitive chorion to form the true chorion and being extremely vascular and lying just beneath the porous shell, it serves as the most important organ of foetal respiration. The allantois in the chick is an extremely vascular organ, the network of small vessels in the wall being composed of radicals of the allantoic or umbilical vessels of the embryo. Soon after the allantois begins to develop, two branches—the umbilical arteries—are given off from the aorta near its caudal end. These pass ventrally through the body wall of the embryo and thence out via the umbilicus to break up into extensive networks of capillaries in the mesodermal layer of the allantois. The capillaries converge to form the um- bilical veins which pass into the embryo via the umbilicus and thence cephalad to the heart. During the incubation period of the chick there are two extraembryonic sets of blood vessels. One set, the vitelline (omphalomesenteric) vessels (p. 238), is concerned with carrying the yolk materials to the growing embryo. The other set, the umbilical (allantoic) vessels, is chiefly concerned with respiration and carrying waste products to the allantois, but is probably in part concerned with conveying the albumen tothe embryo. When the chick is hatched, and the foetal membranes are of no further use and disappear, the extraembryonic por- tions of the blood vessels also disappear. The intraembryonic portions persist, in part, as certain vessels in the adult organism. The Chorion or Serosa.—This membrane is but little developed in the chick as compared with Mammals, especially the Placentalia. Its mode of origin as the outer leaves of the amniotic folds, cut off from the amnion by dorso-medial extension of the mesoderm and body cavity, has been described (p. tot). It consists, as there shown, of extraembryonic ectoderm and parietal mesoderm (Fig. 96). As first formed it is confined to the immediate region of the embryo and of the amnion to which it is later loosely attached. It soon extends ventrally around the yolk where it forms what is sometimes designated the skin layer of the yolk sac. The relation of the outer layers of the allantois to the chorion has been described on page 106, and is illustrated in Fig. 99. FETAL MEMBRANES IN MAMMALS. The development of the foetal membranes in Mammals presents no such uniformity as is found in Birds and Reptiles where it was possible to describe their formation in the chick as typical for the two classes. In the different Mammals much variation occurs, not only in the first appearance of the mem- branes but also in their further development and ultimate structure. In some forms (rabbit, for example) the amnion develops in a manner very 108 TEXT-BOOK OF EMBRYOLOGY. similar to that in the chick; that is, by a dorsal folding of the somatopleure. There is, however, no head fold unless a temporary structure known as the proamnion be considered as such. The entire rabbit amnion is formed by an extension over the embryo of the tail amniotic fold. In other forms (bat and probably man) the amnion and amniotic cavity arise im sifu over the embryonic disk, without any folding of the somatopleure. Yolk is almost entirely lacking in most Mammals, but the yolk sac is always present although it soon becomes a rudimentary structure. The fact that the yolk sac is always present points toward the conclusion that Mammals are descended from animals which possessed large ova with abundant yolk. Asa matter of fact the lowest Mammals, the Monotremes, possess large ova with large quantities of yolk. These are deposited by the female, are developed in a parchment-like shell, and are carried about in the brood-pouch. The allantoic sac in many Mammals is a very rudimentary structure which, as in the chick, always arises as an evagination from the caudal end of the gut. The allantoic blood vessels, however, become vastly important since they here not only carry off waste products from the embryo, as in Reptiles and in Birds, but also assume the function of conveying nutriment from the mother to the embryo. In assuming this new function they are no longer concerned with the allantoic sac proper but enter into a new relation with the chorion. The chorion is the most highly modified and specialized of all the mam- malian foetal membranes. In some cases (the rabbit, for example) it arises in connection with the amnion, as in the chick, by a dorsal folding of the somato- pleure. In other cases (bat and probably man) it arises ata very early stage, partly as a differentiation of the superficial layer of the morula, partly as extraembryonic parietal mesoderm which develops later. In all cases where the embryo is retained in the uterus (except Marsupials) it forms a most highly specialized and complex structure which, in connection with the allantoic vessels, establishes the communication between the mother and the embryo. For the sake of clearness it seems best to describe first the earlier stages of the foetal membranes in some case where the development resembles that of the chick; then later to consider the more specialized types of development, the ultimate structure of the membranes, especially the chorion, and their relation to the embryo and the mother. Amnion, Chorion, Yolk Sac, Allantois, Umbilical Cord.—Referring back to the mammalian blastoderm when it consists of the three germ layers, it will be remembered that the embryonic disk forms the roof, so to speak, of a large cavity—the yolk cavity or cavity of the blastodermic vesicle (Fig. 82); that the ectoderm of the disk is continuous with a layer of cells which extends around the vesicle—the extraembryonic ectoderm; that the entoderm of. the disk is continuous with the entoderm lining the cavity of the vesicle; that the FETAL MEMBRANES. 109 mesoderm extends peripherally beyond the disk between the ectoderm and entoderm (Fig. 89). It will be remembered also that the mesoderm later splits. into two layers—the parietal and visceral, of which the parietal plus the ecto- derm forms the somatopleure and the visceral plus the entoderm forms the Embryonic disk—_ An qiotie fold Coelon.-. See 2 See, a - oct NS caeeetetecs Lae os Sy 5 en Umbilecat cora Fic. 100.—Diagrams representing six stages in the development of the foetal membranes inamammal. Modified from Kélliker. The ectoderm is indicated by solid black lines; the entoderm by broken lines; the mesoderm by dotted lines and areas. splanchnopleure; and that the cleft between the two layers is the body cavity or ccelom. In further development, along with the differentiation of the embryonic body, the somatopleure begins to fold dorsally at a short distance from the 110 TEXT-BOOK OF EMBRYOLOGY. body (Fig. 100, 2). The folds—ammiotic folds—appear cranially, laterally and. caudally. These folds continue to grow dorsally (Fig. 100, 3) and finally meet and fuse above the embryo (Fig. 100, 4). They then break through along the line of fusion so that the extraembryonic body cavity which has been carried up dorsally over the embryo in the amniotic folds becomes continuous across the mid-dorsal line. A double membrane or rather two membranes are thus formed which extend over the embryo. The outer membrane is the chorion and is composed from without inward of ectoderm and parietal mesoderm. The inner membrane is the amnion and is composed from without inward of parietal mesoderm and ectoderm (Fig. 100, 5). Between the amnion and the chorion is a portion of the extraembryonic body cavity, which, as already mentioned, was carried dorsally with the amniotic folds (Fig. roo, 2, 3, 4 and 5). Sclerotome Myotome Neural Ectoderm Piles Parieta1 PPper, mesoderm limb bud Neural tube Ceelom Visceral mesoderm Entoderm Primitive Pronephric aorte tubule Fic. ro1.—Transverse section of a dog embryo with 19g primitive segments. Bonnet. Section taken through sixth segment. In the manner just described the amnion becomes a sac which at first en- tloses the embryo laterally, and then laterally and dorsally (Fig. ror). Later as the embryo becomes constricted off from the underlying cavity, the amnion encloses it entirely except over a small area on the ventral side where the embryo is attached to the yolk sac (Fig. roo, 3, 4 and 5). While the amnion is being formed, the mesoderm continues to extend around the vesicle between the ectoderm and the entoderm. At the same time it splits into parietal and visceral layers, of which the parietal is applied to the ectoderm, and the visceral to the entoderm. In this way the extraembryonic body cavity gradually extends farther and farther around the vesicle until finally the somatopleure is completely separated from the splanchnopleure (Fig. too, 3, 4 and 5). The extraembryonic somatopleure now forms a com- plete wall for the vesicle and constitutes the chorion. The extraembryonic splanchnopleure forms a complete wall for the yolk cavity and constitutes the wall of the yolk sac. The proximal portion of the yolk sac becomes constricted FETAL MEMBRANES. 111 to form the yolk stalk which connects the yolk sac with the ventral side of the embryonic body (Fig. 100, 5). While the processes just described have been taking place, an evagination ap- pears pushing out from the ventral side of the caudal end of the gut (Fig. 100, 4). This evagination grows out into the extraembryonic body cavity (exo- coelom), pushing before it the visceral layer of mesoderm, thus giving rise to a thin-walled sac which communicates with the gut—the allantois (Fig. 100, 5). At this stage the embryonic body, with its surrounding amnion and appended yolk sac and allantois, lies within the large vesicle formed by the chorion. Up to this point the development resembles that in the chick. In succeeding stages a new connection is established between the embryo and the chorion in the following manner: The amnion enlarges and fills relatively more of the cavity within the chorion, while the yolk sac becomes smaller and the yolk stalk much attenuated (Fig. roo, 6). At the same time the allantois also becomes attenuated and its distal end comes in contact with the chorion (Fig. 100, 6). The growth of the amnion results in the pushing together of the attenuated yolk stalk and allantois so that they lie parallel to each other (Fig. 100, 6), and are together invested by a portion of the amnion. As already described, both yolk stalk and allantois are composed of entoderm and mesoderm while the amnion is composed of mesoderm and ectoderm. Con- sequently when the three structures come together and fuse, there is formed a mass of mesoderm which contains the entoderm of the yolk stalk or vitelline duct and the entoderm of the allantois or allantoic duct, and which is sur- rounded by the ectoderm of the amnion. The fusion of these three structures in this region thus produces a slender cord of tissue which forms the union between the embryo and the chorion and which is known as the umbilical cord (Fig. 100, 6). In Mammals the yolk sac contains little or no yolk and consequently can furnish but little nutriment for the embryo; but the union of the allantois with the chorion, mentioned in the preceding paragraph, allows the allantoic blood vessels to come into connection with the chorion. And since in Mammals the chorion is the means of establishing the communication between the embryo and the mother, the allantoic (umbilical) vessels assume the function of carrying nutrient materials to the embryo and also of carrying away from the embryo its waste products. (See p. 242.) Further Development of the Chorion. Up through the stages which have been described the correspondence in the ‘development of the foetal membranes in Reptiles, Birds and Mammals is clear. From now on, the course of development in Mammals becomes more and more divergent. The extensive development of the yolk and yolk sac with its 8 112 TEXT-BOOK OF EMBRYOLOGY. vascular system in the egg-laying Amniotes has been noted. This is dependent upon the fact that the embryo very early in its existence loses its nutritional con- nection with its mother and is therefore dependent for its food upon the yolk stored up within the egg. This condition obtains up through the lowest order of Mammals, the Monotremes, which are egg-laying animals. The Marsupials give birth to young of very immature development. In these two orders of Mammals the foetal membranes present essentially the same condition as in Birds and Reptiles. The chorion in Marsupials, however, lies in close ap- position to the vascular uterine mucosa and perhaps provides for the passage of Chorion Uterine glands Blood vessels Muscularis Fic. 102.—Vertical section through wall of uterus and chorion of a pig. Photograph. Note especially the close apposition of the chorionic and uterine epithelium (and compare with Fig. 103); note also the enlarged blood vessels in the uterine mucosa. nutrition from the mother to the embryo. In all higher Mammals, however, no eggs are laid and the embryo early acquires an intimate nutritional relation to its mother. This relation is maintained until the embryo has reached a com- paratively advanced stage of development. As would be expected therefore, there take place, coincidently with the change in nutritional relation between mother and embryo, and dependent upon this changed relation, the already noted decrease in, or entire loss of, yolk and at the same time the development of a special organ of relation between embryo and uterus. This organ is devel- oped mainly from the chorion which becomes highly specialized as compared with the very simple chorion described in the chick. FETAL MEMBRANES, 113 In some Mammals (e.g., pig, horse, hippopotamus, camel) there develops a more intimate relation between the chorion and the uterine mucosa. In the pig, for example, the chorionic vesicle becomes somewhat spindle-shaped, and, except at its tapering ends, its surface is closely applied to the surface of the uterine mucosa. On that portion of the chorion which is in contact with the uterine mucosa small elevations or projections develop and fit into correspond- ing depressions in the mucosa. These projections involve the epithelial layer (ectoderm) of the chorion and the adjacent connective tissue (mesoderm) (Fig. 102). Furthermore, the chorionic epithelial cells and the uterine epithelial Blood vessel in chorion Chorionic epithelium Uterine epithelium Blood vessel in uterine mucosa Fic. 103.—From section through wall of uterus and chorion of a pig, showing close relationship between the epithelium of the uterus and that of the chorion. Photograph. cells acquire very intimate relations in that the ends of the former become rounded and fit into depressions in the ends of the latter (Fig. 103). The allantois and allantoic vessels in the pig afford a good example of the transition from the respiratory and excretory functions which they almost .ex- clusively possess in Reptiles and Birds, to the additional nutritional function of these vessels in Mammals. The allantoic sac becomes large and applies itself to the inner surface of the chorion, so that the blood vessels of the allantois also grow into and ramify in the mesodermal layer of the chorion. ‘This brings the allantoic (umbilical) blood vessels containing the foetal blood closer to the uterine vessels containing the maternal blood. The two sets of vessels never come in contact, however, being always separated by the chorionic and uterine epithe- 114 TEXT-BOOK OF EMBRYOLOGY. lium and also by some connective tissue of the chorion and of the uterine mucosa (Fig. 103). Food materials for the embryo must, therefore, pass through the connective tissue and the two epithelial layers in order to get from the maternal to the foetal blood; and waste products from the embryo must also pass through the same tissues to get from the foetal to the maternal blood. When the foetal membranes of the pig are expelled at birth, the rudimentary chorionic villi simply withdraw from their sockets in the uterine mucosa and the chorion , is cast off, leaving the uterine mucosa intact. In other Mammals, the attachment of the chorion to the mucous membrane of the uterus is restricted to certain definite, highly specialized areas. This _means that the villi which at first developed over the entire chorion, disappear from the greater part of it. Those villi which remain are limited to a definite area or areas and develop extensive arborizations. Moreover, they do not — Amnion mY P, y Pa oN Belly stalk Fic. 120.—Dorso-lateral view of human embryo with fourteen pairs of primitive segments (2.5 mm.). Kollmann, yolk sac. As the body and yolk sac enlarge, the constriction becomes relatively deeper until the yolk sac is attached to the ventral side of the body by a slender cord—the yolk stalk (Fig. 123). While in the earlier stages there is an active bending of the margins of the disk, in the later stages the body grows rapidly in size, especially in length, and extends out beyond the yolk sac (Fig. 120). This makes it appear that the yolk stalk is becoming smaller. As a matter of fact, the diminution in the relative size of the yolk stalk is more apparent than real, the apparent diminution being caused largely by the rapid increase in size of the embryonic body and yolk sac. There is, however, a considerable distance where fusion occurs in the midventral line as the two lateral body walls meet to form 142 TEXT-BOOK OF EMBRYOLOGY. the ventral body wall. This line of fusion is significant in its relation to certain malformations (Chap. XIX). Preceding the processes which establish the cylindrical form of the body, there are changes in the relation of the amnion to the chorion. Primarily, the entire dome-like roof of the amniotic cavity is attached to the chorion (Fig. go, A). In further development, however, the extraembryonic mesoderm between the trophoderm of the chorion and the ectoderm of the amnion splits farther back over the embryo, leaving the latter attached at its caudal end to the chorion by a mass of mesoderm—the so-called belly stalk (Figs. 90, B, and 85). Following the above mentioned changes in the amnion, chorion, yolk sac and embryonic disk, the amnion continues to enlarge and thus draws the belly Cephalic flexure Branchial arches Branchial grooves Heart Yolk sac Dorsal flexure Amnion ~— * Belly stalk Chorion Y 2 @ Fic. 121.—Human embryo 2.15 mm, long. His. stalk under the embryonic body and brings it closer to the yolk sac. Finally, as the yolk stalk becomes longer and more slender, the belly stalk and yolk stalk unite and become completely surrounded by the amnion. There is thus formed a cord-like structure—the umbilical cord—which is attached to the ventral side of the body (Figs. go, D, and 100; see also p. 132). The changes which occur in the simple cylindrical body, after it is once formed, consist of the differentiation of the head, neck and body regions and the development of the extremities. Even in Eternod’s embryo (Fig. 119) the cephalic end has become proportionately larger than the rest of the body and projects somewhat beyond the yolk sac. This marks the beginning of the DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY,- 143 head. ‘The extreme end of the head region is bent ventrally almost at a right angle to the long axis of the body, the bend being known as the cephalic flexure. On the ventral side of the body and cranial to the attachment of the yolk sac there is a rather large protrusion which indicates the position of the heart: Between the protrusion and the bent part of the head there is a deep depres- sion—the oral fossa. A series of bilaterally symmetrical structures appear in the body region along the sides of the neural tube. These are the primitive segments (mesodermic somites). All these features are even more clearly shown in Fig. 120, which represents Cephalic flexure Naso-frontal process Maxillary process Oral fossa a Mandibular process Branchial groove [ Branchial arch II Ventral aortic trunk Primitive segments Branchial groove I Mandibular process Maxillary process Eye Naso-optic furrow Nasal pit * Heart Lower Liver Upper Umbilical Yolk stalk limb bud limb bud cord Fic. 124.—Human embryo with 28 pairs of primitive segments (7.5 mm.). Photograph. ventral side of the body, originally caused by the heart, is now more prominent owing to the fact that the rapidly growing liver also protrudes ventrally. In this particular case the yolk sac seems unusually large. The yolk stalk has become enclosed for about half its length within the umbilical cord. After the stage just described the dorsal flexure becomes still less prominent, the body of the embryo being less curved (Fig. 125). The cervical flexure remains distinct, so that the head is bent at a right angle to the long axis of the body. ‘Two slight depressions have appeared on the dorsum of the embryo— the occipital depression just cranial to the cervical flexure, the cervical depression DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 147 just caudal to the cervical flexure. The cervical depression becomes more con- spicuous in later stages and finally persists as the depression at the back of the neck in the adult. The maxillary process is more prominent than in the preceding stages, as is also the naso-optic furrow. The second arch has become larger and has grown over the third and fourth, thus completely hiding them, but a depression known as the precervical sinus is left just caudal to the second arch. The first branch- ial groove is relatively large and marks the site of the external auditory meatus, while the surrounding portions of the first and second arches in part are destined to give rise to the external ear. Cervical flexure Occipital depression “ya Cervical depression Cephalic flexure Dorsal flexure Sacral flexure Fic. 125,—Human embryo 11 mm. long (31-34 days). His The distal portion of the upper limb bud has become flattened, and four radial depressions mark the boundaries between the digits. The lower limb bud is now divided by means of a constriction into a proximal and a distal portion. In development the upper limb is always slightly in advance of the lower. The rotundity of the abdomen, due to the rapidly growing heart and liver, is more pronounced than in the preceding stages. Fig. 126 shows a stage in which the crescentic form of the body, as seen in profile, is not so apparent. This is due principally to the partial straightening of the cervical flexure and to the greater rotundity of the abdomen. ‘The 148 TEXT-BOOK OF EMBRYOLOGY. cervical depression is deeper, and the neck region in general is fairly well differentiated. The ventral part of the first branchial arch has fused with the ventral part of the second, leaving the dorsal part of the first groove open to form the ex- ternal auditory meatus. The parts surrounding the meatus bear more resem- blance to the concha of the ear. The mandibular process of the first arch has become differentiated in part into the lower lip and chin regions. ‘The ventral (distal) end of the maxillary process represents the region of the upper lip. The Fic. 126. Fic, 127. Fic, 126.—Human embryo of 15.5 mm. (39-40 days). His. Fic. 127.—Human embryo of 16 mm (42-45 days). His. nose is apparent as a short process extending from the fore-brain region toward the upper dip. The limb buds are turned more nearly at right angles to the long axis of the body. The radial depressions which were present on the flattened distal por- tion of the upper limb in the preceding stage are now continuous with depres- sions around the distal border. Similar radial depressions are also present on the distal portion of the lower limb. The tail is smaller in proportion to the rest of the embryo. DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 149 After the stage shown in Fig. 126 the cervical flexure continues to dimin- ish, so that the head comes to lie nearly in a direct line with the body (Fig. 127). The rotundity of the abdomen diminishes owing to the fact that the heart and liver grow more’slowly relatively to the body as a whole. The tail, which was still a prominent feature in Fig. 125, continues to become less prominent in the succeeding stages (Figs. 127, 128, 129, 130). This is not due so much to an actua] atrophy of the tail as to an increase in the size of the buttocks. In the adult the only remnant of the tail is the coccyx. Fic. 128. Fic, 128.—Human embryo of 17.5 mm. (47-51 days). His. Fic. 129.—Human embryo of 18.5 mm. (52-54 days). His. Fic. 130,—Human embryo of 23 mm. (2 months). His. During the second month of development the external genitalia become very prominent and the sexes can be easily differentiated. By the end of the second month the embryo has acquired a form which resembles in a general way the form of the adult (Fig. 130). Frorn this time on it is customary to speak of the growing organism as a fetus. Branchial Arches—Face—Neck. At a very early stage (embryos of 2-4 mm.) certain peculiar structures appear in that part of the embryo which is destined to become the face and neck regions. They are at first noticeable as slit-like depressions nearly at right angles to the long axis of the body. In an embryo 2.15 mm. long two of these depressions are visible (Fig. 121). Shortly after this a third and then a fourth 150 TEXT-BOOK OF EMBRYOLOGY. appears. At the same time elevations appear between the succeeding depres- sions, the first elevation appearing cranial to the first depression. (Compare Figs. 122, 123.) The elevations are the branchial arches and the depressions are the branchial grooves. Corresponding elevations and depressions also mark the Fic. 131. Fic, 132. Fic, 131.—Human embryo of 78 mm. (3 months). Minot, Fic, 132.—Human embryo of 155 mm, (123 days). Minot. interior of the pharynx, so that the portions. of the wall of the pharynx which correspond to the grooves are thin as compared with those portions which cor- respond to the arches. The arches develop in order from the first to the fourth; consequently they are successively smaller from-the first to the fourth (Fig. 122). The conditions DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 151 change rapidly, so that in embryos of 9-10 mm., the third and fourth arches have sunk inward, thus producing a depression known as the precervical sinus. Soon after this the second arch enlarges, grows over the sinus, and, fusing with the underlying arches, fills up the depression. The ventral end of the first arch fuses with the ventral part of the second across the ventral part of the first groove. The dorsal part of the first groove is thus left open and becomes the external auditory meatus. A part of the second arch, together with a part of the first arch bounding the first groove on the cranial side, is transformed into the concha of the ear (Figs. 123, 125, 126). The first branchial arch becomes the largest and undergoes profound changes which are extremely important in the de- velopment of the face region. Earlier in this chapter (p. 143) it was stated that the cephalic flexure caused the fore-brain to project ventrally at a right angle to the long axis of the body, and that between the pro- jecting fore-brain and the heart a distinct depression or pit—the oral fossa—was pres- ent. Soon after the appearance of the first arch a strong process—the maxillary process —develops on its cranial side (Fig. 122). The main portion of the arch, which may be now called the mandibular process, rapidly increases in size, extends ventrally and finally meets and fuses with its fellow of the opposite side in the midventral line (Fig. 134). The result of the enlargement of the first arch and its process is that they 7 ai boa a are interposed between the heart and the as al Ga, alae sc ari fore-brain vesicle, thus bounding the oral fossa laterally (Fig. 122). During this time the heart is gradually moving caudally. Meanwhile a process—the naso-frontal process—grows ventrally from the medial portion of the fore-brain region and comes in contact laterally with the maxillary process. Along the line of contact a furrow is left, which extends obliquely to the region of the optic vesicle and is known as the naso- optic furrow (Fig. 134). ~The various structures which have been mentioned bound the oral fossa which has become a deep quadrilateral pit. Cranially (above) the fossa is bounded by the broad, rounded, unpaired naso-frontal process; caudally (below) it is bounded by the mandibular processes; laterally it is bounded by the maxil- 152 TEXT-BOOK OF EMBRYOLOGY. lary processes, and to a slight extent by the mandibular processes. Between the maxillary and mandibular processes on each side a notch marks the angle of the mouth. As development proceeds these structures become more elaborate and enter into more intimate relations with one another. The naso-frontal process extends farther downward toward the mandibular processes, so that the oral fossa becomes more nearly enclosed and the entrance to it reduced to a crescent-shaped slit—the mouth slit. At the same time two secondary processes develop on each side from the naso-frontal process. One of these—the medial nasal process—forms near the medial line; the other—the /ateral nasal process—forms more laterally (Figs. 135,136). Between the two processes there Cerebral hemisphere Lat. nasal process Nasal pit Eye Med. nasal process Naso-optic furrow Angle of mouth Maxillary process Mandibular 4 ad Fic. 134.—Ventral view of head of 8 mm. human embryo. His. is a depression—the nasal pit—which marks the entrance to the future nasal cavity. The maxillary process on each side grows farther toward the medial line and comes in contact with the lateral and medial nasal processes. At this stage all the elements which enter into the fundamental structure of the face region are present. Further development consists essentially of fusions between these various elements. The two medial nasal processes come closer together to form the single medial process which gives rise to the medial portion of the upper lip and to the adjoining portion of the nasal septum. The maxillary process on each side fuses with the corresponding lateral and medial nasal processes. This fusion obliterates the naso-optic furrow and also shuts off the communi- cation between the mouth slit and the nasal pit (Figs. 136, 137). The lateral nasal process gives rise to the wing of the nose; the maxillary process gives rise DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 153 to the major part of the cheek and the lateral portion of the upper lip. The fusion between the maxillary and nasal processes, as seen on surface view, is coincident with and a part of the separation of the nasal cavity from the oral cavity (see page 320). The nose itself is at first a broad, flat structure, but later becomes elevated above the surface of the face, with an elongation and a narrowing of the bridge. Mid-brain — Cerebral hemisphere f Lat. nasal process Eye Nasal pit 3 Naso-optic furrow Med. nasal process rt . Maxillary process Angle of mouth ' Mandibular process Branchial grooves Branchial arch II Fic. 135.—Ventral view of head of 113 mm. humanembryo. Radl. The lower jaw, lower lip and chin are formed by the mandibular processes of the first branchial arch (Figs. 134, 136, 137). At first the chin region is rela- tively short, but broad in a transverse direction. Later it becomes longer and a transverse furrow divides the middle portion into lower lip and chin (Fig. 137). The Extremities. The limb buds appear in human embryos about the end of the third week as small, rounded protuberances on the ventro-lateral surface of the body. The upper limb buds arise just caudal to the level of the cervical flexure, the lower opposite the sacral flexure (Figs. 123, 124). The upper appear first, the lower following shortly, and the difference in time in the appearance of the upper and lower buds is followed by a difference in degree of development, the upper extremities maintaining throughout fcetal life a slight advance in develop- ment over the lower. 154 TEXT-BOOK OF EMBRYOLOGY. During the fourth week the limb buds become elongated, and each bud becomes divided by a transverse constriction into a proximal and a distal por- tion (Figs. 124, 125). The proximal portion remains cylindrical, while the ) Naso-frontal process a) Naso-optic furrow Mouth slit =——_———i nn Branchial groove I \ Fic. 136.—Ventral view of head of 13.7 mm. human embryo. His. distal portion becomes somewhat broader and considerably flattened. Dur- ing the fifth week the digits appear (see below). During the sixth week the proximal portion of each bud is subdivided by a transverse constriction into two segments (Fig. 127). Thus each extremity as a whole is divided into three (external ear) E ot al Fic. 137.—Ventral view of head of human embryo of 8 weeks. His. segments—each upper, into arm, forearm and hand, each lower, into thigh, leg and foot. The anlagen of the digits (fingers and toes) appear, during the fifth week, in the broader, flattened distal portions of the limb buds. The boundaries be- DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 155 tween the anlagen are marked by radial depressions on the flat surfaces; the anlagen themselves are the elevations between the depressions (Figs. 125, 126). The anlagen grow rapidly in thickness and length, thus producing not only an apparent deepening of the radial depressions but also indentations around the distal free borders of the limb buds (Fig. 126). The depressed areas produce a web-like structure between the digits, resembling the web in some aquatic animals. The web does not keep pace with the digits, however, and is soon confined to the proximal ends of the latter. In length the fingers grow slightly more rapidly than the toes and thus become somewhat longer. From the seventh week on, the thumb and great toe become more and more widely sepa- rated from the index finger and the second toe respectively (Figs. 128, 130, 131). As the limb buds become elongated during the earlier stages of development, they assume a position with their long axes nearly parallel with the long axis of the body, and are directed caudally (Fig. 125). In later stages they are directed ventrally and their long axes are nearly at right angles to the long axis of the body (Fig. 126). The radial margins of the upper extremities are turned . toward the head, as are the tibial margins of the lower. The palmar surfaces of the hands and the plantar surfaces of the feet are turned inward or toward the body. The elbow is turned slightly outward and toward the tail, the knee slightly outward and toward the head. From these conditions it may be con- cluded that the radial side of the upper extremity is homologous with the tibial side of the lower; that the palmar surface of the hand is homologous with the plantar surface of the foot; and that the elbow is homologous with the knee. In order to acquire the position relative to the body as found in postnatal life, the extremities must undergo further changes. These consist essentially of tortions around their long axes. The right upper extremity turns to the right, the right lower turns to the left. The left upper extremity turns to the left, the left lowef turns to the right. At the same time the extremities rotate through an angle of ninety degrees and again come to lie parallel with the long axis of the body. The result is that the radial sides of the upper extremities are turned outward (away from the sagittal plane of the body) and the tibial sides of the lower are turned inward (toward the sagittal plane of the body). In the upper extremity this is, of course, the supine position in which the radius and ulna are parallel. Age and Length of Embryos. Acr.—Certain general conclusions regarding the age of embryos have been formulated by His (Anatomie menschlicher Embryonen, 1882) and accepted for the most part by embryologists. These as stated by His are as follows: 1. Development begins at the time of impregnation, that is, at the moment when the male sexual element enters the ovum and fertilizes it. 156 TEXT-BOOK OF EMBRYOLOGY. 2. The time the ovum leaves the ovary is determined by the menstrual period, but the rupture of the (Graafian) follicle is not necessarily coincident with the beginning of hemorrhage; it may occur two or three days before or it may occur during hemorrhage. 3. The egg is not capable of being fertilized at any point in its course from the ovary to the uterus, but only in the upper part of the oviduct. 4. The spermatozoa which have entered the female sexual organs must await the ovum in the upper part of the oviduct, and can retain their vitality here for several days or possibly for several weeks; the time of cohabitation, therefore, does not stand in direct relation to the age of the embryo. 5. In the majority of cases the age of the embryo can be estimated from the beginning of the first menstrual period which has lapsed. It is possible, how- ever, for menstruation to occur after fertilization of the ovum. 6. The age of the embryo can be expressed thus: age = X —M, or age = X —M —28. X is the date of the abortion and M is the beginning of the last menstrual period. The second formula is used where it is necessary to estimate from the beginning of the first period which has lapsed. ; There is no doubt whatever that the age of the embryo must be dated from the time of fertilization of the ovum; but owing to the fact that the time of fertilization of the human ovum is not known, the exact age cannot be deter- mined. Even when the date of coitus and the time of cessation of the menses are known, the uncertainty regarding the time of ovulation and the time re- quired by the spermatozoa to reach the upper end of the oviduct must be taken into consideration. It is now generally conceded that ovulation and menstruation are coincident in the majority of cases, but, on the other hand, ovulation is known to occur sometimes independently of the menstrual periods (see also p. 30). In addition to the uncertainty regarding the time when development begins there is also an uncertainty as to the time when the embryo ceases to develop. For in most cases the embryos are abortions and the death of the embryo does not necessarily precede immediately its expulsion from the uterus. It is convenient, however, for practical purposes, to have some means of approximating the age of anembryo. His’ formule serve to determine the age within certain limits. It is obvious from these formule that there is a possibility of an error of twenty-eight days in the estimate. Yet in the earlier stages of development (during the first three months) the error can be corrected after examination of the embryo, since there is no difficulty in recognizing the differ- ence, for example, between an embryo two weeks old and one six weeks old. LencTH.—Many German authors employ two different methods for measuring embryos at different periods. One of these methods they use in measuring embryos between 4 and 14 mm., when the body is much curved. DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 157 The length of the embryo is considered as the length of a straight line drawn from the apex of the cervical flexure to the apex of the sacral flexure (neck- rump length, Nackensteisslange; see Fig.124). During the second month and later, or in embryos of more than 20 mm., the body becomes more nearly straight and the measurement is taken along a straight line from the apex of the cephalic flexure to the apex of the sacral flexure (crown-rump length, Scheitel- steisslange; see Fig. 126). Owing to the changes in curvature of embryos during development, no one system of measurement will give uniform results for all stages. In this country it is the general practice to measure the greatest length of the embryo, in its natural attitude, along a straight line. The measurement does not of course include the extremities. At certain stages this length corresponds with the neck-rump length, at other stages with the crown-rump length, at still other stages with neither. RELATION OF AGE To LENGTH.—Not infrequently the history of an embryo is not obtainable, and in such cases the age must be inferred from what is known concerning the relation of the age to the length of the embryo. The age can be computed approximately by this means, although there is a possibility of error. Embryos of the same age are not necessarily of the same length, since conditions of nutrition, etc., determine not only the size of the embryo but also the degree of its development. In the later stages of development the limit of error is not so important, but in the younger stages the difference of a day or two means much, His estimated the ages of a number of embryos from available data as follows: Embryos of 2-24 weeks measure 2.2-3 mm. (neck-rump length). Embryos of 24-3 weeks measure 3-4.5 mm. (neck-rump length). Embryos of 3} weeks measure 5-6 mm. (neck-rump length). Embryos of 4 weeks measure 7-8 mm. (neck-rump length). Embryos of 44 weeks measure 10-11 mm. (neck-rump length). Embryos of 5 weeks measure 13 mm. (neck-rump length). More recent researches on the rate of development in the lower Mammals tend to show that development proceeds relatively slowly during the earliest stages, and then goes on with increasing rapidity for a time. In the rabbit, for example, it has been shown that the embryonic disk is but slightly differentiated at the seventh and eighth days, while at the tenth day the embryo possesses branchial grooves and primitive segments. If this peculiarity in the rate of development occurs in the human embryo, the ages assigned to the earlier embryos by His must be increased. Mall’s formule for estimating age, deduced from observations on a large number of embryos, are as follows: In embryos of 1-100 mm. the age in days 158 TEXT-BOOK OF EMBRYOLOGY. can be expressed fairly accurately by the square root of the length multiplied by 100 (y/length inmm.x 100). In embryos between roo and 220 mm. the age in days is about the same as the length in millimeters. Some of the most important embryos which have been described are listed in the accompanying table, no pretense being made of giving a complete list. The table is compiled largely from the more extensive tables of Mall and merely serves to indicate some of the younger embryos with fairly well- known histories, from which certain conclusions have been drawn concerning the relation of age to length. The periodicals in which descriptions may be found are given with the authors’ names in ‘“‘ References for Further Study” at the end of this chapter. 1 | Number | of days be- Number of days Lensth of N. 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AS dramas AG tae ea 5 ee Lyhaveeds | Ok 4 ga uth 2t | Mall.........) QOS 5 aatuindaphes. ea GBs ima Dew aperidiy AS x Baars: ee 22 TiSieiiw ine emis DIK Wak wes need oe Gite gaggia 20 (CR) tates va OF irene But | Fs FS cline noe 23 | Meyer AS sex 28nes ve gece! sea ee ye hanes a2 23 eves Hy Dic aeeasneauedye Ce 5 eee ee OOns 2 22y as Bi scinies ne oe BB anes Pomoc 25 | His.......... BOR 27 susgeseeausas Olmos aca BBN as asain B3e now een Woy artiaiees 20) Hisscccuacany Bk Costu we sey tee (oh Senn eee BI eas aac es BIS bee gen 13:03 22254 Normal, Abnormal and Pathological Embryos. In the majority of cases of spontaneous abortion it is not possible to examine the uterus; but in those cases where it is possible, examination frequently shows abnormal or pathological conditions. As might be expected, the embryos obtained from abnormal or pathological uteri very frequently show anom- alous conditions or pathological changes, or both. Since many of the DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 159 human embryos obtained are the results of spontaneous abortions, there is reason to suspect that such embryos are not normal. To the physician, as well as to the embryologist, it is important, therefore, that there should be some criteria for differentiation between normal and abnormal or pathological embryos. Gross anomalies, or monstrosities, such as cases in which the head or some other member of the body is lacking, or in which the head is disproportionately large or disproportionately small, or in which two embryos are directly united, or in which the foetal membranes are partially lacking, or in which the mem- branes are present and the embryo wholly or partially lacking, and many other anomalous conditions, can, of course, be recognized at once. Extensive pathological changes or processes of disintegration in the tissues of the em- bryo or foetal membranes are also easily recognized. But there are many less obvious anomalies and pathological conditions which, nevertheless, are im- portant. Such cases are most difficult to differentiate. The foetal membranes not infrequently are useful in determining whether an embryo has followed the normal course of development. During the first month the amnion invests the embryo rather closely when development is normal. If the amniotic sac is disproportionately large, however, it is a mark of abnormal or pathological changes. In some cases an amniotic sac 50 to 60 mm. in diameter contains an embryo but a few millimeters in length. In the earlier stages of development, before the amnion enlarges sufficiently to reach the chorion, there is present a delicate network of fibrils, the magma reticulare, which is attached to both chorion and amnion and which serves as a sort of anchor for the amnion. In abnormal or pathological cases the magma reticu- lare may become wholly or partially fluid or granular, or may become greatly increased in amount. It may even extend through the amnion and reach the embryo itself. Normal human as well as other mammalian embryos in the fresh condition are more or less transparent, and such structures as the heart, the larger blood vessels, the liver, and the brain vesicles can be seen through the skin. If the embryo has been dead for some time or has undergone pathological or degen- erative changes, the transparency is lost. Where pathological or degenerative changes in the embryo or its membranes are suspected but cannot be definitely determined by macroscopic examination, recourse may be had to sectioning and staining. PRACTICAL SUGGESTIONS. Since the earlier stages of human embryos in any condition are not readily procured, all embryos which come into the hands of physicians or others should be preserved and turned over to someone who can use them in the study of development. Abnormal or pathological It 160 TEXT-BOOK OF EMBRYOLOGY. embryos are often extremely valuable, for many anomalous conditions in postnatal life can be explained on the ground of unnatural developmental conditions. Obstetricians and gynecologists can render great service to embryology by saving curettings or the entire uterus in cases where pregnancy is suspected or known to occur and turning them over to a com- petent embryologist. The earliest stages are especially valuable. Never should a human embryo, normal or abnormal, under any circumstances be thrown away. When the uterus is removed where there is suspected pregnancy, it should always be saved. Within as short a time as possible, carefully open the uterus by a ventral median incision. If pregnancy has gone beyond the first month, the membranes and embryo are easily seen. During the earliest stages of pregnancy, especially during the first part of the first month it is sometimes very difficult to locate the embryo in the uterus. The most likely position is on the dorsal wall. It may show only as a scarcely visible elevation in the mucous mem- brane. If the little elevation is once recognized, cut out the block of uterine wall containing it. Fix the block in some good fluid, such as Orth’s fluid and embed carefully in paraffin. Cut serial sections at right angles to the inner surface of the uterine mucosa. The sections may be stained as desired, Weigert’s hematoxylin and eosin giving good results (see Appendix). The most valuable human embryos in the earliest stages have all been obtained in a similar manner. Embryos three to eight weeks old may be fixed with the membranes intact. Put the specimen in a large quantity of strong alcohol (the alcohol of druggists is never too strong).- The volume of the alcohol should be at least ten times the volume of the specimen. A 4 per cent. solution of formalin (one volume of the commercial formalin—which is a 4o per cent. solution of formaldehyde gas in water—to nine volumes of water) may be used if alcohol cannot be obtained at once. The specimen should not be left in formalin, however, longer than a few days, for it is likely to become somewhat blackened owing to changes in the blood. As soon as possible, it should be put in Orth’s fluid for a day or two, and then put through graded alcohols up to 80 per cent. Kleinenberg’s mixture is also an excellent fixative for young embryos. In embryos eight to twelve weeks old the membranes should be opened before fixing. Strong alcohol may be used as a fixative, as in the earlier stages, but usually causes considerable shrinkage. A better plan is to put the embryo in Orth’s fluid for a few days, the length of time depending upon the size of the embryo, and then to put it through the graded alcohols up to 80 per cent. As mentioned in the preceding paragraph, 4 per cent. formalin may be used as a fixative, but should be followed in a few days by Orth’s fluid and the graded alcohols. Zenker’s fluid is frequently used as a fixative for embryos but always causes some shrinkage. Aside from the shrinkage, it gives very good results. (See Appendix.) If embryos of twelve weeks or more are to be studied histologically, they should be opened by a ventral medial incision before fixing. It is well also to make a few incisions in the skull. If it is desired, organs or parts of organs may be removed and fixed by them- selves. Orth’s or Zenker’s fluid may be used with good results. For gross preparations, embryos may be fixed as suggested in the ,preceding paragraphs. Then, if occasion requires, they can be studied histologically at a later period. Gross preparations which are not likely to be used histologically can be fixed and preserved indefi- nitely in 4-10 per cent. formalin, with practically no shrinkage, although there is a possi- bility of discoloration due to changes in the blood. As complete histories as possible of all embryos should be obtained and recorded. The younger stages should always be carefully measured before fixing. It is also advisable to DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 161 have the embryos photographed. In short, all possible data concerning an embryo should be obtained, not only for use in studying it as an individual but also for use in comparing it with other embryos. Models of embryos can be made from serial sections by means of the wax reconstruction method. (See Appendix.) For class study on the external form of the body, very useful preparations can be made by mounting whole small pig embryos or parts of embryos in glycerin jelly in small flat dishes. The dishes can be sealed and handled freely. References for Further Study. vaN BENEDEN, E,: Recherches sur les premiers stades du développement du Murin (Ves- pertilio murinus). Amat. Anz., Bd. XVI, 1899. Bryce, T. H., and TEacuer, J. H.: An Early Human Ovum Imbedded in the Decidua. Mac Lehose & Sons, Glasgow, 1908. Ecker, A.: Beitrége zur Kenntniss der dusseren Formen jiingster menschlichen Embryo- nen. Archiv. f. Anat. u. Physiol., Anat. Abth., 1880. ETERNOD, A. C. F.: Communication sur un oeuf avec embryon excessivement jeune. Arch, ttal. de Biol. Suppl. 12 et 14, 1894. ' Evernop, A. C. F.: Sur un oeuf humain de 16.3 mm. avec embryon de 2.1 mm. Arch. des. sct. phys. et nat., Vol. II, 1896. ; His, W.: Anatomie menschlicher Embryonen. With Atlas. 1880-1885. His, W.: Die Entwickelung der menschlichen und tierischen Physiognomien. Arch. }. Anat. u. Physiol., Anat. Abth., 1892. Jan6sik, J.: Zwei junge menschliche Embryonen. Arch. f. mik. Anat., Bd. XXX, 1887. KerBeL, F.: Ein sehr junges Menschliches Ei. Arch. f. Anat. u. Physiol., Anat. Abth., 1890. Kerpet, F.: Entwickelung der fusseren Korperform der Wirbeltierembryonen. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere. Bd. I, Teil I, 1906. Keiser, F., and Erze, C.: Normentafel zur Entwickelungsgeschichte des Menschen. Jena, 1008. Kerset, F., and Mat, F. P.: Manual of Human Embryology. Vol. I, 1910. Ko iMAnn, J.: Die Kérperform menschlicher normaler und pathologischer Embryonen. Arch. f. Anat. u. Physiol., Anat. Abth. Suppl., 1889. Kotrmann, J.: Handatlas der Entwickelungsgeschichte des Menschen. Jena, 1907. Lropotp, G.: Ueber ein sehr junges menchliches Ei. Leipzig, 1906. Matt, F. P.: A Human Embryo Twenty-six Days Old. Jour. of Morphol., Vol. V, “1891. en F. P.: A Human Embryo of the Second Week. Anat. Anz., Bd. VIII, 1893. Maxt, F. P.: Human Embryos. Wood’s Reference Handbook of the Medical Sciences, Vol. ITI, rgor. Mever, H.: Die Entwickelung der Urnieren beim Menschen. Arch. }. mik, Anat., Bd. XXXVI, 1890. Perers, H.: Ueber die Einbettung des menschlichen Eies, und das friiheste bisher bekannte menschliche Placentarstadium. Leipzig und Wien, 1899. Rast, C.: Die Entstehung des Gesichtes. I. Heft. Leipzig, 1902. Folio. Retcuert, B.: Beschreibung einer friihzeitigen menschlichen Frucht. Abhandl. preuss. Akad., Berlin, 1873. 162 TEXT-BOOK OF EMBRYOLOGY. SELENKA, E.: Studien itber die Entwickelungsgeschichte der Tiere; (Menschenaffen). Wiesbaden, 1908. Parts 8 to ro. von Sper, GraF: Beobachtungen an einer menschlichen Keimscheibe mit offener Medullarrinne und Canalis neurentericus. Arch. j. Anat. u. Physiol., Anat. Abth., 1889. von SpEE, GraF: Ueber frithe Entwickelungsstufen des menschlichen Eies. Arch. }. Anat. u. Physiol., Anat. Abth., 1896. STUBENRAUCH' Inaug. Dissert. Miinchen, 1889. TxHompson, A.: Contributions to the History of the Structure of the Human Ovum Before the Third Week after Conception, with a Description of Some Early Ova. Edin- burgh Med. and Surg. Journal, Vol. III, 1839. PART I. ORGANOGENESIS.° CHAPTER IX. THE DEVELOPMENT OF THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. -All the connective or supporting tissues of the body, except neuroglia, are derived from the mesoderm. This does not imply, however, that all the mesoderm is transformed into connective tissues; for such structures as the endothelium of the blood vessels and lymphatic vessels, probably blood itself, the epithelium lining the serous cavities, smooth and striated muscle, and a part of the epithelium of the urogenital system are derived from mesoderm. Primitive groove Ectoderm Mesoderm Entoderm es) Fic. 138.—Transverse section of chick embryo of 27 hours’ incubation. Photograph, The origin of the mesoderm itself has been discussed elsewhere (p. 85). In this connection it is sufficient to recall that it is situated between the ectoderm and entoderm and consists of several layers of closely packed cells (Fig. 138). The axial portion in the neck and body regions becomes differentiated into the primitive segments. At the same time a cleft (the coelom) separates the more peripheral portion into a. parietal ‘and a visceral layer (Figs. 139 and 141). In the head region where, in the higher animals, there is little or no indication of 165 166 TEXT-BOOK OF EMBRYOLOGY. Neural Primitive Ectoderm tube segment i i s L q “Oy : oe % 4 f t q Ectoderm Notochord Visceral Coelom Mesoderm Fic. 139.—Transverse section of chick embryo (2 days’ incubation). Photograph. The parietal mesoderm (lying above the ccelom) is not labeled. The two large vessels under the primitive segments are the primitive aorta. Spaces separating germ layers are due to shrinkage, Mesoderm Neural tube (mesenchyme) Ectoderm Pharynx Entoderm Fic. 140.—Transverse section through head region of chick embryo of 42 hours’ incubation. Photograph. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 167 segments and ccelom, the mesoderm simply fills in the space between the ectoderm and entoderm (Fig. 140). Portions of the mesoderm in all these regions are destined to give rise to connective tissues. Each primitive segment soon becomes differentiated into three parts—the sclerotome, cutis plate and myotome (Fig. 142). Of these, only the sclerotome and cutis plate are directly concerned in the formation of connective tissues, the myotomes giving rise to striated voluntary muscle. The sclerotomes are destined to give rise to the Neural ="==- Primitive segment Intermediate 7 cell mass ochord oe) “he Entoderm @s i g sone ed ate ca i \ Ectoderm cell mass ~~~*_ , ~----4 ) Prim. ® gut Visceral ey ( Parietal mesoderm 7 . ee : ie Fi 1k Y Coclom', --& “ 4 i pep es it 3 i Vig) ody wa. sothelium ~ Umbilical vein Fic, 141.—Transverse section of human embryo with 13 primitive segments; section taken through the 6th segment. Kollmann. vertebre and other forms of connective tissue in their neighborhood, the cutis plates to a part, at least, of the corium of the skin. The parietal and visceral layers of the mesoderm (except the mesothelium lining the ccelom) and the mesoderm of the head region are destined to give rise to the various types of connective tissue forming parts of the other organs of the body. HISTOGENESIS. The sclerotomes and cutis plates at first constitute parts of the primitive segments, and are composed of epithelial-like cells with little intercellular sub- stance. The intercellular substance gradually increases in amount so that the TEXT-BOOK OF EMBRYOLOGY. 168 Myotome { Cutis plate Sclerotome Myotome 4 (Muscle plate = | __ Upper j limb bud Pronephros —-- “i fy y 2 Parietal mesoderm=--\\-- {+ \@ YY A Umbilical aie dics Vein Intestine =~ Visceral mesoderm Fic. 142.—Transverse section of human embryo of the 3rd week. Sc/.1, Break in myotome at point where sclerotome is closely attached. Kollmann, Neural tube Intersegmenta! — artery Intersegmental —- ' | artery | Qe Age oe ¢ be ft ee (Pa 6 Bene Fic. 143.—Three primitive segments from sagittal section of human embryo of the 3rd week. Kollmann, THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 169 cells become more widely separated from one another, at the same time assum- ing oval or spindle shapes and then irregular branching forms (Fig. 144). The rest of the mesoderm, except the mesothelium, also undergoes a similar transformation so that structurally its cells are indistinguishable from those derived from the sclerotomes and cutis plates. Thus the tissue from which the connective tissues in general are derived is composed at one stage of irregular branching cells, with a relatively large amount of a homogeneous substance filling the interstices among the cells. The question of the relation of these cells to one another has not been settled ED Fic. 144.—Mesenchymal tissue from somatopleure of a 5 mm. human embryo. Mesothelium is shown along lower border of figure. By some it is held that they are simply individual units, structurally independ- ent of one another. By others it is maintained that the branches of each ’ © cell anastomose with branches of neighboring cells, to form a syncytium, and that the syncytial character is retained in the connective tissue derivatives (Mall). That intercellular substance is derived originally from the cell can scarcely be denied. All the cells of the organism are derived from the fertilized ovum. As soon as two or more cells are formed by segmentation of the ovum, they are either simply in apposition or else they are united by something in the nature of a “cement” substance which must have been derived from the cells themselves. In the connective tissues this intercellular ground substance is a prominent 170 TEXT-BOOK OF EMBRYOLOGY. feature, and while it may increase independently of the cells it must primarily have a cellular origin. Fibrillar Forms.—The first type of connective tissue to be derived from the embryonic non-fibrillar form is areolar tissue. Areolar tissue is composed of comparatively few cells and much intercellular substance, the latter in turn Fic. 145.—Fibril forming cells from fresh subcutaneous tissue of head of chick embryo. Boll, being composed of fibers and “ground substance.” The fibers are of two kinds, “white” or fibrillated and “‘yellow”’ or elastic. To this type of tissue in the embryo the term embryonic connective tissue has been applied. The origin of the fibers is an unsettled question. Some investigators hold that they are derived from the homogeneous “ground substance” by a process Fic. 146.—Connective tissue (mesenchymal) cells from larval salamander. Flemming. of differentiation (Ranvier, Merkel). The view that is best supported by direct observation, however, is that the fibers are derived from the cells (Boll, Spuler, Flemming). The cytoplasm at the periphery of the cells and their processes becomes differentiated into extremely delicate fibrillee which become grouped into bundles (fibers) and then become separated from the THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 171 parent cells and lie free in the “ ground substance” (Figs. 145, 146). This applies to both fibrillated and elastic fibers and the same cell may produce both kinds of fibers, i. e., the same cell that produces fibrillated (““white’’) fibers may also produce elastic (“yellow”) fibers. The fibers, although not derived primarily from the “ground substance,” probably do increase in size by intus- susceptive growth. Thus the “ground substance,” while probably not capable of producing fibers, is an active factor in their further growth. Fic. 147.—Longitudinal section of developing ligament from finger of : human foetus of 6 months. Photograph. In any type of connective tissue where the fibers form the most characteristic feature, such as the looser forms (areolar, reticular) or such as the denser forms (fascia, tendons, ligaments), the structure depends upon the secondary ar- rangement of the fibers and not upon any peculiarity of origin. In areolar tissue, for example, the fibers are derived from the cells, as described above, and become so arranged as to look haphazard. In fascia, tendons and ligaments the fibers arise in the same manner but come to lie parallel, with the cells en- closed among them in more or less distinct rows (Fig. 147). Adipose Tissue.—Adipose tissue is a form of connective tissue in which the fatty element replaces to a great extent the cytoplasm in many of the embryonic connective tissue cells. It always develops in close relation to blood 172 TEXT-BOOK OF EMBRYOLOGY. vessels, and first appears in the axilla and groin about the thirteenth week. It is formed in other places at later periods, even during adult life, but the mode of development is always the same. In some of the cells in the neigh- borhood of small blood vessels minute droplets of fat are deposited. The origin of the fat isnot known. The droplets become larger, other smaller ones appear, and finally all of them coalesce to form a single large drop which practi- cally fills the cell. The result of this is that the remaining cytoplasm is pushed outward and forms a sort of pellicle around the fat. The nucleus also is crowded outward and comes to lie flattened in the pellicle of cytoplasm (Fig. Small artery Fic. 148.—Developing fat from subcutaneous tissue of pig embryo 5 inches long. Small artery breaking up into capillary network: groups of fat cel!s developing in embryonic connective tissue 149). Atthe same time the whole fat cell increases in size and forms a relatively large structure. Fat cells usually develop in groups or masses around blood vessels (Fig. 148). The neighboring groups gradually enlarge and approach each other, but do not fuse, thus leaving more or less fibrous connective tissue between them, which constitutes the interlobular tissue seen in adult adipose tissue. Among the individual cells in a lobule there is also a small amount of fibrous tissue present. From the mode of development a small artery usually affords the blood supply for each lobule. Cartilage.—tIn the different kinds of cartilage the matrix probably repre- THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 173 sents a modification of the “ground substance” of the original embryonic tissue. The fibers in the matrix are probably derived from the cells in the same manner as the fibers in the fibrillar forms of connective tissue (Fig. 150). Osseous Tissue.—Here again the basis for development is embryonic connective tissue, although in one type of development cartilage precedes the bone. Two types of ossification are recognized—intramembranous and intra- cartilaginous or endochondral. In intramembranous ossification calcium salts are deposited in ordinary embryonic connective tissue. In intracartilagi- Embryonic connective tissue Fic. 149.—Developing fat from subcutaneous tissue of pig embryo 5 inches long. Fat (stained black) developing in embryonic connective tissue cells. At the right are five individual cells showing stages of development from an embryonic cell to an adult fat cell. nous ossification hyalin cartilage first develops in the same general shape as the future bone and the calcium salts are afterward deposited within the mass of cartilage. It is customary to speak also of another type of ossification—sub- periosteal—in which the calcium salts are deposited under the periosteum. INTRAMEMBRANOUS OSSIFICATION. This is the type of ossification by which many of the flat bones of the skull and face are formed. ‘The region in which these bones are to develop consists of embryonic connective tissue. At certain points in this region bundles of connective tissue fibers become impregnated with calcium salts. Such areas are known as calcification centers. In each of these areas the cells increase in num- ber, the tissue becomes very vascular and some of the cells, becoming more or less round or oval, with distinct nuclei and a considerable amount of cytoplasm, 174 TEXT-BOOK OF EMBRYOLOGY. Endoplasm with “‘white’’ fibers Ground “ \ . e : es { substance \ ott eames \ tie ” i Sane 4K Ectoplas J fi J ke a dei es f y Elastic fibers Fic. 150.—Connective tissue cells from intervertebral disk of calf embryo; showing origin of “White” and elastic fibers in protoplasm of cells. Ectoplasm represents a modified part of the protoplasm. Hansen. ‘ : Periosteum " Osteoblasts Osteogenetic tissue Trabecule of bone ‘| Periosteum Fic, 151,—Vertical section through frontal bone of human fcetus of 4 months, (Intramembranous ossification.) Photograph. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 175 arrange themselves in single, fairly regular rows along the bundles of calcified fibers. The differentiated cells are known as osteoblasts (bone formers), and the whole tissue is now known as osteogenetic tissue. Under the influence of the osteo- blasts a thin layer of calcium salts is deposited between the osteoblasts and the calcified fibers. In this way the first true bone is formed, and the calcification center becomes an ossification center. Successive layers or lamelle of calcium salts are laid down and some of the osteoblasts become enclosed between the lamellz to form the bone cells (Figs.1§1 and 152). The spaces in which the bone cells lie are the lacune. At the same time the fibers also are enclosed within the bone and give it its characteristic fibrous structure (Fig. 152). Such a process results in the formation of irregular, anastomosing trabecule of bone. The spaces among the trabecule are known as primary marrow Osteogenetis tissue Osteoclast Lacunz Bone Calcified fibers Osteoblasts Fic, 152.—From vertical section through parietal bone of human feetus of 4 months. Bone cells not shown in lacune. (Intramembranous ossification.) Spaces and contain osteogenetic tissue (Fig. 151). This type of bone, consisting of irregular, anastomosing trabeculae and enclosed marrow spaces, is known as spongy bone. ‘The spongy bone thus formed is covered on its outer side by a layer of connective tissue which from its position is called the periosteum (Fig. 151), and which represents a part of the original embryonic connective tissue membrane in which the bone was laid down. During its development the periosteum becomes an exceedingly dense fibrous membrane which is closely applied to the surface of the bone. In a growing embryo, provision must be made for increase in the size of the cranial cavity to accommodate the growing brain. This is accomplished in the following manner: On the inner surface of the newly formed bone, large multinuclear cells appear, which are known as osteoclasts (bone destroyers). The osteoclasts are unusually large cells with a large number of nuclei and abundant cytoplasm, and in sections can be seen lying in depressions in the I2 176 TEXT-BOOK OF EMBRYOLOGY. bone—Howslip’s lacune (Fig. 152). They apparently possess the power of dissolving bone tissue. While the destruction of bone by the osteoclasts is going on on the inner surface, new bone is being formed on the outer surface, especially under the periosteum where the osteoblasts are most numerous. Thus the layer of bone gradually comes to lie farther and farther out and the cranial cavity is enlarged. So long as the cranial cavity continues to enlarge Cartilage Intracartilaginous bone Subperiosteal Z bone Blood vessels ial whi: Periosteum = __#/& (perichondrium) y > Ossification center = | Calcification zone ~~ s Fic, 153.—Longitudinal section of one of the metatarsal bones of a sheep embryo. (Intracartilaginous ossification.) the new bone is of the spongy variety, but toward the end of development the trabecule become thicker and finally come together to form the compact bone characteristic of the roof of the skull. The fact that the new bone laid down during the enlargement of the cranial cavity is laid down under the periosteum has led to the term subperiosteal ossification. ‘The process is essentially the same as in the original intramembranous ossification. INTRACARTILAGINOUS OSSIFICATION. In this type of ossification hyalin cartilage is first formed in a shape which corresponds very closely to the shape of the future bone. For example, the femur is first represented by a piece of hyalin cartilage which develops from THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 177 the original embryonic connective tissue. On the surface of the cartilage a membrane of dense fibrous connective tissue, known as the perichondrium, develops (Fig. 153). In most cases, ossification begins about the middle of the piece of cartilage, corresponding to the middle of the shaft of a long bone (Fig. 153). The cell spaces enlarge and in some cases the septa of matrix between the enlarged spaces break down, so that several cells may lie in one space. The cell spaces radiate from a common center, but a little later they come to lie in rows parallel with the long axis of the mass of cartilage. During these early changes lime salts are deposited in the matrix of the cartilage in this region, and the portion so involved is known as a calcification center. So far the process is preparatory to actual bone formation. Then small blood vessels from the perichondrium (periosteum) grow into the cartilage, Periosteal bud Blood vessel Cartilage cell spaces Nae Fic. 154.—From section of one of the tarsal bones of a pig embryo. Showing periosteal bud pushing into the cartilage at the ossification center. (Intracartilaginous ossification.) carrying with them some of the embryonic connective tissue. These little ingrowths of connective tissue and blood vessels are known as periosteal buds (Fig. 154). The septa between the enlarged cartilage cell spaces break down still further, forming still larger spaces into which the periosteal buds grow. Many of the connective tissue cells are transformed into osteoblasts—oval or round cells with distinct nuclei and a considerable amount of cytoplasm—and with the fibers and blood vessels constitute osteogenetic tissue (Fig. 155). The cartilage cells in this region disintegrate and disappear, and the cavity formed by the coalescence of the cell spaces constitutes the primary marrow cavity (Fig. 155). From the primary marrow cavity osteogenetic tissue pushes in both directions toward the ends of the cartilage. The transverse septa between the enlarged cartilage cell spaces break down, leaving a few longitudinal septa which form the walls of long anastomosing channels which are continuous with 178 TEXT-BOOK OF EMBRYOLOGY. the primary marrow cavity. The osteoblasts arrange themselves in rows along the septa of calcified cartilage and a thin layer or lamella of calcium salts is deposited between them and the cartilage. Successive lamelle are deposited in the same manner and some of the osteoblasts become enclosed to form bone cells (Fig. 156). The cartilage in the center gradually disappears. This region where bone formation is going on is known as an ossification center (Fig. 1§3) and the irregular anastomosing trabecule of bone with the enclosed marrow spaces constitute primary spongy bone. From this time on, ossification gradually progresses toward each end of the cartilage, and at the same time a special modification of the cartilage precedes it. Nearest the ossification center the cartilage cell spaces become enlarged and Cartilage cell spaces (primary marrow space) Disintegrating ae 5 }] | cartilage cells SY / Ss 2 | eB HS. . A pA / ee vo LA ) Cartilage cell ye —— S spaces \_ re BON Blood vessel ———+— Trabecula of cartilage Osteogenetic tissue in primary marrow space Fic, 155.—From same section as Fig. 153; showing osteogenetic tissue pushing into the cartilage and breaking it up into trabeculz. (Intracartilaginous ossification.) arranged in rows and contain cartilage cells in various stages of disintegration. Some of the septa break down, leaving larger, irregular spaces; the remaining septa become calcified (Fig. 153). Passing away from the center of ossifica- tion, there is less enlargement of the cell spaces and they have a tendency to be arranged in rows transverse to the long axis of the cartilage; there is also a lesser degree of calcification. The region of modified cartilage at each end of the ossification center passes over gradually into ordinary hyalin cartilage and is known as the calcification zone. It always precedes the formation of bone as the latter process moves toward the end of the cartilage (Fig. 153). Along with the type of ossification just described subperiosteal ossification also occurs (Fig. 153). Beneath the periosteum (perichondrium) is a layer of connective tissue the cells of which are transformed into osteoblasts. They THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 179 deposit layers of calcium salts on the surface of the cartilage in the same manner as around the trabeculz inside the cartilage. The transformation of the spongy bone into compact bone is peculiar in that the former is dissolved and then replaced by new bone. This dissolution is brought about by the action of the osteoclasts—large mul- tinuclear cells the origin of which is not known. By the process of dis- solution the marrow spaces are increased in size and are known as Hlaver- sian spaces. Within these spaces new bone is then deposited layer upon layer, under the influence of the osteoblasts, until the Haversian spaces are reduced to narrow channels, the Haversian canals. The layers of bone are the Haver- stan lamelle. ‘The interstitial lamelle in compact bone have two possible origins. They may be the remnants of certain lamelle of the original spongy Blood vessel Bone Cartilage Bone cell Cartilage cell aa] Cartilage cell space Connective Bone Osteogenetic Osteoblasts tissue cells tissue Fic. 156.—From same section as Fig. 153; showing bone deposited around one of the trabecule of cartilage. (Intracartilaginous ossification.) bone which were not removed in the enlargement of the primary marrow spaces, or they may be parts of early formed Haversian lamella which were later more or less replaced by other Haversian lamellz. The fact should be emphasized that although it is convenient to describe three types of bone formation, the three do not differ essentially from one another. ‘The similarity of intramembranous and subperiosteal ossification has already been noted (p. 176). In both these types the bone is developed within a membrane of embryonic connective tissue by a transformation of this tissue into osteogenetic tissue and then of the latter into bone. The only way in which intracartilaginous bone formation differs from the other two types is that cartilage is first formed within the membrane in the same general shape as the future bone. But it must be remembered that it is only im this cartilage that bone is developed and not from it, the bone being produced by osteogenetic 180 TEXT-BOOK OF EMBRYOLOGY. tissue which in turn is derived from the embryonic connective tissue brought into the cartilage by the periosteal bud. GROWTH OF Bones.—The way in which the cranial cavity enlarges has been described on page 175. While the process of enlargement is going on, the individual bones increase in size principally by the addition of new bone along their edges. Intracartilaginous bones grow both in diameter and in length. It has already been stated that the primary spongy bone formed in cartilage is dis- solved and that new bone is deposited under the periosteum. This naturally brings about an enlargement of the primary marrow cavity and at the same time an increase in the diameter of the bone asa whole. From this it is obvious that the compact bone of the shaft of a long bone is of subperiosteal origin, the intracartilaginous bone having been completely absorbed. A B C Fic. 157.—Diagram representing growth in diameter of a long bone. Modified from Flourens. The fact that the osseous tissue bordering the marrow cavity is absorbed and that new bone is deposited under the periosteum can be quite clearly demonstrated. A young growing animal is fed for a few weeks on madder, which colors all the bone formed during that time a distinct red. If the animal is then killed and sections made of the long bones, the outer part of the latter will appear a distinct red. Another growing animal is fed on madder for a few weeks, then allowed to live a few weeks longer without madder. Then if it is killed and sections made of the bones, the red bone is found to be covered with a layer of uncolored bone which was deposited after the madder feeding had been stopped. If a young growing animal is fed on madder for a time and then allowed to live long enough without madder, the red bone will be found lining the marrow cavity. (See Fig. 157.) Growth in length of the long bones takes place in a different manner. The primary center of ossification is situated near the middle of the piece of cartilage, and ossification proceeds in both directions toward the ends of the cartilage to produce the diaphysis or shaft of the bone. In each end of the cartilage there appears a secondary center from which ossification proceeds in all directions to produce the epiphysis. Between the shaft and epiphysis a disk of cartilage remains, and here, so long as the bone is growing, new cartilage continues to be formed. At the same time new bone is being formed in the new cartilage, THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 181 principally in the part next the shaft. This produces an elongation of the shaft, the two epiphyses being carried farther and farther apart, and conse- quently a lengthening of the bone as a whole. When the bone reaches the required length, the cartilage disk diminishes and finally is wholly replaced by bone, being represented in the adult only by the epiphyseal line. (See Fig. 158). Marrow.—The forerunner of marrow is the osteogenetic tissue in the pri- mary marrow spaces, which in turn is derived from embryonic connective tissue (Fig. 155). During the development of bone, great numbers of osteoblasts are Frc. 158.—Longitudinal section from head of femur of young dog. Photograph. The head of the femur is shown in the upper part of the figure, the end of the shaft in the lower part. Between the two the lighter line represents the cartilage between the primary center of ossification (shaft) and the secondary center (epiphysis, head), and marks the site of the epiphyseal line. The lighter portion covering the head represents the cartilage bordering the joint cavity. constantly being differentiated from the connective tissue cells and many of these ultimately become bone cells. When development ceases, osteoblasts cease to become differentiated. When dissolution of bone becomes necessary, osteoclasts appear. Their origin is not known with certainty. One view is that they are derived from leucocytes, another is that they are derived from the endothelium of blood vessels. Their relation to the myeloplaxes (giant cells) in adult marrow is also a matter of doubt, though it is possible that the two forms are identical. Leucocytes appear in the marrow at an early stage, but whether they arise in situ or are brought in primarily by the blood vessels is not known. 182 TEXT-BOOK OF EMBRYOLOGY. There is also just as much uncertainty in regard to the origin of red blood cells im the marrow. At an early stage nucleated red cells are present, and from this time on, the marrow affords a place at least for their proliferation, for in the adult marrow all the nucleated forms are found, as well as the non-nucleated. The origin of the marrow cells—myelocytes—is not known. The fibrous part of the osteogenetic tissue assumes a reticular structure and forms the reticulum of the marrow. In young marrow there is little or no fat present, but in later life many of the connective tissue cells are transformed into fat cells (p. 172), so that these form the greater part of the marrow. Such a process oc- curs most extensively in the shaft of the long bones and gives rise to “yellow” marrow. In the heads of the long bones, in the ribs, and in the short bones the marrow retains its earlier character and is known as “red” marrow. THE DEVELOPMENT OF THE SKELETAL SYSTEM. The Axial Skeleton. The Notochord.—The notochord (chorda dorsalis) constitutes the primitive axial skeleton of all Vertebrates, yet it differs from the other skeletal elements in that it is a derivative of the entoderm. Inman it is merely a tran- sient structure and disappears early in foetal life, leaving but a slight trace of itself in the intervertebral disks. In embryos of 2-3 mm. the cells of the Anlage of notechord===--— -< Fic. 159.—From transverse section of human embryo with 8 pairs of primitive segments (2.69 mm.). Kollmann. entoderm just ventral to the neural groove become slightly differentiated (Fig. 159) and then form a groove with a ventral concavity. The groove closes in, becomes constricted from the parent tissue (entoderm) and lies just ventral to the neural tube, where it soon becomes surrounded by mesodermal tissue. This structure is the notochord and constitutes a solid, cylindrical cord of cells extending from a point just caudal to the hypophysis to the caudal extremity of the embryonic body. In embryos of 17-20 mm. the mesodermal tissue around the notochord becomes modified to form the chordal sheath. On account of its position the notochord naturally becomes embedded in the developing vertebral THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 183 column, extending through the bodies of the vertebra and the intervertebral disks. The cells are at first of an epithelial nature (Fig. 159), but those within the vertebral bodies become vacuolated and broken up into irregular, multinu- clear masses which then disappear. The cord is thus first interrupted in the vertebra, leaving only the segments within the intervertebral disks. Later these segments also undergo degenerative changes, but persist as the so-called pulpy nuclet. While the notochord is morphologically the forerunner of the axial skeleton, and persists as a whole in Amphioxus, and in part in Fishes and Amphibia, in the higher forms it is almost exclusively an embryonic structure with little or no functional significance. It differs in origin from the true skeletal elements and becomes involved with them only to disappear as they develop. Perichordal sheath Cleft between two vertebral anlagen Intersegmental ew artery i | —Notochord Parts of two adjacent sclerotomes Fic. 160,—Five myotomes and sclerotomes from sagittal section of human embryo of § mm. Bardeen, Each sclerotome is differentiated into a looser cephalic part and a denser caudal part, the two being separated by a cleft (fissure of von Ebner). The Vertebrae.—The changes which occur in the ventro-medial parts of the primitive segments to form the sclerotomes have already been described: At the same time it was stated that the vertebre, with the other types of connective tissue around them, were derived from the mesenchymal tissue of the sclerotomes (p. 167; see also Fig. 142). The segmentally arranged masses forming the sclerotomes are separated by looser tissue in which the intersegmental arteries develop. The arteries mark the boundaries between the sclerotomes (Fig. 160). About the third week of development the caudal part of each sclerotome con- denses to form a more compact mass of tissue, and a little later becomes separated from the cephalic part by a small cleft (Fig. 161). From the denser caudal part a secondary mass of tissue grows medially and meets and fuses with its fellow of the opposite side, thus enclosing the notochord. The medial mass. 184 TEXT-BOOK OF EMBRYOLOGY. thus formed may be considered as the anlage of the body of a vertebra. Another secondary mass also grows dorsally between the myotome and the spinal cord, forming the anlage of the vertebral arch. A third mass grows ventro-laterally to form the costal process (Figs. 162 and 163). The looser tissue of the cephalic part of each sclerotome also sends an extension medially to surround the notochord, and fills up the intervals between the succeeding denser (caudal) parts. The looser part also forms a sort of membrane between the succeeding vertebral arches. ‘The tissue between the denser caudal part and the looser cephalic part of each sclerotome is destined to give rise to an imlervertebral fibrocartilage. While the denser tissue forming the caudal part of each sclero- Dermis Notochord % Cleft f Intersegmental artery Myotome Perichordal sheath Intervertebral disk Interdiscal membrane Spinal nerve Fic. 161.—Six myotomes and sclerotomes from sagittal section of human embryo of 6 mm, Bardeen, Compare with Fig. 160, tome probably gives rise to the greater part of a vertebra, the looser tissue of the cephalic part is also involved in the formation of the cartilaginous body, as will be noted again in the following paragraph. The peculiar feature of the process is that the denser caudal part of a sclerotome becomes associated with the looser cephalic part of the next succeeding sclerotome, so that each vertebra is derived from parts of two adjacent sclerotomes and not fromasingle sclerotome. This naturally brings about an aliernation of vertebre and myotomes (Fig. 161). So far the anlagen of the vertebre are in the so-called blastemal stage. Following the blastemal stage and beginning in human embryos of about 15 mm., comes the cartilaginous stage in which the mesenchymal anlagen of the vertebre are converted into embryonic hyalin cartilage. In the body of each vertebra a center of chondrification appears in the looser tissue of the caudal THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 185 part and gradually enlarges and involves the denser cephalic part. It is to be noted that the denser tissue of the cephalic part of a vertebral body corresponds to the caudal part of asclerotome. Two chondrification centers appear, one on Arch of vertebra Not Costal process otochord Body of vertebra Aorta Mesonephros Stomach Liver Fic. 162.—Transverse section (dorsal part) of pig embryo of 1; mm. Photograph. each side of the medial line, but the two soon fuse around the notochord to form a single center. In addition to the center in the body of the vertebra, one also appears in each half of the vertebral arch, and one in each costal process Arch of vertebra Interdorsal membrane Bodies of Notochord vertebra Costal process Fic. 163.—Models of three vertebra in the blastemal stage; from an embryo of 11 mm. Bardeen. Fig. 164). All these centers then enlarge and unite to form a single mass of cartilage which corresponds quite accurately in shape to the future bony vertebra. Processes then grow out from the vertebral arch. These represent 186 TEXT-BOOK OF EMBRYOLOGY. the transverse and articular processes (Fig. 165). Each half of a vertebral arch meets its fellow of the opposite side dorsal to the spinal cord, and from the point of meeting the spimous process grows out. The costal processes do Costal process Body of Costal process (rib) vertebra (rib) Aorta Pleural cavity Liver Csophagus Lung Fic. 164,—Transverse section (dorsal part) of pig embryo of 35 mm. Photograph. not retain their connection with the body of the vertebra, but break away and become the rib cartilages, as will be noted again in connection with the development of the ribs. Following the cartilaginous stage is the stage of ossification in which the Arch of vertebra Post. articular process Transverse process Fic. 165.—Models of the 6th, 7th and 8th thoracic vertebra of an embryo of 33 mm. (dorsal view). Bardeen. On the right the cartilage is shown, on the left the surrounding fibrous tissue. vertebre become ossified and acquire the adult condition. Ossification begins during the third month of foetal life and extends over a long period, even up to the age of twenty-five years. A single center of ossification appears in the body THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 187 of each vertebra, and following this a center in each half of the vertebral arch (Fig. 166). Osseous tissue then gradually replaces the cartilage. The two halves of an arch fuse dorsal to the spinal cord during the first year of post- natal life, thus completing the bony arch. The arch fuses with the body of the _____—-— Spinous process Articular process Transverse process Lat. ossif. center Med. ossif. cente Fic. 166.—Thoracic vertebra and ribs of human embryo of 55 mm. (middle of grd month). Kollmann’s Ailas. Cartilage indicated by stippled areas, ossification centers by irregular black lines. vertebra between the third and eighth years. ‘Thus it is seen that the process of ossification is a slow one, and this is even more striking when one considers the formation of the secondary centers. For at about the age of puberty a secondary center appears in each of the cartilages that cover the ends of the vertebre, pro- Spinous process Transverse process Articular process Body of vertebra Upper epiphyseal plate Fic. 167.—Lumbar vertebra (lateral view) showing secondary centers of ossification, Sappey. ducing disks of bone—the epiphyses. A secondary center also appears in the cartilage on the tip of each spinous process and transverse process, and in the lumbar vertebre one appears also on the tip of each articular process (Fig. 167). The epiphyses unite with the vertebra any time between sixteen and twenty- 188 TEXT-BOOK OF EMBRYOLOGY. five years. About the twenty-fifth year. the sacral vertebre unite to form a single mass of bone, and a similar union also takes place between the more or less rudimentary coccygeal vertebra. While the general plan of development is practically the same in all the vertebree, there are a few noteworthy modifications. The greatest modification is in the atlas and epistropheus (axis). The entire atlas is formed from the denser caudal part of a sclerotome. The lateral mass and the posterior (dorsal) arch represent the vertebral arch. The anéerior (ventral) arch represents the hypochordal bar, a plate of cartilage which develops in all vertebre ventral to the notochord but disappears in all except the atlas. A body also develops but instead of forming part of the atlas it unites with the body of the epistro- pheus to form the dens (odontoid process) of the latter. Clavicle Suprasternal cartilage Sternal bar 7th rib Fic. 168.—Ventral view of developing sternum of human embryo of 30 mm. (beginning of 3rd month). Ruge, Kollmann’s Ailas. The various digaments of the vertebral column are derived from the embry- onic connective tissue surrounding the vertebra. The embryonic connective tissue in the clefts separating the developing vertebre is transformed into the intervertebral fibrocartilages. The Ribs.—It has been stated in a previous paragraph that the costal proc- esses arise as outgrowths from the denser caudal parts of the sclerotomes; that they grow in a ventro-lateral direction and consequently are at first connected with and are parts of the bodies of the vertebre (Figs. 162 and 165). These costal processes are the anlagen of the ribs, and they continue to grow ventrally until they practically encircle the body, the ventral ends of a number of them fusing in the medial line to form the sternum. The primary junctions between the costal processes and vertebre are dissolved, and the embryonic connective tissue in this region gives rise to the costo-vertebral ligaments. The dissolu- tion of the junctions leaves the ribs simply articulating with the vertebra. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 189 A chondrification center appears in-each costal process, shortly after that in “the body of the vertebra, and from this point the formation of cartilage gradually extends throughout the entire rib. Ossification: begins during the third month at a center which is situated near the angle of the rib (Fig. 166). At the age of eight to fourteen years a second- ary center appears in each capitulum and tuberculum, and subsequently fuses with the rest of the rib at the age of fourteen to twenty-five years. As the tuberculum develops, the transverse process of the corresponding vertebra grows ventrally and caudally to meet it and form the articulation. The ribs ‘reach the highest degree of development in the thoracic region where one develops on each side, corresponding to each vertebra. The first seven or eight thoracic ribs extend almost to the mid- ventral line and are attached to the sternum; the last four or five become successively shorter and are only indirectly or not at all attached to the sternum. In the cervical region the ribs do not reach a high degree of development. - Their tips simply fuse with the transverse processes of the vertebre and their heads with the bodies of the vertebre, leaving a space—the foramen transversarium—through which the vertebral vessels pass. The seventh cervical rib may, however, reach a fairly high degree of development. In the lumbar region also the ribs are reduced to small pieces of bone which are firmly united with the transverse processes and form the accessory processes. In the sacral region the rudimentary ribs unite to form the lateral part (pars lateralis) of the sacral bone. After the blastemal | stage there are no indications of ribs in the coccygeal region. In the blastemal stage, however, there is a small bit of tissue Fic. 169.—Sternum of which probably represents the anlage of a rib, but soon {howing Soap: showing centers of fuses with the transverse process. ossification. Seven The Sternum.—The sternum is formed by the fusion oe oie ade a oa of the ventral ends of the first eight or nine thoracic ribs. Ein tag a A longitudinal bar is first formed on each side of the medial line by the fusion of the ventral ends of the ribs on each side; then the two bars unite in the medial line to form a single piece of cartilage (Figs. 168 and 169). Subsequently the last one or two ribs become separated from the sternum, leaving only seven or eight connected with it. At the cephalic end of the sternum two separate pieces of cartilage—episternal cartilages—appear, with which the clavicles articulate (Fig. 168). These usually unite with the longi- tudinal bar to form a part of the manubrium, but they may remain separate and ossify to form the suprasternal bones (ossa suprasternalia). 190 TEXT-BOOK OF EMBRYOLOGY. Ossification begins in the sternum about the end of the fifth month of foetal life. In each of the two cephalic segments a single center appears; caudal to the second segment a series of paired centers appears, and later the centers of each pair fuse into a single center (Fig. 169). The paired centers possibly represent epiphyses of the ribs. Sometimes, however, the centers appear as a single series, that is, with no indication of a paired character. The ossification of the most cephalic segment, along with the episternal cartilages, produces the manubrium sterni. Ossification of the following six or seven segments and their union produce the corpus sterni. The bars formed from the most caudal ribs (excluding the false ribs) form the «yphoid process. This process remains car- Olfactory organ Nasal septum Hypophysis Olfactory organ Visual organ Hypophysis #- Visual organ Prechordal plate Prechordal plate Auditory organ Auditory organ Parachordal plate Sa (parachordal) Notochord Notochord otocho Fic. 170. Fic. 171. Fic. 170.—Diagram of first stage in the development of the cartilaginous primordial cranium. Wiedersheim. Fic. 171.—Diagram of later stage of same. Wéiedersheim. tilaginous for a long period, and may be single, perforated, or bifurcated, de- pending upon the degree of fusion between the two primary bars. The Head Skeleton.—Topographically the skeleton of the head appears as the cephalic part of the axial skeleton. Structurally it is decidedly different, for it is adapted to different conditions. The neural tube here becomes differ- entiated into the brain with its many and dissimilar parts. In connection with the brain the complicated sense organs (nose, eye and ear) arise. A part of the alimentary tract and portions of the visceral arches are also inclosed within the head. The head skeleton is specially modified to accommodate these highly developed organs, and becomes extremely complicated. In general the skeleton in any part of the body adapts itself to the other structures and not the other structures to the skeleton. The anlage of the skull is a mass of embryonic connective tissue which sur- THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 191 rounds the cephalic end of the notochord, extends from there into the nasal region and also extends around the sides and dorsal part of the neural tube (brain). Unlike the anlage of the vertebral column, the anlage of the skull shows no distinct division into primitive segments. The only indications of a segmental character are referred to in a succeeding paragraph (small print, P- 193). The next step in the development of the skull is the appearance of cartilage in certain regions of the embryonic connective tissue. On account of the com- plicated arrangement of the cartilage in the human skull, it is best to consider Palatoquadrate Nasal fossa Preorbital process Palatoguadrate Meckel’s cartilage Roof of skull #—— Marginal bar Prechordal plate — Prootic incisure Hypophysis f — Jugular foramen i Foramina (VII Nerve) Prechordal plate Palatoquadrate _f aa (4 Hyomandibular Post. basal fenestra Notochord Otic (auditory) capsule Synotic tectum Fic. 172.,—Primordial cranium of Salmo salar (salmon) embryo of 25 mm. Dorsal view. Gaupp. Compare with Fig. 171 and note further elaboration of parts surrounding the sense organs. first its more simple arrangement in the lower Vertebrates. In these there ap- pear in the embryonic connective tissue around the cephalic end of the notochord two bilaterally symmetrical pieces of cartilage, which extend as far as the hypophysis. Then two other bilaterally symmetrical pieces appear, extending from the hypophysis to the nasal region. Subsequently all these pieces fuse into a single mass which extends from the cephalic end of the vertebral column to the tip of the nose, enclosing the end of the notochord and, to a certain ex- tent, the ear, eye and olfactory apparatus. There is left, however, an opening for the hypophysis. From this mass of cartilage, chondrification extends into the embryonic connective tissue along the sides and roof of the cranial 13 192 TEXT-BOOK OF EMBRYOLOGY. cavity, so that the brain and sense organs are practically enclosed. To this capsule the term cartilaginous primordial craniwm has been applied. (See Figs. 170, 171, 172.) In the higher Vertebrates, chondrification is limited to the basal region of the skull, while the side walls and roof are formed later by intramembranous bone. Meckel’s cartilage Malleus Sella turcica Incus Dorsum sellae Int. acoustic pore Foramina / (VII Nerve) Jugular foramen —/ ff Auditory Subarcuate fossa capsule Foramen Foramen (XII Nerve) Large occipital foramen Occipital (foramen magnum) (synotic tectum) Fic. 173.—Dorsal view of primordial cranium of human embryo of 80 mm. (3rd month). Gaupp, Hertwig. The membrane bones of the roof of the skull have been removed. Through the large occipital foramen can be seen the first three cervical vertebrze. In the human embryo chondrification occurs first in the occipital and sphenoidal regions, and then gradually extends into the nasal (ethmoidal) region. A little later it spreads somewhat dorsally in the occipital and sphenoidal regions to form part of the squamous portion of the occipital and the wings of the sphenoid. At the same time cartilage develops in the embryonic connective tissue surround- THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 193 ing the internal ear to form the periotic capsule which subsequently unites with the occipital and sphenoidal cartilages. The pieces of cartilage thus formed con- stitute the chondrocranium. In connection with the development of the caudal part of the occipital cartilage there is an interesting feature which is at least indicative of a segmental character. In some of the lower Mammals there are four fairly distinct condensations of embryonic connective tissue just cranial to the first cervical vertebra, corresponding to the first cervical nerve and the three roots of the hypoglossal. These condensations bear a general resemblance to the primitive segments and indicate the existence of four vertebra which are later taken up into the chondrocranium. In the human embryo the condensations are less distinct, but the existence of a first cervical and a three-rooted hypoglossal nerve in this region suggests an original segmental character. If this is true, then the base of the human skull is formed from the unsegmented chondrocranium plus four vertebree which become incorporated in the occipital region. Ala magna (sphenoid) Optic foramen Ala parva (sphenoid) Foramen (VII Nerve) Incus Nasal capsule Nasal septum Maxilla Vomer ge \, Styloid process / : | Palate bone Malleus \ Cochlear fenestra Mandible Foramen (XII Nerve) Meckel’s cartilage Cricoid cartilage PK Thyreoid cartilage Fic. 174.—Lateral view of primordial cranium of human embryo of 80 mm. (3rd month). Gaupp, Hertwig. The membrane bones of the roof of the skull have been removed. Compare with Fic. 173. The maxilla, vomer, palate, and mandible are membrane bones. In addition to the chondrocranium, other cartilaginous elements enter into the formation of the skull, all of which are derived from the visceral arches. Not all the arches, however, produce cartilage; for in the maxillary process of the first arch, which forms the upper boundary of the mouth, cartilage does not appear, and the bones which later develop in it are of the membranous type. The mandibular process of the first arch produces a rod of cartilage—Meckel’s cartilage. This gives rise, at its proximal end, to a part of the auditory ossicles, but the cartilage in the jaw proper soon wholly or almost wholly disappears. The cartilage of the second arch becomes connected with the skull in the region 194 TEXT-BOOK OF EMBRYOLOGY, of the periotic capsule. The cartilages of the other three arches are only indirectly connected with the skull and will be considered later. Figs. 173 and 174 show the condition of the chondrocranium in a human embryo of 80 mm. (third month). Although at first glance it seems exceedingly complicated, a careful study and comparison of the various parts will aid the student in his comprehension of the cartilaginous foundation upon which the skull is built. OSSIFICATION OF THE CHONDROCRANIUM. In the human feetus ossification begins in the occipital region during the third month. Four centers appear which correspond to the four parts of the adult occipital bone (Fig.175). (x) An unpaired center situated ventral to the foramen magnum. From this center ossification proceeds in all directions to Interparietal (of lower forms) 2. Squamous part A, (intramemb.) Squamous \" Part \ Kerkringius’ bone Squamous part (intracartilag.) —Lateral part Basilar part Fic. 175.—Occipital bone of human embryo of 21.5 cm. Kollmann’s Atlas. form the pars basilaris (basioccipital). (2 and 3) Two lateral centers, one on each side. From these, ossification proceeds to produce the partes laterales (exoccipital) which bear the condyles. (4) A center dorsal to the foramen magnum. ‘This produces the pars sqwamosa (supraoccipital) as far as the supe- rior nuchal line. Beyond this line the pars squamosa is of intramembranous origin. (See p. 196.) At birth the four parts are still separated by plates of cartilage. During the first or second year after birth the partes laterales unite with the pars squamosa, and about the seventh year the pars basilaris unites with the rest of the bone. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 195 In the sphenoidal region ossification begins at a number of centers which, as in the occipital region, correspond generally to the parts of the adult Sphenoid bone (Fig. 176). (x and 2) About the ninth week an ossification center appears on each side in the cartilage which corresponds to the ala magna (alisphenoid). (3 and 4) About the twelfth week a center appears on each side which corresponds to the ala parva (orbitosphenoid). (5 and 6) A short time after this a center appears on each side of the medial line in the basal part of the cartilage, and the two centers subsequently fuse to produce the corpus (basisphenoid). (7 and 8) Lateral to each basal center, another center appears which represents the beginning of the Jingula. (9 and 10) Finally two centers appear in the basal part of the cartilage, in front of the other basal centers, and then fuse to form the presphenoid. As in the case of the oc- cipital bone, not all of the adult sphenoid is of intracartilaginous origin; for the Ala parva Ala magna Lingula Lingula Pterygoid process Corpus (basisphenoid) Fic. 176.—Sphenoid bone of embryo of 34-4 months. Sappey. The parts that are still cartilaginous are represented in black. upper anterior angle of each ala magna is of intramembranous origin, as are also the medial and lateral laminz of the pterygoid process. The pterygoid hamulus, however, is formed by the ossification of a small piece of cartilage which de- velops on the tip of the medial lamina. The fusion of these various parts oc- curs at different times. The lateral pterygoid lamina unites with the alisphe- noid before the sixth month of foetal life; about the sixth month the lingula fuses with the basisphenoid, and the presphenoid with the orbitosphenoid. ‘The alisphenoid and medial pterygoid lamina fuse with the rest of the bone during the first year after birth. The union of the basisphenoid and basioccipital usually occurs when the growth of the individual ceases, though the two bones may remain separate throughout life. In the region of the periotic capsule, several centers ‘of ossification appear in the cartilage during the fifth month. During the sixth month these centers unite to form a single center which then gradually increases to form the pars petrosa and pars mastoidea of the adult temporal bone. The mastoid process is 196 TEXT-BOOK OF EMBRYOLOGY. formed after birth by an evagination from the pars petrosa, and is lined by an evaginated portion of the mucosa of the middle ear. The other parts of the temporal bone are of intramembranous origin, except the styloid process which represents the proximal end of the second branchial arch. In the ethmoidal region, conditions become more complicated on account of the peculiarities of the nasal cavities, and on account of the fact that the cartilage is never entirely replaced by bone, and that “membrane” bones also enter into more intimate relations with the “cartilage” bones. The ethmoidal cartilage at first consists of a medial mass, which extends from the presphenoid region to the end of the nasal process, and of a lateral mass on each side, which is situated lateral to the nasal pit (Fig. 174). Ossification in the lateral mass on each side produces the ethmoidal labyrinth (lateral mass of ethmoid). It is perhaps not quite correct to say that ossification produces the ethmoidal labyrinth, for at first there is only a mass of spongy bone with no indication of the honey-combed structure characteristic of the adult. The latter condition is produced by a certain amount of dissolution of the bone and the growth of the nasal mucosa into the cavities so formed. By the same process of dissolution and ingrowth of nasal mucosa the superior, middle and inferior conche (turbinated bones) are formed. The medial mass of cartilage begins to ossify after birth and then only in its upper (superior) edge. It forms the lamina perpendicularis and crista galli and extends into the nose as the nasal septum. The lower (inferior) edge remains as cartilage until the vomer, which is a membrane bone (p. 198), develops, after which it is partly dissolved. The lamina cribrosa (cribriform plate) is formed by bony trabecule which extend across between the medial mass and the lateral masses and surround the bundles of fibers of the olfactory nerve. MEMBRANE BONES OF THE SKULL. Under this head we shall consider only those bones which develop apart from the visceral arches, those which involve the arches being considered later. It has been seen that by far the greater parts of the bones forming the base of the skull are of intracartilaginous origin. On the other hand, those forming the sides and roof of the skull are largely of intramembranous origin. In the case of the occipital bone, two centers of ossification appear in the membrane dorsal to the supraoccipital, and the bone so formed begins to unite with the supra- occipital during the third month of foetal life. At birth the union is usually complete, though for a time an open suture may persist on each side. The bone derived from the two centers forms that part of the occipital squama which is situated above the superior nuchal line; the part below the line is of intracarti- laginous origin (p. 194). The adult occipital is thus a composite oe partly of intramembranous, partly of intracartilaginous origin. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 197 The temporal is also a composite bone, the petrous and mastoid parts and the styloid process being of intracartilaginous origin, while the temporal squama and the tympanic part are of intramembranous origin. During the eighth week of foetal life a center of ossification appears in the membrane in the temporal region, and the bone formed from this center subsequently unites with the petrous part and becomes the temporal squama. Another center ap- pears in the membrane to the outer side of the periotic capsule and produces a ring of bone around the external auditory meatus, which fuses with the petrous Parietal = rag lait Frontal fontanelle Occipital --fA s—(i‘“‘ : Styloid process---------------------- zZ “Sy Soran ae Onn Oe Mandible ¥N \ St Galil Bioneers eet Sa TEE C= aE SEN PADI Oe : sadpaeiaies Pear. = tel Meckel’s cartilage Hyoid (greater horn) .-------.---------------------- —_ PR A Eee Hyoid (lesser horn) a Se becuosessececeesas se Thyreoid Cricoid --- ----------------- 57-77 - fj Fic. 177.—Diagram of skull of new-born child. Combined from AfcM urrich and Kollmann. White areas represent bones of intramembranous origin; dotted areas represent bones (not derived from branchial arches) of intracartilaginous origin; black areas represent derivatives of branchial arches. part and forms the tympanic part of the adult bone. It gives attachment at its inner border to the tympanic membrane. While the union of the different parts begins during foetal life, it is usually completed after birth. The sphenoid bone is also composed of parts which have different origins. The body, small wings and large wings are of intracartilaginous origin, the plerygoid process of intramembranous origin. About the eighth week of development a center of ossification appears in the mesenchyme in the lateral wall of the posterior part of the nasal cavity and gives rise to the medial pterygoid lamina. On the tip of the latter a small piece of cartilage appears in 198 TEXT-BOOK OF EMBRYOLOGY. which ossification later takes place to form the pterygoid hamulus (p. 195). The lateral pterygoid lamina is also of intramembranous origin and fuses with the medial lamina, the two lamine forming the plerygoid process which subse- quently unites with the body of the sphenoid. (See Fig. 176.) In the ethmoidal region, only the vomer is of intramembranous origin. An ossification center appears in the embryonic connective tissue on each side of the perpendicular plate (lamina perpendicularis) and these two centers produce two thin plates of bone which unite at their lower borders and invest the lower part of the perpendicular plate. The portion of the latter thus invested undergoes resorption. The frontal and parietal bones are purely of intramembranousorigin. About the eighth week two centers of ossification, one on each side, appear for the frontal. The bones produced by these centers unite in the medial line to form the single adult bone. In the event of an incomplete union an open suture remains—the metopic suture. A single center of ossification appears for each parietal bone at about the same time as those for the frontal. The union of the bones which form the roof and the greater part of the sides of the skull does not occur till after birth. The spaces between them constitute the sutures and fontanelles so obvious in new-born children (Fig. 177). A single center of ossification appears in the embryonic connective tissue for each zygomatic, lachrymal and nasal bone, all of which are of intramem- branous origin. Bones DERIVED FROM THE BRANCHIAL ARCHES. The first branchial arch becomes divided into two portions. One of these, the maxillary process, is destined to give rise to the upper jaw and much of the upper lip and face region. The other, the mandibular process, is destined to give rise to the lower jaw, the lower lip and chin region, and two of the auditory ossicles. The angle between the two processes corresponds to the angle of the mouth, and the cavity enclosed by the processes is the forerunner of the mouth and nasal cavities. (See Fig. 134, also p. 151.) So far as the skeletal elements are concerned, cartilage develops only in the mandibular process where it forms a slender bar or rod known as Meckel’s cartilage. Only a small part of this becomes ossified, the greater portion of the mandible being of intramem- branous origin. No cartilage develops in the maxillary process. This probably indicates a condensation of development in man and the higher animals, for among the lower animals cartilage precedes the bone. In man the maxilla and palate bone also are of intramembranous origin. The palate bone develops from a single center of ossification which appears at the side of the nasal cavity in embryos of about 18 mm. This center THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 199 represents the perpendicular part, the horizontal part appearing in embryos of about 24 mm. as an outgrowth from the perpendicular and not as a separate center of ossification. The orbital and sphenoidal processes also represent out- growths from the primary center and appear much later. Opinions regarding the development of the mawilla are at variance. One view is that it arises from five centers of ossification. One of these centers gives rise to that part of the alveolar border which bears the molar and premolar teeth; a second center forms the nasal process and that part of the alveolar bor- der which bears the canine tooth; a third produces the part which bears the incisor teeth; and the two remaining centers give rise to the rest of the bone. All these parts effect a firm union at an early stage, with the exception of the part bearing the incisor teeth which remains more or less distinct as the incisive bone (premaxilla, intermaxilla). Another view arising from recent work on Incisive bone Upper lip (intermaxillary) ——- tis = Lip groove Primitive choana —+ Pg! Cut surface Palatine processes Fic. 178.—Head of human embryo of 7 weeks. His. Ventral aspect of upper jaw region. Lower jaw and tongue have been removed. human embryos is that there are primarily only two ossification centers; one of these gives rise to the incisive bone, the other to the rest of the maxilla (Mall). These centers appear at the end of the sixth week (embryos of 18 mm.). A very important feature in the development of the maxilla is its agency in separating the nasal cavity from the mouth cavity. The palatine process of the bone grows medially and meets and fuses with its fellow of the opposite side in the medial line, the two processes together thus constituting about the an- terior three-fourths of the bony part of the hard palate. It should be observed, however, that the palatine processes do not meet at their anterior borders, for the incisive bone is insinuated between them (see Figs. 178, 179). 200 TEXT-BOOK OF EMBRYOLOGY. The incisive bone is probably not derived from the maxillary process of the first visceral arch, but from the fronto-nasal process. The question thus arises as to whether it is derived from both the middle and lateral nasal processes or only from the middle. According to Kolliker’s view, the lateral nasal process takes no part in the formation of the incisive bone. It is derived from the middle process, hence genetically it is a single bone on each side. According to Albrecht’s view the incisive bone is genetically composed of two parts, one derived from the lateral, the other from the middle nasal process. While the matter is not one of great importance merely from the standpoint of development, it has an important bearing on the question of certain congenital malformations, e.g., hare lip, and will be discussed further under that head (p. 216). In the mandibular process of the first visceral arch, the mandible develops as a bone which is partly of intramembranous and partly of intracartilaginous origin. In the first place a rod of cartilage, known as Meckel’s cartilage, forms the core of the mandibular process and extends from the distal end of the process to the temporal region of the skull, where it passes between the tympanic Medial line ay Incisive bone Canine alveolus Molar alveolus y/ Ys 5 yy t & He ys We ‘Ze Uf at Liye ye Min. Incisive suture Palatine process HI Sy Fic. 179.—Ventral aspect of hard palate of human embryo of 80 mm. Kollmann’s Atlas. m Palate bone <49- (horizontal part) bone and the periotic capsule and ends in the tympanic cavity of the ear Fig. 174). During the sixth week of foetal life, intramembranous bone begins to develop in the mandibular process. In the region of the body of the mandible the bone encloses the cartilage, but in the region of the ramus and coronoid process the cartilage lies to the inner side of the bone. Development is further complicated by the appearance of cartilage in the region of the middle incisor teeth and on the coronoid and condyloid processes. These pieces of cartilage form independently of Meckel’s cartilage and subsequently are replaced by the bone which constitutes the corresponding parts of the mandible. The part of Meckel’s cartilage enclosed in the bone disappears; the part to the inner side of the ramus is transformed into the sphenomandibular ligament. (See Fig. 180.) In each half of the second branchial arch a rod of cartilage develops, which extends from the ventro-medial line to the region of the periotic capsule. The THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 201 proximal end of this rod is then replaced by bone which fuses with the temporal bone and forms the styloid process. The distal (ventral) end is replaced by bone which forms the lesser horn of the hyoid bone. Between the styloid proc- ess and the lesser horn, the cartilage is transformed into the stylohyoid liga- ment (see Figs. 177 and 180). In each half of the third branchial arch a piece of cartilage develops and subsequently is replaced by bone to form the greater horn of the hyoid bone. The two horns become connected at their ventral ends by the body of the hyoid bone which is also a derivative of the third arch. Later the lesser horn fuses with the greater horn to bring about the adult condition (Fig. 180). In the ventral parts of the fourth and fifth arches pieces of cartilage develop Incus Malleus Temporal squama Zygoma Mandible Tympanic ring Stylohyoid lig. Cricoid cartilage Thyreoid cartilage | Meckel's cartilage Hyoid cartilage (greater horn) Fic. 180.—Lateral dissection of head of human fcetus, showing derivatives of branchial arches in natural position. Kollmann’s Atlas. and form the skeletal elements of the larynx. A more detailed account of these will be found under the head of the larynx (p. 363). The, auditory ossicles are also derived largely from the branchial arches, the incus and malleus being derived from the proximal end of Meckel’s cartilage (first arch), the stapes having a double origin from the second arch and the embryonic connective tissue surrounding the periotic capsule. But since they form inte- gral parts of the organ of hearing, a discussion of their formation is best in- cluded in the development of the ear (p. 599). The accompanying table indicates the types of development in the different bones of the head skeleton. 202 TEXT-BOOK OF EMBRYOLOGY. B Of Intracartilaginous Of Intramembranous Derived from Visceral ones Origin Origin Arches Occipitale. Pars basilaris. Squama occipitalis above Pars lateralis. sup. nuchal line. Squama occipitalis below sup. nuchal line. Temporale. | Pars mastoidea, Pars tympanica. Processus styloideus (second Pars petrosa, with proc-| Squama temporalis. arch) essus styloideus. Sphenoidale. | Corpus. Processus pterygoideus, ex- Ala parva. cept hamulus pterygoi- Ala magna. deus. Hamulus pterygoideus. Ethmoidale. | Crista galli. Lamina cribrosa. Lamina perpendicularis. Labyrinthus ethmoidalis. Vomer. Vomer. Parietale. Parietale. Frontale. Frontale. Lacrimale. Lacrimale. Nasale. Nasale. Zygoma. Zygoma. Maxilla. Maxilla, with incisivum. | Maxilla,exceptincisivum( ?) (first arch). Palatinum. | Palatinum. Palatinum. Mandibula. | Processus condyloideus, ; Processus condyloideus,ex-| Mandibula (first arch). tip of. cept tip. Processus coronoideus, Processus coronoideus, ex- tip of. cept tip. Corpus, distal end of. Corpus, except distal end. Ramus Hyoideum. | Hyoideum Cornu majus (third arch). Cornu minus (second arch). Corpus (third arch). Ossicula Incus. Basis stapedis. Incus (first arch). auditus. Malleus. Malleus (first arch). Stapes, except basis (?). Stapes, except basis (second arch). (?) The Appendicular Skeleton. The growth of the limb buds and their differentiation into arm, forearm and hand, thigh, leg and foot, along with the rotation which they undergo during development, have been discussed in the chapter on the external form of the body (p. 153). The metameric origin of the muscles of the extremities is THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 203 discussed in the chapter on the muscular system (Chap. XI). It has been seen that the greater part of the axial skeleton is derived from the sclerotomes, is preformed in cartilage, and maintains its segmental character throughout life. It has also been seen that the head skeleton is in part preformed in cartilage, isin part of intramembranous origin, and shows but a trace of segmental character, and that only in the occipital region at a very early stage. The appendicular skeleton is derived wholly from the embryonic connective tissue which forms the cores of the developing extremities, and shows no trace of a segmental character. Here also, as in the axial skeleton, three stages may be recognized—a blastemal, a cartilaginous (Fig. 181), and a final osseous. Acromion Coracoid process Scapula ies Pee SSR Cae ore Radius RCS ae | Metacarpal I Large multangular Humerus (trapezium) Navicular (scaphoid) Lunate (semilunar) Small multangular (trapezoid) Metacarpal IV Ba Capitate (os magnum) -| Triquetral (cuneiform) Hamatate (unciform) log Fic. 181.—Cartilages of left upper extremity of a human embryo of 17 mm. Hagen. In the region of the shoulder girdle a plate of cartilage appears in the em- bryonic connective tissue which lies among the developing muscles dorso-lateral to the thorax. This plate of cartilage is the forerunner of the scapula, and in general resembles it in shape. During the eighth week of foetal life a single center of ossification appears and gives rise to the body and spine of the scapula. After birth certain accessory centers appear and produce the coracoid process, the supraglenoidal tuberosity, the acromion process, and the inferior angle and verte- bral margin (Fig. 182). Later the supraglenoidal fuses with the coracoid and forms part of the wall of the glenoid cavity. About the seventeenth year the single center formed by the union of these two fuses with the rest of the scapula. 204 TEXT-BOOK OF EMBRYOLOGY. At the age of twenty to twenty-five years all the other accessory centers unite with the rest of the scapula to form the adult bone. There are two views concerning the development of the clavicle: one that it is of intracartilaginous origin, the other that it is of intramembranous origin. Ossification begins during the sixth week, possibly from two centers. It is true that the cartilage that appears around the centers is of a looser character than the ordinary embryonic cartilage, but whether the centers appear i cartilage seems not to have been determined. At the age of fifteen to twenty years a sort of secondary center appears at the sternal end of clavicle and fuses with the body about the twenty-fifth year. The humerus, radius and ulna are preformed in cartilage (Fig. 181) and develop as typical long bones. Ossification begins in each during the seventh Coracoid process Glenoidal fossa Bone Cartilage’ —— an ne ov ue” Fic, 182.—Scapula of new-born child, showing primary center of ossification, and cartilage (lighter shading) in which secondary centers appear. Bonnet, week at a single center and proceeds in both directions to form the shaft. During the first four years after birth epiphyseal centers appear for the head, greater and smaller tubercles, trochlea and epicondyles. All these secondary centers unite with the shaft of the humerus when the growth of the individual ceases. In the case of the radius and ulna a secondary center appears at each end of each bone to form the epiphysis; and in the ulna another secondary center appears to form the olecranon. (For the growth of bones, see page 180). The carpal bones are all preformed in cartilage (Fig. 181) but their develop- ment is somewhat complicated owing to the fact that pieces of cartilage appear which subsequently may disappear, or ossify and become incorporated in other bones. Primarily seven distinct pieces of cartilage develop and become ar- ranged transversely in two rows; these represent seven of the carpal bones. The proximal row consists of three large pieces which are the forerunners of the navicular (radial, scaphoid), lwnate (intermediate, semilunar) and ¢riquetral THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 205 (ulnar, pyramidal, cuneiform). The distal row is composed of four elements which are the forerunners of the large multangular (trapezium), small multangu- lar (trapezoid), capitate (os magnum), and hamatate or hooked (unciform). In addition to the cartilages mentioned, several others also appear in an inconstant way in different individuals. ‘Two of these are important. One appears on the ulnar side of the proximal row and is the forerunner of the pisiform; the other is situated between the two rows and may either disappear entirely or fuse with the navicular. Ossification does not begin in the carpal cartilages until after birth; it begins in the hamatate and capitate during the third year, in the Phalanges Metacarpals Large multangular Capitate Hamatate Navicular Triquetral Lunate Radius Ulna ese Fic. 183.—Skiagram of right hand of 5 year old girl. (Courtesy of Dr. Edward Leaming). The ossification centers are indicated by the darker areas. others at later periods, and is completed only when the growth of the individ- ual ceases. The fact that the hamatate ossifies from two centers indicates that it is probably derived phylogenetically from two bones. Comparative anatomy teaches that the accessory cartilages in the human wrist are repre- sentatives of structures which are normally present in the lower forms. The metacarpals and phalanges are preformed in cartilages which correspond in shape to the adult bones. A center of ossification appears in each cartilage and produces the shaft of the bone. Only one epiphysis develops on each metacarpal and phalanx. In each metacarpal it develops at the distal end, TEXT-BOOK OF EMBRYOLOGY. 206 Vi / ao / { § / , Crural nerve ig 5 — Pubic bone (cartilage) ‘g i — Obturator nerve Ischium Ischiadic nerve Fic. 184.—Cartilage of right side of pelvic girdle of a human embryo of 13.6 mm. (5 weeks). Petersen. The numerals indicate the vertebrie; the first sacral being opposite the ilium. i>. f pote ber y i peo ) 7 Pi Tlium pa. tars ee, \ { \ P——_s Crur: re \ eh Crural nerve \ WA ~ eed \ , \ = \ ‘ \ + t.——— Pubie bone (cartilage) \ : \ & 42 | . y \ ie ) ee ) \ ea \ }7> ot 2 nerve \ 1 \ e ey Jbturator nerve X a4 cet eee ) Se) aad : \ ~ Ischium “\ ~ Vn \ X \ Ischiadic nerve As \ \ S chips Fic. 185.—Cartilage of right side of pelvic girdle of a human embryo of 18.5 mm. (8 weeks). Petersen. The numerals indicate the vertebra; the first and second sacral being opposite the ilium. Compare with Fig. 184. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 207 except in the thumb where it appears at the proximal end. In each phalanx it develops at the proximal end (Fig. 183). The skeletal elements of the lower extremities, including the pelvic girdle, are of intracartilaginous origin. Each hip bone (os coxe, innominate bone) is pre- formed in cartilage which, in a general way, resembles in shape the adult bone. The ventral part of the pubic cartilage does not at first join the zschial; but by the eighth week the junction is complete, leaving dorsal to it the obturator foramen. In the earliest stages the long axis of the cartilage is nearly at right angles to the vertebral column, and the ilium lies close to the fifth lumbar and first sacral vertebre; later (eighth week) the long axis lies nearly parallel with the vertebral column and the whole cartilage has shifted so that the ilium is associated with the first three sacral vertebre (Figs. 184 and 18s). Ischium Pubic bone Acetabulum Cartilage Fic. 186 —Right os coxe (innominate bone) of new-born child. Bonnet. Bone is indicated by darker areas, cartilage by lighter areas. Ossification begins at three centers which correspond to the ilium, ischium and pubis; the center for the ilium appears during the eighth week, the centers for the ischium and pubis several weeks later (Fig. 186). The process of ossifi- cation is slow, and is far from complete at the time of birth, for at that time the entire crest of the ilium, the bottom of the acetabulum and all the region ventral to the obturator foramen are cartilaginous. During the eighth or ninth year the ventral parts of the pubis and ischium become partly ossified, but up to the time of puberty the pubis, ischium and ilium remain separated by plates of car- tilage which radiate from a common center at the bottom of the acetabulum. Soon after this, the three bones unite to form the single os coxe, leaving only the crest of the ilium, the pubic tubercle and the sciatic tuber (tuberosity of the ischium) cartilaginous. In each of these regions an accessory ossification cen- 14 208 TEXT-BOOK OF EMBRYOLOGY. ter appears and finally fuses with the corresponding bone about the twenty- fourth year. The femur, tibia and fibula are preformed in cartilage. In the femur a center of ossification appears about the end of the sixth week and gives rise to the shaft; similar centers appear in the tibia and fibula during the seventh and eighth week, respectively. In the femur a distal epiphyseal center appears shortly before birth, and during the first year after birth a proximal center appears for thehead. These centers do not unite with the shaft until the individ- ual ceases to grow. The great and lesser trochanters also have accessory ossifica- tion centers. In the tibia the center of ossification for the proximal epiphysis appears about the time of birth, the one for the distal during the second year. In Fibula------ ~--2eTibia Calcaneus -*7-7> --Talus Cuboid --- a wowedlasssces Navicular Cuneiform III «--f---F-“f TA. +-b---7--e? Cuneiform I Sa Serias Cuneiform II fide Z ra Metatarsals f---f---"- 9-4 ---7' Fic. 187.—Diagram of cartilages of left leg and foot of human embryo of 17 mm, Hagen. the fibula the epiphyseal centers appear during the second and sixth years after birth. The cartilage of the patella appears during the third or fourth month of foetal life, and ossification begins two or three years after birth. The bones of the tarsus, like those of the carpus, are preformed in pieces of cartilage which are arranged in two transverse rows. The proximal row con- sists of three pieces, one at the end of the tibia (tibial), one at the end of the fibula (fibular), and the third between the two (intermedial). At an early stage the tibial and intermedial fuse to form a single piece of cartilage which corre- sponds to the ¢alus (astragalus) bone. The fibular cartilage corresponds to the calcaneus (os calcis). The distal row is composed of four pieces of cartilage which correspond to the first cuneiform (internal), second cuneiform (middle), third cuneiform (external), and cuboid (Fig. 187). Between the two rows is a THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 209 piece of cartilage which corresponds to the mavicular (scaphoid). Ossification begins relatively late in the metatarsals. A center for the calcaneus appears during the sixth month of foetal life, and one for the talus shortly before birth. Centers appear in the cuboid and third cuneiform during the first year after birth, and in the first cuneiform, navicular and second cuneiform in order during the third and fourth years (Figs. 188 and 189). At the age of puberty ossifica- tion is nearly complete in all the metatarsals. In the talus two centers, cor- responding to the tibial and intermedial, appear, but soon fuse into a single center. Occasionally the intermedial remains separate and forms the trigonum. Calcaneus Tals et OE Ter Cuboid --- £8 Cuneiform III *~f-~--- & Metatarsals Phalanges Fic. 188.—Ossification centers in foot of a child 9 months old. Hasselwander. An accessory center appears in the calcaneus at the insertion of the tendon of Achilles. The metatarsals and phalanges develop in a manner corresponding to the metacarpals and phalanges (of fingers). Ossification begins in the metatarsals about the ninth week, in the first row of (proximal) phalanges about the thirteenth week, in the second row about the sixteenth week and in the third row (distal) about the beginning of the ninth week. Epiphyseal centers ap- pear from the second to the eighth year after birth. Development of Joints. The embryonic connective tissue from which the connective tissues, includ- ing cartilage and bone, are developed, at first forms a continuous mass. When cartilage appears it may form a continuous mass, as in the chondrocranium, or 210 TEXT-BOOK OF EMBRYOLOGY. it may form a numberof distinct and separate pieces, as in the vertebral column, the pieces being united by a certain amount of the undifferentiated embryonic connective tissue. SYNARTHROSIS. Syndesmosis.—When ossification begins at one or more centers, either in cartilage or in embryonic connective tissue, the centers grad- ually enlarge and approach each other, and the bone so formed comes in contact with the bone formed in neighboring centers. (a) In a case where more than one center appears for any single adult bone, they may come in contact and fuse so completely that the line of fusion becomes indistinguishable. (b) In the case of oe \ “=~ Talus (astragalus) Cuneiform II Cuboid ~~ -\- Bae LS o x 2 cee PS pS = ers ‘ DBS re aN Fic. 191.—Longitudinal section of finger of human embryo of 26 mm. (2 months), showing beginning of joint cavity between adjacent ends of phalanges. (Photograph from preparation by Dr. W. C. Clarke.) Fic. 192,—From longitudinal section of finger of child at birth, showing developing joint cavity between adjacent ends of phalanges. The darker portion at each end of the figure indicates the ossification center in the phalanx, the end of the latter (lighter area) being yet cartilagi- nous. The dark bands at each side of the joint indicate developing ligaments. Photograph. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 213 cavity are the most highly differentiated, the cell bodies being large and ap- parently swollen, and there is gradually less differentiation as the distance from the surface increases, until finally they merge with the ordinary type of con- nective tissue cells of the joint capsule (Clarke). The more mobile joints of the body are all representatives of this type. Joint cavity Synovial membrane Fic, 193.—From longitudina! section of finger of child at birth, showing joint cavity and synovial membrane between adjacent ends of the first metacarpal and proximal phalanx. Other description same as in Fig. 192. Photograph. Anomalies. Tue AXIAL SKELETON. Tur VERTEBRE.—The number of cervical vertebra in man is remarkably constant. Cases where the number is but six are extremely rare. The thoracic vertebree may vary in number in different individuals from eleven to thirteen, twelve being the usual number. The /umbar vertebra may vary from four to six, five being the usual number. The sacral vertebre, fused in the adult to form the sacrum, are usually five in number, sometimes four, sometimes 214 TEXT-BOOK OF EMBRYOLOGY. six. Occasionally a vertebra between the lumbar region and sacral region— lumbo-sacral vertebra—possesses both lumbar and sacral characters, one side being fused with the sacrum, the other side having a free transverse process. Variation occurs frequently in the coccygeal vertebre; four and five are present with about equal frequency, more rarely there are only three. The total number of true (presacral) vertebree may be diminished by one or increased by one. In the former case the first sacral is the twenty-fourth ver- tebra, and, if the number of ribs remains normal, there are only four lumbar vertebre. In case the total number is increased by one, the first sacral is the twenty-sixth vertebra, and there are twelve thoracic and six lumbar or thirteen thoracic and five lumbar. From these facts it is seen that variation occurs most frequently in the more caudal portion of the vertebral column—in the lumbar, sacral and coccygeal regions. According to a hypothesis advanced by Rosenberg, the sacrum in the earlier embryonic stages is composed of a more caudal set of vertebre than those which belong to it in the adult, and during development lumbar vertebre are converted into sacral and sacral vertebra into coccygeal. In other words, the hip bone moves headward during development and finally becomes attached to vertebre which are situated more cranially than those with which it was _pri- marily associated. Thischange in the position of the pelvic attachment, and the corresponding reduction in the total number of vertebra, during the develop- ment of the individual (i.e., during ontogenetic development) is believed to correspond to a similar change in position during the evolution of the race (i.e., during phylogenetic development). According to Rosenberg, variation in the adult is due largely to a failure during ontogeny to carry the processes of reduction in the number of vertebra as far as they are usually carried in the race, or to their being carried beyond this point. The coccygeal vertebre apparently represent remnants of the more exten- sively developed caudal vertebra in lower forms. In human embryos of 8 to 16 mm., when the caudal appendage is at the height of its development, there are usually seven anlagen of coccygeal vertebre. During later development this number becomes reduced by fusion of the more distally situated anlagen to the smaller number in the adult. This process of reduction varies in different in- dividuals, so that five or four, rarely three, coccygeal vertebre may be the result. In cases where children are born with distinct caudal appendages there is no good evidence that the number of coccygeal vertebre is increased, although the coccyx may extend into the appendage. THe Rrps.—Occasionally in the adult a rib is present on one side or on each side in connection with the seventh cervical vertebra (cervical rib), or in connection with the first lumbar vertebra (lumbar rib). There seems to be no THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 215 case on record where cervical and lumbar ribs are present in the same individual. The cervical rib may vary between a small piece of bone connected with the transverse process of the vertebra and a well developed structure long enough to reach the sternum. ‘There are also great variations in the size of the lumbar rib. In case the number of ribs is normal, the last (twelfth) may be rudimentary. The eighth costal cartilage not infrequently unites with the sternum. Oc- casionally the seventh costal cartilage fails to fuse with the sternum, owing to the shortening of the latter, but meets and fuses with its fellow of the opposite side in the midventral line. The above mentioned anomalies can be referred back to aberrant develop- ment. Primarily costal processes appear in connection with the cervical, lum- bar and sacral vertebre. Normally these processes fuse with and finally form parts of the vertebre (p. 189). In some cases, however, the seventh cervical or the first lumbar processes develop more fully and form more or less distinct ribs. As an explanation of these variations in the number of ribs, it has been sug- gested that there is a tendency toward reduction in the total number of ribs, and that supernumerary ribs represent the result of a failure to carry the reduction as far as the normal number. Incase the twelfth rib is rudimentary, the reduction has been carried beyond the normal limit. This hypothesis is a corollary to the hypothesis regarding the variations in the number of vertebra. (See under “The Vertebre.’’) THE STERNUM.—Certain anomalous conditions of the sternum can also be explained by reference to development. The condition known as cleft sternum, in which the sternum is partially or wholly divided into two longitudinal bars by a medial fissure, represents the result of a failure of the two bars to unite in the midventral line (p. 189, see also Fig. 168). This is sometimes associated with ectopia cordis (p. 285). The xyphoid process may also be bifurcated or perforated, according to the degree of fusion between the two primary bars (p. 190). Suprasternal bones may be present. They represent the ossified episternal cartilages which have failed to unite with the manubrium (p. 190). Morpho- logically the suprasternal bones possibly represent the omosternum, a bone situated cranially to the manubrium in some of the lower Mammals. Tue Heap SKELETON.—The skull is sometimes decidedly asymmetrical. Probably no skull is perfectly symmetrical. The condition which most fre- quently accompanies the irregular forms of skulls is premature synosteosis or premature closure of certain sutures. The cranial bones increase in size prin- cipally at their margins, and when a suture is prematurely closed the growth of the skull in a direction at right angles to the line of suture is interfered with. Consequently compensatory growth must take place in other directions. Thus if the sagittal suture is prematurely closed and transverse growth prevented, 216 TEXT-BOOK OF EMBRYOLOGY. increase occurs in the vertical and longitudinal directions. This results in the vault of the skull becoming heightened and elongated, like an inverted skiff, a condition known as scaphocephaly. After premature closure of the coronal suture, growth takes place principally upward and gives rise to acrocephaly. In case only one-half the coronal or lambdoidal suture is closed, the growth is oblique and results in plagiocephaly. A suture—the metopic suture—sometimes exists in the medial line between the two halves of the frontal bone, a condition known as metopism. ‘This is due to an imperfect union of the two plates of bone produced by the two centers of ossification in the frontal region (p. 198). Certain malformations in the face region and in the roof of the mouth are brought about by defective fusion or complete absence of fusion between certain structures during the earlier embryonic stages. The maxillary process of the first branchial arch sometimes fails to unite with the middle nasal process (Kolliker’s view, p. 200; see also Fig. 136). The result is a fissure in the upper lip, a condition known as hare lip, which may or may not be accompanied by a cleft in the alveolar process of the maxilla, extending as far as the incisive (palatine) foramen. The same result may be produced by a defective fusion between the middle nasal process and the lateral nasal process (Albrecht’s view, p. 200% see also Fig. 136). Hare lip may be either unilateral (single) or bilateral (double), accordingly as defective fusion occurs on one or both sides, but never medial. Occasionally the palatine process of the maxillary process fails to meet not only its fellow ot the opposite side, but also the vomer (see Fig. 179). The result is a cleft in the hard palate, a condition known as cleft palate. This malforma- tion may be unilateral or bilateral, but not medial. Sometimes the cleft extends into the soft palate where it occupies, however, a medial position. Cleft palate may accompany hare lip, or either may exist without the other, depending upon the degree of fusion between the processes mentioned above. In bilateral hare lip, with or without cleft palate, the incisive (intermaxillary) bone is sometimes pushed forward by the vomer and projects beyond the surface of the face, a condition known as ‘“‘wolf’s snout.” The causes underlying the origin of harelip and cleft palate are very obscure. Tuer APPENDICULAR SKELETON. Tue Humervus.—On the medial side of the humerus, just proximal to the medial condyle, there is not infrequently a small hook-like process directed distally—the supracondyloid process. This process represents a portion of bone which in some of the lower mammals (cat, for example) joins the internal condyle and completes the supracondyloid foramen, through which the median nerve and brachial artery pass. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 217 THE CarPaL Bones.—Occasionally an os centrale is present in addition to the usual carpal bones. It is situated on the dorsal side of the wrist between the navicular, capitate and small multangulum. In the embryo an additional piece of cartilage is of constant occurrence in this location, but usually disappears during later development; in cases where it persists, ossification takes place to form the os centrale. In some of the apes the os centrale is of constant occurrence: in the adult. THe FEemur.—The gluteal tuberosity (ridge) sometimes projects like a comb, forming the so-called third trochanter, a structure homologous with the third trochanter in the horse and some other mammals. THE TarsaL Bones.—Cases have been recorded in which the total number of tarsal bones was reduced, owing to congenital synosteosis (fusion) of the calcaneus (os calcis) and scaphoid (navicular), of the talus (astragalus) and calcaneus, or of the talus and scaphoid. Occasionally an additional bone—the trigonum—is present at the back of the talus. In the embryo, the talus ossifies from two centers which normally fuse at an early stage into a single center. The trigonum probably represents a bone produced by one of the centers which has remained separate. PotypactyLy.—This anomaly consists of an increase in the number of fingers or toes, or both. Any degree of variation may exist from a supernum- erary finger or toe to adouble complement of fingers ortoes. Thecauses under- lying the origin of such anomalies are not clear. Some assign the supernumer- ary digits to the category of pathological growths or neoplasms, linking them with partial duplicate formations. Others explain the extra digits on the ground of atavism or reversion to an ancestral type. The latter explanation assumes an ancestral type with more than five digits. But neither zodlogy nor paleon- tology has found any vertebrate form, above the Fishes, which normally pos- sesses more than five digits on each extremity. Consequently one must refer to the Fishes for some ancestral type to explain the existence of more than five digits. Going back so far in phylogenetic history, no certainty whatever can be attached to the origin of supernumerary digits, for it is not even known from what fins the extremities of the higher forms are derived. Still another view regarding the origin of supernumerary digits is that they are due to certain ex- ternal influences among which the most important is the mechanical impression of amniotic folds or bands. This, however, could not be the sole cause of polydactylism, since such malformations are common in amphibian embryos where no amnion is present. PRACTICAL SUGGESTIONS. Embryonic Connective Tissue.—The most easily obtained material for the study of the development of embryonic connective tissue is the chick embryo. From about the beginning of the second day to the end of the third day of incubation, the differentiation of the meso- 218 TEXT-BOOK OF EMBRYOLOGY. dermal cells, the formation of the primitive segments, and the further differentiation of the primitive segments into sclerotomes and myotomes may be studied in successive stages. The embryos are removed from the egg, fixed in Flemming’s fluid or Zenker’s fluid, sectioned transversely in paraffin, stained with Heidenhain’s haematoxylin and mounted in xylol damar (see Appendix). A counterstain with a weak solution of fuchsin (o.s per cent. in distilled water) is of value in studying the fibers. This stain is used after the hematoxylin, and the sections are then rinsed in distilled water before passing them into alcohol. Sections of the umbilical cord of any mammalian embryo, prepared by the above technic, are also very instructive. Primitive Segments.—The primitive segments, from which the vertebre, etc., are derived, can be studied in transverse sections of chick embryos during the second and third days of incubation. The material is prepared as described above under the head of embryonic connective tissue. For a comprehensive picture of the series of primitive segments, sagittal sections should also be prepared. Blastemal Stage of the Skeletal System.—Pig embryos of 12 to 14 mm. are fixed in Zenker’s fluid or in Bouin’s fluid, cut transversely in celloidin or paratiin, stained with Weigert’s hematoxylin and eosin, and mounted in xylol-damar (see Appendix). The anlagen of the vertebra: appear as condensations in the embryonic connective tissue. Similar condensa- tions in the extremities indicate the anlagen of the appendicular skeleton, and in the region of the base of the skull the anlage of the chondrocranium. By the use of serial sections, and the method of plastic reconstruction, models of the anlagen of the bones may be made. (For method of reconstruction, see p. 638.) Cartilaginous Stage of the Skeletal System.—Pig embryos of about 35 mm., prepared as de- scribed above under the head of the Blastemal Stage, show the cartilaginous elements which precede true bone in the vertebra, ribs, base of the skull, and certain portions of the appen- dicular skeleton. In order to get a comprehensive idea of the relation of the cartilaginous elements to one another, it is often necessary to make models of parts or of the whole of the cartilaginous skeleton by the method of plastic reconstruction (p. 638). Serial sections are of course necessary for this. Stage of Ossification. (a) Intramembranous Bone.—Small pieces, including the skin and dura mater, are removed from the skull cap of a four-month human feetus or of a four-inch pig embryo, fixed in Orth’s fluid, hardened for several days in alcohol, decalcified in 1 per cent. hydrochloric acid (several days), washed in running water to remove the acid, sectioned at right angles to the surface in celloidin or paraffin, stained with Weigert’s hematoxylin and picro-acid-fuchsin, and mounted in xylol-damar. (b) Intracartilaginous and Subperiosteal Bone.—Remove the forearms and legs of foetal pigs of five to six inches, and prepare by the technic given in the preceding paragraph, cutting the sections parallel to the long axes of the developing bones. In preparing specimens for the study of either intramembranous or intracartilaginous de- velopment, both fixation and decalcification can be done at the same time. Put the fresh material in Bouin’s picro-formalin-acetic mixture and let it remain for a week, with one or two changes of the fluid; then wash in several changes of alcohol (40 to 50 per cent.). A few drops of ammonia added to the alcohol facilitates the removal of the picric acid. Further treatment is the same as after any ordinary fixation. The results are good. The earlier stages in the ossification of the vertebra and the base of the skull can be studied in pig embryos of 40 to 50 mm. The embryos are fixed, and at the same time decalci- fied (see preceding paragraph), in Bouin’s fluid. They are sectioned transversely in THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 219 celloidin, stained with Weigert’s hematoxylin and eosin and mounted in xylol-damar (see Appendix). Ossification of the vault of the skull can be studied in pig embryos of 40 mm. and longer by means of the technic described under Intramembranous Bone (p. 218). Ossification of the appendicular skeleton can be studied in the extremities of pig embryos of three inches and longer, by means of the technic for Intracartilaginous Bone (p. 218). Models of parts or of the whole of the osseous skeleton can be made by the method of plastic reconstruction. For this, of course, serial sections are necessary (see p. 638). Transparent Preparations.—A method which renders the tissues more or less transparent and differentiates the bone is extremely useful in studying the development of the osseous skeleton. Embryos of any kind are put into strong alcohol and left indefinitely—the longer the better. They become much shrunken, but that is not injurious. They are then put into a 3 per cent. aqueous solution of potassium hydrate (KOH). If the solution becomes colored by the pigment from the blood it should be changed as often as necessary. The embryos become quite transparent in a short time, a few days or more, depending upon their size. They are then put carefully into equal parts of glycerin and water for a day or two, then into stronger glycerin, and finally preserved in pure glycerin. The tissues, except bone, are quite transparent; the bone remains white, and the entire osseous skeletal system, so far as it is developed at any particular stage, can be seen clearly. For technic to demonstrate the growth of bones, see small print in the text, page 180. Fat.—F ix bits of tissue from the axilla and groin of a five- or six-inch foetal pig in 1 per cent. osmic acid for twenty-four hours. Wash in running water for several hours and preserve in pure glycerin. ‘Tease small pieces of tissue on a slide and mount in glycerin. References for Further Study. Apotput, H.: Ueber die Variationen des Brustkorbes und der Wirbelsdule des Menschen. Morph. Jahrbuch, Bd. XXIII, 1905. Bape, P.: Die Entwickelung des menschlichen Skeletts bis zur Geburt. Arch. f. mik. Anat., Bd. LV, 1900. BARDEEN, C. R.: Numerical Vertebral Variations in the Human Adult and Embryo. Anat. Anz., Bd. XXV, 1904. BaRDEEN, C. R.: Studies of the Development of the Human Skeleton. American Jour. of Anat., Vol. IV, 1905. BARDEEN, C. R.: The Development of the Thoracic Vertebre in Man. American Jour. of Anat., Vol. IV, 1905. Barrets, M.: Ueber Menschenschwanze. Arch. j. Anthropol., Bd. XII. Bernays, A.. Die Entwickelungsgeschichte des Kniegelenkes des Menschen mit Bemerkungen iiber die Gelenke im allgemeinen. Morph. Jahrbuch, Bd. IV, 1878. Bott, F.: Die Entwickelung des fibrillaren Bindegewebes, Arch. j. mik. Anat., Bd. VIII, 1872. Boxk, L.: Beziehungen zwischen Skelett, Muskulatur und Nerven der Extremitaten, etc. Morph. Jahrbuch, Bd. XXI, 1894. Bonnet, R.: Lehrbuch der Entwickclungsgeschichte. Berlin, 1907. Bravs, H.: Die Entwickelung der Form der Extremitdten und des Extremitatenskeletts. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. III, Teil IT, 1904. Fawcett, E.: On the Early Stages in the Ossification of the Pterygoid Plates of the Sphenoid Bone of Man. Anat. Anz., Bd. XXVI, 1905. 290 TEXT-BOOK OF EMBRYOLOGY. Fawcett, E.: Ossification of the Lower Jawin Man. Jour. Amer. Med. Assoc., Bd. XLV, 1905. : ese E.: On the Development, Ossification and Growth of the Palate Bone. Jour. of Anat. and Physiol., Bd. XL, 1906. FiemMinc, W.: Die Histogenese der Stiitzsubstanzen der Bindesubstanzgruppe. In Hertwig’s Handbuch der vergleich, u. experiment. Entwickelungslehre der Wirbeltiere, Bd. III, Teil II, 1gor. Fiemminc, W.: Morphologie der Zelle. Ergebnisse der Anat. u. Entwick., Bd. VII, 1897. 5 aaa E.: Alte Probleme und neuere Arbeiten tiber den Wirbeltierschidel. Ergebnisse der Anat. u. Entwick., Bd. X, rgot. Gavpp, E.: Die Entwickelung des Kopfskeletts. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. Ill, Teil I, 1905. GEGENBAUR, C.: Die Metamerie des Kopfes und die Wirbeltheorie des Kopfskeletts. Morph. Jahrbuch, Bd. XIII, 1887. GREFENBERG, E.: Die Entwickelung der Knochen, Muskeln und Nerven der Hand und der fiir die Bewegungen der Hand bestimmten Muskeln des Unterarms. Anat. Hefte, Heft XC, 1905. Hacen, W.: Die Bildung des Knorpelskeletts beim menschlichen Embryonen. Arch. f. Anat. u. Physiol., Anat. Abth., 1900. HANSEN, C.: Ueber die Genese einiger Bindegewebsgrundsubstanzen. Anat, Anz., Bd. XVI, 1899. HASSELWANDER, A.: Untersuchungen tiber die Ossification des menschlichen Fuss- skeletts. Zeitschr. f. Morphol. u. Anthropol., Bd. V, 1903. HeErtwic, O.: Lehrbuch der Entwickelungsgeschichte des Menschen u. der Wirbeltiere. Jena, 1906. Jaxopy, M.: Beitrag zur Kenntniss des menschlichen Primordialcraniums. Arch. f. mik. Anat., Bd. XLIV, 1894. KerBeEL, F.:; Ueber den Schwanz des menschlichen Embryo. Arch. f. Anat. u. Physiol., Anat. Abth., 1891. KEIBEL, F.: Zur Entwickelungsgeschichte der Chorda bei Saugern. Arch. j. Anat. u. Physiol., Anat. Abth., 1889. Ke1BEL, F., and MAtt, F. P.: Manual of Human Embryology, Vol. I, 1910. Chap. XI. KyELLBERG, K.: Beitrage zur Entwickelungsgeschichte des Kiefergelenks. Morph. Jahrbuch, Bd. XXXII, 1904 Kotimann, J.: Entwickelung der Chorda dorsalis bei dem Menschen. Avat. Anz., Bd. V, 1890. Kotimann, J.: Lehrbuch der Entwickelungsgeschichte des Menschen. Jena, 18098. KoLiMann, J.: Handatlas der Entwickelungsgeschichte des Menschen. Jena, 1907. Matt, F. P.: The Development of the Connective Tissues from the Connective-tissue Syncytium. American Jour. of Anat., Bd. I, 1902. Matt, F. P.: On Ossification Centers in Human Embryos less than One Hundred Days Old. American Jour. of Anat., Bd. V, 1906. McMorricu, J. P.. The Development of the Human Body. Philadelphia, 1907. Paterson, A.: The Sternum: Its early Development and Ossification in Man and Mammals. Jour. of Anat. and Physiol., Vol. XXXV, got. PETERSEN, H.: Untersuchungen zur Entwickelung des menschlichen Beckens. Arch. j. Anat. u. Physiol., Anat. Abth., 1893. Rast, C.: Theorie des Mesoderms. Morph. Jahrbuch, Bd. XV, 1889. THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 221 RosENBERG, E.: Ueber die Entwickelung der Wirbelséule und das Centrale carpi des Menschen. Morph. Jahrbuch, Bd. I, 1876. SCHAUINSLAND, H.: Die Entwickelung der Wirbelsiiule nebst Rippen und Brustbein. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. III, Teil TI, 1905. SputeR, A.: Beitriige zur Histologie und Histogenese der Binde-und Stiitzsubstanz. Anat. Hejte, Heft XXI, 1896. Tuitenius, G.: Untersuchungen iiber die morphologische Bedeutung accessorischer Elemente am menschlichen Carpus (und Tarsus). Morph. Arbeiten, Bd. V, 1896. Tuomson, A.: The Sexual Differences of the Foetal Pelvis. Jour. of Anat. and Physiol., Vol. XXXTII, 1899. Tornier, G.: Das Entstehen der Gelenkformen. Arch. f. Entw.-Mechanik, Bd. I, 1895. WALDEYER, W.: Kittsubstanz und Grundsubstanz, Epithel und Endothel. Arch. j. mk. Anat., Bd. LVII, 1900. Weiss, A.: Die Entwickelung der Wirbelsiule der weissen Ratte, besonders der vorder- sten Halswirbel. Zetischr. j. wissensch. Zool., Bd. LXIX, igor. ZIMMERMANN, K.: Ueber Kopfhéhlenrudimente beim Menschen. Arch. f. mik. Anat., Bd. LII, 1899. CHAPTER xX. THE DEVELOPMENT OF THE VASCULAR SYSTEM. THE BLOOD VESSELS AND BLOOD. Inasmuch as the blood vessels form such an extensive and extremely com- plicated system, it would be obviously beyond the scope of this book to con- sider the details of the development of all parts of this system. A somewhat detailed discussion of the heart and of the larger vascular trunks will be given but in the case of the smaller vessels very brief statements must suffice. The Heart. Despite the fact that in the adult the heart is so intimately associated with the blood vessels and blood, it begins to develop quite independently, and is said to begin to beat before the vessels containing the blood are connected with it. It is one of the first organs to appear, and at a very early stage assumes the function which it maintains throughout the life of the organism. The primitive heart is a very simple structure, consisting merely of a tube whose wall is capable of contractile activity. One end of this tube is attached to the arterial system and the other to the venous system. This simple form is elaborated more and more during development until it reaches the complicated structure characteristic of the adult. The heart has a peculiar origin in that it arises as two separate parts or anlagen which unite secondarily. In the chick, for example, it appears during the first day of incubation, at a time when the germ layers are still flat. The coelom in the cephalic region becomes dilated to form the so-called primitive pericardial cavity (parietal cavity), and at the same time a space appears on each side, not far from the medial line, in the mesodermal layer of the splanchno- pleure (Fig. 194). These spaces at first are filled with a gelatinous substance in which lie a few isolated cells. These cells then take on the appearance of en- dothelium and line the cavities, and the mesothelium in this vicinity is changed into a distinct, thickened layer of cells. Now by a bending ventrally of the splanchnopleure the cavities or vessels are carried toward the midventral line (Fig. 194). The bending continues until the entoderm of each side meets and fuses with that of the opposite side, thus closing in a flat cavity—the fore-gut. The entoderm ventral to the cavity breaks away and allows the medial walls of the two endothelial tubes to come in contact. These walls then break away and the tubes are united in the midventral line to form a single tube (Fig. 194), 222 THE DEVELOPMENT OF THE VASCULAR SYSTEM. 223 which extends longitudinally for some distance in the cervical region of the embryo. The mesothelial layers of opposite sides meet dorsal and ventral to the endothelial tube, forming the dorsal and ventral mesocardium (Fig. 194). In the meantime the cephalic end of the tube has united with the arterial system, Pri. Mp. pericard. cay, SEE Endolheliu. Efeechedion Phary 9x Dors. mesocardivyy Myocardiur ae (Mésoth eliuvm) pericard. ca vity Endoca rdtug C (Endothelium) Fic. 194.—Diagrams showing the two anlagen of the heart and their union to form a single structure; made from camera lucida tracings of transverse sections of chick embryos. In C the ventral mesocardium has disappeared (see text). and the caudal end with the venous system; and ina short time the dorsal and ventral mesocardia disappear and leave the heart suspended by its two ends in the primitive pericardial cavity. The conditions at this point may be sum- marized thus: The heart is a double-walled tube—the inner wall composed of 15 224 TEXT-BOOK OF EMBRYOLOGY. endothelium and destined to become the endocardium,. the outer wall of a thicker mesothelial layer and destined to become the myocardium—the two walls separated by a considerable space. The organ hangs, as it were, in the primi- tive pericardial cavity (coelom), connected at its cephalic end with the ventral aortic trunk and at its caudal end with the omphalomesenteric veins. In all Mammals thus far studied the principle of development in the earlier stages is essentially the same as in the chick. The double origin of the heart is even more marked because of the relatively late closure of the fore-gut. There are no observations on the origin of the heart in human embryos, but it is Dorsal aortic root Gut (pharynx) Dorsal mesocardium Pericardial cavity (caclom) Endocardium (endothelium) Myocardium Fic. 195.—Transverse section of a human embryo of 2.69 mm. von Spee, Kollmann’s Atlas. reasonable to assume that it has the same double origin as in other Mammals, although in embryos of 2 to 3 mm. the organ has already become a single tube (Figs. 195 and 196). At this stage the tube is somewhat coiled. The origin of the endothelium of the heart (endocardium) is not known with certainty. Some investigators in their researches among the lower Vertebrates have suggested that itis derived primarily from the entoderm; others have sug- ° gested a possible derivation from both entoderm and mesoderm; still others hold that it is derived from the mesenchyme (mesoderm). It seems to be undis- puted, however, that the muscular wall of the heart (myocardium) is a derivative of the mesothelium (mesoderm). THE DEVELOPMENT OF THE VASCULAR SYSTEM. 225 _ While the double origin of the heart is characteristic of all amniotic Vertebrates (Reptiles, Birds, Mammals), in all the lower forms the organ arises as a single anlage. In the region of the fore-gut the two halves of the ccelom are separated by a ventral mesentery which extends from the gut to the ventral body wall, and which is composed of two layers of mesothe- lium with a small amount of mesenchyme between them. In the mesenchyme a cavity Oral fossa ——_ Ventral aortic, _ trunk Ventricle——~ is | ——. Ventricle Atrium Ant. cardinal vein __! Diaphragm — Duct of Cuvier — Duct of Cuvier Umbilical vein — -— Liver | -— Duct of liver Fic, 196.—Ventral view of reconstruction of human embryo of 2.15 mm. His. The ventral body wall has been removed. The vessels (in black) at the sides of the duct of the liver are the omphalomesenteric veins. appears and is lined by a single layer of flat (endothelial) cells. This cavity extends longi- tudinally for some distance in the cervical region and with its endothelial and mesothelial walls constitutes the simple cylindrical heart. On the dorsal side it is connected with the gut by a portion of the mesentery which is called the dorsal mesocardium; on the ventral side it is connected with the ventral body wall by the ventral mesocardium (Fig. 197). Thus Entoderm Mesoderm (visceral) Dorsal mesocardium Endothelium Mesoderm (parietal) Heart - Pericard. cavity (ccelom) Ventral mesocardium Ectoderm Fic. 197.—Ventral part of transverse section through the heart region of Salamandra maculosa embryo with 4 branchial arches, - Rabi. the heart is primarily a single structure.. The difference between the two types of develop- ment is not a fundamental one but simply depends upon the difference in the germ layers. In the lower forms the germ layers are closed in ventrally from the beginning, and the heart appears in a medial position. In the-higher forms the germ layers for a time remain spread out upon the surface of the yolk or yolk sac, and the heart begins to develop before they close in on the ventral side of the embryo. Consequently the heart arises in two parts which are carried ventrally by the germ layers and unite secondarily. 226 TEXT-BOOK OF EMBRYOLOGY, The further development of the heart consists of various changes in the shape of the tube and in the structure of its walls. At the same time the dila- tation of the ccelom (primitive pericardial cavity) in the cervical region is of im- portance in affording room for the heart to grow. In the chick, for example, the tube begins, toward the end of the first day of incubation, to bend to the right; during the second day it continues to bend and assumes an irregular S-shape. This bending process has not been observed in human embryos, but other Mammals show the same process as the chick. In a human embryo of 2.15 mm. the S-shaped heart is present (Fig. 196). The venous end, into which the omphalomesenteric veins open, is situated somewhat to the left, ex- tends cranially a short distance and then passes over into the ventricular portion. The latter turns ventrally and extends obliquely across to the right side, then bends dorsally and cranially to join the aortic bulb which in turn joins the ventral aortic trunk in the medial line. The endothelial tube, which is still separated from the muscular wall by a considerable space, becomes somewhat Vent. aortic trunk Ventricular portion ff Fic. 198.—Ventral view heart of human embryo of 4.2mm. His, The atria are hidden behind the ventricular portion. constricted at its junction with the aortic bulb to form the so-called fretum Halleri. During these changes the heart as a whole increases in diameter, especially the ventricular portion. Gradually the venous end of the heart moves cranially and in embryos of 4.2 mm. lies in the same transverse plane as the ventricular portion. The latter lies transversely across the body (Fig. 198). At the same time two evaginations appear on the venous end, which repre- sent the anlagen of the atria. In embryos of about 5 mm. further changes have occurred, which are represented in Fig. 199. The two atrial anlagen are larger than in the preceding stage and surround, to a certain extent, the proxi- mal end of the aortic trunk. As they enlarge still more in later stages, they come in contact, their medial walls almost entirely disappear, and they form a single chamber. The ventricular portion of the heart becomes separated into a right and a left part by the interventricular furrow (Fig. 199); the right part is the anlage of the right ventricle, the left part, of the left ventricle. At the same time the atrial portion has moved still farther cranially so that it lies to the cranial THE DEVELOPMENT OF THE VASCULAR SYSTEM. 227 side of the ventricular portion. The venous and arterial ends of the heart have thus reversed their original relative positions. At this point it should be noted that the atrial end of the heart is connected with the large venous trunk formed by the union of the omphalomesenteric veins and the ducts of Cuvier— the sinus venosus. During the changes in the heart as a whole, certain changes also occur 1n the endothelial and muscular walls. The walls of the atria are composed of com- pact plates of muscle with the endothelium closely investing the inner surface. The walls of the ventricular portion, on the other hand, become thicker and are composed of an outer compact layer of muscle and an inner layer made up of Right atrium’ Left atrium Right ventricle # Left ventricle Interventricular furrow Fic. 199.—Ventral view of heart of human embryo of 5mm. His. trabecule which are closely invested by- the endothelium. Everywhere the endothelium is closely applied to the inner surface of the myocardium, the space which originally existed between the endothelium and mesothelium being obliterated. The embryonic heart in Mammals in the earlier stages resembles that of the adult in the lower Vertebrates (Fishes). The atrial portion receives the blood from the body veins and conveys it to the ventricular portion which in turn sends it out through the arteries to the body. The circulation is a single one. This condition changes during the fcetal life of Mammals with the development of the lungs. The same transition occurs in the ascending scale of development in the vertebrate series in those forms in which gill breathing is replaced by lung breathing. ‘The change consists of a division of the heart and circulation, so that the single circulation becomes a double circulation. In other words, the heart is so divided that the lung (pulmonary) circulation is separated from the general circulation of the body. This division first appears in the Dipnoi (Lung Fishes) and Amphibians in which gill breathing stops and lung breathing begins, although here the division is not complete. In. Reptiles the division is complete except for a small direct communication between the ventricles 228 TEXT-BOOK OF EMBRYOLOGY. Fig. 200 represents the dorsal half of the heart at a stage when all the chambers are in open communication, and shows the conditions in a single cir- culation but with the beginning of a separation. The atria are rather thin- walled chambers, the ventricles have relatively thick walls. Between the Septum spurium Atrial septum (septum superius) Opening of sinus venosus Right atrium Left atrium Atrio-ventricular canal Right ventricle Ventricular septum Left ventricle Fic. 200,—Dorsal half of heart (seen from ventral side) of a human embryo of romm, His, atrial and ventricular portions is a.canal—the atrio-ventricular canal—which affords a free passage forthe blood. From the cephalic side of the atrial por- tion a ridge projects into the cavity. This ridge represents a remnant of the original medial walls of the two atria and marks the beginning of the future Septum: stiperius ~- oes] see tee UE arsieecsia) Sn ee een nnn Foramen ovale Sinus venosus —------- = [eae 4 ----> Atrial septum \ “v7 > Left atrium Valvule venose Right atrium J : --~ Atrio-ventricular valves Right ventricle enante ene Atrio-ventricular canals Ventricular septum : ----- Left ventricle oe Fic. 201.—Dorsal half of heart showing chambers and septa. (Semidiagrammatic.) Modified from Born, atrial septum. The opening of the sinus venosus is seen on the dorsal wall of the right atrium. Primarily both atria communicated directly with the sinus venosus, but in the course of development the opening of the latter migrated to the right and at this stage is found in the wall of the right atrium. The opening THE DEVELOPMENT OF THE VASCULAR SYSTEM. 229 is guarded, as it were, by a lateral and a medial fold the significance of which will be described later. The ventricular portion also shows a ridge projecting from the caudal side, which corresponds to the interventricular groove and represents the beginning of the ventricular septum. The Septa.—The further changes are largely concerned with the separation of the heart into right and left sides, and with the development of the valves. The atria become separated by the further growth on the cephalic side, of the ridge which has already been mentioned and which is known as the sepium superius (Figs. 200 and 201). This septum grows across the cavity of the atria until it almost reaches the atrio-ventricular canal, forming the septum atriorum. A portion of the septum then breaks away, leaving the two atria still in com- Sinus venosus \_ y Atrial septum ~ / Pulmonary vein Left valvula venosa —--F-=}- gag oe rai a i X i Right valvula venosa ~-~-:ghesteens seen mener tte : ' ee eh —— Teft atrium coum! ? P a Left atrio- F reateiele Rok = Right veatrele ventricular canal Right atrigg —_* Nipeurs st eo ventricular cana Tyee, / . ; 3° — Left ventricle / Right ventricle ~ ‘ i Interventricular furrow Ventricular septum Fic. 202.—Dorsal half of heart (ventral view) of rabbit embryo of 5.8 mm. Born, munication. This secondary opening is the foramen ovale which persists throughout foetal life but closes soon after birth. The atrio-ventricular canal also becomes divided into two passages by a ridge from the dorsal wall and one from the ventral wall uniting with each other and finally with the septum atriorum (Fig. 201). ‘Thus the two atria would be completely separated if it ‘were not for the foramen ovale. During the separation of the atria, a division of the ventricular portion of the heart also occurs. On the caudal side of the ventricular portion a septum ap- pears and gradually grows across the cavity forming the sepium ventriculorum (Figs. 200 and 201). This septum is situated nearer the right side and is in- dicated on the outer surface by a groove which becomes the sulcus longitudinalis 230 TEXT-BOOK OF EMBRYOLOGY. anterior and posterior. The dorsal edge of this septum finally fuses with the septum dividing the atrio-ventricular canal, but for a time its ventral edge re- mains free, leaving an opening between the two ventricles (Figs. 202 and 203). This opening then becomes closed in connection with the division of the Aorta *---------- ao Pulmonary artery Aortic septum .--______# y Interventricular opening ----—4— --- Aorta Right atrio-ventricu- u -_s -. Left atrio-ventricular orifice lar orifice ~ 7 Right ventricle ~~) ~ Left ventricle Ventricular septum ~- + Fic, 203.—Ventricles and proximal ends of aorta and pulmonary artery of a 7.5 mm. human embryo. Lower walls of ventricles have been removed. Kollmann’s Atlas. aortic bulb and ventral aortic trunk. On the inner surface of the aortic trunk, at a point where the branches which form the pulmonary arteries arise, two ridges appear, grow across the lumen and fuse with each other, thus dividing the vessel into two channels. ‘This partition—the septum aorticum (Fig. 204)— gradually grows toward the heart through the aortic bulb and finally unites with Fic. 204.—Diagrams representing the division of the ventral aortic trunk into aorta and pulmonary artery and the development of the semilunar valves. Hochstetter. the ventral edge of the ventricular septum, thus closing the opening between the two ventricles. Corresponding with the edges of the septum aorticum, a groove appears on each side of the aortic trunk and gradually grows deeper and ex- tends toward the heart, until finally the trunk and aortic bulb are split longitudi- THE DEVELOPMENT OF THE VASCULAR SYSTEM. 231 nally into two distinct vessels, one of which is connected with the right ventricle and becomes the pulmonary artery, the other with the left ventricle and becomes the proximal part of the aortic arch (Fig. 203). The result of the formation of these various septa is the division of the entire heart into two sides. The atrium and ventricle of each side are in communication through the atrio- ventricular foramen, the two sides are in communication only by the fora- men ovale which is but a temporary opening. After the opening of the sinus venosus is shifted to the right atrium, the left atrium for a short period has no vessels opening into it. As soon, however, as the pulmonary veins develop, they form a permanent union with the left atrium (Fig. 202). At first two veins arise from each lung, which unite to form a single vessel on each side; the two single vessels then unite to form a common trunk which opens into the left atrium on the cephalic side. As development pro- ceeds, the wall of the single trunk is gradually absorbed in the wall of the atrium, until the single vessel from each side opens separately. Absorption continu- ing, all four veins, two from each lung finally open separately. This is the con- dition usually found in the adult. A partial failure in the absorption may leave one, two, or three vessels opening into the atrium. Such variations are not infrequently met with in the pulmonary veins. The Valves.—If all the passageways between the different chambers of the heart and the large vascular trunks were to remain free and clear, there would be nothing to prevent the blood from flowing contrary to its proper course. Consequently five sets of valves develop in relation to these orifices, and are so arranged that they direct the blood in a certain definite direction. These ap- pear (a) at the openings of the large venous trunks into the right atrium, (b) at the opening between the right atrium and right ventricle, (c) at the opening between the left atrium and left ventricle, (d) at the opening between right ventricle and pulmonary artery and (e) at the opening between the left ven- tricle and aorta. No valves develop at the openings of the pulmonary veins into the left atrium. (a) The sinus venosus (which is formed by the union of the large body veins) opens into the right atrium on its cranial side, as has already been mentioned (p. 228). By a process of absorption, similar to that in the case of the pul- monary veins, the wall of the sinus is taken up into the wall of the atrium. The result is that the vena cava superior, vena cava inferior, and sinus coronarius (a remnant of the left duct of Cuvier) open separately into the atrium. As the sinus is absorbed, its wall forms two ridges on the inner surface of the atrium, one situated at the right of the opening and one at the left (Figs. 201 and 202). These two ridges—valvule venose—are united at their cranial ends with the septum spurium (Fig. 200), a ridge projecting from the cephalic wall of the atrium. The septum spurium probably has a tendency to draw the two valves 232 TEXT-BOOK OF EMBRYOLOGY. together and prevent the blood from flowing back into the veins. The left valve and the septum spurium later atrophy to a certain extent and probably unite with the septum atriorum to form part of the limbus fosse ovalis (Vieus- senii). The right valve is the larger and in addition to its assistance in prevent- ing a backward flow of blood into the veins, it also serves to direct the flow to- ward the foramen ovale. As the veins come to open separately, the cephalic part of the right valve disappears; the greater part of the remainder becomes the valuula vene cave inferioris (Eustachii) and during feetal life directs the blood toward the foramen ovale. In the adult it becomes a structure of variable size. A small part of theremainder of the right valve forms the valvula sinus coronarii (Thebesii) which guards the opening of the coronary sinus. (b) and (c) The valves between the atrium and ventricle on each side develop for the most part from the walls of the triangular atrio-ventricular opening (ostium atrio-ventriculare). Elevations or folds appear on the rims of the openings and project into the cavities of the ventricles where they become attached to the muscle trabecule of the ventricle walls (Figs. 205 and 206). On the right side three of these folds appear, and develop into the valvula Valve Cavity of ventricle Muscle trabecule Papillary muscles Trabecule carnez Fic. 205.—Diagrams representing the development of the atrio ventricular valves, chorde tendinez, and papillary muscles. Gegenbaur. tricuspidalis which guards the right atrio-ventricular orifice. On the left side only two folds appear, and these become the valuula biscuspidalis (mitralis) which guards the left atrio-ventricular orifice. These valves, which are at first muscular, soon change into dense connective tissue. The muscle trabecule to which they are attached also undergo marked changes. Some become con- densed at the ends which are attached to the valves into slender tendinous cords —the chorde tendineg, while at their opposite ends they remain muscular as the Mm. papillares; others remain muscular and lie in transverse planes in the ventricles, or fuse with the more compact part of the muscular wall, or form irregular, anastomosing bands and constitute the trabecule carnee (Fig. 205). (d) and (e) The valves of the pulmonary artery and aorta develop at the point where originally the endothelial tube was constricted to form the fretum Halleri (p. 226) where the ventricular portion of the heart joined the aortic THE DEVELOPMENT OF THE VASCULAR SYSTEM. 233 bulb. Before the aortic trunk and bulb are divided into the aortic arch and pulmonary artery, four protuberances appear in the lumen (Fig. 204). The septum aorticum then divides the two which are opposite so that each vessel receives three (Fig. 204). These then become concave on the side away from the heart, in a manner which has not been fully determined, and at the same Dorsal aortic roots Amnion Upper limb bud Atrial septum Right atrium Right atrio- . ventricular Left atrium (tricuspid) valves Right ventricle eieataes ventricular (bicuspid) valves Ventricular septum Left ventricle Pericardial cavity Fic 206,—Transverse section of pig embryo of 14 mm, Photograph. time enlarge so that they close the lumen. Those in the pulmonary artery are known as the valuule semilunares arteri@ pulmonalis, those in the aorta as the valuule semilunares aorie. Changes after Birth.—The migratory changes of the heart from its origi- nal position in the cervical region to its final position in the thorax will be con- sidered in connection with the development of the pericardium (Chap. XIV). ‘With the exception of the septum atriorum, the heart acquires during fcetal life practically the form and structure characteristic of the adult (Fig. 207), Solong 234 TEXT-BOOK OF EMBRYOLOGY. as the individual continues to grow, the heart, generally speaking, increases in size accordingly. This increase takes place by intussusception in the endocardium and myocardium. At the time of birth the two atria are in communication through the foramen ovale which is simply an orifice in the atrial septum (Fig.208). Thus the blood which is brought to the right atrium by the body veins is al- lowed to pass directly into the left atrium, thence to the left ventricle, and thence is forced out to the body again through the aorta. A certain amount of blood also passes from the right atrium into the right ventricle and thence into the pulmonary artery; but this blood does not enter the lungs but passes directly into the aorta through the ductus arteriosus (Fig. 207). After birth the lungs begin Left carotid artery dnnomunate artery ON Neen Palbis ae Ae cote Left subclavian artery Branches of right _ sip JO oe i) Ne ee eee ene Ductus arteriosus pulmonary artery Arch of aorta ------- \. Branches of left Pulmonary artery----- 3 we pulmonary artery Right auricular appendage-- -~- Left auricular appendage Left ventricle Right ventricle -—- Saleen! Descending aorta Fic. 207.—Ventral view of heart of foetus at term. Kollmann’s Atlas. to function and the placental blood is cut off, so that the right atrium receives venous blood only and the left arterial blood only. If the foramen ovale were to persist it would allow a mingling of venous and arterial blood. Consequently the foramen ovale closes soon after birth and the two currents of blood are com- pletely separated. At the same time the ductus arteriosus atrophies and be- comes the ligamentum arteriosum. Consequently there is no direct communica- tion between the pulmonary artery and aorta. Certain features of development have an important bearing on the theories regarding the physiology of the heart, particularly on the theory that the heart is an automatic organ. Whether the theory that the heart beats automatically, ice., independently of stimuli from the nervous system, is true or not, it is a fact that in the embryo it begins to beat before any THE DEVELOPMENT OF THE VASCULAR SYSTEM. 235 nerve cells appear in it and before any nerve fibers are connected with it. At least no technic has yet been devised by which it is possible to demonstrate nerve cells in, or fibers connected with it, at the time when it begins to perform its characteristic function. And, furthermore, at the time when the heart begins to beat, no heart muscle cells are developed. This last fact seems to indicate an inherent contractility in the mesothelial cells which form the anlage of the myocardium. Sup. vena cava~-—-—-- — ee ' Inf. vena cava a xn ; ? Pulmonary veins A uy Right atrium~ — seers \ Left atrium Right ventricle .___ K Left ventricle Taf, vena cava Fic. 208.—Dorsal half of foetal heart. Bamm, Kollmann’s Atlas. The Vessels. Origin.—It has already been stated at the beginning of this chapter that the . heart and blood vessels arise independently, and only secondarily come into close relationship. The heart has its origin inside the embryo; the first vessels and first blood cells, on the other hand, have their origin in the extraembryonic area of the germ layers. In the chick embryo toward the end of the first day of incubation, the peripheral part of the area opaca presents a reticulated appear- ance when seen from the surface (Fig. 209). Sections of the blastoderm show that this appearance is due to thickenings in the mesoderm, and that at this stage there is no ccelom in this region (Fig. 210). The thickenings, however, are situated rather nearer the entodermal side, and after the mesoderm splits 236 TEXT-BOOK OF EMBRYOLOGY. a Fic, 209.—Surface views of chick blastoderms. Ruickert, Hertwig a, Blastoderm with primitive streak and head process; showing blood islands (dark spots in crescent- shaped area in lower part of figure). b, Blastoderm with 6 pairs of primitive segments. Reticulated appearance is due to blood islands (dark spots) and to developing vessels the entire reticulated area being the area vasculosa, Ectoderm i) Mesoderm Entoderm 4) (yolk cells) | Blood island Fic, 210.—Section of blastoderm (area opaca) of chick of 27 hours’ incubation. Photograph. THE DEVELOPMENT OF THE VASCULAR SYSTEM. 237 into visceral and parietal layers, come to lie in the visceral (splanchnic) layer (Fig. 211). They arecomposed of masses of cells known as blood islands which are the anlagen of both the blood cells and the endothelium of the blood vessels. The superficial cells of an island become transformed into flat cells which gur- round the remaining cells of the island and form the endothelial walls of a primitive blood vessel (Fig. 211). Anumber of these vessels then anastomose to forma net-work of channels containing, at intervals, groups of cells which repre- sent the central cells of the blood islands, and which are the forerunners of the Colom Parietal mesoderm Ectoderm Visceral mesoderm Blood islands Fic, 211.—Section of blastoderm of chick of 42 hours’ incubation. Photograph. The cells of the blood islands are differentiated into nucleated red blood cells (erythroblasts) and the endo- thelium. of the vessels. : ‘ red blood cells. Around the border of the area vasculosa the vascular channels then unite to form a vessel—the sinus terminalis—which is continuous except at the head end of the embryo (Fig. 212). During the second day of incubation the vascularization of the splanchnic layer of mesoderm gradually extends through the area pellucida toward the embryo (Fig. 212). Some of the channels become larger and form arteries and veins which extend into the embryo and * finally unite with the heart in a definite way (p. 224). The question of the growth of blood vessels is not yet settled. Are they formed by a progressive differentiation of the mesenchymal tissue, or do they grow out as sprouts or buds from vessels already present? It is obvious that the first vessels. in the area opaca are formed im situ by a differentiation of the tissue already present. In the vascularization of the area pellucida, do the new vessels represent independently formed structures which ‘unite secondarily with those of the area opaca, or are they the result of outgrowths from the primary vessels in the area opaca? ‘This same question arises in regard to the for- ‘mation of the first vessels inside the embryonic body, and in fact in regard to the formation 238 TEXT-BOOK OF EMBRYOLOGY. of new vessels so long as any tissue of the body continues to grow and become vascular. In the formation of the larger vascular trunks in the body there is evidence to support the view that the vessels arise independently, for in some cases at least cavities, lined by flat cells appear in the mesenchyme, which are at first quite unconnected, and only secondarily unite to form a continuous vessel. On the other hand, in the case of the formation of granu- lation tissue or any new tissue, including pathological growths, it is usually held that the Fic. 212.—Surface view of chick embryo with 18 pairs of primitive segments, including the area vasculosa. Riickert, Hertwig. The reticulum indicates the blood vessels; the dark spots in the vessels being blood islands The darker line at the border of the figure represents the sinus terminalis. new vessels arise as outgrowths from other vessels: The endothelium of a vessel proliferates and grows out as a slender process which becomes hollow and thus forms a capillary con- tinuous with the original vessel from which it is an outgrowth. Some of the larger channels of the area vasculosa converge to form a single vessel on each side, which enters the embryonic body through the splanchno- pleure and joins the caudal end of the heart (p. 224). These two vessels are known as the omphalomesenteric (vitelline) veins (Fig. 213). Other channels of THE DEVELOPMENT OF THE VASCULAR SYSTEM. 239 the area vasculosa converge to form another pair of vessels, the omphalomesen- teric (vitelline) arteries, which extend into the embryo through the splanchno- pleure until they reach a point ventro-lateral to the notochord, thence extend cranially and caudally as the two primitive aorte. Later as the germ layers close in ventrally, these fuse longitudinally, except in the cervical region, to form the single dorsal aorta. The proximal ends of the omphalomesenteric arteries also fuse into a single trunk which may then be considered as a branch of the aorta. The double portion of the aorta in the cervical region sends a Aortic arches Sinus terminalis Right vitelline vein xu ae Left vitelline artery Right vitelline artery Left vitelline vein Fic. 213.—Diagram of the vitelline (yolk) circulation of a chick embryo at the end of the third day of incubation. Balfour, branch ventrally on each side through each branchial arch, forming the aortic arches. "The aortic arches on each side then unite ventrally to form the ventral aortic root, and the two ventral aortic roots unite in the medial line to form the single ventral aortic trunk which joins the cranial end of the heart (p. 224). Thus the vitelline (or yolk) circulation is completed. And from this time on, the area vasculosa gradually enlarges, as the mesoderm extends farther and farther around the yolk, until finally it surrounds the entire mass of yolk. In Mammals, as in the chick, the vascular anlagen first appear in the extra- 16 240 TEXT-BOOK OF EMBRYOLOGY. Fic, 214.—Surface view of area vasculosa of a rabbit embryo of 11 days. van Beneden and Julin, The vessel around the border is the sinus terminalis; the two large vessels above the embryo are the vitelline (omphalomesenteric) veins; the two large vessels converging below the embryo are the vitelline (omphalomesenteric) arteries. Dors. aortic root _ and aortic arches Ant. cardinal vein — Amnion Post. cardinal Fic. 215.—Human embryo of 3.2 mm. His. The arrows indicate the direction of the blood current. THE DEVELOPMENT OF THE VASCULAR SYSTEM. 241 embryonic area. In the rabbit they begin to develop about the eighth day. A reticulated area appears in the peripheral part of the area opaca and in a short time shows an anastomosing network of channels, in which lie the blood islands. The network then gradually extends toward the embryo and some of the channels converge to form a pair of omphalomesenteric veins and a pair of omphalomesenteric arteries, which behave in the same manner as in the chick Int. carotid artery Vertebral artery —Duct of Cuvier Post. cardinal Vitelline vein vein Vitelline artery Umbilical vein Umbilical arteries Post. cardinal vein Fic, 216.—Reconstruction of a human embryo of 7mm. Mail. Arteries represented in black. A.V., Auditory vesicle; B, bronchus; L, liver; K, anlage of kidney; J, thyreoid gland; IIIJ-XIJ. cranial nerve roots; 1, 2, 3) 4, branchial grooves; x, 8,12, 5 (on spina] nerve roots), 1st and 8th cervical, 12th dorsal, 5th lumbar spinal nerves respectively. Dotted outlines represent limb buds. (Fig. 214). The area vasculosa then gradually enlarges until it embraces the entire yolk sac. “There are no observations on the early formation of the vascular anlagen in the human embryo, but it is reasonable to suppose they appear in the same manner as in other Mammals. In an embryo in which there is no trace even of a neural plate (Peters’ embryo, see p. 90 and Fig. 82), the vascular area 242 TEXT-BOOK OF EMBRYOLOGY. embraces the entire yolk sac, and this fact indicates a precocious development of the vascular anlagen. In an embryo of 3.2 mm. the vitelline circulation is complete. A study of Fig. 215 will assist in understanding the similarities between the early form of circulation in the human embryo and the other forms which have been considered. It is thus seen that the earliest form of circulation in man, as well as in lower forms, is associated with the yolk sac. In animals below the Mammals, where a large amount of yolk is present, the vitelline circulation is of great importance in supplying the growing embryo with nutritive materials. In Mammals, where there is little yolk present, the Gut Yolk stalk Umbilical vein Amnion __ Umbilical artery Umbilical vein Allantois Amnion Chorionic villi Fic, 217.—Diagram of the umbilical vessels in the belly stalk and chorion. Aollmann’s Atlas, vitelline circulation is of short duration and minor importance, yet those por- tions of the vessels inside the embryo play a part in the further development of the vascular system. Again, in Reptiles and Birds, a second circulation, as it were, develops in connection with the allantois, and persists during incubation, since the allantois is a reservoir for the waste products of the body. In Mam- mals, however, the allantois is rudimentary, its place being taken by the pla- centa which establishes the communication between the embryo and the mother; and the vessels which correspond to the allantoic vessels in Reptiles and Birds become associated with the placental circulation. Two arteries, one on each THE DEVELOPMENT OF THE VASCULAR SYSTEM. 243 side, arise from the dorsal aorta in the lumbar region of the embryo and pass out through the belly stalk (later the umbilical cord) to end in the chorionic villi. These are known as the allantoic or umbilical arteries. The blood from the chorionic villi is carried back to the embryo by the umbilical veins which, although a single trunk in the umbilical cord, pass cranially through the body wall, one on each side, and open into the ducts of Cuvier (Figs 216 and 217). The course and distribution of the blood vessels in the placenta have been considered in the chapter on foetal membranes (see p. 131). The Arteries.—The simplest form of the arterial system is as follows: The single dorsal aorta extends from the cervical region to the caudal end of the embryo and is situated in the medial line ventral to the notochord. In the cervical region a branch extends cranially on each side of the medial line, the two forming the dorsal aortic roots. From the latter, other branches arise and Dors. aortic root “-~°""~-- 7777-7 eee A BA Ps : i Nel i Vent. aortic root “XY | Vent. aortic trunk © - Dors. aortic root > “—- @sophagus - Trachea ‘Wigoebre nag ai asians: Pulmonary artery Fic. 218.—From reconstruction of aortic arches (1, 2, 3, 4, 6,) of left side and pharynx of a5 mm. humanembryo. Tandler. I-IV. Inner branchial grooves. pass ventrally, one in each branchial arch, forming the aortic arches. These unite ventrally on each side to form a single vessel, the ventral- aortic root, and the two roots unite to form the single ventral aortic trunk which joins the cranial end of the heart. Somewhat caudal to the middle of the embryo a branch of the aorta passes ventrally through the mesentery as the omphalomes- _enteric artery which enters the umbilical cord. Still farther caudally the paired umbilical (allantoic) arteries are given off from the aorta and pass out into the umbilical cord (Figs. 215, 216, 217). The conditions which exist at this stage in the region of the aortic arches in mammalian embryos are indicative of the conditions which persist as a whole or in part throughout life in the lowest Vertebrates. The changes which occur in Mammals, however, are profound and the adult condition bears no resem- blance to theembryonic. Yet certain features in the adult are intelligible only from a-knowledge of their development. In the human embryo six aortic arches 244 TEXT-BOOK OF EMBRYOLOGY. appear on each side. The first, second, third, and fourth pass through the corresponding branchial arches. The fifth arch, which is merely a loop from the fourth, seems to pass through the fourth branchial arch. The sixth aortic arch passes through the region behind the fourth branchial. All these arches are present inembryosof 5mm. (Fig.218). InFishesand larval Amphibians, where the branchial arches develop into the gills, the aortic arches are broken up into capillary networks which ramify in the gills, and the ventral aortic root becomes the afferent vessel, the dorsal aortic roots the efferent vessels. In the higher Vertebrates and in man the aortic arches begin, at a very early period, to undergo changes; some disappear and others become portions of the large Vent. aortic roots ‘lL ce l i ay yt, II Iv _—— IV v GEN fama g) Vv ; VI «(RL Na TD Ventral aortic trunk Dorsal aortic roots V/ ——"£ mS Subclavian arteries Aorta Ce Fic, 219.—Diagram of the aortic arches of a Mammal. Modified from Hochstetter. arterial trunks which leave the heart. In connection with the following description, constant reference to Figs. 219 and 220 will assist the student in understanding the changes. The first and second arches soon atrophy and disappear. The third arch on each side becomes the proximal part of the internal carotid artery, while the continuation of the dorsal aortic root, cranially to the third arch, becomes its more distal part. The continuation of the ventral aortic root cranially to the third arch, becomes the proximal part of the external carotid artery, while the portion of the ventral aortic root between the third and fourth arches becomes the common carotid artery. The portion of the dorsal aortic root between the THE DEVELOPMENT OF THE VASCULAR SYSTEM 245 third and fourth arches disappears. The fourth aortic arch on the left side enlarges and becomes the arch of the aorta (arcus aorte) which is then continued caudally through the left dorsal aortic root into the dorsal aorta. On the right side, the fourth arch becomes the proximal part of the subclavian artery. Since the third, fourth, fifth, and sixth arches really leave the ventral aortic trunk as a single vessel, it will be seen that these changes bring it about that the common carotid and subclavian on the right side arise by a common stem, the innomi- nate artery, which in turn is a branch of the arch of the aorta. On the left side, Common carotid arteries . i ti ight Int, carotid artery (right) Int. carotid artery (left) Ext. carotid artery (right) - Ext. carotid artery (left) TIT Int. carotid IV Arch of aorta Int. carotid IIT Subclavian IV Vv VI Innominate artery VI Ductus arteriosus Subclavian artery (right) Pulmonary artery Subclavian artery (left) Aorta Fic. 220.—Diagram representing the changes in the aortic arches of a Mammal, ; Compare with Fig. 219. Modified from Hochstetter. for the same reason, the common carotid is a branch of the arch of the aorta. The fifth aortic arch from the beginning is rudimentary and disappears very early. The sixth arch on each side undergoes wide changes. A branch from each enters the corresponding lung. On the right side the portion of the sixth arch between the branch which enters the lung and the dorsal aortic root disap- pears, as does also that portion of the right dorsal aortic root between the -subclavian artery and the original bifurcation of the dorsal aorta. On the left side, however, that portion of the sixth arch between the branch which enters the lung and the dorsal aortic root persists until birth as the ductus 946 TEXT-BOOK OF EMBRYOLOGY. arteriosus (Botalli). This conveys the blood from the right ventricle to the aorta until the lungs become functional (Fig. 207); it then atrophies and be- comes the ligamentum arteriosum. In the meantime the septum aorticum has divided the original ventral aortic trunk into two vessels (see p. 230); one of the vessels communicates with the left ventricle and is the proximal part of the arch of the aorta, the other communicates with the right ventricle and be- comes the large pulmonary artery (Fig. 203). In human embryos of 10 mm. the dorsal aortic root on each side gives off several lateral branches—the segmental cervical vessels (Fig. 221). The first of these (first cervical, suboccipital), which arises nearly opposite the fourth aortic arch, is a companion, as it were, to the hypoglossal nerve, and sends a branch cranially which unites with its fellow of the opposite side inside the skull to form the basilar artery. The basilar artery again bifurcates and each branch Int. carotid artery Vertebral artery a... Segmental cervical artery \ ‘ . ‘ Namnneeececcceccere -~- Pulmonary artery Fic, 221.—Diagram of the aortic arches (III, IV, VI) and segmental cervical arteries of aromm. human embryo. His. unites with the corresponding internal carotid by means of the circulus arteriosus (Fig. 223). The other segmental cervical vessels arise from the aortic root at intervals, the eighth arising near the point of bifurcation of the aorta. Ina short time a longitudinal anastomosis appears between these segmental arteries, which extends as far as the seventh (Fig. 222). The proximal ends of the first six disappear, and the longitudinal vessel forms the vertebral artery which then opens into the aortic root through the seventh segmental artery, and which is continued cranially as the basilar artery (Fig. 223). The seventh (it is held by some to be the sixth) segmental artery becomes the subclavian, and conse- quently the vertebral opens into the subclavian, as in the adult (Fig. 222). But it should be borne in mind that the right subclavian artery is more than equiva- lent to the left, since the proximal part of the former is made up of the fourth aortic arch and a part of the aortic root (see Figs. 219 and 220). Further- more, changes occur in the position of the heart during development, which THE DEVELOPMENT OF THE VASCULAR SYSTEM. 247 alter the relations of the vessels. The heart migrates from its original position in the cervical region into the thorax, and this produces an elongation of the carotid arteries and an apparent shortening of the arch of the aorta; conse- Iyt. carotid Vert. art. Subclaviag art | C, + a ti sp REE Fic. 222.—Diagram illustrating the formation of the vertebral and superior intercostal arteries. The broken lines represent the portions of the original segmental vessels that disappear. Modified from Hochstetter. quently the subclavian artery on the left side arises relatively nearer the heart. The arteries of the brain arise as branches of the internal carotid and circu- lus arteriosus. The anterior cerebral artery and the middle cerebral artery arise 3 a Circulus arteriosus ¥ Middle cerebral i artery Basilar artery Int. carotid artery Ant. cerebral artery Lay ‘ Fic. 223.—Brain and arteries of a human embryoofgmm. Mail. primarily from a common stem which in turn is a branch of the most cranial part of the internal carotid (Figs. 223 and 224). The posterior cerebral artery arises as a branch of the circulus arteriosus (Fig. 224). 248 TEXT-BOOK OF EMBRYOLOGY. From the point of its bifurcation to its caudal end the aorta gives off paired, segmental branches which accompany the segmental nerves. The last (eighth) cervical branch and the first two thoracic branches undergo longitudinal anas- tomoses, similar to those between the first seven cervical, to form the superior intercostal artery (A. intercostalis suprema) which opens into the subclavian (Fig. 222). The other thoracic branches persist as the intercostal arteries; the lumbar branches persist as the Jumbar arteries. At the same time anastomoses are formed between the distal ends of the intercostal and lumbar arteries in the ventro-lateral region of the body wall, which give rise, on the one hand, to the internal mammary artery and, on the other hand, to the inferior epigastric artery. Of these two the former opens into the subclavian, the latter into the external iliac. By a further anastomosis the distal ends of the internal mammary and inferior epigastric are joined, thus forming a continuous vessel from the sub- Confluence of sinuses Post. cerebral vein (sup. petrosal sinus) Inf. sagittal sinus Sup. sagittal sinus Circulus arteriosus Post. cerebral artery Transverse sinus Basilar artery Ant. cerebral artery Int. jugular vein Int. carotid artery Fic. 224.—Brain, arteries and veins of a human embryo of 33 mm. Mail. clavian to the external iliac (Fig. 225). It is interesting to note that while originally all the lateral branches of the aorta are arranged segmentally, many of them lose their segmental character and are replaced or supplemented by longitudinal vessels. In addition to the lateral segmental branches of the aorta, which have been described, other branches develop which carry blood to the viscera. A num- ber of these, or possibly all, are also primarily segmental vessels, although they lose every trace of their segmental character during development. The first of the visceral branches to appear is the omphalomesenteric artery which arises from the ventral side of the aorta and which has been mentioned in connection with the vitelline circulation. Originally it passes out through the mesentery and follows the yolk stalk to ramify on the surface of the yolk sac. But since the yolk sac is of slight importance, the distal part of the artery soon disappears, while the proximal part becomes the superior mesenteric artery (Fig. 226). The THE DEVELOPMENT OF THE VASCULAR SYSTEM. 249 celiac artery arises from the ventral side of the aorta a short Gistance cranially to the omphalomesenteric (Fig. 226) and gives rise in turn to the gastric, hepatic and splenic arteries. The inferior mesenteric artery also arises from the ventral side of the aorta some distance caudal to the omphalomesenteric (Fig. 226). In the early stages these visceral arteries arise relatively much farther cranially than in the adult. During development they gradually migrate caudally to their normal positions. Int. mammary artery ~--~-~----~----f----- -— Intercostal artery Inf, epigastric artery ------------4---- --- — Femoral artery Umbilical artery Fic. 225.—Diagram of human embryo of 13 mm., showing the mode of development of the internal mammary and inferior epigastric arteries. Mall. Other branches of the aorta develop in connection with the urinary and genital organs, and, with the possible exception of the renal arteries, they are primarily segmental in character. Several branches supply the mesonephroi, but when the latter atrophy and disappear the vessels also disappear. The renal arteries do not develop until the kidneys have practically reached their final position in the embryo, and then they arise directly from the aorta. Dur- ing the development of the genital glands several pairs of branches from the aorta supply them with blood. Later the majority of these vessels disappear, one pair only persisting as the internal spermatic arteries which differ in accord- ance with the sex of the individual. In both sexes they are at first very short; 250 TEXT-BOOK OF EMBRYOLOGY. in the female, as’the ovaries move farther into the pelvic region, they become considerably elongated to form the ovarian arteries; in the male, with the descent of the testes, they become very much elongated to form the ¢esticular arteries. The fourth (or fifth?) pair of segmental lumbar arteries primarily gives rise to the vessels which supply the lower extremities, viz., the iliac arteries. These then would be serially homologous to the subclavians. But certain changes occur in this region, which are due to the relations of the um- bilical arteries. The latter, as has already been noted, arise as paired branches of the aorta in the lumbar region, pass ventrally through the genital cord (Chap. XV) and then follow the allantois (urachus) to the umbilical cord. 6 al (eee --- Aorta Coeliac artery -en~-eeece-H-n sates sees Sup. mesenteric | (vitelline) artery "* * 7777 \ = Duodenum SS, aoa ay aw TeW rea S 3 \) Umbilical artery +e" ~eeere- = Fic. 226.—Diagram of the visceral arteries in a human embryo of 12.5 mm. Tandler. Numerals indicate segmental arteries. During foetal life they carry all the blood that passes to the placenta. At an early period a branch from each iliac artery anastomoses with the corresponding umbilical, and the portion of the umbilical artery between the aorta and the anastomosis then disappears. This makes the umbilical artery a branch of the iliac; and the blood then passes from the aorta into the proximal part of the iliac which becomes the common iliac artery of the adult. At birth, when the umbilical cord is cut, the umbilical arteries no longer carry blood to the placenta, and their intraembryonic portions, often called the hypogastric arteries, persist only in part; their proximal ends persist as the superior vesical arteries, while the portions which accompanied the urachus degenerate to form the /ateral umbilical ligaments. THE DEVELOPMENT OF THE VASCULAR SYSTEM. 251 So far as a complete history of the growth of the arteries of the extremities is concerned, knowledge is lacking. The facts of comparative anatomy and the anomalies which occur in the human body have led to certain conclusions which have been largely confirmed by embryological observations; but much more work on the development of the arteries is yet necessary to complete their history. The extremities represent outgrowths from several segments of the body, and the nerve supply is derived from several segments, but the blood is furnished by a single segmental vessel in each extremity. In the upper ex- tremity the subclavian, which represents the seventh cervical branch of the aortic root, is the primary vessel from which all the other vessels are derived. In Brachial artery*-"--rror-grpo rr rere Brachial artery Superficial radia artery RaeS= Median artery Median artery ---~ Interosseous artery Interosseous artery ----- 7 ated Ulnar artery Ulnar artery aaa. | ~--> Radial artery Fic. 227.—Diagrams showing (A) an early and (B) a late stage in the development of the arteries of the upper extremity. McMurrich. the lower extremity the common iliac, which represents the fourth (or fifth ?) lumbar branch of the aorta is the primary vessel. In the upper extremity the subclavian grows as a single vessel to the wrist and then divides into branches corresponding to the fingers. In the forearm it lies between the radius and ulna. In a short time a branch is given off just distal to the elbow and accompanies the median nerve. As this branch in- creases, the original vessel in the forearm diminishes to form the volar inter- osseous artery; andat the same time the branch unites again with the lower end of the interosseous, takes up the digital branches and becomes the chief vessel of the forearm at this stage, forming the median artery. Later, however, it di- minishes in size as another vessel develops, the uJnar artery, which arises a short 952 TEXT-BOOK OF EMBRYOLOGY. distance proximal to the origin of the median and, passing along the ulnar side of.the forearm, unites with the median to form the superficial volar arch. From the artery of the arm, which is called the brachial artery, a branch develops about the middle and extends distally along the radial side of the forearm. A little later another branch grows out from the brachial just proximally to the origin of the ulnar and extends across to, and anastomoses with, the first branch. Then the portion of the first branch between its point of origin and the anasto- mosis atrophies, leaving only a small vessel which goes to the biceps muscle. The second branch and the remaining part of the first branch together form the radial artery (Fig. 227) (McMurrich). Femoral artery «- fl A p=. Sciatic artery Sciatic artery J --4- Tete S: Femoral artery “oh Popliteal artery “SSCT - 1 4 HJ}-- -- ~ Popliteal artery f- N — Ant. tibial artery '+--4-—-- Peroneal artery Peroneal artery _ ri Right hep —_—eE J ce Vent. pancreas duct eo) ; ley Gall { 22 a .i_—__- Duodenum bladder ~ | = Fic. 312.—From a reconstruction of the anlagen of the liver and pancreas and a part of the stomach and duodenum of a human embryo of 4 weeks. Felix. that the hepatic cylinders, which are comparable with the smaller ducts and terminal tubules of other glands, anastomose, and in that the blood vessels are broken up by the growth of these cylinders. Fic. 313.—From a reconstruction of the anlagen of the liver and pancreas and the stomach of a human embryo of 8mm. Hammar. ‘D.P., Dorsal pancreas; Du., duodenum; D.V., ductus venosus; G.B., gall bladder; R.1., right lobe of liver; S., stomach; V. P., ventral pancreas, This mode of development establishes what is known as a sinusoidal circulation, which differs from the ordinary capillary circulation. The sinusoids are produced by the growth of the trabeculz of the developing organ into large vessels and the breaking up of the latter DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 349 into smaller vessels. It is obvious that a sinusoidal circulation is purely venous or purely arterial. Furthermore, development of this nature leaves comparatively little connective tissue within the gland, another feature characteristic of the liver. All the blood carried to the liver by the omphalomesenteric veins must follow the tortuous course of the sinusoids before being collected again and passed on to the heart. When the umbilical veins come into connection with the liver they also join in the sinusoidal circulation. Subsequently, however, a more direct channel—the ductus venosus—is established and persists for a Suprarenal glands Aorta Inf. vena cava Mesonephros Dorsal mesogastrium Coelom (greater omentum) Stomach .. Ductus Ventral mesogastrium choledochus (lesser omentum) Liver Right side Left side Fic. 314.—Tranverse section of a r4 mm, pig embryo, through the region of the stomach. Photograph. The arrow points into the bursa omentalis. short time. This is probably due to the large volume of blood brought in by the umbilical veins. Finally the ductus venosus disappears and the sinusoidal circulation remains as the permanent form. (For the development of the veins in the liver see p. 262.) The lobes of the liver develop in a general way in relation to the great venous trunks which at one time or another pass into or through the gland. The anlage of the organ grows into the ventral mesentery, subsequently be- coming enclosed in the septum transversum. In so doing it encounters the omphalomesenteric veins, and forms, in relation to the latter, two incompletely separated parts which have been called the dorso-lateral lobes. When the umbilical veins enter the liver a more ventral, medial mass is formed. This becomes incompletely separated into two parts which give rise to the permanent 350 TEXT-BOOK OF EMBRYOLOGY. right and left lobes. The right becomes the larger. The right umbilical vein loses its connection with the liver (p. 264). After birth the left, which lies be- tween the right and left lobes, degenerates into the round ligament of the liver. The other lobes arise secondarily as outgrowths from the right primary dorso- lateral lobe, the caudate (lobe of Spigelius) from its inner (medial) surface, the quadrate from its dorsal surface. The liver as a whole grows rapidly and by the second month is relatively large. During the third month it fills the greater part of the abdominal cavity. After the fifth month it grows less rapidly and the other intraabdominal organs overtake it, so to speak, although at birth it forms one-eighteenth the total weight of the body. After birth it actually diminishes in size. The right lobe is from the beginning larger than the left, and after birth the predominance increases. Histogenesis of the Liver.—The hepatic part (pars hepatica) of the liver anlage is derived from the entodermal lining of the gut and constitutes a mass of cells with no lumen. From this mass, solid bud-like evaginations grow into the mesentery, break up the omphalomesenteric veins into smaller channels and form trabecule, or hepatic cylinders (p. 348). The latter anastomose freely with one another and are composed of polyhedral, darkly staining cells with vesicular nuclei (Fig. 315, A). Lumina begin to appear in the cylinders about the fourth week as small cavities which communicate with the cavity of the gut. The hepatic cylinders are the forerunners of the hepatic cords or cords of liver cells. There are two views as to the manner of transformation. The older view is that the cylinders gradually become stretched, the number of cells in cross-section becoming less until it is reduced to two. Between these two lies the lumen of the cord or the so-called “bile capillary” (Fig. 315, B). The other view is that branches from the sinusoids grow into the cylinders and sub- divide them into hepatic cords. As stated above; the hepatic cylinders are at first composed of darkly stain- ing, polyhedral cells with vesicular nuclei. These are the liver cells proper. Later other small spherical cells, with dense nuclei, appear and during the fourth month become very numerous (Fig. 315, A). From this time on, they grow less in number and at birth have practically disappeared. Earlier investi- gators considered them as developing liver cells. Further study on the develop- ment of the blood, however, has led others to consider them as erythroblasts (p. 271). Since they are inside of the hepatic cylinders, they either wander in from the intertrabecular blood vessels or lie in intratrabecular vessels. The latter supposition accords with the view that the cylinders are broken up into hepatic cords by the ingrowth of branches from the sinusoids. The development of the lobules of the liver, producing the peculiar relations DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 351 between the parenchyma of the gland and the blood vessels, has not been clearly and completely demonstrated. In young embryos the branches of the hepatic veins are surrounded by comparatively little connective tissue. The branches of the portal vein are surrounded by a considerable amount which subdivides the liver into lobules but not in the same manner as in the adult. The trabecule possess no radial character and there are several so-called central veins in each lobule. The changes by which these primary lobules are sub- divided into the permanent ones do not take place until after birth. The branches of the portal vein, with the surrounding connective tissue, invade the Fic. 315.—Sections of the liver of (A) a human feetus of 6 months and (B) a child of 4 years. Toldt and Zuckerhandl. McMurrich. be, Bile “capillary”; e, ervthroblast; he, hepatic cylinder (in A), cord of liver cells (in B). primary lobules and divide them into a number of secondary lobules, corre- sponding to the original number of central veins. At the same time the hepatic cords (which have been formed meanwhile) become arranged radially around the central veins in the characteristic manner. The hepatic artery grows into the liver secondarily and its branches follow the course of the branches of the portal vein. Degeneration of the liver cells occurs in the region of the left triangular liga- ment, the gall bladder and the inferior vena cava. The bile ducts may, how- ever, withstand the degenerative processes and persist as the vasa aberrantia of the liver. The cause of the degeneration is possibly the pressure brought to bear by other organs. The Development of the Pancreas. The epithelium of the pancreas, like that of the liver, is a derivative of the ‘entoderm. It arises from two (or three) separate anlagen, one dorsal and one 23 352 TEXT-BOOK OF EMBRYOLOGY. (or two) ventral. The dorsal anlage appears first as a ridge-like evagination from the dorsal wall of the gut, slightly cranial to the level of the liver (Figs. 311 and 312). It appears about the same time as the liver or a little later. The mass of cells grows into the dorsal mesentery and becomes constricted from the parent epithelium except for a thin neck which becomes the duct of Santorini (Fig. 316). A little later two other diverticula appear, one from each side of the common bile duct. It is uncertain whether only one or both of these Stomach Liver Cystic duct Gall bladder Ductus choledochus Ventral pancreas Acces. pancr. | duct (Santorini) Fic, 316. Dorsal pancreas Stomach Ductus choledochus Dorsal pancreas Acces. pancr. duct —— Cystic d (Santorini) ystic duct Duodenum Gall bladder Ventral pancreas with pancr. duct (Wirsung) Fic. 317. Fics. 316 and 317—From models of the developing liver and pancreas of rabbit embryos of 8 mm. and Jo mm., respectively. Both seen from the right side. Hammar, Bonnet. take part in the formation of the pancreas, but it seems most probable that the left one disappears entirely. The right diverticulum continues to develop and becomes constricted from the parent epithelium, leaving only a thin neck which becomes the duct of Wirsung. The smaller ventral pancreas grows to the right and then dorsally in the mesentery (Fig. 318), passing over the right surface of the portal vein, until it meets and fuses with the proximal part of the larger dorsal pancreas. The fusion takes place in the sixth week, and the two anlagen then form a single DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 353 mass. A communication is established between the two ducts, and the dorsal duct (Santorini) usually disappears, leaving the ventral (Wirsung) as the per- manent duct opening into the ductus choledochus. Ina general way it may be said that the ventral anlage gives rise to the head, the ‘dorsal anlage to the body and. tail of the pancreas (compare Figs. 316 and 317). As the pancreas grows into the dorsal mesentery it comes to lie in the dorsal mesogastrium between the greater curvature of the stomach and the vertebral column, and since the dorsal mesogastrium at first lies in the medial sagittal plane, the pancreas is similarly situated. After the sixth week, how- ever, as the stomach changes its position (p. 337), the pancreas is carried along Inf. vena cava Mesonephros Ceelom Greater omentum Dorsal pancreas : (dorsal mesentery) Portal vein Ventral pancreas Duodenum Ductus choledochus . Liver Right side Left side Fic. 318.—From a transverse section through the region of the duodenum of a pig embryo of 14mm. Photograph. -with the mesogastrium and comes to lie in a transverse plane, with its head to the right and embedded in the bend of the duodenum, and its tail reaching to the spleen on the left. The organ as a whole is at first movable along with the mesentery, but when it assumes its transverse position it lies close to the dorsal abdominal wall. The mesentery then fuses with the adjacent peritoneum (see p. 382), and the pancreas is firmly fixed. The connective tissue of the pancreas is derived from the mesodermal tissue of the mesentery. As the processes or buds which form the ducts and terminal tubules grow out from the primary masses, they penetrate the mesodermal tissue and are surrounded by it. Groups of tubules form lobes and lobules, and the entire gland is surrounded by a capsule of connective tissue. 354 TEXT-BOOK OF EMBRYOLOGY. Histogenesis of the Pancreas.—The masses of entodermal cells forming the anlagen of the pancreas develop further by a process of budding, which goes on until finally a compound tubular gland is produced. According to Fic. 319.—Sections of the developing pancreas of a guinea-pig embryo of 12 mm. (a); of 33 mm. (b); of Torpedo marmorata (c). Helly, ¢, Capillaries; Dg, ducts; Gz, duct cells; Lz, Langhans’ cells. The cells in ¢ show distinct zymogen granules some investigators the primary evaginations are hollow, their lumina being continuous with the lumen of the gut. According to others they are solid at first and acquire their lumina secondarily. The same uncertainty exists in regard to the later outgrowths or buds. DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 355 The early entodermal cells proliferate, and the resulting cells change ac- cording to their position in the gland. Those lining the larger ducts become high columnar, with more or less homogeneous cytoplasm; those lining the intermediate (intercalated) ducts become low; those lining the terminal secret- ing tubules become pyramidal and more highly specialized, and also acquire certain constituents—the zymogen granules (Fig. 319, c)—-which vary with the functional activities of the gland. The centro-tubular cells in the terminal tubules are probably to be explained on a developmental basis. While a few maintain that they are “wandering” cells, it is quite generally accepted that they are simply continuations of the flat cells lining the intermediate ducts, the result being that the cells of the terminal tubules seem to spread out over the ends of the intermediate ducts in the form of cap-like structures. It was once thought that the islands of Langerhans were derived from the mesodermal tissue. Recently it has been pretty clearly demonstrated that they are derived from entoderm. In guinea-pig embryos of 5 to 6 mm., at a time when the dorsal pancreas has merely begun its constriction from the gut, certain cells in the mass.appear darker and slightly larger than the others. They show darker areas of cytoplasm around the nuclei, and later the darker areas extend throughout the cells and the nuclei become larger and more vesicular. When lumina appear in the outgrowths or buds, these cells occupy a position on or near the surface of the buds (Fig. 319, a). In further development they tend to sepa- rate themselves from the buds and collect in clumps (Fig. 319, b). Capillaries then penetrate the clumps and break them up into the trabecule of cells char- acteristic of the islands of Langerhans (Fig. 319, c). Studies on the development of the islands in the human pancreas indicate a similar origin and mode of development. Anomalies. One of the most striking anomalies of the organs of alimentation is found in connection with a more general anomalous condition known as transposition of the viscera (situs viscerum inversus). The transposition may be so complete that the minor asymmetries normally present on the two sides are all repeated in reverse order, the functions of the organs being unimpaired. As regards the alimentary tract, this means that the position of the stomach is reversed in the abdominal cavity; that the duodenum crosses from left to right; that the various coils of the jejunum and ileum occupy positions opposite to the normal; that the cecum and ascending colon are situated on the left side and the descending. colon on the right; and that the larger lobe of the liver lies on the left side. The other visceral organs are transposed accordingly, the heart being inclined to- ward the right side, the left lung consisting of three lobes and the right of two, 356 TEXT-BOOK OF EMBRYOLOGY. the left kidney being lower than the right, etc. Such cases are not uncommon, two hundred being on record. Various theories as to the causes of transposition of the organs have been advanced. In the most plausible of these the anomalous condition is consid- ered as due to the influence of the large veins in the embryo. It seems best, therefore, to consider first the transposition of the heart (dextrocardia, referred to on page 285). After the two anlagen unite in the midventral line, the heart constitutes a simple straight tube which lies in a longitudinal direction in the primitive peri- cardial cavity, and which is joined caudally by the two omphalomesenteric veins and cranially by the ventral aortic trunk (p. 224). Normally the left omphalomesenteric vein is the larger and pours a greater quantity of blood into the heart tube than the right. This condition is regarded as the primary factor in the deflection of the tube toward the right side (p. 226; also Fig. 196). If the conditions were reversed, that is, if the right omphalomesenteric vein were the larger and poured the greater quantity of blood into the heart tube, the pri- mary bend of the latter would be toward the left side. Consequently the heart would continue to develop in the transposed position and eventually come to lie on the side opposite to the normal. Although dextrocardia is very frequently associated with transposition. of the abdominal organs, it is not necessarily so, for there are cases of the latter in which the heart occupies the normal position. Consequently it seems that further influences must be present to account for transposition of the abdominal organs when the thoracic organs are normal. A number of investigators have emphasized the importance of the influence of the large venous trunks in the abdominal region, especially on the position of the liver and stomach. Primarily the omphalomesenteric veins pass cranially through the mesen- tery. Later they form two loops or rings around the duodenum. Then the left half of the upper ring and the right half of the lower disappear, the common venous trunk thus following a spiral course around the duodenum (p. 265; also Fig. 239). This primary relation of the omphalomesenteric vein is retained in the relation of the portal vein to the duodenum. The stomach lies to the left of the portal vein. After the allantoic (placental) circulation is established the umbilical veins pass cranially in the lateral body walls. After the veins come into connection with the liver, the right atrophies and the left increases in size and becomes the single large umbilical vein of later stages (p. 264; also Fig. 240). The right lobe of the liver becomes the larger. If, as is maintained by some investigators, the usual position of the stomach and liver is due to the persistence of the left venous trunks, a persistence of the right venous trunks would afford a plausible explanation of the transposition of these organs. It is not unreasonable to attribute also the transposition of the DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 357 other abdominal organs directly or indirectly to the persistence of the right venous trunks. Certainly a reversal in the position of the stomach would cause a reversal in the position of the duodenum. If these conditions are the real ones, the fact that the thoracic organs can be transposed without a transposition of the abdominal organs, or vice versa, is accounted for. The primary bend of the heart tube occurs at a very early period, before the changes in the vessels in the region of the liver. Conse- quently a reversal of the conditions of the omphalomesenteric at a very early stage only would be likely to affect the heart. The principal changes in size of the venous trunks in the abdominal region take place after their channels have been broken up in the liver. In other words, the modifications in the veins in the liver occur after the definite relations of the heart have been established. Therefore the transposition of the abdominal organs may take place after the heart has begun to develop normally. THE Movutu.—Anomalies in the mouth region, due to defective fusion of the processes that bound it, have been considered elsewhere (p. 216). ; Anomalies of the tongue sometimes arise as the result of imperfect develop- ment of one or more of its anlagen. Imperfect development of the tuberculum impar results in total or partial lack of the anterior part. Defects in the root are probably due to imperfect development of one or both of the paired anlagen (p. 321). Malformations of the lower jaw (micrognathus, agnathus) are usually accompanied by malformations of the tongue, both structures being derived largely from the first pair of branchial arches. THE PHarynx.—The pharynx is the seat of cysts, fistula and diverticula which have been considered in connection with the anomalies in the region of the branchial arches and grooves (Chap. XIX). The thyreoid gland is not infrequently the seat of certain anomalies that arise as the result of abnormal development. Persistent portions of the thyreo- glossal duct, the upper end of which is indicated by the foramen cecum lingue, may give rise to cystic structures extending to the region of the hyoid bone. Persistent portions of the duct may even give rise to accessory thyreoid (supra- hyoid, prehyoid) glands (p. 333; also Fig. 298). Considerable variation also exists in the isthmus and lateral lobes of the thyreoid, due to variation in the manner of development of the medial anlage. Impaired development of the shymus gland sometimes leads to cysts which come to lie in the anterior mediastinum. Tue (sopHacus.—Very rarely the cesophagus is entirely lacking, being represented by a mere cord of tissue. More frequently it is defective in certain parts. The atresia may begin just below the pharynx or just above the stomach, the intermediate portion being composed of a cord of fibrous tissue. Occasion- ally the non-atretic portion opens into the trachea. Possibly this represents 358 TEXT-BOOK OF EMBRYOLOGY. an imperfect separation between the primitive gut and the anlage of the respiratory system (p. 362). Tue Stomacu.—Occasionally the stomach is smaller than the normal. It may even be a narrow tube resembling the other portions of the gut, owing to lack of dilatation. Other congenital malformations, apart from transposition (p. 355), are very rare. Tue InrEstrnEs.—One of the most common anomalies is the persistence of the proximal end of the yolk stalk, forming Meckel’s diverticulum (see p. 117). This usually is attached to the ileum about three feet from the cecum. In ex- ceptional cases it retains its lumen and, when the stump of the umbilical cord disappears, forms a congenital umbilical fistula. Usually, however, the diver- ticulum is shorter and ends blindly. Occasionally it becomes constricted from the intestine and forms a cystic structure. (See also Chap. XIX.) Congenital stenosis and atresia may occur in different regions of the intestine, the duodenum being the most common site. Normally the lumen of the duodenum becomes closed for a brief period during development (p. 339), and congenital closure of the lumen may represent a persistence of the early em- bryonic condition. A conspicuous malformation is the persistence of the cloaca. ‘The septum which normally separates the latter structure into rectum and urogenital sinus fails to develop, thus leaving a common cavity (see Figs. 361 and 362). In addition to this the cloacal membrane may fail to rupture and the cloaca be- come much distended. More often the septum develops in part, leaving only a small opening between the rectum and urogenital sinus. After the latter undergoes further development, the rectum comes to open into the urethra or bladder, or into the vagina or uterus. Atresia of the anus is not infrequently met with. The cloacal (or anal) membrane fails to rupture and the rectum ends blindly. In other cases the rectum opens into the urogenital sinus, as described in the preceding paragraph. Occasionally the lumen of the rectum is closed—airesia recti—and the gut ends blindly some distance from the surface, being connected with the anal region by a cord of fibrous tissue. Variations in the position of the intestinal loops, apart from transposition (p. 355), are of frequent occurrence. It is not customary to include these varia- tions among malformations (see p. 340). The cecum (and appendix) and colon present some striking variations. The cecum may be situated high up in the abdominal cavity, the ascending colon being absent. Or it may be situated at any intermediate point between that and its usual position in the right iliac fossa. These variations are due to different degrees of development of the ascending colon (p. 34r). Tue Liver.—Congenital malformations of the liver are rare. The most DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 359 frequent, apart from transposition, include anomalies in the size and number of lobes. Accessory lobes may occur within the falciform ligament. One case of lack of development of the gall bladder has been observed. Stenosis of the bile passages is occasionally met with. THE PaNcREAS.—Occasionally accessory glands are found in the intesti- nal or gastric wall. These probably represent aberrant portions of the main gland, and may give rise to cystic structures. Very recently, however, a number of intestinal diverticula have been observed in certain mammalian embryos and also in human embryos.* Although the history of these unusual diverticula has not been traced, their presence may offer a clue to the origin of accessory pancreatic structures. The ducts of the pancreas are subject to distinct variations, which, however, are not usually considered as anomalies. Not infrequently the duct of the dorsal anlage (duct of Santorini) persists and opens directly into the duodenum. It may persist along with the duct of the ventral anlage (duct of Wirsung), or the latter may disappear (p. 353; compare Figs. 316 and 317). PRACTICAL SUGGESTIONS. The Primitive Gut.—The formation of the primitive gut can be studied in chick embryos prepared according to the technic on page 385, under the head of the Coelom and Common Mesentery. The Mouth and Pharynx.—The external appearance of the oral pit and the branchial ‘arches and grooves can be studied in very young pig embryos (6 mm. or less). The speci- mens should be preserved im toto in 4 per cent. formalin, and for class purposes can be mounted in glycerin jelly (see Appendix). Occasionally young human embryos (8 mm. or less) are obtained, in which the branchial arches and grooves show clearly. For the study of the internal structure, sections of course are necessary. Pig embryos afford very good material and are easily obtained. The specimens may be fixed in Zenker’s or Bouin’s fluid, cut in celloidin or in paraffin, and stained with hematoxylin and eosin. Usually it is advisable to cut serial sections. The stages given in the following paragraphs are perhaps the most convenient for the study of the different organs. The tongue can be seen in its earlier stages in sagittal sections of pig embryos of 12 to 18mm. The development of the teeth can be followed in embryos of 20 to 100 mm. The most. convenient way is to remove the jaws, fix for several days in Bouin’s fluid, which at the same time decalcifies, and cut sections transversely to the long axes of the jaws. Very beautiful preparations can be obtained in this way. The anlagen of the salivary glands can be seen in embryos of 5 mm. The anlagen of the thyrecid and thymus are very clearly shown in embryos of 10 to 15 mm. The esophagus can be seen in any transverse section in the thoracic region of an embryo of any stage after the formation of the primitive gut. The Stomach and Intestine.—The development of these organs can be studied in transverse sections of pig embryos of from 6 mm. up to any size that is not too large to section. Zenker’s and Bouin’s fluid are both good fixatives. It is best to cut transverse serial sections, although sections taken at intervals are very instructive. Hematoxylin and eosin give a *Lewis, Thyng. 360 TEXT-BOOK OF EMBRYOLOGY. good differential stain. The later stages of development can be followed in very carefully made gross dissections. Wax reconstructions of one or two of the earlier stages are useful. These can be made in conjunction with reconstructions of the mesenteries (p. 385). The Liver and Pancreas.— Very young embryos (3 to 6 mm.) are necessary in studying the primary evaginations. Transverse sections are taken just cranial to the umbilicus. Later stages can be studied in specimens prepared according to the technic in the preceding paragraph. In fact the same sets of specimens can be used in the study of the stomach, omenta, duodenum, liver, and pancreas, since these structures lie so nearly in the same transverse plane. The histogenesis of any of the organs of alimentation can be studied after the technic given above. Histological structure is well preserved by Bouin’s fluid, but even. better by Flemming’s fluid. The sections should be cut thin. Haematoxylin and eosin give a good differential stain after fixation in Bouin’s fluid. After Flemming’s fluid Heidenhain’s hema- toxylin should be used. References for Further Study. Bett, E. T.: The Development of the Thymus. American Jour. of Anat., Vol. V, 1906 Berry, J. M.: On the Development of the Villi of the Human Intestine. Anat. Anz. Bd. XVI, 1900. Bonnet, R.: Lehrbuch der Entwickelungsgeschichte. Berlin, 1907. Born, G.: Ueber die Derivate der embryonalen Schlundbogen und Schlundspalten bei Saugetiere. Arch. 7. mik. Anat., Bd. XXII, 1883. Bracuet, A.: Die Entwickelung und Histogenese der Leber und des Pancreas. Ergeb- nisse der Anat. u. Entwick., Bd. VI, 1897. CulEvitz, J. C.: Beitrage zur Entwickelungsgeschichte der Speicheldriisen. Arch. f. Anat. u. Physiol., Anat. Abth., 1885. CHoronscHitzky: Die Entstehung der Milz, Leber, Gallenblase, Bauchspeicheldriise und des Pfortadersystems bei den verschiedenen Abteilungen der Wirbeltiere. Anat. Hefte, Bd. XIII, 1900. Fox, H.: The Pharyngeal Pouches and their Derivatives in the Mammalia. Am. Jour. of Anat., Vol. VIII, No. 3, 1908. Fusari, R.: Sur les phénoménes, que l’on observe dans la muqueuse du canal digestif durant le développement du feetus humain. Arch. ital. Biol., T. XLII, 1904. GoprerT, E.: Die Entwickelung des Mundes und der Mundhéhle mit Driisen und Zunge; die Entwickelung der Schwimmblase, der Lunge und des Kehlkopfes der Wirbeltiere. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere. Bd. II, Teil I, 1902. Hammar, J. A.: Einige Plattenmodelle zur Beleuchtung der friiheren embryonalen Leberentwickelung. Arch. jf. Anat. u. Physiol., Anat. Abth., 1893. Hammar, J. A.: Allgemeine Morphologie der Schlundspalten beim Menschen. Entwick- elung des Mittelohrraumes und des dusseren Gehérganges. Arch. f. mik. Anat., Bd. LIX, 1902. Hammar, J. A.: Das Schicksal der zweiten Schlundspalte. Zur vergleichenden Em- bryologie und Morphologie der Tonsille. Arch. j. mik. Anat., Bd, LXI, 1903. HEtty, K.: Studien itiber Langerhanssche Inseln. Arch. f. mik. Anat., Bd. LXVII, 1907. Hertwie, O.: Lehrbuch der Entwickelungsgeschichte der Wirbeltiere und des Menschen. Jena, 1906. DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 361 Henpricxson, W. F.: The Development of the Bile Capillaries as Revealed by Golgi’s Method. Johns Hopkins Hosp. Bull., 1898. His, W.: Anatomie menschlicher Embryonen. Leipzig, 1880-1885. His, W.: Die Entwickelung der menschlichen und tierischen Physiognomien. Arch. }. Anat. u. Physiol., Anat. Abth., 1892. Koun, A.: Die Epithelkérperchen. Ergebnisse der Anat. u. Entwick., Bd. IX, 1899. Kotimany, J.: Die Entwickelung der Lymphknétchen in dem Blinddarm und in dem Processus vermiformis. Die Entwickelung der Tonsillen und die Entwickelung der Milz. Arch. j. Anat, u. Physiol., Anat. Abth., 1900. Kottmann, J.: Lehrbuch der Entwickelungsgeschichte des Menschen. Jena, 1808. Kotimann, J.: Handatlas der Entwickelungsgeschichte des Menschen. Jena, 1907. Matt, F. P.: Ueber die Entwickelung des menschlichen Darmes und seiner Lage beim Erwachsenen. Arch. f. Anat. u. Physiol., Anat. Abth. Suppl., 1897. Maurer, F.: Die Entwickelung des Darmsystems. In Hertwig’s Handbuch der ver- gleich. u. experiment. Entwickelungslehre der Wirbeltiere., Bd. II, Teil I, 1902. McMorricg, J. P.: The Development of the Human Body. Third Ed. Philadelphia, 1907. Pearce, R. M.: The Development of the Islands of Langerhans in the Human Embryo. American Jcur. of Anat., Vol. II, 1903. Prersot, G. A.: Teratology. In Wood’s Reference Handbook of the Medical Sciences, Vol. VII, 1904. Potzt, A.: Zur Entwickelungsgeschichte des menschlichen Gaumens. Anat. Hejte, 1905. Rose, C.: Ueber die Entwickelung der Zahne des Menschen. Arch. jf. mik. Anat., Bd. XXXVIII, 189r. Srerpa, A.: Ueker Atresia ani congenita und die damit verbundenen Missbildungen. Arch, j. klin. Chir., Bd. LXX, 1903 Stour, P.: Ueber die Entwickelung der Darmlymphknétchen und iiber die Riickbildung von Darmdriisen. Arch. }, Anat. u. Physiol., Anat. Abth., 1898. TANDLER, J.: Zur Entwickelungsgeschichte des menschlichen Duodenum in friihen . Embryonalstadien. Morph. Jahrb., Bd. XXIX, 1900. Toxpt und ZUCKERHANDL: Ueber die Form und Texturveriinderungen der mensch- lichen Leber wihrend Wachsthums. Sitzungsber. d. kaiser. Akad. d. Wissensch., Wien. Math.-Naturwiss. Klasse., Bd. LXXII, 1875. TourNEUX ET VERDUN: Sur les premiers développements de la Thyroide, du Thymus et des glandes parathyroidiennes chez /homme. Jour. de. ? Anat. et. dela Physiol., T. XXXIII, 1897. CHAPTER XIII. THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. The anlage of the respiratory system appears in human embryos of about 3.2 mm. A hollow, linear evagination—the lung groove—develops on the ventral side of the cesophageal portion of the primitive gut, extending caudally a short distance from the region of the fourth inner branchial groove. It was once thought that the evagination developed along practically the entire length of the cesophagus anlage, but more recent researches seem to prove that it is confined to the cephalic end. The lung groove soon becomes separated from Pharynx Branchial arches (pharynx) oedema? . Lung \ 4—— Liver +—— Stomach Pancreas Common mesentery Mesonephros ain eae ao Allantoic duct Caudal gut Beer ae ie ae Kidney bud a Fic. 320,—Sagittal section of reconstruction of a human embryo of 5mm. His, Kollmann. Belly stalk Hind-gut the gut by a constriction which appears at the caudal end and gradually pro- gresses forward. Thus there is formed a tube which lies ventral to the gut and which opens upon the floor of the latter at the boundary line between the cesophagus and pharynx (Figs. 320 and 284). From this simple tube the entire respiratory system develops. The cephalic end gives rise to the /arynx, the opening into the gut being the aditus laryngis. ‘The middle portion gives rise to the trachea. ‘Two outgrowths from the caudal end of the tube, which appear about the time of separation from the 362 THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 363 cesophagus, develop into the bronchi and their continuations—the lungs. The epithelial lining of the system is of course derived from the entoderm. The various kinds of connective tissue are derived from the mesoderm, since the anlage grows into the mesodermal tissue of the ventral mesentery. The Larynx. The opening from the gut into the respiratory tube becomes surrounded by a U-shaped elevation—the furcula—which lies in the floor of the pharynx with its open end directed caudally. Toward the end of the first month each side of the opening (aditus laryngis) becomes elevated, forming the arytenoid ridge. From each of these a secondary elevation arises, forming the cunei- form ridge. The arytenoid ridges come so close together that they practically close the opening except at its cephalic side (Fig. 321). Along with the develop- ment of these ridges the apical portion of the furcula becomes a distinct trans- Tuberculum impar s. Epiglottis ¢ Aryepiglottic ridge Arytenoid ridge Cuneiform ridge Aditus laryngis Cuneiform ridge Fic. 321.—From a reconstruction of the larynx of a human embryo of 28 days. Seen from above. Kallius, verse fold at the cephalic rim of the opening. This fold is the anlage of the epigloitis, Laterally the epiglottic fold becomes continuous with the arytenoid ridges, forming the aryepiglottic ridges (Fig. 321). During the fourth month a groove-like depression appears on the medial side of each arytenoid ridge, gradually becomes deeper, and leaves on each side of it a fold or lip which bounds the opening. The external lips—those nearer the pharynx—form the superior or false vocal cords; the internal lips form the true vocal cords. At the same time the opening into the larynx, which’ was closed by the arytenoid ridges, is reestablished. ‘The depression between the vocal cords on each side becomes still deeper to form the ventricle, and a further outgrowth from the ventricle produces the appendage of the ventricle (the laryn- geal pouch). 364 TEXT-BOOK OF EMBRYOLOGY. ‘ The mesodermal tissue external to the epithelium (entoderm) of the larynx gives rise to the various kinds of connective tissue including the laryngeal cartilages. By the end of the fourth week condensations appear in the mesen- chymal tissue, which are the forerunners of the cartilages, but true cartilage does not appear until the seventh week. The anlagen of the éhyreoid cartilage Sup. hy. Thyr. B Fic 322.—From reconstructions of the mesenchymal condensations which represent the hvoid and thyreoid cartilages in an embryo of 4o days. A, Ventral view; B, lateral view from right. Kallius, Inf. hy., Inferior (greater) horn of hyoid; Sup,hy., superior (lesser) horn of hyoid; Thyr., thyreoid. The portions indicated by black lines represent chondrification centers, are two mesenchymal plates, one on each side, which are bilaterally sym- metrical and correspond to the lateral parts of the adult cartilage (Fig. 322, A). These plates gradually grow ventrally and unite and fuse in the midventral line (Fig. 323). ‘Twocenters of chondrification appear in each plate (Fig. 322, A,) as, . Pharynx metre w Muscle Arytenoid cartilage - Thyreoid cartilage Copula Fic. 323.—From a transverse section through the pharynx and larynx of a human embryo of 48mm. Nicolas. and enlarge until the entire plate is converted into cartilage, the middle part becoming elastic in character, the rest hyalin. Originally the cephalic edge of each thyreoid plate is connected with the inferior horn of the hyoid cartilage (Fig. 322, B). This connection is subse- quently lost, but a remnant of the connecting cartilage persists as the ériticeous THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 365 cartilage in the lateral hyothyreoid ligament. The anlagen of the arytenoid cartilages develop in the arytenoid ridges as condensations of the mesenchyme, which later are converted into true cartilage (Fig. 323). The apex and vocal process of each arytenoid become elastic, the main body becomes hyalin. The corniculate cartilages (cartilages of Santorini) are split off from the cephalic ends of the arytenoids and are of the elastic variety. The cricoid cartilage, like the others, is preceded by a condensation of mesenchyme. Chondrifica- tion begins on each side and then progresses around dorsally and ventrally until a complete hyalin ring is formed. From its developmental resemblance to the tracheal rings, the cricoid is sometimes regarded as the most cephalic of that series. The epiglottic cartilage develops in the epiglottic ridge as two sepa- rate pieces which subsequently fuse.. It is of the elastic variety. The cuneiform cartilages (cartilages of Wrisberg) are split off from the two pieces of the epi- glottic, and are of the elastic type. Attempts have been made to determine which branchial arches are represented by the laryngeal cartilages. It seems quite definitely settled that the thyreoid is derived in part, at least, from the fourth arch. There is much doubt as regards the others, for there is great difficulty in determining their derivation in the human embryo, since the arches disappear at such an early stage. Furthermore, some of these cartilages may represent arches which are present in lower forms but do not appear in the higher Mammals. The larynx is situated much farther cranially in the foetus and in the new- born child than in the adult. In a five months foetus it extends into the naso- pharyngeal cavity, whence it migrates caudally to its adult position. The laryngeal skeleton becomes ossified during postnatal life. Ossification begins in the thyreoid and cricoid cartilages ‘at the age of eighteen to twenty years, and in the arytenoids a few years later. Three centers appear in the thyreoid —one on each side near the inferior cornu and one in the medial line between the two wings. In the cricoid, ossification begins near the upper border on each side, in the arytenoids at the lower borders. Ossification usually begins earlier and proceeds more rapidly in the male than in the female. As an example of the explanation which Embryology offers of certain peculiarities of structure in the adult, the case of the recurrent laryngeal nerve may be cited. The heart and aortic arches are primarily situated in the cervical region. At that time a branch of the vagus on each side, passes behind the fourth aortic arch to reach the larynx. As the heart and arches recede into the thorax, the nerve is pulled caudally between its origin and termination, so that in the adult the left nerve bends around the arch of the aorta and the right around the subclavian artery. The Trachea. The portion of the original tube between the larynx and the two caudal out- growths which form the bronchi and lungs, develops into the trachea. It lies ventral to the cesophagus and is surrounded by mesodermal tissue which is 366 TEXT-BOOK OF EMBRYOLOGY. destined to give rise to the connective tissue, includng the cartilage, of the adult trachea (Figs. 284 and 320). The development of the tracheal rings is very similar to that of the laryngeal cartilages. During the eighth or ninth week con- densations appear in the mesenchyme, which are later transformed into hyalin cartilage. The rings are not complete but remain open on the dorsal side. At birth the trachea is collapsed, the ventral side being concave and the dorsal ends of each ring being in contact. After respiration begins it is dilated and becomes more or less rigid. Ossification of the tracheal rings begins in the male at the age of about forty years, in the female at about sixty. The glands of the trachea represent evaginations from the epithelial linings. The Lungs. As has been stated (p. 362), the caudal end of the original tube evaginates to form two hollow buds which are the beginnings of the two lungs (Fig. 324). The evagination takes place soon after or even along with the separation of the lung groove from the gut. The right bud soon gives rise to three secondary Aorta Mesonephros Diaphrag. lig. Upper limb bud of mesonephros Pleural cavity Csophagus Bronchi Body cavity Heart Pericardial cavity Fic. 324.—Transverse section of a 14mm. pig embryo, at the level of the upper limb buds, showing especially the two bronchi. buds, the forerunners of the three lobes of the right lung. The left bud gives rise to two secondary buds, the forerunners of the two lobes of the left lung (Fig. 325). The primary buds may be said to represent the two bronchi arising from the trachea, the five secondary buds to represent the bronchial rami which extend into the five lobes of the lungs. Successive evaginations from each of the five buds take place and form an extensive arborization for each lobe (Figs. 326 and 327). THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 367 The manner in which the bronchial rami branch is not definitely known. Some maintain that the branching is dichotomous, that is, each bud gives rise to two equal buds and each of these to two others, and so on. In order to as- sume the adult form, however, one of the buds places itself in line with the preceding bud or bronchus while the other places itself as a lateral outgrowth. Others hold that the growth is monopodial, that is, that the original bud grows in a more or less direct line and the others develop as lateral outgrowths. When Upper right lobe Trachea Middle right lobe Upper left lobe Mesoderm (mesenchyme) Lower right lobe Fic, 325.—Anlage of lungs of a human embryo of 4.3 mm. His. the evaginations that produce the bronchial rami are completed, each éerminal (respiratory) bronchus subdivides into three to six narrow tubules, the alveolar ducts. The latter again branch into several wider compartments, the aéria, from which several air sacs are given off. The walls of the air sacs are evagi- nated to form many closely set air cells which represent the ultimate branches of the air passages of the lungs. Trachea Left bronchus Right bronchus: Bronchial ramus Mesoderm (mesenchyme) Bronchial ramus Fic. 326.—Anlage of lungs of a human embryo of 85mm. His. While there is a general tendency toward bilateral symmetry in the various sets of bronchial rami, the lobes of the lungs are asymmetrical. This asym- metry is indicated in the five secondary buds that arise from the two primary, since three arise on the right side and only two on the left. The three on the right represent the upper, middle and lower lobes of the right lung (Fig. 325). The upper is known as the eparierial from the fact that its bronchus lies dorsal 24 368 TEXT-BOOK OF EMBRYOLOGY. to the pulmonary artery. No lobe develops on the left side corresponding to the upper (eparterial) on the right. There is a possibility that it is absent in order to allow the arch of the aorta to migrate caudally as it normally does (see p. 247). One of the larger ventral bronchial rami of the left lung is ab- sent, owing to the inclination of the heart toward the left side; but as a compensa- tion the corresponding ramus of the right lung develops more extensively and projects into the space between the pericardium and diaphragm as the infracardiac ramus. From the fact that the anlage of the respiratory system is enclosed within the mesentery between the gut and the pericardial cavity, and that its caudal end becomes enclosed within the dorsal edge of the septum transversum, it is obvious Trachea Pulmonary artery : Left bronchus Right bronchus Upper right bronch,. ramus Upper left bronch, ramus Middle right bronch. ramus Lower left branch vulmonary vein Lower right bronch. ramus Lower left Mesoderm bronch. ramus (mesenchyme) Fic. 327.—Anlage of lungs of a human embryo of 10.5 mm, is. that the lungs will push their way into the dorsal parietal recesses or pleural cavities (Figs. 328 and 333). The way in which the lungs and pleural cavities enlarge and separate the pericardium from the body wall on each side and from the diaphragm is described on page 378 (see Figs. 334 and 335). The mesoder- mal tissue that surrounds the primary lung buds is in part pushed before the numerous outgrowths and in part remains among them (Figs. 325, 326, 327). The part around the lungs, with its covering of mesothelium, comes to form the visceral layer of the plewra which closely invests the entire surface of the lungs and dips down between the lobes. At the roots of the lungs it is continuous with the parietal layer of the pleura lining the inner surface of the pleural cavi- ties. The mesodermal tissue among the bronchi and their terminations gives rise to the connective tissue that separates the lobes and lobules and invests all the structures in the interior of the lungs. This connective tissue at first con- THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 369 stitutes a large part of the lungs, but as development proceeds, the more rapid growth of the respiratory parts results in the relatively small amount of connective tissue characteristic of the adult lung. Changes in the Lungs at Birth.—At birth the lungs undergo rapid and remarkable changes in consequence of their assuming the respiratory function. These changes affect their size, form, position, texture, weight, etc., and furnish probably the only certain means of distinguishing between a still-born child and one that has breathed. In the foetus at term the lungs are small, possess rather sharp margins and lie in the dorsal part of the pleural cavities. Diaphragm Lungs Pleural cavities Fic. 328.—Transverse section of a pig embryo of 35 mm., showing the developing lungs (bronchial rami surrounded by mesoderm). The cesophagus is seen between the two lungs; above the cesophagus is the aorta. The dark mass in the lower part of the figure is the liver, Photograph, After respiration they enlarge, fill practically the entire pleural cavities and naturally become more rounded at their margins. The introduction of air into the air passages converts the compact, gland-like, foetal lung into a loose, spongy tissue. The specific gravity is changed from 1.056 to 0.342. While there is a gradual increase in the weight of the lungs during development, there is a very sudden increase at birth when the blood is freely admitted to them through the pulmonary arteries. The weight of the lungs relative to that of the body changes from about 1 to 70 before birth, to about 1 to 35 or 4o after birth. 370 TEXT-BOOK OF EMBRYOLOGY. Anomalies. Tue Larynx.—The larynx may be excessively large or unusually small. Occasionally the epiglottic cartilage consists of two pieces, indicating a failure of the two anlagen to fuse (p. 364). Similar defects may occur in the other cartilages that are derived from more than one anlage. The ventricle on either side may be abnormally large with an exaggerated appendage (laryngeal pouch). * This condition resembles that in the anthropoid apes. THE TRACHEA.—The trachea is sometimes absent, in which case the bronchi arise immediately below the larynx, indicating a failure on the part of the original tube to elongate. The trachea may be abnormally short. Rarely there is a direct communication between the trachea and cesophagus, probably due to an incomplete separation of the lung groove from the gut (p. 362). The cartilaginous rings may vary in number as a result of abnormal splittings and fusions. Tue Lunes.—Rarely the eparterial bronchial ramus on the right side arises as a branch of the trachea and not as a branch of the bronchus (p. 367). This condition is normal in certain Mammals (ox, sheep). Rarely an eparterial bronchial ramus is present on the left side, thus producing a third lobe for the left lung. In some animals an eparterial ramus is normally present on each side, the larger bronchial rami thus being bilaterally symmetrical. Varia- tion in size and number of lobes is not infrequent. Supernumerary or acces- sory lobes, formed either by evaginations from the original anlage or by in- dependent evaginations from the gut, are met with in rare cases. Occasionally some portion of either lung is defective. The bronchial bud that would normally give rise to the lung tissue in that region fails to develop properly, and the result is a number of rami, without the ultimate terminations, surrounded by vascular tissue. The rami may remain normal or may become dilated and form large bronchial cysts. PRACTICAL SUGGESTIONS. The anlage of the respiratory system can be seen in chick embryos about the beginning of the third day of incubation, or in young mammalian embryos (pig embryos of 6-8 mm.). Fix in Zenker’s fluid or in Bouin’s fluid, cut transverse sections in the cervical region and stain with Weigert’s hematoxylin and eosin. Time can be saved by staining 2m toto with borax-carmin, but the differentiation is not so good. Either technic can be used in follow- ing succeeding stages of development, so long as the embryos are not too large for con- venience in cutting sections. The structure and relations of the developing pleura can also be seen in these sections. In fact it is possible to use the same sets of sections for the study of the heart, pericardium, lungs and pleura. When the embryos are too large to section 7 foto, remove the lungs and subject them to the above technic. Very interesting comparisons can be made between sections of lung tissue from a still-born child and from one which has breathed, but died shortly after. THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 371 References for Further Study. Bonnet, R.: Lehrbuch der Entwickelungsgeschichte. Berlin, 1907. Fut, J. M.: The Development of the Lungs. American Jour. of Anat., Vol. VI, 1906. Géprert, E.: Die Entwickelung des Mundes und der Mundhéhle mit Driisen und Zunge; die Entwickelung der Schwimmblase, der Lunge und des Kehlkopfes der Wirbeltiere. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. II, Teil I, 1902. Hertwic, O.: Lehrbuch der Entwickelungsgeschichte des Menschen und der Wirbel- tiere. Jena, 1906. His, W.: Zur Bildungsgeschichte der Lungen beim menschlichen Embryo. Arch. }. Anat. u, Physiol., Anat. Abth., 1887. Katiius, E.: Beitrage zur Entwickelungsgeschichte des Kehlkopfes. Anat. Hefte, Bd. IX, 1897. Ko.rMann, J.: Lehrbuch der Entwickelungsgeschichte des Menschen. Jena, 1898. KoLLmann, J.: Handatlas der Entwickelungsgeschichte des Menschen. Jena, 1907. McMourricu, J. P.: The Development of the Human Body. Third Ed., 1907. Prersot, G. A.: Teratology. In Wood’s Reference Handbook of the Medical Sciences, Vol. VII, 1904. SYMINGTON, J.: On the Relations of Larynx and Trachea to the Vertebral Column in the Foetus and Child. Journ. of Anat. and Physiol., Vol. IX. CHAPTER XIV. THE DEVELOPMENT OF THE CCELOM (PERICARDIAL PLEURAL AND PERITONEAL CAVITIES), THE PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM, AND MESENTERIES. In the Chapter on the development of the germ layers, it is stated that the peripheral part of the mesoderm splits into two layers, an outer or parveial, and an inner or visceral (Fig. 81; see also p. 87). The parietal layer of mesoderm and the ectoderm constitute the somatopleure. The visceral layer and the entoderm constitute the splanchnopleure (Fig. 81). The cleft or cavity that appears between the parietal and visceral layers is the ca@lom or body cavity and is lined with a layer of flattened mesodermal cells known as the mesothelium. It will be remembered that in the earlier stages of development a portion of the embryonic disk becomes constricted off from the yolk sac to form the simple cylindrical body (p. 141). Along each side of the axial portion of the germ disk, and also at its cephalic and caudal ends, the germ layers bend ven- trally and then medially until they meet and fuse in the midventral line (p. 141). In this way a part of the somatopleure forms the lateral and ventral portions of the body wall Fig. 141). At the same time the axial portion of the entoderm is bent into a tube which is closed at both ends—the primitive gui—and is then pinched off from the rest of the entoderm except at one point, where the cavity of the gut remains in communication with the cavity of the yolk sac. The splanchnic mesoderm adjacent to the entoderm on each side comes in contact and fuses with the corresponding portion from the opposite side, thus forming a sheet of tissue which encloses the primitive gut and also forms a partition be- tween the two parts of the ccelom. This sheet of tissue is the common mesentery and is attached to the dorsal and ventral body walls along the medial line. The portion between the gut and the dorsal body wall is the dorsal mesentery, the portion between the gut and the ventral body wall is the ventral mesentery. Thus the gut is suspended in the common mesentery (Figs. 235 and 320). When portions of the somatopleure and splanchnopleure are bent ventrally the coelom between the portions is naturally carried with them. This part of the ccelom thus becomes enclosed within the cylindrical body and constitutes the intraembryonic or simply the embryonic celom (body cavity proper). The part of the ccelom which, while the germ layers were still flat, was situated more peripherally constitutes the extraembryonic ccelom or exocelom (extraembryonic 372 PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 373 body cavity). From the nature of the bending process, the embryonic ccelom is divided into bilaterally symmetrical parts by the common mesentery (Fig. 235). These two simple cavities are the forerunners of all the serous cavities of the body. The various partitions between the serous cavities, the walls of the cavities and the mesenteries proper are all derived from the somatic and splanchnic mesoderm with its covering of mesothelium. While the foregoing would represent a typical case of early coelom and mesentery formation, there are certain modifications and peculiarities in the higher Mammals and in man. In all cases the splitting of the mesoderm to form the ccelom proceeds from the periphery of the germ disk toward the axial portion (p. 89). In the human embryo the bending ventrally and fusing of the germ layers to form the cylindrical body begins in the anterior region of the disk and is accomplished there before the splitting of the mesoderm is com- pleted. The peripheral splitting has resulted in the formation of the exoccelom, but at the time when the ventral fusion of the germ layers takes place, the split- ting has not extended axially to a sufficient degree to form the intraembryonic celom. The latter, which appears later in this region, never communicates laterally, therefore, with the exoccelom. Caudal to this region the ccelom is formed as in the typical case. The more anterior part of the coelom on each side is thus primarily a pocket-like cavity. It communicates with the rest of the ccelom at about the level of the yolk stalk. In the region of the fore-gut, the future cesophagus, no distinct mesentery is formed, but the fore-gut remains broadly attached to the dorsal body wall. A ventral mesentery is lacking from a point just cranial to the yolk stalk to the caudal end of the gut. There are no ¢ccelomic cavities in the branchial arches, the ccelom extending only to the last branchial groove. In very young human embryos the primitive segments contain small cavities. These cavities soon disappear, being filled with cells from the surrounding parts of the segments. Whether they represent isolated portions of the coelom is not certain. In the lower Vertebrates, the cavities of the primitive segments regularly communicate with the ccelom, and in the sheep the cavities of the first formed segments are continuous with the ccelom. In the head there is no cavity analogous to the ccelom in the body. In but one human embryo have any cavities in the head resembling those of the primitive segments been observed (see p. 301). The Pericardial Cavity, Pleural Cavities and Diaphragm. The pericardial and pleural cavities and diaphragm are so closely related in their development that they must be considered together. Ih the region just caudal to the visceral arches, where the two anlagen of the heart appear, the embryonic ccelom becomes dilated at a very early stage to form the primitive pericardial cavity (parietal cavity of His). After the two anlagen of the heart 374 TEXT-BOOK OF EMBRYOLOGY. unite to form a simple tubular structure (p. 222; also Fig.. 194), the latter is suspended in the cavity by a mesentery which consists of a dorsal and a ventral part, a dorsal and a ventral mesocardium. By these the cavity is at first divided into two bilaterally symmetrically parts. The mesocardia soon disappear and leave the heart attached only to the large vascular trunks which suspend it in the single pericardial cavity. The early pericardial cavity is simply the cephalic end of the embryonic ccelom and is therefore directly continuous with the rest of the coelom. As mentioned on p. 373 it does not, however, at any time communicate laterally with the extraembryonic ccelom. The communication between the pericardial cavity and the rest of the em- bryonic ccelom is soon partly cut off by the development of a transverse fold —the sepium transversum. This septum is formed in close relation with the omphalomesenteric veins. These vessels unite in the sinus venosus at the caudal end of the heart, whence they diverge in the splanchnic mesoderm. Fic. 329.—Transverse sections of a rabbit embryo, showing how the omphalomesenteric veins (vom) push outward across the ccelom and fuse with the lateral body wall, forming the ductus pleuro-pericardiacus (rp, to) 3;@m, amnion. Ravn, They are thus embedded in the mesodermal layer of the splanchnopleure, and as the latter closes in from either side to form the gut, the vessels form ridge-like projections into the ccelom. As the vessels increase in size, the ridges become so large that the splanchnic mesoderm is pushed outward against the parietal mesoderm and fuses with it (Fig. 329). Thus a partition is formed on each side, which is attached on the one hand to the mesentery and on the other hand to the ventral and lateral body walls, and which contains the omphalomesenteric veins. It is obvious that these partitions, forming the septum transversum, close the ventral part of the communication between the pericardial cavity and the rest of the ceelom. The dorsal part of the communication remains open on each side of the mesentery as the ductus pleuro-pericardiacus (dorsal parietal recess of His) (Figs. 329 and 330). As the heart develops it migrates caudally, and by corresponding migration the pericardial cavity draws the ventral edge of the septum transversum farther caudally, so that the cephalic surface of the latter faces ventrally and cranially. PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES, 375 In other words, the septum comes to lie in an oblique cranio-caudal plane. The pericardial cavity therefore comes to lie ventral to the ductus pleuro-pericardiaci The latter—one on each side of the mesentery—are two passages which com- Ventral aortic trunk Pericardial cavity Se :.. Dorsal mesocardium Lateral mesocardium Sinus venosus ~ Duct of Cuvier Pericardium i ~— Left umbilical vein Septum transversum q i z Left omphalomes, vein Liver Ductus pleuro-pericardiacus Ductus choledochus ee Stomach Yolk stalk = Byes ; inves] Peritoneal cavity vi 8, 3 ase Ae - Fic, 330.—From a model of the septum transversum, liver, etc., of a human embryo of 3mm, His, Kollman. municate on the one hand with the pericardial cavity and on the other hand with the peritoneal cavity, while they themselves form the cavities into which the lungs grow—the pleural cavities. (Compare Figs. 330, 331 and 332.) Ductus pleuro- -—..Ductus pleuro- pericardiacus am . pericardiacus “ y Duct of Cuvier oF 4 eN Nea Pericardial cavity Fic. 331.—View (in perspective) of the pericardial cavity and ductus pleuro-pericardiaci of a rabbit embryo of 9 days.. Ravn. The pleural cavities also become separated from the pericardial cavity, ap- parently through the agency of the ducts of Cuvier. The anterior and posterior cardinal veins on each side unite to form the duct of Cuvier which then extends 376 TEXT-BOOK OF EMBRYOLOGY. from the body wall through the dorsal free edge of the septum transversum to join the sinus venosus (Fig. 330). This free edge is pushed farther and farther into the ductus pleuro-pericardiacus (Fig. 331) until it meets and fuses Pleural cavity | __ Lateral mesocardium Lateral mesocardium --- “~~ Dorsal mesocardium — Heart Pericardial cavity -- Fic. 332.--View (in perspective) of the pericardial and pleural cavities of a human embryo of 7.5mm. Kollmann. The arrow points through the opening which forms the communication between the pleural and peritoneal cavities, and which is eventually closed by the pleure-peritoneal membrane. with the mesentery or posterior mediastinum. This process thus produces a septum between each pleural cavity and the pericardial cavity. The septum transversum early acquires still more complicated relations Pleuro-peritoneal membrane{ Pleuro-peritoneal membrane Mesentery of Csophagus inf. vena cava Inferior vena cava ~f---1 Dorsal mesogastrium és j Mesonephros +- Fic, 333.—Ventral view (in perspective) of parts of the lungs, pleural cavities, peritoneal cavity, and the pleuro-peritoneal membranes in a rat embryo. Rav, from the fact that the liver grows into its caudal part (Fig. 330). It may, for this reason, be divided into a caudal part in which the liver is situated and which furnishes the fibrous capsule (of Glisson) and the connective tissue of the liver, and a cephalic part which may be called the primary diaphragm. These two parts at first are not separate, the separation taking place secondarily. After PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 377 the separation between the pericardial cavity and the pleural cavities, the latter for a time remain in open communication with the rest of the ccelom or-peritoneal cavity. The lungs, as they develop, grow into the pleural cavities (Fig. 332) until their tips finally touch the cephalic surface of the liver. At this point folds grow from the lateral and dorsal body walls (Fig. 333) and unite ventrally with the primary diaphragm and medially with the mesentery. These folds— the plewroperitoneal membranes—separate the pleural cavities from the perit- oneal cavity and completethediaphragm. ‘Thus the diaphragm, from thestand- Fic. 335. Fic. 334.—Transverse section through the thoracic region of a rabbit embryo of 15 days. Hochstetter. Fic. 335.—Transverse section through the thoracic region of a cat embryo of 25 mm. Hochstetter. I.v.c,, Inferior vena cava; Inf.-c. l., infracardiac lobe of lung; L., lung; Oc.. cesophagus; Pc. cav., pericardial cavity; Pl.cav., pleural cavity; Pl.-p.m., pleuro-pericardial membrane; Pu.-h.1., pulmo-hepatic recess. ’ point of development, consists of two parts: a ventral part which is the cephalic portion of the original septum transversum, and a dorsal part which develops later from the body wall and is the closing membrane between the peritoneal and pleural cavities. ‘The musculature of the diaphragm is considered in the chapter on the muscular system (p. 300). While the foregoing structures are being formed, decided changes take place in their positions and relations. At first the heart lies far forward in the cervi- cal region near the visceral arches. Laterit migrates caudally and the pericardial 378 TEXT-BOOK OF EMBRYOLOGY. cavity comes to occupy much of the ventral part of the thorax, the pericardium having extensive attachments to the ventral body wall and to the cephalic sur- face of the primary diaphragm (Fig. 330). The diaphragm is much farther forward than in the adult and is broadly attached to the cephalic surface of the liver. The principal changes which bring about the adult conditions are the growth of the lungs, the separation of the diaphragm from the liver, and the caudal migration of the diaphragm itself. With the development of the lungs, the pleural cavities 42 necessarily enlarge and push their way ventrally. In so doing they split the pericardium away from the lateral body walls and likewise from the dia- phragm (compare Figs. 334 and 335). Thus the pericardial cavity comes to be confined more and more closely to the medial ventral position. The separation of the liver from the primary diaphragm is caused by changes in the peritoneum which at first covers the caudal, lateral and ventral surfaces of the liver. The cephalic surface of the liver, as stated above, is covered by the primary diaphragm itself. The peritoneum is reflected from the surface of the liver on to the diaphragm, and at the line of reflection a groove appears on each side, extending from the midventral line around as far as the attachment of the liver to the diaphragm. The Fic. 336.—Diagram showingthe grooves gradually grow deeper, the peritoneum position of the diaphragm in J 1ching its way, as a flat sac, between the two human embryos of different a ame Mal ee structures, until the separation is almost complete. € OSItIONS are ose Shown. x ia Snbeyseol Mall’s collection There is left, however, an area of attachment (except KO, which is a ro,2 . . : pea bnibsryo bt the Paeeslicte between the liver and diaphragm, over which the tion); XIT beinganembryo of peritoneum is reflected, the ligamentum coronarium XIX. of 5 eles thay aa hepatis. In the medial line there is also left a IX, of 17 mm.; XLII, of 5 broad thin lamella which is attached to the dia- mm.; VI, of 24 mm. The 3 numerals on the right indicate Phragm, the liver and the ventral body wall. This a is a remnant of the ventral mesentery and forms the ligamentum suspensorium (falciforme) hepatis. In its free caudal edge is embedded the ligamentum teres hepatis which is closely related to the umbilical vein (see p. 265). The diaphragm itself, during its development, migrates from a position in the cervical region, where the septum transversum first appears, to its final position opposite the last thoracic vertebre. During the. migration the plane of direction also changes several times, as may be seen in Fig. 336. PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 379 The Pericardium and Pleura.—Since the pericardial cavity represents a portion of the original ccelom, the lining of the cavity must be a derivative of either the parietal or the visceral layer of mesoderm orof both. The common mesentery in which the heart develops is derived from the visceral layer. Con- sequently the epicardium is a derivative of the visceral mesoderm (Fig. 194). The pericardium is derived from three regions of mesoderm. The greater part is derived from the parietal mesoderm, since the body wall which is com- posed of parietal mesoderm is also primarily the wall of the pericardial cavity. A small dorsal portion is probably derived from the mesoderm at the root of the dorsal mesocardium (Fig. 194). The septum transversum primarily forms the caudal wall of the pericardial cavity, and, since the septum is a derivative of the visceral layer, the caudal wall is derived from this layer. The three portions are, of course, continuous. The lungs first appear in the common mesentery as an evagination from the primitive gut (Fig. 320, p. 362). As they develop further they grow into the pleural cavities, pushing a part of the mesentery before them. This part of the mesentery thus invests the lungs and forms the visceral layer of the pleura which is therefore a derivative of the visceral mesoderm. The parietal layer of the pleura is a derivative of the parietal mesoderm, since the wall of the pleural cavity is primarily the body wall. The lining of all these cavities is at first composed of mesothelium and mesenchyme. The latter is transformed into the delicate connective tissue of the serous membranes, and the mesothelium becomes the mesothelium of the membranes. The Omentum and Mesentery. From the septura transversum (or diaphragm) to the anus the gut is sus- pended in the ccelom (or abdominal cavity) by means of the dorsal mesentery. This is attached to the dorsal body wall along the medial line and lies in the medial sagittal plane (Fig. 301; compare with Fig. 235). Onthe ventral side of the gut a mesentery is lacking from the anus to a point just cranial to the yolk stalk (p. 373). There is, however, a small ventral mesentery extending a short distance caudally from the septum transversum. On account of its relation to the stomach this is known as the ventral mesogastrium (Fig. 301). These two sheets of tissue, the dorsal and ventral mesenteries, are destined to give rise to the omenta and mesenteries of the adult. Owing to the enormous elongation of the gut and its extensive coiling in the abdominal cavity, the primary mesen- teries are twisted and thrown into many folds which enclose certain pockets or burse. Furthermore, certain parts of the gut which are originally free and movable become attached to other parts and to the body walls through fusions of certain parts of the mesentery with one another and with the body walls. 380 TEXT-BOOK OF EMBRYOLOGY. The Greater Omentum and Omental Bursa.-—A small part of the gut caudal to the diaphragm is destined to become the stomach, and the portion of the mesentery which attaches it to the dorsal body wall is known as the dorsal mesogastrium (Fig. 301). The latter is inserted along the greater curvature of the stomach and lies in the medial sagittal plane so long as the stomach lies in this plane. When the stomach turns so that its long axis lies in a transverse direction and its greater curvature is directed caudally (p. 337), the dorsal mesogastrium changes its position accordingly. From its attachment along the dorsal body wall it bends to the left and then ventrally to its attachment along the greater curvature of the stomach. Thus a sort of sac is formed dorsal to the stomach (Figs. 337 and 338). ‘This sac is really a part of the abdominal or Lesser Lesser omentum omentum b-Stomach Stomach Bile duct - Duodenum Greater Duodenum Greate: Cmentun Common omentum Meso- Transverse mesentery colon mesocolon Czcum Oommen Desc. mesentery’ mesocolon Small Caecum: Bi Desc. colon intestine Desc. Appendix _} Small colon Mesentery intestine Yolk stalk Yolk stalk Rectum Rectum Fic. 337. Fic. 338. Fic. 337.—Diagram of the gastrointestinal tract and its mesenteries at an early stage. Ventral view. Hertwig, Fic. 338.—Same at a later stage Hertwig, The arrow points into the bursa omentalis. peritoneal cavity and opens toward the right side. The ventral wall is formed by the stomach, the dorsal and caudal walls by the mesogastrium. The cavity of the sac is the omental bursa (bursa omentalis); the mesogastrium forms the greater omentum (omentum majus). The opening from the bursa into the general peritoneal cavity is the epiploic foramen (foramen of Winslow). (Fig. 314.) From the third month on, the greater omentum becomes larger and gradually extends toward the ventral abdominal wall, over the transverse colon, and then caudally between the body wall and the small intestine (Figs. 339 and 340). The portion between the body wall and intestine encloses merely a flat cavity continuous with the larger cavity dorsal to the stomach. From the fourth month on, the omentum fuses with certain other structures and becomes less free. The dorsal lamella fuses with the dorsal body wall on the left side and PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 381 with the transverse mesocolon and transverse colon (Fig. 341). During the first or second year after birth the two lamelle fuse with each other caudal to the transverse colon to form the greater omentum of adult anatomy. Diaphragm... Lesser omentum.__ mY Pancreas-.__ Bursa omentalis-~._ Stomach Greater omentum---- Duodenum ---- Transverse mesocolon- Transverse colon - Mesentery of ___- small intestine Small intestine -----4-/--~~~ FIG. 339. Diaph.---+ er eer hee L. oment.,__| L. oment. “""~ (h.-g. lig.) B. oment. ---- G.-col. * | lig. -- G. oment.-4 Mes. ~~ Fic. 340. Fic. 341. Fics. 339, 340 and 341.—Diagrams showing stages in the development of the bursa omentalis, the greater omentum, and the fusion of the latter with the transverse mesocolon. Diagrams represent sagittal sections. For explanation of lettering in Figs. 340 and 341 see Fig. 339. The Lesser Omentum.—It has already been noted that the liver grows into the caudal portion of the septum transversum (p. 376). Since the ventral mesentery in the abdominal region, or the ventral mesogastrium, is primarily 382 TEXT-BOOK OF EMBRYOLOGY. directly continuous with the septum transversum, it is later attached to the liver. In other words it passes between the liver and the lesser curvature of the stomach and also extends along the duodenal portion of the gut for a short ‘distance (Fig. 301). As the stomach turns to the left the ventral mesentery is also drawn toward the left and comes to lie almost at right angles to the sagittal plane of the body, forming the lesser omentum (omentum minus) or the hepato- gastric and hepatoduodenal ligaments of the adult (Figs. 341 and 342). The Mesenteries.—So long as the intestine is a straight tube, the dorsal mesentery lies in the medial sagittal plane, its dorsal attachment being practi- cally of the same length as its ventral (intestinal) attachment. As development proceeds, the intestine elongates much more rapidly than the abdominal walls, and the intestinal attachment of the mesentery elongates accordingly. When the portion of the intestine to which the yolk stalk is attached grows out into the proximal end of the umbilical cord (p. 339), the corresponding portion of the mesentery is drawn out with it (Fig. 301). When the intestine returns to the abdominal cavity and forms the primary loop, with the cecum to the right side (p. 340), its mesenteric attachment is carried out of the medial sagittal plane. This results in a funnel-shaped twisting of the mesentery (Figs. 337 and 338). The portion of the mesentery which forms the funnel is destined to become the mesentery of the jejunum, ileum, and ascending and transverse colon, and is attached to the dorsal body wall at the apexof the funnel (Fig. 337, 338, 342). This condition is reached about the middle of the fourth month. Up to this time the mesentery and intestine are freely movable, that is, they have formed no secondary attachments. From this time on, as the intestine continues to elongate and forms loops and coils, the mesentery is thrown into folds, and certain parts of it fuse with other parts and with the body wall. Thus certain parts of the intestine become less free or less movable within the abdominal cavity. The duodenum changes from the original longitudinal position to a more nearly transverse position and, with its mesentery—the mesoduodenum—tuses with the dorsal body wall, thus becoming firmly fixed. Since the mesoduode- num fuses with the body wall, the duodenum has no mesentery in the adult. The pancreas, which is primarily enclosed within the mesoduodenum, also becomes firmly attached to the dorsal body wall (compare Figs. 339 and 340). The mesentery of the transverse colon, or the transverse mesocolon, which lies across the body ventral to the duodenum (Figs. 338 and 342), fuses with the ventral surface of the latter and with the peritoneum of the dorsal body wall. In this way the dorsal attachment of the transverse mesocolon is changed from its original sagittal direction to a transverse direction (Figs. 340 and 341). The mesocolon itself forms a transverse partition which divides the peritoneal cavity into two parts, an upper (or cranial) which contains the stomach and liver, and PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 383 a lower (or caudal) which contains the rest of the digestive tube except the duodenum. The mesentery of the duodenum and pancreas changes from a serous membrane into subserous connective tissue, and these two organs as- sume the retroperitoneal position characteristic of the adult (Fig. 339). The mesentery of the descending colon, or the descending mesocolon, lies in the left side of the abdominal cavity, in contact with the peritoneum of the body wall (see Fig. 342). It usually fuses with the peritoneum, and the descending Dors. mesogastrium Lesser omentum y ’ (hep.-gast. lig.) : —— Spleen Bile duct Mesoduodenum ___ Greater omentum Transv. colon Mesocolon ——;.-# pet \ ; Duo.-jej. flexure t-y Asc, colon Desc. colon Desc. mesocolon Appendix Yolk stalk Medial line Fic. 342.—Gastrointestinal tract and mesenteries in a human embryo. The arrow points into the bursa omentalis, Kollmann, colon thus becomes fixed. After the ascending colon is formed, the ascending mesocolon usually fuses with the peritoneum on the right side (see Fig. 342). In a large percentage (possibly 25 per cent.) of individuals, the fusion between the peritoneum and the ascending and descending mesocolon is incomplete or wanting. The sigmoid mesocolon bends to the left to reach the sigmoid colon, but forms no secondary attachments. It is continuous with the mesorectum which maintains its original sagittal position. A sheet of tissue—the mesoappendix— continuous with and resembling the mesentery, is attached to the cecum and vermiform appendix (Fig. 342). It probably represents a drawn out portion of 25 384 TEXT-BOOK OF EMBRYOLOGY. the original common mesentery, since the cecum and appendix together are formed as an evagination from the primitive gut. Normally the mesentery of the small intestine forms no secondary attach- ments, but is thrown into a number of folds which correspond to the coils of the intestine. The secondary attachments of the greater omentum and the fusion of the two lamella have been described earlier in this chapter (p. 380). The mesen- teries of the urogenital organs are considered in connection with the develop- ment of those organs (Chapter XV). The Peritoneum.—The thin layer of tissue—composed of delicate fibrous connective tissue and mesothelium—which lines the abdominal cavity and is re- flected over the various omenta, mesenteries and visceral organs, is derived wholly from the mesoderm. The lining of the ccelom is composed of mesothe- lium and mesenchyme. The latter gives rise to the connective tissue of the serous membranes, and the mesothelial layer becomes the mesothelium of these membranes. Anomalies. THE PericarpiuM.—Anomalous conditions of the pericardium are usually, although not necessarily, associated with anomalies of the heart. They may also be associated with defects in the diaphragm. Displacement of the heart (ectopia cordis) is accompanied by displacement of the pericardium. The heart sometimes protrudes through the thoracic wall, and, as a rule, in such cases is covered by the protruding pericardium. In extensive cleft of the thoracic wall (thoracoschisis, Chap. XIX) the pericardium may be ruptured. Tue DiapHRAGM.—The most common malformation of the diaphragm is a defect in its dorsal part, occurring much more frequently on the left than on the right side. ‘The defect may affect but a small portion or may be extensive, the peritoneum being directly continuous with the parietal layer of the pleura. Such defects are due to the imperfect development of the pleuro-peritoneal mem- brane which normally grows from the dorso-lateral part of the body wall and fuses with the edge of the primary diaphragm, thus separating the pleural and and peritoneal cavities (p. 377). The most conspicuous result of defects in the dorsal part of the diaphragm is diaphragmatic hernia, in which parts of the stomach, liver, spleen and intestine project into the pleural cavity, either free or enclosed in a peritoneal sac. Defects in the ventral part of the diaphragm, due to imperfect development of portions of the septum transversum, are not common. ? THE MESENTERIES AND OmENTA.—Extensive malformations of the mesen- teries apparently do not occur without extensive malformations of the digestive tract. One of the most striking anomalous conditions is a retained embryonic PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 385 simplicity of the mesentery, concurrent with corresponding simplicity,in the loops and coils of the intestine. In this anomaly the intestine has failed to arrive at its usual complicated condition and the mesentery has not undergone the usual processes of folding and fusion (p. 382 e¢ seq.). Minor variations in the mesenteries and omenta are probably due’ largely to imperfect fusion of certain parts with one another and with the body wall. It is not uncommon to find the ascending or descending colon, or both, more or less free and mov- able. This condition is due to imperfect fusion of the mesocolon with the body wall (p. 383). Ifthe greater omentum is wholly or partially divided into sheets of tissue, the two primary lamelle have failed to fuse completely (p. 381). This divided condition is normal in many Mammals. PRACTICAL SUGGESTIONS. The Calom and Common Mesentery—Chick embryos afford excellent material for the study of the formation of the ccelom and common mesentery. Use successive stages during the latter part of the first day and during the second day of incubation. Remove the embryo from the egg, being careful to keep it as nearly as possible in the natural position, fix in Flemming’s or in Zenker’s fluid, cut transversely in paraffin and stain with Heidenhain’s hematoxylin. Time can be saved by staining iz toto in borax carmin, but the differentia- tion is not so clear. The various stages in the development of the ccelom and mesentery are well shown. If serial sections are cut, one can often trace the successive stages in one embryo- by examining sections in succession through the series, for the bending of the germ layers begins in the head region and gradually progresses toward the tail. The Primitive Pericardial Cavity —The dilatation of the ccelom in the cervical region to form the primitive pericardial cavity can be seen in transverse sections of chick embryos of the latter part of the first day of incubation. Prepare the specimens as directed in the pre- ceding paragraph. The specimens prepared for the study of the ccelom and mesentery will serve this purpose if sections from the cervical region are selected. The Septum Transversum.—The early stages of the septum can be seen in sections of chick embryos of the second day of incubation, prepared as directed above. It is best to cut serial sections. By tracing the omphalomesenteric veins, the ridges formed by the veins can be seen projecting into the ccelom. If later stages are examined, the ridges will be seen to extend across the ccelom and to fuse with the ventro-lateral part of the body wall. The ridges form the anlage of the septum. ' Later Stages.—The study of the later stages becomes more difficult as the structures increase in complexity. Chick or mammalian embryos in different stages are fixed in Zenker’s fluid or Bouin’s fluid, sectioned transversely in paraffin, and stained with Weigert’s hematoxylin and eosin. It is best to cut serial sections, and time can be saved by staining in toto with borax-carmin if the embryos are not too large (not more than 10 mm.). While much can be learned by examining a series of sections, it is advisable to recon- siruct from serial sections (see Appendix) the developing organs in one or two stages. The models thus obtained are extremely useful in understanding the relations of the structures. After the embryo has attained a considerable size, very careful gross dissections will often prove instructive. 386 TEXT-BOOK OF EMBRYOLOGY. References for Further Study. Bracuet, A.: Recherches sur le développement du diaphragme et du foie. Jour. c P Anat. et de la Physiol., T. XXXII, 1895. Broman, J.: Die Entwickelungsgeschichte der Bursa omentalis und ahnlicher Reces: bildungen bei den Wirbeltieren. Wiesbaden, 1904. Broman, I.: Ueber die Entwickelung und Bedeutung der Mesenterien und der Kérper hdhlen bei den Wirbeltieren. Evgebnisse der Anat. u. Entwick., Bd. XV, 1906. Brossikr, G.: Ueber intraabdominale (retroperitoneale) Hernien und Bauchfelltascher nebst einer Darstellung der Entwickelung peritonealer Formationen. Berlin, 189. Hertwic, O.: Lehrbuch der Entwickelungsgeschichte des Menschen und der Wirbeltiere Jena, 1906. Keise1, F., and MALL, F. P.: Manual of Human Embryology, Vol. I, 1910. Kraatscu: Zur Morphologie der Mesenterialbildungen am Darmkanal der Wirbeltiere Morph. Jahrbuch, Bd. XVITI, 1892. Koutimann, J.: Lehrbuch der Entwickelungsgeschichte des Menschen. Jena, 1898. KoLiMann, J.: Handatlas der Entwickelungsgeschichte des Menschen. Bd. II, 1907. MALL, F. P.: Development of the Human Ceelom. Jour. of Morphol., Vol. XII, 1897. Prersot. G. A.: Teratology. In Wood’s Rejerence Handbook of the Medical Sciences. 1904. Ravn, E.: Ueber die Bildung der Scheidewand zwischen Brust- und Bauchhdhle in Saugetierembryonen. Arch. f. Anat. u. Physiol., Anat. Abth., 1889. STRAHL and Carius: Beitriége zur Entwickelungsgeschichte des Herzens und der Korperhohlen. Arch. j. Anat. u. Physiol., Anat. Abth., 1889. Swakn, A.: Recherches sur le développement du foie, du tube digestif, de l’arritre- cavit® du peritoine et du mésentére. Premiére partie, Lapin. Jour. de P Anat. et de la Phystol., T. XXXIII, 1896. Seconde partie. Embryons humains. T. XXXII, 1897. Toipt, C.: Bau und Wachstumsverdénderung der Gekrése des menschlichen Darm- ‘kanals. Denkschr. der kais. Akad. Wissensch. Wien. Math.-Naturwissen. Classe, Bd. XLI, 1879. CHAPTER XV. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. No other system in the body presents such peculiarities of development as the urogenital system. In the first place, it is exceedingly complicated on ac- count of its many parts. It is derived from both mesoderm (mesothelium and mesenchyme) and entoderm. The urinary portion develops into a great com- plex of ducts for the carrying off of waste products. The genital portion in. both sexes becomes highly specialized for the production and carrying off of the sexual elements. In the second place, instead of one set of urinary organs: developing and persisting, three sets develop at different stages. The first set (the pronephroi) disappears in part, but leaves certain structures whichare used, so to speak, in the development of thesecond. Thesecond set (the meso- nephroi) disappears for the most part, leaving,-however, some portions which are taken up in the development of the genital organs and other portions which persist as rudimentary structures in the adult. The third set (the metanephroi or kidneys) develops in part from the second and in part is of independent origin. -These conditions afford one of the most striking examples of the repe- tition of the phylogenetic history by the ontogenetic, or, in other words, of von Baer’s law that an individual, in its development, has a tendency to repeat its ancestral history; for the first and second sets of urinary organs in the human embryo represent systems that are permanent in the lower Vertebrates. In the third place, the ducts of the genital organs are not homologous in the two sexes. In the male the ducts (deferent duct, duct of the epididymis, efferent ductules) are derived from the second set of urinary organs; in the female they (the oviducts) are derived from other ducts which develop in the second set of urinary organs, but which are not functionally a part of the latter. THE PRONEPHROS. The pronephros, with the pronephric duct, is the first of the urinary organs to appear. In embryos of 2-3 mm. there are two pronephric tubules on éach side, situated at the level of the heart. Although their mode of origin has not been observed in the human embryo, it is probable, judging from observations on lower Vertebrates, that they arise as evaginations of the mesothelium. The part of the mesothelium involved is that adjacent to the intermediate cell mass (Fig. 343). (The intermediate cell mass is the portion of the mesoderm interven- ing between the primitive segments and the unsegmented parietal and visceral 387 388 TEXT-BOOK OF EMBRYOLOGY. layers; p. 84.) The more cephalic of the two tubules becomes hollow and opens into the ccelom; the more caudal is merely a solid cord of cells. Neither tubule forms any connection with the pronephric duct. At each side of the root of the mesentery a small elevation, which projects into the ccelom, probably represents a rudimentary glomerulus. A glomerulus in the lower Vertebrates, where the pronephros develops to a much greater degree than in Mammals, contains tortuous vessels derived from branches of the aorta (Fig. 344). The mesonephros (p. 389), beginning to develop almost as soon as the pro- nephros and in the same relative position, forms a ridge which projects into the celom. The pronephric tubules thus become embedded in the mesonephric ridge. The pronephric duct begins to develop about the same time as the tubules. It appears as a longitudinal ridge on the outer side of the intermediate cell mass Selerotome Myotome New Ectoderm a es Parietat limb bud mesoderm Neural tube a : S Visceral mesoderm Entoderm Primitive Pronephric aorte tubule Fic. 343.—Transverse section of a dog embryo with 19 primitive segments. Bonnet, Section taken through sixth segment. at the level of the heart and projects into the space between the mesoderm and ectoderm. ‘The ridge is at first solid but soon acquires a lumen, and gradually extends to the caudal end of the embryo where it bends medially to open into the gut. The origin of the caudal portion of the duct is a matter of dispute. It comes in contact and fuses with the ectoderm, but whether in the higher ani- mals the fusion is secondary or signifies a derivation from the ectoderm has not been determined. When first formed, the entire duct lies on the outer side of the intermediate cell mass, but later becomes embedded in the mesonephric ridge (p. 391). The pronephric tubules are but transient structures and have no functional significance in man and the higher Vertebrates. The ducts, however, remain and become the ducts of the second set of urinary organs, the mesonephroi. The significance of the pronephros can be understood only by reference to the conditions in the lower animals. In the latter the pronephros acquires a relatively higher degree of de- THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 389 velopment than in the higher forms, The tubules are segmentally arranged and are present in many’segments of the body. They open at their outer ends into the ducts, and at their inner ends into the coelom through ciliated funnel-shaped mouths or nephrostomes. Little masses of mesoderm, containing tortuous vessels derived from branches of the aorta, form glomeruli which project into the ccelom. Waste products are removed from the blood through the agency of the glomeruli and are collected in the ceelom. They are then taken up by the pronephric tubules and carried away by the ducts. In some of the Round Worms there is not even a longitudinal duct, but the tubules open directly on the outer surface of the body. In the lowest Fishes all the tubules on each side open into a longitudinal duct which opens into the cloaca. In these lower forms of animal life the pronephroi constitute the permanent urinary apparatus. In the ascending scale the mesonephroi appear (higher Neh. Fic. 344.—Diagram of the pronephric system in an amphibian. Bonnet. Ceal., Ceelom; Glom., glomerulus, containing ramifications of a branch of the aorta; Nch., notochord; Pron. ¢., pronephric tubule. Fishes, Amphibia) and assume the function of carrying off waste products. The prone- phroi also develop, but to a lesser degree. Still higher in the scale (Reptiles, Birds, Mam- mals) the kidneys (metanephroi) appear and the mesonephroi lose their functional sig- nificance. But even in the very highest Mammals the pronephroi appear, in a very rudimen- tary form, in each individual in the earliest embryonic stages, thus repeating the ancestral history. THE MESONEPHROS. The mesonephroi, which constitute the second set of urinary organs, appear in embryos of 2.6-3.0 mm., immediately following the pronephroi. They be- gin to develop just caudal to the pronephric tubules and in the same relative position as the latter, that is, in the intermediate cell mass. Condensations* appear in the mesenchyme and become more or less tortuous. At their inner ends they form secondary connections with the mesothelium and at their outer ends they join the pronephric duct which now becomes the mesonephric (or Wolffian) duct. The cells acquire an epithelial character, lumina appear, and the tortuous mesenchymal condensations thus become true tubules. Their connections with the mesothelium soon disappear (Fig. 345). *The term “condensation” is here used to mean increased density of tissue due mainly to proliferation of cells. 390 TEXT-BOOK OF EMBRYOLOGY. After the tubules are formed, other condensations of the mesenchyme appear near their inner ends. A branch from the aorta enters each condensation and breaks up into a number of smaller vessels which ramify inside, the entire structure thus becoming a glomerulus. Each glomerulus pushes against the corresponding tubule, the latter becoming flattened and then growing around the glomerulus. In this way the glomerulus becomes surrounded by two layers of epithelium, except at the point where the vessels enter, and the wholestructure —the Malpighian corpuscle—resembles very closely a renal corpuscle of the adult Roof Spinal , plate ganglion Amnion Post. cardinal vein Mesonephric (Wolffian) duct Blood vessel Glomerulus Mesonephric (Wo.ffian) ridge Coelom Mesentery Body wall with umbilical vein Intestine — Fic. 345.—From a transverse section of a sheep embryo of 21 days (15 mm.), showing the developing mesonephros, Bonnet. kidney. Waste products are removed from the blood through the agency of the glomeruli and are carried to the ducts by the mesonephric tubules (Fig. 345). The tubules themselves increase in length and become much coiled. Sec- ondary and tertiary tubules also develop and become branches of the primary. Whether these develop from condensations of the mesenchyme or as buds from the primary tubules has not been determined. Each tubule consists of two parts—(z) a dilated part around the glomerulus, composed of large flat cells and forming Bowman’s capsule, and (2) a narrower coiled part leading from THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 391 the glomerulus to the duct and composed of smaller cuboidal cells (F ig. 345). The primary mesonephric tubules are arranged segmentally, one appearing in each segment as far back as the pelvic region. Thus the intermediate cell mass may be considered as a series of nephrotomes, corresponding to the sclerotomes and myotomes. The segmental character is soon lost, however, owing to the inequality of growth between the mesonephros and the other seg- mental structures, and to the development of the secondary and tertiary tubules. As stated above, the first mesonephric tubules appear immediately caudal to Mid-brain =s4—Fore-brain Hind-brain Branchial groove I — Mesonephros Body wall , 4 Genital eminence Lower limb bud ———— i ‘ Fic. 346.—Human embryo of 5 weeks. The ventral body wall has been removed to disclose the mesonephroi. Kollmann. the pronephros. From this point their formation gradually progresses in a caudal direction as far as the pelvic region. By the further development of the primary and by the addition of the secondary and tertiary tubules and the glomeruli, the mesonephros as a whole increases in size and forms a large structure which projects into the ccelom on each side of the body, forming the so-called mesonephric or Wolffian ridge. It reaches the height of its develop- ment in the human embryo about the fifth or sixth week, at which time it ex- tends from the region of the heart to the pelvic region (Fig. 346). Each organ 392 TEXT-BOOK OF EMBRYOLOGY. is attached to the dorsal body wall by a distinct mesentery which, at its cephalic end, also sends off a band to the diaphragm—the diaphragmatic ligament of the mesonephros. The peritoneum is reflected over the surface of the meso- nephros, and on the ventro-medial side the mesothelium becomes thickened to form the genital ridge (p. 407; Figs. 314 and 346). ‘The mesonephric ducts are embedded in the lateral parts of the organs and extend throughout practically their entire length. Since the ducts are identical with the pronephric ducts, they open at first into the caudal end of the gut, or cloaca (p. 388; Fig. 360). At a little later period, when the urogenital sinus is formed, they open at the junction of the latter with the bladder (Fig. 363). Still later they open into the ie “= Appendage of epididymis Appendage tj 4 of testicie ‘, .... Mesonephric duct (duct of epididymis) t -)— -Paradidymis Testicle neewiesud eer Urogenital sinus Fic. 347.—Diagram representing certain persistent portions of the mesonephros in the male (see table). Kollmann. sinus itself (p. 403). A description of their further development is best deferred to the section on the male genital organs, since they become the genital ducts (p.,420). The mesonephroi function as urinary organs during the period of their existence in the embryos of all higher Vertebrates. Excretory products are con- veyed directly to the tubules by means of the glomeruli instead of being de- posited in the ccelom and then taken up by the tubules, as is the case in func- tional pronephroi (p. 389). The main excretory ducts are the same as in the pronephroi. Aside from the vessels in the glomeruli the mesonephroi are ex- ceedingly vascular organs. Large and small branches of the posterior cardinal veins ramify among the tubules (Figs. 314 and 232). The blood undergoes THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 893 purifying processes in its close contact with the tubules and is returned to the heart by the posterior cardinals, or, after the cephalic ends of the latter atrophy, by the subcardinals and the inferior vena cava (see p. 258; also Fig. 232, B). There is thus present a true renal portal system, similar to the hepatic portal system. : ; Although the mesonephroi become large functional organs durirg the earlier stages of development, they atrophy and disappear for the most part, coinci- dently with the appearance and development of the kidneys. The degeneration begins during the sixth or seventh week and goes on rapidly until, by the end of the fourth month, little remains but the ducts anda few tubules. The degenera- Fic. 348.—Diagram representing certain persistent portions of the mesonephros : in the female (see table). Epo. 1., Longitudinal duct of the epodphoron; Epo, ¢., transverse ductules of the epodphoron; O. t.a., ostium abdominale tube; Ovd., oviduct; X represents a small duct which, if present, leads from the epodphoron to oné of the fimbriz of the oviduct. tive processes consist of (1) an ingrowth of connective tissue among the tubules, (2) atrophy of the epithelium of the tubules, and (3) atrophy of the glomeruli. The portions which remain differ in the two sexes, and since the remnants are taken up in the formation of the male and female genital organs it seems best to discuss them more fully under those heads (pp. 417, 420). The accor panying table, however, will give a clue to their fate (see also Figs. 347 and 348). A more comprehensive table will be found on p. 427. Male Female Efferent ductules (vasa efferentia) | Epodphoron ‘ Cephalic part Mesonephros P , { Paradidymis Vasa aberrantia Deferent duct Duct of mesonephros Ejaculatory duct’ Gartner’s canals: Seminal vesicles Caudal part \ Parodphoron _ The significance of the mesonephroi, which, as well as the pronephroi, are present in the embryos of ail higher Vertebrates, can be understood only by referring to the conditions in the lower Vertebrates. In the majority of the Fishes and in the Amphibia the mesonephroi con- stitute the functional urinary organs of the adult and possess essentially the same structure as us 394 TEXT-BOOK OF EMBRYOLOGY. in the embryos of higher forms. Beginning in the Reptiles and continuing up through the series of Birds and Mammals, another set of urinary organs-—the kidneys—develops. The mesonephroi also develop in these forms, even to a high degree, thus repeating the ancestral history, but retain their original function only in the earlier embryonic stages. THE KIDNEY (METANEPHROS). The kidneys are the third set of urinary organs to develop. They assume the function of the mesonephroi as the latter atrophy, and constitute the per- manent urinary apparatus. Each kidney is derived from two separate anlagen which unite secondarily. The epithelium of the ureter, renal pelvis, and straight renal tubules (collecting tubules) is derived from the mesonephric duct Mesonephros Mesonephric duct A ; — = Metanephric blastema Metanephric blastema - Bladder (inner zone) Primitive renal pelvis Fig. 351 Cloacal membrane. Urete Fic. 349.—From a reconstruction of the anlage of the kidney (metanephros), etc., of a human embryo at the beginning of the 5th week. Sc/ireiner, by a process of evagination. The convoluted renal tubules and glomeruli are derived directly from the mesenchyme, and in this respect resemble the meso- nephric tubules and glomeruli. The Ureter, Renal Pelvis and Straight Renal Tubules.—During the fourth week (in embryos of about 5 mm.) a small, hollow, bud-like evagination appears on the dorsal side of each mesonephric duct near its opening into the cloaca. The evagination continues to grow dorsally in the mesenchyme toward the vertebral column, and at the same time becomes differentiated into two parts, a narrow stalk and a dilated terminal portion. The stalk is the forerunner of the ureter, the dilated end is the primitive renal pelvis (Figs. 349 and 351). When. the dilated end reaches the ventral side of the vertebral THE DEVELOPMENT OF THE UROGENITAL SYSTEM, 395 column it turns and grows cranially between the latter and the mesonephros. The stalk (or ureter) elongates accordingly (Fig. 350). About the fifth week, four evaginations from the primitive renal pelvis appear —one cephalic, one caudal and two central (Figs. 3 50 and 352). These may be considered as straight renal tubules of the first order. The distal end of each then enlarges to form a sort of ampulla, and from each ampulla two other evaginations develop, forming tubules of the second order. F rom the ampulla of each secondary tubule two tertiary tubules grow out; and this process con- Cephalic evagination Mesonephros x 7 M$ Metanephric blaste= a Central evaginations \ Mesonephric duct ee > Caudal evagination Junction of meson. duct and ureter Fic. 350.—From a reconstruction of the anlage of the kidney, etc., of 4 human embryo of 11.5 mm. Schreiner. ‘ tinues in a similar manner until twelve or thirteen divisions occur, the final divisions occurring during the fifth month. The tubules grow into the mesen- chyme which surrounds the pelvis and which forms the so-called metanephric blastema, or nephrogenic tissue (Fig. 351). If the straight tubules were to remain in this condition, only four would open directly into the pelvis, corresponding with the four primary evaginations. In the adult, however, many hundreds open into the pelvis; consequently extensive changes of the early condition must take place. These changes are similar to 396 TEXT-BOOK OF EMBRYOLOGY. the process by which the proximal ends of some of the blood vessels come to be included in the wall of the heart (p. 231). The proximal ends of the tubules become wider, the pelvis swells out, and the walls of the tubules become in- cluded in the wall of the pelvis. In certain parts of the pelvic wall this process goes on until deep bays—the calyces—are formed, into which a large number of tubules open. In the other parts of the wall the process does not go so far, thus leaving promontories—the renal papillg—upon which larger tubules or papil- lary ducts open. The adult renal pelvis thus consists of the primitive pelvis plus the proximal ends of the straight tubules. Metanephric blastema Primitive renal pelvis Ureter Mesonephric duct Intestine —— # Bladder Fic. 351.—From a transverse section of a human embryo at the beginning of the 5th week. The plane of the section is indicated in Fig. 349. Schreiner. . The Convoluted Renal Tubules and Glomeruli.—As stated above, the metanephric blastema or nephrogenic tissue surrounds the renal pelvis and the straight tubules. Itrepresents a condensation of the mesenchyme and is destined to give rise to the convoluted tubules and glomeruli. The cells of the blastema in the region of the ampulle of the terminal straight tubules acquire an epithelial character and become arranged in solid masses (Fig. 353). Each mass unites with an ampulla and acquires a lumen, which becomes continuous with the lumen of the straight tubule, then elongates and forms an S-shaped structure (Figs. 354 and 355). The loop of the S nearer the straight tubules elongates still more and grows toward the pelvis, parallel with the straight THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 397 tubules, to form Henle’s loop. The part between Henle’s loop and the straight tubule elongates and becomes convoluted to form the proximal part of a con- voluted renal tubule (second convoluted tubule). The part between the distal end and Henle’s loop elongates and becomes convoluted to form the distal part of a convoluted renal tubule (first convoluted tubule) (Figs. 356 and 357). To avoid confusion it may be well to call attention to the fact that what has here been called the proximal part of a convoluted tubule corresponds with what is usually described as the second or distal convoluted tubule, and that the distal part of a convoluted tubule corresponds with the first or proximal convoluted tubule. In histology the distal and proxi- mal convoluted tubules are spoken of in relation to the renal corpuscle, but in development it is more convenient to speak of the terminal part of a tubule as its distal part. Fe Cephalic evagination Central — evaginations Caudal evagination Ureter Fic. 352.—From a model of the primitive renal pelvis and the evaginations which form the cephalic, central and caudal straight renal tubules of the first order. Human embryo of 43 months. Compare with Fig. 350. Schreiner A glomerulus develops in connection with the extreme distal end of a con- voluted tubule or, in other words, with the distal loop of the S (p. 396). There occurs here a further condensation of the mesenchyme, into which grows a branch from the renal artery. This, as the afferent vessel of the glomerulus, breaks up into several arterioles, each of which gives rise to a tuft of capillaries. These tufts are separated from one another by somewhat more mesenchymal tissue than separates the capillaries within a tuft. The tufts with the asso- ciated mesenchymal tissue constitute a glomerulus, and it is the mesenchymal septa between the tufts that give to the glomerulus its characteristic lobulated appearance. The capillaries of each tuft empty into an arteriole, and the several arterioles unite to form the efferent vessel of the glomerulus, which passes out along side of the afferent vessel. The renal tubule becomes flattened on the side next the condensation of the mesenchyme, and as the glomerulus develops, the epithelium of the tubule grows around it except at the point where the blood 398 TEXT-BOOK OF EMBRYOLOGY. vessels enter and leave. Thus a double layer of epithelium comes to surround the glomerulus, the space between the two layers being the extreme distal part of the lumen of a renal tubule. Thedinner layer is closely applied to the surface Anlagen of convoluted renal tubules Renal pelvis Capsule Anlage of convoluted renal tubule Ampulla of straight renal tubule Fic. 353.—Sagittal section of the anlage of the left kidney in a rabbit embryo of 15 days. Schreiner. The straight renal tubules (sections of which are shown) are embedded in the metanephric blastema, Condensations of the latter form the anlagen of the convoluted renal tubules. At the left of the figure several mesonephric tubules are shown. Met. bl. Con. r. t. A\HR 9g ray § Wahel Fic. 354.—From a section of the kidney of a human foetus of 7 months. Schreiner. Amp., Ampulla of a straight renal tubule; Con. 7. t., anlagen of convoluted renal tubules, above and between which are two ampulla (compare Fig. 355); met. bl., metanephric blastema. of the glomerulus and even dips down into the latter between the tufts. The outer layer forms Bowman’s capsule, the flat epithelium of which passes over into the cuboidal epithelium of the “neck” of the tubule, and this in turn is THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 399 — Ampulla of straight tubule & Henle’s loop Distal part of convoluted tubule Bowman's capsule Les) iss) mn mn Proximal part of 2 convoluted tubule Prox. convoluted tubule — __ Distal part of Dist. convoluted tubule— convoluted tubule Henle's loop: Satt Nacke Bowman's capsule Prox. convoluted tubule Dist. convoluted tubule Prox. convoluted tubule Henle’s loop _- Dist. convoluted tubule Prox. convoluted tubule~ \ Bowman's capsule Straight tubule Dist. convoluted tubule Bowman's capsule Ascending \ arm of Henle's loop Descending J Fic. 357. Fics. 385, 356 and 357.—From reconstructions of convoluted renal tubules in successive stages of development. Stéoerk, 26 400 TEXT-BOOK OF EMBRYOLOGY. continuous with the pyramidal epithelium of the distal convoluted tubule. The entire structure is a renal corpuscle. The formation of renal corpuscles begins in embryos of 30 mm. and continues until after birth. The Renal Pyramids and Renal Columns.—The tubules arising from the four primary evaginations of the renal pelvis together form four distinct groups or primary renal (Malpighian) pyramids—one cephalic, one caudal, and two central. The central pyramids are crowded in between the end pyramids, (cephalic and caudal) and do not develop as rapidly as the latter which soon bend around toward the ureter, thus resulting in the formation of the convex side of the kidney and a depression or hilus opposite (compare Figs. 352 and 358). Between these four pyramids the mesenchyme remains for some time as Primary renal pyramid Primary renal column Primary renal pyramid Primary renal column Caudal straight tubule - A) Primary renal pyramid Fic. 358.—Frontal section of the kidney of a human fcetus of 32 months (rocm.). Hauch, rather distinct septa, forming the primary renal columns (columns of Bertini) which are marked by corresponding depressions on the surface of the kidney and extend to the renal pelvis. The four primary pyramids may be considered as lobes (Fig. 358). Itshould also be stated that the parts of the tubules derived from the mesenchyme form the bases of the renal pyramids. Be- tween the groups of straight tubules derived from evaginations of the second or third order (see p. 395) there are also septa of mesenchyme which divide each primary pyramid into two or three secondary pyramids. These septa may be considered as secondary renal columns (Fig. 359). Thus the entire kidney is divided into from eight to twelve secondary pyramids. Tertiary renal columns then divide incompletely the secondary pyramids into tertiary pyra- THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 401 mids. These are apparent on the surface of the kidney and constitute the surface lobulation, but are not clearly defined in the interior. The formation of renal papille (p. 396) corresponds to the formation of pyramids only to a certain point, for some of the tertiary pyramids appear only near the surface and consequently do not have corresponding papilla. This accounts for the fact that frequently the number of pyramids apparent on the surface does not correspond with the number of papillz. The surface lobula- tion is very plainly marked in kidneys up to and for a short time after birth. It then disappears and the surface becomes smooth. At the same time the con- nective (mesenchymal) tissue of the renal columns is largely replaced by the Secondary renal column Secondary f renal Pyramid Secondary renal : x column Primary ae renal — column Fic. 359.—Frontal section of the kidney of a human foetus of 19 weeks (17.5 cm.). auch, epithelial elements of the gland so that in the adult kidney the columns are not clearly defined. The capsule of the kidney is derived from the mesenchyme which surrounds the anlage of the organ (Fig. 353). Thismesenchyme is transformed into fibrous connective tissue and a small amount of smooth muscle, forming a layer which closely invests the kidney and dips into the hilus where it surrounds the blood vessels and the end of the ureter. The connective tissue and muscle of the ureter are also derived from the mesenchyme. CorTEX AND MrpULLA.—As the convoluted renal tubules develop in the metanephric blastema (p. 396), they form a cap-like mass around the group of 402 TEXT-BOOK OF EMBRYOLOGY. straight tubules. This is the beginning of the renal cortex. A true cortex, however, can be spoken of only after the appearance of the glomeruli (in embryos of 30 mm.). Its peripheral boundary is the capsule, and the renal corpuscles nearest the pelvis mark its inner boundary. The mass of straight tubules forms the bulk of the medulla. It does not at this stage contain Henle’s loops, the latter developing later (during the fourth month). Both cortex and medulla increase until the kidney reaches its adult size. The cortex increases relatively faster than the medulla up to the seventh year; after this the increase is practically equal. The medullary rays are probably secondary formations, being formed by groups of straight tubules which grow out into the cortex; later. ascending arms of Henle’s loops are added to these groups. Some of the glomeruli of the first generation are much larger than any found in the adult. In some of the lower Mammals these “giant” glomeruli disappear and it is probable that the same occurs in the human embryo. Some of the tubules also degenerate and disappear. The cause of these phenomena is not known. Changes in the Position of the Kidneys.—As has already been described (p. 394), the kidney buds first grow dorsally from the mesonephric ducts toward the vertebral column. They then grow cranially, with a corresponding elongation of the ureters, and in embryos of 20 mm. they lie for the most part cranial to the common iliac arteries. This migration continues until the time of birth when the cephalic ends of both kidneys reach the eleventh thoracic ver- tebra. Whenthe kidneys begin to move cranially the hilus is directed caudally. Later they rotate and the hilus is turned toward the medial sagittal plane. Since the ureter, renal pelvis and straight tubules develop from the mesonephric ducts, and since the convoluted tubules and glomeruli develop directly from the same tissue as the mesonephric tubules, namely, the mesenchyme, the renal tubules may be said to represent the third generation of urinary tubules. But no definite reason for the appearance of the third generation can be given. The atrophy of the mesonephroi would, of course, make necessary the compensatory development of new structures; but this only carries the problem a step further back, for the cause of the atrophy of the mesonephroi is not clear. In regard to this atrophy, however, there is a suggestion of a cause in the fact that in the Amphibia the mesonephroi are in part used for conveying the sexual elements, which leaves the meso- nephroi less free to function as urinary organs. Possibly the loss of freedom to function leads to the development of new siructures—the kidneys—in the higher forms (Reptiles, Birds and Mammals). In these forms the kidneys assume the urinary function after the early embryonic stages, and only the ducts and a part of the tubules of the mesonephroi persist in the male to convey the sexual elements. Thus the persistent parts of the mesonephroi as- sume a new function as the old one is lost. But, on the other hand, complications arise on account of the fact that in the female the sexual products are carried off by another set of ducts (the Miillerian ducts), which develop in both sexes but disappear in the male, while the mesonephroi and their ducts disappear almost entirely. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 403 THE URINARY BLADDER, URETHRA AND UROGENITAL SINUS. As described elsewhere, the allantois appears at an early stage as an evagi- nation from the ventral side of the caudal end of the primitive gut (Fig. 282), grows out into the belly stalk, and finally becomes enclosed in the umbilical cord (p.118). As the embryo develops, the proximal end of the allantois becomes elongated to form a stalk or duct which extends from the caudal end of the gut to the umbilicus (Fig. 285). The portion of the gut immediately caudal to. the attachment of the allantoic duct becomes dilated to form the cloaca which at first is a blind sac, its cavity being separated from the outer surface of the embryo by the cloacal membrane (Fig. 360). ‘The latter is composed of a layer of entoderm and a layer of ectoderm, with a thin layer of mesoderm between. The cloaca then becomes separated into two parts—a larger ventral part which forms Intestine Kidney bud Mesonephric duct Urachus \ Cloaca _—___ a 4 Cloacal membrane : = BS >, i= Caudal gut —{4 . Notochord Neural tube Fic. 360.—From a model of the cloaca and the surrounding structures in a human embryo of 6.5mm. Keibel. the urogenital sinus and a smaller dorsal part which forms the rectum. This is accomplished by a fold or ridge which grows from the lateral wall into the lumen and meets and fuses with its fellow of the opposite side. The fusion be- gins at the cephalic end, in the angle between the allantoic duct and the gut, and gradually proceeds caudally until the separation is complete as far as the cloacal _.The mass of tissue forming the partition is called the uro- old (Fig. 361). The openings of the mesonephric ducts, which primarily ere situated in the lateral cloacal wall (p. 392), are situated after the separation in the dorso-lateral wall of the urogenital sinus (compare Figs. 360, 361, 362). ~~ During the separation of the urogenital sinus from the rectum, certain changes take place in the proximal ends of the mesonephric ducts and ureters. The ends of the ducts become dilated and are gradually taken up into the wall of thesinus. This process of absorption continues until the ends of the ureters are included, with the result that the ducts and ureters open separately, the latter 404 TEXT-BOOK OF EMBRYOLOGY. slightly cranial and lateral to the former. (Compare Figs. 362 and 363.) This condition is reached in embryos of 12 to14mm. The point at which these two sets of ducts open marks the boundary between a slightly larger cephalic part of the sinus, the anlage of the bladder, and a smaller caudal part which becomes the urethra and urogenital sinus (Fig. 363). After the second month the bladder becomes larger and more sac-like, and the openings of the ureters migrate farther cranially to their final position. The lumen of the bladder is at first continuous with the lumen of the allantoic duct, but the duct degenerates into a solid cord of cells, the urachus. The latter degenerates still further and finally remains only as the middle umbilical liga- Urorectal fold Mesonephric duct Kidney bud. = Cloaca ve , Brin La — Intestine Urogenital sinus ——+% ie Rectum Cloacal membrane — -audal gut Fic. 361.—From a model of the cloacal region of a human embryo slightly older than that shown in Fig. 360. Keibel. The arrow points to the developing partition (urorectal fold) between the rectum and urogenital sinus. The opening of the mesonephric duct into the urogenital sinus is indicated by a small seeker. ment. It seems quite probable that the bladder is derived almost wholly from the cloaca. A small part arises from the inclusion of the ends of the mesoneph- ric ducts. If any part is derived from the allantoic duct, it is only the apex. After the bladder begins to enlarge, the adjacent portion of the urogenital sinus becomes slightly constricted. This marks the beginning of the urethra. In the female the constricted part represents practically the entire urethra. In the male it represents only the proximal end, the other portion developing in connection with the penis (p. 428). The urogenital sinus is narrow and tubular at its junction with the urethra; more distally it is wider and is shut off from the exterior by the cloacal membrane. After the embryo reaches a length of 16 to 17 mm., the membrane ruptures and the sinus opens on the surface. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 405 The narrow part of the sinus is gradually taken up into the wider, resulting in the formation of a sort of vestibule. In both sexes the urethra opens into the deeper end of the vestibule. In the male the mesonephric (seminiferous) Mesonephric ducts SSS” iY Si) Be Colom we Miser — Primitive renal pelvis Cloaca | —_—_———_-: Wy Pe a EN , (undivided portion) » ee i Cloacal membrane -———————— SS “aaa ees Rea “7——~ Rectum Fic. 362.—F'rom a reconstruction of the caudal end of a human embryo of 11.5 mm, (44 weeks). Keisbel. 6 Par Le Z ie { ae 5 f § = — Ovary Umbilical artery ~~, _ Xo f \ ee y | sees u | * A 52 — Broad ligament ey ¥ |e ot uterus Bladder — ae 2 = : i | | | ; i} —- Mullerian duct Symphysis pubis Urogenital sinus ey ~ Mesonephrie duct y, i. iy | Ureter Genital tubercle “~ Recto-uterine Urethra---—-=-# excavation Fic. 363.—From a reconstruction of the caudal end of a human embryo of 25 mm. (84-9 weeks). Urogenital sinus —4f Cloaca Levi Genital tubercle ia Genital ridge o Opening of cloaca Fic. 378.—Diagrammatic representation of the urogenital organs in the “indifferent” stage. Hertwig. as the processus vaginalis peritonei, but continues to burrow through the body wall and causes an elevation in the skin which is destined to become one side of the scrotum (see p. 430). The opening of the peritoneal sac into the body cavity is the inguinal ring. In its descent the testicle passes through the inguinal ring and comes to lie in the elevation in the skin or scrotum (ninth month). Whether its passage into the scrotum is the result of actraction by the gubernaculum is not certain. The inguinal ring then closes by apposition of its walls and the testicle lies in a closed sac which has been pinched off, so to speak, from the body cavity Fig. 376). Kidney Appendage of testicle __| (hydatid of Morgagni) Epididymis Testicle Paradidymis Deferent duct Gubernaculum testis Ureter Seminal vesicle ————_4 Deferent duct ra Epididymis ——-# Millerian duct Kidney Hydatid Oviduct (fimbriz) Epodéphoron Ovary Paroéphoron Oviduct Epoéphoron Ovary —— if Ovarian ligament ——~_-7—* Mesonephric duct Round ligament Uterus These diagrams should be compared with Fig. 378. The dotted lines represent the organs in the male and the mesonephric duct in the female, which ducts disappear for the most part). 425 THE DEVELOPMENT OF THE UROGENITAL SYSTEM. Apex of bladder Bladder Opening of ureter Urethra Opening of ejacul. duct Prostate Urethra Sinus prostaticus Fic. 379. Ureter ‘Urethra Vestibulum vagine Vagina Fic. 380. Fic. 379.—Diagram of the development of the male genital organs from the “indifferent” anlagen, Hertwig. Fic. 380.—Diagram of the development of the female genital organs from the Hertwig. ‘Indifferent ” anlagen. relative positions they occupy in the adult (with the exception of the Miillerian duct in the 426 TEXT-BOOK OF EMBRYOLOGY. Since the testicle is invested by peritoneum from the beginning of its develop- ment, it must be understood that in its passage into the scrotum it passes along under the peritoneum. Consequently when it reaches the scrotum it is sur- rounded by a double layer of peritoneum, the éwnica vaginalis propria. The descent of the testicle also produces marked changes in the course of the deferent duct. Primarily the (mesonephric) duct extends cranially from the urogenital sinus in a longitudinal direction. But as the testicle migrates, the cephalic end of the duct is drawn caudally so that in the adult the deferent duct extends cranially from the scrotum to the ventral side of the urinary bladder and then bends caudally again to open into the urethra. Descent of the Ovaries.—The ovaries undergo a change of position cor- responding to the descent of the testicles, although the change is not so extensive. Primarily the Miillerian and mesonephric ducts lie in a ridge on the surface of the mesonephros (p. 417). As the mesonephros and its duct atrophy, the Miil- lerian duct (oviduct) comes to lie in a fold, the mesosalpinx, which is attached to the mesovarium (Fig. 373). Atthe same time the mesovarium becomes directly continuous with and really a part of the inguinal ligament. The latter cor- responds, of course, to the gubernaculum testis, and plays a rdéle in the descent of the ovaries. It may be conveniently divided into three parts, (1) a cephalic part which is attached to the hilus of the ovary, (2) a middle part which ex- tends from the ovary to the uterus, forming the ovarian ligament, and (3) a cau- dal part which extends from the uterus to the inguinal region, forming the round ligament of the uterus (Fig. 377). The round ligament pierces the body wall and is attached to the corium of the skin. At the point where it passes through the body wall there is a slight evagination of the peritoneum, the diverticulum of Nuck, which corresponds to the processus vaginalis peritonei in the male. The ovaries gradually migrate caudally from their original position into the false pelvis (third month) and thence into the true pelvis (at birth). Obviously no traction can be exerted upon them by the round ligament (or caudal part of the inguinal ligament), since the latter extends from the uterus to the inguinal region. ‘Their descent into the pelvic seems to be due to the unequal growth of the ovarian ligaments, or in other words, to the fact that the ovarian liga- ments grow proportionally less than the surrounding parts. During their descent the ovaries become embedded in the broad ligaments of the uterus, which represent further development of the peritoneal folds of the genital cord. In this way the mesovarium becomes merged with the broad ligament. On pages 424 and 425 are three diagrammatic representations of the changes that take place in the genital systems of the two sexes. Fig. 378 represents the “indifferent” stage in which all the embryonic structures are present; Fig. 379 represents the changes that occur in the male; Fig. 380 represents the THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 427 changes that occur in the female. A careful study of the diagrams will assist the student materially in understanding the processes of development which have been described in the preceding paragraphs. Below is a table that is meant to set forth briefly the various structures which belong to the internal genital organs in the two sexes, and which are derived from the structures in the “indifferent” stage. The words in italics: are the names of structures that persist in a rudimentary form. Indifferent Male Female Ovarian (Graafian) follicles é ied a , with ova. onvoluted seminiferous tubules with spermatozoa. . ? a Ade Straight seminiferous tubules . } iF ete testis . Germinal epithelium (meso- thelium) Rete cords. Part of stroma of testicle Part of stroma of ovary. Efferent ductules (vasa efferentia) Epoéphoron, transverse duc- Mesonephros Gepheee pent Appendage of epididymis tules. eamivdal part Paradidymis (organ of Giraldes :) Paroéph P Aberrant ductules(vasa aberrantia) a midis) . Morgagni) (?) Mesonephric duct Deferent duct (vas deferens) { Epodphoron, longitudinal Ejaculatory duct duct. Seminal vesicle . Gértner’s canals. Morgagni’s appendage of testicle (hydatid of Morgagni) Miillerian duct . 4 Oviduct. Uterus. Prostatic utricle (uterus masculinus) | Vagina. I Duct of epididymis (vas =| Vesicular appendage (of “ Fimbriz of oviduct Inguinal ligament of meso- . : Ovarian ligament. nephros } Gubernaculum testis (Hunteri) eee ligament of uterus. prostatic part . . Urethra. Linephea ae part . } Lone of vagina. Urogenital sinus Prostate . . | Prostate. Bulbo-urethral gland (Cowpers) Larger vestibular gland (Bar- tholin’s. THE EXTERNAL GENITAL ORGANS. In addition to the internal organs of generation, to which the description has thus far been confined, certain other structures appear on the outside of the body to form the external genitalia. In the case of these also there is an “indif- ferent’? stage from which the courses of development diverge in the two sexes. During the sixth week a depression appearing on the ventral surface of the caudal end of the body indicates the position of the cloacal membrane (p. 403). This becomes surrounded by a slight elevation, produced by the thickening of the mesoderm which is known as the genital ridge (Fig. 381). The cephalic 428 TEXT-BOOK OF EMBRYOLOGY. + side of the ridge becomes raised still farther above the surface, forming a dis- tinct protrusion, the genital tubercle. The tubercle continues to increase in size, and the distal end forms a knob-like enlargement. Along the ventral (or rather caudal) side a groove appears, which extends distally as far as the base of the enlarged end. The ridges along the sides of the groove increase in size and form the genital folds. In the meantime a second pair of elevations appears lateral to the genital folds to form the genital swellings (Fig. 382). After the cloacal membrane ruptures, a single opening is produced which leads from the exterior into the cloaca. This opening is then separated by the further growth of the urorectal fold (p. 403) into the opening of the urogenital tract and the anal opening. The caudal part of the fold then enlarges to form the perineal body, which serves to push the anus farther away from the genital ridges. The latter, together with the genital tubercle and swellings, all of which lie in the immediate vicinity of the urogenital opening, constitute the anlagen of the external genital organs (Fig. 383). These at this time are in the “indifferent” stage, from which development proceeds in one of two directions, accordingly as the embryo is a male or a female. Up to the fourth month there is little difference between the structures in the two sexes. After this the differences become more and more obvious. In the female the changes in the originally “indifferent” structures are comparatively slight. The genital tubercle grows slowly and becomes the clitoris. The enlarged extremity becomes more clearly marked off from the other part to form the glans clitoridis. ‘The skin covering the glans is converted by a process of folding into a sort of prepuce. The genital folds, which bound the opening of the urogenital tract, become elongated and form the labia minora. ‘The opening of the urogenital tract is the vestibulum vagine. The genital swellings enlarge still more than the genital folds, by a deposition of a considerable mass of fat in the mesenchyme, and become the labia majora. The latter are the structures (mentioned on p. 424) which mark the points at which the inguinal ligaments of the mesonephroi pierce the body wall, and are homologous with the scrotum in the male (Figs. 384 and 385). In the male the “indifferent” anlagen undergo more extensive changes than in the female. The genital tubercle continues to grow more rapidly and forms the penis, which is homologous with the clitoris. The enlarged extremity becomes the glans penis, and an extensive folding of the skin over the glans forms the prepuce. The groove on the caudal or lower side of the tubercle elongates as the latter elongates and becomes deeper. Finally the ridge (or genital fold) on each side of the groove meets and fuses with its fellow of the opposite side, thus enclosing within the penis a canal—the penile portion of the urethra, ‘The groove is primarily continuous with the opening of the uro- genital tract, and as the fusion takes place the penile portion forms a direct Gen, a. Gen. tub. Gen. sw. Gen. f. Fic. 386. Fies. 381-386.—Stages in the development of the external genital organs. Kollmanw’s Allas, Fic. 381, “indifferent ” stage—embryo of 17 mm.; Fig. 382, “indifferent”? stage—embryo of 23 mm.; Fig. 383, “indifferent ” stage—embryo of 29 mm. (beginning of 3d month); Fig. 384, female embryo of 7o mm, (11 weeks); Fig. 385, female embryo of 150 mm. (16 weeks); Fig. 386, male embryo of 145 mm, (16 weeks). An., Anus; Cl., clitoris; Clo. and gen. j., cloaca and genital folds; C/. 1., cloacal membrane; Ext., lower extremity; Gen. }., genital folds; Gen. r., genital ridge; Gen. sw., genital swelling; Gen, tub., genital tubercle; Gl. ~., glans penis; Lab. ma., labium majus; Lab. m7., labium minus; Ra., raphé of scrotum; {Ser., scrotum; Ta., tail; Ug. s., urogenital sinus; Umb. c., umbilical cord, 430 TEXT-BOOK OF EMBRYOLOGY. continuation of the internal (membranous and prostatic) portion of the urethra. The genital swellings also fuse and form the scrotwm, the line of fusion in the medial line becoming the raphé (Fig. 386). Primarily the inguinal ligaments of the mesonephroi are attached to the corium of the skin in the genital swellings, and as the testicles descend they pass through the inguinal ring into thescro- tum. Ina sense the scrotum represents an evagination of the body wall THE DEVELOPMENT OF THE SUPRARENAL GLANDS. Although the suprarenal glands do not logically come under the head of the urogenital system, being neither functionally nor developmentally a part of the latter, it is most convenient to consider them in this chapter. In Mammals including man these glands are composed of two parts which can be differentiated histologically and topographically—the cortex and medulla. The cortex is composed of trabecule and spheroidal masses of cells Pheochrome cells _ Nerve fibers . \. Pheochrome Connective Sympathetic cells tissue ganglion cells Fic. 387.—Section of a sympathetic ganglion in the coeliac region of a frog (Rana esculenta), showing differentiating phaochrome cells, Giacomini. which do not have a strong affinity for the ordinary cytoplasmic stains and which contain granules of a fat-like substance known as lipoid granules. The medulla is composed of irregularly arranged sympathetic ganglion cells and other granular cells which, after treatment with chrome salts, acquire a peculiar brownish color. ‘The brown cells are known as chromaffin (or pheochrome) cells and their granules as chromaffin (or phzochrome) granules. As cortex and medulla are distinct anatomically, they are also distinct developmentally, being derived from two distinct and different parent tissues which unite secondarily. Furthermore, it is an interesting fact that in the lower Vertebrates (Fishes) the two parts remain permanently separate; that in the ascending scale of animal life (Amphibia, Reptiles, Birds) they become more closely associated; and that finally (in Mammals) they unite to form a single glandular structure. In Mammals the phylogenetic history is repeated with remarkable THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 431 precision during the development of an individual: The two parts arise sepa- rately, come closer together, and finally unite. The Cortical Substance.—The cortex is of mesothelial (mesodermal) origin. In embryos of five to six mm. the mesothelium at the level of the cephalic third of the mesonephros proliferates and sends buds or sprouts into the mesenchyme at each side of the root of the dorsal mesentery. These sprouts soon lose their connection with the parent mesothelium and unite with one another to form a rather compact mass of epithelial-like cells ventro-lateral to the aorta (Fig. 314). Frequently the two masses fuse across the medial line ventral to the aorta. They constitute the anlagen of the cortical substance of cea Medulla Medulla 7 (Phzochrome cells) Cortex > Fic, 388.—From a transverse section of a 40 mm. pig embryo, showing the growth of the medullary substance into the cortical substance of the suprarenal gland. The vessel in the center of the figure is the aorta. Wéesel. the two suprarenal glands. From the fact that in the lower forms they remain separate from the medullary substance and lie between the urinary organs, they are known as the interrenal organs. The Medullary Substance.—A little later than the appearance of the cortical anlage, the cells of some of the developing sympathetic ganglia become differentiated into two types—(z) the so-called sympathoblasts which develop into sympathetic ganglion cells, and (2) pheochromoblasts which are destined to give rise to the pheochome or chromaffin cells (Fig. 387). Hence the chromaffin cells are derivatives of the ectoderm, since the ganglia are of ectodermal origin. They soon become more or less separated from the ganglia, migrate to the 28 432 TEXT-BOOK OF EMBRYOLOGY. region of the cortical anlagen and then penetrate the latter in cord-like masses (Fig. 388). Finally these masses unite in the interior of the cortical substance to form a single compact mass (Fig. 389). Along with the phzochrome masses, sympathoblasts also are carried in and give rise to the sympathetic ganglion cells within the gland. The two types of cells together constitute the medullary substance. In the lower forms the pheochrome masses remain separate from the cortical substance and are known as the suprarenal organs. In Mammals the two sets of organs (interrenal and suprarenal) unite to form the suprarenal gland. Med. Cor. Cor.! Fic. 389.—Section of the suprarenal gland of a 119 mm. pig embryo. Cor, Cortex; Cor.t, some cortical substance in the center of the gland; Med., medulla. Wiesel, At the time when the mesonephros is fully developed, the cortical substance forms a small oval body near its cephalic end. During the union of the cortex and medulla and the atrophy of the mesonephros, the suprarenal gland becomes more closely associated with the cephalic end of the kidney, and by the middle of the third month has practically reached its adult position. During the third month and the first half of the fourth month the glands increase in size and become relatively large structures, larger in fact than the kidneys. From the fourth month on, they grow proportionately less than the neighboring organs, and by the sixth month are about half as large as the kidneys. At birth the ratio of their weight to that of the kidneys is about 1:3; in the adult about 1:28. While perhaps in a normal course of development all the anlagen are united in the adult suprarenal gland, it is not unusual to find accessory structures in various places. Some of these consist of cortical tissue only and are usually THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 433 found in or near the capsule of the gland. Others may consist of both cortical and medullary substances, and are found in the vicinity of or embedded in the kidneys, in the retroperitoneal tissue near the kidneys, in the walls of neighbor- ing blood vessels, or associated with the internal genital organs—in the rete testis or epididymis, or in the broad ligament. These accessory structures may arise inde- pendently of the main gland, or they may be portions of the main gland which were separated during the union of the different anlagen of the latter and were carried away in the descent of the genital glands. In addition to the chromaffin tissue which enters into the formation of the main gland or of accessory glands, there are other small masses of this tissue which remain permanently associated with some of the prevertebral and peripheral sympathetic ganglia. s.. SBIR ee tana Recent researches have shown that the Carotid Skein _the developing phzo- (glomus caroticum, intercarotid ganglion, carotid gland), ¢hrome masses in a : : ue human foetus of 50 which formerly was believed to be a derivative of the mm. 4, Aorta; N, nee ee cae : : cortical substance (in- epithelial lining of one of the branchial grooves, is of terrenal gland); U, sympathetic origin and that the cells acquire the charac- rere" &, neetam. teristic chromaffin reaction. These facts indicate that it is closely allied with the medullary substance of the suprarenal gland. Anomalies. Tue KipneEvs.—Rarely is there congenital absence of both kidneys. More often there is a high degree of aplasia in both organs in otherwise well-developed children. In either case death necessarily soon follows. Not infrequently one kidney, usually the left, is poorly developed or absent and a compensatory enlargement of the other exists. Such malformations are due to deficient development of the organs, but the causes underlying the deficient development ‘are obscure. One of the most common malformations is the abnormal position of one or both kidneys (ectopia of the kidneys)... Usually they occupy a position lower than the normal in the abdominal cavity, which indicates that they have failed, during development, to migrate forward to the normal limit (see p. 402). Very rarely one or both organs migrate beyond the normal limit, in which case they occupy positions cranial to the normal. Not infrequently the lower ends of the two kidneys are fused across the medial line, giving rise to the so-called “horseshoe kidney.” Two renal pelves and ureters are usually present. Occasionally the fusion is so extensive 434 TEXT-BOOK OF EMBRYOLOGY. that a single flat mass is formed. This occupies a medial position or lies at either side of the medial line, and may be situated at the normal level or lower. The renal pelvis may be single or double, with one or two ureters. In cases of double ureters and pelves it seems most likely that the anlagen of the kidneys have fused secondarily, that is, after the evagination from the mesonephric ducts (p. 394). In cases where the pelvis and ureter are single, the fusion may have occurred secondarily, although there is the possibility that only a single anlage appeared. Occasionally in children and even in adults the kidneys show a distinct lobulation. This is due to the persistence of the lobulation that normally exists in the foetus (p. 4cr). The kidney may be more or less movable owing to laxity of the surrounding tissue, or it may be floating, in which case it has a distinct mesentery. These cases should be distinguished from those in which similar conditions have been acquired, usually as the result of trauma. Congenital cysts of the kidney are not uncommon. ‘They vary in size and number, sometimes being so numerous that they crowd out the greater part of the renal tissue. Rarely they are so large and numerous that the affected organ fills a large part of the abdominal cavity, resulting in serious or even fatal disturbances of the functions of other organs. There are three views con- cerning the origin of these cysts. (1) They may be the result of dilatation of certain renal tubules derived from the nephrogenic tissue, which failed to unite with the straight tubules (p. 396). (2) Inflammation in the medulla of the foetal kidney may effect a closure of the lumina of some of the tubules, with subsequent dilatation of the portions (tubules or renal corpuscles) that are cut off from communication with the renal pelvis. (3) Normally some of the renal corpuscles and tubules degenerate (p. 402) ,and the cysts may arise as dilatations of incompletely degenerated corpuscles or tubules or both. While these views appear reasonable, none of them has been proven. All three views express possibilities, and there is no good reason for believing that any one of them expresses the only possibility. Tue Ureters.—The renal pelvis is sometimes absent, the calyces uniting to form two or more tubes which in turn unite to form the ureter. This prob- ably is the result of abnormal branching of the ureter during development and the failure of the ends of the branches to become dilated. Occasionally the ureter is double or triple throughout the whole or a part of its length. The most reasonable explanation of two or three complete ureters on either side is that two or three separate evaginations arose from the mesonephric duct (p. 394.) Where the tube is double in only a part of its length, an abnormal branching of the single original evagination is indicated. Atresia of one or both ureters is occasionally met with. This probably THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 435 represents a secondary constriction after the ureter is formed since both evag- inations are hollow from the beginning (p. 394), but the cause of the constric- tion is not understood. The atresia results in dilatation of the portion of the ureter on the side toward the kidney. Abnormal situations of the openings are sometimes seen, the explanation of which is to be found in the relations of these tubes to the mesonephric ducts, to the cloaca, and to the Miillerian ducts. Jn the male the ureters may open into the seminal vesicles, the prostatic urethra, or the rectum. If one recalls that the ureter arises as an evagination from the mesonephric duct near the opening of the latter into the cloaca (p. 394), that the cloaca becomes separated into a dorsal part (the rectum) and a ventral part (the urogenital sinus) (p. 403), and that the proximal end of the mesonephric duct is so far taken up into the wall of the urogenital sinus (or bladder) that the ureter opens separately (p. 403), itis readily seen that any interference with these normal processes of development will result in abnormal opening of the ureter. If the ureter does not become separated from the mesonephric duct, it will open into the deferent duct. (vas deferens), the latter being the proximal part of the mesonephric duct. And since the seminal vesicle is an outgrowth from the proximal end of the meso- nephric duct, the opening of the ureter is likely to be associated with the vesicle. If the separation between the ureter and mesonephric duct is complete, but the opening of the ureter does not migrate cranially on the wall of the bladder, the opening comes to lie in the wall of the prostatic urethra. If the wall (urorectal fold) separating the urogenital sinus and rectum is situated too far dorsally, the opening of the ureter comes to bein the wall of therectum. (Con- sult Figs. 360, 361, 362, 363.) In the female the ureters may open into the urethra, the vagina, or the uterus. The explanation of the opening into the urethra is the same as in the male (see preceding paragraph). The opening into the genital tract is probably to be explained on the ground that the ureters fail to migrate cranially along the wall of the urogenital sinus to the bladder, and as the fused ends of the Miillerian ducts enlarge to form the uterus and vagina, the openings of the ureters are taken up into their walls. Tur BLappER.—Absence of the bladder is very rare. Abnormal small- ness, due to imperfect dilatation of the urogenital sinus (p. 404), is not infre- quent. The urachus, which represents the portion of the allantoic duct between the bladder and the umbilicus (p. 404), not infrequently persists as a whole or in part, giving rise to certain anomalous conditions in the region of the middle umbilical ligament. The urachus may persist as a complete tube, lined with epithelium, thus forming a means by which urine can escape at the umbilicus. This condition is usually associated with obstruction of the 436 TEXT-BOOK OF EMBRYOLOGY. urethra and is known as uracho-vesical fistula. The urachus may degenerate in part, leaving disconnected portions which frequently become dilated to form cysts. Vesical fissure, the most serious malformation of the bladder, is associated with fissure of the lower abdominal wall. The edges of the cleft in the bladder are continuous with those of the cleft in abdominal wall, the integument being continuous with the lining of the bladder. In some cases the bladder is everted through the cleft, and the cleft may even be so extensive as to involve the external and internal genital organs. Vesical fissure is much more com- mon in the male than in the female. No very satisfactory explanation of this malformation has yet been given. It is in some way connected with imperfect formation of the ventral abdominal wall resulting from influences acting at a very early period of development. THE URETHRA in both sexes may be abnormally small or abnormally large or partly occluded, owing to faulty development of the urogenital sinus. In the male the penile portion also may be malformed, being represented merely by a furrow on the lower side of the penis. This condition, known as hypo- spadias, is due to the incomplete fusion or lack of fusion between the genital folds along the lower side of the genital tubercle (p. 428). In extreme cases the de- fect may involve the scrotum and extend back as far as the prostate gland, the two halves of the scrotum being separated. Epispadias, in which the urethral cleft extends along the upper side of the penis (or the clitoris) is rare, and is usually associated with vesico-abdominal fissure. Its mode of origin is not understood. THE TEsTICLES.—One of the most common malformations affecting the male genital glands is the condition known as chryptorchism, in which the glands, instead of descending into the scrotum, are retained within the ab- dominal cavity. One or both testicles may be affected. They may occupy their original position far forward in the abdominal cavity or may be situated near the inguinal canal, or may lie at some intermediate point. The malposi- tion is due to a failure in the normal descent into the scrotum (p. 423). The cause of the failure is obscure. Not infrequently the ectopic testicles atrophy or fail to develop properly at puberty. Congenital absence of one or both testicles is rare. More frequently the gland or efferent system of ducts is defective in part, owing to imperfect development. Jn case of absence of the testicles the individual is small and poorly developed; when the glands are imperfectly developed the individual is effeminate. Cysts which are sometimes met with in the epididymis are possibly due to dilatation of incompletely degenerated portions of the mesonephric tubules or Miillerian ducts. Teratoid tumors and chorio-epitheliomata are occasionally THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 437 found in the testicle. For a further discussion of these see chapter on Terato- genesis (XIX). THE Ovaries.—Congenital absence of both ovaries is rare; defective development of one is more common. Either anomaly may occur with or without defects in the other genital organs. Occasionally the ovaries remain rudimentary, their function as egg-producing organs never being assumed. Malpositions, due to partial or complete failure in the normal descent into the pelvis (p. 426), are not infrequent. Sometimes, on the other hand, they descend to the inguinal canal and may even pass through the latter into the labia majora. Ovarian cysts occur frequently. Some of these (follicular cysts) may arise during postnatal life as dilatations of Graafian follicles.. Others probably arise during foetal life in the same manner. Certain other forms of ovarian tumors, known as cystadenomata, are possibly to be considered as derivatives of the epithelium of the medullary cords which in normal cases disappear entirely (p. 410; also Fig. 366). A discussion of the origin of teratoid tumors of the ovary will be found in the chapter on Teratogenesis (XIX). THE Ovipucts, UTERUS AND Vacina.—Absence of the oviducts is usually associated with malformations of other parts of the genital tract. On the other hand, normal oviducts may be present in conjunction with defective uterus and vagina. Atresia may occur at the uterine or fimbriated end, or at any intermediate point. ‘ The majority of the malformations of the uterus and vagina can be at- tributed to defective processes of development in the caudal ends of the Miiller- ian ducts. It will be remembered that the caudal ends of these ducts normally fuse to form a single medial tube which opens into the urogenital sinus, and which constitutes the anlage of the uterus and vagina (p. 419; Fig. 363). It is obvious that any defect in this fusion will result in some degree of duplicity in the two organs in question. The fusion may be almost complete, the result- ing abnormality being merely a small pocket which forms, at each side of the fundus, a continuation of the cavity of the uterus. There may be a greater degree of imperfection in the fusion, resulting in a partial division of the uterus into two horns—bicornuate uterus. The wall between the two Miillerian ducts may remain patent in the entire uterine portion of the tract, thus giving rise to a bipartite uterus. If the wall between the ducts remains intact throughout both uterine and vaginal portions, the result is a complete division of the utero- vaginal tract—uterus didelphys. Occasionally the uterine portion of one Miillerian duct may fail to develop properly and becomes a solid cord, resulting in an unicornuate uterus. Not infrequently the uterus remains rudimentary—infantile uterus. This anomaly is usually accompanied by stenosis of the vagina. Stenosis or other 438 TEXT-BOOK OF EMBRYOLOGY. defects in the vagina may occur, however, when the uterus is normal. In rare instances the hymen is absent; in other cases it closes the entrance to the vagina —a condition known as imperforate hymen. Malformations of the uterus and vagina resulting from persistence of the cloaca and atresia of the anus are mentioned on page 358. HERMAPHRODITISM. This condition implies a combination of the male and female sexual organs in one individual, accompanied by a blending ot the general characteristics of the two sexes When such an individual possesses both ovary and testicle, the condition is known as true hermaphroditism; when the individual possesses ovaries or testicles, the condition is known as false hermaphroditism. TruE HERMAPHRODITISM.—The presence of both ovary and testicle in one individual is one of the rarest anomalies in man. Furthermore, one or both of the organs are sexually immature. Three forms can be recognized (Klebs): 1. Lateral hermaphroditism, in which an ovary is present on one side and a testicle on the other; 2. Unilateral hermaphroditism, in which both ovary and testicle are present on one side, either ovary or testicle, or neither, on the other side; 3. Bilateral hermaphroditism, in which both ovary and testicle are present on both sides. In all these cases the general character of the body is of an intermediate type, sometimes tending toward the male, sometimes toward the female. The external genitalia are also of an intermediate type, with hypospadias, small penis, separate scrotal halves, and small vaginal orifice. The uterus usually shows some degree of duplicity. FatsE HERMAPHRODITISM.—In this type of hermaphroditism, in which either ovaries or testicles are present in an individual with mixed general sexual characteristics, two varieties can be recognized: 1. Masculine false hermaphroditism, the more common, in which testicles are present but the external genitalia and general character of the body approximate the female; 2. Feminine false hermaphroditism, in which ovaries are present but other- wise male characteristics predominate. The causes underlying the origin of hermaphroditism are among the most obscure in teratogenesis. It is well known that up to the fourth or fifth week the anlagen of the sexual glands are histologically “indifferent,” and later be- come differentiated into ovaries or testicles (p. 408). Since the secondary sexual characteristics are dependent upon the development of the primary, they also are brought out later. If the “indifferent” glands give rise to both ovaries THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 439 and testicles, true hermaphroditism is the result; if they give rise to either ovaries or testicles but the external genitalia and general characteristics develop in the opposite direction, false hermaphroditism is the result. Thus the her- maphroditic condition is potentially present in every individual during the earlier stages of development; the most remarkable fact is that it is not more common. Recent researches in cytology have added a new phase to the question of the origin of hermaphroditism. Accessory chromosomes have been demonstrated in the ova and spermatozoa of many species of insects (McClung, Wilson, Morgan) and in ova and pollen of dicecious plants (Correns). It has been suggested that these have some significance in the determination of sex, the female elements containing the additional chromatin elements (see p. 416). Carrying this a step further, Adami has suggested that “hermaphroditism is based upon aberration in the distribution of the chromosomes in either the ovum or the spermatozoon.” PRACTICAL SUGGESTIONS. The Pronephros.—Being a very rudimentary structure in the higher Vertebrates (especially in Mammals), the pronephros can be studied to the best advantage in embryos of some of the lower forms, such as the frog. The embryos can readily be obtained in the spring in ponds; or perhaps a better method is to collect the eggs during the early cleavages (which can be seen with the naked eye or with the aid of a hand lens) and allow them to develop in water in the laboratory, selecting successive stages for preservation. Beginning when the embryos are three to four mm. long, remove the gelatinuous capsules and. fix in Flemming’s fluid. Successive stages up to any length desired (twelve to fifteen mm.) should be prepared in the same manner. Cut transverse serial sections in paraffin and stain with Heidenhain’s hematoxylin. The embryos of four to five mm. will show the early stages, longer ones, of course, the later stages, those of about 12 mm. being especially good. The developing pronephroi are found in the mesoderm lateral to the primitive segments. The ducts, in the same relative position, can be traced through the series of sections to the tail region where they then bend ventrally and medially to open into the gut. For technic, see Appendix. The Mesonephros.—Chick embryos at the end of the second and during the third and fourth days of incubation are most convenient for the study of the early development of the mesonephros. Remove the embryo from the egg, fix in Zenker’s fluid, cut transverse serial sections in paraffin and stain with Weigert’s hematoxylin and eosin. Time can be saved by staining im toto with borax-carmin, but the differentiation is less complete. The mesonephroi appear in the intermediate cell mass at about the level of the heart, development thence progressing caudally. The mesonephric ducts, which are identical with the pronephric ducts, can be traced caudally until they turn ventrally and medially and open into the caudal end of the gut. In these embryos the formation of the glomeruli can be studied, likewise the increasing intimacy between the posterior cardinal and subcardinal veins and the mesonephric tubules. By examining series of sections, the segmental mesonephric branches of the aorta can be seen. The mesonephroi at the height of their development can be studied in human embryos. 440 TEXT-BOOK OF EMBRYOLOGY. of 5 to 6 weeks (about 20 mm.) or in pig embryos of 25 to 35 mm. Fix the pig embryos, for example, in Zenker’s fluid or Bouin’s fluid, cut transverse sections in paraffin and stain with Weigert’s hematoxylin and eosin. Serial sections are, of course, necessary if one wishes to trace the various structures or to make reconstructions. ‘The mesonephroi at this stage extend from the level of the stomach to the pelvic region and fill a considerable portion of the abdominal cavity. The mesonephric duct lies in the peripheral part of the organ, forming a rather wide tubule; the Miillerian duct lies near by, forming a much smaller tubule; the two ducts in the Jater stages are embedded in an elevation on the surface of the organ. On the medial side of the mesonephros a distinct projection constitutes the genital gland. The Kidney.—Fix a pig embryo of about 17 mm. in Zenker’s fluid or Bouin’s fluid. Unless the embryo is to be used for the study of other structures in the anterior region, remove the anterior half. Cut transverse serial sections of the posterior half in paraffin and stain with Weigert’s hematoxylin and eosin. The anlagen of the kidneys are found at the level of the caudal ends of the mesonephroi. They are situated lateral to the aorta and are composed of irregular tubules surrounded by dense mesenchymal tissue. The tubules represent the straight renal tubules which have grown out from the renal pelvis; the latter is usually a large, more centrally located space. The dense mesenchymal tissue is the nephrogenic tissue (metanephric blastema) which gives rise to the convoluted renal tubules. By following the series of sections caudally, the renal pelvis can be traced to the ureter and the latter to its opening into the mesonephric duct. Further development of the kidney can be studied in sections of older embryos, prepared by the same technic as those used for the earlier stages (see above). In more advanced stages the anlagen of the convoluted renal tubules are seen as very dense portions of the mesenchymal tissue, lying in contact with the straight tubules. The cells become epithelial in character and join those of the straight tubules. Wax reconstructions are valuable adjuncts in the study of the kidney. One should be made of a kidney and ureter at an early stage (pig embryo of 17 mm.) and another of a group of renal tubules at some later stage. The Bladder, etc.—It is very difficult, by studying sections alone, to get a comprehensive view of the changes that occur in the bladder, in the proximal ends of the ureters and meso- nephric ducts, and in the urethra. Some idea of the interrelation of these structures may be gained by tracing, in serial sections of a 17 mm. pig embryo, the ureter to the mesonephric duct, the latter to the urogenital sinus, and the urogenital sinus in one direction to the exterior and in the other direction to the urachus. Prepare sections as under “The Kidney” (see above). At the same time the openings of the Miillerian ducts on the dorsal side of the uro- genital sinus should be noted, and the ducts traced forward through the genital cord, in company, with the mesonephric ducts, and along the surface of the mesonephroi. A wax reconstruction of all these parts is very instructive. In larger embryos, the various structures can be identified by very careful dissection. Dissections of human embryos of two months or more are especially valuable. The Genital Ridge.—A single stage in the development of the genital ridge serves to indicate its origin from the mesothelium and its primary location. Fix a pig embryo of 10 to 12 mm. in Flemming’s fluid, making an incision in the abdominal wall to give the fixa- tive access to the interior. Cut transverse sections in paraffin through the embryo at the level of the lower part of the liver, and stain with Heidenhain’s hematoxylin. The genital ridge is found on the medial side of the mesonephros and is composed of small, dark cells and larger, clearer sex cells. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 441 The Ovary.—If the individual is to be a female, specific changes occur in the genital tidge which lead to the formation of the ovary (see page 408 in text). The beginning of these changes can be seen in pig embryos of 25 to 30 mm. (or human embryos of about 25 mm.), prepared according to the technic given under ‘The Genital Ridge” (see p. 440). The ovary forms a fairly large ridge attached to the medial side of the mesonephros by the meso- varium, but is relatively shorter than the original genital ridge. Cords of epithelial cells (the sex cords) extend into the underlying mesenchymal tissue and are composed of small, dark cells and larger, clearer cells (the primitive ova, sex cells). It should be borne in mind that these cords of cells are not Pfliiger’s egg cords (which are formed later), but that the ones in the anterior end of the ovary are the forerunners of the rete cords, the ones in the middle region the forerunners of the medullary cords. For the study of the formation of Pfliiger’s egg cords, egg nests and primary Graafian follicles, remove the ovary from a human foetus of seven to eight months, or from any mam- malian embryo at a corresponding stage of development, fix in Flemming’s fluid, cut thin sections in celloidin and stain with Heidenhain’s hematoxylin. For the study of the later development of the Graafian follicles, fix the ovary of an adult cat or dog in Orth’s fluid, cut celloidin sections through the entire organ and stain with Weigert’s hematoxylin and eosin. These sections are usually satisfactory for the study of corpora lutea. , The Testicle—The beginning of the changes that differentiate the testicle from the ovary can be studied in sections of male pig embryos of 25 to 30 mm., prepared according to the technic given under “‘The Genital Ridge.” As in the ovary (see above), trabecule of epithelial cells extending into the underlying mesenchymal tissue constitute the sex cords, but the sex cells (forerunners of the spermatogonia) are less clear and resemble the undiffer- entiated epithelial cells. The sex cords in the anterior end of the testicle are the forerunners of the rete testis (which unites with the mesonephric tubules) and the straight tubules; those in the middle region are the forerunners of the convoluted seminiferous tubules. Sections of a testicle at a later period (at birth, for example) are very instructive. Fix the gland in Orth’s fluid, cut longitudinal sections of the entire organ through the rete testis, including also the epididymis, and stain with Weigert’s hematoxylin and eosin. The descent of the ovaries and testicles can be demonstrated in dissections of human em- bryos of three months and more. If the dissections are carefully made, the bladder and urachus, the urogenital sinus, and the various ligaments can be identified. References for Further Study. Apam, J. G.: The Principles of Pathology. Vol. I, 1908. AIcHEL, O.: Vergleichende Entwickelungsgeschichte und Stammesgeschichte der Nebennieren. Arch. f. mik. Anat.. Bd. LVI., 1900. ALLEN, B. M.: The Embryonic Development of the Ovary and Testis in Mammals. Am. Jour. of Anat., Vol. III, 1904. BEarD, J.: The Germ-cells of Pristiurus. Anat, Anz., Bd. XXI, 1902. BEARD, J.: The Morphological Continuity of the Germ Cells in Raja batis. Anat. Anz., Bd. XVIII, 1900. Bonnet, R.: Lehrbuch der Entwickelungsgeschichte. Berlin, 1907. E1crenmann, C. H.: On the Precocious Segregation of the Sex-cells of Micrometrus ageregatus. Jour of Morphol., Vol. V, 1891. Fetrx, W.: Entwickelungsgeschichte des Excretions-systems. frgebnisse der Anat. &@. Entwick., Bd. XIII, 1903. 442 TEXT-BOOK OF EMBRYOLOGY. Fetrx, W., and Buuier, A.: Die Entwickelung der Harn- und Geschlechtsorgane. In Hertwig’s Handbuch d. vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. III. Teil I, 1904. Gace, S. P.: A Three Weeks’ Human Embryo, with Especial Reference to the Brain and Nephric System. Am. Jour. of Anat., Vol. IV, 1905. Geruarpt, U.: Zur Entwickelung der bleibenden Nieren. Arch. f. mik. Anat., Bd. LVII, 1901 Hertwic, O.: Lehrbuch der Entwickelungsgeschichte des Menschen und der Wirbel- tiere. Jena, 1906. Hitt, E. C.: On the Gross Development and Vascularization of the Testis. Am. Jour. of. Anat., Vol. VI, 1907. Huser, G. C.: On the Development and Shape of the Uriniferous Tubules of Certain of the Higher Mammals. Am. Jour. of Anat., Vol. IV, Suppl., 1905. Keiser, F.: Zur Entwickelungsgeschichte des menschlichen Urogenitalapparatus. Arch. f. Anat.u. Physiol., Anat. Abih., 1896. Koun, A.: Das chromaffine Gewebe. Ergebnisse der Anat, u. Entwick., Bd. XII, 1903, Koriman, J.: Lehrbuch der Entwickelungsgeschichte des Menschen. Jena, 1898. Koriman J.: Handatlas der Entwickelungsgeschichte des Menschen. Jena, 1907, Bd. II. MarcHanp, F.: Missbildungen. In Eulenburg’s Real-Encyclopddie der gesammten Heilkunde, Bd. XV, 1897. McMourricu, J. P.: The Development of the Human Body. Philadelphia, 1907. Miwor, C. S.: Laboratory Text-book of Embryology. Philadelphia, 1903. Morcan, T. H.: The Cause of Gynandromorphism in Insects. Am. Naturalist, Vol. XLI, 1907. NacEL, W.: Ueber die Entwickelung des Urogenitalsystems des Menschen. Arch. f. Mik, Anat., Bd. XXXIV, 1889. NacEL, W.: Ueber die Entwickelung der Urethra und des Dammes beim Menschen. Arch. f. mik. Anat., Bd. XL, 1892. NaGEL, W.: Ueber die Entwickelung des Uterus und der Vagina beim Menschen. .1rch. f. mik. Anat., Bd. NXXVII, 1891. Prersot, G. A.: Teratology. In Wood’s Reference Handbook of the Medical Sciences, Vol. VII, 1904. Pott, H.: Die Entwickelung der Nebennierensysteme. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. III, Teil I, 1905. Rasi, C.: Ueber die Entwickelung des Urogenitalsystems der Selachier. Morphol. Jahrbuch, Bd. XXIV, 1896. Theorie des Mesoderms. Ueber die erste Entwickelung der Keimdriise. Morphol. Jahrbuch, Bd. XXIV, 1896. ‘ SCHREINER, H. E.: Ueber die Entwickelung der Amniotenniere. Zeitschr. f. wissensch. Zoologie, Bd. LXXI, 1902. Soutiz, A.: Sur le méchanisme de la migration des testicules. Comp. Rend. de la Soc. ‘de Biol., Paris, Ser. 10, T. II, 1895. Sovtie, A.: Recherches sur le développement des capsules surrénales chez les vertebrés supérieurs. Jour. de. P Anat. et de la Physiol., T. XXXIX, 1903. SToERK, O.: Beitrag zur Kenntnis des Aufbaus der menschlichen Niere. Anat. Hefte, Bd. XXIII, 1904. TANDLER, J.: Ueber Vornieren-Rudimente beim menschlichen Embryo. Anat. Hefte, Bd. XXVIII, 1905. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 443 Taussic, F. J.: The Development of the Hymen. Am. Jour, of Anat., Vol. VIII, 1908. WIESEL, J.: Ueber die Entwickelung der Nebennieren des Schweins, besonders der Marksubstanz. Anat. Hefte, Bd. XVI, 1900. Witson, E. B.: Studies on Chromosomes. Jour. of Exp. Zoél., Vol. II, 1905, Vol. III, 1906. Vol. VII, 1909. WINTWARTER, H.: Recherches sur l’ovogenése et l’organogénése de |’ovaire des Mammi- féres. Arch. de Biol., T. XVII, 1900. Woops, F. W.: Origin and Migration of the Germ-cells in Acanthias. Am. Jour. of Anat., Vol. I, No. 3, 1902. CHAPTER XVI. THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. The integument consists of the skin and certain accessory structures. The skin is composed of the dermis (or corium) and the epidermis. The accessory structures comprise the hairs, nails, sudoriferous glands, sebaceous glands, and mammary glands. The epidermis (or epithelial layer) and all the accessory structures are derived from the ectoderm; the dermis is mesodermal in its origin. Other appendages of the skin—such as scales, feathers, claws, hoofs, and horns—which are found only in the lower animals, are ectodermal derivatives and belong in the same class as the accessory structures in man. The Skin. Tur EpipermMis.—The embryonic ectoderm consists primarily of a single layer of cells (Fig. 81). During the latter part of the first month, the single layer gives rise to two layers, of which the outer is composed of irregular flat cellsand is known as the epitrichium or periderm, the inner or basal, of larger cuboidal cells which are the progenitors of the epidermal cells and of the acces- sory structures. The epitrichial cells later become dome-shaped and acquire a vesicular structure, the nuclei becoming less distinct. They persist until the middle of foetal life and are then cast off and mingle with the secretion of the newly formed sebaceous glands as a constituent of the vernix caseosa (see p. 449). The epidermal cells, constantly increasing in number, soon come to form several layers (4 to 6 in the sixth month). The innermost layer rests upon the base- ment membrane and is composed of cuboidal or columnar cells rich in cytoplasm} the outer layers consist of irregular cells with clearer contents and less distinct nuclei. As development proceeds, the basal layer gives rise to several layers which together constitute the stratum germinativum. The cells of the innermost layers are constantly proliferating and thus forming new cells which are pushed toward the surface. During the seventh month keratohyalin granules appear in two or three layers which are then known collectively as the stratum granu- losum. The clearer cells of the superficial layers undergo a process of de- generation by which their contents are transformed into a horny substance, the nuclei becoming fainter and finally disappearing. These modified or degen- erated cells, which are constantly being cast off and replaced by others from 444 THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 445 the deeper layers, constitute the stratum corneum (Fig. 392). In the thick epidermis, on the palms of the hands and the soles of the feet, for example, a few layers of cells just outside of the stratum granulosum become specially modified (keratinized) to form the stratum lucidum. THE Drruis.—In the first month the dermis is represented by closely ar- ranged, spindle-shaped mesenchymal (mesodermal) cells underlying the epidermis, and is separated from the latter by a delicate basement membrane. This mesenchymal tissue gives rise to fibrous connective tissue which, about the third month, becomes differentiated into two layers—the dermis proper and the deeper subcutaneous tissue. The papille develop as little projections of the dermis which grow into the stratum germinativum of the epidermis. In some of these, many blood vessels appear, while in others nerve endings Eponychium Root of nail Nail Sole plate Phalanx II Sweat glands Fic. 391.—Longitudinal section through the end of the middle finger of a 5 months human feetus. Bonnet. (tactile corpuscles of Meissner) develop, thus giving rise to vascular and nerve papille. Usually a considerable amount of fat develops in the subcutaneous tissue. Some of the mesenchymal cells of the dermis are transformed into smooth muscle cells which are found in connection with the hairs (arrectores pilorum), in the scrotum (tunica dartos), and in the nipples. The dermis has generally been considered as a derivative of the cutis plates (p. 167) which, with the myotomes, constitute the outer walls of the primitive segments, but it is probable that the outer walls of the segments are trans- formed wholly into muscle tissue (McMurrich). The pigment in the dermis develops in the form of granules in the connect- ive tissue cells; that in the epidermis appears as granulesin the cells of the deeper layers (white races) or of all the layers (dark races). Whether the pigment in the epidermis arises independently or is carried from the dermis by wandering cells is not known. 446 TEXT-BOOK OF EMBRYOLOGY. The Nails. The nails are derivatives of the epidermal layer of the ectoderm, and cor- respond morphologically to the claws and hoofs of lower animals. The epidermis on the end of each finger and toe forms a thickening, known as the primitive nail, which is encircled by a faint groove (Zander). This occurs about the ninth week. Later the nail area migrates to the dorsal side of the digit and becomes somewhat sunken below the surface of the surrounding epithelium (Fig. 391). These observations have led to the conclusion that primarily the nails in man occupied positions on the ends of the digits, cor- responding to the positions of the claws in lower forms. Furthermore, the fact that the nails (or their anlagen) are at first situated on the ends of the digits and subsequently migrate dorsally would exvlain the innervation of the nail region by the palmar (and plantar) nerves. Strat. corneum il ' Epidermis Strat. germinativum if LS. Hair germ ggf-------- Hair papilla Con, tis. follicle 7 Hair germ ~ Hair papilla Connective tissue follicle Fic. 392.—Vertical section of the skin of a mouse embryo of 18 mm., showing early hair germs, Jfaurer. After the dorsal migration of the nail area, the epithelium and dermis along the proximal and lateral edges become still more elevated to form the nail wall, the furrow between the latter and the nail being the nail groove. At the distal edge of the nail area, the epithelium becomes thickened to form the so-called sole plate, which is probably homologous with the more highly developed sole plate in animals with hoofs or claws. The epithelium of the nail area increases in thickness, and, as in the skin, becomes differentiated into three layers (Fig. 391). The outer layers of cells become transformed into the stratum corneum. The cells of the next deeper layers, which acquire keratin granules and constitute the stratum lucidum, degenerate and give rise to the nail sub- stance. Thus the nail is a modified portion of the stratum lucidum. The layers of epithelium beneath the nail form the stratum germinativum, which, with the subjacent dermis, is thrown into longitudinal ridges. THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 447 After its first formation, the nail is covered by the stratum corneum and the epitrichium, the two together forming the eponychium. The epitrichium soon disappears; later the stratum corneum also disappears with the exception of a narrow band along the base of the nail. The formation of nail substance begins during the third or fourth month in the proximal part of the nail area. The nail grows from the root and from the under surface in the region marked by the whitish color (the lunula). New keratinized cells are added from the subjacent stratum germinativum and be- come degenerated to form new nail substance which takes the place of the old as the latter grows distally. The Hair. The hairs, like the nails, are derivatives of the epidermal layer of the ecto- derm. In embryos of about three months, local thickenings of the epidermis appear (beginning in the region of the forehead and eye-brows) and grow obliquely into the underlying dermis in the form of solid buds—the hair germs (Fig. 393, I, II). As the buds continue to elongate they become club-shaped and the epithelium at the end of each molds itself over a little portion of the dermis in which the cells have become more numerous and which is known as the hair papilla (Fig. 392). As the epidermal bud grows deeper, its central cells become spindle-shaped and undergo keratinization to form the beginning of the hair shaft; the peripheral layers constitute the anlage of the root sheath (Fig. 393, HI, IV). The hair shaft grows from its basal end, new keratinized cells being added from the epithelium nearest the papilla as the older cells are pushed toward the surface of the skin. The surface cells of the hair shaft become flattened to form the cuticle of the hair (Fig. 393, V). The hairs appear above the surface about the fifth month. Of the cells of the root sheath, those nearest the hair become scale-like to form the cuticle of the root sheath; the next few layers become modified (keratinized) to form Huxley’s and Henle’s layers. Outside of these is the stratum germinativum, the basal layer of which is composed of columnar cells resting upon a distinct basement membrane. The stratum germinativum is continued over the tip of the papilla, where its cells give-rise to new cells for the hair shaft (Fig. 393, V)- The connective tissue around the root sheath becomes differentiated into an inner highly vascular layer, the fibers of which run circularly, and an outer layer, the fibers of which extend along the sheath. The two layers together con- stitute the connective tissue follicle. The first formed hairs, which are exceedingly fine and silky, develop in vast numbers over the surface of the embryonic body and are known collectively as the lanugo. ‘This growth is lost (beginning before birth and continuing during 29 448 TEXT-BOOK OF EMBRYOLOGY. the first and second years after), except over the face, and is replaced by coarser hairs. These in turn are constantly being shed during the life of the individual oe tee y BE 60a, ® ‘a:0 OF CLP ES eee Shae SS ey III ma S¢0 Le Kee SLA 1 tee f oats ie FIG. 393.—Five stages in the development of a human hair. Siéhr, z, Papilla; 6, arrector pili muscle; c, beginning of hair shaft; d, point where hair shaft grows through epidermis; e, anlage of. sebaceous gland; /, hair germ; g, hair shaft; #. Henle’s layer; 7, Huxley’s layer; , cuticle of root sheath; J, inner root sheath; m, outer root sheath in tangential section; 1, outer root sheath; 0, connective tissue follicle, and replaced by new ones. ‘The new hairs probably in most cases develop from the old follicles, the cells over the old papille proliferating and the newly THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 449 formed hairs growing up through the old sheaths. In some cases, however, new follicles are formed directly from the epidermis and dermis. In some of the lower Mammals, new hair germs appear as outgrows from the sheaths of old follicles, thus giving rise to tufts of hair. The arrectores pilorum muscles arise from the dermal (mesenchymal) cells and become attached to the follicles below the sebaceous glands. The Glands of the Skin. THE SEBACEOUS GLANDs.—These structures usually develop in connection with hairs. From the root sheath a solid bud of cells grows out into the dermis (Fig. 393, IV) and becomes lobed. The central cells of the mass undergo fatty degeneration and the products of degeneration pass to the surface of the skin through the space between the hair and its root sheath. The more peripheral cells proliferate and give rise to new central cells which in turn are transformed into the specific secretion of the gland, the whole process being continuous. On the margins of the lips, on the labia minora and on the glans penis and prepuce, glands similar in character to the sebaceous glands arise directly from the epidermis independently of hairs. THE SuDoRIFEROUS GLANDS.—The sweat glands begin to develop during the fifth month as solid cylindrical growths from the deeper layers of the epider- mis into the dermis (Fig. 391). Later the deeper ends of the cylinders become coiled and lumina appear. The lumina do not at first open upon the surface but gradually approach it as the deeper epidermal layers replace the more superficial. : Tue VERNIX CasEosA.—During feetal life the secretion of the sebaceous glands becomes mingled with the cast-off epitrichial and epidermal cells to form the whitish oleaginous substance (sometimes called the smegma embryonum) that covers the skin of the new-born child. It is collected especially in the axilla, groin and folds of the neck. Tue Mammary GLANDS. In embryos of six to seven mm., or even less, a thickening of the epidermis occurs in a narrow zone along the ventro-lateral surface of the body (Strahl). In embryos of 15 mm. this thickening, known as the milk ridge, extends from the upper extremity to the inguinal region (Kallius, Schmidt). Later the caudal end of the ridge disappears, while the cephalic portion becomes more prominent. The further history of the ridge has not been traced, but in embryos considerably older the anlage of each gland is a circular thickening of the epidermis in the thoracic region, projecting into the underlying dermis. It seems most probable that this local thickening represents a portion of the original ridge, the remainder 450 TEXT-BOOK OF EMBRYOLOGY. having disappeared. Later the central cells of the epidermal mass become cornified and are cast off, leaving a depression in the skin (Fig. 394). In em- bryos of 250 mm. a number of solid secondary buds have grown out (Fig. 395). These resemble the anlagen of the sweat glands, to which they are generally considered as closely allied (Hertwig, Wiedersheim and others), and represent the excretory ducts. Continued evaginations from the terminal parts of the excretory ducts form the lobular ducts and acini. The acini, however, are scarcely demonstrable in the male, and not even in the female until pregnancy. Lumina appear by a separation and breaking down of the central cells of the ducts and acini, the peripheral cells remaining as their lining. Nipple ; Epitrichium depression Dermis Fic. 394.—Vertical section through the anlage of the mammary gland of a human foetus of 16 cm. Bonnet. Late in foetal life, or sometimes after birth, the original depressed gland area becomes elevated above the surface to form the nipple. The excretory ducts (15 to 20 in number) which at first opened into the depression, thus come to open on the surface of the nipple. In the area around the nipple—the areola—numerous sudoriferous and sebaceous glands develop, some of which come to open into the lacteal ducts. Sometimes rudimentary hairs appear. Other glands—known as areolar glands (of Montgomery)—resembling rudi- mentary mammary glands also develop from the epidermis of the areola. After birth the mammary glands continue to grow slowly in both sexes up to the time of puberty. After this they cease to grow in the male, and then atrophy. In the female, growth of the glandular elements goes on, but very slowly, and usually a considerable amount of fat develops in the surrounding tissue, causing the enlargement of the breasts. The Mammary Glands of Pregnancy.—Even in the female, as stated before, acini are scarcely demonstrable until pregnancy. The mamma consists THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 451 mostly of connective tissue and fat, with scattered groups of duct-like tubules. During pregnancy the tubules give rise to the acini by a process of evagination, the cells increasing in number by mitosis. Toward the end of pregnancy each excretory duct and its smaller ducts and acini form a distinct lobe with a rela- tively small amount of connective tissue. The epithelium is low or cuboidal, and fat begins to accumulate, in the seventh or eighth month, as droplets in the basal parts of the cells. The droplets increase in number and in size, approach- ing the inner end of the cell, until finally the cell is practically filled. At the beginning of lactation the fat escapes into the lumen of the acinus, leaving a bit of ragged cytoplasm with a nucleus. This regenerates into a cell capable of Stroma (dermis) Stroma Fic. 395.—Vertical section of the anlage of the mammary gland of a human feetus of 23 cm. Nagel. further activity; and it is probable that the same cell may become filled with fat and discharge its contents several times during lactation. During pregnancy and lactation the acini also contain leucocytes which have wandered through the epithelium from the surrounding tissue. These contain fat droplets and are known as colostrum corpuscles. At the end of lactation the acini atrophy and disappear, the lobules becoming masses of connective tissue and fat, which contain groups of duct-like tubules and which are so closely joined with one another that they are indistinguishable as lobules. Anomalies. ANOMALIES OF THE SKIN.—The epidermis may develop to an abnormal de- gree over the entire surface of the body, forming a horny layer which is broken only where the skin is folded by the movement of the members of the body— a condition known as hyperkeratosis. Or the abnormal development may give rise to irregular patches of thick epithelium—ichthyosis. In either case, hairs and sebaceous glands arc usually absent over the affected areas. 452 TEXT-BOOK OF EMBRYOLOGY. Occasionally pigment develops in excess over larger or smaller areas of the skin, giving rise to the so-called nevi pigmentost. In some cases, on the other hand, there is total or almost total lack of pigment in the skin and hair (usually accompanied by defective pigmentation of the iris, chorioid and retina)— a condition known as albinism. ‘There are also instances of partial albinism. The influence of heredity in albinism is doubtful, for albinos are usually the children of ordinary parents. The angiomata (lymphangiomata, hemangiomata) found in the skin are due to dilated lymphatic or blood channels, the color in hemangiomata being due to the hemoglobin in the blood. Dermoid Cysts.—The congenital dermoid cysts not infrequently found in or under the skin are usually situated in or near the line of fusion of embryonic structures, as in the region of the branchial arches, along the ventral body wall and on the back. During the fusion of adjacent structures, portions of the epidermis become constricted from the parent tissue and come to lie in the der- mis, where they continue to grow and produce cystic masses and sometimes give rise to hairs and sebaceous glands. This type of dermoid is to be dis- tinguished from that found for example in the ovary, in which derivatives of all three germ layers are present (see Chap. XIX). ANOMALIES OF THE EPIDERMAL DERIVATIVES.— Occasionally hair develops in profusion over areas of the skin that naturally possess only a fine, silky growth, such, for example, as a woman’s face. Or nearly the entire body may be covered by an unusual amount of hair. Such conditions—known as hyper- trichosis—possibly represent the persistence and continued growth of the lanugo (p. 447) and in this sense are to be regarded as the result of arrested development (Unna, Brandt). Congenital absence of the hair (kypotrichosis, alopecia) is a rare anomaly and is usually accompanied by defective develop- ment of the teeth and nails. Sebaceous cysts, generally regarded as due to accumulation of secretion in the sebaceous glands, sometimes probably represent remnants of displaced pieces of epidermis apart from the hairs (Chiari). Supernumerary mammary glands (hypermastia) and nipples (hyperthelia) are not infrequently present in both males and females. They are usually situated below the normal mamme (rarely in the axillary region), in a line drawn from the axilla to the groin, and probably represent persistent and abnormally de- veloped portions of the milk ridge (see p. 449) In very rare cases a super- numerary gland develops in some other region (even on the thigh). If the mammary glands are morphologically allied to the sweat glands (p. 450), these misplaced mamme are suggestive of anomalous development of some of the sweat gland anlagen. THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 453 PRACTICAL SUGGESTIONS. For study of the early epidermal and epitrichial layers and dermis, fix a pig embryo of 25 to 30 mm. in Zenker’s or Bouin’s fluid, cut sections in celloidin, and stain with Weigert’s hematoxylin and eosin (see Appendix). For later stages of the skin and the formation of hair follicles and papille, cut pieces from the skin and deeper tissues of a pig embryo of 5 to 6 inches and treat according to the above technic, cutting sections at right angles to the surface of the skin. If the hair follicles are far enough advanced, the anlagen of the sebaceous glands can be seen as buds on the root sheaths. The eyelids of a human fcetus of about three months afford especially good material for the study of sebaceous glands. For early stages of the developing nails, treat the ends of the fingers and toes of a human foetus of three months according to the technic given above, cutting longitudinal sections at right angles to the surface of the nails. For the later stages (formation of nail substance, etc.) treat the ends of the digits of an embryo of five months as above. The anlagen of the sweat glands can also be seen as cylindrical growths of the epidermis into the dermis. The milk ridge shows very clearly in young pig embryos (20 mm. or more). Transverse sections of the ridge, prepared according to the technic given in the first paragraph, are very instructive, as are also sections of the mammary glands of a new-born child. References for Further Study. Brouwa: Recherches sur les diverses phases du développement et de l’activité de la mammelle. Arch, de Biol., T. XXI, 1905. Bonnet, R.: Die Mammarorgane im Lichte der Ontogenie und Phylogenie. Lrgebitisse d. Anat. u. Entwick., Bd. II, 1892; Bd. VII, 1898. Katuus, E.: Ein Fall von Milchleiste bei einem menschlichen Embryo. Anat. Hefte, Bd. VIII, 1897. KEIBEL, F., and Matt, F. P.: Manual of Human Embryology, Vol. I, 1910. Krause, W.: Die Entwickelung der Haut und ihrer Nebenorgane. In Hertwig’s Handbuch d. vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. II, Teil I, tgo2. Oxamura, T.: Ueber die Entwickelung des Nagels beim Menschen. Arch. f. Der- matol. u. Syphilol., Bd. XXV, 1g00. Prersot, G. A.: Teratology. In Wood’s Reference Handbook of the Medical Sciences, Vol. VII, 1904. Scumipt, H.: Ueber normale Hyperthelie menschlicher Embryonen und iiber die erste Anlage der menschlichen Milchdriisen iiberhaupt. Morphol. Arbeiten, Bd. XVII, 1897. ScHULTzE, O.: Ueber die erste Anlage des Milchdriisen Apparates. Anat. Anz., Bd. VIII, 1892. Stour, P.: Entwickelungsgeschichte des menschlichen Wollhaares. Anat. Hefte, Bd. XXIII, 1903. Srraut, H.: Die erste Entwickelung der Mammarorgane beim Menschen. Verhandl. d. Anat: Gesellsch., Bd. XII, 1898. ZANDER, R.: Bie frithesten Stadien der Nagelentwickelung und ihre Beziehungen zu den Digitalnerven. Arch. f. Anat. u. Physiol., Anat. Abth., 1884. CHAPTER XVII. THE NERVOUS SYSTEM. BY OLIVER S. STRONG. GENERAL CONSIDERATIONS. There are certain features of the nervous system in general and particularly of the vertebrate nervous system, the comprehension of which makes the processes of development of the nervous system in man more intelligible. First, the nervous systems of the lower Vertebrates are in many respects simpler than those of higher forms and their variations throw light upon the causes which determine neural structures. Second, as the nervous systems of all Vertebrates develop from the same germ plasm, there are resemblances between certain features of both the embryonic and adult systems of lower vertebrates and certain developmental stages in the higher. Certain struc- tures met with in lower adult forms may be regarded as representing stages of arrested development—although specialized and aberrant in many respects —of structures found in higher forms. Vestigial structures in the developing nervous systems of higher forms may be regarded as recurring developmental necessities in the attainment of the adult form. Stated in the most general terms, codrdination of bodily activities in response to both external and internal conditions is the biological significance of the nervous system. This implies a transmission of some form of change from one part to another or, in other words, conduction. This functional necessity is shown structurally in the elongated form of the histological elements of the nervous system. That such changes habitually pass along each element or neurone in some one direction seems to find a natural structural expression in the receptive body and dendrites of the neurone, and in its long transmitting axone, It is also evident that codrdination can only be performed by a transmission of a change from some given structure either back to that structure or to some other structure to cause a responsive change. We thus have not only: in the vertebrate, but at a very early stage in the invertebrate nervous system, a dif- ferentiation into afferent and efferent components, the two together usually being termed the peripheral nervous system. The histological elements of these components are the afferent and efferent peripheral neurones. All structures which are so affected as to transmit the change to the afferent peripheral neu- 454 THE NERVOUS SYSTEM. 455 rones may be conveniently termed receptors, those structures affected by the efferent peripheral neurones may be termed effectors (Sherrington). Receptors include various “sensory”’ structures whose principal function appears to be to limit to some particular kind of stimulus the changes affecting the afferent nervous elements connected with them. LEffectors include various structures (muscles, glandular epithelia) whose activities are influenced by the nervous system (Fig. 396). A primitive nervous mechanism, thus composed of (r) afferent peripheral neurones which transmit the stimulus from a receptor to (2) efferent peripheral neurones which in turn transmit the stimulus to an effector, is a simple, two-neurone reflex arc (Fig. 396). At the same time these neurones, as they increase in number, are obviously brought into relation with each other with more economy of space by having Receptor &¥ Effector Fic. 396.—A two-neurone reflex arc in a Vertebrate. gg. Ganglion, van Gehuchten common meeting places. This, together with the factor noted below, leads to the concentration of an originally diffuse nervous system, spread out principally in connection with the outer (ectodermal) surface, into a more centralized (ganglionic) type of nervous system, which at the same time has in part re- treated from the surface layer (ectoderm) frora which it was originally derived (Fig. 397). Furthermore, when we consider the great number of receptors and effectors in even simple forms, it is apparent that for effective codrdination there must be a considerable degree of complexity of association between the afferent and efferent neurones. ‘These associations may be to some extent accomplished by various branches of the afferent and efferent neurones coming directly into various relations with each other, but it is also evident that when a certain 456 TEXT-BOOK OF EMBRYOLOGY. degree of complexity is reached, such an arrangement would necessitate an extraordinary number of afferent and efferent neurones or an extraordinary development of branches of each where they connect. Accordingly we find a second category of neurones, the intermediate or central neurones which mediate Lumbrieus Nereis Vertebrata re N Fic. 397.—Illustrating the withdrawal from the surface of the bodies of the afferent peripheral neurones. After Reéztus. between the afferent and efferent peripheral neurones. ‘These central neurones, together with portions of peripheral neurones in immediate relation with them, form, in all fairly well differentiated nervous systems, including those of all Vertebrates, the central as distinguished from the peripheral nervous system. Fic. 398.—A three-neurone reflex arc. van Gehuchien. x, Afferent peripheral neurone; 2, intermediate or central neurone; 3, efferent peripheral neurones. The change or stimulus would now pass from receptor through (1) afferent peripheral neurones, (2) intermediate neurones, (3) efferent peripheral neu- rones to effector. This arrangement constitutes a three-neurone reflex arc THE NERVOUS. SYSTEM. 457 (Fig. 398), and is evidently capable of complicated combinations which may be further increased in complexity by the intercalation in the arc of other intermediate neurones. Finally, in the central nervous system certain struc- tures consisting of intermediate neurones are developed which represent the mechanisms for certain coérdinations of the highest order. Such are the higher coérdinating centers (suprasegmental structures of Adolf Meyer). As a result of the preceding, it follows that in seeking the explanation for various nervous structures there must always be kept in mind, first, their correla- tion with peripheral structures and, second, the degree of development of the central coérdinating mechanism represented by the intermediate or central neurones. The most important features common to the nervous systems of all Vertebrates owe their uniformity either to a corresponding uniformity in the peripheral receptors and effectors, or to a uniformity in the codrdinations of the stimuli received and given out by the central nervous system. Variations in structure are due to variations of either the peripheral or central factor above mentioned. In the lower Vertebrates the former factor plays a relatively more important part than in the higher Vertebrates, the central apparatus being simpler; while in the development of the higher vertebrate nervous systems the dominating factor is the increasing complexity of the central mechanism. The superiority of the nervous system of man does not consist, in the main, of supe- riority in sense organs or motor apparatus, but in the enormous development of the intermediate neurone system. GENERAL PLAN OF THE VERTEBRATE NERVOUS SYSTEM. The Vertebrate is an elongated bilaterally symmetrical animal progressing in a definite direction, primitively perhaps by alternating lateral contractions performed by a segmented lateral musculature. Associated with these char- acteristics are the bilateral character of the nervous system and its zransverse segmentation, shown by its series of nerves, a pair to each muscle segment. The definite direction of progression involves a differentiation of the forward extremity of the animal, such as the location there of the mouth and respiratory apparatus and the development there of specialized sense organs, the nose, eye, ear, lateral line organs, and taste buds, which increase the range of stimuli received by the animal and thereby render possible a greater range of responsive activities in obtaining food and in reproduction. As a natural outgrowth of these specializations, the highest development of the central codrdinating mechanism also takes place at the forward end or head. ‘This concentration and development of various mechanisms in the anterior end is usually termed cephalization, and is a tendency exhibited also by various groups of Inverte- brates in which the same general conditions are present. The typical vertebrate nervous system, then, consists of a bilateral central 458 TEXT-BOOK OF EMBRYOLOGY. nervous system connected by means of a series of segmental nerves with per- ipheral structures (receptors and effectors) and exhibiting at its anterior ex- tremity a higher development and specialization in both its peripheral and central parts. The general features of the typical vertebrate nervous system are best revealed by a brief examination of certain stages in its development. The entire nervous system, except the olfactory epithelium and parts of certain ganglia (see p. 459), is derived ontogenetically from an elongated plate of thickened ectoderm, the xeural plate. This plate extends longitudinally in the axis of the developing embryo, its position being usually first indicated externally by a median groove, the neural groove (Fig. 410), the edges of the plate being elevated into the neural folds (Fig. 411). The neural folds are continuous around the cephalic end of the plate, but diverge at the caudal end, enclosing between them in this region the blastopore. Even at this stage, the neural plate is usually broader at its cephalic end, thereby indicating already the future differentiation into brain and spinal cord (Fig. 413). The neural folds now become more and more elevated (Fig. 412), presumably due in part to the growth of the whole neural plate, and finally meet dorsally and fuse, thus forming the neural tube (Figs. 72 and 429). The fusion of the lips of the neural plate to form the neural tube usually begins somewhere in the middle region of the plate and thence proceeds both forward and backward (Fig. 119). The last point to close anteriorly is usually considered as marking the cephalic extremity of the neural tube, and is called the anterior neuropore. Even before the neural plate closes to form the tube, there is often a differen- tiation of cells along each edge, forming an intermediate zone between the neural plate and the non-neural ectoderm (Fig. 429). As the neural plate becomes folded dorsally into the neural tube these two zones are naturally brought together at the point of fusion of the dorsal lips of the neural plate. The two zones thus brought together are not included in the wall of the neural tube, but form a paired or unpaired ridge of cells lying along its dorsal surface. This ridge of cells is called the neural crest (Fig. 429). Later, each half of the neural crest separates from the other half and from the neural tube and passes ventrally down along the sides of the tube, at the same time becoming trans- versely divided into blocks of cells (Fig. 434). These masses of cells are the rudiments of the cerebrospinal ganglia and differentiate into the afferent per- ipheral neurones, and into some at least of the efferent peripheral visceral neu- rones (sympathetic) as well as some other accessory structures (see pp. 496 to sor). The peripheral processes of these ganglion cells (afferent peripheral nerve fibers) pass to the receptors, the central processes (afferent root fibers) enter the dorsal part of the nerve tube (Fig. 430). In the case of the special sense organs there is an interesting tendency on the part of portions of the neural THE NERVOUS SYSTEM. 459 tube, either evaginations (optic vesicles, olfactory bulbs), or ganglia, to fuse with ectodermal thickenings (placodes) at the site of the future sense organs, There appear to be often two series of ganglionic placodes in the head, a dorsal (suprabranchial) series and a ventral (epibranchial) series, the latter being often known as gill cleft organs. The former appear to be especially connected with the development of the acustico-lateral system, the latter prob- ably with the gustatory (see p. 469). (Fig. 399). The bodies of the efferent q q a) 016.0 iyat og 49 A a Suprabranchial placode --e# $e ; @ Fic. 399.—Transverse section through the head of a 7 day Ammoccetes in the region of the trigeminal ganglion. von Kupffer. neurones (except the sympathetic) remain in the neural tube, lying in its ventral half, and send their axones out as the efferent peripheral nerve fibers to the effectors. The formation of the neural plate and its closure into a tube are the em- bryological expression of the above noted tendency of highly specialized neural structures to concentrate and withdraw from the surface (p. 455). The same is true of the less highly specialized placodes, in which this process is not carried so far. The neural plate may thus be regarded as the oldest placode. The afferent peripheral neurones would naturally originate from the borders of this plate, such portions being the last to separate from the non-neural ectoderm or outer surface. They may be regarded as the youngest portions, phylo- genetically, of the plate, and there seems to be some variation among Chordates as to the degree of inclusion of the afferent peripheral neurones in the plate. In the neural tube thus formed, there can be distinguished four longitudinal 460 TEXT-BOOK OF EMBRYOLOGY. plates or zones: A ventral median plate (floor plate), a dorsal median plate (roof plate), where the fusion occurred, and two lateral plates (e.g., Fig. 442). Two points are to be noted: First, that the neural plate is a bilateral struc- ture and the future development of the tube will naturally take place principally in the side walls or lateral plates of the formed tube; second, that the primary connection between the two side walls is the ventral median plate, the dorsal median plate having been produced by a secondary fusion. ‘This being the case, the ventral connection between the two lateral plates will naturally be more extensive and possibly more primitive than the dorsal. ‘The ventral and dorsal median plates do not usually develop nervous tissue, but bands of vertical elongated ependyma cells. In places the roof plate expands into thin mem- branes which are covered with vascular mesodermal tissue forming chorioid plexuses, such as the chorioid plexuses of the lateral, third and fourth ventricles (Fig. 408). Fic. 400.—Scheme of a median sagittal section through a vertebrate brain before the closure of the neuropore. von Aupffer. A., Archencephalon; D., deuterencephbalon; J/s., medulla spinalis (spinal cord); cd., notochord; cn., neuronteric canal; ek., ectoderm; ¢1., entoderm: J., infundibulum; 7., neuropore; pv., ventral cephalic fold; ¢p., tuberculum posterius. It has already been seen that even at its first appearance the neural plate exhibits a differentiation into an anterior expanded part, the brain, and a posterior narrower part, the spinal cord. After closure, in many Vertebrates at least, a three-fold division can be made out: (1) A caudal part of the neural tube, the spinal cord, which gradually expands cranially into (2) the caudal part of the brain (deuterencephalon, v. Kupffer) (Fig. 400). These two parts lie above the notochord and all the typical cerebrospinal nerves are connected with them. (3) Cranially, at the anterior end of the notochord, the brain wall expands ventrally forming the third portion (archencephalon). At the forward extremity is seen the anterior neuropore. The deuterencephalon is thus an epichordal part of the brain, while the archencephalon is prechordal. At the boundary between the two is a ventral infolding of the brain wall—the ventral cephalic fold (plica encephali ventralis). At this stage the brain resembles that of Amphioxus in many respects. From each side wall of the archencephalon THE NERVOUS SYSTEM. 461 an evagination appears, the optic vesicle (Fig. 414) which develops into the retina and optic nerve. In the next stage (Fig. gor), there is a tendency for the neural tube to bend ventrally around the anterior end of the notochord. This bending is the cephalic flexure. At the same time the dorsal wall above the cephalic fold. be- comes expanded and is marked off from that part of the dorsal wall lying caudally by a transverse constriction, the rhombo-mesencephalic fold, and from the part of the dorsal wall lying cranially by another transverse fold at the site of the future posterior commissure. The middle part of the brain, the roof of which is thus marked off, is the mid-brain or mesencephalon. Its floor is the middle projecting part of the ventral cephalic fold. The cephalic expansion of the brain, practically the former archencephalon, is now the Fic. 401.—Scheme of a median sagittal section through a vertebrate brain after the formation of the three primary brain expansions. von Kupffer, P., prosencephalon; M., mesencephalon; R., rhombencephalon; Ms., spinal cord; cw., chiasma emi- nence; J., infundibulum; /t., lamina terminalis; pv., ventral cephalic fold; px., processus neuroporicus; ~7., rhombo-mesencephalic fold; r.*, unpaired olfactory placode; re., recessus (pre-?) opticus; ¢., tuberculum posterius. fore-brain or prosencephalon and the caudal expansion is the rhombic brain or rhombencephalon. These three primary brain expansions (“vesicles”), the fore-brain, mid- brain and rhombic brain, are constant throughout the Vertebrates. Beginning at the location of the former neuropore (processus neuroporicus) and passing caudally along the floor of the fore-brain we have the lamina terminalis or end- wall of the brain, containing a thickening which indicates the site of the future anterior (cerebral) commissure, next the recessus preopticus, then another thick- ening, the chiasma eminence, and finally a diverticulum, the recessus postopticus and injundibulum (Fig. 401). At a later stage (Fig. 402), there appear two evaginations in the roof of the fore-brain, the anterior epiphysis or paraphysis and the posterior epiphysis or epiphysis proper (pineal body). Immediately caudal to the paraphysis is a transverse infolding of the brain roof, the velum transversum. ‘The line aa 462 TEXT-BOOK OF EMBRYOLOGY. (Fig. 402) extending from this fold to the optic recess indicates the location of a fold in the side walls in some forms and is taken by some as the boundary be- tween two subdivisions of the fore-brain, the end-brain or telencephalon and the inter-brain or diencephalon. Cranial to the epiphysis proper, is a commissure in the dorsal wall (commissura habenularis) connecting two structures which develop in the crests of the side walls, the ganglia habenule. From the dorsal part of the telencephalon is developed the paliium. The ventral anterior part evaginates toward the olfactory pit, its end receiving the olfactory fibers. This region is often termed the rhinencephalon. ‘Thickenings of the basal lateral walls of the telencephalon form the corpora striata. Fic. 402.—Scheme of a median sagittal section through a vertebrate brain showing the five-fold division of the brain. von Kupffer. T., Telencephalon; D., diencephalon; M., mesencephalon; Mt., metencephalon; A//., myelence- phalon; ¢., cerebellum; cc., cerebellar commissure; ch., habenular commissure; cp., posterior commissure; cw., chiasma eminence; ¢., epiphysis; e?., paraphysis; J., infundibulum; /¢., lamina terminalis; p1., processus neuroporicus; pr., rhombo-mesencephalic fold; pv., ventral cephalic fold; re., recessus (pre-) opticus; si., sulcus intraencephalicus posterior; ¢tp.. tuber- culum posterius. The lines aa., dd and ff indicate the boundaries between four divisions. tw The roof of the mesencephalon finally develops the ‘“‘optic lobes.” The thickened part of the roof lying immediately caudal to the rhombo-mesen- cephalic fold develops into the cerebellum. The part of the tube of which this forms the roof is often called the hind-brain or metencephalon, while the rest of the rhomhbencephalon is then termed the after-brain or myelencephalon. The roof of this portion, which has become very thin in the course of its development, forms the epithelial part of the tela chorioidea of the fourth ventricle. The con- stricted portion of the tube between the rhombic brain and mid-brain is the asthmus. The above subdivisions of the three primary expansions into five parts (end-, inter-, mid-, hind- and after-brains), especially the subdivisions of the rhombic brain, do not have the morphological value of the three primary THE NERVOUS SYSTEM. 463 divisions but have a certain value for descriptive purposes. The cavities of the brain are the ventricles and their connecting passages, namely, the third ventricle of the diencephalon and the fourth ventricle of the rhombencephalon, the. two being connected by the mid-brain cavity (aqueductus Sylvii). The telencephalon usually develops a more or less paired character, its cavities being then paired diverticula of the unpaired fore-brain cavity and known as the lateral ventricles. Before the closure of the brain part of the neural tube, transverse constric- tions appear across the neural plate. The transverse rings into which the Fic, 403.—Chick embryos; 1, of 22 hours’ incubation; z, of 24 hours; 3, of 254 hours; 4, of 26 hours, showing respectively 2, 5, 6, and 7 primitive segments. Hill, cp., Caudal limit of fore-brain; fr., caudal limit of mid-brain; w., first primitive segment; ps., primitive streak; 1-11, neuromeres, : tube, when completed, is thus divided are known as neuromeres. They are held to represent a primitive segmentation of the head, similar, perhaps, to that exhibited by the spinal nerves and segmental somatic musculature (primi- tive segments) of the trunk. The neuromeres may appear before the head somites. ‘To what extent they correspond to the somites or to the visceral segmentation (p. 467) and also to the cranial nerves is a matter of dispute. Concerning their number there have been various views, the evidence inclining to three in the fore-brain, two in the mid-brain and six in the rhombic brain (Fig. 403). ‘Their presence and number are most in doubt in the cephalic end of the tube, the highly modified prosencephalon. 30 464 TEXT-BOOK OF EMBRYOLOGY. The general features of the vertebrate nervous system which especially ‘Iluminate conditions met with in the human nervous system are the following: (t) The correlation between the peripheral structures (receptors and effectors) and the nervous system. (2) The distinction between the epichordal and. pre- chordal portions of the brain. The latter (fore-brain) is, in accordance with its anterior position (comp. p. 457), the most highly modified part of the neural tube. (3) The distinction between the segmental and suprasegmental parts of the brain (Adolf Meyer).* The segmental part of the brain is that portion in more immediate connection with peripheral segmental structures. Its epi- chordal part is spinal-like and most clearly segmental. Its prechordal part, both as to its peripheral and central portions, is so highly modified that its segmental character is more obscure. It and the rest of the prechordal brain are most conveniently treated together as fore-brain. The suprasegmental parts of the brain, or higher coérdinating centers, are the cerebellum, mid- brain roof and the pallium (cerebral hemispheres). Their general functional significance has been mentioned (p. 457). Some of their general structural characteristics are: First, that they are each expansions of the dorso-lateral walls of the neural tube; second, that in them the neurone bodies are placed externally and in layers (cortex), the nerve fibers (white matter) lying within; third, that each appears to have originally had an especially close relation with some one of the three great sense organs of the head, the olfactory, visual or acustico-lateral system; fourth, that each is connected with the rest of the brain by bundles of centripetal and centrifugal fibers, and often there are specialized groups of neurone bodies in other parts of the brain for the origin or recep- tion of such bundles. Each higher center has also its own system of association neurones. It will accordingly be most convenient to consider: (z) the spinal cord, (2) the segmental part of the epichordal brain, (3) the cerebellum, (4) the mid- brain roof, (5) the prosencephalon. Spinal Cord and Nerves. As already brought out, there are two principal morphological differences between the afferent and efferent peripheral neurones. First, the neurone bodies of the former are located outside the neural tube, while the neurone bodies of the latter lie within the walls of the neural tube. Second, the afferent *This distinction apparently ignores the fact that the primitive neuromeric segmentation of the neural tube involves its dorsal as well as its ventral walls and thus ‘‘ suprasegmental ” as well as “ seg- mental” structures were originally segmental. This may be granted, but while the demonstration of the primitive segmentation of the neura! tube may be valuable as showing the primitive mechan- ism which has undergone later modifications, the importance of such later modifications renders the above distinction necessary. The main significance of the nervous system is its associative character and its progressive development is not as a segmental, but as a more and more highly developed associating mechanism. THE NERVOUS SYSTEM. 465 nerves enter the dorsal part of the lateral walls of the tube, while the efferent nerves leave the ventral part of the lateral walls, their neurone bodies lying in this ventral part. The effect of this upon the structural arrangements within the tube is the production in the tube of two columns of neurone bodies, a dorsal gray column for the. reception of the dorsal or afferent roots and a ventral gray column containing the efferent neurone bodies. Another important differentiation arises apparently from the important physiological difference in general character between the activities of what may Fic. 404.—Transverse section through the body of a typical Vertebrate. showing the peripheral (segmental) nervous apparatus. Froriep. Small dots, afferent visceral neurones; coarse dots, afferent somatic neurones; dashes, efferent visceral (ventral root and sympathetic) neurones; dines, efferent somatic neurones. Darm, gut; Gegl..spin., spinal ganglion; Cg. vert., vertebral sympathetic ganglion; Ggl. mesent., mesenteric sympathetic ganglion. The peripheral sympathetic ganglionic plexuses (Auer- bach and Meissner) aré not shown. Musé.. muscle; Rad. dors., dorsal root; Rad. vent., ventral root; R. comm., white ramus communicans. Two sympathetic neurones are represented as intercalated in the visceral efferent pathway. It is doubtful if there should be more than one. be termed the internal (visceral or splanchnic) and the external (somatic) struc- tures. Internal activities are to a certain extent independent of activities which have to do more with the reactions of the organism to the external world, and consequently their nervous mechanisms have a more or less independent character, forming what is often called the autonomic (sympathetic) system. This independence is exhibited structurally by the intercalation in the per- ipheral pathway of additional neurones, whose bodies form visceral ganglia 466 TEXT-BOOK OF EMBRYOLOGY. connected in various ways among themselves and probably having their own reflex arcs or plexuses. These ganglia are nevertheless to some extent under the control of the efferent neurones of the central nervous system, some of which send their axones to such ganglia (Fig. 404). ‘There are thus in the central nervous system two categories of efferent peripheral neurones, those innervating visceral structures via sympathetic ganglia and those innervating somatic structures. The bodies of the somatic efferent neurones are located in the ventral gray matter of the nerve tube, while the bodies of the splanchnic efferent neurones are believed to occupy more central and lateral positions in the lower half of the gray matter of the neural tube (Fig. 4o4). It is uncer- tain whether there are similar afferent splanchnic neurones in the sympathetic ganglia, and thus distinct from those in the spinal ganglia, or whether these all lie in the spinal ganglia and are consequently not fully differentiated from the somatic afferent neurones. The muscular segmentation of the trunk has already been mentioned and also the corresponding segmental arrangement of the spinal nerves. Local extensions of this musculature and of its overlying cutaneous surface in the form of fins and limbs cause corresponding increase in the size of those seg- ments of the cord innervating them. ‘This is due to the increased number of afferent fibers and consequent increase in the dorsal white columns and in the receptive dorsal gray columns, also to the increase in the number of efferent peripheral neurones whose bodies occupy the ventral gray column (e.g., cervi- cal and lumbar enlargements). (Compare also the differentiation in the cervical cord and lower medulla of the columns and nuclei of Goll for the lower extremities and those of Burdach for the upper extremities). In general, the intermediate neurones of the cord fall into two categories; intersegmental (ground bundles), connecting cord segments, and those send- ing long ascending bundles to suprasegmental structures (see pp. 472 and 473.) The Epichordal Segmental Brain and Nerves. The principal peripheral structures which exert a determining influence on the structure of the epichordal brain are: The mouth, the respiratory apparatus (gills and later lungs), and two specialized sensory somatic structures, the acustico-lateral system and the optic apparatus. In the gills we have essentially a series of vertical clefts forming communica- tions between the pharynx and the exterior, the intervals between the clefts being the gill arches. The musculature of the gill arches is morphologically splanchnic (pp. 302 and 311). The gill or branchial musculature is in closer relations with stimuli from the external world than is the visceral musculature of the body. As a result of this the former is not of the smooth involuntary THE NERVOUS SYSTEM. 467 type, like the visceral musculature of the body, but is of the striated voluntary type, like the somatic musculature. The branchial receptors are naturally visceral in character and there is also in this region a series of specialized visceral receptors, the end buds of the gustatory system. The development of this whole specialized visceral apparatus in this region of the head has appar- ently caused a corresponding reduction of the somatic musculature. The musculature of the mouth is also splanchnic, the mouth itself being regarded by many morphologists as a modified pair of gill clefts which has re- placed an older mouth lying further forward in the region of the hypophysis. The existence of this series of gill clefts has naturally caused a branchiomeric or splanchnic segmentation of the musculature of this region as opposed to the somatic muscular segmentation seen in the trunk. Whether these two kinds of segmentation correspond in this region is uncertain. (In this connection see Fig. 428 and p. 496.) In the acustico-lateral system three parts may be distinguished: (1) a remark- able series of cutaneous sense organs, extending in lines over the head and body and known as the lateral line organs; (2) the vestibule, including the semicircu- lar canals; (3) the cochlea (organ of hearing proper—Corti’s organ). In the higher Vertebrates, the lateral line organs have disappeared, owing to a change from a water toa land habitat; the labyrinth has remained unchanged, and the cochlea has undergone a much higher development and specialization. Regarding the optic apparatus, it is sufficient to point out here that its motor part, the eye muscles, is usually taken to represent the sole remaining somatic musculature belonging to the head proper. The peripheral nerves of the epichordal part of the brain have fundamen- tally the same arrangements as the spinal nerves, namely, the peripheral af- ferent neurone bodies are separate from the nerve tube, forming ganglia, while the bodies of the efferent neurones are located centrally in the morphologically ventral portions of the lateral walls of the nerve tube. There are, however, important differences, clearly correlated with the peripheral differentiations and specializations outlined above, and affecting the afferent and efferent nerves. First to be considered is the afferent part of the trigeminus (Figs. 405 and 406). -The peripheral branches of the ganglion (semilunar or Gasserian ganglion) of this nerve innervate that part of the external (somatic) surfaces of the head (skin and stomodeal epithelium) which have not been encroached upon by the spinal afferent nerves. This nerve is accordingly more strictly comparable with the afferent spinal nerves. The central processes of the semilunar ganglion cells, after entering the brain, form a separate descending bundle, the spinal V. It is interesting to note that the terminal nucleus of this bundle of fibers is the morphological continuation in the brain of the dorsal gray column of the cord. The extensiveness of the area innervated by TEXT-BOOK OF EMBRYOLOGY. 468 (‘90b “Brq “yo ‘tasks onoyyeduds 94} 0) SurBuojaq Bysues ay} Jog) saarou Teurds omy ys1y ay) Jo sjoor [etjWaa puke [esiop ay} “4 "a pue “4p ‘saarau yeydto30-ourds ‘90 $aaroU [BIOISIA ‘X “Wa ssayoueiq jeryouesq-isod pur -o1d owl A-JI $Is[9 Joao Buryioy pue pajeuorySued ‘searou jwwpoueiq oy} ‘y-1Xy :snBea ayy, ‘(slsowojsvue s,uosqose[) ITA Jo. youesq oun -ered ay} YM Sursourojseue (‘XT “uv ‘@) aarau aunered sy pue saysueiq Teryouviq-jsod pu -aid sy yy ‘(x y) snadudreydosso3 aul vauryeyed ‘774 ‘ud Sue ‘ue ‘me ‘soyIueiq JOJOUr STI YIM ‘seqipuemMody “Xy ‘uolsae8 ayepnajues °F ‘uF ‘ruedurdy epsoyo “2 *9 Surpnypout ‘774 ‘radoid ST[RIORY AY, “wiaysds eradstA-orqouriq Jualaya ay} SOP 491094 pUe (XT 94} 1Oy 4 yanp) waysds (suYyouRlds) yerodslA-orypsuEiq JuAIEye oy L—'sop 1317 ‘aarou (re[nguisea) Atoupne “FIT A ‘ILA Jo styerogiadns snormyey3ydo “774 ‘s ‘Jo WM (2) uorsauUOD [eInsstutUIOD sh pure ‘(7°s) yours perodwiaj-eidns s3t YIM ‘aarou stperaqel “yw “7 TTA JO youriq rejnqrpueur [euta}xe “xa ‘put S]TA jo youesq stpeoong ‘774 ‘gong “wnoisnoe Jaqn} ayy ‘('p 7) Jajueo swt pue waysds aur] [eraye] ey —‘Suzpoys anbiqgo ‘snpunjoid snoiwiyeyyydo “¢ ‘do ‘sueyixeur “xu {stepqipueu “pus Suoy8ued urpasser) OF -F ‘(aasau YA ‘Teuruasty) waysds a1vUIOs |BIOUaT jualaye ay [—y901g ‘pepeys Apusregip ore saatou Surureusol ay, ‘soarou [elueIs qIxXIs pue ‘YIINO} ‘pry? ‘puooes Ysry aya 74 ‘ar ‘ery ‘77 ‘7 ‘siyapo yeryourigq ‘4-7 fapends “s ‘eyeipenb-ojejed “ *¢ SueB10 A1ojo¥yO “o “yo fagepnivs speypoyy “9 ‘yu (“Meaysaapat |] Woy paytpoyy) "USg & UL Soadou JeTUBID ay) Jo sayouerq fedrousd ayy Suraoys weiserq— Sor ‘ont S % ENED: SN ARAN NGAY ARIAL VA g-- aioe: THE NERVOUS SYSTEM. 469 the trigeminus may be partly due to disappearance or specialization of anterior somatic nerves and also to the growth of the head. The organs of the lateral line are innervated by a quite distinct system of ganglionated afferent nerves whose central connections are nearly identical with those of the acoustic (Fig. 405). With the disappearance of the lateral line organs and the specialization of the cochlear part of the ear vesicle, there is a disappearance of the lateral line nerves (comp. Figs. 405 and 406) and a well- marked division of the acoustic nerve into vestibular and cochlear portions, the former innervating the older vestibulo-semicircular canal portion, the latter, the more recent cochlea. Centrally, the vestibular nerve forms also a descend- ing bundle of fibers and has its own more or less specialized terminal nuclei. The latter is also true of the cochlear nerve. The afferent portions of the facial, glossopharyngeal and vagus nerves in- nervate the splanchnic receptors of the pharyngeal and branchial surfaces as well as of a large part of the viscera. The facial, glossopharyngeal and vagus also innervate the specialized splanchnic receptors, the gustatory system men- tioned above. This system of taste buds has a very extensive development in certain lower Vertebrates, especially the Bony Fishes. In the latter the system of nerves innervating these structures is naturally much more extensive and its central terminations and nuclei cause important modifications of the medulla. In Mammals the remnants of this system are represented by the taste buds in the mouth, the nerves innervating them being the chorda tympani branch of the facial and the lingual branch of the glossopharyngeal (Fig. 406). The central branches of the ganglia of these three nerves, after entering the brain, form a descending bundle of fibers, the ¢vactus solitarius (or communis). The somatic musculature of the head, as above mentioned, is usually taken to be represented by the eye muscles and, later, the tongue muscles. The tongue is one of the newer structures, rising in importance with the change to a land habitat, and its muscles are probably an invasion from the neck region caudal to the branchial arches (p. 322). The eye muscles are innervated by the III, IV and VI cranial nerves, the tongue muscles by the XII which is a more recent addition to the cranial nerves. All of these nerves are charac- terized by having their neurone bodies located in the most medial (morpholog- ically most ventral) portions of the lateral brain walls, and they all, except the IV, emerge near the mid-ventral line. In these respects they resemble the major or somatic part of the ventral spinal roots. (For illustration see Figs. 427, 405 and 406). The splanchnic musculature of the jaws and the branchial arches is inner- vated by the efferent portions of the V, VII, IX, X (and XI). The neurone bodies or nuclei of origin of these nerves lie more laterally than those of the III, IV, VI and XII, and their axones also leave the nerve tube more laterally TEXT-BOOK OF EMBRYOLOGY. 470 ‘aarou Ted} eproys ay} Aq paryerojsed Arejpxeurqns ‘4 fuosqodef Jo sisoumoiseue ay} Aq jeasudreydossoys pure petoe} ay} YA pa}oauu0s {9110 ‘y faarou yessoajad [eioysodns ayy Aq [etsey oy} YIM payauu0. st yoy ‘auryeyed ouayds ‘|| fArerpro ‘| -oeaypdmcds ay) mosf paatsap D1,SuUDE) ‘sadtau yeutds oi} 151g 9Y} JO S}ooI [eIJUIA pu [esIop ‘ary ‘ay pue ‘pry “py *8JOOI [esiop [eIssaa sw ‘pry fpessojsod4y ‘a zry “Caossaace yeurds ‘yx “youwiq praBudrel Soy Oy ‘yy yey ‘suorsiaipqns JOYUN] Sli YUM “BIadsIA O} YDUBIG ‘yyy fsnBea Sy ‘saAToU [BIRT pure jeadudreydosso/3 ay} Suljsvouu0s ‘uosqooe[ jo sisowojseut “ypr ‘youviq peadudreyd ‘yy woyg pure ‘qenSuy ‘yy “Suez {pea8udreydossos ‘xyz *gAdau (1B9]q909 pur Iejnq ~ysea) Atonpne “777A *(SMIpaursayur snaiau) yersez jo uorjiod ywasaye “weg ‘sapasnut [eryouerg tf} 0} pue Iva aq} Jo sapIsnuI 94} 0} sayoUeIq “WA ‘sapsnur [etoey ayy 10F (cz yy) snxajd oy} 0} asia soals sayeunig ul yor ‘yun (renqipuewody) urew ‘774 ‘oye “KET f(YO) ree a|ppru ay} ySnosy} spuayxa speuumeyy UL Yrs Jo jred ‘tueduis] epsoys “ha "YD ‘(FTA ‘Jod) ouyeped ay se ({1) uorSue3 aujeped-ousyds aq} puosaq PIEM1O} Spueixe pue ‘Teloey oy} Jo uOTod Justaye oy] Wolz sastze Yoram ‘aarou pesorjod peroysedns solew “yy ‘uoySued oyemoues “ay ‘{yeey ‘77 A ‘youriq jetodura}-ojnarine “mp fsajosnu prokyoréu ay} pur qoquaargq ay} Jo ATjaq yeruaa ayy Ajddns yop sayouezgq quataya “py “arg ‘uryo oy sayouesq ‘wayyy $(-€7-YD) yeIoey ayy Jo yueIq ruedurd} epsoyo oy) YM pue (4 wg) uottod JusIoNa ay} YWA paysuuos st Jsye] ey, “(4 “Fu2T) penBuy e pue ‘(DY) Buds JOMOLIeU OY] UL Ie_NqIpueuUT ® OJUl SUIPIAIp pouren-jse] oy} ‘(4 ‘punyy) IeMqIpuem pue ‘(4 wvopy) Arepxew “(A ‘Y¢O) orwueyiydo oi ‘sayoueIq 901y) SIT YM ‘feuruTesLy ‘4 “ACIS Yep “X Jusroye ayy pue ‘youq papeys st XT erage oy} yey? Waoxe ‘Sob “Bry yt puodsai09 0} ‘Supeys Jo spury ywarayIp Aq paysinsurjsip ore saarau Joyjo ay, “oAdau jusonpge ‘7A ‘Aru (TeaTY301}) oNayTed ‘4 7 foAIauU IOJOWIO|NI0 ‘7 Jy ‘eatau odo ‘77 ‘aarau Atojoeyo ‘7 ‘onSuo} ‘7 Soqipuew “puny fa8essed ueioeisna-ourdiudy “() fade “Pp ‘ueB10 A1ojIeT]O “sou “avd (ujaysaapa?,\) WOI, PEegTPOY) “BIOTUUTY 9Y} Ul S2ArOU [wrUeID oy} Jo UOTINGIySIp ayI SuIMOYs WIeIsel~[—gor “org OMX KH 5 pees SE THE NERVOUS SYSTEM. 471 along with the incoming afferent fibres. These nerves all exhibit a character- istic segmental arrangement corresponding to that of the gill clefts. The VII, IX, and the various nerves making up the X, divide dorsal to the cor- responding gill clefts into prebranchial and postbranchial branches, also giving off suprabranchial branches. The efferent element, or component, forms a part of each postbranchial branch. These relations are shown clearly in the accompanying diagrams (Figs. 405 and 406). Part of the vagus also innervates the viscera and this nerve is thus divisible into branchial and visceral portions. Two peculiarities may be noted in regard to these splanchnic nerves: First, that the afferent portions have ganglia resembling those of the spinal nerves; second, that the branchial efferent portions consist simply of one neurone proceeding all the way from the nerve tube to the muscle innervated, thus resembling the somatic rather than the visceral nerves of the trunk. As al- ready noted (p. 466), these nerves regulate activities somatic in character but involving splanchnic structures. It is thus seen that the dominating factor is functional rather than morphological—present functional necessities modify those of the past. With the change from a water to a land habitat and the accompanying disappearance of gills and appearance of lungs, we have various suppressions and modifications of the branchial musculature (Fig. 406). There are two striking specializations of the branchial musculature. One is the origin of the facial (mimetic) musculature in the highest Vertebrates. This is derived from the muscles of the hyoid arch, innervated naturally by extensions of the facial nerve. The other is a specialization of muscles, probably of the caudal branchial arches, into cervico-cranial muscles (head-movement), innervated by what may be considered a caudal extension of the vagus nerve, namely, the spinal accessory (p. 503). The splanchnic laryngeal musculature and its nerves show a certain degree of specialization (sound-production) in higher forms. The efferent V is naturally a large constant nerve, in correlation with the uniformly developed jaw musculature in all jaw-bearing (gnathostome) Vertebrates (Figs. 405 and 406). These various changes in peripheral structures are thus due either to environmental influences or to developments within the central nervous system (p. 457). One of the most important en- vironmental influences is the change from a water to a land habitat. The influence of the central nervous system is shown in the further development and specialization of a number of peripheral structures as motor “‘instru- ments” of suprasegmental mechanisms. The effects, then, of the peripheral arrangements upon the arrangements within the neural tube are: (1) The formation of separate tracts and terminal nuclei for (a) the unspecialized somatic afferent V nerve (spinal V and posterior 472 TEXT-BOOK OF EMBRYOLOGY. horn); (b) the specialized somatic vestibular nerve (descending or spinal VIII and various terminal nuclei) and also the cochlear nerve and its various termi- nal nuclei; (c) the splanchnic afferent nerves (tractus solitarius and its terminal nuclei). (2) The separation of the efferent neurone bodies lying in the neural tube into two main longitudinal series of nuclei (a) the somatic efferent nuclei, occupying a more medial position, their axones emerging from the neural tube as medial ventral nerve roots; (b) the splanchnic efferent nuclei occupying a more lateral position, their axones emerging laterally and forming mixed roots with the incoming afferent fibers (Fig. 407). se Fic. 407.—Diagram of a transverse section through the lower human medulla showing the origin of the X and XII cranial nerves. Schafer. g, Ganglion cell of afferent vagus sending central arm (root fiber) to solitary tract (f. s.) and col- lateral to the nucleus of the solitary tract (f.5s.7.). It is not certain that the axones of the cells of this terminal nucleus take the course indicated in the figure. 7. amb., nucleus am- biguus and d. ». X, dorsal efferent nucleus of the vagus, both of which send out axones as the efferent root fibers of the vagus. These two represent the lateral or splanchnic efferent nuclei of this region. ». XII, nucleus of the hypoglossus the axones of which pass out medially as efferent root fibers of the XII. This nucleus represents the medial or somatic efferent nuclei of this region. /.s.. tractus solitarius or descending roots of vagus, glossopharyngeus and facial; d. V., descending spinal root of the trigeminus; ¢., restiform body; v., inferior olivary nucleus (“olive”); pyr.. pyramid. The intermediate neurones of the epichordal segmental brain, as well as of the cord, fall into two general systems. One of these is the system of intersegmental neurones, connecting various segments of the segmental brain and cord. This system may be collectively termed the ground bundles (of the cord) and reticular formation (of the brain). ‘These neurones may be regarded as not only furnishing the various reflex communications between the afferent and efferent cerebrospinal peripheral neurones, but as also forming a system upon which the descending neurones from the higher coérdinating centers (suprasegmental structures) act, before the efferent peripheral neurones are reached. This system may thus be regarded in general as more closely associ- THE NERVOUS SYSTEM. 473 ated with the efferent than with the afferent peripheral neurones. Certain tracts in this system and their nuclei of origin have reached a considerable degree of differentiation, due principally to association with higher centers. Among these differentiated reticulo-spinal tracts may be mentioned the medial longitudinal fasciculus, the rubro-spinal tract, and the various tracts from Deiters’ nucleus. The other system consists of nuclei which are associated with the afferent axones as their terminal nuclei, the axones of which form long afferent tracts to suprasegmental structures. Especially well-marked differ- entiations of nuclei and tracts of this system are usually due both to its con- nections with peripheral structures and with the higher centers. The principal afferent suprasegmental tracts to the cerebellum are mentioned below (p. 473). Those to mid-brain roof and (via added neurones) to pallium are the medial fillet or lemniscus from the nuclei of the columns of Goll and Burdach, the lateral lemniscus from the cochlear terminal nuclei and other ascending tracts from terminal nuclei of peripheral afferent neurones. The Cerebellum. The other great factor (see p. 457) affecting the structure of the epichordal brain is the development in it of two higher coérdinating centers or supraseg- mental structures, the cerebellum and optic lobes. The cerebellum is a develop- ment of the dorsal part of the lateral walls of the tube just caudal to the isthmus and was probably primarily developed in correlation with the acustico-lateral system, especially with the lateral line and vestibulo-semicircular canal portions (p. 467). Due probably to the fact that it is thus an important “equilibrating’’ mechanism, the cerebellum has acquired other important con- nections besides its original ones with the acustico-lateral system. In the vertebrate series it is especially developed in all active balancing forms (Fig. 408). In Mammals it has acquired important connections with the greatly enlarged pallium (cerebral hemispheres), in accordance with its general regulative in- fluence (static and tonic) upon motor reactions. The great development of the cerebellum has profoundly modified the anatomical arrangements of the rest of the brain and cord, owing to its numerous and massive connections. The fol- lowing important masses of gray matter and fiber bundles may be mentioned as cerebellar afferent connections: Clarke’s column cells, and other cells in the cord, and the spino-cerebellar tracts; the lateral nuclei, inferior olives and the restiform body in the medulla; part of the pes pedunculi, the pontile nuclei and middle peduncle of the cerebellum. The superior cerebellar peduncle to the red nucleus, together with tracts to Deiter’s nucleus, belong to the cerebellar efferent connections. The cortico-pontile portion of the pes, the pontile nuclei and the middle peduncle represent the most recently developed cerebral con- nections (comp. pp. 477-479 and Fig. 409). 474 TEXT-BOOK OF EMBRYOLOGY. The Mid-brain Roof. This expansion of the dorsal part of the neural tube constitutes a higher coérdinating center for impulses received by various somatic nerves—spinal, cochlear and optic. Owing to its being, in all forms below Mammals, the principal visual center, the optic part (optic lobes) varies in proportion to the development of the eye, animals with poorly developed eyes having small optic lobes. In Mammals, the optic part (anterior corpora quadrigemina or col- liculi) is relatively less important, owing to a taking over of a portion of its coérdinating functions by the neopallium (pp. 477, 479), but the cochlear part (posterior corpora quadrigemina or colliculi) has increased in importance, owing to the rise of the cochlear organ (organ of Corti). The centripetal and centrifugal connections of the mid-brain roof are not so massive or extensive and consequently do not modify the other parts of the brain and cord as pro- foundly as do those of the cerebcllum. It sends descending tracts to after- brain and cord segments. The Prosencephalon. The division of this part of the brain into the telencephalon and diencephalon has already been indicated (p. 462). In the diencephalon may be noted (1) the absence of the notochord ventral to the brain, thereby permitting a ventral ex- pansion of the brain walls, the hypothalamus, associated with an organ not well understood, the hypophysis; (2) certain more or less vestigial structures, such as the pineal eyes (epiphyses), and other primitive structures, such as the ganglia habenule, in the dorsal part, this dorsal portion being collectively termed the epithalamus; (3) nuclei in (1) and (2) connected with olfactory and gustatory tracts; (4) receptive nuclei for the optic tract and the cochlear path from the posterior colliculus; (5) receptive nuclei for secondary tracts from the end stations of more caudal somatic ganglia (nuclei of Goll and Burdach and medial lemniscus). The last two (4 and 5) constitute the thalamus and increase in importance in the higher Vertebrates (see p. 477, Fig. 409). In the telencephalon there may be roughly distinguished an anterior and basal part, the rhimencephalon, in especially intimate relations with the olfactory nerve; a thickening of the basal wall, the corpus striatum;and a thinner-walled dorsal part, the pallium. The latter may be regarded in a sense as a dorsal develop- ment of the corpus striatum and first appears as a distinct structure in the Amphibia. The peripheral or segmental apparatus which are connected with the pros- encephalon are the highly modified optic and olfactory organs. While the optic apparatus primarily originates from the prechordal brain, in the lower Verte- brates its highest coédrdinating center, as mentioned above, lies partly in the THE NERVOUS SYSTEM. 475 epichordal portion (optic lobes). It is possible that this connection is secon- dary and contingent upon two functional necessities, the importance of cor- relation with stimuli coming via more caudal nerves (cochlear and spinal nerves), and the innervation of its motor apparatus by epichordal nerves, the III, IV and VI. With the development of the neopallium in Mammals (see p. 477) and the consequent projection of visual stimuli upon it, the lower pre- chordal (thalamic) centers form part of the newer pathway to the neopallium and thus increase in importance, while the optic lobes recede, assuming the position of a reflex center, especially for the visual motor apparatus. The olfactory nerves enter the anterior extremity of the brain and are con- nected by secondary and tertiary tracts with regions lying more caudally, where in some cases the olfactory stimuli are associated with gustatory and probably with visual stimuli. One of these regions is the hypothalamus which receives both olfactory and gustatory tracts (Herrick). More dorsal olfactory pathways pass to the epithalamus. Both epithalamus and hypothalamus give rise to de- scending systems which doubtless ultimately reach efferent nuclei. In fact, this part of the brain presents, apparently, a complicated primitive mechanism for the correlation especially of olfactory and gustatory stimuli, also to some extent of visual stimuli and stimuli via the trigeminal nerve, the whole forming a sort of oral sense, probably controlling the feeding activities (Edinger). The next factor in the further development of this part of the brain is the rise in importance of the pallium upon which at first are projected mainly olfactory stimuli (Fig. 408). A further and still more extensive development of the pallium arises when other kinds of stimuli are projected to a considerable extent upon it, thus giving rise to a distinction between the older olfactory pallium (archipallium) and the newer non-olfactory pallium (neopallium). The latter appears first in the lateral dorsal portion of the pallial wall and by its subsequent development the archi- pallial wall is rolled inward upon the mesial surface of the hemispheres. Further changes consist in the extension caudally of this portion pari passu with the extension caudally of the neopallium and then the practical obliteration of its middle portion by the great neopallial commissure, the corpus callosum (Fig. 408, G and H). In addition to the increasing projection of stimuli from all parts of the body upon the neopallium and the consequent increase in centripetal fiber termina- tions and in centrifugal neurone bodies lying in its walls, a second factor in the development of the neopallium is the enormous increase of its association neurones. It is the latter feature which especially distinguishes the human from other mammalian brains. The biological significance of these changes lies in the fact that there is thus produced a mechanism not only for the association of all kinds of stimuli, but ’ 476 TEXT-BOOK OF EMBRYOLOGY. << 0. 8 SELACHIAN (Torpeds) “* G oRNitTHORRYNCHUS Fic. 408.—A-F (Edinger) are sagittal sections showing structures lying in the median line and also paired structures (e.g., pallium) lying to one side of the median line. The cerebellum is black. It is doubtful whether the membranous rovf in A indicated as pallium is strictly homologous with that structure in other forms. In B, Pallium indicates prepallial structures. Aq. Syl., Aqueductus Sylvii; Basis mesen., basis mesencephali; Bulb. olf., bulbus olfactorius; Corp. striat., corpus striatum; Epiph., epiphysis; G.A., ganglion habenule; Hyp., hypophysis;. Infund., infundibulum; Lam, ¢t., lamina terminalis; Lob. elect.. lobus electricus; L. vagi, lobus vagi; L. opt., mid-brain roof; Med. obl., medulla oblongata; Opt., optic nerve; Pl. chor., plexus chorioideus; Rec. inf., recessus infundibuli; Rec. mam., recessus mammnillaris; Saccts vasc., saccus vasculosus; Sp. ¢., spinal cord; venir., ventricle; v.m.a., velum medullare anterius; v.m. p., velum medullare posterius. G and H show the mesial surface of the cerebral hemispheres in a low (G) and high (H) Mammal. G. Elliot Smith, Edinger, slightly modified. The exposed gray matter of the olfactory regions is shaded, the darker shade indicating the archi- pallium (preterminal area and hippocampal formation), the lighter shade indicating the rhinencephalon, which consists of the anterior and the posterior (principally pyriform) olfactory THE NERVOUS SYSTEM. 477 also for very complex codrdinations between these stimuli. In this way an extensive symbolization and formulation of individual experience (memory, language, etc.) can take place. The formulated experience of one generation can be immediately transmitted (by education in the broad sense of the term) to the plastic late-developing neopallia of the next generation. In this way a racial experience may be rapidly built up without the direct inter- vention of the slow processes of heredity and natural selection and each gen- eration profit by the accumulated experience of past generations to a much greater extent. The nervous mechanism, the pallium, is provided by in- heritance; experience-is not inherited but “learned.” The pallial associative mechanisms are continuously modified by their activities, thus affecting the character of subsequent pallial reactions (associative memory). Such reac- tions are usually termed psychical or conscious, as distinguished from the reflex reactions of other parts of the nervous system. In the course of these developments the pallium or cerebral hemispheres have enormously increased in size until in man they overlap all the other parts of the brain. Naturally the extensive connections of the neopallium with the rest of the brain have profoundly modified the latter. Among the new struc- tures which have on: this account been added to the older structures of the rest of the brain, the following may be mentioned: (1) The centripetal connections of the neopallium, consisting mainly of what are usually termed the thalamic radi- ations. ‘These consist essentially of a system of neurones passing from the above mentioned termini in the thalamus of general somatic, acoustic and optic ascending systems to certain areas in the cerebral hemispheres. In this system we can distinguish (a) the continuation of the fillet (general somatic) to the cen- tral region (somesthetic area) of each hemisphere; (b) the optic radiation from the lower- thalamic optic center (lateral geniculate body) to the calcarine (visual) area of the hemisphere; (c) the acoustic radiation from the medial geniculate body of the thalamus to the upper temporal region (auditory area) of the hemisphere. Associated with these last two connections are the increase lobes. In Amphibia and Reptiles the hippocampal formation includes all or nearly all of the mesial surface. As the early neopallium appears in the lateral hemisphere walls, the neo- pallial commissural fibers first pass across the median line in the ventral or anterior com- missure. With the increase of the neopallium and its extension on the mesial hemisphere walls, its commissural fibers pass across more dorsally vie the archipallial or fornix com- missure (psalterium) forming the neopallial commissure or corpus callosum, the great de- velopment of which nearly obliterates the anterior hippocampal formation, Com, ant., Anterior commissure; corp. callosum, corpus callosum; Fimbr., fimbria; Fiss. hippo- campi, hippocampal fissure; Lam. ¢., lamina terminalis; Lob. olf. ant., anterior olfactory lobe; Leb. pyriformis, pyriform lobe; Psalt., psalterium (fornix commissure); Sept. pell., septum pellucidum; Tub. olf., tuberculum olfactorium. Only a part of the gray (cortex) of the hip- pocampal formation appears, as the gyrus dentatus, on the mesial surface; the remainder forms an eminence, the cornu Ammonis, on the ventricular surface. This invagination is indicated externally by the hippocampal fissure. The exposed fiber bundle forming the edge of this formation (fimbria) passes forward (fornix and its commissure) and thence descends, as the anterior pillar of the fornix, behind the anterior commissure, The anterior pillar is partly indicated by a few lines in this region in the figure. 478 TEXT-BOOK OF EMBRYOLOGY Ss PH. 5 w_) Cc & 5 g g 4 7 4 ¢ F.C.-P oe “Sa e “s Lo) Sheer loi maha. e Nea 7% sto 2% SS, BRACH. Pon, — oe oe oe a, ~_ ~ RAD, ANT, Sm RAO. POST, SP. GANG, Fic. 409.—Principal afferent and efferent suprasegmental pathways (excepting the archipallial con- nections, the efferent connections of the mid-brain roof and the olivo-cerebellar connections). Neopallial connections are indicated by broken lines. Intersegmental connections are omitted. Some peripheral elements are indicated. Each neurone group (nucleus and fasciculus) is in- dicated by one or several individual neurones. Decussations of tracts are indicated by an X. ac., Acoustic radiation, from medial geniculate body to temporal lobe; br. conj., brachium con- THE NERVOUS SYSTEM. 479 of the geniculate bodies and the diminution of the mid-brain in importance already alluded to (p. 474). (2) The centrifugal connections consisting of (a) the pyramids passing from the precentral area of each hemisphere to various lower efferent neurones, or neurones affecting the latter, and forming part of the internal capsule and pes pedunculi; (b) fibers from various parts of the hemis- phere, forming the greater part of the rest of the internal capsule and pes, and terminating principally in the pontile nuclei whence a continuation of this system (the fibers of the middle peduncle), passes to the cerebellar hemisphere. The great increase in size of the cerebellar hemispheres, of the contained nuclei dentati, and probably of the superior cerebellar peduncles are further effects of this new connection, which has already been alluded to (see Cere- bellum, p. 473), (Fig. 409.) . Another important effect of the development of the pallium is the assump- tion by man of the upright position, due both to the specialization of the hand to execute pallial codrdinations and its consequent release from locomo- tion, and also to the overhanging of the eyes by the enlarged cranium. The great increase of cerebellar connections may be partly due to the new problems of equilibrium connected with the upright position. GENERAL DEVELOPMENT OF THE HUMAN NERVOUS SYSTEM DURING THE FIRST MONTH. One of the earliest stages in the development of the human nervous system is shown in the 2 mm. embryo of about two weeks (Fig. 410). This shows the stage of the open neural groove. The appearance of a transverse section of the neural plate, groove and folds, in other forms, is shown in Figs. 411 and 412. The neural folds now become more and more elevated and finally meet, thus forming the neural tube as previously described (p. 458). The fusion of the neural folds begins in the middle region and thence extends cranially and cau- junctivum (superior cerebellar peduncle); brach. pon., brachium pontis (middle cerebellar peduncle); b.g.7., brachium quadrigeminum inferius (a link in the cochlear pathway); c. g.J., lateral or external geniculate body; c. g.m., medial or internal geniculate body; c. quad., cor- pora quadrigemina; /. cort.-sp., cortico-spinal fasciculus (pyramidal tract); f.c. p.-f. frontal cortico-pontile fasciculus (from frontal lobe); jf. ¢.-p.t., temporal cortico-pontile fasciculus (from temporal lobe); f.¢.-p.0., occipital cortico-pontile fasciculus (from occipital - lobe); J. cun., fasciculus cuneatus (column of Burdach); /f. grac., fasciculus gracilis (column of Goll); f. s.-¢., tract from cord to mid-brain roof and thalamus (sometimes included in Gowers’ tract); f.sp.-c.d., dorsal spino-cerebellar fasciculus (tract of Flechsig); f. sp.-c.v., ventral spino-cerebellar fasciculus (tract of Gowers, location of cells in cord uncertain); lem. lat., lateral lemniscus or lateral fillet; lemniscus med., medial lemniscus or fillet (the part to the thalamus is mainly a neopallial acquisition); .coch., cochlear nerve; n. cun., (terminal) nucleus of the column of Burdach; x. grac., nucleus of the column of Goll; 2. dent., nucleus dentatus; ~. opt., optic nerve; .7., nucleus ruber (red nucleus); pes ped., pes pedunculi (crusta); pulv. thal., pulvinar thalami; pyr., pyramid; rad. ant., ventral spinal root; rad. post,. dorsal spinal root; rad. opt., optic radiation (from lateral geniculate body, and pulvinar (?), .to calcarine region); somaes., bundles from thalamus to postcentral region of neopallium; sp.gang., spinal ganglion; ¢hal., thalamus, br 480 TEXT-BOOK OF EMBRYOLOGY. dally. The stage of partial closure of the neural tube is shown in Eternod’s figure of a human embryo of 2.1 mm. (Fig. 413, 6). This order of closure in- dicates; to some extent, the order of subsequent histclogical development; the extreme caudal and cephalic extremities are more backward than the parts which close first. The last point to close anteriorly marks, as stated previously (p. 458), the cephalic extremity of the neural tube and is the anterior neuropore. As indicated in Eternod’s embryo, the anterior end of the neural plate is broader even before its closure; thus when the tube is completed its anterior end is more expanded. This expansion is the future brain, the narrower caudal portion Yolk sac — f Neurenteric canal Primitive groove Belly stalk Fic. 410.—Dorsal view of human embryo, two millimeters in length, with yolk sac, von Spee, Kollmann. The amnion is opened dorsally. being the future spinal cord. Before the closure of the brain part of the tube the beginnings of the three primary brain vesicles are also indicated (Fig. 120). At this stage the neural plate shows no differentiation into nervous and sup- porting elements. The neural tube is composed of the two Jaderal walls and the median roof and floor plates (comp. p. 460) (Figs. 345 and 442). The appearance of the anterior end of the neural tube with the closure com- pleted, except the anterior and posterior neuropores, is shown in the model of one half of the tube. The external appearance and also the inner surfaces are shown in Figs. 414 and 415. At this stage the cephalic flexure (see p. 461) is already quite pronounced, the cephalic end of the brain tube being bent ven- THE NERVOUS SYSTEM. 481 trally at about a right angle to the longitudinal axis of the remaining portion of the tube. This bending begins before the closure of the cephalic part of the neural tube (Fig. 120). From each side of the brain near the cephalic ex- tremity is an evagination of the brain wall, the beginning of the optic vesicles. Neural Neural Neural fold groove fold Ectoderm CHEN, I a Chorda anlage Entoderm Fic. 411.—Transverse section through dorsal part of embryo of frog (Rana fusca), Ziegler. x, Groove indicating evagination to form mesoderm. The process of evagination and consequently the location of the vesicle begins before the closure of the tube. Dorsal and anterior to the optic vesicles can be seen a slight unpaired pro- trusion of the dorsal wall, the beginning of the pallium. The area basal to itand Neural Prim. Intermed. groove seg. cell mass Parietal and visceral mesoderm ’ . Ectoderm io (epidermis) Chordal plate Ceelom Entoderm Blood vessels Fic. 412.—Transverse section of dog embryo with ten pairs of primitive segments. Bonnet, extending a short distance into the anterior wall of the optic vesicle is the site of the future corpus striatum (Figs. 414 and 415). Caudal to the pallium and separated from it by a slight constriction (in- dicated best by the ridge on the inner wall) is another protrusion of the dorsal wall, the roof of the diencephalon. Still further caudally and separated from the 482 TEXT-BOOK OF EMBRYOLOGY. roof of the diencephalon by another slight constriction is another expansion of the dorsal wall, the roof of the mid-brain or of the mesencephalon which arches over the cephalic flexure. It is separated by another constriction (plica rhombo-mesencephalica) from the rhombic brain or rhombencephalon, which latter tapers into the cord. A ventral bulging of the rhombencephalon indicates the future pons region (Figs. 414 and 415). Cerebral plate Amnion—; Heart Ant. entrance to prim. gut (Ant. “Darmpforte’’) Neural tube Yolk sac 7 (cut edge) Yolk sac Primitive segment —— Neural fold Post. entrance to ; prim. gut (Post. ——7§ Neural groove ““Darmpforte’’) Belly stalk S0/ Neural fold Primitive groove a b Fic. 413.—(a) Ventral view; (b) dorsal view of human embryo with 8 pairs of primitive segments (2.11 mm.). Efernod. From modcls by Ziegler. ; In b the amnion has been removed, merely the cut edge showing; in a the yolk sac has been removed. Even at this early stage the cavity of the caudal part of the rhombencephalon is expanded dorsally due to an expansion of the roof plate, which forms only the narrow dorsal median part of the rest of the tube. This expansion reaches its maximum about opposite the auditory vesicle. The principal changes in form during the next two weeks are the following (Figs. 416 and 472): The cephalic flexure becomes still more pronounced so _that the anterior end of the neural tube is folded back upon the ventral side of the rest of the brain, an effect probably enhanced by the expansion of the THE NERVOUS SYSTEM. Fic. 414.—Lateral view of the outside of a model of the brain of a human embryo two weeks old. His. Diencephalon Pallium Neuropore Corpus striatum Mesencephalon P.f. Optic evagination mbo- mesencephalic fold Ventral cephalic fold (Seesel’s pocket) Pons region Rhombencephalon Fic. 415.—Lateral view of inner side of the same model shown in Fig. 414. Hizs, P.f. is the ridge corresponding to the peduncular furrow on the outer side. 483 484 TEXT-BOOK OF EMBRYOLOGY. ventral wall of the anterior portion (Figs. 416 and 472). In the space thus enclosed the dorsum sell is subsequently formed. Associated with this increase of the cephalic flexure is an increased prominence of the mid-brain roof. The pontine flexure has begun, there being now a bending of the whole tube in the pons region, the concavity of the bend being dorsal. At the same time there is a corresponding tendency for the roof of the rhombencephalon to become shorter and wider. There is also a further thinning of the above mentioned expanded portion of the roof plate in this region, and associated with this a thrusting of the thick lateral walls outward at the top so that they come to lie almost flat instead of vertically as in the cord. From the cord to the place of greatest width above mentioned, this dorsal thrusting apart Fic. 416.—Profile view of a model of the brain of a human embryo during the third week. His. A, Optic vesicle; A.v., auditory vesicle; By, pons region; H, pallium; Hh, cerebellum; J, isthmus; Af, mid-brain; N and Rf, medulla; NK, cervical flexure; Pm, mammillary region; Tr, in- fundibulum; Z, inter-brain or diencephalon. of the lateral rhombic walls obviously becomes more and more pronounced. In front of this region of greatest width, the roof plate becomes narrower and the dorsal parts of the walls (alar plates) form the rudiment of the cerebellum, the rest of the rhombic brain forming the medulla oblongata. Each lateral wall of the rhombic brain is now divided into a dorsal longitudinal zone or plate (alar plate) and a ventral zone or plate (basal plate) by a longitudinal furrow along its inner surface, the sulcus limitans. A study of the external appearances and transverse sections of this part of the brain tube will make these relations clear (Figs. 456, 436 to 439 and 427). Neuromeres are also present at this stage (see p. 496). In the meantime the neural tube has also become bent ventrally at the junction of the brain and cord, forming the cervical THE NERVOUS SYSTEM. 485 flexure. The pallium has increased in size and now forms a considerable prominence on the brain tube. Its boundaries are also much more clearly marked off (see Fig. 471). On the inner side of the tube, the area below the bulging of the pallium is the corpus striatum. Externally, just below the bulging, we have the region where the olfactory lobes are differentiated. The proximal part of the optic evagination has become longer and narrower. The ventral expansion of the diencephalon is the hypothalamus, the portion of the diencephalon dorsal to the latter being the éhalamus. Two slight protrusions of the ventral wall of the hypothalamus have appeared; the caudal one is the mammillary region, the anterior one the infundibulum. ‘The cavity of the diencephalon (third ventricle) is connected by the mid-brain cavity (iter or aqueductus Svlvii) with the rhombic brain cavity or fourth ventricle. HISTOGENESIS OF THE NERVOUS SYSTEM. The neural plate is at first a simple columnar epithelium. The various processes by which this is converted into the fully formed nervous system are: (1) cell proliferation; (2) cell migration; (3) cell differentiation. These proc- esses are not entirely successive in point of time, but overlap each other. Cell division is present from the first, increases to a certain period in development and then practically ceases; cell migration is partly a necessary concomitant and resultant of cell division, and cell differentiation is in part due to the growth of the cytoplasm and is in part a result of environmental differences produced by these processes. In development the following stages may be distinguished: (z) Stage of indifferent epithelium; (2) appearance of nerve elements (neurones) and resulting differentiation into supporting and nerve elements; (3) growth of neurones and resulting differentiation and development of (a) peripheral neurones, (b) lower intermediate or intersegmental neurones, (c) neurones of higher centers and neurone groups in connection with them (supra- segmental neurones). These stages do not occur simultaneously throughout the whole neural tube, some parts being more backward in development than others (p. 480). In general the spinal cord and epichordal segmental brain are most advanced in development. Furthermore, the ventral part of the brain tube precedes the dorsal. The most backward part of the whole neural tube is the pallium. The various phases of form-differentiation of the neurone are (1) the development of the axone and, later, of its branches; (2) the growth of the dendrites; (3) the formation of accessory coverings or sheaths, the neurilemma and the myelin (medullary) sheath. The principal iernal differentiations are (1) the appearance of the neurofibrils; (2) the chromophilic bodies of Nissl; (3) pigment. These latter may all be regarded as products of the nucleus and undifferentiated cytoplasm of the nerve-cell. 486 TEXT-BOOK OF EMBRYOLOGY. Epithelial Stage. Development of Neuroglia. From the very first, the neural plate exhibits dividing cells similar to those seen in the non-neural ectoderm. The cell divisions are indirect and the mitoses are confined to the outer part of the ectoderm, occurring between the outer ends of the resting epithelial cells (Fig. 417). These dividing cells have been termed by His germinal cells. When the neural tube is formed, the mitoses are still confined to the outer. now the luminal, surface, this being a general phenomenon in developing epithelial tubular structures. As a result the daughter nuclei migrate away from the lumen. In the most advanced parts of the neural tube (see p. 485), the mitoses in- crease in number up to about the fourth to sixth week of development, and then diminish and finally nearly disappear about at the end of two months. At about the time the blood vessels penetrate the tube, the mitoses are no longer entirely confined to the proximity of the lumen. As a result of proliferation, the epithelial wall very early assumes the ap- pearance of a stratified epithelium—at least there are several strata of nuclei. There are at this stage in many forms two layers, an outer or marginal layer, free of nuclei, and an inner or nuclear layer (Figs. 418 and 419). In a human embryo, however, of about two weeks this division into layers is yet hardly evident, though there are several strata of nuclei. Apparently these layers are not well-marked until the radial arrangement of the myelospongium, as described below, has become more pronounced. Accompanying the above changes, changes also manifest themselves in the character of the cells. At about the time of the closure of the neural tube, the cell boundaries become indistinct and finally practically obliterated, thus form- ing a syncytium, the myelospongium. At the same time, the syncytium becomes very alveolar in structure and a general spongioplasmic reticulum is formed (Figs. 418 and 419) by the anastomosing denser strands (trabecule) of protoplasm. At a very early stage (two weeks), these trabecule unite along the inner and outer walls of the neural tube forming imternal and external limiting mem- branes. The nuclei of the neural tube have at first an irregular arrangement in the reticulum, at least in the human embryo. This is followed by a more radial arrangement of both nuclei and protoplasmic filaments (Fig. 420), form- ing nucleated radial masses of protoplasm—the sponglioblasts (Figs. 419 to 422). There is some dispute as to the loss, complete or incomplete, of identity of the epithelial cells in the formation of the spongioblasts. According to Hardesty, they are formed by a collapse of the epithelial cells and a rearrange- ment of their denser parts into axial filaments. The radial arrangement does not extend into the outer part of the neural tube which, retaining its irregular reticular character, is now non-nucleated in the human embryo and forms the THE NERVOUS SYSTEM. 487 Fic. 426. Fic. 417.—From the neural tube of an embryo rabbit shortly before the closure of the tube. g, Germi- nal or dividing cell; m, peripheral zone, position of the later marginal layer. His. ‘ Fic. 418.—Pig of 5 mm., unflexed. Just after closure of the neural tube. Segment of a vertical section of the lateral wall of the tube. g, Germinal cells; m, beginning of marginal dayer; mli, internal limiting membrane; 7, radial columns of protoplasm, The resting nuclei lie in the inner or nuclear layer. Hardesty. ' 488 TEXT-BOOK OF EMBRYOLOGY. marginal layer. The increase in the thickness and circumference of the walls of the tube and the resulting tensions may be a factor in this arrangement of the protoplasmic filaments. At the boundary between the marginal and nuclear layers the reticulum appears to be especially dense. With the further increase and development of the nervous elements (see p. 492) the radial arrangement of the spongioblasts noted above becomes more and moreobliterated. As shown by Golgi preparations, in their migration from the lumen (Fig. 422) the spongioblasts lose their connection with the lumen, m mn Ze ep mili aa “a 7 78) ; < e ah S = a ae h ; 7 cee 1 ma 6 GT or IS 4 é' xf 2 iS nan y i me TX. \ \ <5 . rf By Ne LA be” é. & ff te Z a Mer, A Rees A ‘ Re % NY AE mv Fe ss \ mle" SA ag es Fic. 421.—Hardesty. Combination drawing from sections of pig of 15 mm. The upper part is from a section of the same stage as the lower but stained by the Golgi method. By migra- tion and differentiation the mantle layer has been formed. The cells remaining near the lumen form the ependyma layer (ep.). 6, Boundary between mantle and marginal layers; ep, ependyma; mi and mle, internal and external limiting membranes; mv, differently arranged mid-ventral portion of the marginal layer; 7, radial filaments; cs, connective tissue syncytium. : their peripheral processes become abbreviated and disappear, and they finally differentiate into the irregular branching neuroglia cells (Fig. 423). According to Hardesty, there is simply a general nucleated mass which changes form pari passu with changes in the enclosed differentiating nervous elements, finally assuming shapes dependent upon the character of the spaces between the formed nervous elements. An exception to this is a layer of nucleated elements which remain next the lumen and form the ependyma cells which still Fic. 419.—Pig of 7 mm., unflexed. Segment from the ventro-lateral wall of the neural tube; g, Germinal cells; mi, internal limiting membrane; mile, external limiting membrane r, radial, axial filaments of the syncytial protoplasm; , beginning of pia mater. Hardesty. Fic. 420.—Pig of 10 mm., “‘crown-rump”’ measurement. Segment from lateral wall of neural tube. b, boundary between nuclear layer and marginal layer (m). Other references same as in 419. Hardesty. a indicates the zone in which the dividing cells are located. Later, it is composed of the inner ends of the ependyma cells (column layer of His). 489 THE NERVOUS SYSTEM. -uON “¢ ‘gyeitut of unZaq savy YT Jo Tafonu ox} Jo autos sisv{qorSuods jo sjusureyy [eIpes ‘4 ‘1ayeuT vid ‘¢ ‘roke] apjueur ‘ume (sake] peurSreur ‘wa SeuApuoda ‘da ‘ss0ke] apucur pue [euTsreut usamjaq Alepunoq uo ‘sasseUl pojvaponu ‘sjsviqorsuods urejias Jo uorei8rur Suruurgeq Surmoys ‘ Bid -vrur $$ Jo pico jo s[[B@A\ [eIe}e] Jo SjusuIges Jo SIUIMVIpP pautquIoD “Kysapav FJ —cev “OL 490 TEXT-BOOK OF EMBRYOLOGY. send radial extensions into the wall of the neural tube (Figs. 421 and 422). These cells develop cilia projecting into the lumen. A still later differentiation in the supporting elements of the tube is the ap- pearance of neuroglia fibers—a product of the spongioblastic protoplasm, but differing from it chemically (Fig. 423). The exact relation of these neuroglia fibers to the nucleated neuroglia cells in the adult is a matter of dispute. Fic. 423.—Hardesty. Combination drawing from transverse sections of the spinal cord of 20 cm. pig. Showing the first appearance of neuroglia fibers. u, Neuroglia cell as shown by the Benda method of staining; a’, similar cell by the Golgi method; } and 0’, non-nucleated masses; d, free nuclei; e and f, differentiating neuroglia fibers; s, ‘seal-ring’’ cells, envelop- ing myelinating nerve-fibers. With the penetration of blood vessels into the neural tube a certain amount of mesodermal tissue is brought in. How much of the supporting tissue of the nervous system is derived from the mesoderm is uncertain, but it is most probable that it is relatively small in amount and is confined principally to the connective tissue of the walls of the blood vessels. Early Differentiation of the Nerve Elements. It has been seen that some of the actively dividing cells (germinal cells) at first simply increase the ordinary epithelial elements of the tube which in turn form the myelospongium, the spongioblasts and finally the ependyma and the neuroglia. Other daughter cells produced by the division of the germinal cells THE NERVOUS SYSTEM. 491 differentiate into nerve cells as described below. Still others probably migrate outward as indifferent cells, which later proliferate and form cells which differ- entiate into neuroglia and nerve cells. According to recent researches (Cajal), by means of the silver stain of Cajal the first indication of the differentiation of cells into nerve cells is the appear- ance of neurofibrils in the cytoplasm of cells near the lumen. The part of the cell in which the neurofibrils first appear is called the fibrillogenous zone (Held) and is usually in the side furthest from the lumen. The cells in which these appear are apparently without processes, and are accordingly termed apolar cells (Cajal). (Fig. 424.) Fic. 424.—Section through the wall of the fore-brain vesicle of a chick embryo of 3} days. Cajal. A, 6 and c¢, Differentiating nerve cells in apolar stage, the neurofibrils are black; a, cell in a stage transitional to the bipolar stage; B, bipolar cells; « (at lower right corner), cone of “ growth” of developing axone; e, tangential axone. The cells in the bipolar stage have migrated out ward, but the neuroblast or mantle layer has not yet been differentiated. The next step in the development of many, but probably not all, of these cells is their transformation into.bipolar cells by the outgrowth of two neurofibrillar processes, one directed toward the lumen, the other, usually thicker, toward the periphery, the cell body at the same time beginning to migrate outward (Fig. 424). This bipolar stage may be regarded as conditioned to some extent by the radial arrangement of the other elements, due in turn partly to the original epithelial structure and partly, possibly, to tensions produced by the growth of the tube. It is also interesting as recalling conditions in sensory epithelia and in the cerebrospinal ganglia. The bipolar stage is most common probably in those parts where the elements show a radial arrangement in the adult. Such are the layered cortices of the mid-brain and pallium. Nerve cells maintaining a con- nection, by central processes, with the luminal wall have been described in lower Vertebrates. This connection may be explained as due to a persistence of the central processes of cells in the bipolar stage. 492 TEXT-BOOK OF EMBRYOLOGY. The next stage is a monopolar stage produced by the atrophy of the luminal process. Cells in this stage are the neuroblasts of His, the peripheral processes being the developing axones (Fig. 42 5). As seen in ordinary stains, the above differentiation of the neuroblasts is marked by a corresponding differentiation of the nuclear layer into an inner layer retaining its previous characteristic radial arrangement, and an outer layer characterized by fewer nuclei more irregularly arranged. The latter layer is the mantle, or neurone layer (Fig. 442). There are now three layers: (1) inner (nuclear), (2) mantle (neurone) and (3) marginal. The mantle layer is thus produced by the migration and differentiation of cells into neuroblasts. While this process may begin near the lumen (apolar nerve Fic. 425.—Dorsal portion of the lumbar cord of a chick embryo of three days. Cajal. A, B, Cells in the apolar stage with fibrillogenous zones; B shows transition to the bipolar stage; E, further advanced bipolar cell; G, ceils in monopolar stage or neuroblasts of His; a, giant cone of growth. These cells have migrated to the outer part of the nuclear layer, thereby forming the beginning of the mantle layer. cell of Cajal) and progress as the cell has moved somewhat further away (bipolar stage), the monopolar stage is probably reached only when such cells form a part of the mantle layer. In other words, the mantle layer is created by the migra- tion to a certain location and differentiation to a certain stage of the primitive nerve cells. The mantle layer, as previously stated, probably also contains indifferent cells which may by further proliferation and subsequent differentia- tion become either glia or nerve cells.* The looser arrangement of the cells of the mantle layer is probably in some measure due to the growth of the dendrites which appear soon after theaxones. It may be also due to the beginning vascularization of the tissues with resulting transudates (His) which usually, however, begins somewhat later. The association in time of vascularization and further growth *It is _an open question as to how late in development these ‘‘extraventricular’’ cell-divisions, in- volving “indifferent” cells, may occur. The neuroglia cells, however, like other supporting elements, preserve this capacity of division indefinitely, as shown by the increase in neuroglia cells in patho- ogical conditions. THE NERVOUS SYSTEM. 493 of neurocytoplasm (dendrites) is significant. When the cell-proliferation near the lumen has ceased, the supply of new cells ceases, and as the cells of the inner layer continue to differentiate into cells of the mantle layer, the inner layer, being no longer replenished from within, is reduced to the single layer of cells which remain behind as ependyma cells (p. 488). ; Differentiation of the Peripheral Neurones of Cord and Epichordal Segmental Brain. Efferent Peripheral Neurones. The differentiation of a mantle or neurone layer from the outer part of the original nuclear layer is practically universal throughout the whole neural tube. It appears first and is conse- quently most advanced, however, in the ventral part of the lateral walls of the cord and epichordal brain. The axones of neuroblasts occupying the basal plate of this region of the neural tube grow out through the external limiting mem- Fic. 426.—Ventral part of wall of lumbar cord of 7o-hour duck embryo, showing efferent root fibers first emerging from cord (combined from two sections). Cajal. A, Spinal cord; B. perimedullary space; C, meningeal membrane; 4, b, cones of radially directed axones; c,d, cones of transversely directed axones; D, bifurcated cone; E, F, cones crossing perimedullary space; G, aberrant cones. brane and emerge as the efferent ventral root fibers. The appearance of these early root fibers in the duck is shown in Fig. 426. The process is similar in the human embryo and begins about the third week. The neurones thus differentiated are the efferent peripheral neurones. In some forms, at least, cells appear to migrate out from the tube along with the efferent root fibers. Their fate is not certain, but they probably either metamorphose into the neurilemma cells or possibly form part of the sympa- thetic ganglia (see p. 499). In general the questions affecting the differentiation 494 TEXT-BOOK OF EMBRYOLOGY. of the efferent fibers are the same as for the afferent and are further dealt with later (pp. 499-502). The majority of the efferent root fibers pass to the differentiating somatic muscles which they innervate, forming specialized terminal arborizations (the motor end plates). The fibers to the dorsal musculature form, together with the afferent fibers (p. 497), the dorsal branch of the peripheral spinal nerve ; others form part of the ventral branch which sends a branch mesially toward the aorta. Some of the fibers of the mesial branch take a longitudinal course. This mesial branch is the white ramus communicans and terminates in the various sympathetic ganglia which are later formed along its course (p. 498). Fic. 427.—Diagram (lateral view) of the brain of a 10.2 mm, human embryo (during the fifth week), showing the roots of the cranial nerves. His. III, Oculomotor; IV, Trochlear; V, Trigeminus (2, efferent root, s, afferent root); VI, Abducens; VII, Facial; VIII, Acoustic. (c, cochlear part, v, vestibular part); IX, Glossopharyngeus; X, Vagus; XJ, Spinal accessory; XII, Hypoglossus. of., Auditory vesicle; Rh.d., rhombic lip. The two series of efferent roots (medial and lateral) are clearly shown. (Comp. Figs. 263, 265, 432 and 404.) The fibers to the sympathetic ganglia are the visceral (splanchnic) fibers of the ventral root. There are a few other fibers which grow dorsally from neuroblasts in the ventro-lateral walls of the cord and thence out via the dorsal root (Fig. 430). They also are probably visceral. In the cord the splanchnic fibers, with the exception above noted, issue with the somatic fibers in a common ventral root. In the epichordal segmental brain, however, there is a differentiation of the efferent neuroblasts of the basal plate into two series of nuclei, a medial and a lateral. The medial series consists of THE NERVOUS SYSTEM. 495 the nuclei of the XII, VI, IV and III cranial nerves, and their axones grow out as medial ventral root fibers (except the IV) (Fig. 427) to the differenti- ating muscles of the tongue and eyeball which they respectively innervate. These muscles are probably somatic and their nerves are the somatic efferent cranial nerves corresponding with the greater part of the fibers of the ventral roots of. the cord (compare p. 469). The /ateral series consists of the nuclei of the efferent portions of the roots of the XI, X, IX, VII and V cranial nerves and their axones grow out as lateral roots (Fig. 427) to the differentiating striated branchial (splanchnic) muscles (sternocleidomastoideus, trapezius, e~-N.trigem. (motor) jt —-N.trigem. (sens) +N. Pacialis ,acusticus N.abducens fromm N, HY poglossus Fic. 428—Diagram of the floor of the 4th ventricle of a ro mm. human embryo, illustrating the rhombic grooves and their relations to the cranial nerves. ‘The point of attachment of the acoustic and the sensory root of the trigeminal nerve is shown by dotted circles; the motor nuclei are represented by heavy dots. Streeter. pharynx, larynx, face and jaw) and also to muscles of the viscera (via sympa- thetic?). The lateral nuclei and their roots are thus splanchnic. (Cp. pp. 302-3, 469, 471.) Their root fibers, with the incoming afferent fibers, form the mixed roots of these nerves. The positions of these various nuclei and their roots are clearly indicated in Figs. 427, 436-439, 447 and 451 and require no further description. Additional details are mentioned in connection with the afferent cranial nerves. In the region of the vagus nerve, there are differentiated two series of lateral nuclei, a ventro-lateral (nucleus ambiguus X) and a dorso-lateral (dorsal efferent nucleus X) (comp. Fig. 407).. Fig. 452 32 496 TEXT-BOOK OF EMBRYOLOGY. apparently indicates the beginning of this differentiation. The significance of the dorso-lateral nucleus is uncertain. It possibly sends fibers to the sympathetic system. At about this period six transverse rhombic grooves are plainly marked in the floor of the fourth ventricle, standing in relation with the nerves of this region (Fig. 428). They are ordinarily regarded as neuromeric, but the above relation would indicate that they have primarily a branchiomeric character (Streeter). It will be noticed that each of the three main ganglionic masses of this region (p. 502) corresponds to two of the grooves. (Comp. p. 463). The further development of the efferent neurones exhibits phases common to many other nerve-cells with a large amount of cytoplasm (somatochrome cells). The further development of the neurofibrils of cell body and dendrites “ Neural crest we Ectoderm A Toren Neural plate segment Fic. 429.—Three stages in the closure of the neural tube and formation of the neural crest (spinal ganglion rudiment). From transverse sections of a human embryo of 2.5 mm, (13 pairs of primitive segments, 14-16 days). von Lenhossék, is, according to some observations, at first confined to the peripheral portions, leaving a clear zone in the vicinity of the nucleus. The chromophilic sub- stance first appears as distinct granules about the end of the second month, there being apparently a diffuse chromophilic substance present before this period. The chromophilic granules also are first differentiated in the per- ipheral portions of the cell. A still later differentiation is the pigment, which probably does not appear till after birth. This increases greatly in amount in later years and is then an indication of senility of the nerve-cell. Afferent Peripheral and Sympathetic Neurones.—It has already been mentioned (p. 458) that in the closure of the neural tube certain cells forming an intermediate band between the borders of the neural plate and the non- neural ectoderm are brought together by the fusion of the lips of the plate THE NERVOUS SYSTEM. 497 and form a ridge on the dorsal surface of the mental tube, this ridge being known as the neural crest (Fig. 420). In the SPINAL CORD, at three weeks, the neural crest has separated from the cord and split into two longitudinal bands. The ventral border of each band shows a transverse segmentation into rounded clumps of cells, forming the rudiments of the spznal ganglia which later become completely separated. The efferent roots have begun to develop but the afferent roots appear later (fourth week, Fig. 434). The cells composing these rudiments are polyhedral or oval rather than columnar and proliferation still proceeds among them A differentiation of these cells soon begins. Some, usually larger cells Fic. 430—Part of a transverse section through the cord and spinal ganglion of a 56-hour chick embryo (combined from two sections). Cajal. A, Efferent cell of dorsal root; B, cone of growth of central process (afferent dorsal root fiber) of spinal ganglion cell; C, bifurcation of afferent root fibers in cord, forming beginning of dorsal funiculus or dorsal white column of cord. begin to assume a bipolar shape. Their central processes grow toward the dorsal part of the lateral walls (alar plate) of the neural tubé which they enter (Fig. 430), becoming afferent (dorsal) root fibers. These fibers enter the mar- ginal layer and there divide (Figs. 430 and 441) into ascending and descend- ing longitudinal arms which constitute the beginning of the dorsal (posterior) juniculus of the cord. The peripheral processes of the developing ganglion cells grow toward the periphery, uniting with the ventral root and forming with it the various branches of the peripheral spinal nerve (compare Figs. 263, 265, 432 and 404). Other peripheral branches pass as a part of the white ramus communicans to the sympathetic ganglia through which they 498 TEXT-BOOK OF EMBRYOLOGY. proceed to the visceral receptors. These latter fibers are thus visceral afferent fibers. It is now known that the spinal ganglion is a much more complicated struc- ture and has more forms of nerve cells than was formerly realized. The dif- ferentiation into these various types has not yet been fully observed. The bipolar cells, however, become unipolar in the manner shown in Fig. 431. The cell body first becomes eccentrically placed with reference to the two proc- esses and then, as it were, retracts from them, remaining connected with them by a single process. This change may economize space. According to most authorities, many of the cells of the neural crest do not cease their migration by forming spinal ganglia, but undifferentiated cells Fic. 431.—Section of spinal ganglion of 12-day chick embryo. Cajal. Showing various stages of the change from the bipolar to the unipolar condition. A, B, Unipolar cells; C, D, F, G, cells in transitional ztage; E, bipolar cell; HZ, immature cell. The neuro- fibrils are well shown. wander still further ventralward and form, probably also undergoing still further proliferation, the rudiments of the various sympathetic ganglia, becom- ing subsequently differentiated into the sympathetic cells. By this migration there is first formed a longitudinal column of cells ventral to the spinal ganglia (Fig. 433) and, later, in relation with the white communicating rami (Fig. 432). This column becomes segmented (seventh weck), forming ultimately the ganglia of the vertebral sympathetic chain. In the meanwhile, the cells of the column proliferate in places, forming rudiments which, by migra- tion and further differentiation, form the ganglia of the various prevertebral sympathetic plexuses (cardiac, coeliac, pelvic, etc.). Further migrations lead to the formation of the ganglia of the peripheral plexuses (Auerbach, Meissner, THE NERVOUS SYSTEM. 499 etc.). All these ganglia, probably, are innervated by fibers from the white ramus, along whose course they apparently migrated. The axones of their cells pass to visceral structures either in the same segment or, via the longi- tudinal chain, to those of other segments. Some also join the branches of the peripheral spinal nerves (gray ramus). Fibers of the white ramus also pass longitudinally in the chain to vertebral ganglia of other segments. The possibility previously mentioned (p. 493) of a contribution to the sympa- thetic ganglia by cells migrating out along with the ventral roots must be kept in mind. It would seem a priori more probable that these latter would furnish the efferent sympathetic cells, but the efferent cells predominate in the sym- Spinal cord —~-- Spinal ganglion ~----- : Mixed spinal nerve ~--~- - : Myotome manos Sympathetic ganglion ----~ Fic. 432.—From a transverse section of a chick embryo of 44 days. Neumayer. pathetic and must thus be regarded as derived partly or wholly from the neural crest which furnishes at least the major part of all the sympathetic cells. It seems probable that not all the cells of the neural crest form nerve cells, but some, usually smaller cells, become closely applied to the spinal ganglion cells, forming amphicytes, while others (lemmocytes) wander out along the nerve fibers and become the neurilemma cells, forming the neurilemma. ‘These cells in this case would be quite strictly comparable to the glia cells of the neural tube. According to another view, the neurilemma cells are of mesodermal origin. While this point cannot be considered entirely determined, it seems fairly certain that in some types at least the former view is correct, removal of the neural crest having resulted in the formation of efferent nerves without 500 TEXT-BOOK OF EMBRYOLOGY. neurilemma cells (Harrison). The modification into neurilemma cells seems to be accomplished by their enveloping the axones and becoming closely applied to them. The peripheral nerve grows toward the periphery as a bundle of fibers which forms, as seen in many stains, a common fibrillated mass, dividing at its extremity into the develop- ing branches of the nerve. The lemmocytes closely envelop each of these growing tips, but proximally only envelop the main nerve trunk (Bardeen). The final clear separation of Spinal ganglion rudiment Notochord a. @ 8 —Sympathetic ganglion rudiment * we HN, ‘ Fic. 433.—From a transverse section through a shark (Scyllium) embryo of 15 mm., showing the origin of the sympathetic ganglion. Onodi. In mammals the cells are more scattered and their origin from the spinal ganglion rudiment not so clear. the fibrillated mass into the individual nerve fibers is accomplished, according to Gurwitsch, by these accompanying cells forming septa within the mass and finally enveloping each axone as its neurilemma sheath. Growth in bundles appears to be characteristic also of the axones (tracts and fasciculi) of many neurone groups in the central nervous system. Owing to the presence of these migrating cells as well as of mesodermal cells, the peripheral nerves in their earlier stages appear cellular in character; later the fibrous elements predominate, the nuclei becoming more scattered and changing into the flatter nuclei characteristic of the neurilemma (Fig. 432). According to one view (Balfour), the nerve fibers themselves are differentiated from the cyto- THE NERVOUS SYSTEM. 501 plasm of these cell-strings and are thus multicellular structures. Still another view is that of Hensen, according to which the fibers are a differentiation in situ from preéxisting syncytial bridges uniting the parts connected subsequently by the formed nerve fibers. This differentiation may not be primarily con- nected with the neuroblasts (Apathy, Paton). An intermediate view between this and the outgrowth view of His is that of Held, according to which the neurofibrillar substance is an outgrowth from the neuroblast body, or at least a differentiation proceeding from that body, but always within the preéxisting cellular bridges of Hensen. The differentiating fiber is thus always intracel- lular instead of intercellular as according to the His-Cajal view. The experi- ments of Harrison above alluded to, in which the accompanying migrating cells were eliminated and naked axones (axis-cylinders) nevertheless developed, ap- parently disposes of the cell-string theory of Balfour. The growth of the fibers in the marginal layer of the central nervous system is also unfavorable to this theory. The apparently proven capacity of growing axones to find their way through foreign tissues (aberrant regenerating nerve fibers, Cajal), through ventricular fluid (Cajal), and even through serum (Harrison) seems to throw the weight of evidence in favor of the view of His. The latter is the view adopted in this description, though many of the most important facts of development are not perhaps entirely irreconcilable with any of these views. The general conception of the neurone is affected by these questions and the related question of anastomoses between the nervous elements, whether present at all, and if present, whether primary or secondarily acquired. From the above it would seem that the cells of the neural crest have the capacity of differentiating into afferent neurones, efferent (sympathetic) neurones and supporting cells. Other cells of the neural crest differentiate into the chromaffine cells of the suprarenal glands and similar structures (p. 430). There are several views as to the development of the myelin sheath. Ac- cording to one view (Vignal), it is.a product of the neurilemma cells, being formed in a manner analogous to the formation of fat by fat cells. Accord- ing to Wlassak, the various substances composing the myelin (fat, lecithin and protagon) are first found in the central nervous system in the protoplasm of the spongioblasts, their probable original source being the blood of the meningeal blood vessels. Later, the myelin is laid down around the axones, appearing first as drops or granules. The same process takes place in the peripheral nervous system. The supporting elements of the nervous system thus would have a chemical as well as a mechanical function. Another view (Gurwitsch) is that the myelin is a product of the axone and is, at its first appearance, quite distinct from the neurilemma cells. As the appearance of the myelin sheath is a final stage in the development of the neurone, the various neurone systems would naturally become myelinated in about the same sequence 502 TEXT-BOOK OF EMBRYOLOGY. a in which their axones develop. This is probably true in a general way, but the development of both axones and sheaths requires further study before any law can be exactly formulated. Coarse fibers apparently become medullated early; the sheaths of such fibers being usually thicker. Although the myelin sheath is apparently an accessory structure, its formation is of great importance, not only from the above reason, but also because its appearance possibly indicates the assumption by the neurone of its capacity for the precise performance of its final functions. The functional significance of the myelin sheath is not, however, entirely clear. Its importance is enhanced by the fact that its integrity depends upon the integrity of its neurone and that we possess precise stains for demonstrating both its normal and abnormal conditions. In the region of the RHOMBENCEPHALON, the neural crest very early exhibits a division into three masses: a glossopharyngeo-vago-accessorius, an acustico- facialis, and a trigeminus. These masses soon become separated from each other and from the neural tube, the glossopharyngeus also showing a partial separation from the vago-accessorius mass (Fig. 434). The vago-accessorius group, at about three weeks, is a mass of cells much larger at the cranial end and continuous by a narrow band of irregular cells with the spinal neural crest. The cranial end of the mass shows a partial division into a dorsal and ventral part. The former becomes the ganglion of the vagus root, the latter the ganglion of the trunk (nodosum). The glosso- pharyngeus mass likewise shows a division into a dorsal group of cells, the future ganglion of the root and a ventral group, the future ganglion of the trunk (petrosum). The two ventral groups are associated with epidermal thickenings (placodes), but it is doubtful whether any ganglion cells are derived from the thickenings. These thickenings probably represent the uuckenings associated in water-inhabiting Vertebrates with the development of certain sense organs, either lateral line or epibranchial (see p. 459). At this stage there are no afferent fibers, the cells not yet being differentiated into neurones. Some fibers found among the cells are efferent (see p. 495). The glossopharyngeus cells lie in the region of the third branchial arch, the vagus in the region of the fourth. _ During the fourth and fifth weeks the processes of the cells begin to develop (Fig. 434), and the cell masses finally become definite ganglia with afferent root fibers passing into the neural tube and peripheral processes passing outward, forming, with the associated efferent fibers, the peripheral branches of the nerves in question (Fig. 435). The root and trunk ganglia of the vagus and glosso- pharyngeus, respectively, are also now connected by fiber bundles instead of cellular strands. At the same time there is a diminution of cells in the caudal part of the vago-accessorius group, this part finally being composed almost ex- clusively of efferent fibers emerging from the lateral surface of the medulla and cord. A few groups of cells (accessory root ganglia) persist, however, and develop THE NERVOUS SYSTEM. 503 into ganglion cells, some being found there at birth (Streeter). This would in- dicate the presence of a small and hitherto undetected afferent element in the spinal accessory nerve, which is usually regarded as purely efferent. The spinal accessory nerves are thus identical with the vagus in their early development and consist at first of a homologous series of efferent roots and ganglia. This Ms 4 Ix-x-x1. gang. crest, Opthal div. ( Sup max. div: N.masticatorius. \ 1 G —Z Ix. Inf max.div: Gang. petros. Nilarv9. sup. Gang.nodos. Fic. 434.—From a reconstruction of the peripheral nerves in a human embryo of 4 weeks (6.9 mm.), Streeter. ITI-XII, II to XIE cranial nerves; C.J, D.I., L.I., S.I., rst cervical, rst dorsal, rst lumbar, and ist sacral nerves, respectively; 1, 2, 3, branchial arches; O¢. v., auditory vesicle; [X-X-XI gang. crest, ganglionic or neural crest of EX, X and XI cranial nerves. Fiber masses are represented by fine lines, ganglion cell masses by dots. indicates that the spinal accessory might be regarded as a specialized part of the vagus extending caudally into the cord (Streeter) (see p. 471).* From this point on, the further development of the efferent fibers of the X and XI nerves and of the peripheral processes of their ganglia is the further * According to another view (Bremer), the spinal accessory nuclei and roots are to be regarded as representing a specialization of lateral nuclei of the ventral gray column of the cord whose root fibers pass in the dorsal branches of the spinal nerves to the dorsal trunk musculature (p. 494, comp. Fig. 404). According to this view, the-muscles innervated by the XI would be somatic. The possible homology of the lateral efferent nuclei and roots of the medulla with those dorsal root fibers of the cord which arise from cells in the ventral gray column (p. 494 and Fig. 430) may be mentioned in this connection. 504 TEXT-BOOK OF EMBRYOLOGY. growth of the various branches of these nerves and their connection with the differentiating structures innervated by them. At the same time there is an in- creasing concentration of the cells, thereby forming more definite ganglionic Vesicula auditiva ; ' Ga g. radicis n. 1X Gang. acusticum e e ‘ f f Gang. semilunare n V ‘Gang. petrosum Cerebellum : N.VI. 1 1 i 1 i Gang radicisn,.X : =e N.I0” N.IV N, frontalis--~ Gang. rodos. N. dese. cerv, ‘Ram hyoid. (Ansa hypoglossi) N uasociliaris“"~ musculocutan, axillaris . phrenicus . medianus v. radialis .. ulnaris N. maxillaris ~ N. mandibularis Gang. geniculatum N. chorda tympani Diaphragma’ zs 4 - Hepar.---- f ICo.. N. tibialis-~ N. peroneus” Tubus digest, 7” R. posterio + R, terminalis lateralis ‘ R. terminalis anterior N. femoralis | N, obturatorius Mesonephros i 1 1 i 1 1 1 1 1 i) { Nn, ilioing, et hypogastr. Fic, 435.—Lateral view of a reconstruction of a to mm. human embryo, showing the origin and distribution of the peripheral nerves. The ganglionic masses are represented by darker and the fiber bundles by lighter shading. For purposes of orientation the diaphragm and some of the viscera are shown. The arm and leg are represented by transparent masses into the substance of which the nerve branches may be followed. Streeter. THE NERVOUS SYSTEM. 505 masses. “The changes taking place are similar to those exhibited in the differentiation of the spinal nerves (p. 497). The central relations of the nerves of this region of the medulla are shown in Fig. 436. (Comp. Fig. 407). The glossopharyngeus at the same time develops its branches, most of the peripheral fibers running in the third arch (lingual branch). Somewhat later (12 to 14 mm. embryo) another bundle (tympanic branch) (Fig. 435) passes for- ward to the second arch. This forms the typical branchiomeric arrangement in which there is a forking of the nerve into prebranchial and postbranchial branches, the latter being larger and containing the efferent element (see p. 471 and Fig. 40s). a Tractus solitarius -~ -[f ee a (in marginal layer) Al Sulcus limitans ss: of basal i ~ Inner layer plate - Mantle layer py ~~ Ventro-lat. column (in marginal layer) : if --- Floor plate Fic. 436.—Transverse section through the rhombic brain of a 10.2 mm. human embryo (during the fifth week). X, Vagus; XII, Hypoglossus. His. While the ganglia of the facialis and acusticus are derived from the same mass of cells (p. 502, Fig. 434) and are later still in very close apposition, it must be remembered that they are totally different in character. At four weeks they are differentiated from each other (Fig. 437). The relations of the two ganglia are shown in Figs. 435 and 437. It is probable that the ganglion of the facial (geniculate ganglion) shows an early differentiation into dorsal and ventral parts similar to the ganglia of the IX, and X, and also has associated placodes. The peripheral branches of the cells of the geniculate ganglion develop into the great superficial petrosal and chorda tympani. Both of these nerves enter into secondary relations with the V. There is some doubt as to whether the chorda is a prebranchial or postbranchial nerve (Fig. 435; also compare p. 469 and Figs. 405 and 406). 506 TENT-BOOK OF EMBRYOLOGY. The VII, IX and X are, as already mentioned, branchial (splanchnic) nerves and the central processes of their ganglia all have a common destina- tion; they grow into the lateral surface of the medulla oblongata, enter the marginal layer of the alar plate, and there bend caudally, forming a common descending bundle of fibers in the marginal layer, the éractus solitarius . (Figs. 436 and 470; see also pp. 469, 472). The acoustic ganglionic mass is elongated at an early stage, and is in con- nection with an ectodermal thickening (placode) which gives rise to the acoustic ( Ai BAUS tbh eee NN Ue a) ve ait ae LB “pate eis TE ya lt \ Wes VLG-c, 2 ee Fic. 437.—Transverse section through the acoustic region of the rhombic brain of a 10.2 mm. human embryo, VJ, Abducens and its nucleus; VII G. g., geniculate ganglion; VIII G.c., cochlear ganglion of acoustic nerve; VIJIG.v., vestibular ganglion of VIII nerve. His. Floor plate receptors (p. 598). From the upper part of the mass a bundle of peripheral processes forms a branch which subsequently innervates the ampulle of the superior and lateral semicircular canals and the utricle, while from the lower part a branch develops to the ampulla of the posterior canal and to the saccule. The nerve and ganglion (ganglion of Scarpa) is thus at first vestibular and at this stage the cochlear part of the ear vesicle is not indicated as a separate out- growth. As the lower border of the vesicle grows out into the cochlea, the lower border of the ganglion becomes thickened and develops into the cochlear ganglion (the ganglion spirale). It will be recalled that the vestibular part of THE NERVOUS SYSTEM. 507 the ear is the older part phylogenetically, the cochlea being a more recent special- ized diverticulum of the older structure. (See p. 599 and Figs. 512 and 513.) The central processes of the acoustic ganglionic mass first develop from the upper part, forming the vestibular nerve root which enters the marginal layer of the medulla. A portion at least of its fibers bends caudally, forming a de- scending tract. The central processes of the cells of the cochlear ganglion, forming the cochlear nerve root, pass dorsally, cross the vestibular ganglion and enter the medulla dorsal and lateral to the vestibular root fibers (Fig. 437). Roof plate Fic. 438.—Transverse section through the rhombic brain in the region of the trigeminus (V) nerve of a 10.2mm. human embryo. u.W., Spinal V; G.G., Gasserian ganglion; V.m., efferent root of V nerve. His. The trigeminus is the most anterior of the ganglionic masses (Fig. 434). Embryological evidence has been brought to show that it consists of two or more nerves which subsequently fuse. Placodes have also been described. It is possible that such placodes represent those belonging to the most anterior division of the lateral line system in lower forms, and probably in this case would not properly belong to the V (comp. Fig. 405). From the ganglionic mass (Gasserian or semilunar ganglion) the three principal branches—oph- thalmic, maxillary and mandibular—are formed, the two latter passing into the 508 TEXT-BOOK OF EMBRYOLOGY. Roof plate Floor plate Fic. 439.—Transverse section through the trigeminal region of the rhombic brain of a ro.2 mm, human embryo. a. IV., Spinal V; V.s., Gasserian ganglion; V.m., part of efferent root of Vonerve. His. Fic. 440. Part of a transverse section through the rhombic brain of a chick embryo toward the fourth day, showing the trigeminal roots. Cajal. A, part of the efferent (masticator) nucleus of the V; B, efferent root of the V; C, bipolar cells of the Gasserian ganglion; D, beginning of descending tract (spinal V) formed by the central processes of C, THE NERVOUS SYSTEM. 509 maxillary process and mandibular arch, respectively (Fig. 435). The central processes, forming the afferent root (portio major) of the V, enter the marginal layer of the alar plate of the rhombencephalon and form a descending bundle, the spinal V (Figs. 438, 439, 440 and 470). The trigeminus exhibits its spinal-like character in the behavior of its visceral portion (comp. p. 498). Cells of the ganglionic mass migrate further peripherally and form sympathetic ganglia (ciliary, otic, sphenopalatine (?) submaxillary(?) ). As in the cord, the question has arisen whether efferent roots may not also contribute a portion. Cells have been described as migrat- ing with the oculomotor root fibers and forming part of the ciliary ganglion (Carpenter). Besides those already described (cerebrospinal, sympathetic), the only other peripheral neurones of the nervous system are connected with the PRos- ENCEPHALON and are a part of the eye and mose. The visual receptors (rods and cones) and peripheral afferent neurones (bipolar cells) appear to be repre- sented by portions of the retina and are described elsewhere (Chap. XVIII). In the nose there is first a placode (p. 459) from which neuroblasts develop. Some of these migrate toward the neural tube and probably differentiate into lemmocytes, a few becoming ganglion cells.* The majority of the neuroblasts remain in the olfactory epithelium, sending their axones (jiJa olfactoria) into the olfactory bulb, the peripheral afferent olfactory neurones thus apparently displaying the primitive ectodermal location of afferent peripheral neurones (p. 455 and Fig. 397). (Comp. p. 591.) Development of the Lower (Intersegmental) Intermediate Neurones. It has already been seen how, by migration and by differentiation of the cells during migration, the nucleated layer comprising the greater part of the thick- ness of the wall of the neural tube is differentiated into two layers—an inner nucleated layer retaining its earlier characteristics, and an outer nucleated (mantle) layer, composed largely of the differentiating neuroblasts and characterized in ordinary staining by more widely separated nuclei. It has also been seen that this differentiation takes place earlier and more rapidly at first in the ventral part of the lateral walls (basal plate), and that the first cells to migrate and differentiate are those whose axones grow out through the neural wall and pass out as the ventral root fibers. Not much later than the above differentiation of the efferent peripheral neurones, axones of other neuroblasts also grow toward the periphery of the tube but do not pass beyond its wall. Such neuroblasts become intermediate * The latter are probably transient, but possibly in some forms persist as the ganglion cells of the nervus terminalis of Pinkus. 510 TEXT-BOOK OF EMBRYOLOGY. neurones (p. 456). The migrating bodies of these neuroblasts are checked at the inner boundary of the marginal layer, but their growing axones enter the marginal layer and there, apparently on account of their inability to penetrate the external limiting membrane, turn cranially or caudally, or bifurcate, and form longitudinal ascending and descending fibers. These longitudinal fibers constitute a part of the future white columns (see p. s14), and their cells are therefore often called column cells. Many axones from such cells in all parts of the lateral walls (heteromeric or commissural column cells) pursue a ven- tral course through the mantle layer, arching around near the periphery and Fic. 441.—Part of a section through the lumbar spinal cord of a 76-hour chick embryo. Cajal. A, Ventral root; B, spinal ganglion; C, bifurcation of dorsal root fibers forming beginning of dorsal funiculus; @, b,c, neuroblasts showing various stages of differentiation into intermediate neurones, some, at least, (¢) becoming heteromeric column cells; d, efferent neurone. crossing the floor plate, ventral to the lumen, to become longitudinal ascending and descending fibers in the marginal zone of the opposite side. These early decussating axones form, in the cord, the beginning of the anterior commissure (Fig. 441). Other neuroblasts, the axones of which do not cross the median line, become tautomeric column cells. It is about this time that the afferent root fibers enter the marginal layer of the dorsal part (alar plate) of the lateral wall and form in the marginal layer the various bundles of longitudinal fibers above described (dorsal funiculus, tractus solitarius, descending vestibular, and spinal V) (Figs. 441, 442, 436, 437; THE NERVOUS SYSTEM. 511 439, 440 and 470). In the cord the ascending arms grow to a greater length than the descending. In the rhombic brain the reverse is usually the case. The longitudinal fibers of the afferent roots and of the intermediate neurones thus form an external layer occupying the marginal layer of the neural tube. This is the beginning of the differentiation into white and gray matter, i.e., into that part of the neural tube containing only the axones of the neurones and into that part containing the cell bodies and the beginnings and termina- tions of the axones. The terminations of axones are formed by a turning of the longitudinal fibers into the mantle layer or gray matter to form there terminal arborizations. Later, the longitudinal fibers develop branches (col- laterals) which also pass into the gray matter. The differentiation of the white matter is completed several months later by the myelination of the nerve fibers. The longitudinal axones of intermediate neurones which are formed at this period in the cord and epichordal brain are located ventrally near the median line. These medial tracts occupy the position of the future medial longitu- dinal fasciculi, the reticulo-spinal and ventral ground bundles, and may be regarded on both comparative anatomical and embryological grounds as a primitive system of long and short ascending and descending tracts mediating between cerebrospinal afferent and efferent peripheral neurones, and not having at this period connections with the higher centers. Other more lateral tracts of this character are formed somewhat later, the whole forming the beginning of the reticular formation + ventro-lateral ground bundle system (compare Figs. 442, 449, 452 and 454). While merging more or less imperceptibly into the following stages, it may in a general way be said that at this stage of development there is differentiated what might be termed the primary and probably the oldest codrdinating mech- anism of the nervous system, most clearly segmental in character and having general features common not only to all Vertebrates, but to many Invertebrates. It is characterized by afferent and efferent peripheral neurones arranged seg- mentally and connected longitudinally in the central nervous system by crossed and uncrossed intersegmental intermediate neurones. (Compare pp. 472 and 473). At the anterior end of this part of the nervous system (epichordal segmen- tal brain) there are also exhibited differentiations due to fundamental vertebrate differentiations in the peripheral receptive and effective apparatus. Some of these are: (1) The differentiation of the splanchnic (visceral) receptive and motor apparatus, giving rise in the nervous system to (a) a separate system of afferent root fibers (tractus solitarius) including the more specialized gustatory apparatus; (b)a distinct series of lateral efferent nuclei. (2) The concentra- tion of the non-specialized somatic afferent innervation into one nerve (tri- 33 512 TEXT-BOOK OF EMBRYOLOGY. geminus and its central continuation, the spinal V). (3) The specialized somatic sense organ, the ear, with its older vestibular and newer cochlear divisions with central continuations of its nerves, including a vestibular descending tract. These differentiations of the peripheral afferent apparatus lead to the later formation of special terminal nuclei for their central continuations and second- ary tracts from these nuclei to suprasegmental structures (p. 473, Fig. 409). The peripheral and intermediate neurones of the more highly modified cranial end of the tube, or FORE-BRAIN, appear to lag behind in development, but in its basal part the neuroblasts are beginning to be differentiated (fifth week). In the development of the eye, the brain wall is evaginated, carrying with it the future retina comprising, apparently, the sensory epithelial cells or receptors (rods and cones), the afferent peripheral neurones (bipolar cells of retina) and the receptive or primary intermediate neurones (ganglion cells of retina and optic nerve). The histogenesis of these elements is dealt with elsewhere, but it may be pointed out here that the axones of the ganglion cells of the retina grow toward the inner side of the optic cup (away from the original luminal surface), pass thence in the marginal layer of the optic stalk, undergo a partial ventral decussation (optic chiasma) in the floor plate, and terminate in certain thalamic nuclei (lateral geniculate bodies) and in the roof of the mid-brain. ‘The so-called optic nerve is thus obviously a central, secondary tract. The development of this tract does not apparently take place until a later period than the differentiation of the earlier secondary tracts of the cord and rhombic brain (after the sixth week). In the case of the olfactory organ, it has already been seen that the peripheral neurones develop at first apart from the neural tube and send their axones into the olfactory bulb. The latter is an evagination of the neural tube which receives the olfactory fibers, thereby constituting a complicated terminal nucleus for the latter. The axones of bulb cells (the mitral cells) which pass along the stalk of the bulb are thus the secondary tract of this system. Many of them decussate in the anterior commissure. Secondary (and tertiary) olfactory tracts find their way to caudal parts of the rhinencephalon and to hypothalamus, thalamus and epithalamus, forming, with other tracts, a highly modified prechordal intersegmental mechanism (p. 544). Other olfactory tracts proceed to the suprasegmental archipallium which develops efferent bundles to the segmental brain. The embryological development of the peripheral apparatus, especially of its receptive portions, as shown by the various separate ganglionic rudiments (Fig. 434) and placodes, exhibits a segmental character which, though not in all respects primitive, is of practical value. These segments are (Adolf Meyer): (1) The olfactory apparatus, nose, without efferent elements. (2) THE NERVOUS SYSTEM.” 513 The visual apparatus, eye, with the eye-moving III and IV mid-brain nerves as its efferent portion. (3) The general sensory apparatus of the surfaces of the head and mouth, the afferent trigeminus, with the jaw-moving efferent trigeminus. (4) The auditory (and vestibular) apparatus, the ear (VIII nerve), with the VI (turning the eye to the source of sound) and VII (ear and face muscles) efferent nerves. In the latter, the original ear-moving apparatus has been replaced largely, in man, by the muscles of expression. (5) The visceral segment (IX, X, and XII nerves), not indicated externally in forms with- out gills. The afferent portion is concerned with taste and visceral stimuli, the efferent with tasting, swallowing, sound-production and other visceral functions. Overlapping with other segments is due to i¢s visceral as opposed theiy somatic chracter. ‘The apparent dislocation shown by the abducens is due to its common use by more than one segment. Caudal to this is the mechanism for head movement (N. XJ), its afferent por- tion being the upper spinal nerves. Following this, there is the segmental series of spinal nerves which in places shows a tendency to fuse (plexuses) into larger segments (phrenic segment, limb segments). All such modifications are expressions of more recent functional adjustments modifying preéxisting ones. These segments may be regarded as a series of reflex arcs, each one of which may have a certain amount of physiological independence but which are associated by intersegmental neurones. The latter class of intermediate neurones probably effects certain groupings of various efferent neurones, fur- nishing mechanisms which secure harmonious responses of groups of effectors involved in certain definite reactions (e.g., limb-movements, associated eye movements). These effector-associating mechanisms may be acted on di- rectly (reflex) by afferent neurones or by the efferent arms of suprasegmental mechanisms. Superadded to this segmental apparatus are the suprasegmental mechanisms which develop later, the pallium being the last to be completed. ‘These re- ceive bundles from the segmental nervous system and send descending bundles to the intersegmental neurones (pp. 464, 472 and 473 and Fig. 409). FURTHER DIFFERENTIATION OF THE NEURAL TUBE. The Spinal Cord. From this time on, differences of structure between cord and epichordal segmental brain become more marked and make it more convenient to treat their later development separately. The ventral half of the cord for a con- siderable period maintains its lead in development. At four weeks (Fig. 442) this lead is not.so pronounced as in the immediately following period. At this stage it will be noticed that the lumen is narrower in the ventral part, 514 TEXT-BOOK OF EMBRYOLOGY. as if due to the greater thickening of the ventral walls (basal plates). The increase of the mantle layer (gray) of the basal plate marks the beginning of the ventral (anterior) gray column or horn. ‘The increase in the basal plate may be partly due to neuroblasts migrating from the alar plate. These would be intermediate neurones. The development of the mantle layer at the expense of the inner layer, due to differentiation and migration of the cells of the latter, is well shown, but is more marked in the following stages. As already mentioned, the axones of the heteromeric cells, many of which lie in the dorsal half of the lateral walls, after decussating (anterior commis- Beginning of dorsal funiculus Dorsal root? ad ca Mantle layer** : Meningeal membrane Ventral root (from neuroblasts*= of mantle layer) Fic. 442—Half of a transverse section of the spinal cord of « 4 weeks, (6.9 mm.) human embryo. Dp, Roof plate; Bp, floor plate. His, sure), form longitudinal fibers in the marginal layer along the ventral surface of the opposite side, mostly mesial to the emerging ventral roots (Fig. 442). These longitudinal fibers are the beginning of the ventral (anterior) white columns or funiculi of the cord. The sides of the tube between the dorsal and ventral roots contain at first only a few longitudinal fibers—the beginning of the ventro- lateral funiculi. Their number soon rapidly increases, the fibers apparently coming from ventrally located tautomeric cells. The dorsal root fibers, as stated before (p. 497), form small round bundles in the marginal layer of the dorsal halves (Fig. 442). This is the beginning of the dorsal (posterior) white columns or funiculi, THE NERVOUS SYSTEM. 515 At four weeks there are blood vessels in the mesodermal tissue surrounding the neural tube. Branches of these soon penetrate the tube itself. From its first appearance in the cord as an oval bundle, during the fourth week, the dorsal funiculus steadily increases in size, forming a ‘‘root zone” in the marginal layer of the dorsal half, but not reaching the roof plate (Fig. 443). This increase in size is probably produced in part by the addition on its inner side of overlapping ascending arms of dorsal root fibers from lower Partly differentiated mantle layer Mantle layer Dorsal funiculus NY (post. white column) *~_ Dorsal root ~._, Marginal furrov Dorsal spinal artery/ Arcuate fibersz--f- Lateral gray, column (lat. horn)'| Meningeal— membrane o* FE z os x % 0% 0 © Oo Ventral root A: Fic. 443.—Half of a transverse section of the spinal cord of a 44 weeks (10.9 mm.) humanembryo. His. A.s., Artery in ventral longitudinal sulcus; A. sp. ¢@., ventral (anterior) spinal artery; Bp, floor plate; Dp, roof plate; I./., inner layer. The faint inner outline is the outline of the cord proper. cord segments. The mantle layer of this part contains an increasing number of cells forming curved or arcuate fibers. (Fig. 443-) The increase in the mantle cells of the dorsal part marks the beginning of the dorsal (posterior) gray column or horn (terminal nucleus of the dorsal root fibers). Later, other cells become differentiated from the inner layer which do not apparently form arcuate fibers (Fig. 443) and which subsequently become part of the posterior horn. It is possible that the axones of some of these cells form the compara- 516 TEXT-BOOK OF EMBRYOLOGY. tively small ground bundles of the dorsal funiculus. During this period of development of the dorsal portions of the lateral walls the latter have ap- proached each other, reducing the dorsal part of the lumen to a slit. The roof plate has undergone a slight infolding (Fig. 444). Ventral to the dorsal roots there is a groove running along each side of the cord (marginal furrow of His). At four and one-half weeks the number of fibers of the ventro-lateral funiculus has greatly increased and another groove has appeared parallel and ventral to the marginal furrow and forming the dorsal boundary of the ventro- Dors. funiculus --------. Dors. gray column (post. horn) = ---- Dors. root 4. \ __... Marginal furrow Intermediate plate Central cana‘ =} aah -+ Ventro-lat. funiculus ~/---~- + Vent. gray column (ant. horn) SSrsS sss - Vent. root Vent. funiculus Hees s2 == -"--- (ant, white column) Vent. sp. artery Fic. 444.—Half of a transverse section of the spinal cord of a human embryo of 18.5 mm. (74 weeks). His. lateral funiculus (cylinder furrow of His) (Figs. 443 and 444). The portion of the lateral wall lying between these two grooves or furrows forms an intermediate plate which contains few fibers in its marginal layer at this period, and is thus backward in development. Grooves appear on the luminal wall, apparently corresponding approximately to the outer grooves. ' The further growth of the dorsal funiculi and the concomitant growth of the associated gray matter, i.e., of the cells of the adjoining mantle layer, proceed until we have the conditions shown in Figs. 444 and 445. At the same time there is a further approximation of the dorsal portions of the lateral THE NERVOUS SYSTEM 517 walls so that the widest part of the lumen is further ventral. At about eight weeks the portion of the wall near the median line, which has formed a ridge by the apposition of the two inner layers and the roof plate (Fig. 444 Y), and is uncovered as yet with fibers, differentiates a marginal layer (eight and one-half weeks, Fig. 445) into which fibers grow forming, on each side, in the upper part of the cord, the column of Goll or fasciculus gracilis (Fig. 446). Many of these fibers, at least, are the ascending arms of caudal dorsal root fibers, which are thus added mesially to the continuations of upper cord roots. It will Rudiment of funiculus gracilis Dorsal funiculus (cuneatus) --- Dors. root \--+ Marginal furrow -4-2 Cylinder furrow Intermediate plate -- - - Lat. gray column Central canal - ++ Ventro-lat. funiculus Vent. long. sulcus | i § Lig 2 2 ' ‘ Seyareslaeierearavanaiare ceararatrais - Vent. funiculus Vent. sp. art. Fic. 445.—Half of a transverse section of the spinal cord of a human embryo of 24 mm, (84 weeks). His. be noted that there is now a massive dorsal gray column and that the original oval bundle has extended around on the mesial side of this gray column. While these changes are taking place, the dorsal portions of the lateral walls have fused, probably beginning at the most dorsal part, thus forming the dorsal ‘septum. This may be accompanied by a certain amount of rolling in from the dorsal part indicated by the direction of the ependyma cells (Fig. 445). The growth of the ventral funiculi and gray columns results in the appearance and subsequent increasing depth of the ventral longitudinal fissure. The cord now resembles the adult cord in many features, having well-marked white* and *The term “white” column is used for convenience. The funiculi do not become “white” until their fibers become myelinated during the sixth month. 518 TEXT-BOOK OF EMBRYOLOGY. gray columns, but contains a disproportionately small amount of fibers. A further and later change consists in a rolling inward, as it were, of the dorsal gray column so that it becomes separated from the ventral gray column, and that portion of it formerly facing dorsally comes to face more mesially, the roots entering more dorsally. This change may be due partly to the development of the intermediate plate which has in the meantime taken place. In this plate axones of tautomeric cells have begun to form the limiting layer of the lateral funiculus. From the cells of the intermediate plate are formed the neck of the dorsal gray column, also the cells of Clarke’s column and the Funiculus gracilis Dors, funiculus (ctuneatus) Saati Steines Dors. gray column = Dors. root ‘Central canal Sink ANS | Vent. long. sulcus ----+ Vent. sp. artery Fic. 446.—Half of a transverse section of the spinal cord of a human fcetus of about 3 months. His. processus reticularis. In the course of these developments, the ventro-lateral ground bundles, formed primarily by heteromeric and tautomeric cord cells, receives various accessions. These are first the long descending inter- segmental tracts from epichordal brain nuclei in the formatio reticularis which as they proceed down the cord naturally overlap externally the ground bundles already formed there. They include the medial longitudinal fasciculi, tracts from Deiters’ nuclei and the rubro-spinal tracts which occupy the ventro- lateral funiculi external to the ground bundles. In the lateral funiculi there are also added the ascending tracts from cord nuclei to suprasegmental structures. THE NERVOUS SYSTEM. 519 These are the dorsal s pino-cerebellar tracts from Clarke’s columns, ventral spino- cerebellar tracts, and tracts to mid-brain roof and thalamus (spino-tectal and thalamic). Finally (fifth month) the descending tracts from the pallium are added, the direct and crossed cortico-spinal (pallio-spinal or pyramidal) tracts, the latter being thrust, as it were, into the lateral funiculus. The development of the cord, then, is produced by (r) the proliferation of the epithelial cells and the formation of the nuclear and marginal layers; (2) the multiplication, differentiation and growth of the neuroblasts (mantle layer) ; (3) the development of the ventral roots; (4) formation of the funiculi (white columns when myelinated) by the growth into the marginal layer of (a) dorsal root fibers of the cord, the ascending arms of which overlap those root fibres entering higher cord segments, (b) cord neuroblasts forming intersegmental (ground bundle) tracts next to the gray matter, (c) descending intersegmental tracts from the segmental brain, representing continuations principally of cere- bellar efferent tracts, (d) afferent suprasegmental tracts from cord nuclei, (e) descending pallio-spinal tracts. In addition to this, there are general factors of growth, such as increasing vascularization, increasing amount of neurone cytoplasm (especially dendrites), increased size of axones and, finally, the acquisition by the latter of myelin sheaths. The vertebral column grows faster in length than the inclosed spinal cord. The result of this is that the caudal spinal nerves making their exit through the intervertebral foramina are, so to speak, dragged caudalward and instead of proceeding outward at right angle to the cord, pass caudally to reach their foramina. The leash of nerve roots thus formed, lying within the caudal part of the vertebral column, constitutes the cauda equina. The coverings of the cord retain their original connections at the caudal end of the vertebral canal and form a prolongation of the cord membranes enclosing the thin, terminal part of the cord, the filum terminale. The Epichordal Segmental Brain. In the fifth week, the walls of the rhombencephalon are comparatively thin. In the caudal region of the medulla oblongata (p. 484), the dorsal part of each lateral wall is upright and is bent at a considerable angle with the ventral part (basal plate), the groove on the inner surface between the two being the sulcus limitans. The roof of this region is formed by the thin expanded roof plate (Figs. 436-439). Anterior to this, the roof plate is not expanded, the alar plates almost meeting in the mid-dorsal line. This thicker part of the roof is the rudiment of the cerebellum. Its caudal edges are attached to the expanded roof plate (see P- 532). 520 TEXT-BOOK OF EMBRYOLOGY. In front of the cerebellum the tube is narrower and is compressed laterally. This part is the isthmus (Fig. 447). Anterior to this, the roof plate and alar plates expand into the mid-brain roof, the basal and floor plates forming the basal part of the mid-brain. Certain gross changes which from now on take place in the medulla may conveniently be noted here. At about this time (fifth week) the outer borders of the alar plate become folded outward and then downward, being thus turned back on the plate itself (Figs. 452 and 416). This fold is called the primary rhombic lip, and is most marked along the caudal border of the cerebellum. The folds of the lip then fuse, forming a rounded eminence composing the border of the alar plate to which the roof plate is attached laterally. Subsequently, the attachment to the roof plate is shifted dorsally in the medulla, caudally in Fic. 447.—Transverse section through the isthmus of a 10.2 mm, humanembryo, D.JV’, Decussa- tion of trochlear nerve; M/./., marginal layer; Ww. JV, nucleus trochlear nerve. His. the cerebellum. The portion of this lip which thins off into the roof plate is the tenia of the medulla and the posterior velum and tenia of the cerebellum. The thin roof plate itself becomes tbe epithelial part of the tela chorioidea of the fourth ventricle. At the caudal apex of the fourth ventricle a fusion of the lips of the opposite sides forms the obex. A further complication is due to the increasing pontine flexure by which the dorsal walls of the tube are brought close together (Fig. 448). The transverse fold of the tela thus produced is the chorioid fold. At about the same time lateral pocketings outward of the dorsal walls occur just caudal to the cere- bellum which contain portions of the chorioid fold. These are the Jateral recesses. By further growth and vascularization, the mesodermal part of the chorioid fold forms the chorioid plexus of the fourth ventricle (metaplexus). Finally, in the human brain an aperture appears in the caudal portion of the roof of the ventricle—the foramen of Magendie (metapore); and, according to many authorities, one also occurs in the roof of each of the lateral recesses THE NERVOUS SYSTEM. 521 —the foramina of Luschka. The roof of the fourth ventricle, where present, is thus composed of an inner ependymal epithelium—the expanded roof plate of the neural tube—and an outer mesodermal covering containing blood vessels. Other gross changes chiefly involve the basal plate. At the beginning of the fifth week this does not much exceed the alar plate in thickness and is separated from the opposite basal plate by an inner median sulcus (Fig. 452). The basal plate now increases in thickness and thereby both deepens the sulcus and con- tributes to a flattening out of the lateral walls, so that all portions by the sixth week lie approximately in the same horizontal plane (Fig. 454). Later, the floor plate increases in thickness more rapidly and the sulcus becomes shallower (eight weeks) (Fig. 455). The band of vertical ependyma fibers passing through Mesencephaion Epiphysis === eee - Ps Diencephalon : Wie, -..--..------ Isthmus Cerebellum + Transverse fold »\ --Rhombic lip Olfactory lobe - -—--> ey Optic stalk ~- ___= Soe 7 . . Infundibulum Hypophysis Basilar artery Fic. 448.—Lateral view of a model of the brain of a 74 weeks’ (18.5 mm.) human embryo. His. it is the septum medulle. It is bounded on each side by a vertical extension of the marginal layer which for convenience will be referred to as the septal marginal layer (Figs. 453, 454 and 455). The histological condition of this part of the tube at the beginning of five weeks has already been described. The lateral walls consist of an inner layer of closely packed cells, of a mantle layer consisting of efferent neurones and a simple system of intermediate neurones, and an outer marginal layer containing the longitudinal bundles of incoming afferent roots and longitudinal axones of intermediate neurones (see p. 511). It has been seen that this conditign has been brought about by the proliferation of cells near the tube cavity, which . migrate outward, at the same time many of them differentiating into neuro- blasts and nerve cells and thereby forming the mantle layer. As in the cord, the basal plate takes the lead and thus at first outstrips the alar plate, as showa 522 TEXT-BOOkK OF EMBRYOLOGY. in its greater thickness above mentioned. This process likewise terminates sooner in the basal plate, few cell divisions being present there at seven weeks. At about the end of the fifth week (see p. 526) the alar plate begins to develop very rapidly. Its period of proliferation is about terminated at the end of the second month. When the cell proliferation near the ventricle has ceased, the inner layer is reduced by outward migration to a single layer of ependyma cells (compare pp. 492 and 493). ; While the efferent nuclei continue to develop and the central continuations of the afferent neurones continue to grow in length, the principal differentiations now taking place in the rhombic brain are those affecting the intermediate neurone systems. The first of these to be considered is the further differentiation of thesystem of intersegmental neurones (p. 472). The earlier development of this system has been seen to involve especially the basal plate and the further development of the latter leads to the complete differentiation of the formatio reticularis which especially represents this system in the epichordal brain. It has already been seen (p. 511) that many of the intermediate neurones representing the beginning of this system seem to be at first heteromeric and form an internal arcuate system of fibers similar to those seen in the cord (pp. 510, 514). They increase in number toward the median line and are especially numerous in the basal plate, where they, together with the medial efferent neurones (XII and VI cranial nerves), form an eminence of the mantle layer corresponding to the ventral gray column of the cord (Fig. 449). Many of the axones of these cells of the arcuate system cross the septum medulle, thus marking the beginning of the raphé, and form on each side a longitudinal bundle in the septal marginal layer (Fig. 449). These longitudinal bundles correspond to the first formation of the ventral funiculi of the cord. They must not, of course, be confused with the pyramids which appear much later. Whether these longitudinal bundles are also partly formed of axones of tautomeric cells is uncertain. Later, as the anterior horn swellings grow and the depth of the septum medulle and of the septal marginal layers increases (compare p. 521), more longitudinal fibers appear in the latter, the new ones apparently being added ventrally. Others also appear more laterally in the marginal layer (Figs. 453, 454 and 455). (Compare cord, p. 514.) At this time, also, fibers enter the marginal layer bordering the surface (as distinguished from the septal), pass along parallel with the surface, cross the septum, and proceed to various parts of the marginal layer of the opposite side. These fibers are the first external arcuate fibers as opposed to the preceding internal arcuate fibers which traverse the mantle layer (gray) in the arcuate part of their course (Fig. 453). The majority of the longitudinal fibers entering the septal marginal layers during the second month occupy approximately the position of the future THE NERVOUS SYSTEM. 523 mesial formatio reticularis alba (white reticular formation) and correspond in position to the fibers of the medial longitudinal fasciculi and reticulo-spinal tracts in the adult medulla, representing probably the same system as the medial part of the ventro-lateral funiculi of the cord (medial longitudinal fasciculi, reticulo-spinal and ventro-medial ground bundles of the cord). The medial longitudinal fasciculi are in part descending fibers from higher levels described later. Tenia ————____ aes Marginal layer Mantle layer Alar plate Inner layer Tractus solitarius Sulcts limitans N.X. (Medullary XI) »<——_—__——~ Basal plate Internal arcuate fibers (in beginning gray reticular formation) N. XII Ventral funiculus Floor plate (beginning of form, retic. alba) Fic. 449.—Half of a transverse section of the medulla of a 10.2 mm. human embryo. Ais. In the basal plate, between the medial and lateral efferent nuclei, there are, even at the beginning of the fifth week, not only the efferent neurones and the heteromeric (commissural) neurones already mentioned, but other neuroblasts whose axones have a radial direction, i.e., toward the periphery. (Figs. 449 and 452.) The interlacing of these with the arcuate fibers forms the first indication of the formatio reticularis grisea (gray reticular formation). Later, longitudinal fibers are present here, giving rise to a condition more fully corresponding to that in the adult, analogous also to the condition in the lateral funiculi of the cord, especially in the processus reticularis. §24 TEXT-BOOK OF EMBRYOLOGY. In the region cf the auditory segment an important neurone group appears which is possibly a differentiation of the extreme dorso-lateral portion of the basal plate. This is Deiters’ nucleus, which apparently receives vestibular and cerebellar fibers and sends uncrossed descending bundles along the outer lateral part of the reticular formation and also ascending and descending crossed and uncrossed fibers along its outer mesial portion (part of the medial longi- tudinal fasciculus). This nucleus thus represents, apparently, like the nucleus ruber and nucleus of Darkschewitsch (below), a differentiated portion of the intersegmental neurones in especial connection with suprasegmental efferent fibers which thereby act on many brain and cord segments. The great development of the reticular formation here and caudally possibly causes a ventro-lateral displacement of the contained nucleus ambiguus and efferent facial nucleus and consequently the arched or hook-shaped course of Geny facialis forward 5 oe H Hl med sulcus med.suleus ! med.sulcus A B e Fic. 450.—Diagram illustrating the development of the genu of the facial nerve in the human embryo. The drawings show the right facial nerve and its nucleus of origin, in three stages: the youngest, A, being a 10 mm. embryo, and the oldest, C, a new-born child. The relative position of the abducens (I) nerve is represented in outline; its nerve trunk is not shown, as the structures represented are seen from above. Streeter. their root fibers as seen in transverse section (Streeter). At the same time, the nucleus of the VI, which originally was caudal to the VII, migrates cranially, carrying the facial efferent roots with it. ‘This gives rise to the genu facialis (Streeter, Fig. 450). In the mid-brain (Fig. 451), what appears to represent the basal plate forms an eminence, the tegmental swelling. Later there is differentiated from this the reticular formation of this region, containing various nuclei and traversed by radial, longitudinal and arcuate fibers, many of the latter arising from the later differentiating dorsal portions (corpora quadrigemina) of the lateral mid-brain walls. An important neurone group of the reticular forma- tion system which appears in this region is the nucleus of Darkschewitsch. Its descending axones form a part of the medial longitudinal fasciculus and probably appear at the end of the first month. The nucleus ruber is probably differentiated from the forward extremity of the tegmental swelling which over- Japs into a prechordal region (Fig. 463). Its axones (crossing as Forel’s decus- sation and forming the rubro-spinal tract) probably develop early. This THE NERVOUS SYSTEM. 525 neurone group apparently owes its great development principally to its close association with the cerebellum. These two long descending intersegmental tracts as they grow downward envelop the differentiating reticular formation of more caudal regions of brain (and cord) and thereby come to occupy an external position in the fully differentiated reticular formation. The reticular formation is thus composed of a gray portion containing the neurone bodies and shorter tracts and a white portion composed of the longer tracts. Axones from certain nuclei (especially N. ruber, N. of Darkschewitsch and N. of Deiters) form long, principally descending, tracts which envelop the gray reticular formation mesially (medial longitudinal fasciculus including fibers from nuclei of Darkschewitsch and Deiters as well as other reticulo- spinal fibers) and laterally (rubro-spinal, lateral uncrossed tract from Deiters’ cect Alar plate - Marginal layer --- Nucleus N, III Root fibers N. III Fic. 451.—Tzansverse section through the mid-brain of a 10.2 mm. human embryo. His, nucleus and other reticulo-spinal fibers) and constitute the white reticular formation. These long tracts descend to the cord and there similarly envelop its ventro-lateral ground bundles. While the above differentiation of the reticular formation has been taking place, changes in the alar plate have begun which lead to the formation of terminal nuclei of peripheral afferent nerves, as well as terminal nuclei of other tracts, all of which send fiber bundles to suprasegmental structures. The formation of the receptive nuclei of the afferent nerves of peripheral (segmental) structures is complicated by the fact that the central continuations of the peripheral afferent nerves are not confined to their own respective seg- ments but form longitudinal tracts which continue to grow upward (columns of Goll and Burdach) or downward (descending solitary, vestibular and trigeminal tracts) passing’ into other segments and overlapping externally structures already in process of formation there. In each segment, then, the terminal nuclei of the afferent nerves of that segment must be distinguished from the 526 TEXT-BOOK OF EMBRYOLOGY. terminal nuclei of afferent elements from other segments. ‘The latter are external or added to the former and are differentiated from additional prolifer- ations of neuroblasts of the alar plate. In addition to these nuclei, there are certain nuclei forming links between the two great suprasegmental structures, the pallium and cerebellum. These nuclei are the olive* and pons nuclei, both of which form afferent cerebellar bundles and which are differentiated by still further proliferations and migrations of alar plate neuroblasts. It has already been seen that the afferent peripheral nerves (IX and X) of the visceral segment form (together with descending fibers of the VII) the tractus solitarius. This is at first (5th week) short, but in six weeks has reached the cord. The éerminal nucleus of the tractus solitarius is differentiated from the neuroblasts of the medial portion of the alar plate. The course of the axones of this nucleus is not known. Judging from comparative anatomical grounds, they would not follow the fillet pathway (C. J. Herrick). The most caudal part of this nucleus is the nucleus commissuralis at the lower apex of the fourth ventricle. The formation of the other terminal nuclei lying in the region of this seg- ment is begun by the further developments of the alar plate already alluded to. These are initiated by an expansion and consequent folding of its border (formation of the rhombic lip, p. 520), followed by further cell-proliferation, leading to fusion of these folds and copious formation of neuroblasts in this region. These neuroblasts represent fresh accessions to the neuroblasts already formed in the mantle layer of the more medial part of the alar plate. This latest development of the border portions of the alar plate is the last step in the progressive development of the neural tube from the medial portion (basal plate) to the lateral (dorsal) border of the lateral walls of the tube where further development ceases at the attachment to the roof plate (tenia). (Fig. 452.) Many of the neuroblasts of the rhombic lip region migrate ventrally. Some of those from the medial part of the swelling produced by the fusion of the rhombic lip folds (p. 520) migrate along the inner side of the tractus -solj- tarius, while those from the lateral part of the swelling pass outside the tractus, which becomes thereby enclosed in the mantle layer (Fig. 453). Many of these neuroblasts continue their journey, passing along the outer side of the differ- *This is conjectural. The origin of fibers to the inferior olivary nuclei is not known. The most conspicuous tract to the olive is von Bechterew’s central tegmental tract. Purely @ priori con- siderations might be adduced in favor of this being considered a descending tract from thalamic nuclei which in turn receive pallio-thalamic fibers. It may, however, arise from lower optic centers. fit is, perhaps, an open ‘question whether the formation of the lip is a fundamental feature in this last proliferation and invasion of neuroblasts from the border of the alar plate. The promi- nence of the rhombic lip in man is the early embryological expression of the future great develop- ment of parts subsequently formed from this portion of the neural wall, especially the cerebellum and neurone groups in connection with it. THE NERVOUS SYSTEM. 527 entiating formatio reticularis, until they are arrested at the septal marginal layer (Figs. 454 and 455). From these neuroblasts which remain in situ near the dorsal border are de- veloped the nucleus gracilis and nucleus cuneatus, The axones of these nuclei form internal arcuate fibers which decussate and form a bundle of longitudinal fibers in the opposite septal marginal layer ventral to the reticularis alba. This tract is the medial fillet whose fibers appear during the second month and is one of the afferent paths to suprasegmental structures (mid-brain roof Inner rhombic furrow Tractus solitarius Inner layer N. X (medullary XI) Mantle layer Marginal layer Basal plate Beginning of gray reticular formation Floor plate F.r.a, N. XII Internal arcuate fibers (forming septum medullz) Fic. 452.—Hialf of a transverse section of the medulla of a 9.1 mm. human embryo (during the fifth week). His. The arrow is in the inner median sulcus. F.7.a., beginning of white reticular formation. and pallium). Other neuroblasts, which probably migrate further, form the substantia gelatinosa of Rolando, Axones of this group also form tracts repre- senting afferent paths to suprasegmental structures (pallium). Neuroblasts which migrate further form, as already mentioned, afferent cerebellar con- nections. Those migrating to the septal marginal layer form there an L-shaped mass mesial to the root fibers of the XII cranial nerve (Fig. 455). This is the medial accessory olive. Fresh groups of neuroblasts, added laterally to these in streaks, form the inferior olivary nucleus, while others which have not advanced so far form the lateral nucleus. Axones of the olivary neuroblasts 34 528 TEXT-BOOK OF EMBRYOLOGY, padi (olivo-cerebellar fibers) pass across the median line (seventh or eighth week) to the opposite dorsal border where they, together with axones from the lateral nuclei and the continuation from the cord of Flechsig’s tract, form (end of the second month) the bulk of the restiform body (Fig. 455). At three months the olives have acquired their characteristic folded appearance. Owing to the later development and ventral migration of the alar plate neuroblasts, there are thus formed the various nuclei which lie external to the reticular formation in the adult. The continuations of ascending spinal cord _ Outer part of rhombic lip migration Inner part of r. l. mig. Inner layer Tractus solitarius Marginal layer Mantle layer Ext. arcuate fibers Iat. arcuate fibers Septum Beginning white N. XII Gray reticular medullz reticular formation formatioa Fic. 453.—Half of a transverse section of the medulla of a 10.5 mm, human embryo (end of fifth week). His. tracts (Flechsig and Gowers) occupy the most external position on the lateral sur- face, and other cord continuations (medial fillets) the most external mesial positions. Later, however (fifth month), there is added ventral to the fillets the descending cortico-spinal fibers (pyramids). Their decussation takes place at the cervical flexure. By the external accessions from the alar plate above described, forming terminal nuclei of overlapping tracts from above (especially the nucleus of the spinal V), the tractus solitarius becomes buried, as it were, hence its deep position in the adult. The great development of the reticular formation may contribute to this result. As the trigeminus is the most cephalic rhombic THE NERVOUS SYSTEM. 529 segment, its descending fibers are not overlapped by fibers from above and therefore occupy the most external position of all these descending peripheral systems. Mantle layer Inner layer Gray ret. form Restiform furrow Rhombic lip migration | Ext. arc. fib. in marg. layer N. XII F.ra.v. Septum medullz Fic. 454.—Half of a transverse section through the medulla of a 13.6 mm. human embryo (beginning of sixth week). His. Fv. a., Beginning of white reticular formation in dorsal part of septal marginal layer. Another bundle has tormed more ventrally (F. 7. a. v.). Inner layer Roof plate Tractus solitarius Ee SS FA are eae ai oo Rhomic lip ye a Restiform E ‘ Y) body ee Spinal V Formatio reticularis — < grisea Neuroblasts from alar plate Marginal layer Sh Wee Sas. Neuroblasts from alar plate N. XII Septum (Rudiment of accessory olive) medulla Fic. 455.—Transverse section through the medulla of an 8 weeks’ human embryo. His. The terminal nuclei belonging to the auditory (acustico-facialis-abducens) segment are those of the vestibular and cochlear portions of the VIII nerve. 530 TEXT-BOOK OF EMBRYOLOGY. The development of these nuclei is not fully known, but they are derived from the alar plate, except possibly Deiters’ nucleus (see p. 524), the nuclei of the later formed cochlear nerve occupying the more external position. The ves- tibular nuclei apparently send axones both to cerebellum and reticular formation. The cerebellum itself may be regarded as primitively a receptive vestibular structure (p. 473) and probably receives vestibular root fibers. ‘The axones of the cochlear nuclei pass across the median line, along the ventral border, of the reticular formation (second half of second month), forming the trapezium. On the lateral boundary of the opposite reticular formation they ascend, form- ing the lateral fillet, to the suprasegmental posterior corpus quadrigeminum. Accessions are received from the superior olive, in which some of the trapezium fibers terminate. The alar plate of this segment also forms the substantia gelatinosa and the anterior portions of the olivary nuclei in this region. The various remaining tracts assume the same positions as further caudally. Later, the pyramids are added ventrally to the fillet, and the great develop- ment of the pons leads to its covering the ventral surface of part of this region. Owing to the late development of the pons and pyramids, the trapezium is thus uncovered and lies on the ventral surface of the rhombic brain during the third month. It is permanently uncovered in the dog and cat. In the érigeminus segment, the terminal nucleus of the afferent portion of this nerve is probably similarly formed from the alar plate. Its axones decus- sate, probably joining the fillet, and proceed to the thalamus, which is connected with the pallium. Descending axones from cells in the mid-brain roof form part of the trigeminus known as its descending or mesencephalic root. The view has been advanced (Meyer, Johnston) that these are afferent neurones equivalent to certain dorsal horn cells found in some adult and embryonic Vertebrates and representing spinal ganglion cells which have become included in the neural tube instead of becoming detached with the rest of the neural crest (compare p. 459). In front of the lateral recess another extensive development of the alar plate occurs, evidenced by the large rhombic lip of this region. The neuroblasts thus differentiated form the enormously developed pontile nuclei whose axones pass across the median line (fifth month) to the opposite cerebellar hemisphere, forming the middle cerebellar peduncle or brachium pontis. The pons extends over the ventral surface of the cephalic part of the medulla and over the ventral " surface of part of the mid-brain. It receives fibres from various parts of the neopallium, which form a great part of the pes pedunculi or crusta. A still greater development of the alar plate forms the cerebellum. In the mid-brain region, the reticular formation already described (p. 524) is enveloped ventrally and laterally by the upward extension of the medial and THE NERVOUS SYSTEM. 531 lateral fillets, the whole comprising the tegmentum. Ventral to this are added later the pons and the descending cortico-pontile, cortico-bulbar and cortico- spinal bundles forming here the pes pedunculi or crusta (probably during the fifth month). The alar plate of the mid-brain region forms the corpora quadrigemina (mid-brain roof). The further changes in the gross morphology of the medulla are due mainly to further growth of structures already present. The nuclei of the dorsal col- umns by their increase cause the swellings on the surface of the medulla known as the clava and cuneus, and likewise by their increase in size cause a secondary dorsal closing in of the caudal apex of the fourth ventricle which formerly extended to the cervical flexure. The tuberculum of Rolando is produced by the growth of the terminal nucleus of the spinal V, and the restiform body largely by the development of the afferent cerebellar fibers (Fig. 457). The growth of the olivary nuclei produces the swellings known as the olives. The above mentioned accession of the descending cerebrospinal tracts to the ventral surface is indicated by the pyramids. In the floor of the ventricle there is a longitudinal ridge each side of the median line occupied by swellings produced by the nucleus of the XII and, further forward, the nucleus of the VI, together with other nuclei (intercalatus, funiculus teres and incertus, Streeter) which are not well understood. The furrow forming the lateral boundary of this area is usually taken to be the representative of the sulcus limitans and consequently the area in question would be the basal plate. Lateral to it is a triangular area with depressed edges—the ala cinerea. It represents a region where portions of the vago- glossopharyngeal nuclei (dorsal efferent and terminal nuclei of fasciculus solitarius) lie near the surface. Possibly a secondary invasion by surrounding more recently differentiated nuclei may account for their apparent partial retreat from the surface. It is possible that the ala cinerea may be regarded not so much as a part of the alar plate, but that it—or rather the branchial nuclei involved in its formation—represents an independent intermediate region corresponding to the intermediate region in the cord (J. T. Wilson). The remaining portion of the alar plate, in the floor, is apparently represented principally by the acoustic, especially the vestibular, field. In the development of the segmental brain there are thus the following overlapping stages: (1) The differentiation of the inner, mantle and marginal layers. (2) The primary neural apparatus, consisting of (a) the peripheral segmental neurones, the central processes of the afferent neurones entering the alar or receptive plate, the efferent neurone bodies forming two main series of nuclei in the basal plate, and (b) intersegmental neurones composing the reticular formation in which the long tracts occupy external positions. (3) 532 TEXT-BOOK OF EMBRYOLOGY. The further differentiation, from the alar plate, of terminal nuclei for the afferent peripheral segmental neurones, the axones of the terminal nuclei forming afferent tracts to suprasegmental structures. These tracts and other later forming afferent suprasegmental tracts with their nuclei are laid down external to the reticular formation. (4) Formation of efferent (chiefly thala- mic(?) mid-brain and cerebellar) suprasegmental tracts which act upon the intersegmental neurones or reticular formation. (5) Accession at a late stage of development of a descending system of fibres from the neopallium. These lie ventral to the preceding structures. The Cerebellum. It has already been pointed out that at an early period (three weeks) the anterior boundaries of the thin expanded roof plate of the rhombic brain form two lines converging anteriorly to the median line where the roof plate is represented by the usual narrow portion connecting the two alar plates (Fig. 456). It has also been pointed out that the pontine flexure produces on the dorsal surface a deep transverse fold in this thin roof, into which vascular tissue grows later forming the chorioid plexus (Fig. 448). At this stage, the continuations of the alar plates of the medulla form two transverse bands which, when viewed laterally, are vertical to the general longitudinal axis of this part of the brain (Fig. 448). At the same time, the rhombic lips are formed along the caudal border of these bands and the latter become thickened into the two rudiments of the cerebellum, a considerable portion of which may be derived from the lips. These rudiments are thus two transverse and vertical swellings and are connected across the median : line by the roof plate. The attachment (tania) of the Fic. 456. ee, view : of that part of the alar plate of this region to the roof plate of the fourth brain caudal to the ventricle is at first along its caudal edge. Later, by the cephalic flexure (human embryo of 3d folding back and fusion of this border to form the rhom- week, 2.15mm.). Hh. el ans F * Cerebellum; J, isth. bic lips, the attachment is carried forward. Still later, mus; JZ, mid-brain; : +t 7 Ei. eae by the growth of the cerebellar rudiment, it is rolled Compare with Fig. backward and under, as described below. The rudi- A ta ments subsequently fuse across the median line, thus forming externally a single transverse structure, but internally a paired dorsal median projection of the lumen marks the location of the uniting roof plate (comp. Fig. 458). THE NERVOUS SYSTEM. 533 While the structure thus formed expands enormously in a lateral direction, in its subsequent development its greatest growth is in a longitudinal direction. The effect of this is that the continuations of the cerebellum forward (velum medullare anterius) and backward (velum medullare posterius) into the adjoining, brain walls of the isthmus and medulla are comparatively fixed points and are completely overlapped by the spreading cerebellum, producing an appearance in sagittal section as though they were rolled in under the latter structure (comp. Fig. 408, F). Another result of this longitudinal growth is the formation of fis- sures running across the organ, transversely to the longitudinal brain axis. First, lateral incisures separate two caudal lateral portions, the flocculi (Fig. 457), the median continuation of which, the module, is finally rolled in on the under side of the cerebellum as explained above. Another transverse fissure, the primary fissure, beginning in the median part and extending laterally, sepa- - Cerebellar hemisphere ~ Vermis Fic. 457.—Dorsal view of the cerebellum and medulla of a 5 months’ human foetus, Kollmann. rates an anterior lobe from a middle lobe, the former comprising the future lin- gula, centralis and culmen and their lateral extensions. The anterior portion is rolled forward under the anterior part of the cerebellum. Another trans- verse fissure next appears in the median part (secondary fissure) which later ex- tends (peritonsillar) to the floccular incisure, and thereby completes the de- marcation of a posterior lobe, including not only the flocculus and nodule, but also the tonsilla and uvula, which are also rolled backward and under. The result of this transverse fissuration would be the production of a cerebellum resembling that of certain forms below Mammals where the cerebellum is well developed (Selachians, Birds). A complicating factor, however, is the great growth of certain lateral portions of the middle lobe, forming the future cere- bellar hemispheres (Fig. 457), which causes also a lateral overlapping and rolling inward of adjoining parts. This growth is the chief factor in the division of the cerebellum into vermis and hemispheres and is correlated with the devel- opment of the neopallium (p. 473 and Fig. 409) : 534 TEXT-BOOK OF EMBRYOLOGY. The early histological development of the cerebellum has been most closely studied in Bony Fishes (Schaper) and there is every reason to suppose that the processes taking place in the human cerebellum are essentially thesame. In that part of the alar plate forming the rudiment above described, the cells pro- liferate, forming first a nuclear layer with the dividing cells along its ventricular surface, and a non-nucleated outer or marginal layer. Later, owing to begin- ning migration and differentiation, there is formed the usual mantle layer, representing a differentiation of part of the original nuclear layer and thereby forming the three layers: an inner, a mantle and a marginal. The outer cells of the mantle layer increase in size and differentiate into the cells of Purkinje, smaller cells within forming the granular layer. The earliest stage of differ- entiation of the Purkinje cells has not been accurately described, but the axones Fic. 458.—Diagram representing the differentiation and migration of the cerebellar cells in a teleost. The arrows indicate the migration of cells from the borders of the cerebellar rudiment into the marginal layer; these cells probably all differentiate into nerve cells. Clear circles, indif- ferent cells; circles with dots, neuroglia cells (except in marginal layer); shaded cells, epithelial cells; circles with crosses, epithelial cells in mitosis (germinal cells); black cells, neuroblasts; L lateral recess; M, median furrow, above which is roof plate; R, floor of 4th ventricle (IV). Schaper. of the neuroblasts evidently proceed (end of fifth month) toward the ventricular surface instead of entering the marginal layer. In this way the fibrous layer (white matter) comes to lie within instead of on the outer surface as in the cord, and, tosome extent, in the medulla, There is thus formed the outer gray matter or cortex. The axones of the Purkinje cells form the great bulk of the centrifu- gal fibers of the cerebellar cortex. The marginal layer becomes ultimately the outer or molecular (plexiform) layer of the adult cerebellum. It has been seen that in the other parts of the tube development begins in the medial parts of the lateral plates and thence advances toward their dorsal borders, which actively develop after the corresponding stages have ceased in the medial portions. The same is true of the cerebellar rudiment. In this, the edges which border on the thin roof plate, i.e., those parts adjoining the lateral recesses, the main roof of the fourth ventricle and the roof plate inter- posed between the two original lateral cerebellar rudiments, are the last to pro- THE NERVOUS SYSTEM. 535 liferate. The cells thus formed spread into the marginal layer of the earlier developed parts and by further proliferation form a nucleated layer of consider- able thickness (Fig. 458). This complication is apparently essentially similar to that described above in the development of the medulla. From the cells of this invasion are formed a part, at least, of the granule cells, as well as the basket cells and other cells which remain in the marginal (molecular) layer. These are all association cells of the cerebellum. The cerebellum reaches its full histological development very late; after birth in many Mammals. These last postnatal stages of development naturally. gk Fic. 459.—Scheme showing the various stages of position and form in the differentiation of granule cells from the outer granular layer. Cajul.- A, Layer of undifferentiated cells; B, layer of cells in horizontal bipolar stage; C, partly formed molecular (plexiform) layer; D, granular layer; b, beginning differentiation of granule cells; ¢, cells in monopolar stage; d, cells in bipolar stage; e,/, beginning of descending dendrite and of unipolarization of cell; g,4,7, different stages of unipolarization or formation of single process connecting with the original two processes; 7, cell showing differentiating and com- pleted dendrites; &, fully formed granule cell. involve principally those cells proliferated last and which lie in the mar- ginal layer. These have been studied by means of the Golgi method in new-born Mammals by Cajal and others. The majority of these cells form granule cells by means of a progressive migration and differentiation, as shown in the accompanying Fig. 459. Each cell first develops a single horizontal process, then another, thus becoming a horizontal bipolar cell. Following this, the cell body migrates past the Purkinje cells into the granular layer, remaining in connection with the original processes by a single process. ‘There are thus formed the axone of the granule cell with its bifurcation into two horizontal pro- cesses, the parallel fibers of the molecular layer. This mode of formation is thus 536 TEXT-BOOK OF EMBRYOLOGY. Dd similar to the unipolarization of the cerebrospinal ganglion cell. The dendrites begin to be formed during the migration, branch when the cell body reaches the granular layer and there finally attain the adult form. Other undifferentiated cells in the marginal layer send out horizontal processes the collaterals of which envelop the Purkinje cell bodies, and form the baskets. The place vacated, so to speak, by the migrating granules, is filled at the same time by the developing dendrites of the Purkinje cells. These at first show no regularity of branching, but subsequently differentiate into the definite branches of the adult condition, at the same time advancing toward the periphery (Fig. 460). When they z rv a Fic. 460.—Section through cerebellar cortex of a dog a few days after birth, showing the partial development of the dendrites of two cells of Purkinje. Cajal. A, external limiting membrane; B, external (embryonic) granule layer; C, partly formed molecular (plexiform) layer; D, granular layer; a, body of cell of Purkinje; b, its axone; c, and d, col- laterals with terminal arborizations (e). reach this, the migration of the granules is completed and the molecular layer is definitely formed. This condition, evidenced by the disappearance of the outer granular layer, is usually reached in Mammals within two months after birth, but in man not until the sixth or seventh year. There are observations indicating that animals possessing completely developed powers of locomotion and balancing at birth have more completely differentiated cerebella at that time. The axones of the Purkinje cells form many embryonic collaterals which are afterward reduced in number. Of the centripetal fibers to the cerebellum, those from the inferior olives cross the median line of the medulla about the seventh or eighth week, and thence advance to the vermis, reaching their final destination during the third THE NERVOUS SYSTEM. 537 month. The fibers from the pontile nuclei (middle peduncle) do not develop until considerably later (end of the fourth month), the time of their reaching their destination in the cerebellar hemispheres not being definitely known. Many at least of the centripetal fibers do not reach their full development in Mammals till birth or after. Some of these fibers (climbing fibers) form arbor- izations around the inferior (axone) surface of the Purkinje cell bodies and later creep upward, enveloping the upper surface instead, and finally the den- dritic branches. Other centripetal fibers (mossy fibers) ramifying in the granular layer are varicose fibers, at first otherwise smooth. From the vari- cosities a number of branches are given off which later become abbreviated and modified into the shorter processes of the adult condition. This final differ- entiation occurs simultaneously with the final differentiation of the dendrites of the granule cells with which they come into connection. The glia elements apparently develop in a manner essentially similar to their development else- where. The development of the internal nuclei of the cerebellum has not been thoroughly investigated. The nucleus dentatus is well developed at the end of the sixth foetal month. Eminences passing forward and ventrally along the sides of the isthmus are the earliest indications of the superior peduncles, formed later by the axones of the cells of these nuclei. Corpora Quadrigemina. The mid-brain roof is an expansion of the alar plate of the mid-brain. Later this differentiates into the anterior and posterior corpora quadrigemina. In the former, by the usual ventricular mitoses (germinal cells), a nuciear layer is formed with a non-nucleated marginal layer external to it which becomes the outer or zonal layer. Still later the neuroblast or mantle layer is differen- tiated, there being an unusually thick inner layer. The further development has not been closely studied in man. Owing to the diminished importance of the anterior corpora quadrigemina (p. 474) the neuroblasts do not differ- entiate into the well marked “spread out” layers characteristic of the optic lobes of many Vertebrates. This is probably due to a lack of development of their association neurones. The fibers of the optic tracts grow toward the anterior corpora quadrigemina in the marginal layer forming the anterior brachia. When they reach the anterior corpora quadrigemina, they leave the marginal layer and penetrate the gay matter forming the most external fiber layer. The medial (and some lateral) lemniscus fibers enter more deeply than the optic. Neuroblast axones grow toward the ventricle, turn internally to the lemniscus fibers, cross (Mey- nert’s decussation), and proceed as the predorsal tracts to the segmental brain and cord, lying ventral to the medial longitudinal fasciculi. 538 TEXT-BOOK OF EMBRYOLOGY. The Diencephalon. The stage of development of the diencephalon at four weeks has already been mentioned (p. 485). (Figs. 461, 471 and 472.) In the lateral walls the principal feature is the presence of a furrow, the sulcus hypothalamicus, which begins ventrally as an extension of the optic recess and extends dorsally and caudally toward the mid-brain. A branch of it extends to the posterior part of the foramen of Monro. This is the sacus Monrot. The sulcus hypothala- micus is sometimes regarded as the representative in this region of the sulcus imitans. It is doubtful whether it has the same morphological value as the latter. Such a comparison is seen @ priori to be difficult when it is considered that this region is in the most highly modified part of the brain tube, lacking Fic. 461.—Transverse section through the diencephalon of a 5 weeks’ human embryo. Dp., Roof plate; Ma., mammillary recess; P.s. hypothalamus; S.M., sulcus hypothalamicus; Th., thalamus. His. motor peripheral apparatus, and that it is also the end region of the tube where all longitudinal divisions would naturally merge. The sulcus deepens till the end of the second month (Fig. 467). Later it becomes shallower, but appears to persist till adult life. The region of the diencephalon ventral to the sulcus, as already mentioned, is termed the pars subthalamica or hypothalamus. The ventral part of the optic stalk forms a transverse groove in the floor, the pre- optic recess, caudal to which is a ridge or fold, the chiasma swelling, in which the fibers of the optic chiasma later appear.* Caudal to this is the recess or invagi- nation of the floor, representing the postoptic recess and the beginning of the infundibulum (Figs. 462 and 463). Its extremity later becomes extended into the infundibular process, the posterior part of which in the fifth week comes into contact with the hypophyseal (Rathke’s) pouch. This is a structure formed * According to Johnston, the chiasma is formed in front of the optic recess which would then be represented by the postoptic recess. In this case the chiasma would be regarded as falling in the region ° the telencephalon instead of forming the optic part of the hypothalamus (comp. Figs. 402 and 471). THE NERVOUS SYSTEM. 539 from the stomodeal epithelium and is connected with the latter by a stalk. The pouch, which is at first a flat structure, develops two horns which envelop Ant. corp. quad. Pineal Anterior (ant. colliculus) region brachium Pallium Ant. \ olfact. lobe Post. Optic stalk Hypophyseal pouch Mammillary Lateral Tuber region geniculate cinereum body Fic. 462.—Lateral view of a model of the brain of a 10.2 mm. human embryo (middle of 5th week), His. the infundibulum. The cavity of the end of the infundibular process becomes nearly shut off from the rest of the infundibular cavity. The process penetrates the upper part of the pouch and then bending reaches its posterior surface and Diencevhalon Thalamus Pineal region Paliium Mesencephalon Foramen of Monro Tegmental swelling Sulcus hypothal- amicus Ant. olfact. lobe 77 ¥ 2 Mammillary region Post. olfact. lobe Hypothalamus Lamina terminalis Tuber cinereum Corpus striatum , Recessus Hypophyseal Recessus | (pre?) opticus pouch infundibuli Fic. 463.—Median view of the right half of a model of the brain of a 10.2 mm. human embryo (middle of 5th week). Compare Fig. 462. His. ends blindly. In the second half of the second month epithelial sprouts, which become very vascular, begin to appear, first in the lateral parts of the pouch, 540 TEXT-BOOK OF EMBRYOLOGY. next the brain, and then extending through the pouch and finally nearly oblit- erating its cavity (third month). The shape of the organ (the hypophysis) formed by the union of these two parts is subsequently changed by its relations to surrounding parts. Its posterior lobe is derived from the infundibular por- tion, its anterior lobe from the pouch. An expansion of the floor of the brain caudal to the infundibulum has been mentioned as the mammillary region. Subsequently there is formed from its cephalic part another evagination, the tuber cinereum. ‘The mammillary region forms the mammillary bodies. The region caudal to the mammillary region later receives many blood vessels, thereby becoming the posterior perforated Space. At the end of the fourth week the roof plate of the diencephalon is smooth. At about this time the greater part of the roof expands, forming a median longitudinal ridge (Fig. 464). This ridge, which remains epithelial throughout life, is broader at its anterior end where it passes between the beginning pallial hemispheres. As the roof plate expands further, the anterior part is next thrown into longitudinal folds. The ridge forms the epithelial lining of the tela chorioidea of the third ventricle (diatela). By further growth and vas- cularization of its mesodermal covering at the beginning of the third month, there is formed the chorioid plexus of the third ventricle (diaplexus). Lateral extensions of the tela form the chorioid plexuses of the lateral ventricles (see p- 554). In the fifth week a protrusion appears at the caudal end of the median ridge which is the beginning of the epiphysis. Soon after this, the furrow which forms its caudal boundary extends forward along the upper part of the sides of the walls, marking off a fold which is the lateral continuation of the median protrusion. From the median protrusion is later formed the pineal body, while from the lateral folds are formed the pineal stalk, and in front the habenula, with its contained nucleus (ganglion) habenule, and the stria medullaris, Still further caudally, the anterior part of the mid-brain forms a horseshoe-shaped fold the arms of which extend forward over the dien- cephalon, ventral to the pineal folds. The median part of this fold forms the anterior corpora quadrigemina. From its lateral extensions are formed the anterior brachia of the anterior corpora quadrigemina, the pulvinar and the lateral and medial geniculate bodies, all of which (pulvinar ?) later receive optic fibers. The transverse furrow which forms the boundary between the rudi- ments of the pineal body and of the anterior corpora quadrigemina marks the location of the future posterior commissure (F igs. 464, 465 and 466). The part of the roof anterior to the pineal fold, as already stated, forms the tela chorioidea of the third ventricle. Certain folds appear in it, however, which are more clearly indicated in later stages of embryonic development than in the adult and which probably represent structures already mentioned THE NERVOUS SYSTEM. 541 as common to the vertebrate brain (“‘cushion”’ of the epiphysis, velum trans- versum, paraphysis?) (p. 461 and Fig. 4o2). From the above it is evident that at the close of the fifth week the rudiments of the various parts of the diencephalon are already well marked. These rudiments are principally indicated by foldings of the walls, there being no very strongly marked differences of thickness except the early differentiation between the median and lateral plates. From this time on, both general and local Lamina terminalis Cavity of ant. olfact. lobe Angulus prethalamicus Anterior arcuate fissure Cavity cf post. olfact. lobe (a) (b) Chorioid fold \ 4 (c) Pe: ay \ Corpus striatum Hippocampal fissure Roof plate of diencephalon f \ Lateral geniculate body E | * | t Pineal region Ant. corp. quad. (ant. colliculus) (extending forward into ant. brachium) Fic. 464.—Dorsal view of a model of the brain of a 13.6 mm. human embryo (beginning of 6th week). The’ dorsal part of the pallium on each side has been removed. Compare with Figs. 465 and 466. His. a thickenings of the lateral walls occur. This indicates a rapid proliferation of the cells, especially a differentiation of the nerve cells and consequent forma- tion of masses of gray and white matter. Another factor affecting the dien- cephalon is the subsequent etowth backward over it of the cerebral hemispheres. During the second month, the lateral walls become ubaien, forming @ prominence on the inner surface of each side. ‘This reduces much of the cavity of the third ventricle to a cleft and in the third or fourth month a fusion of 542 TEXT-BOOk OF EMBRYOLOGY. a portion of these two projections takes place, forming the commissura mollis or massa intermedia. ‘The condition at this stage is shown in Fig. 467. Later Ant. corp. quad. Diencephalon Pallium Tegmental ) —— swelling “A ; y Beginning of w - f f Sy. Mammillary { i j ossa DY.Vu dies X s . y Ant. \ olfact. Tuber ‘i pH Es lobe cinereum ~. “ & : : . Post. Wh : Optic stalk Infundibulum Hypophyseal pouch. Fic. 465.—Lateral view of the model of the brain of a 13.6 mm. human embryo (beginning of 6th week). , Beginning of frontal lobe; T, beginning of temporal lobe. His. this protrusion thrusts the lateral structures above described (the pulvinar, geniculate bodies and brachia) to the side, the cavity of the lateral geniculate Eptthatamus (Corpus pineale) | Mctathalamus (Corpora geniculata) Thalamus : Fissura ge * : _ Corpora quadrigemina chorioidea:. /_ Pallium .;., { " 4 2% >-Pedunculus cerebri JTelencephalo » Rhinencephalon” =~“ Corpus striatum Suleus hypothalamicus a Hypothalamus ~ Chiasma opticum Cerebelluun Pons [Varoli] Fossa rhomboidea & spina Fic, 466.—From a model of the brain of a 13.6 mm, human embryo, right half, seen from the left side. His, Spaltehols. body being obliterated. The prominence itself extends to the tegmental swell- ing (see Figs. 467-8) and there thus arises the possibility of direct connections THE NERVOUS SYSTEM. 543 between these two structures. ‘There can, then, be distinguished in the dien- cephalon three regions, a hypothalamic region, as already described, an epithala- Hippocampal fissure Chorioid fissure Angulusprethalamicus Foramen of Monro-> Hypothalamic region Mammillary region Ant. arcuate fissure Preterminal area Ant. olfact. lobe Olfactory nerve Post. olfact. lobe R.o. Hypophysis Lamina terminalis Fic. 467.—Median sagittal section of the brain of a 74 weeks’ human embryo. Ag. S., Aqueductus Sylvii; C.c., fold between mid- and interbrain; C.m., commissura mollis; C.s., corpus stri- atum; H.b., tegmental swelling; R.g., geniculate recess; R.i., recessus infundibuli; R.o., recessus (pre-?) opticus; S.., habenular evagination; S.M/., sulcus hypothalamicus; 5. p., pineal evagination; T.7., thalamus. His. mic region comprising the pineal body, ganglia habenulz and related structures, and finally the shalamus proper. In the latter, the geniculate bodies already Thalauus Palliun Epithalamus (Corpus pineale) Metathalamus (Corpora geniculaia) 2th : y 2 Drencephaiey): sae sone ( Ss gess eséncephala z & . Télencép Corpus striatum. | ies Rhinencephalon . Pars optica hypothalami , Chiasma opticum™” .” Hypophysis”” Pars mamillaris bypothalami* Pons (Varolil* Fic, 468.—Brain of a human foetus in the 3d month, right half, seen from the left. His, Spalteholz. mentioned constitute a metathalamic portion, while the portion derived from the thickened part, which is continuous anteriorly with the corpus striatum, 35 544 TEXT-BOOK OF EMBRYOLOGY, differentiates various nuclei, especially those which receive the general somatic sensory fibers (medial lemniscus or fillet), and other nuclei in relation to definite centers of the pallium. The thalamus is thus strongly developed, owing to its containing the nuclei which receive the general sensory (ventro-lateral nuclei), acoustic (medial geniculate bodies), and optic (lateral geniculate bodies) systems of fibers and which in turn send fibers (thalamic radiations) to the pallium. These thalamic nuclei do not receive fibers probably until after the middle of the second month. About this time the thalamic radiations begin to be formed from the thalamic nuclei and grow toward the corpus striatum which they reach toward the end of the second month. With the first appearance of the cortical Epithalamus (Corpus pincale) Corpora Ng -quadrigemina : Thalamus Pallium # Telencephalpn 43 Bhinencephaion cori ae As yf : Recessus opticus | ee Chiasma opticum ag Recessus infundibuli ” f Infundibulum Pedunculus cerebri ~_Velum medul- lare anterius Cerebellum Ventriculus quartus Pops [Naroli] Myelen- -- Medulla oblongata ephaion Fic. 469.—-Adult human brain, right half, seen from the left, partly schematic. Spalteholz. layer of the developing neopallium (see p. 549) they penetrate the corpus stria- tum and pass to the cortex, forming the beginning of the internal capsule, and corona radiata, It has already been pointed out (p. 474) that the great develop- ment of the thalamus and its radiations is more recent phylogenetically and is due to the newly acquired connections with the neopallium. Before the development of these neopallial connections, other tracts have begun to appear which represent older epithalamic and hypothalamic connec- tions existing practically throughout the Vertebrates (pp. 474 and 475). Some of the hypothalamic connections are the mammiullo-tegmental fasciculus which appears early in the second month, the thalamomammillary fasciculus (Vicq d’Azyr’s bundle), which appears later, and the bundles from the rhinen- cephalon (p. 512) and archipallium (columns of the fornix, middle of fourth month, p. 558). In the hypothalamic region is also differentiated the corpus THE NERVOUS SYSTEM. 545 Luysii, connected by fiber bundles with the corpus striatum and tegmentum. Epithalamic connections are represented by bundles from anterior olfactory regions (siria medullaris, seventh week), by the commissura habenularis, and by bundles to caudal regions (fasciculus retroflexus of Meynert to the interpedun- cular ganglion, middle of second month). (pp. 474 and 512.) The posterior commissure fibers are formed early in the second month in the fold between mid- and inter-brain (Fig. 467). (Fig. 470). - SS eee Teac. yi \ a / ™m. Sy } f Ma. ¢ \ \ ~ = | St. i \ ) FCS \ / \ i : // Sea J / / / P _— Zi ae | / | Sie / / key oes: ZF \ { L nN | \ | a 4K To f 0 vei y |W x } x A fom \ B. \ Vv. 3) A Sy re x | | WE oe. 7 / wi. : ) 4 ( Fic. 470.—Construction of the brain of a 19 mm. human embryo (74 weeks), showing the stage of development of some of the principal fiber-systems. Hs. C.c., posterior commissure; F.s., tractus solitarius; F.t., fasciculus spinalis trigemini (spinal V); K, nuclei of dorsa! funiculi of cord; L., medial longitudinal fasciculus; M., fasciculus retro- flexus of Meynert; Ma,., mammillary bundle; x. 7., nervus intermedius; O., olive; Ol., olfactory nerve; 5S., fillet; S¢., stria medullaris thalami; T., thalamic radiation; T.v., tractus opticus; V, Gasserian ganglion; VII, facial nerve and geniculate ganglion; VIIT, ganglia of acoustic nerve; IX, N. glossopharyngeus; X, N. vagus. The Telencephalon (Rhinencephalon, Corpora Striata and Pallium). To understand the development of this part of the brain it is necessary to keep firmly in mind certain relations which are laid down at a comparatively early stage. Some of these relations are shown in the diagram of the inner sur- face of a model of a brain of four weeks. At this stage the pallium is unpaired, i.e., there is no median furrow separating the two halves of the pallial expansion. The various boundaries of the pallium in one sideare (1) the median line uniting 546 TEXT-BOOK OF EMBRYOLOGY the two halves of.the pallial expansion (Fig. 471, bc); (2) the boundary line or line of union with the thalamus lying caudally (pallio-thalamic boundary) (Fig. 471, cd); (3) the boundary between pallium and corpus striatum (strio- pallial boundary) (Fig. 471, 0¢). T he boundaries of the future corpus striatum are (1) the median (Fig. 471, 20), (2) the strio-pallial (Fig. 471, bd), (3) the strio-thalamic or peduncular (Fig. 471, de) and (4) the strio-hypothalamic (Fig. 471, ae). The internal prominence which is the rudiment of the corpus striatum, has three limbs or crura, (1) a ridge proceeding forward (anterior crus), which corresponds externally to the furrow (external rhinal furrow) forming the lateral boundary of the anterior olfactory lobe, (2) a middle crus Thalanus ye te, oe Prosencephaton (Fore -brain) Palliuns . “4 /.. Peduneulus cerebri Brachium conjunctivum - and velum medullare apterius Rhinencephalon+- <7 fy" hy y Metencephalon : 7 (Midd brain) neg sie Corpus striatum Pars optica hypothalami “ Pars mamillaris hypothalami_ ,.; if. ” Zencephalon Pons [Varolil efor" (Lozenge - shaped en drain) Cerebelluin Pars ventratts --}- Mb... Saleus limitans--|--4Hf---... 4 Fic. 471.—From a model of the brain of a human embryo at the end of the first month, right half, seen from the left. His, Spaltehols. corresponding to the constriction separating the two olfactory lobes, and (3) a posterior crus corresponding to the posterior boundary of the posterior olfactory lobe. This latter is merged with the earlier furrow separating the telencephalon from the thalamus and hypothalamus (peduncular furrow). What may be called the main body of the corpus striatum, from which these limbs radiate, soon becomes expressed externally by a shallow depression in the lateral sur- face of the hemispheres immediately dorsal to the olfactory lobes. This depression is the first indication of the fossa Sylvii (Fig! 465). The boundaries of the pallial hemisphere above indicated are identical with the boundaries of the future foramen of Monro. The median lamina uniting the two halves of the pallium and the two corpora striata may be termed the lamina terminalis and represents the roof plate and floor plate of this region. The point of meeting of the roof plate and floor THE NERVOUS SYSTEM. 547 plate at the end of the tube is often taken to be at the recessus neuroporicus; and the lamina terminalis or end wall of the neural tube, more strictly speaking, is limited to the median wall ventral to this point. Here it will be understood as including the median wall to the point where the pallio-thalamic boundary begins, marked later by the angulus prethalamicus of His (see p. 554 and Fig. 480). Rhinencephalon.—-The term rhinencephalon is a convenient one for those basal structures of the fore-brain which are in most intimate connection with the olfactory nerve. The term has been extended by some to include the pallial olfactory structures. For descriptive purposes it is here used in the more limited sense. At the fourth week, as already indicated (p. 546, Fig. 472), thereisaslightlongi- tudinal furrow on the external surface, marking the ventral limit of the pallial Fic. 472.—Lateral view of outside of brain shown in Fig. 471. His. eminence. The part of the brain ventral to this furrow is the rhinencephalon, Somewhat later the latter becomes better marked off, the fissure forming its boundary on the lateral surface being thé external rhinal fissure (Fig. 462). Later the mesial side is also marked off by an extension of the fissure around on the mesial side (medial rhinal fissure) and also by a notch, the incisura prima, a continuation of which later ascends along the middle part of the median surface of the hemispheres and is known as the anterior arcuate fissure (fissura prima of His). (Fig. 480.) The existence of a fissura prima in early stages, however, is doubtful. At about this time, the rhinencephalon shows a beginning division into anterior and posterior portions, the anterior and posterior olfactory lobes, the whole structure assuming a bean-shape (comp. p. 549) (Fig. 465). On the lateral surface immediately above this constriction is the beginning concavity in the lateral surface of the hemispheres which marks the 548 TEXT-BOOK OF EMBRYOLOGY. earliest appearance of the fossa Sylvii. The external rhinal fissure, as it becomes more pronounced, may be regarded as an extension forward of the fossa (anterior crus of the corpus striatum). On the mesial surface the incisura prima marks this constriction. With the further curvature of the hemispheres, the anterior lobe becomes bent back under the posterior (third month), but later is again directed forward. It contains a diverticulum of the fore-brain cavity. The cavity of the posterior lobe is not so well marked off and is bounded by the corpus striatum and the inward projection of the incisura prima. (Figs. 462, 463, 465, 466 and 480.) The olfactory nerve at the end of five weeks has reached the anterior lobe on its ventral and posterior side. The lobe develops into the receptive centers for the nerve—the olfactory bulb; into the stalk in which the secondary olfactory : 2 =— dngula Gyrus olfact. medialis Gyrus olfact. lat. Gyrus olfact. medius Gyrus diagonalis _Gyrus ambiens Gyrus semilunaris Cerebellum Olive Fic. 473.—Ventral view of the brain of human foetus at the beginning of the 4th month. Aollmann. tract proceeds; and also into a triangular area where the tract divides—the trigonum. ‘The posterior olfactory lobe develops into the anterior perforated space and an eminence known as the lobus pyriformis which becomes reduced later (comp. Fig. 408,GandH). From it is developed the gyrus olfactorius later- alis, connected with the lateral division of the olfactory tract and the gyri ambiens and semilunaris (Fig. 473). On the mesial wall, the posterior lobe is especially connected with the region between the anterior arcuate fissure and the lamina terminalis (trapezoid area of His, parolfactory or preterminal area of G. Elliot Smith) (Fig. 480). Part of this mesial region represents the anterior portion of the archipallium (comp. Fig. 408, G and H and p. 512). Corpora Striata and Pallium.—The leading feature of the development of this part of the brain is the great expansion of the pallial hemispheres. That part of the brain wall marked externally by the fossa Sylvii and internally by the body of the corpus striatum, and especially that part where the corpus striatum THE NERVOUS SYSTEM. 549 is continuous with the thalamus (peduncular part), may be considered as a fixed point from which the pallial walls expand in all directions, anteriorly, dorsally and posteriorly, i.e., in both transverse and longitudinal directions. At first, this expansion causes the pallial hemispheres to assume a bean-shape with the hilum at the fixed point (Fig. 465). The anterior end curves downward and forms the frontal lobe with its enclosed cavity (anterior horn of the lateral ven- tricle). The posterior end curves downward caudally and forms the temporal lobe with the descending horn of the lateral ventricle. At the same time, owing to the great expansion in a transverse plane of each pallial eminence, the median lamina uniting them (Figs. 463 and 464) not sharing in this growth, there are formed the hemispheres with their cavities, the lateral ventricles, and the great longitudinal fissure between the hemispheres. Later, , vascular mesodermal tissue fills this fissure, forming the falx cerebri. The paired cavities of the pallium are connected with the unpaired end-brain cavity (aula) by the foramina of Monro, the boundaries of which are the same as those of the pallium described above (p. 545). At first the walls of the telencephalon, like those of other parts of the tube, are epithelial in character and nearly uniform in thickness. By proliferation there is formed a several-layered epithelium differentiated into an inner nuclear layer and an outer marginal layer. Later a mantle layer is differen- tiated. ‘The hemispheres are late in development and until the end of the second month the walls are thin and simply show the above three layers. Toward the end of the first month a greater activity in cell proliferation takes place in the basal portion of the telencephalon which thickens into the corpus striatum. At eight weeks there first appears on the external surface of the corpus striatum, a cortical layer of cells lying next the marginal layer and sepa- rated from the inner layer by an intermediate layer comparatively free of cells and known as the fibrous or medullary layer (see p. 561). ‘The differentiation thus begun extends gradually around the circumference of the hemispheres until the mesial surface is reached. This differentiation permanently ceases at the medial pallial margin. The cortical layer does not extend as far as the medullary layer, thus leaving an uncovered medullary layer on the mesial hemisphere wall. As a result of this, there is in this region, passing toward the median line, (1) a region covered with a cortical layer (limbus corticalis of His); (2) an uncovered medullary layer (limbus medullaris); (3) a fibrous transitional zone (the tenia) passing into (4) a membranous zone, the roof plate. This process resembles that taking place in other parts of the neural tube, in which there is the same progressive development from the ventral portion of the lateral wall to the dorsal border of the same, where the latter passes into the roof plate which is either ependymal or expanded into a thin membrane. 550 TEXT-BOOK OF EMBRYOLOGY. The longitudinal growth of the hemispheres naturally affects the form of a number of its structures. As already mentioned, this growth consists in an elongation around a fixed point, which may be regarded as located on its ven- tral border, the result of this being a curving down in front and behind this point. This is especially marked in the caudal half which thereby becomes curled first ventrally and then forward, thus forming a spiral. This growth in length is interstitial, 7. e., due to expansion of the intermediate parts, and pari passu with it there is an elongation not only of the corpus striatum and structures in the mesial hemisphere wall (hippocampal formation, corpus callo- sum, chorioid plexus of lateral ventricle), but also of adjacent thalamic struc- tures (stria terminalis or semicircularis), as described later. Fic. 474.—View of the inside of the lateral wall of anterior part of fore-brain. Human embryo of about 44 weeks. His. C, Corpus striatum; H, pallium; h. R, posterior olfactory lobe; L, lamina terminalis: O, re- cessus (pra-?) opticus; R. 7., recessus infundibuli; S. %.. sulcus hypothalamicus; St, hypo- thalamus; 7, thalamus; v. R., anterior olfactory lobe. The early divisions of the corpus striatum have been mentioned, and also the relations of its parts with the rhinencephalon. The anterior end of the corpus striatum at this period and later shows a longitudinal division into three portions, a lateral, a middle and a medial, due to the original division into three limbs described above (p. 545). (Figs. 474, 475, and 476.) With the elongation backward of the hemisphere the corpus striatum also becomes elongated, being drawn out and curled around the peduncle or stalk of the hemisphere and forming a thickening along the elongated wall. This caudal prolongation of the striatum is its cauda (tail) and extends to the tip of the in- ferior horn (Figs. 475 and 476). The medial portion of the corpus striatum forms a triangular projection (Figs. 464 and 466) the edge of which is directed toward the foramen of Monro. THE NERVOUS SYSTEM. 551 The stalk of the hemisphere has already been mentioned as including that part where corpus striatum and thalamus meet. In this region, according Fic. 475.—View of inside of the lateral wall of lateral ventricle of a human fcetus at beginning of third month. His. Bb, bulbus olfactorius; C./., lateral limb of corpus striatum; C.m,, medial segment (consisting of the middle and inner limbs) of the corpus striatum. The furrow between these two parts opens into the anterior olfactory lobe; AR/., posterior olfactory lobe; L. f., frontal lobe; L. o., occipital lobe; Og., olfactory nerve; R. 7., recessus infundibuli; R. 0., recessus (pre-?) opticus; St., stalk of hemisphere (strio-thalamic junction); V.J., lateral ventricle; v.Ri.+Bb, anterior olfactory lobe, to some, there is a fusion of the striatum, the medial wall of the hemisphere and the anterior part of the thalamus. According to others, the increase in bulk of Medial wall Lateral ventricle Chorioid plexus of lateral ventricle ‘Caudate nucleus (head) Lamina terminalis Tenia thalami alae: j Medial wall Caudate nucleus (tail) Pineal body Trigonum subpineale \——— Median sulcus Cerebellum Mesencephalon Myelencephalon Fourth ventricle Fic. 476.—Dorsal view of the brain of a 3 months’ (45 mm.) human foetus. The dorsal part of each cerebral hemisphere has been removed. Kollmann. this region is produced by a simple thickening of the walls, thus causing a flat- tening out or shallowing of the grooves marking the junctions of striatum and TEXT-BOOK OF EMBRYOLOGY. Fic. 477.—1, 2 and 3, Schematic horizontal sections through human embryonic fore-brains at dif- ferent stages of development; 4, vertical section through fore-brain at about same stage as 1. Goldstein. a, That part of the lateral ventricle lying between the corpus striatum and the junction of medial hemisphere wall and thalamus (leading into the inferior horn); b, furrow or trough between mesial hemisphere wall and thalamus, produced by backward extension of hemisphere; c. 7., internal capsule; F.M., foramen of Monro; h, external surface at junction of mesial hemi- sphere wall and thalamus; Sér., corpus striatum; Th., thalamus; U, place where mesial hemi- sphere wall continues into the thalamus wall (junction of hemisphere wall and thalamus); U:, place where mesial hemisphere wall is continuous with lateral hemisphere wall. In 1, owing to the thickening of U and growth of the corpus striatum, these two are brought into apposition, as indicated by the dotted lines on the right, and apparently fuse, obliterating a and producing the condition shown in 2 and 3. In 2 and 3 the position of the former space a is indicated by the dotted lines a—a’ By comparison with 4, it will be seen that this obliteration by apparent fusion is actually produced by a filling up from the bottom of a (in- dicated faintly by dotted lines on the right in 4). The thickening of the walls at this region also produces a shallowing of b (indicated by dotted lines on the right in 1). The principal cause of this general thickening is the passage of the fibers of the thalamic radiation to the oe aie and, later, of fibers from hemisphere to pes, forming the internal capsule (4, 2 and 3). THE NERVOUS SYSTEM. 593 thalamus on the ventricular surface, and between medial hemisphere wall and thalamus externally (Fig. 477). The effect is much the same whether accom plished by apposition and fusion or by interstitial thickening, massive con- nections being formed which consist mainly of fibers connecting hemispheres and thalamus, the foramen of Monro at the same time being changed in form to a slit. From the metathalamic region the fibers of the optic and acoustic pathways grow forward into the hemispheres (see also p. 544), entering more caudally and forming the retro- and sublenticular portions of the internal capsule (comp. p. 544). That part of the thalamic radiation from the anterior portion of the thalamus (fillet pathway) also forms a part of the internal capsule as described on p. 544. Later, the internal capsule is completed by the growth * Medial wall - Caudate nucleus Chorioid fissure Internal capsule —— Lentiform nucleus — Mesencephalon Lateral wall —= Pedunculus cerebri > Cerebellum Vee a ae Myelencephalon Ty Chiasma ———______ Recessus infundibuli Fic. 478.—Lateral view of the brain of a 3 months’ (42 mm.) human feetus. The lateral wall of the left cerebral hemisphere has been removed. His, Kullmann. from the pallium of descending fibers from the neopallial cortex, through the striatum to the pes. By these various traversing fibers the striatum is divided into the nucleus lenticularis or lentiformis and the nucleus caudatus. The posterior arm of the internal capsule is formed by fibers passing between and thus separating thalamus and lenticularis (Figs. 477 and 478). THe ARCHIPALLIUM. During the fifth week, following the stage shown in Figs. 471 and 472, the pallial evaginations or hemispheres have become much moré pronounced and consequently the foramina of Monro much better defined. A comparison will show that the boundaries of the foramen of Monro are essentially unaltered. Anteriorly it is bounded by the medial wall connecting the two hemispheres, posteriorly by the boundary between pallium and thalamus, ventrally by the corpus striatum and junction of it and thalamus (Figs. 463 and 479). 554 TEXT-BOOK OF EMBRYOLOGY. At the beginning of the sixth week the foramen of Monro has changed some- what in shape. The pallio-thalamic part of its boundary passes forward and forms the above-mentioned (p. 547) acute angle (angulus prethalamicus) with that part of the wall uniting the two hemispheres (lamina terminalis). The latter wall descends to the region of the optic recess. The inferior part of the foramen is partly closed by the medial part of the corpus striatum as already described. (Comp. Figs. 479, 464 and 466.) In the ependymal mesial wall of the hemispheres just below the tenia, described above, there arises a folding inward, which begins anteriorly near the angulus prethalamicus and proceeds caudally along the upper (pallio-thalamic) border of the foramen of Monro. This infolding is the chorioid fissurc. In the ependymal mesial wall there are Pallium II ventricle Chorioid fissure Mesodermal tissue, forming later the chorioid plexus. Foramen of Monro Corpus striatum Pharynx Eye Tongue Fic. 479.—Transverse section through fore-brain of a 16mm. embryo (six to seven weeks). His. now the following: limbus chorioideus (the infolded part) and a small strip of the ependyma wall below the fold, the lamina infrachorioidea (Fig. 480). This invagination soon becomes very deep, resulting in the formation of a double- layered ependymal fold (the chorioid fold, plica chorioidea) lying in the lateral ventricle over the corpus striatum (Figs. 479, 464 and 482). Later, vascular mesodermal tissue passes in from the falx between the lips of this fold and thereby forms the chorioid plexus of the lateral ventricles. The chorioid fissure is at first quite short, but becomes elongated (Fig. 481) with the above-described posterior elongation of the hemisphere of which it is a part, and thus extends into the inferior horn of the temporal lobe. (Figs. 481 and 482.) Toward the end of the second month, according to some authorities (His), but not until considerably later, according to others (Hochstetter, Goldstein), another furrow appears in the limbus corticalis above and parallel to the chori- THE NERVOUS SYSTEM. 555 oid fissure, and known as the posterior arcuate fissure. This fissure does not extend at first as far forward as the chorioid, but extends farther caudally, arching downward in the temporal lobe around the caudal end of the chorioid fissure (Fig. 481). The posterior arcuate fissure is a total fissure, involving the whole wall and producing a fold on the inner surface of the medial hemisphere wall (plica arcuata). The temporal or caudal part of this whole formation persists in the adult without much further change. The fissure here becomes the hippocampal fissure separating the fascia dentata from-the gyrus hippocam- pus; the part rolled in by the hippocampal fissure produces the eminence in the lateral ventricle known as the cornu ammonis or hippocampus major; Prhl--~ wRh Vine Fetr i Rh Fic. 480,—Diagram of a graphic reconstruction of the mesial hemisphere wall of a 16 mm. human embryo (about six weeks). -His, Ziehen. Cavities are dotted, cut surfaces are lined. Apt, Angulus prethalamicus; Air, preterminal area; Fpr, anterior arcuate fissure (fissura prima); Frhl, mesial termination of lateral rhinal fissure; Rh, posterior olfactory lobe (tuberculum olfactorium + substantia perforata anterior); Lt, lamina terminalis (lined); Vmr, depression between the two olfactory lobes; vRh, anterior olfactory lobe (bulbus olfactorius + tractus olfactorius + trigonum olfactorium). the edge of the limbus corticalis forms the fascia dentata; the limbus medullaris or exposed fibrous part is the fimbria which is continued by its thinning edge or tenia fimbrie into the ependymal or epithelial portion (lamina chorioidea) of the chorioid plexus of the lateral ventricle. The chorioid plexus is attached by the tenia chorioidea and lamina infrachorioidea (here the lamina affixa) to the brain wall, usually near the junction of corpus striatum and thalamus, thereby forming a part of the wall of the inferior horn of the lateral ventricle. At this line of junction of thalamus and hemisphere wall is formed the siria terminalis. The fimbria is continuous anteriorly with the posterior pillar of the fornix. (Fig. 482.) The anterior part of the hippocampal formation above described undergoes 556 TEXT-BOOK OF EMBRYOLOGY. Corpus callosum Hippocampal fissure 1 ! 3 i e | / : : f f fe fq A ‘ | Hippocampal fissure Lamina terminalis | Chorioid fissure Anterior commissure Beginning anterior column of fornix Fic. 481.—-Graphic reconstruction of the mesial hemisphere wall of a human foetus (fourth month). His, from Quain’s Anatomy. cand v, Anterior and posterior parts of preterminal area; Ji, lamina infrachorioidea; /cm, limbus or border of mesial hemisphere wall (gyrus dentatus and fimbria) between hippocampal and chorioid fissures; P, ‘‘ stalk”? of hemisphere. fale Fic. 482 —Diagram of a transverse section through month) to show the relations of the margins of from Quain’s Anatomy. Cs., corpus striatum; f.., limbus medullaris (fimbria); fa., limbus corticalis hippocampal fissure; Th., thalamus the fore-brain of a human foetus (fourth the mesial walls of the hemispheres. His, (gyrus dentatus); 2.f., THE NERVOUS SYSTEM. 557 further modifications, due principally to the development of commissural fibers in this region. Some of these commissural fibers connect the representatives on each side of the hippocampus (limbus corticalis) of this region, forming the fornix commissure, but most of them (corpus callosum) connect the rest of the cortical areas (neopallial areas) of the two hemispheres. There are two views regarding the formation of these commissures. Ac- cording to one view, the first commissural fibers appear in the upper (dorsal) part of the lamina terminalis. The latter subsequently expands pari passu r Corpus callosum Callosal (continuation of hippocampal) fissure Pornix (continuation of fimbria) | Vv Calcarine fissure actory stalk Optic@pmmissure (chiasma) 1 ies Lamina terminalis | Anterior commissure Uncus 5 re fissure Fic. 483.—Graphic reconstruction of the mesial hemisphere wall of a 120 mm, foetus (end of four months). His, from Quain’s Anatomy. b, Fimbria; cs , cavity of septum pellucidum (‘“‘fifth” ventricle, ventricle of Verga) 3 lcm, limbus corticalis (gyrus dentatus);. P, stalk of hemisphere; v, outline of cavity of hemisphere (lateral ventricle). : with the expansion of the corpus callosum. The commissural fibers are thus confined to the original walls connecting the two hemispheres. According to the other view, there is a secondary fusion of the mesial hemisphere walls and in these fusions the fibers cross. The first fibers appear during the third month and form at first a small band in the upper part of the lamina terminalis (Fig. 481). These fibers come partly from the limbus corticalis (fornix commissural fibers) and partly from other parts of the cortex (callosal fibers), in either case traveling along the intermediate layer. According to the fusion view, the exposed intermediate layers (limbi medullares) fuse where the fibers cross. This fusion can easily be imagined by conceiving the- opposite surfaces in 558 TEXT-BOOK OF EMBRYOLOGY. question to be brought together in the upper part of Fig. 482. It is more prob- able, though, that not only the first fibers cross in the lamina terminalis, but that the later ones also cross in extensions of the latter. There are three views regarding the further development of the corpus callosum. The first is that all parts are represented at this stage, future growth being by intussusception of fibers; the second is that the part first formed represents the genu, the rest being added caudally; the third (His) is that this first formed part represents the middle portion of the callosum, both anterior (genu and rostrum) and posterior (splenium) portions being subsequently added (Figs. 481 and 483). This latter view is indicated in Fig. 483, the later additions being shaded darker. As the callosal fibers connect the limbi medullares, the limbus corticalis and the arcuate fissure, corresponding to the gyrus dentatus and hippocampal fissure of the temporal lobe, lie dorsal to the callosum. The limbus corticalis is reduced to a mere vestige (indusium griseum and sirie Lancisi) on the dorsal surface of the corpus callosum the fissure becoming the callosal fissure. The part of the limbus medullaris ventral to the corpus callosum, corre- sponding to the fimbria of the temporal lobe, forms the posterior pillars and body of the fornix. These relations are shown in the following table from His (slightly modi- fied): Upper callosal region Hippocampal region Upper lip of arcuate, Gyrus cinguli Gyrus hippocampi fissure ' Arcuate fissure Fissura corporis cal- losi | Cortical covering of. callosum (indusium: griseum and strie Lancisi) Fissura hippocampi Limbus Corticalis Cortical layer of low- Gvrus dentatus ‘er lip of arcuate fissure Limbus Medullaris Medullary part of Callosum and fornix Fimbria lower lip Tenia Tenia fornicis Tenia fimbrie Lamina chorioidea Plica chorioidea Plica chorioidea Lamina — infrachorio- Lamina affixa Tenia chorioidea idea Fibers from the hippocampus enter the fimbria and pass forward in the pos- terior pillars and body of the fornix. In or near the lamina terminalis these fibers of the fornix descend, forming the anterior pillars of the fornix, and thence pass back of the anterior commissure and caudally to the mammillary region. THE NERVOUS SYSTEM. 559 They are joined by fibers from the dorsal surface of the callosum (fornix longus), i.e., from the vestigial hippocampal formation, many of which also - descend in front of the anterior commissure to the rhinencephalon. The trian- gular mesial area (sepium pellucidum) included between callosum and fornix probably represents an extended part of the lamina terminalis or “commis- sure-bed,” in which a cavity is formed, the so-called fifth ventricle and ventricle of Verga. A remnant of the hippocampal formation at the anterior end of the callosum is represented by the gyrus subcallosus (Fig. 483). THE NEOPALLIUM. The hippocampal or cornu ammonis formation and preterminal area represent the older part of the pallium (archipallium) comp. pp. 475 and 476. This part of the pallium is olfactory in character, being mainly a higher center for the reception of secondary and tertiary olfactory tracts. In its extension backward and partial obliteration by the corpus callosum, its embryologic presents a striking similarity to its phylogenetic development (compare p. 475.) The rest of the pallial hemispheres (neopallium) are occupied by the non- olfactory higher centers. The further growth of the neopallial hemispheres leads to their extension backward, overlapping the caudal portions of the brain tube. In the course of this extension the occipital lobe and its cavity, the posterior horn of the lateral ventricle, are formed. The growth of various portions of the hemisphere sur- face is unequal, producing folds (convolutions) and fissures. This folding may be partly due to growth in a confined space, but especially important is the relation between gray and white matter. The gray matter, containing not only fibers but also neurone bodies, remains spread out in a comparatively thin layer, probably to accommodate associative connections. The white matter, on the other hand, increases in thickness. This leads to a folding of the outer layer. The position of these folds is probably partly determined by the local histological differentiation and growth of various cortical areas (p. 564). Only some of the earliest and most important of these folds will be mentioned here. It has been seen (p. 546) that early in the development of the pallium a shallow depression appears on the external lateral surface of each hemisphere, the fossa Sylvii (Fig. 484). The bottom of this is the future msula. It is ex- ternal to the corpus striatum and does not grow as rapidly as the parts bound- ing it, which consequently overlap it, forming its opercula. These bounding walls are formed by the fronto-parietal lobe on its upper side, by the temporal on its lower, and by the orbital on its anterior. The temporal and fronto- parietal opercula begin about the end of the fifth month, the temporal at first 36 560 TEXT-BOOK OF EMBRYOLOGY. growing more rapidly but later the fronto-parietal, thereby changing the direction of the Sylvian fissure from an oblique to the more horizontal angle characteristic of man as compared with the ape. In the meanwhile the development of the frontal lobe leads to its also overlapping the insula. If the Parietal lobe —————_=—+ Frontal lobe Insula Occipital lobe Bulbus olfactorius f Ne ; a “ \ Gyrus olfactor. lat. Mesencephalon Cerebellum \ - . Gyrus semilunaris Gyrus ambiens Fic. 484.—Lateral view of the brain of a human feetus at the beginning of the 4th month. Kollmann. frontal lobe fully develops, it forms a U-shaped operculum between the fronto- parietal and the orbital, if it does not so fully develop it forms a V-shaped operculum, and a still less developed condition is shown by a Y-shaped arrange- ment in which the frontal lobe does not completely separate the fronto-parietal Sulcus corp. callosi Corpus callosum | Splenium Gyrus cinguli | i Fissura parieto-occip. Cavum septi pellucidi Lamina rostralis Area parolfactoria (preterminalis) N. olfact. | | Fiss. rhinica N. optic. Lob. temp. Fic, 485.—Median view of the Jeft half of the train of a human feetus at the end of the 7th month. Kollmann. and orbital opercula. The opercula cover the fore-part of the Sylvian fossa during the first year. Conditions of arrested development are thus indicated by the Y-shaped anterior ascending branch of the Sylvian fissure coupled with an absence of the pars triangularis and also by a partial exposure of the island THE NERVOUS SYSTEM. 561 of Reil. In the ape the frontal operculum is absent and the island of Reil partly exposed. Toward the end of the third month the calcarine fissure appears, producing on the ventricular surface the eminence known as the calcar avis. At the beginning of the fourth month the parieto-occipital fissure unites with it forming the cuneus. The parieto-occipital reaches the superior border of the hemi- spheres by the sixth or seventh month. At the sixth month the fissure of Rolando (central fissure) appears. The condition of the surface of the hemisphere at the end of the seventh month is shown in Figs. 485 to 488. The early histogenetic development of the pallial wall, resulting in the dif- ferentiation into the usual ependymal, mantle and marginal layers, has been mentioned. (Fig. 489). The next stage, already alluded to (p. 549), marks a Gyrus front. med. Sulcus front. sup. Gyrus front. inf. — Sulcus front. inf. Gyrus front. sup. — Sulcus precentralis Gyrus przcent. precentrali Sulcus centralis Gyrus cent. post. | Sulcus postcentralis ~ - Lobulus par. sup. © Lobulus par. inf, Fissura parieto-occipit. Lobus occipitalis — Fic. 486.—Dorsal view of the cerebral hemispheres of a human foetus at the end of the 7th month. Kollmann. difference in development between the pallium, as well as other supraseg- mental structures, and the rest of the walls of the neural tube. This stage consists apparently in a further migration outward of the neuroblasts and their accumulation under the marginal layer, forming,-at eight weeks, a definite layer of closely packed cells, the-beginning of the cortex (Fig. 490). Later neuroblast migrations probably add to this layer. It has already been men- tioned that the fibers of the thalamic radiation appear in the pallial walls about this time. They proceed internally to the cortical layer and thus mark the beginning of the fiber layer (medullary layer) which by later myelination becomes the white matter of the hemispheres. The extension of the process of differentiation of the cortical layer from the region of the corpus striatum over the rest of the pallium has also been men- tioned (p. §49). It is probable that the afferent pallial fibers (thalamic radia- tion) in their growth keep pace with this process. Those fibers from the lateral 562 TEXT-BOOK OF EMBRYOLOGY. geniculate bodies proceed to the occipital region, those from the medial genicu- late bodies to the temporal, and those from the ventro-lateral thalamic nuclei (continuation of the medial fillet) to the future postcentral region. The afferent pallial fibers are often termed the afferent or ascending projection fibers. Sulcus | eet centralis Lobus parietal. sup. —— ee ~~ Y ken > Sulcus front. inf. Region of gyrus sup- ey : ramarg. and angular. '\—— Ramus post. Ramus ant. asc. 4 S es Fissura Sylvii Sulcus tempor. med. a s -4 | i \: : 4 | | | Post. pole_. { | gad of cerebrum ~*~ pear eee le Lobus temporalis Gyrus temp. sup. Gyrus temp. med. Fic. 487.—Lateral view of the right cerebral hemisphere of a human fcetus at the end of the 7th month. Aollmann. The axones of the neuroblasts of the cortical layer grow inward, entering the medullary layer. Their peripherally directed processes become the apical dendrites of the pyramid cells into which most of the cortical cells differentiate. According to Mall and Paton, this change of direction in the growth of the axone is due to a turning of the cell axis during its outward migration. It would seem Sulcus orbitalis ——~* 8 — Sulcus olfactorius Insula — , ae j + — Lobus olfactorius Gyrus olf. lat. — co ial | ttn ~- Chiasma Gyrus semilun. -f es pA A fo ee = 4 4. P Gyrus ambiens ~~ , : Pyramid "7 i. - Cerebellum Medulla ~ pars —— Post. pole of cerebrum Fic, 488.—Ventral view of the brain of a human fcetus at the beginning of the sixth month. Refsius, Kollmann, more probable that the cells retain an original bipolar character and that the inner processes differentiate into axones instead of the cells going through a monopolar stage (pp. 491 and 492 and Figs. 424 and 425). The axones of the cortical cells form either efferent or descending projection fibers, proceeding to THE NERVOUS SYSTEM. 563 other parts of the nervous system, or crossed (callosal) and uncrossed association fibers, connecting various cortical areas of the hemispheres. The basilar dendritic processes of the pyramid cells and the axone collaterals develop last. Many details of development of the cells in Mammals are not completed until after birth (Fig. 491). @ B 6: nde 3 2 4° g ee EB: zag ef 5g gf Fic. 489. Fic. 490 Fic. 489.—Section through the pallial wall of a two months’ human foetus. His, Cajal. a, Layer of germinal cells; 5, nuclear layer; c, mantle layer; d, marginal layer; e, germinal cell Fic, 490.—Section through the pallial wall of a human fcetus at the beginning of the third month. His, Cajal. a, Layer containing germinal cells; b, fibrous (medullary) layer (rudimentary white matter); ¢, layer of neuroblasts forming rudimentary cortical gray matter; ¢d, marginal layer (future molecular layer); e, germinal cell; f, g, neuroblasts with radial processes. Spongioblasts and myelo- spongium are shown on the right side. During the fourth and fifth foetal months the cortical layer shows a differen- tiation into a denser outer and an inner layer. During the sixth and seventh months a differentiation and grouping of the nerve cells begins which results in the formation of six cortical layers (Brodmann). These are: (1) the zonal 564 TEXT-BOOK OF EMBRYOLOGY. layer (marginal layer, molecular layer of adult), (2) the external granular layer (layer of small pyramid cells of adult), (3) pyramid layer (medium and large pyramid cells), (4) internal granular layer, (5) ganglionic layer (internal pyra- mid cells), (6) multiform layer (polymorphous cells). By various local modifi- cations of this six-layered cortex the differentiation of the various histological areas of the adult cortex is brought about. In the calcarine region of the occipital lobe, in the sixth month, the internal granular layer differentiates into Notts earns Fic. 491.—Section through cortex of a mouse foetus before birth, showing later stages of differentiation of pyramid cells. Golgi method. Cayal. a, large pyramid cells; b,c. medium-sized and small pyramid cells; d, beginning collaterals of, e, axis-cylinders or axones; f, horizontal cell of molecular layer. Basal dendrites of pyramid cells are beginning to appear. two layers between which is formed the line of Gennari which contains termi- nations of the fibers from the lateral geniculate bodies, representing the visual pathway. This area is the visual cortex. In the temporal (future transverse gyri) and postcentral regions, areas are differentiated which mark the re- ception of the terminations of the fibers of the acoustic and somesthetic (medial fillet) pathways. These areas are thus, respectively, the auditory cortex and the somesthetic (general bodily sensation) cortex. (Cf. Fig. 409.) In the precentral region, the internal granular layer becomes merged with THE NERVOUS SYSTEM. 565 the adjoining layers and practically disappears, the two inner layers become more or less fused and in them certain cells develop to a great size forming the layer of giant pyramid cells. It is the axones of these cells, in all probability, which proceed as the pyramidal tracts through the middle part of the internal capsule and pes to the epichordal segmental brain and cord. The area in which these cells lie is the motor cortex (cf. Fig. 409). Descending axones de- velop similarly from cells in the calcarine area, possibly here also from large pyramidal cells of the fifth and sixth layers (solitary cells of Meynert), which probably pass to the anterior colliculus (operating there upon, reflex eye mechanisms). In the whole pallium there are thus four great projection fields, differen- tiated both by their histological structure and their connections. These are (z) the archipallial olfactory area with mesial ascending and descending connections; (2) the visual; (3) the acoustic; (4) the somatic. ‘The systems of projection fibers of the three neopallial fields are lateral. The visual and acoustic fields repre- sent certain specialized and concentrated groups of receptors (rods and cones, hair cells of organ of Corti) upon which stimuli of a certain definite nature (light and sound waves), from distant objects, are focussed by means of acces- sory apparatus (eye, ear). The somatic area represents receptors scattered . over the whole organism. In the visual and acoustic mechanisms, the efferent element is small or lacking in both peripheral apparatus and cortical areas, in the somatic the efferent element is large and is represented cortically by an area (motor, precentral area) distinct from that of the receptive portion (somzs- thetic, postcentral area). Gustatory and other visceral areas have not been well determined (vicinity of archipallium ?). These four primary sensori-motor fields are probably the first differentiated of the various pallial cortical areas. This is evidenced by the myelination (comp. p. 501) which first involves the projection fibers of these areas (at or soon after birth, Flechsig), the afferent projection fibers probably myelinating before the efferent (Figs. 492 and 493). The process of myelination next spreads over areas adjoining the primary areas, the intermediate areas of Flechsig. Descending projection fibers from these areas in the frontal, temporal and occipital lobes are probably represented by the cortico-pontile systems of fibers, securing cerebellar regulation of pallial reactions. The presence of other fibers connecting with thalamic nuclei is probable, but knowledge of their develooment and connections is very incomplete. The cells whose axones form descending or efferent projection fibers con- stitute only a small fraction of the cortical cells. The great majority are asso- ciation cells whose axones, or collaterals, pass across the median line in the lamina terminalis as the callosal fibers already mentioned (p. 5 57) or pass 566 TEXT-BOOK OF EMBRYOLOGY. to distant or near parts of the same hemisphere. In general, these develop later than the projection neurones and the completion of their development is carried to a much later period. Variations which arise in their differentiation and ar- rangement probably contribute largely to the formation of various histological areas which develop at different periods. These local inequalities of growth probably constitute a factor in the production of the convolutions appearing later than those already mentioned in connection with the primary areas. The last areas to myelinate, the terminal areas of Flechsig, are poor in projection fibers and are thus composed largely (entirely ?, Flechsig) of association cells. It is the extent of these last developing areas which constitutes the principal difference between the human cortex and that of related forms. These pallial = LUMLEY Y ae ao y y, ETE is Ly iy : SY OK AOE Lop le MAE Gms; Uff P23. Ye oo > Fic. 492,—Diagram of cortical areas of mesial surface of pallium as determined by the myelogenetic method Flechsig, from Quain’s Anatomy. For explanation see Fig. 493. areas are those which continue to grow in human development. Myelination in the cortical areas may continue for twenty years or so. It is a significant fact that the last areas to develop are comparatively poor, even when completely developed, in both cells and fibers (Campbell). The association neurones thus probably follow the same order of development as the projection systems: As their development spreads from the primary receptive areas (perceptions ?), the incoming stimuli receive a more and more extended associative “setting” (psychologically, the “meaning” or “significance” of perceptions ?), extensive associations between the various areas being provided by the extension of their development to the terminal areas (rendering possible the association of symbols: mental processes ?). THE NERVOUS SYSTEM. 567 The general biological significance of this late development of the pallium and especially of its associative mechanisms has already been alluded to. These “added” parts of the nervous system are the most modifiable mechan- isms of the human organism; they are those mechanisms which perform its newest and most highly adaptive adjustments. The other parts of the ner- vous system are fixed at birth, but the cerebral hemispheres are still plastic for the reception and recording of individual experience. Such experience symbolized and formulated (spoken, written, etc.) is transmitted to the next generation, as already pointed out (p. 477). An example of the far-reaching consequences of this capacity of the pallium is the prolonged period of infancy and education of man. Fic. 493.—Diagram of cortical areas of lateral surface of pallium as determined by the myelogenetic method. Flechsig, from Quain’s Anatomy. The numerals indicate, in a general way, the order of myelination. The primary areas (1-10) are indicated by dots, the intermediate areas (11-31) by oblique lines and the terminal or final areas (32-36) by clear spaces. Anomalies. Those anomalies of the nervous system involving more general develop- mental anomalies (cyclopia, anencephaly, cranioschisis, spina bifida, etc.) are dealt with in the chapter on Teratogenesis (XIX). Owing to the fact that the nervous system consists of parts which are more or less separated, and yet con- nected and interdependent, it is in certain respects affected differently from the other organs when portions of it are injured or inhibited in development. Thus an injury or inhibition in development of one part of the nervous system may, because of the dependence upon this part of other perhaps distant parts, affect the development of the latter. Even in the adult, injury of an axone leads to the 568 TEXT-BOOK OF EMBRYOLOGY. disappearance of that portion of the axone distal to the point of injury; it may also lead to the disappearance of the entire neurone where regeneration is not possible. Such an injury during development will not only cause a disappear- ance of the whole neurone, but it may also lead to the disappearance of other neurones forming links in the same functional pathway. Thus a develop- mental defect involving the central area will not only lead to absence of the pyramidal tract, but also to partial atrophy of the corresponding fillet bundles. When one cerebellar hemisphere fails to develop, there results a correlated defect in its centripetal and centrifugal pathways. The opposite inferior olive is practically absent, as is also the central tegmental tract leading to that olive. The pontile nuclei of the opposite side, the middle peduncle leading from them to the affected cerebellar hemisphere, and the fibers in the pes which pass to the pontile nuclei in question are likewise suppressed, and the superior peduncle and red nucleus are absent or reduced. In this case it is evident that the correlated atrophy affects at least two neurones in the pathways leading to and from the cerebellum. This illustrates the far-reaching character of cor- related developmental defects in the nervous system arising from the nature of the connections between various portions of the system. PRACTICAL SUGGESTIONS. Very instructive pictures (surface views) of the neural tube (the brain vesicles, the spinal cord, and the relation of the latter to the primitive streak) can be seen in chick embryos during the second day of incubation. Remove the blastoderm from the egg, fix in Zenker’s fluid, stain im toto with borax-carmin, and mount im ioto in xylol-damar. It is also interesting to examine the blastoderm in the fresh condition. Even in chick embryos of later stages, many of the general features of the developing nervous system can be seen in gross mounts prepared as above. For details of structure, sections of embryos must be made. For this purpose mam- malian embryos are usually available. The sections, if properly prepared, will serve a two- fold purpose, viz.: the study of histological structure and.of gross form. And since in such complicated organs as the nervous system gross form can be studied to the best advantage by means of reconstructions (see Appendix), serial sections should be prepared. Fix the embryos (pig embryos, for example) in Bouin’s fluid, cut serial transverse sections in paraffin, stain with Weigert’s iron hematoxylin and eosin, and mount in xylol-damar. Orth’s fluid and Zenker’s fluid are also good fixatives. Hermann’s fluid and Flemming’s fluid, followed by Heidenhain’s hematoxylin stain, are also useful in the case of small embryos (not more than 8 mm.). While general histological structure can be studied in sections prepared by the ordinary technic (as above), other special methods are necessary for the study of certain structures. Of the special methods, the most important are (1) the method of Golgi, (2) the method of Cajal, and (3) the method of Weigert. All these have been more or less extensively modified. Method of Golgi.—This has many modifications, but the method most in use is that known as the Golgi rapid method. This consists in— 1. Fixing and hardening in potassium bichromate, 3 1/2 per cent., 4 vols. + osmic acid, I per cent., t vol. These proportions may be varied. The time of hardening is very im- THE NERVOUS SYSTEM. 569 portant, but must be specially determined for each specimen. In general it varies from 12 hours (chick embryos of two to three days incubation) to six or seven days (e.g., human cerebral cortex at birth). 2. The specimen is next rinsed in a 3/4 per cent. to per cent. solution of silver nitrate (a previously used solution will answer for this rinsing) and changed until the solution no longer becomes turbid with silver chromate. The specimen is then transferred to a fresh silver nitrate solution of the same strength for 24 hours, preferably in the dark. It is often well to keep the specimen at this stage in a warm chamber at a temperature of from 25° to 30° C. 3. The specimen is next brought directly into 95 per cent. alcohol for an hour or more, the alcohol being changed several times, and then into equal parts alcohol and ether for from 1/4 tor/2anhour. It is next placed in thin celloidin for about 1/2 hour and then for the same length of time in thick celloidin. The specimen is blocked and the celloidin hardened quickly in chloroform. Thick sections (50 to 70 microns) are cut, cleared in oil of origanum Cretici, followed by xylol, and mounted in xylol-balsam or damar, without a cover-glass. It is often of advantage to transfer the sections from the xylol to a dish of xylol- balsam or damar and from this to the slide, thereby avoiding diffusion currents. The slides are then placed in the paraffin bath to hasten the hardening of the balsam. Sections may be mounted under a cover glass if melted hard balsam or damar is used instead of a solution of the same. If the balsam in uncovered specimens becomes wrinkled or cracked, heat applied until the balsam just melts (but does not bubble) will render it smooth again. Material which is to be subjected to the Golgi technic should be cut into small pieces, not exceeding 2 to 3 mm. in thickness, ‘The method shows a few of the neuroblasts and spongio- blasts as black objects, they being filled or encrusted with a black silver compound. Internal structure is of course not shown. ‘The method is capricious and is best used when there is considerable material available. Methods of Cajal.—Of these the most generally useful is as follows: 1. Specimens not more than 5 or 6 mm. in thickness are fixed and hardened for twenty- four hours in strong alcohol or in strong alcohol to every roo c.c. of which from four to twelve drops of strong ammonia have been added. 2. The specimen is next rinsed in distilled water or simply placed in the water until it sinks and then transferred to a 1 1/2 per cent. aqueous solution of silver nitrate. Thestrength of the silver solution may be varied from 1/2 per cent. to 4 per cent., 1 1/2 per cent. being the strength most commonly employed. While in the silver solution the specimens should be kept in the dark and at a temperature of 32° to 38°C. The exact time the specimens should remain in the silver solution must be determined by experiment in each case, but is usually from three to six days. 3. The specimen is next rinsed quickly in distilled water and then placed for twenty- four hours at room temperature in Pyrogallol, x to 2 grams Water, I0O C.C. Formalin, 5 to Io c.c. The solution should be freshly made up and changed after the specimen is put in it if it becomes turbid. 4. The specimen may be embedded in celloidin or paraffin and cut in the usual way and about the usual thickness. Dehydrate, clear in carbol-xylol and xylol, and mount in xylol-damar. 570 TEXT-BOOK OF EMBRYOLOGY. Another in toto silver method is that of Bielschowsky, modified by Paton. This method is more complicated, as is also Bielschowsky’s important method of staining sections. None of the silver methods has an absolutely reliable technic, especially for embryological work. They are particularly designed to bring out the neurofibrils which are often sharply and deeply stained. The non-neurofibrillar portions of the cell-body and its processes are also often, especially in the adult, clearly defined though not so deeply stained. The picture is thus a more general one than that yielded by the methods of Golgi and reveals important details of internal cell structure. It is, however, inferior to the Golgi methods in displaying the finest ramifications of the processes. Weigert’s Method.—For studying myelination, the method of Weigert or one of its modifications is used. The specimen is fixed and hardened in Miiller’s fluid, or preferably fixed for two or three days in Miiller’s fluid 9 vols., formalin 1 vol. (the fluid being changed once or twice), and then hardened for three or four weeks in Miiller’s fluid. After a short washing in water the specimen is carried through graded alcohols, the alcohol being changed until the bichromate no longer colors it. If the specimen is kept in the dark, precipitation of the bichromate is avoided. The specimen is embedded in celloidin or paraffin and sectioned in the usual way. In the unmodified Weigert method, the sections are carried from water into a saturated or half saturated aqueous solution of neutral cupric acetate for twelve to twenty-four hours. They are then rinsed in water until the free cupric acetate is removed, and transferred to a mixture of 9 vols. of No. x solution and 1 vol. of No. 2. No. 1. Water, IO0O C.Cc. Saturated aqueous solution lithium carbonate, 1 or 2 c.c. No. 2. Hematoxylin, Io grams. Alcohol, 95 per cent. or absolute, TOO C.C. Solution No. 2 should be a week or more old. Sections should remain in the above mix- ture for from twelve to twenty-four hours and are then differentiated in the following: Potassium ferricyanide, 2 1/2 grams. Borax, 2 grams. Water, IOO C.c. After differentiation is complete, the specimens should be washed for half an hour or more in water, dehydrated, cleared in carbol-xylol and xylol, and mounted in xylol-damar or balsam. 2 The most important modification of Weigert’s method is the so-called Weigert-Pal method. This method should be used only with celloidin sections. The sections are transferred directly into either a 10 per cent. solution of haematoxylin in alcohol, 1 vol. + water, 9 vols., or into 1 vol. of the hematoxylin solution + 9 vols. of a 1 per cent. or 2 per cent. solution of acetic acid. If the material has been kept in alcohol for some time, it may be advisable, before staining, to mordant for from one to several days in 5 per cent. potassium bichromate or in Miiller’s fluid. Copper bichromate, > per cent. to 3 per cent. (12 hours), is a stronger mordant, but may make the sections brittle. When stained, the sections are rinsed in water and differentiated as follows; First place in a freshly made 1/s per cent. to 1/3 per cent. solution of potassium permanganate, usually about 1/2 minute, then rinse in water and transfer to Potassium sulphite I gram. Oxalic acid, I gram Water, 200 C.C. THE NERVOUS SYSTEM. 571 This completes the differentiation. A very weak solution of sulphurous acid answers as well as or better than the sulphite-oxalic mixture. If differentiation is not sufficient, rinse in water and repeat the process. After differentiation, dehydrate, clear, and mount as usual. It is often advantageous, especially in younger foetuses, to fix in copper bichromate,"3 per cent. to 5 per cent., 9 vols. + formalin 1 vol., and then harden a week or more in copper bichromate, 3 per cent. These specimens should not be over 1/2 an inch in thickness, For all Weigert methods, the most satisfactory fixation may be had by injecting the bichromate-formalin mixture into the blood vessels. Specimens fixed and preserved in formalin may be used for staining by the Weigert method. Mordant pieces in copper bichromate 3 per cent. about a week or use the methods given in Mallory and Wright’s “Pathological Technic.’’ For further details and other modifications of Weigert’s methods, the student is referred to the books on technic mentioned in the Appendix. References for Further Study. BarDEEN, C. R.: The Growth and Histogenesis of the Cerebrospinal Nerves in Mam- mals. Am. Jour. of Anat., Vol. Il, No. 2, 1903. , ’ DEJERINE, J.: Anatomie des centres nerveux. Tome I, Ch. 2 and 3. EDINGER, L.: Vorlesungen iiber den Bau der nervésen Zentralorgane. Seventh Ed. Epincer, L. The Relations of Comparative Anatomy to Comparative Psychology. Jour. of Comp. Neurol. and Psychol., Vol. XVIII, No. 5, Nov., 1908. Frecusic, P.: Einige Bemerkungen iiber die Untersuchungsmethoden der Grosshirnrinde insbesondere des Menschen. Berichten der math.-phys. Klasse d. Kénigl.-Sdchs. Gesellsch. d. Wissensch. 2u Leipzig. 1904. See also Johns Hopkins Hosp. Bull., Vol. XVI, 1905, pp. 45-49. Harvesty, I.: On the Development and Nature of the Neuroglia. Am. Jour. of Anat, Vol. ITI, No. 3, July, 1904. Harrison, R. G.: Further Experiments on the Development of Peripheral Nerves. Am. Jour. of Anat., Vol. V, No. 2, May, 1906. Harrison, R.G.: Observations on the Living Developing Nerve Fiber. Anat. Record, Vol. I, No. 5, 1907. Harrison, R. G.: Embryonic Transplantation and Development of the Nervous System. Anat. Record, Vol. II, No. 9, 1908. Herrick, C. J.: The Morphological Subdivision of the Brain. Jour. of Comp. Neurol. and Psychol., Vol. XVIII, No. 4, 1908. His, W.: Zur Geschichte des menschlichen Rtickenmarkes und der Nervenwurzeln. Abhandl. der math.-phys. Klasse der Kénig.-Sachs. Gesellsch. d. Wissensch., Bd. XIU, 1887. His, W.: Zur Geschichte des Gehirns, sowie der centralen und peripherischen Nerven- bahnen beim menschlichen Embryo. Adhandl. d. math.-phys. Klasse d. Kénig.-Séchs. Geselisch. d. Wissensch., Bd. XIV, 1888. His, W.: Die Neuroblasten und deren Entstehung im embryonalen Mark. Abhandl. d. math.-phys. Klasse d. Kénig.-Sichs. d. Wissensch., Bd. XV, 1890. Also Arch. f. Anat. u. Physiol., Anat. Abth., 1889. His, W.: Ueber die Entwickelung des Riechlappens und des Riechganglions und tiber diejenige des verlingerten Markes. Verhandl. d. Anat. Gesellsch. 2u Berlin, 1889. Also Abhandl. d. math.-phys. Klasse d. Kénig-Sachs. Gesellsch. d. Wissensch., Bd. XV, 1889. His, W.: Die Entwickelung des menschlichen Rautenhirns vom Ende des ersten bis zum 572 TEXT-BOOK OF EMBRYOLOGY. Beginn des dritten Monats. I. verlangertes Mark. Adhandl.d. math.-phys. Klasse d. Kénig.- Sachs. Gesellsch. d. Wissensch., Bd. XVII, 1891. His, W.: Die Entwickelung des menschlichen Gehirns wahrend der ersten Monate. Leipzig, 1904. Jounston, J. B.: The Nervous System of Vertebrates. 1906. KoLiMann, J.: Handatlas der Entwickelungsgeschichte des Menschen. Bd. II, 1907. von Kuprrer, K.: Die Morphogenie des Centralnervensystems. In Hertwig’s Handbuch d. vergleich. u. experiment. Enitwickelungslehre der Wirbeltiere. Bd. II, Teil III, Kap. 8, 1905. Marzsvrc, O.: Mikroskopisch-topographischer Atlas des menschlichen Zentralnerven- systems, 1904, Mever, A.: Critical Review of the Data and General Methods and Deductions of Modern Neurology. Jour. of Comp. Neurol., Vol. VIII, Nos. 3 and 4, 1898. NeEuMAYER, L.: Histo- und Morphogenese des peripheren Nervensystems, der Spinal- ganglien und des Nervus sympathicus. In Hertwig’s Handbuch der vergleich. und expert- ment. Entwickelungslehre der Wirbeltiere, Bd. II, Teil III, Kap. 10, 1906. RAMON y CaJAL, S.: Sur lorigine et les ramifications des fibres nerveuses de la moélle embryonnaire. Anat. Anz., Bd. V, Nos. 3 and 4, 1890. Ramon y Cajat, S.: A quelle époque apparaissent les expansions des cellules nerveuses de la moélle épiniére du poulet? Anat. Anz., Bd. V, Nos. 21 and 22, 1890. Ramon y Cajat, S.: Textura del sistema nervioso del hombre y de los vertebrados. Madrid, 1899-1904. Also translation into French by Azoulay, 1910-11. Ramon y Cajal, S.: Nouvelles observations sur |’evolution des neuroblasts, avec quel- ques rernarques sur l’hypothése neurogenetique de Mensen-Held. Anat, Ans., Bd. XXXII, Nos. 1, 2, 3 and 4, 1908. ScuaPer, A.: Die morphologische und histologische Entwickelung des Kleinhirns der Teleostier. Alorph. Jahrbuch, Bd. XXI, 1894. ScuaPer, A.: Die friihesten Differenzierungsvorgiinge im Centralnervensystems. Arch. f. Entw.-Mechan., Bd. V, 1897. SmitH, G. E.: On the Morphology of the Cerebral Commissures in the Vertebrata, etc. Trans. Linnean Soc. of London, 2d Ser. Zoology, Vol. VIII, Part 12, 1903. See also articles by same author in Jour. of Anat. and Physiol. STREETER, G. L.: The Development of the Cranial and Spinal Nerves in the Occipital Region of the Human Embryo. Am. Jour. of Anat., Vol. IV, No. 1, 1904. STREETER, G. L.: The Peripheral Nervous System in the Human Embryo at the End of the First Month. Am. Jour. of Anat., Vol. VIII, No. 3. ZIEHEN, T'u.: Die Morphogenic des Centralnervensystems der Saugetiere. In Hertwig’s Handbuch der vergleich. u. experiment. Entwickelungslehre der Wirbeltiere. Bd. IT, Teil ITI, Kap. 8, 1905. ZIEHEN, TH.: Die Histogenese von Hirn- und Riickenmark. Entwickelung der Leitungsbahnen und der Nervenkerne bei dea Wirbeltieren. In Hertwig’s Handbuch der vergleich. u. experiment Entwickclungslehre der Wirbelliere, Bd. Il, Teil TIT, Kap. IX, 1905. CHAPTER XVIII. THE ORGANS OF SPECIAL SENSE. THE EYE. The receptive mechanisms of all the general and special sense organs are derived from the ectoderm. With the single exception of the eye, all develop as direct specializations of the ectoderm in the form of the various neuro-epithelia. The eye is peculiar among the sense organs in that its recep- tive cells are not derived directly from surface ectoderm, but only indirectly from the ectoderm after it has become folded in to form the neural canal. The neuro-epithelium of the eye develops as a direct outgrowth from the central nervous system. ‘The retina is a modified part of the brain; the optic nerves correspond to central nervous system fiber tracts. Of the accessory optic ‘structures, the lens, the epithelium of the lids and conjunctiva, the eyelashes, the Meibomian glands and the epithelium of the lacrymal apparatus are of ectodermic origin; the coats of the eye, the sclera and chorioid, and parts of Optic Neural Optic depression plate depression Fic. 494.—Diagram showing location of optic areas before the closure of the neural groove. Modified from Lange. their modified anterior extensions, the cornea, ciliary body and iris, are of mesodermic origin. In the sensory divisions of the other spinal and cranial nerves, with the exception of the olfactory, the cell bodies of the neurones which serve to connect the receptive mechanisms with the brain and cord are located in parts (the sensory ganglia of the cranial and spinal nerves) which have be- come separated from the crests of the neural folds as the latter fuse to form the neural canal. In the eye the cell bodies of these neurones are located in the retina, but the area of ectoderm from which the retina develops first occupies a position along the neural crest analogous to that occupied by the anlagen of the spinal and cranial ganglia. In the case of the retina this area, instead of be- coming split off in the closure of the neural canal, becomes folded into the canal and later pushed out toward the surface in the optic evagination (Figs. 494, 495, 496). 573 574 TEXT-BOOK OF EMBRYOLOGY. The first indication of eye formation is found in the chick at the beginning of the second day of incubation; in the human embryo, at what has been estimated as about the second or third week. At this stage the neural canal is not yet completely closed in and its anterior end shows three primary brain vesicles Optic vesicle area Neural canal Fic. 495.—Diagram showing location of areas shown in Fig. 494 after the formation of the neural canal. Modified from Lange. (p. 480, Fig. 497). The anlagen of the eyes first appear as bilaterally sym- metrical evaginations from the lateral walls of the fore-brain vesicle (Figs. 497 and 498), and are at first large in proportion to the brain vesicle itself. When first formed, the optic evagination opens widely into the fore-brain vesicle (Fig. 498, right side), but as the distal part of the evagination expands more rapidly Lom, bern. Retina Optic stalk Fic. 496. Fic. 497. Fic. 496.—Diagram showing location of the (dark) optic area (see Fig. 495) after the beginning of the formation of the optic cup and optic stalk. Lange. Fic. 497.—Dorsal view of head of chick of 58 hours’ incubation. A/ihalkovics. Lam. term, lamina terminalis; Fb., fore-brain; Opt. v., optic vesicle; AZ.b., mid-brain; #f.b., hind or rhombic brain; H., heart. than the proximal part, there soon results a spheroidal optic vesicle attached to the fore-brain by the narrow optic stalk (Fig. 498, left side). Through the latter the cavity of the optic vesicle and the cavity of the fore-brain are in communi- cation. With the development of the hemispheres, that part of the brain to which the optic stalks are attached becomes the inter-brain (diencephalon). THE ORGANS OF SPECIAL SENSE. 575 The Lens.—As each optic vesicle grows out toward the surface, its outer wall soon comes to lie just beneath the surface ectoderm. The cells of that portion of the ectoderm which overlies the optic vesicle next proliferate and cause a thickening of the ectoderm (Fig. 498, left side). This thickening of the Fore-brain vesicle Surface ectoderm Lens area - Optic vesicle Optic vesicle Fic. 498.—Section through head of chick of two days’ incubation. Duval. The formation of the optic vesicle and stalk appears to be somewhat more advanced - on the left than on the right. ectoderm over the optic vesicle is apparent in the chick embryo of 36 hours in- cubation; in the human embryo it occurs about the third or fourth week and represents the first step in the development of the crystalline lens. The thick- ened portion of ectoderm is known as the Jens area (Fig. 498). The latter next Fore-brain ’ Lens invagination - - - -—& F J ---~- Lens invagination Optic vesicle ~ Optic vesicle Fic. 499.—Section through head of chick of three days’ incubation. Duval. becomes depressed against the outer surface of the optic vesicle forming a distinct lens invagination (Fig. 499). This becomes cup-shaped and then its edges come together and fuse, thus forming the lens vesicle (Fig. 500). At first the lens vesicle is connected with the surface ectoderm, but about the eighth week 37 576 TEXT-BOOK OF EMBRYOLOGY. a thin layer of mesoderm grows in between the lens vesicle and the surface ectoderm, completely separating them (Fig. 501). The ingrowth of the lens vesicle against the outgrowing optic vesicle has the effect as though a small hard ball (the lens vesicle) had been pressed into a larger soft ball (the optic vesicle) Fic. s00.—Showing somewhat later stage in development of optic cup and lens : than is shown in Fig. 499. Duval. (Fig. 502). The lens vesicle pushes the outer wall of the optic vesicle in against the inner wall, the optic vesicle thus becoming transformed into the two-layered optic cup (Figs. 500, 501). Bonnet calls attention to the fact that the two proc- esses, lens formation and the invagination of the optic vesicle to form the optic Conjunctival epithelium ----7 Optic stalk Retina (inner layer of optic cup) ~~ Pigmented layer of retina, ___ (outer layer of optic cup) Fic. 501.—Diagram of developing lens and optic cup. Duval. The cells of the inner wall of the Jens vesicle have begun lo elongate to form lens fibers. The epi- thelium over the lens is the anlage of the corneal epithelium. The mesodermal tissue between the latter and the anterior wall of the lens vesicle is the anlage of the substantia propria cornez, cup, are more or less independent and that it is not correct to describe the lens as actually pushing in the outer wall of the vesicle. As evidence of this is noted the fact that typical optic cup formation may occur in cases where no lens is developed. The optic cup when first formed is not a complete cup, for the THE ORGANS OF SPECIAL SENSE. 577 invagination of the optic vesicle is carried over along the posterior surface of the optic stalk forming the chorioidal fissure (Fig. 502, see also p. 585). The lens area is thicker at its center than at its periphery and when the center of the lens area becomes the bottom of the lens depression and later the posterior wall of the lens vesicle this greater thickness is maintained. In fact, the posterior wall of the vesicle becomes still thicker so that it projects into the cavity of the lens vesicle as an eminence (Fig. 503, g.). In the chick the lens vesicle is hollow. In man and in Mammals generally it is more or less filled with cells. These, however, degenerate and take no part in the formation of the Pigmented layer of retina Nervous layer of retina (outer layer of optic cup) (inner layer of optic cup) Cavity of optic vesicle Rim of optic cup. Optic furrow . Lens Hyaloid artery | Optic furrow Hyaloid artery entering cavity of vitreous Fic. 502.—Model showing lens and formation of optic cup. A piece has been removed from the upper part of cup,to show the cavity of the optic vesicle and the position of the inner layer of the cup (nervous layer of retina). Bonnet. permanentlens. Comparing the posterior with the anterior wall of the lensat this stage, the latter is seen to be composed of a single layer of cuboidal cells, the an- lage of the anterior epithelium of the lens (Figs. 501, 503, g,/,7). This layer passes over rather abruptly into the posterior wall which consists of a single layer of greatly elongated lens cells, the anlagen of the lens fibers. The lens fibers con- tinue to elongate until by the end of the second month they touch the anterior epithelium, thus completely obliterating the cavity of the lens vesicle (Fig. 505). A small cleft containing a few drops of fluid, the liquor Morgagni, may remain between the anterior epithelium and the lens fibers. When the lens fibers are first formed, the longest fibers are in the center and the fibers gradually get shorter toward the periphery of the lens where they pass over into the anterior epithelium (Fig. 503). As the lens develops, the periph- 578 TEXT-BOOK OF EMBRYOLOGY. eral fibers elongate more rapidly than the central, with the result that in the fully developed lens the central fibers are the shortest, forming a sort of core around which the now longer peripheral fibers extend in much the same manner as the layers of an onion (Fig. 505). The ends of the fibers meet on the anterior and posterior surfaces of the lens, along more or less definite lines which can be seen Fic. 503.—Successive stages in the development of the lens in the rabbit embryo. Rabi. a, b,c, d, and e, are from embryos of from 11} to 12 days; f, at end of rath day; g, during the 13th day; h, between the 13th and r4th days; i, from an embryo of 11 mm. on surface examination and which are known as sutural lines. ‘The lens fibers are at first all nucleated and as the nuclei are situated at approximately the same level in all the fibers, there results a so-called nuclear zone (Fig. 503, 2). Later the nuclei disappear. The sutural lines become evident about the fifth month and mark the completion of the lens formation, although lens fibers continue to be formed throughout foetal and in postnatal life, probably by proliferation THE ORGANS OF SPECIAL SENSE 579 and differentiation of the cells of the anterior epithelium, in the region where the latter pass over into thelens fibers. (The successive stages in the development of the Jens are shown in Fig. 503.) The lens capsule becomes differentiated during the third month. It is con- sidered by some as derived from the lens epithelium and of the nature of a cuticular membrane, by others as a product of the surrounding connective tissue. . By the extension of mesodermic tissue in between the lens and the surface ectoderm, the lens becomes by the end of the sixth week completely surrounded by a layer of vascular connective tissue. This is known as the tunica vasculosa lentis, and receives its blood supply mainly from the hyaloid artery (Fig. 505) which is a foetal continuation of the arteria centralis retine (p. 585). Branches from the hyaloid artery break up into a capillary network which covers both anterior and posterior surfaces of the lens. That part of the tunica vasculosa which covers the anterior surface of the lens is known as the membrana pupillaris. After the earlier and more rapid formation of lens fibers ceases, the hyaloid artery begins (about the seventh month) to undergo regressive changes, and at birth is normally absent. Rarely more or less of the tunica vasculosa fails to degenerate, and if the part which persists is the membrana pupillaris there results a malformation known as congenital atresia of the pupil. The Optic Cup.— The way in which the optic vesicle becomes transformed into the optic cup has been partially described in considering the development of the lens (p. 576). The growing lens vesicle appears to push in the outer wall of the optic vesicle while at the same time the edges of the latter are extending around the lens vesicle, until what was originally the outer wall of the optic vesicle lies in apposition with the original inner wall, the cavity of the primary optic vesicle thus becoming completely obliterated (Fig. 504). In this way the optic vesicle is transformed into a two-layered thick-walled cup, the cleft be- tween the two layers corresponding to the cavity of the primary vesicle. This cup is at first entirely filled with the developing lens (Fig. 504). As the cup in- creases in size faster than the lens, the contiguous walls of the cup and lens become separated, the cavity thus formed being the cavity of the vitreous humor (Fig. 505). There seems to be no question but that in Mammals a small amount of mesoderm at first separates the optic evagination from the lens area of the surface ectoderm. This apparently disappears, however, so that the two are in direct contact. It is still an open question whether a thin layer of mesoderm grows in between the edges of the cup and the lens at or just before the beginning of the formation of the vitreous. The lens now no longer fills the optic cup but lies in the mouth of the cup, while at the same time the margin of the cup is extending somewhat over its outer surface, where with the meso- derm it ultimately gives rise to the ciliary body and iris, and forms the 580 TEXT-BOOK OF EMBRYOLOGY. boundary of the pupil. The remainder of the two-walled optic cup becomes the retina. The Retina.—Of the two layers which form the wall of the optic cup (p. 579), the outer (away from the cavity) forms the pigmented layer, while the inner forms the remainder of the retina (Figs. 501, 505). Soon after the formation of the optic-cup, it is possible to distinguish a boundary zone—the future ora serrata— between the larger posterior part of the retina or nervous retina and the smaller anterior non-nervous part which becomes the retinal portion of the ciliary body Vascular mesoderm Ectoderm Remains of optic vesicle cavity . Lens anlage Lens invagination Pigmented layer of retina (outer layer of optic cup) AG, - RGAE T=) PAIS Ee 3 y Vascular mesoderm Wall of brain vesicle * Fic. 504.—Section through optic cup and lens invagination of chick of fifty-four hours’ incubation. Lange. Between the lens anlage and the pigmented layer of the retina is the broad inner layer of the optic cup, the anlage of the remainder of the retina. and iris. While the optic cup is forming, its two layers are both rapidly in- creasing in thickness by mitotic division of theircelis. Especially is this true of the inner layer over that region which is to become the nervous retina, andit is the rather abrupt transition between the thicker nervous retina and the com- paratively thin non-nervous anterior extension of the retina that forms the ora serrata. The invagination which gives rise to the two-layered optic cup thus differen- tiates what may be called the two primary layers of the retina, the pigmented layer, and a broad layer from which are to develop all the other layers of the retina. THE ORGANS OF SPECIAL SENSE. 581 (Figs. 501, 505). Further development consists in a gradual differentiation, within the broad layer, of the various retinal elements and consequent demarcation of the layers which constitute the adult retina. The next layer to differentiate is the innermost layer of the retina, or layer of nerve fibers. This appears during the sixth or seventh week as a thin, clear, faintly striated zone containing a few scattered nuclei. What remains of the original inner layer of the cup has now become a comparatively thick layer with numerous chromatic and actively dividing nuclei. It may be conveniently designated the primitive nuclear layer. Surface epithelium of eyelid Eyelid (upper) Chorioid , 4 Pigmented layer Corneal epithelium Gf rating y' Conjunctival Split between epithelium retinal layers Substantia Retina, except propria cornee pigmented layer Lens Vitreous Tunica vasculosa Jentis Nerve fiber layer of retina Anterior epithe- lium of lens Be TIN Conjunctival sac He i liege Be SS AN wor Cee NES Wieene Hyaloid arzery Central artery of retina Optic nerve Fic. 505.—Horizontal section through eye of human embryo of 13-14 weeks. Modified from Lange. The similarity in development between the retina and wall of the neural tube is to be noted. Thus the layer of nerve fibers appears to correspond quite closely to the marginal layer of the central nervous system, while the primitive nuclear layer is probably homologous with the mantle layer (pp. 486, 492). There is a similar correspondence between the retina and the central nervous system in regard to their early cellular development, the retinal cells early showing a differentiation into neuroblasts and spongiobiasts (pp. 486, 492). About the end of the eighth week the inner part of the primitive nuclear layer differentiates into the layer of ganglion cells (Fig. 506, h). These are large cells and with their processes constitute the chird or proximal optic neurone. They can be first distinguished in the fundus of the cup and gradu- ally extend to the ora serrata. They are the first of the cellular elements of the adult retina which can be definitely recognized as such. From each cell, two kinds of processes develop, dendrites, which ramify in this and in the more external layers of the retina, and an axone which grows toward the cavity of the eye and becomes a fiber of the layer of nerve fibers, whence it continues into 582 TEXT-BOOK OF EMBRYOLOGY. . the optic stalk as one of the fibers of the optic nerve. The layer of ganglion cells is thickest in an area situated somewhat lateral to the attachment of the optic stalk and known as the area centralis. It is distinguishable about the end of the fourth month. In the center of the area centralis the retinal layers become thin to form the fovea centralis which develops toward the end of foetal life. The macula lutea with its yellow pigment does not develop until after birth. The retina at this stage thus consists of four layers which from within out- ward are (r) the layer of nerve fibers, (2) the layer of ganglion cells, (3) the nuclear layer, (4) the pigmented layer (see Fig. 507). Z 2 Vet 2 k a Vy \e d ¢ s if ke @\ ex Cc tf a z A Fic. 506.—Diagram of the development of the retinal cells. Kallius, after Cajal, a, Cone cells in unipolar stage; 6, cone cells in bipolar stage; c, rod celJs in unipolar stage; d, rod cells in bipolar stage; e, bipolar cells; f and 7, amacrine cells; g, horizontal cell; #, ganglion cells; k, Muller’s cells or fibers; /, external limiting membrane. The further development of the retina consists largely of a differentiation of the cells of the nuclear layer. This is extremely complex and our knowledge of it meager. From the cells of this layer develop (1) the rod and cone cells, (2) the bipolar cells, (3) the tangential or horizontal cells, (4) the amacrine cells, (5) Miiller’s cells or fibers. The differentiation of these cells and their processes also results in the demarcation of the following layers of the adult retina; (r) the layer of rods and cones, (2) the outer limiting membrane, (3) the outer nuclear layer, (4) the outer molecular layer, (<) the inner nuclear layer, (6) the inner molecular layer, (7) the inner limiting membrane (see Fig. 508). Miiller’s cells or the sustentacular cells (Fig. 506, 2) develop from spongio- blasts which lie toward the inner limit of the nuclear layer. This accounts for the location of the nucleated portions of Miiller’s cells. Processes of these cells grow toward both surfaces of the retina until they reach the positions of the future outer and inner limiting membranes where they are believed to spread out THE ORGANS OF SPECIAL SENSE. 583 horizontally and unite to form these membranes. Other spongioblasts develop into other types of glia cells, mainly spider cells, which are most numerous in the layer of ganglion cells and in the layer of nerve fibers. The rod and cone cells are first recognizable as unipolar cells (F ig. 506,¢, c). Thé single process of each extends outward as far as the outer limiting mem- brane. About as soon as these cells are recognizable, a differentiation between the rod cells and the cone cells can be made by-their reactions to the Golgi silver stain, the cone cells impregnating much more completely than the rod cells. Processes next grow out from the inner ends of the cells so that they become bipolar (Fig. 506, 8,2). Both rod and cone cells are at first distributed throughout the entire nuclear layer, but later they become arranged in a dis- tinct layer just beneath the outer limiting membrane. Each cell next gives rise to or acquires at its outer end an expansion which extends through Layer of nerve fibers Layer of nerve cells Inner molecular layer Inner nuclear layer Outer undifferentiated layer Fic. 507.—Vertical section through retina of a four months’ human embryo. Modified from Lange. the outer limiting membrane into the pigmented layer. As the pigmented cells give off pigmented processes which extend inward among the outer ends of the rods and cones, the layer of retina just beneath -the pig- mented layer consists of the outer ends of the rod cells, the tips of the cone cells, and the extensions of the pigmented cells. The nucleated portions of the rod and cone cells form the outer nuclear layer. ‘Though the layer of rods and cones and the outer nuclear layer present the appearance in hematoxylin- eosin stained specimens of two distinct layers, it is evident from their develop- ment and structure that they should be regarded as a single neuro-epithelial layer. The apparent separation into two layers is due to the interposition of the outer limiting membrane, through tiny holes in which the rod and cone-cells extend. The inwardly directed processes of the rod and cone cells are their axones. ‘These cells constitute the first or distal optic neurone. The bipolar cells (Fig. 506, e), which with their processes constitute the middle or second optic neurone, also develop from cells of the nuclear layer 584 TEXT-BOOK OF EMBRYOLOGY. and are probably bipolar at the time that the rod and cone cells are in the. unipolar condition. Reference fo the two bipolar cells shown in Fig. 566, e, e, shows that at this stage in their development their outwardly directed processes extend to the outer limiting membrane. These processes must either actually shorten or else fail to grow in length proportionately as the retina increases in thickness, for in the mature retina they end in relation with the centrally (inwardly) directed processes (axones) of the rod and cone cells. According as they are in relation with rod cells or cone cells, they are known as rod bipolars or cone bipolars. The retinal layer in which the axones of the rod and cone Inner limiting membrane Layer of nerve fibers Layer of nerve cells Inner molecular layer § horizontal cells bipolar cells amacrine cells Inner nuclear tayer Outer molecular layer Outer nuclear layer Outer limiting membrane Layer of rods and cones Layer of pigmented epithelium Fic. 508.—Vertical section through retina of a five and one-half months’ human embryo, Modified from Lange. cells and the dendrites of the rod and cone bipolars intermingle is the ouder molecular layer of the adult retina. Itis first distinctly recognizable as a mo- lecular layer about the end of the fifth month (Fig. 508). The development of the outer molecular layer separates the originally single nuclear layer into two layers, an outer composed of the nuclei of the rod and cone cells and an inner composed of the nucleated bodies of the rod and cone bipolars, of the horizontal cells (Fig. 506, g) and of the amacrine cells (Fig. 506, j and 7), all of which can- be recognized in Golgi specimens by the end of the seventh month. The rod and cone bipolars and probably most of the otuer cells of the inner nuclear layer send their axones centrally to lie in contact with the dendrites and bodies of the ganglion cells. THE ORGANS OF SPECIAL SENSE. 585 With the development of the cells of the inner nuclear layer and their proc- esses, there differentiates the inner molecular layer which separates the inner nuclear layer and the layer of ganglion cells. It consists mainly of ramifica- tions of the dendrites and axones of cells the bodies of which lie in the inner nuclear layer and in the layer of ganglion cells. (Fig. 508.) The Chorioid and Sclera.—These develop wholly from the mesoderm. The way in which the mesoderm grows in between the lens and the surface and surrounds the optic cup has been described (p. 576). That part of the meso- derm lying immediately external to the retina develops very early a close- meshed capillary network. This appears before there is any definitely limited sclera and may be considered the anlage of the chorioid. Somewhat later the mesoderm which lies just to the outside of the chorioid takes definite shape as the external fibrous tunic of the eye or sclera. The Vitreous.—The manner in which the vitreous humor is formed has been the subject of much controversy and remains still undetermined. As already noted in describing the development of the lens (p. 595), the latter is at first in direct contact with the inner layer of the retina (Fig. 504). Thelens and the retina separate as the vitreous forms between them. During the develop- ment of the lens the arteria centralis retin does not stop, as in the adult, with its retinal branches, but continues across the optic cup as the hyaloid artery to end in the vessels of the tunica vasculosa lentis. Some investigators consider the vitreous a transudate from these blood vessels. As the chorioidal fissure closes, some mesodermic tissue is enclosed with the artery, and some investigators consider the vitreous a derivative of this mesoderm. In Birds the formation of the vitreous humor begins before either mesoderm or blood vessels have penetrated the optic cup, and Rabl suggests-that the vitreous may be a secretion of the retinal cells. Bonnet describes a double origin of the vitreous, differentiating between a retinal vitreous and a mesoderm vitreous. According to Bonnet, the primary vitreous body begins its formation before the closure of the chorioidal fissure. This primary vitreous appears at the time of formation of the optic cup, is a fibrillated secretion of the retinal cells, and fills in the vitreous space with a feltwork of fine fibrils. With the formation of the optic cup and the closure of the chorioidal fissure this type of vitreous forma- tion ceases and a secondary vitreous body formation takes place from the cells of the pars ciliaris retin. This is also fibrillated and there develops at this time the so-called hyaloid membrane which closely invests the vitreous. Among the fibers of the vitreous body appears the vitreous humor. Up to this point the vitreous is entirely non-cellular. There next grow into it mesodermal cells which have reached the vitreous through the chorioidal fissure along with the hyaloid artery. To what extent these cells are used up in the formation of the blood vessels of the vitreous and to what extent they remain as connective tissue 586 TEXT-BOOK OF EMBRYOLOGY. cells of the mature vitreous after the blood vessels have degenerated is not known. As already noted, the vitreous is at first crossed by the hyaloid artery which supplies the developing lens (p. 579). As lens formation becomes less active the artery becomes less important and by the end of the third month begins to atrophy. At birth nothing remains of it, but in its former course the vitreous is somewhat more fluid than elsewhere and this is known as the hyaloid canal (canal of Cloquet). The Optic Nerve.—Referring to the description of the optic evagination it will be recalled that the optic vesicle maintains its connection with the brain by means of the optic stalk (p. 574). The latter is hollow and connects the cavity of the optic vesicle with the cavity of the brain. When the invagination of the optic vesicle to form the optic cup occurs (p. 576, Fig. 502), the invagination is carried along the posterior surface of the optic stalk toward the brain, and just ‘as the invagination of the optic vesicle results in the obliteration of the cavity of the vesicle, so the invagination of the optic stalk results in an oblitera- tion of its lumen. In Mammals the invagination of the optic stalk extends only part way to the brain, to the point where the artery enters. The chorioidal fissure closes about the seventh week. The optic stalk consists of supportive elements only, and serves as a track along which nerve fibers extend to connect the retina and brain. Nerve fibers appear in the optic stalk about the fifth week. They appear first around the periphery and apparently crowd the neuroglia nuclei toward the center, so that the stalk at this stage may be said to consist of a mantle layer and a marginal layer, apparently analogous to these layers in the retina and brain. The nerve fibers gradually invade the entire stalk so that by the end of the third month the stalk has become transformed into the optic nerve among the fibers of which the original supportive elements of the stalk are still represented by neuroglia cells. Much difference of opinion has existed in regard to the origin of the optic nerve fibers, whether they are processes of retinal cells which end in the brain or processes of brain cells which end in the retina. It is now quite generally accepted that most of the fibers of the optic nerve are the axones of neurones the cell bodies of which are situated in the ganglion cell layer of the retina. These axones pass centrally into the layer of nerve fibers, which they form, and con- verge toward the optic nerve. Through the latter they pass to their terminations in the external geniculate bodies, optic thalami and anterior corpora quadri- gemina. According to Cajal and others, some centrifugal fibers are present in the optic nerve. These are processes of cells situated in the above-mentioned nuclei, and terminate in the retina. They are fewer in number and of later development than the centripetal fibers. As the mesodermic anlagen of the chorioid and sclera are present before THE ORGANS OF SPECIAL SENSE. 587 the nerve fibers begin to grow into the optic stalk, the fibers must pass through these two coats in their exit from the eye. ‘There results the fenestrated cross- ing of the optic nerve by these two coats, known as the lamina cribrosa. The optic nerve fibers are medullated but have no neurilemmz. They are supported by neuroglia. ‘The connective tissue sheaths which enclose the optic nerve are direct extensions of the meninges. These structural peculiarities accord with the peculiarities already described in the development of the nerve. Attention has been called to the fact (p. 573) that just as the retina should be considered a modified and displaced portion of the central nervous system—of brain cortex—so the optic nerve should be considered not as a peripheral nerve, but as analogous to a central nervous system fiber tract. The Ciliary Body, Iris, Cornea, Anterior Chamber.—Anteriorly where they come into relation with the lens and are so arranged as to admit light to the retina, all three coats of the eye are extensively modified. Thus the retina is continued anteriorly as the pars ciliaris retine and pars iridica retine, the chorioid as the stroma of the ciliary body and iris, the sclera as the cornea. THE Crtrary Bopy Anp Irts.—Both primary retinal layers (the two layers of the optic cup) are continued anteriorly as the non-nervous retinal layer of the ciliary body and iris. The outer pigmented layer consists at first of several layers of pigmented cells, but later becomes reduced to a single layer of pigmented cells which do not, however, possess pigmented processes extend- ing inward as do the analogous cells of the nervous retina. The abrupt tran- sition at the ora serrata where the thick pars optica retinz passes over into the pars ciliaris retinze has been mentioned (p. 580). The inner layer of the primi- tive retina (optic cup) extends over the ciliary body and iris as a single layer of cells. These remain non-pigmented over the ciliary body, but over the iris acquire pigment so that the two layers form the pigmented layer of the iris. The mesodermic tissue which forms the stroma of the ciliary body and iris is derived from the mesoderm lying between the lens and the surface ectoderm. This separates into two layers enclosing between them the anterior chamber of the eye, and it is from the posterior of these two layers that mesodermic tissue extends into the ciliary body and iris. It is continuous with the mesoderm of the tunica vasculosa lentis. During the fourth month the ciliary body under- goes foldings to form the ciliary processes. These foldings at first involve also the iris, but the iris folds soon (end of fifth month) disappear, while the ciliary processes become more prominent. Of the smooth muscle tissue found in the ciliary body and iris, the dilator and contractor pupille are, according to Bonnet, derived from the cells of the pigmented layer of the retina, i.e., from ectoderm. ‘The ciliary muscle, on the other hand, develops from mesoderm. These muscles become well developed during the seventh month. 588 TEXT-BOOK OF EMBRYOLOGY. The suspensory ligament of the lens, or zonula Zinnii, first appears about the end of the fourth month. By some the fibers of the suspensory ligament are believed to differentiate from the vitreous, by others they are considered as derived from the pars ciliaris retine. Spaces among the fibers of the ligament enlarge and coalesce to form the canal of Petit. Tue CorNEA.—The way in which the mesoderm grows in between the lens vesicle and the surface ectoderm has been described (p. 576). This mesoderm forms a thin almost homogeneous layer containing very few cells. Later that part of the layer which lies against the lens becomes more cellular and vascular, so that it is possible to distinguish between an outer homogeneous non-vascular layer and an inner cellular vascular layer. The former is the anlage of the cornea. Between the two layers vacuoles appear and coalesce to form the anterior chamber of the eye or cavity of the aqueous humor. Subsequent growth of the iris subdivides this chamber into an anterior and a posterior portion. The chamber separates the cornea from the pupillary membrane portion of the tunica vasculosa lentis. Bounding the chamber anteriorly and so forming the posterior layer of the cornea there develops a single layer of flat cells, the so-called “endothelium” of Descemet. Over the surface of the cornea the ectoderm remains and gives rise to a stratified squamous epithelium four to eight cells thick, the anterior corneal epithelium. Just beneath the epithelium a layer of corneal tissue retains its original homogeneous character and forms the anterior elastic membrane or membrane of Bowman. The posterior elastic membrane or membrane of Descemet is usually considered a cuticular derivative of the “‘endothelium.”’ Throughout the rest of the cornea —substantia propria cornee—cells develop, either by proliferation of the few cells originally present or from cells which grow in from the surrounding cellular mesoderm, and become arranged parallel to the surface as the fixed connective cells of the cornea. The Eyelids.—After the lens vesicle becomes separated from the surface ectoderm, the latter folds over above and below to form the first rudiments of the upper and lower eyelids. Each fold consists of a core of mesoderm and a covering of ectoderm. From the mesoderm develop the connective tissue elements of the lids including the tarsal cartilage. From the ectoderm develop the epithelial structures of the lids, the epidermis, the eyelashes and the glands. The edges of the lids gradually approach each other and about the beginning of the third month the epithelium of the upper lid becomes adherent to that of the lower, thus completely shutting in the eyeball. This condition obtains until just before birth. The eyelashes develop in the same manner as other hairs (p. 447). The Meibomian glands, glands of Moll and the lacrymal glands develop, during the period the lids are adherent, as solid cords of ectoderm which grow THE ORGANS OF SPECIAL SENSE. 589 into the underlying mesoderm where they ramify to form the ducts and tubules. The anlagen of the ducts and tubules of these glands are thus at first solid cords of cells, their lumina being formed later by a breaking down of the central cells of the cords. At the inner angle of the conjunctiva there develops beneath the eyelid folds a third much smaller fold. This becomes the plica semilunaris which in man is a rudimentary structure, but in many of the lower Vertebrates, especially Birds, forms a distinct third eyelid, the so-called nictitating mem- brane. A few hair follicles and sebaceous glands develop in a portion of this fold forming the lacrymal caruncle. The Lacrymal Duct. At a certain stage in development, a groove bounded by the maxillary process and the lateral nasal process extends from the eye to the nose (Fig. 136). This is known as the naso-optic furrow. The ectoderm (epithelium) lying along the bottom of this groove thickens about the sixth week and forms a solid cord of cells. As development proceeds.and the parts close in, this cord of ectoderm becomes enclosed within the mesoderm,.excepting at its ends where it remains connected with the surface ectoderm of the eye and nose, respectively. By a breaking down of the central cells of this cord a lumen is formed and the cord becomes a tube, the lacrymal duct. The primary con- nection of the lacryma! duct is with the upper lid, but while the lumen is being formed an offshoot grows out to the under eyelid to form the inferior branch of the lacrymal duct. THE NOSE. The anlage of the organ of smell is apparent in human embryos of about three weeks as two thickenings of the ectoderm, one on each side of the naso- frontal process. To these thickenings the term olfactory placodes has been applied (Kupffer). A little later (in embryos of about four weeks), the placodes become depressed below the surface, the depressions themselves being the nasal pits or fosse (see p. 152; also Fig. 123). The placodes, which are destined to give rise to the sensory epithelium, thus come into closer relation with the olfactory lobes of the brain (rhinencephalon) which represent out- growths of the fore-brain (telencephalon) (see p. 508). ~ As described in connection with the development of the face, the lateral nasal process arises on the lateral side, the medial nasal process on the medial side, of each nasal pit (p. 152 e¢ seq.; also Fig. 134). Of these processes, the lateral is destined to give rise to the lateral nasal wall and the wing of the nose, the medial to a part of the nasal septum (see p. I 32). As development pro- ceeds, the epithelium (ectoderm) of the nasal fosse grows still deeper into the subjacent mesoderm, the fosse thus becoming converted into the nasal sacs, which lie above the oral cavity. According to Hochstetter and Peter, the 590 TEXT-BOOK OF EMBRYOLOGY. nasal sacs are not at first in communication with the oral cavity, but lie above, and are separated from it by a plate of tissue which gradually becomes thinned out along the deeper part of the sacs to form the bucco-nasal membrane (Hoch- stetter). Later (in embryos of 15 mm.), the bucco-nasal membrane ruptures and the deep ends of the sacs thus come to open into the mouth cavity, the openings being known as the primitive choanen. In front of the primitive choanen, the nasal passages (formerly the nasal sacs) are separated from the mouth cavity by a plate of tissue, known as the primitive palate (Fig. 5009). The latter is produced by the fusion of the maxillary process with the lateral and medial nasal processes (see p. 152), the outer nares thus being somewhat separated from the border of the mouth. The further separation of the nasal passages from the oval cavity has been described in connection with the development of the mouth (p. 318) and the Lateral nasal process Outer nasal opening Maxillary process Eye Primitive choanen Palatine process Fic. 509.—From a model of the anterior part of the head of a 15 mm. humanembrvo. The lower jaws (mandibular processes) have been removed. Peter. aevelopment of the palatine processes of the maxilla. It may be repeated briefly, however, that from each maxillary process a horizontal extension grows across between the oral and nasal cavities until it meets and fuses with its fellow of the opposite side and with the nasal septum in the medial line, thus forming the palate which is continuous with the primitive palate mentioned above. (See Figs. 178 and 510.) In this way the nasal cavities or chambers become separated from the oral cavity, but remain in communication with the pharyn- geal cavity through the posterior nares. The nasal cavities increase enormously in size and the epithelial surface in extent, owing to (1) the formation of the palate alluded to above, (2) the develop- ment of the nasal conche which has been described on page 196, and (3) the development of accessory cavities—maxillary, frontal and sphenoidal sinuses, which represent evaginations, so to speak, from the nasal cavities. Probably correlated with the above-mentioned increase in extent of the nasal chambers is the fact that in lung-breathing Vertebrates the chambers THE ORGANS OF SPECIAL SENSE. 591 -have acquired.a.secondary function. In these forms the nose is not only an apparatus for receiving olfactory stimuli, but also serves to convey air to and from the lungs; it is in a sense a respiratory atrium. The sensory epithelium which the olfactory nerves supply is limited to relatively small areas in the supe- rior conch and nasal septum, ‘Stratified columnar ciliated epithelium lines all other parts of the cavities. ° _. Studies on the development of the olfactory nerve shave led to diverse opinions, but the investigations of His and Disse go to show that the fibers are processes of cells derived from the thickened ectoderm or olfactory placodes. In human embryos of about four weeks some of the cells i in.the upper part of the nasal fossa become modified to form the neuro- -epithelium. From the Nasal septum Jacobson’s organ Inferior concha Nasal cavity Jacobson’'s cartilage Palatine process , Oral cavity Fic. 510—From a section through the head of a human embryo of 28 mm., showing the nasal ferns: the nasal cavities, the oral cavity, and the palatine processes. Peter. peripheral pole of each cell a short slender process grows out to the surface a the epithelium. From the opposite pole a:slender process (the axone) grows centrally until it penetrates the olfactory lobe, where it ends in contact with the dendrites of the first central neurone of the olfactory tract. Most of these cells remain in the epithelial layer, but a few wander into the subjacent mesoderm and become bipolar cells which resemble the bipolar cells of the embryonic nets root ganglia (p. 509). Other epithelial cells of the nasal fossa are ‘converted into the sustentacular cells of the olfactory areas. ; Jacobson’s organ arises at the beginning of the third month as a Se out- "pocketing of the’ epithelium: on the lower anterior part of the nasal septum & 1g. 510). ‘This: evagination grows backward as a slender sac along the nasal "septum for a distance of several millimeters: and ends blindly. In the adult the sac degenerates and often disappears. In some of the lower Mammals 38 592 TEXT-BOOK OF EMBRYOLOGY. 2 Jacobson’s organ develops to a greater degree, and some of the epithelial cells send out processes which pass to the olfactory lobes. THE EAR. The ear of higher Vertebrates consists of three parts—the internal, middle, and external. Of these, the internal is the sensory portion proper ‘and, so far as the epithelial elements are concerned, is of ectodermal origin, but secondarily becomes embedded in the subjacent mesoderm. It constitutes a complicated and highly specialized structure for the reception of certain stimuli that are to be conveyed to the central nervous system. From a functional standpoint it may be divided into the portion composed of the semicircular canals and their ap- pendages, which is concerned in receiving and transmitting stimuli destined Fic. 511.—Hialf of a transverse section through the region of the developing ear of a sheep embryo of 13 mm. ° Béttcher. Aud. ves., Auditory vesicle; Co. gang., cochlear ganglion; End. ap., endolymphatic appendage; Rh.br., rhombic brain. for the static and equilibration centers in the central nervous system, and the cochlear portion, which is concerned in receiving and transmitting auditory stimuli. The middle and outer ear represent modified portions of the most cranial of the branchial arches and grooves, and constitute an apparatus for conducting sound waves to the cochlear portion of the inner ear. The Inner Ear.—In embryos of 2 to 4 mm., the ectoderm becomes some- what thickened over a small area lateral to the still open neural groove in the region of the future hind-brain. This thickening is often spoken of as the auditory placode (see p. 506). Owing to more rapid growth of the cells in the deeper layers of the placode, it soon becomes converted into a cup-shaped depression which is known as the auditory pit. The edges of the pit fold in and fuse and the pit thus becomes the auditory vesicle (otocyst), which finally becomes constricted from the parent ectoderm and lies free in the sub- jacent mesoderm (Fig. 511). THE ORGANS OF SPECIAL SENSE. 593 At this stage (embryos of 4 to 5 mm.) the auditory vesicle is an oval or spherical sac the wall of which consists of two or three layers of undifferen- tiated epithelial cells. It lies against the neural tube and is connected with the latter by the acoustic ganglion (Fig. 512, a). About the same time an evagina- _tion appears on the dorsal side of the auditory vesicle, forming the anlage of the endolymphatic appendage (Fig. 512, a, b, c). The evagination continues to elongate and comes to form a club-shaped structure, the distal end of which becomes flattened to form the endolymphatic sac, the narrower proximal portion constituting the endolymphatic duct (Fig. 512 a-n). The epithelium, which at first consisted of two or three layers of cells, becomes reduced to a single layer. In the chick the endolymphatic appendage is formed out of the original union between the ectoderm and the auditory vesicle (Keibel, Krause). In Reptiles and Amphibia (Peter, Krause) and in man (Streeter), on the other hand, this appendage develops independently of the union, appearing on the dorsal side of the seam of closure in the auditory vesicle. In embryos of about 6 mm. the auditory vesicle (apart from the endolymph- atic appendage) becomes differentiated into two portions or pouches—a bulging, triangular one above, which is connected with the endolymphatic appendage, and a more flattened one below. The former is the vestibular pouch, the latter the cochlear pouch (Fig. 512, bf). Between the two is a portion of the vesicle which is destined to give rise to the saccule and utricle, and which may be called the atrium (Streeter). Properly speaking, the atrium is a division of the vestibular pouch. The cochlear pouch is phylogenetically a secondary diver- ticulum which develops from the atrium, appearing first in the lowest land- inhabiting Vertebrates (Amphibia). As mentioned above, the vestibular pouch early assumes the form of a triangle, with the apex toward the endolymphatic appendage. The three borders of the triangle form the anlagen of the semicircular canals and bear the same interrelation as the latter. At the same time a vertical groove (the lateral groove) appears between the anlage of the posterior canal and the posterior end of the lateral canal (Fig. 512, 0, d). The formation of the semicircular canals is shown in Fig. 512, g-k. The edges of the triangular vestibular pouch expand and become more or less crescentic in shape. The two walls in the concavity of each crescent come together and then break away (Fig. 512, g, j, absorp. focus), thus leaving the rim of the crescent as a canal attached at its two ends to the utricle. The breaking away affects first the superior, then the posterior, and finally the lateral canal. During these gross changes the epithelium becomes reduced to a single layer of cells. 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