TEXT-BOOK OF THE EMI RYOLOGY OF MAN AND MAMMALS TEXT-BOOK OF THE EMBRYOLOGY or MAN AND MAMMALS BY DE. OSCAE HEETWIG Professor extraordinarlus of Anatomy and Comparative Anatomy, Director of the IL Anatomical Institute or the University of Berlin TRANSLATED FROM THE THIRD GERMAN EDITION BY EDWARD L. MARK, Pn.D Hcrsey Professor of Anatomy in Harcard University SStitlr 339 um in tire &e*t ml) 2 : SWAN SONNENSCHEIN & CO., LIM. NEW :YOft£: ' 'MS jrACMILL^N CU "1001 ;,,;;" ht!:lik':fcj v-j-v-:; FJBST EDITION, October 1892. SECOND EDITION, January 1898. THIRD EDITION, March 1901. 4)1959 TEANSLATOE'S PEEFACE. THE rapidly increasing recognition of the importance of Embryology in all morphological studies makes it desirable that the most valuable text-books upon the subject, in whatever language, be made available for those who are beginning its study. Although the English-reading student already has at command a number of text-books upon this subject, it is evident to any one familiar with HERTWIG'S Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere that this work covers the field of Vertebrate Embryology in a more complete and satisfactory way than any book heretofore published in English. Two important objects to be accomplished in a text-book are : first, a clear and methodical exposition of the well-established facts of the science; and, secondly, such a presentation of unsettled questions as shall stimulate the reader to further inquiry and re- search. I believe it is far too common for the second of these aims to be overlooked. The present work fulfils both requirements in an eminent degree, and in its historical surveys exhibits an exceptional fairness of treatment, notwithstanding the author has been one of the foremost contestants in several of the fields reviewed. The summaries which follow the discussions of the several topics serve a useful purpose in directing attention to the more important conclu- sions drawn from each subject. I have aimed to give a clear and accurate reproduction of the author's ideas ; while I have endeavored — not always successfully — to avoid awkward renderings and German idioms, I have preferred to err on the side of a too literal rather than a too liberal translation. There are a few points that demand a brief explanation. The German word Anlage has heretofore been variously rendered into English by rudiment, origin, beginning, basis, foundation, etc., while some writers, recognising the inadequacy of any of these words to express the idea, have incorporated the German word itself in their English. The Anlage of a structure is its beginning or its undifferentiated state — the object in a simple condition which is destined to be followed by a more complicated one. The use of rudiment in this sense is undesirable, because, in the interest of scientific accuracy, it is important to restrict its meaning, as in German, to a structure which is not destined to become more complicated, but which may have been, either ontogeiietically or phylogenetically, even more highly developed than it now is. Origin and beginning are abstract terms, whereas Anlage is more frequently used in the concrete; basis and foundation (Grundlaye) convey a wrong impression — that of the sub- stratum upon which the structure is erected. The need of a new word, which shall be used in the sense of Anlage, is evident. I suggest the adoption of an already existing word,— fundament, — used at present only in a sense with which the proposed usage will not produce confusion. This word has been uniformly employed in the present translation, and the reader will see how readily and naturally it lends itself to this use. Fundament would thus bear the same relation to foundation that Anlage does to Grundlage. I have also departed from authorised usage by sometimes employ- ing for Bindegewebe and Stiitzgewebe the term sustentative (in a mechanical sense) tissue, instead of connective tissue. My reason for this is the narrower meaning of connective as compared with sustentative. In deference to a custom still followed in Human Anatomy, the author, in describing the relative positions of parts, has very generally used anterior and posterior for dorsal and ventral, etc. Instead of converting these expressions into terms which are independent of the temporary position of the organism, as I should have preferred, it has seemed better to indicate the direction by a bracketed word in those cases where a misunderstanding was most likely to occur. It has of course not been necessary to repeat this after each term of direction, but only after the first one of a series, the reader's atten- tion being thus sufficiently directed to the matter to prevent any misconception. The rapid advances in Embryology make it impossible for a book two years old to be a faithful reflection of the science of to-day in all its branches ; there are some topics in which even radical changes must be recognised. I have thought best, however, to reproduce the book as it left the hands of its author, and to content myself with calling the reader's attention to some of the topics in which the most important advances h,i\r U>en made, such as the metamerism of the li<>;ul, and the plan and metamorphoses of the vessels of the visceral arches. TRANSLATORS PREFACE. vii I am under very great obligations to my colleague, Dr. C. B. Davenport, for kind assistance and valuable criticism, but for which many defects of the translation would have been overlooked. I am also indebted to Drs. T. G. Lee, H. B. Ward, and W. McM. Wood- worth for aid in reading portions of the proof. a. L. MARK. CAMBRIDGE, MASS. AUTHOR'S PREFACE TO THE FIRST EDITION. s* Die Entwickelungsgeschichte 1st der wahre Lichttrager f iir Untersuchungen iiber organische Korper." — C. E. v BAEB, "Ueber Entwickelungsgeschichte der Thiere " (Bd. L, 8. 231). THE Embryology of Animals, although one of the youngest shoots of morphological research, has, nevertheless, grown up in the course of sixty years, along with the cell-doctrine and that of the tissues, to a vigorous and stately tree. The comprehension of the structure of organisms has been extended in a high degree by numerous develop- mental investigations. The study of the human body has also derived great advantage from the same. In the newer anatomical text- books (GEGENBAUR, SCHWALBE) Embryology is receiving more and more attention in the description of the separate systems of organs. To what extent many things may be more clearly and attractively described in this manner is best shown by a comparison of the des- criptions of brain, eye, heart, etc., in the older and the more recent anatomical text-books. Although it is generally recognised that Embryology constitutes " a foundation-stone of our comprehension of organic forms/' neverthe- less the attention which its importance warrants is not yet given to it ; it is especially true that it has not become as extensively as it should be a component of well-rounded medical and natural-history instruction, to which it is indispensable. The cause of this is perhaps in part to be sought in the fact that in student-circles the study of Embryology is often held to be especially difficult and a comprehension of it to be laborious. And thus many do not venture into this apparently obscure realm. But ought the development of an organism to be really more difficult to comprehend than the complicated finished structure ? To a certain extent this was the ca>e at a time when the most divergent and contradictory opinions prevailed concerning many of the most important processes of development, such as the formation of the germ-layers, the protovertebrse, etc., which the lecturer had to AUTHORS PREFACE TO THE FIRST EDITIOX. IK take into account, and when many processes were not vet understood in their essence and their significance. But, thanks to the results of Comparative Embryology, the number of the unintelligible processes has been every year diminished, and in the same ratio the study of Embryology even for the beginner has been rendered easier. At least, it is not in any way an essential feature of the process of development that it should be more difficult to understand than the structure of the completed form. For every development begins with a very simple condition, from' which the more complicated is gradually derived and by which it is explained. Inasmuch as I have for twelve years pursued the study of Embry- ology with especial interest, both in annually recurring academic lectures and in a series of scientific investigations, the desire has been awakened in me to acquire for Embryology a broader and more secure foundation in education, and to procure for it admission into larger circles of medical men and well-educated naturalists. As the result of this there has come into existence the book which is before us, in which the especial problem has been to make the complicated structure of the human body more intelligible through the knowledge of its development. For the solution of this problem I have in the present text-book placed the comparative method of investigation in the foreground. I do not thereby find myself in any way in opposition to another direction of embryological research, which places the objective point in the physiological or mechanical explanation of the form of the animal body. Such a direction I hold to be fully warranted, and I believe that, instead of being opposed to a comparative-morphological direction, it can be of the most permanent value to it in the solution of its problems. One will find that I have here given fall attention to the mechanico-physiological explanation of forms. Compare the sections on cell-division and Chapter IV., " General Discussion of the Principles of Development," in which the laws of unlike growth and the processes of the formation of folds and evaginations are treated. In the presentation of the separate processes of development, in the main the important things only have been selected, the sub- sidiary left out, in order thus to make the introduction into embryological study easier. In the case of fundamental theories I have gone into their history extensively, because it is of great interest, and under certain circumstances operates as a stimulus, for one to see in what way the state of a scientific question for the- time being has been attained. In pending controversial questions. 2 AUTHORS PREFACE TO THE FIRST EDITION. I have, it is true, employed chiefly as the foundation of my pre sentation the views which appear to me the most entitled to acceptance, but have not left unmentioned opposing conceptions. Numerous figures in the text, as well as some colored plates, will contribute materially to the easier comprehension of the various developmental processes. I submit, then, this text-book to physicians and to students of medicine and the natural sciences, with the desire that it may promote and facilitate the study of Embryology in wider circles, and that it may thereby contribute to a deeper insight into the structure of our own bodies. OSCAR HEBTWIQ. JENA, October 1888. AUTHOR'S PREFACE TO THE SECOND EDITION. THE friendly reception which the " Text-book of the Embryology of Man and Mammals " has found, is an indication of the increased interest which this branch of Morphology now meets with. Even more than a year ago, after the first part of the text-book appeared and while the second part was in the press, the necessity of preparing a second edition became evident. In this edition fundamental changes have not been undertaken ; the text has, however, undergone an expansion in some places, owing to the attention given to several works which have recently appeared. This has been the case with the section on the first developmental processes of the egg ^ WEISMANN, BLOCHMANN) ; that on the origin of the vascular system (RABL, RUCKERT) ; that on the development of the foatal membranes (DuvAL, OSBORN) ; and that on the human placenta (KASTSCHENKO, WALDEYER, HUGE). As the second part of the text-book has just appeared, it has been possible to incorporate it in the second edition without alteration. It has, furthermore, seemed to me expedient in the second edition to distribute at the ends of the several chapters the synopses of the literature, which in the first edition were brought together at the close of the whole work. Finally, there has been added an index of subjects, by which a more rapid orientation concerning the separate topics will be facilitated; this will increase the usefulness of the work. May the book in this form make for itself new friends, not only among students of medicine and the natural sciences, but also with all those who have a fondness for and a comprehension of studies in natural science. ObCAE HERTWIG. JENA, February 1888. AUTHOR'S PREFACE TO THE THIRD EDITION. IN the two yeais which have elapsed since the appearance of the second edition of this text-book, our knowledge of the embryology of Vertebrates has experienced many important enrichments, thanks to the numerous investigations which are annually published. There- fore, as the problem of preparing a third edition of the text-book confronted me, I was compelled to make extensive changes in many places. Thus the second and third chapters, concerning the processes of fertilisation and cleavage of the egg, have undergone expansion, owing to the presentation of the important discoveries which have been made on the the egg of Ascaris megalocephala. I have given an entirely new wording to the ninth chapter on the development of connective substance and blood, also to the sections on the origin of the urinary organs and the development of the peripheral nervous system, and, finally, to the account of the development of the heart and the venous system. Also at other places one will often recognise the hand of improvement. The third edition has been essentially improved by the addition of thirty new figures, which I have taken from the investigations of VAN BENEDEN, BOVERI, DUVAL, FLEJIMING, HERMANN, His, BORN, GEGENBAUR, NAGET,, VAN WIJHE, GRAF SPEE, BONNET, and KEIBEL. Through the friendliness of Professor VAN BENEDEN I was also put in a position to employ for my text-book three figures out of his hitherto unpublished extensive work on the development of the germinal layers of the Rabbit. By means of the increase in the number of figures I hope that I have been able to render still easier the comprehension of many of the processes of development. And so I close the preface to the third edition by expressing my thanks to all those who have rendered me friendly aid, and especially to the publisher, who in the further equipment of the text-book has met my wishes with the greatest willingness. OSCAR HERTWIG. BERLIN, March 1890. CONTENTS. PAGE INTRODUCTION 1 MANUALS AND TEXT-BOOKS . ' 4 PART FIRST. CHAPTER I. DESCRIPTION OP THE SEXUAL PRODUCTS 7 THE EGG-CELL . . 7 THE SEMINAL FILAMENTS 19 Historical 23 SUMMARY 27 CHAPTER II. THE PHENOMENA OF THE MATURATION OF THE EGG AND THE PROCESS OF FERTILISATION . . . . . . .30 THE PHENOMENA OP MATURATION 30 Historical .... 35 THE PROCESS OF FERTILISATION . , . . . . .37 Historical 45 SUMMARY . ... 46 CHAPTER III. THE PROCESS OF CLEAVAGE . 51 Historical 69 SUMMARY ... 72 CHAPTER IV. GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT 76 CHAPTER V. THE DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS (GASTR^EA-THEORY) 84 CHAPTER VI. THE DEVELOPMENT OF THE TWO MIDDLE GERM -LAYERS (CCELOM-THEORY) . . .106 SUMMARY 142 CHAPTER VII. HISTORY OF THE GERM-LAYER THEORY 145 CHAPTER VIII. DEVELOPMENT OF THE PRIMITIVE SEGMENTS . . . .161 SUMMARY 169 XIV CONTENTS. CHAPTER IX. PAGE DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD (THE PARABLAST- AND MESENCHYME-THEORIES) . . 170 Historical 189 SUMMARY 191 CHAPTER X. ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY . 194 SUMMABY 206 CHAPTER XI. THE FCETAL MEMBRANES OF REPTILES AND BIRDS . . .206 SUMMARY 220 CHAPTER XII. THE FCETAL MEMBRANES OF MAMMALS .221 SUMMARY . . . . . .238 CHAPTER XIII. THE FCETAL MEMBRANES OF MAN .... .241 (1) THE CHORION 248 (2) „ AMNION ... . ... 250 (3) „ YOLK-SAC . 251 (4) „ DECIDU^S . 252 (5) „ PLACENTA . 258 (6) „ UMBILICAL CORD 268 SUMMARY .... 272 PART SECOND. CHAPTER XIV. THE ORGANS OF THE INNER GERM-LAYER. THE ALIMENTARY TUBE WITH ITS APPENDED ORGANS 281 I. THE FORMATION OP THE MOUTH, THE THROAT-, GILL-, OR VISCERAL CLEFTS, AND THE ANUS 282 II. THE DIFFERENTIATION OF THE ALIMENTARY TUBE INTO SEPARATE REGIONS, AND FORMATION OF THE MESENTERIES 295 III. THE DEVELOPMENT OF THE SEPARATE ORGANS OF THE ALI- MENTARY TUBE 304 A. The Organs of the Oral Cavity : Tongue, Salivary Glands, and Teeth 304 B. The Organs arising from the Pharynx 313 (1) The Thymus .... . .314 (2) „ Thyroid Gland 317 (3) „ Lungs and Larynx 320 C. The Glands of the Small Intestine 324 (1) The Liver ... 324 (2) , Pancreas 332 SUMMARY . . 333 CONTENTS. XV CHAPTER XV. PAGE THE ORGANS OF THE MIDDLE GERM-LAYER . . . .341 I. THE DEVELOPMENT OF THE VOLUNTARY MUSCULATURE . . 342 A. The Primitive Segments of the Trunk 342 B. ,. Head-Segments 351 II. THE DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS . 353 («) The Pronephros and the Mesonephric Duct .... 353 (b) „ Mesonephros (Wolffian Body) . . . . . .359 (0) „ Metanephros (Kidney) . . . . , . .367 (d) „ Miillerian Duct 369 () ... M ,:. Membranous Ear-Capsule into the Bony Labyrinth and the Perilymphatic Spaces . 49S (0) ^ ,., ., Middle and External Ear . . 505 SUMMARY 510 C. The Development of the Organ of Smell . . - .511 SUMMARY 518 IIL THE DEVELOPMENT OF THE SKIN AND ITS ACCESSORY ORGANS 520 (a) The Skin .... 520 (&) ,. Hair 522 O) „ Nails 526 (d) „ Glands of the Skin . ..... 528 SUMMARY . 531 CHAPTER XVII. THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME 538 I. THE DEVELOPMENT OP THE BLOOD-VESSEL SYSTEM . . . 542 A. The first Developmental Conditions of the Vascular System . 542 (a) Of the Heart ..... ... 542 (J) Vitelline 'Circulation, All antoic and Placental Circulation 549 B. The further Development of the Vascular System up to the Mature Condition 553 (a) The Metamorphosis of the Tubular Heart into a Heart with Chambers 553 (&) The Development of the Pericardial Sac and the Dia- phragm 566 (c) Metamorphoses of the Arterial System . 570 (d) ... ,, ' „ Venous ,; . .. „ .577 SUMMARY 588 II. THE DEVELOPMENT OP THE SKELETON 503 A. The Development of the Axial Skeleton 593 (a) The Development of the Vertebral Column . . . 51 Hi (£) „ „ „ Head-Skeleton .... »;<»:; I. Bones of the Cranial Capsule 619 II. „ „ Visceral Skeleton 622 (c) Concerning the Relation of the Head-Skeleton to the Trunk-Skeleton 627 S. The Development of the Skeleton of the Extremities . . 635 (a) Pectoral and Pelvic Girdles 638 (&) Skeleton of the Free Extremity 640 (c) Development of the Joints 644 SUMMARY 647 APPENDIX TO LITERATURE . 658 INTRODUCTION. THE history of the development of the individual, or Ontogeny (Embryology), is the science of the growth of an organism ; it de- scribes the morphological changes which an organism passes through from its origin in the ovum up to its complete maturity, and presents these in their natural connection. We can regard the fertilisation of the egg-cell as the beginning of the process of development for Vertebrates, as it also is for all the rest of the higher animals. In giving an account of the changes of the egg-cell, which begin with fertilisation, one may choose between two different methods. According to one method a particular organism is made the basis of the account, and one describes the changes which its germ under- goes from the moment of fertilisation onward, from hour to hour, and from day to day. It is in this way that the embryology of the Chick has been worked out by C. E. VON BAER in his classical paper, and by FOSTER AND BALFOUR in their " Elements of Embryology." This method has the advantage that the reader acquires a view of the total condition of an organism in the separate stages of its development. A book of that kind is especially suitable for such persons as desire to acquaint themselves, by their own observation, with the embryology of a single animal, as, for example, the Chick, by repeating the investigations of others. It is, on the contrary, less adapted to those who wish to acquire a connected view of the development of the separate organs, as the eye, the heart, the brain, etc. For the formation of these will of course be treated of at different places in describing younger and older embryos. In order to procure a general survey of the course of development of an organ, the reader must consult various places in the text-book, and collect for himself what relates to the subject. For beginners, and for the needs of theoretical instruction ir Embryology, the second method commends itself, in which the separate organs are considered in succession, each for itself, and the changes which a single organ lias to pass through during development are 1 35 INTRODUCTION. set forth connectedly from beginning to end. It is in this way that KO'LLIKER'S " Embryology of Man and the Higher Animals" is written. The second method is, moreover, the only one applicable when the problem is to investigate in a comparative way the development of several organisms, and to fill up the gaps which exist in our know- ledge of one by that which we know concerning nearly related animals. But it is precisely in this position that we find ourselves when we wish to acquire a survey of the development of the human body. An account which should limit itself to that which we know about Man would exhibit numerous and extensive gaps. For up to the present the eye of man has not seen how the human ovum is fertilised, how it divides, how the germ-layers are formed, or how the establishment of the most important organs is effected. It is especially the period of the first three weeks, during which the greatest variety of fundamental processes of development take place,, concerning which we know next to nothing ; there is also little prospect that a change will soon occur in this regard. The time will therefore perhaps never come when a complete embryology of Man in the strict sense of the word will be possible. However, the existing gaps can be filled out in another mannerr and one which is entirely satisfactory. The study of the most widely differing Vertebrates teaches us that they are developed according to a common plan, that the first processes of development agree in all really important points, and that the differences which we encounter here and there are produced by causes of a subordi- nate kind, as, e.g., by the egg's possessing a greater or less amount of yolk. When we see that the establishment of the central nervous system, of the eye, of the spinal column, of the viscera, etc., takes place in Mammals on the whole just as it does in Amphibia, Birds, and Reptiles, the conclusion is near at hand, and justified, that Man also in his development is no excf pt;on to this general phenomenon. Thus in the study of Embryology we are naturally led to the com- parative method. What, owing to the nature of the difficulties, we cannot learn directly about the development of Man, we seek to deduce by the investigation of other Vertebrates. In earlier decennia the Hen's egg was the favorite object, and it is upon this that we possess the most numerous and most complete M'rit s of observations. During the last twenty years research lm> !>een directed to Mammals, — in the investigation of which tho tlillicultifs have to be surmounted, — :is well as to Kq, tiles, INTRODUCTION. 3 Amphibia, Fishes, etc. Only through the observation of such various objects has insight been acquired into many processes, which in their essence remained unintelligible to us from the study of the Chick alone. For it was thus that one first learned to distinguish the important from the accessory and unimportant, and to understand the laws of development in their generality. In this text-book, therefore, I shall not confine myself to a single object, such as the egg of the Hen or the Rabbit, but from more general comparative standpoints shall endeavour to present what, through extensive series of investigations, we have thus far recognised as the rule in regard to the real nature of the processes of fertilisa- tion and cleavage, the formation of the germ layers, etc. However, let no one expect a text-book of comparative Embryo- logy. The purpose and the problem is first of all to learn to com- prehend the development and the structure of the human body. What we know about that has been placed before everything else, and the embryology of the remaining Vertebrates has been cited, and, as it were, fully utilised, only in so far as was necessary for the purpose indicated. In the division of the embryological material proposed by us, ac- cording to the separate systems of organs, there is a long series of processes, with which the development begins, which do not permit of an arrangement, because at the beginning the fundaments of definite, afterwards differentiated organs, are not recognisable in the germ. Before there is any formation of organs, the egg is divided into numerous cells, and these then arrange themselves into a few larger complexes, which have been called the germ-layers, or the primitive organs of the embryo. Further, in the higher Verte- brates there are formed certain organs, which are useful only during embryonic life, and are subsequently lost — namely, the foetal mem- branes and foetal appendages. All of the processes of that nature we shall treat of connectedly, and by themselves. In accordance with this, we can divide our theme into two main sections, the first of which will deal with the initial processes of development and the embryonic membranes, the second with the origin of the separate systems of organs. In order to facilitate for the advanced a more thorough study, and a penetration into embryological literature, a survey of the more important original works is given at the close of the separate chapters. On the other hand, text-books of Embryo- logy may be mentioned in this place. [Compare also the larger monographic works cited at the end of the book.] MANUALS AND TEXT-BOOKS. Valentin, G. Handbuch der Entwicklungsgeschichte des Menschen mit vergleichender Riicksicht der Entwicklung der Saugethiere und Vogel. Berlin 1845. Bischoff. Entwicklungsgeschichte der Saugethiere und des Menschen. Leipzig 1842. Rathke, H. Entwicklungsgeschichte der Wirbelthiere. Leipzig 1861. Kolliker, A. Entwicklungsgeschichte des Menschen und der hoheren Thiere. Academische Vortrage. Leipzig 1861. 2. ganz umgearbeitete Auflage. Leipzig 1879. Kolliker, A. Grundriss der Entwicklungsgeschichte des Menschen und der hoheren Thiere. 2. Auflage. Leipzig 1884. Schenk. Lehrbuch der vergleichenden Embryologie der Wirbelthiere. Wien 1874. Haeckel, E. Anthropogenic oder Entwicklungsgeschichte des Menschen. Leipzig 1874. Dritte Auflage. 1877. Foster, M., and F. M. Balfour. The Elements of Embryology. Part I. (Chick.) London 1874. 2nd edit, by Adam Sedgwick and Walter Heape 1883. German translation by Kleinenberg. Leipzig 1876. His, W. Unsere Korperform und das physiologische Problem ihrer Ent- stehung. Leipzig 1875. Balfour, F. M. A Treatise on Comparative Embryology. London 1880, -81, 2 vols. German translation by Dr. C. Vetter. Jena 1881. Romiti, G. Lezioni di embriogenia umana e comparata dei vertebrati. Siena 1881, -82, -88. Preyer, W. Specielle Physiologic des Embryo. 1883, -84. Hoffmann, C. K. Grondtrekken der vergelijkende Ontwikkelingsgeschie- denis van de gewervelde Dieren. Leiden 1884. Duval, M. Atlas d'Embryologie. Pam 1888. PART FIRST, CHAPTER I. DESCRIPTION OF THE SEXUAL PRODUCTS. EGG-CELL AND SEMEN-CELL. IN most animals, and without exception in all Vertebrates, the •development of a new being can take place only when reproductive elements, produced by two sexually different individuals, — the egg by the female, and the seminal corpuscle or seminal filament by the male, — are at the proper time brought into union as the result of the procreative act. The egg and the seminal filament are simple elementary parts or cells, which are produced in special glandular organs, the egg-cells in the ovary of the female, and the semen-cells in the testis of the male. After the beginning of sexual maturity at definite periods, they detach themselves within the sexual organs from their union with the remaining cells of the body, and form, under suitable conditions of development, among which the union of the two sexual cells is the most important, the starting-point for a new organism. First of all, therefore, we have to acquaint ourselves with tho peculiarities of the two kinds of sexual products. 1. The Egg-cell. The egg is by far the largest cell of the animal body. At a time when nothing was known of its cell-nature, its separate components were given special names, which remain in use even at the present time. The contents were called egg-yolk, or vitellus; the cell-nucleus was called vesicula germinativa, or germinative vesicle, discovered by the physiologist PURKINJE ; the nucleai corpuscles, or nucleoli, were called germinative spots, or maculae germinativce (WAGNER) ; and, finally, the cell-membrane was called the yolk-membrane, or mem- brana vitellina. All these parts vary in not unimportant ways from 8 EMBRYOLOGY. the ordinary condition of the protoplasm and nucleasof most anima) cells. The vitellus (figs. 1 and 3 n.d) rarely appears homogeneous, mucila- ginous, and translucent, like the protoplasm of most cells; it is ordinarily opaque and coarsely granular. This results from the fact that the egg-cell, during its development in the ovary, stores up in itself nutritive materials, or reserve stuff's. These consist of fat, of albuminous substances, and of mixtures of the two, and are described, according to their form, as larger and smaller yolk- spherules, yolk-plates, etc. Later, when the process of development is in progress, they are gradually used up in the growth and for the increase of the embryonic cells. The fundamental substance of the egg, in which the reserve stuffs just now referred to are imbedded, is protoplasm, physiologically the most in- teresting and important of substances, because in it take place, as we infer from many phenomena, the essential life-processes. We must therefore distinguish in the yolk, in accordance with the sug- gestion of VAN BENEDEN, (1) the egg- protoplasm, and (2) the yolk-substance. or deutoplasm, which is of a chemi- cally different nature, and is stored up in the former. When the deposition of reserve materials takes place to a great degree, the really essential substance, the egg-protoplasm, may become almost entirely obscured by it (figs. 3, 4). The protoplasm then fills up the small interstices between the closely packed yolk- globules, yolk-cakes, or lamellae, as mortar does those between the stones in masonry, and appears in sections only as a delicate net- work, in the smaller and larger meshes of which lie the yolk-elements. Only at the surface of the egg is the egg-plasm constantly present as a thicker or thinner continuous cortical layer. The germinative vesicle usually occupies the middle of the egg. It is the largest nuclear structure in the animal body, and its diameter generally increases with the size of the egg. The germinative vesicle (figs. 1, 2) is separated from the yolk by a firm membrane, which may often be distinctly demonstrated, juid which sin rounds various included components : nuclear liquid (Kern- 1.— Immature egg from the ovary of an Echinoderm. The large ger- minative vesicle shows a germinative dot, or nucleolus, in a network of filament*, the nuclear network. DESCRIPTION OF THE SEXUAL PRODUCTS. -— k, saft), nuclear network, and nucleoli. The nuclear liquid is more fluid than the yolk, in the fresh condition usually as clear as water, and when coagulated by the addition of reagents, absorbs only a little or no coloring matter. It is traversed by a network of delicate filaments (kn), which attach themselves to the nuclear membrane. In this network are enclosed nucleoli, or germinative spots (&/), small, for the most part spherical, homogeneous, lustrous structures, which consist of a substance akin to protoplasm — nuclear substance or nuclein. Nuclein is distinguishable from protoplasm — in addition to certain other chemical reactions — especially by the fact that it absorbs with great avidity pigments such as car- mine, haematoxylin, aniline, etc., on account of which it has also received from FLEMMING the name chromatin. The number of the nucleoli in the germinative vesicles of different animals is highly variable, but it is tolerably constant for each species; sometimes there is only a single nucleolus present (fig. 1), sometimes there are several or even very many of them (fig. 2&/"). Accordingly one may with AUERBACH distinguish uninucleolar, plurinucleolar, and multinucleolar germinative vesicles. At their surfaces eggs are surrounded by protective envelopes, the number and condition of which are exceedingly variable throughout the animal kingdom as well as among Vertebrates. It is best to divide them, as LUDWIG has done, according to their method of origin, into two groups, into the primary and the secondary egg- membranes. Primary egg-membranes are such as have been pro- duced either by the egg itself or by the follicular cells within the ovary and the egg-follicle. Those produced by the yolk of the egg are called vitelline membrane', those formed by the follicular epithelium, chorion. All which take their origin outside of the ovary, as a result of secretions on the part of the wall of the oviduct, are to be designated as secondary egg-membranes. In their details the eggs of the various species of animals differ Fig. 2. — Germinative vesicle of a Frog's egg that is still small and immature. It shows very numerous mostly peripheral germinative spots (A/), in a fine nuclear network (kn). m, Nu- clear membrane. ]0 EMBRYOLOGY. from each other in a high degree, so that they must really he con- sidi-ved as the most characteristic for the species of all the kinds of animal cells. Their size, which is due to a greater or 1< ^s ac- cumulation of deutoplasm, varies so extensively that in some species the egg-cells can be only barely recognised as minute dots, where; ,s in others they attain the considerable dimensions of a Hen's egg, or even of an Ostrich's egg. The form is usually globular, more rarely oval or cylindrical. Other variations arise from the method in which protoplasm and deutoplasm are constituted and distrilmt» d within the limits of the egg ; there are in addition the differences of the finer structure of the germinative vesicle and the great variability of the egg-membranes. Some of these conditions are of great significance from their in- fluence on the manner of subsequent development. They have br< n employed as a basis for a classification of the various kinds of eggs. It is most expedient to divide eggs into two chief groups, — into simple and into compound eggs, — the first of which is divisible into several sub-groups. A. Simple Eggs. Simple eggs are such as are developed in an ovary out of M sin^l*- germinal cell. The eggs of all the Vertebrates and most of t.Jie Invertebrates belong to this group. In this chief group there occur, according to the manner in which 'protoplasm and deutoplasm are distributed within the egg, three modifications, which are of very great importance in the determination of the first processes of development. In the simplest case the deutoplasm, which ordinarily is present only to a limited amount in the correspondingly small egg, is more or less uniformly distributed in the protoplasm (fig. 1). In ot lin- en ses there has arisen out of this original condition, in conjunction with :m increase in the bulk of the yolk-material, an inequality in 1 1n- distribution of the two egg-substances previously distinguished, The egg-plasma has accumulated in greater abundance at certain regions of the egg -territory, and the deutoplasma at other regions. ( 'nnsequently, a contrast has arisen between portions of the e^ r. 11 \\hicli are richer, and those which are poorer, in protoplasm. A further accentuation of this contrast exercises an extraordinarily liroad ami profound inflm-nn- on tin- first proo ssi s of developnn nt. which take place in the e^ after fertilisation. That is to say, the change^, which fnrtlur on are < inliraci <1 under the pmc< DESCRIPTION OF THE SEXUAL PRODUCTS. 11 A.P •cleavage, make their appearance only at the region of the egg which is richer in protoplasm, whereas the region which is more voluminous and richer in deutoplasm remains apparently quite unaltered, and is not divided up into cells. By this means the contrast, which was already present in the unsegmented egg, becomes during development disproportionately greater and moro obvious. The one part undergoes changes, is divided into cells, and out of these produces the individual organs ; the other part remains more or less unaltered, and is gradually employed as nutritive material. Following the example of REICHERT, the part of the yolk which is richer in protoplasm, and to which the developmen- tal processes remain confined, lias been designated formative yolk, and the other nutritive yolk. The unequal distribution of formative yolk (vitellus forma- tivus) and of nutritive yolk {vitellus mitritivus) within the egg is accomplished in two dif- ferent ways. In the one case (fig. 3) the formative yolk is accumulated at one pole of the egg as &flat germ-disc (k.sch). Inasmuch as its specific gravity is less than that of the nutritive yolk (n.d) collected at the opposite pole, it is always directed upward, and it spreads itself out on the yolk just like a drop of oil on water. In this case, therefore, the egg has undergone a polar differentiation : when at rest it must always assume a definite position, owing to the unequal weight of the two poles. The dissimilar poles are distin- guished : the upper, lighter pole, with the germ-disc, as the animal (A.P) ', the under, heavier and richer in yolk, as the vegetative pole, (V.P). The polar differentiation of eggs is often encountered in Vertebrates, and is especially prominent in the classes of Bony Fishes, Reptiles, and Birds. In the second case (fig. 4) the formative yolk (b.d) is accumulated over the whole surface of the egg, and surrounds the centrally placed nutritive yolk (n d) as a uniformly thick, finely granular cortical V.P Fig. 3.— Diagram of an egg with the nutritive yolk in a polar position. The formative yolk constitutes at the animal pole (A.P)<\ germ-disc (k.sch), in which the germinutive vesicle (t.6) is enclosed. The nutritive yolk {n.d) fills the rest of the egg up to the vegetative pole (F.P). 12 EMBRYOLOGY. k.h - m Fig. 4.— Diagram of an egg with the nutri- tive yolk in the centre. The germinative vesicle (£.6) occupies the middle of the nutritive yolk (n.d), which is enveloped in a mantle of formative yolk (b.d). layer. The egg exhibits central differentiation, and therefore does not assume a constant position when at rest. As in the former case the yolk was polar in position, so here it is central. Such a condition is never encountered in Verte- brates, but it is characteristic of Arthropods. In order to distinguish the three modifications, BALFOUR has made use of the expressions alecithal, telolecithal, and centrolecithal. He calls those eggs alecithal in which the deutoplasm, in small amount, is uniformly distributed through the protoplasm ; telolecithal, those in which it is accumulated at the vegetative pole ; centrolecithal, those in which the accumulation of deutoplasm has taken place at the centre. In what follows, we shall speak of (1) eggs with uniformly distributed yolk, (2) eggs with polar deutoplasm, and (3) eggs with central deutoplasm. It is now expedient to illustrate what has just been said by typical examples, and for this purpose the eggs of Mammals, Amphibia, Birds, and Arthropods have been selected. We shall also frequently recur to these in the presentation of the subsequent phases of develop- ment. The egg of Mammals and of Man is exceedingly small, since it mea- sures on the average only 0'2 mm. in diameter. It is for this reason that it was not discovered until the present century — in 1827, by CARL ERNST VON BAER. Previously the much larger GBAAFIAN follicle of the ovary, in which the smaller true egg is enclosed, had been erroneously taken for the latter. The Mammalian egg (fig. 5) con- si>< s principally of a finely granular protoplasmic substance, which contains dark, fat-like spherules and granules (deutoplasm), and which is turbid and opaque in proportion to the amount of these. The germinative vesicle (k.b) contains a large germinative dot (k.f), located, together with a few smaller accessory dots, in a nuclear network (k.n). The egg-membrane is called zona pelludda (z.p), because it surrounds the yolk as a relatively thick and clear layer. It is a primary membrane, for it is formed within the GRAAFIAN follicle, by the follicular cells. Under high magnification the zona pellucida DESCRIPTION OF THE SEXUAL PRODUCTS. 13 Fig. 5.— Egg from a Rabbit's follicle which was 2 mm. in diameter, after WALDEYER. It is surrounded by the zona pellucida (z.p), on which there rest at one place follicular cells (J.z). The yolk contains deutoplasmic granules (d). In the germinative vesicle (£.6) the nuclear network (k.ri) is especially marked, and contains a large germinative dot (*./"). (z.p) appears radially striate, since it is traversed by numerous pore- canals, into which, as long as the egg remains in the GRAAFIAN follicle, very fine projections of the follicular cells (f.z) penetrate. These fuse with the egg-plasm, and are probably concerned in the nutrition and growth of the contents of the egg. (RETZIUS.) The human ovum is wonderfully like the egg of Mamnlals in size, in the condition of its contents, and the nature of its membranes. However, it always can be distinguished by means of special, though trifling, characteristics, as the careful investigations of NAGEL have shown. Whereas in the Rabbit lustrous, fat-like spherules render the yolk cloudy, the human ovum retains its transparency during all stages of development, so that one may recognise most ac- curately all its structural details, even on the living object. The yolk is divided into two layers. The inner layer contains principally •cleutoplasm, which produces in this case, contrary to most of the Mammals, only a slight cloudiness ; it consists in part of feebly lustrous, in part of highly refractive fragments, some coarser, some finer ; but it is not possible to recognise the mutual boundaries of EMBRYOLOGY. the individual components, as is the case in other Mammals and lower animals, where one distinguishes with great ease granules and distinct drops. The outer layer or peripheral zone of the yolk is more finely granular and still more transparent than the central part, and contains the gerininative vesicle with a large germinative dot, in which NAGEL was able to observe amoeboid motions. The zona pellucida is remarkably broad ; it is striate, and is separated from the yolk by a narrow (perivitelline) space. There are two or three layers of follicular cells attached to the periphery of the egg when it is set free from the GRAAFIAN follicle. The long diameters of these cells are arranged in a radial direction around the egg, as is general in Mammals, and it is due to this circumstance that they have received the name corona radiata, introduced by BISCHOFF. The human egg without the follicular epithelium measures, on the average, O17 mm. in diameter. The eggs of many Worms, Molluscs, Echinoderms, and Ccelenterates agree with the Mammalian egg in their size, and in the method in which protoplasm and deutoplasm are uniformly distributed through the egg. The eggs oj Amphibia, which were cited as the second example, foim a transition from simple eggs, with uniform distribution of yolk-material, to eggs with distinctly expressed and externally recognisable polar differentiation. Already these have deposited in themselves a large amount of deutoplasm, and have thereby acquired a very considerable size. The Frog's egg, for example, is stuffed full of closely compacted, fatty-looking yolk-lumps (Dotterschollen) and yolk-plates. The egg protoplasm is in part distributed as a network between the little yolk-plates ; in part it forms a thin cortical layer at the surface of the egg. Upon closer examination however, the beginning of a polar differentiation is most distinctly recognisable even here. It manifests itself in this way : at one pole, which at the same time appears black on account of a deposit of superficial pigment, the yolk-plates are smaller and enveloped in more abundant egg-plasm ; and also, nrobably as a consequence of this >liirht differences in specific gravity are distinguishable between t In- pigmented and the unpigmented, or the animal and the vegetative, halves of the egg. The germinative vesicle (fig. 2) lies in the middle of the immature ogg, is exceedingly large, even visible to the naked eye, and multi- nucleolar. inasmuch ;»-> there are a liundred or more large germinativo dots (//) distributed irnmediatpK- under the nuclear membrane.. DESCRIPTION OF THE SEXUAL PRODUCTS. 15" The envelopes exhibit, in comparison with the Mammalian egg, an increase in number, for to the zona pellucida (zona radiata), which is produced in the follicle, there is subsequently added still another, a secondary envelope. This is a thick, viscid, gelatinous layer, which is secreted by the wall of the oviduct, and which becomes swollen in water. The polar differentiation, taken, as it were, in the very process of developing in the case of the Amphibia, is found sharply expressed in our third example, the Bird 's egg. In order to form a correct picture of the condition of the egg-cell in the case of the Hen, or of any *.& k.sch other bird, we must seek it while still in the ovary, at the moment when it has finished its growth, and is ready to be set free from the follicle. It is then ascertained that only the spheroidal yolk, the so- called yellow of the egg, which in itself is an enormously large cell (fig. 6a), is developed in the botryoidal Ovary. It is enclosed in a thin but tive disc ; k.b, germinative vesicle ; tolerably firm pellicle (d.h), the ^^J^^J^ *°* vitelline membrane, the rupture of which is followed by an extrusion of the soft pulpy contents. By careful examination one will discover upon the latter a small white spot, the germinative disc (k.sch), or discus proligerus, also called scar or cicairicula. It has a diameter of about 3 or 4 mm., and consists of formative yolk, — a finely granular protoplasm with small yolk- spherules, — which alone is involved in the process of cleavage. In the flattened germinative disc is also found the germinative vesicle, fig. 6a (k.b) and fig. 6b (x\ which is likewise somewhat flattened and lenticular. The remaining chief mass of the egg-cell is nutritive yolk, which is composed of numberless yolk-spherules united by slight traces of egg-plasm, as though by a cement. Information concerning its finer structure is to be gained from thin sections through the hardened egg, which should be cut perpendicularly to the germinative disc. According to differences in staining and in -elementary composition, there are now to be distinguished the ivhite and the yellow nutritive yolk (fig. 6a). The ivhite yolk (w.d) is present in the egg cell only in a small 16 EMBRYOLOGY. quantity ; it forms a thin layer over the whole surface, the white yolk-rind ; secondly, it is accumulated in somewhat greater quantity under the germinative vesicle, for which it at the same time forms a bed or cushion (PANDER'S nucleus) ; and, thirdly, from this region it U'.y. Fig. 6b. Section of the germ-disc of a mature ovarian Hen's egg still enclosed in the capsule, after BALFOCR. a, Connective-tissue capsule of the egg ; 6, epithelium of the capsule, on the inside of which lies the vitelline membrane reposing upon the egg ; c, granular substance of the germinative disc ; w.y, white yolk, which passes imperceptibly into the finely granular substance of the disc ; x, germinative vesicle enclosed in a distinct membrane, but shrivelled up ; y, space originally occupied by the germinative vesicle, but made empty by its shrivelling up. penetrates in the form of a mortar-pestle into the very centre of the yellow yolk, where it terminates in a knob-like swelling (latebra, PURKINJE). Upon boiling the egg, it is less coagulated, and remains softer than the yellow yolk. In the coagulated condition the latter discloses upon sections a lamellated condition, in that it consists of smaller and larger spherical shells, which envelope the latebra. The two kinds of yolk also differ from each other in respect to the condition of their elementary particles. The yellow yolk consists of soft plastic spherules (fig. 7 A) from 25 to 100 ft in diameter, which acquire a punctate appearance from the presence of numerous exceedingly minute granules. The elements of the white yolk are for the most part smaller (fig. 7 B), and likewise spherical, but contain one or several large highly refractive granules. Fig, 7.— Yolk-elements from the Fowl's egg, after BAI.FOUR. A, Yellow yolk ; B, white yolk. At the boundary between the two kinds of yolk there are presi nt spherules which effect a transition between them. The freshly laid Hen's egg (fig. 8) has a different appearance from that of such an ovarian egg. This results from the fact that there is deposited around the yolk, when it detaches itself from DESCRIPTION OP THE SEXUAL PRODUCTS. 17 the ovary and is taken up by the oviduct, several secondary en- velopes derived from the wall of the oviduct, viz., the white of the egg, or the albumen, the shell-membrane, and the calcareous shell. Each of these parts is formed in a special region of the Hen's oviduct. The latter is divided into four regions : (1) A narrow ciliated initial part, into which the liberated egg is received, and where it is fertilised by the spermatozoa already accumulated there ; (2) a I Fig. 8. — Diagrammatic longitudinal section of an unincubated Hen's egg, after ALT.EN THOMSON. (Somewhat altered.) b.l. Germ-disc ; ic.y. white yolk, which consists of a central flask-shaped mass and a number of concentric layers surrounding the yellow yolk (y.y.) ; v.t. vitelline membrane ; x. a somewhat fluid albuminous layer, which immediately envelopes the yolk ; w. attmmen composed of alternating layers of more and less fluid portions ; ch.l. chalazre ; a.ch. air chamber at the blunt end of the egg — simply a space between the two layers of the shell-membrane ; i.t.m. inner, s.m. outer layer of the shell-membrane ; s. shell. glandular region, covered with longitudinal furrows, from which the albumen is secreted and spread around the yolk in a thick layer \ (3) a somewhat enlarged part, covered with small villi, the cells of which secrete calcareous salts, and thus cause the formation of the shell ; (4) a short narrower region, through which the egg passes rapidly, and without undergoing any further change, when being deposited. The envelopes furnished in succession by the oviduct have the following composition : — The white of the egg, or albumen (w), is a mixture of several aterials : according to chemical analyses, it contains 12% albumen, 18 EMBRYOLOGY. 1-5% fat and other extractive materials, 0*5% salts (potassic chloride, sodic chloride, sulphates, and phosphates), and 86% water. It surrounds the yolk in several layers of varying consistency. There is a layer quite closely investing the latter, which is firmer and especially noteworthy because it is prolonged into two peculiar spirally twisted cords, the chalazce (ch.l), which consist of a very compact albuminous substance, and which make their way through the albumen to the blunt and to the pointed poles of the egg The albumen is enclosed by the thin but firm shell-membrane (s.m) (membrana testae), which is composed of felted fibres. It may be separated into two lamellae — an outer, which is thicker and firmer, and an inner, which is thinner and smooth. Soon after the egg is laid the two layers separate from each other at the blunt pole, and enclose between them a space filled with air (a.cA), — the so-called air-chamber, which continues to increase in size during incubation r and is of importance for the respiration of the developing Chick. Finally, the shell, or testa ($), is in close contact with the shell- membrane; it consists of an organic matrix (2%), in which 98% cal- careous salts are deposited. It is porous, being traversed by small canals, through which the atmospheric air may gain entrance to the egg. The porosity of the calcareous shell is an absolute necessity for the normal development of the egg, since the vital processes in the protoplasm can take place only when there is a constant supply of oxygen. If the porosity of the shell be destroyed, either by soaking it in oil or closing its pores with varnish, the death of the incubated egg ensues in a very short time. B. Compound Eggs. Compound eggs are .found only in a few subdivisions of the invertebrated animals, as in the Cestodes, Trematodes, etc.; they are noteworthy in this respect, that they are produced by the union of numerous cells, which are formed in two different glands of the sexual apparatus of the female, — in the germariuni and in the vitellarium. In the gerniarium is developed the egg-cell in the restricted sense. This is always very small, and consists almost exclusively of egg-plasm. When this cell a* its maturity is set free from its surroundings and comes into the sexual outlets, it is obliged to pass the opening of the vitellarium ; hen- thnv an- associated with it a number of yolk-cells, which, owing to deposition of reserve material in (lie protopliiMn. ,-ippear turbid and coarsely granular,. DESCRIPTION OF THE SEXUAL PRODUCTS. 19 — k and which constitute the dower that is given by the maternal organism to the developing germ on its way. Thereupon the whole is enclosed in one or several secondary egg-membranes, and now constitutes the compound egg, in which, however, the developmental processes manifest themselves exclusively on tjie simple germ cell ; it is that alone which is fertilised and segments, while the yolk-cells gradually degenerate and are employed as nutritive material. Thus in this case also, upon closer examination, the general law, that the descendent organism takes its origin from a single cell of the maternal body, suffers no exception. 2. The Seminal Filaments. In contrast with eggs, which are the largest cells of the animal body, the sperm-cells or sperm -filaments (spermatozoa) are the smallest elementary parts ; they are accumulated in great multitudes in the seminal fluid of the male, but can be recog- nised in it only by the aid of high magnification, A being, for the most part, slender motile filaments. Inasmuch as every cell consists of at least two parts, namely, nucleus and protoplasm, we must look for these parts in this case also. We shall take for description the spermatozoa of Man. In Man the seminal filaments (fig. 9) are about 0*05 mm. long. One may distinguish as head (k) a short but thick region, which marks the anterior end, as tail a long thread-like appendage (s), and between the two a so-called middle piece (m). The head (k) has the form of an oval plate, which is slightly excavated on both surfaces, and is somewhat thinner toward the anterior end. Seen from the side (£) it presents a certain re- semblance to a flattened pear. Chemically considered, it consists of nuclear substance (nuclein or chromatin), as microchemical reactions show. To the head is united, by means of a short part called the middle piece (m), the long thread-like appendage (s), which is com- posed of protoplasm, and is best compared to a flagellum, because, like the latter, it executes peculiar serpentine motions in virtue of its contractile properties. By means of these motions the sper- matozoon moves forwards in the seminal fluid with considerable velocity. Fig. 9.— Mature sper- matozoa of Man, seen in two dif- ferent positions. Each consists of a head (k), a mid- dle piece (r/i), and tail (S). . 20 EMBRYOLOGY. The spermatozoa have often been designated— and it seems to us. with entire justice— as ciliate, or still better as flagellate, cells. The spermatozoa of the remaining Vertebrates have a similar structure to that of Man ; on the whole, the diversity of form which is encountered in the comparative study of the egg-cell in the animal kingdom is wanting here. That spermatozoa are in reality metamorphosed cells cannot be more clearly demonstrated than by their development. According to the extended observations of LA VALETTE and others, each spermatozoon is formed from a single seminal cell or spe> matid, and, to be more precise, the head i# formed from the nucleus, the contractile filament from the protoplasm. The metamorphoses which take place in the development have been investigated with the greatest detail by FLEMMING and HERMANN in the case of Salamandra maculata, the spermatozoa of which are characterised by their very great size. The individual spermatozoon here consists of: (1) a very long head, which has the form of a finely pointed skewer, and takes up stains with avidity; (2) a short cylindrical middle piece, which differs from the first part in chemical properties also ; (3) the motile caudal filament, which in the Salamander exhibits the additional peculiarity that it is provided with a contractile undulating membrane. Of these three regions the skewer-like head, and probably also the middle piece, arise from the nucleus of the spermatid, whereas the contractile filament is differentiated out of the protoplasm. In the development of the head the nucleus of the seminal cell is seen to become more and nore elongated (fig. 10 A, B); at first it takes the form of a p» ar vfig. 10 A k) ; then it grows out into an elongated cone (fig. 10 B k), the base of which serves as the point of attachment for the middle piece (mst). The cone becomes elongated and narrowed into a rod (fig. 11 A, B), which is finally converted into the characteristic form of a skewer. With this elongation of the nucleus the chromatic network becomes more and more dense, and at last assumes a ejuite compact and homogeneous condition, as in the mature spermatozoon. The fundament (Anlage) of the middle piece (figs. 10, 11, A, B, makes its appearance early — when the nucleus begins to elongate — at that end of the nucleus which was called its base, in the form of a small oval body, which at first takes up stains like the head, but afterwards IOM-S this property. Its iir.>t appearance demands still further elucidation. DESCRIPTION OF THE SEXUAL PRODUCTS. 21 Why are the male sexual cells so small and thread-like, and so ilitierently constituted from the eggs ? The dissimilarity between the male and the female sexual cells is explained by the fact that a division of labor has arisen between the two, inasmuch as they have adapted themselves to different missions. Fig. 10 A and B. Initial stages of the metamorphosis of the seminal cell into the seminal filament, after Hi: KM \\v A. Seminal cell with peai '-.-haped nucleus; />'. seminal cell with cone -shaped nucleus ; sc, seminal eell ; £, nucleus witli chromatin network, and nnrleoli (»); mst, boil} out of which the middle piece is de\ eloped ; r, vin:4-like structure, which is in contact \\itli the middle piece, and is claimed to have relation to the formation of the spiral membrane of the filament ; t\ caudal appendage of the seminal filament. Fig. 11 A and B.— Two terminal stages in the metamorphosis of the seminal cell into the seminal filament, after FLEMMINO. k, Nucleus, which has become elongated to form the head of the spermatozoon; mat, its middle piece ; /, its caudal filament. The female cell lias assumed the function of supplying the substances whu-h are necessary for that nutrition and growth of the cell proto- pla.Mn \\hieh a rapid accomplishment of the process of development demands. It has therefore, while in the ovary, stored up in itself yolk-substance, reserve material, for the future; and consequently has Income large and incapable of motion. But inasmuch as it is necessary for the accomplishment of a process of development that union with a second cell from another individual should take f" ice, and since non-motile bodies cannot unite, theiefore the male n;enf has Iveu suitably modified to meet this second requirement. 22 EMBRYOLOGY. For the purpose of locomotion and in order to make possible the union with the non-motile egg-cell, it has become metamorphosed into a contractile filament, and has rid itself completely of all substances, as, for example, yolk-material, which would interfere with this principal requirement. At the same time it has assumed the form best adapted for passing through the envelopes with which, as a means of protection, the egg is surrounded, and for penetrating the yolk. The conditions especially in the vegetable kingdom confirm the accuracy of this interpretation. There are plants of the lowest forms in which the two copulating sexual cells are entirely alike, both being small and motile ; and there are other related species in which a gradual differentiation is brought about by the fact that one of the cells becomes richer in yolk and incapable of motion, while the other becomes smaller and more active. From this it is evident that the stationary egg must now be sought out by the migratory cell. A few physiological statements may be in place in this connection. In comparison with other cells of the animal body, and especially in comparison with the eggs, the seminal filaments are characterised by greater duration of life and power of resistance, a fact which is frequently of importance for the success of fertilisation. The mature spermatozoa, after they are set free from their connection with other cells, remain for months in the testes and vasa deferentia without losing their fertilising power. They also appear to remain active for a long time after having been introduced into the sexual passages of the female, perhaps for several weeks in the case of Man. For some animals this is demonstrable to a certainty. For example, it is known that the semen of Bats remains alive in the uterus of the female during the whole winter ; and in the case of the Fowl it is known that fertilised eggs can be laid up to the eighteenth day after the removal of the Cock. In the presence of external influences semen shows itself to be much more resistent than the egg-cell, which is easily injured or killed. For example, when semen is frozen and then thawed out, the motion of the seminal filaments comes back again. Many salts, if they are employed not too strong, have no deleterious influence. Narcotics in strong concentration, and when employed for a long time, make the filaments motionless, without immediately killing them, because after removal of the injurious substance they can be revived. DESCRIPTION OF THE SEXUAL PRODUCTS. 23 Very weak alkaline solutions stimulate the motions of seminal filaments ; on the contrary, acids, even when they are very dilute, produce death. Accordingly the motion becomes more lively in all animal fluids of alkaline reaction, whereas in acid solutions it soon dies out. HISTORY. —The discovery that egg and seminal filament are simple cells is of far-reaching import for the comprehension of the whole process of develop- ment. In order to appreciate this to its full extent, it will be necessary to make a digression into the historical field. Such a digression will acquaint us with some fundamental transformations, which have affected our conception of the essentials of developmental processes. In the last century, and even in the beginning of the present, ideas about the nature of the sexual products were very indistinct. The most distinguished anatomists and physiologists were of opinion that eggs agreed in their structure in every particular with the grown-up organism, and therefore that they possessed from the beginning the same organs in the same position and con- nection as the latter, only in an extraordinarily diminutive condition. But in- asmuch as it was not possible, with the microscopes of the time, actually to see and demonstrate in the eggs at the beginning of their development the assumed organs, recourse was had to the hypothesis that the separate parts, such as nervous system, glands, bones, etc., must be present, not only in a very diminu- tive, but also in a transparent condition. In order to make the process more intelligible, the origin of the blossoms of plants from their buds was cited as an illustrative example. Just as already in a small bud all the parts of the flower, such as stamens and coloured petals, are enveloped by the green and still unopened sepals, — just as the parts grow in concealment and then suddenly expand into a blossom, so also in the de- velopment of animals it was thought that the already present but small and transparent parts grow, gradually expand, and become discernible. The doctrine which has just been outlined was consequently called the Theory of unfolding, or evolution. However, a more appropriate designation for it is the one intro- duced during recent decennia— preformation theory. For the characteristic feature of this doctrine is, that at no instant of development is there anything new formed, but rather that every part is present from the beginning, or is preformed, and consequently that the very essence of development— tlie be- coming—is denied. " There is no such thing as becoming ! " is the way it is expressed in the " Elements of Physiology " by HALLER. " No part in the animal body was formed before another ; all were created at the same time." As the necessary consequence of a rigid adherence to the preformation theory, it follows, and indeed was formulated by LEIBNITZ, HALLER, and others, that in any germ the germs of all subsequent offspring must be established or included, since the animal species are developed from one another in un- interrupted sequence. In the extension of this box-within-box doctrine (Einschaclitelungslehre) its expounders went so far as to compute how many human germs at the least were concentrated in the ovary of mother Eve, and thereby arrived at the number 200,000 millions. The evolution theory offered a point of attack for a scientific feud, inasmuch as every individual among the higher organisms is developed by means of the cooperation of two separate sexes. When, therefore, the seminal filament as 24 EMBRYOLOGY. well as the animal egg became known, there soon arose the actively discussed question, wlietlier the egg or the seminal filament was the preformed germ. Decennium after decennium the antagonistic camps of the ovlsts and of the animalcidists stood opposed to each other. Those who followed the latter thought they saw, with the aid of the magnifying glasses of the times, the spermatozoa of man actually provided with a head, arms, and legs. The animalculists recognised in the egg only a suitable nutritive soil, as it were, which was necessary to the growth of the spermatozoon. In the face of such doctrines there dawned a new period for Embryology, when in 1759 CASPAR FRIEDRICH WOLFF in his dqctor's dissertation opposed the dogma of the evolution theory, and, casting aside preformation, laid down the scientific principle that what one could not recognise by means of his senses was certainly not present preformed in the germ. At the beginning, so he maintained, tlie germ is nothing else than an unorganised material eliminated from the sexual organs of tlie parent, which gradually becomes organised, but only during the process of development, in consequence of fertilisation. Ac- cording to WOLFF, the separate organs of the body differentiate themselves one after another out of the hitherto undifferentiated germinal material. In individual cases he endeavoured, even at this time, to determine more exactly, by means of observations, the nature of the process. Thus C. F. WOLFF was the founder of the doctrine of epigenesis, which, through the discoveries of the present century, has proved to be the right one.* WOLFF'S doctrine of unorganised germinal matter has been compelled since then to give way to more profound knowledge, thanks to the improved optical aids of recent times, and to the establishment of the cell-theory by SCHLEIDEN and SCHWANN. A better insight into the elementary composition of animals and plants was now acquired, and especially into the finer structure of the sexual products, the egg-cell and the seminal filament. So far as regards the egg-cell, a series of important works began with PURKINJE'S investigation of the Hen's egg in 1825, in whicli the germinative vesicle was described for the first time. This was soon (1827) followed by C. E. V. BAER'S celebrated discovery of the Mammalian egg, which had been hunted for, but always without success. Extensive and comparative investiga- tions into the structure of the egg in the animal kingdom were published in 1836 by R. WAGNER, who also discovered at the same time in the germinative vesicle the germinative dot (macula germinativa). With the establishment of the cell-theory there naturally arose the question as to how far the egg was in its structure to be regarded as a cell,— a question which was for years answered in widely different ways, and which even now from time to time is brought up for discussion in an altered form. Even at that time SCHWANN, albeit with a certain reservation, expressed it as his opinion that the egg was a cell, and the germinative vesicle its nucleus; but others, his co- temporaries (BISCHOFF and others), regarded the germinative vesicle as a cell, * Historical presentations of the theory of evolution and the theory of epigenesis, which are worth the reading, have been given by A. KIRCIIHOKK in his interesting paper, "CASPAR FRIEDRICH WOLFF. Sein Leben mid .seine Bedeutung fiir die Lehre von der oi.unnix-lirii Kntwiekluiitr." Jriiaische Zc\1- fchrift fiir Mi'dlcin und ^'nluni-ix.tcnschaft, Bd. IV., Leipzig, 1868; and by W. His, " Die Theorien der geschlechtlichen Zeugung." Archiv fiir Anthropclogic, Bd. IV. u. V. DESCRIPTION OF THE SEXUAL PRODUCTS. 25 and the yolk as a mass of enveloping substance. A unanimity of views in this matter was brought about only after the general conception of " cell " had received in Histology a more precise definition. This was due especially to more accurate knowledge of the processes of cell-formation gained through the works of XAGELI, KOLLIKER, REMAK, LEYDIG, and others. The interpretation of eggs with separate formative and nutritive yolk, and with partial cleavage, occasioned especial difficulty. Two antagonistic views in this matter have existed for a long time. According to one view, eggs with polar nutritive yolk (the eggs of Eeptiles, Birds, etc.) are compound structures, which cannot be designated as simple cells. Only the formative yolk, together with the germinative vesicle, is comparable with the Mammalian egg ; the nutritive yolk, on the contrary, is something new, superposed upon the cell from without, a product of the follicular epithelium. The spherules of the white yolk are explained as uninuclear and multinuclear yolk-cells. The formative and nutritive yolk together are comparable with the entire contents of the GRAAFIAN vesicle of Mammals. H. MECKEL, ALLEN THOMSON, ECKER, STRICKER, His, and others, have expressed themselves in favour of this view with slight modifications in the details. According to the opposite view of LEUCKART, KOLLIKER, GEGENBAUR, HAECKEL, VAN BENEDEN, BALFOUR, and others, the Bird's egg is just as truly a simple cell as the egg of a Mammal, and the comparison with a G-RAAFIAN follicle is to be rejected. The yolk never contains enclosed cells, but only nutritive components. As KOLLIKER, especially in opposition to His, has shown, the white-yolk spherules contain no structures comparable with genuine cell-nuclei ; and therefore cannot be interpreted as cells. As GEGENBAUR already in 1861 sharply formulated it : " The eggs of Vertebrates with partial cleavage are on that account essentially no more compound structures than those of the remaining Vertebrates; they are nothing else than enormous cells peculiarly modified for special purposes, but which never surrender this their real character." There would be no change in this interpretation, even if it should prove to be that the yolk was formed in part from the follicular epithelium, and was set free from the latter as a sort of secretion. In that event we should have to do with a special method of nutrition of the egg, the cell-nature of which cannot on that account be called in question. Various components of the yolk have received special names. REICHERT first distinguished as formative yolk the finely granular mass, which, in the Bird's egg, contains the germinative vesicle, and forms the germ-disc, because it alone undergoes the process of cleavage, and produces the embryo. The other chief mass of the egg he called nutritive yolk, because it does not break up into cells, and because subsequently, enclosed in a yolk-sac, it is consumed as nutritive material. Afterwards His introduced for these the names chief germ and accessory germ (Haupt- und Nebenkeim) . Whereas the nomenclature of REICHERT and His is applicable only to eggs with polar arrangement of nutritive yolk, VAN BENEDEN (1870) has undertaken the division of the substance of the egg from a more general standpoint. He distinguishes between the protoplasmic matrix of the egg, in which, as in every cell in general, the vital processes take place, and the reserve and nutritive materials, which are stored up in the protoplasm in the form of granules, plates, and balls, and which he designates as deutoplasm. Every egg possesses both components, only in different proportions, in varied forms and distribution. BALFOUR has selected this latter condition as a basis for 26 EMBRYOLOGY. division ; and has consequently made the three groups of alecithal, telolecithal, and centrolecithal eggs, for which I have selected the designation eggs with little or uniformly distributed yolk, eggs with polar, and eggs with central yolk. In recent times investigation has been directed to the finer structure of the germinative vesicle, in which KLEINENBERG (1872) was the first to observe a special protoplasmic nuclear trestle (Kerngeriist) or nuclear network, which since then has been shown by numerous researches to be a constant structure. In the case of the germinative dot I have myself designated two chemically and morphologically distinguishable substances as nuclein and paranuclein, the investigations concerning the importance and the role of which in the develop- ment of the egg are not yet concluded. The history of the spermatozoa begins with the year 1677. A student in Leyden, HAMM, in the microscopic examination of semen, saw the briskly moving bodies, and communicated his observation to his teacher, the celebrated microscopist LEEUWENHOECK, who instituted more accurate investigations, and published them in several papers, which soon attracted general attention. The sensation caused was all the greater because LEEUWENHOECK declared the seminal filaments to be the preexisting germs of animals, and maintained that at fertilisation they penetrated into the egg-cell and grew up in it. Thus arose the school of animalculists. After the refutation of the preformation theory, it was thought that no importance was to be ascribed to the seminal filaments in fertilisation, it being held that it was the seminal fluid that fertilised. Even during the first four decennia of the present century, the seminal filaments were almost universally held to be independent parasitic creatures (spermatozoa) com- parable with the Infusoria. Even in JOH. MULLER'S " Physiology " (1833-40) occurs this statement : " Whether the semen-animalcules are parasitic animals, or animated elements of the animals in which they occur, cannot for the present be answered with certainty." The settlement of the question was accomplished by comparative histological investigations of the semen in the animal kingdom, and by physiological experiment. In two essays — " Beitriige zur Kenntniss der Geschlechtsverhaltnisse und der Samenfliissigkeit wirbelloser Thiere," and " Bildung der Samenfaden in Blaschen " — KOLLIKEB showed that in many animals, e.g., in the Polyps, the semen consists of filaments only, the fluid being entirely absent ; and that in addition the filaments are developed in cells, and consequently are themselves elementary parts of animals. REICHERT discovered the same to be true in Nematodes. By means of physiological experiment it was recognised that seminal fluid with immature and motionless filaments, and likewise mature but filtered semen, did not fertilise. This was decisive for the view that the seminal filaments are the active part in fertilisation, and that the fluid, which is added thereto in the case of the higher animals under complicated sexual conditions, "can be regarded only as a menstruum for the seminal bodies which is of subordinate physiological significance." Since then our knowledge (1) of the finer structure, and (2) of the develop- ment of the seminal filaments, has made further advances. So far as regards the first point, we have learned, especially through the works of LA VALETTE and SCHWEIGGER-SEIDEL, to distinguish between head, middle piece, and DESCRIPTION OP THE SEXUAL PRODUCTS. 27 tail, and to know their different chemical and physical properties. The view expressed by KOLLIKER, that ordinarily the seminal filaments were the metamorphosed and elongated nuclei of the seminal cells, underwent a modifi- cation. According to the researches of LA VALETTE, only the head of the seminal filament arises from the nucleus, the tail, on the contrary, from the protoplasm of the spermatid. Finally FLEMMING brought forward convincing proof that it is only the chromatin of the nucleus that is metamorphosed into the head of the seminal filament. Important investigations concerning the development of the seminal filaments in various animals have recently been made by VAN BENEDEN ET JULIN, PLATNER, HERMANN, and others. SUMMARY. The most important results of this chapter may be briefly sum- marised as follows : — 1. Male and female sexual products are simple cells. 2. The seminal filaments are comparable to flagellate cells. They are usually composed of three portions, head, middle piece, and contractile filament. 3. The seminal filament is developed out of a single cell, the spermatid ; the head, and probably also the middle piece, from the nucleus ; the contractile filament from the protoplasm. 4. The egg-cell consists of egg-plasm and yolk-particles, which are reserve material (deutoplasm), imbedded in it. 5. The quantity and distribution of the deutoplasm in the egg-cell is subject to great variation, and exercises the greatest influence on the course of the first processes of development. (a) The deutoplasm is small in amount, and uniformly dis- tributed in the egg-plasm. (b) The deutoplasm is present in greater quantity, and, in consequence of unequal distribution, is more densely accumulated either at one pole of the egg or in its middle. (Polar and central deutoplasm.) (c) In eggs with polar deutoplasm (eggs with polar differentia- tion) the pole with more abundant deutoplasmic contents is designated as the vegetative, the opposite one as the animal pole. (d) In the case of eggs with polar differentiation, the more abundant protoplasm of the animal pole may be sharply differentiated as germ-disc (formative yolk) from the portion which is richer in deutoplasm (nutritive yolk). The developmental processes take place only in the formative yolk, while the nutritive yolk remains on the whole passive. 28 EM BRYOLOGY. 6- Eggs may be divided into several groups and sub-groups ac- cording to their development from cells of the ovary alone, or from cells of the ovarium and vitellarium, as well as according to the distribution of the deutoplasm, as exhibited in the following scheme : — I. Simple eggs. (Development from cells of the ovary.) A. Eggs with little deutoplasm uniformly distributed through the egg (alecithal*). (Amphioxus, Mammals, Man.) B. Eggs with abundant and unequally distributed deutoplasm. (1) Eggs with polar differentiation (telolecithal), with deuto- plasm having a polar position, with animal and vegetative poles. (Cyclostomes, Amphibia.) (2) Eggs with polar differentiation, which are distinguished from the preceding sub-group by the fact that with them there has been effected a still sharper segregation into formative yolk (germ-disc) and nutritive yolk— into a part which is active during development and a part that is passive. (Eggs having polar differentia- tion with a germ-disc. Fishes, Reptiles, Birds.) (3) Eggs having central differentiation with central deuto- plasm (centrolecithal) and superficially distributed formative yolk (blastema, Keimhaut). (Arthropods.) II. Compound eggs. (Double origin from cells of the ovarium and vitellarium.) LITERATURE. Baer, C. E. von. De ovi mammalium et homiuis genesi epistola. Lipsiae 1827. Beneden, Ed. van. Recherches sur la composition et la signification de 1'oeuf. Mem. cour. de 1'Acad. roy. Sci. de Belgique. T. XXXIV. 1870. Bischoff. Entwicklungsgeschichte des Kanincheneies. 1842. Flemming. Zellsubstanz, Kern- und Zelltheilung. Leipzig 1882. Frommann, K. Das Ei. Realencyclopadie der gesammten Heilkunde. 2. Auflage. Gegenbaur, C. Ueber den Bau und die Kntwicklung der Wirbelthiereier mit partieller Dottcrtheilung. Archiv f. Anat. und Pbysiol. 1861. Guldberg. Beitrag zur Kenntniss der Eierslockseier bei Echidna. Sitzungsb. d. Jena. Gesellsch. (1885), p. 113. Hensen. Die Physiologic der Zeugung. Hermann's Haudbuch der Physio. logie. Bd. VI. Theil IT. 7>/>r/ed through a condition ivithout nucleus, and again acquired a nucleus in )nsequence of fertilisation. The controversial points were cleared up by investigations which BUTSCHLI id the author had undertaken at the same time. I showed in my first " Beitrage zur Kenntniss der Bildung, Befruchtung id Theilung des thierischen Eies," that in all the older writings there been no distinction made between the nucleus of the immature, the iture, and the fertilised egg, but that these nuclei had been of ten confounded id held to be identical, and I first established the differences between germi- itive vesicle, egg-nucleus, and cleavage-nucleus, the latter being the names rtiich were introduced by me. In addition I showed that the disappearance 36 EMBRYOLOGY. of the germinative vesicle and the origin of the egg-nucleus preceded fertilisa- tion, and thus I distinguished between the phenomena of maturation and fertilisation of the egg-cell, which generally had been interchanged and con- founded. I also endeavoured to make it probable that the egg-nucleus descended from the germinative vesicle, and in fact from a nucleolus of the vesicle, and defended the thesis that the egg during its maturation did not pass through a non-nuclear condition. In this I fell into an error : I overlooked,, like all previous observers, the connection between the formation of the polar cells and the disappearance of the germinative vesicle, — a process which it was the more difficult to establish in the object which I studied because it takes place in the ovary. The excellent investigations of BCTSCHLI, which brought the changes of the germinative vesicle into connection with the formation of the polar cells, now made their appearance, supplementing my results. The polar cells were discovered in the year 1848 by FB. MCLLER and LOVEX, and were named by the former directive vesicles (Richtungsblaschen), because they always lie at the place where subsequently the first cleavage-furrow makes its appearance. Their wide distribution in the animal kingdom had also been established by many investigators ; BCTSCHLI was the first, however, to direct attention to the peculiar processes which take place in the yolk, in the interpretation of which he, nevertheless, committed several errors. He maintained that the whole germinative vesicle is converted into a spindle-shaped nucleus, which moves to the surface, and, while becoming constricted in the middle, is thrust outside by the contractions of the yolk in the form of two directive bodies. By this process the egg became non-nuclear, and again acquired a nucleus only in consequence of fertilisation. In two further articles on the Formation, Fertilisation, and Cleavage of the Animal-Egg, I modified the teachings of BCTSCHLI, and brought them into unison with my previous investigations, inasmuch as I pointed out that the germinative vesicle is not as such directly converted into the nuclear spindle, but in part is dissolved : that the spindle takes its origin from the nuclear substance in a manner which it is very difficult to investigate : that the polar cells are formed, not by the elimination of the spindle, but by a genuine process of division or budding ; that in consequence of this the egg is not destitute of a nucleus even after the constricting off of the second polar cell, but that the egg-nucleus arises from the half of the divided polar spindle which remains in the yolk, and therefore, in its ultimate derivation, from components of the germinative vesicle of the immature egg. Soon afterwards BCTSCHLI also interpreted the development of the directive bodies as cell-budding, likewise GIARD and also FOL, who has produced a very extensive and thorough investigation on the phenomena of the maturation of the egg in animals. Recently VAN BEXKDKN. supported by researches on Nematodes, has combatted the interpretation of the process as cell-budding; however, BOVERI and O. Z ACH ARIAS, who have established a complete agreement between the formation of directive bodies and the process of cell-division in the case of the Nematodes also, are unable to subscribe to his conclusion in this matter. As a new advance is to be recorded the discovery by WKISMAXX and by BLOCHMAXX, that in eggs which are developed parthenogenetically only a single jwlur cell arises. If the original obscurity on the morphological side, in which the phenomena MATTEATIOX OF THE EGC, AXD PROCESS OF FEKTTLISATIOK. 37 of the maturation of the egg were enveloped, has been in general cleared up, the same is not the case if we inquire after its physiological nwMring That and a rich accumulation to the interaction of protoplasm and active nuclear substance in the processes of division. Its dissolution is, as it were, the preliminary requirement for the renewed activity of the nuclear contents. But what function sball one ascribe to the polar cells? Concerning this several hypotheses have been proposed. BALFOUB, SEDGWICK MLSOT. TAX BEXEDES, and others, are of opinion that the immature egg, like every other cell, is originally hermaphroditic, and that by the development of polar cells it rids itself of the male constituents of its nucleus, which afterwards are replaced by fertilisation. BALFOTJR thinks that, if no polar cells were formed, parthenogenesis must normally occur. WEISJUAXX, supported by his discovery in the case of eggs developing parthenogenetically (p. 34), ascribes a different function to the first and the second polar cells. He distinguishes m the germinative vedcle two different kinds of p1 **-•!* y which he designates ovogenetic and germinal p1 •*"•** He maintains that by the formation of the first polar cell the ovogenetic plasma is »limiMt«*l from the ovum ; by that of the second polar ceO, half of the germinal plasma. In the latter case the ejected germinal plasma must be replaced by fertilisation. Thff»g hypotheses appear to ™«* upon rloffft1* examination to pr*******! manv vulnerable points. To me appears more promising an interpretation of BUTSCHIJ, who compares the egg, as had already often been done, to the MiQ*|K»«-- the egg must have once poBm-flrofl the capability of dividing **»Mf into many eggs. In the formation of the polar cells, which are eggs that have t"6OC'!£6 T"UCLLI!lr Hilary. T^5 IT TrT^. t— rTr H.Vf r>^rll TT-r >rT • "r - -i "r.^.- -I 1_T?-5 original conditions. Also BOTERI regards the polar cells as mortice eyyt. I have likewise always conceived of the conditions in this manner. & The Process of The union of egg-cell and spermatic cell is designated as the process fertilisation. This process is to be observed, sometimes with great netimes with considerable ease, according to the choice of for experimentation. The investigator ordinarily en- great difficulties meases where the ripe eggs are part, if not the whole, of their development is effected within ducts of the maternal organism. In such cases the fertili- also must evidently take place in the duets of the female sexual into which the semen is introduced in the act of internal fcrtiJ^atio* takes place in nearly all Vertebrates tbegre^er part of th^Fishesandmaiiy Amphibia. Usually the and the spermatozoa meet, in the case of Man and Mammals, in EMBRYOLOGY. the beginning of the oviduct ; likewise in the case of Birds they meet in the first of the four regions previously (p. 17) distinguished, and at a time when the yolk is not yet surrounded with its albuminous envelope and calcareous shell. In contrast to internal fertilisation stands external fertilisation r which is the simpler and more primitive method, and which occurs in the case of many Invertebrates that live in the water, as well as ordinarily in Fishes and Amphibia. In this method, while male and female keep near together, both kinds of sexual products, which are for the most part produced in great number, are evacuated directly into the water, where fertilisation takes place outside of the maternal Fig. 17 A, B, C.— Small portions of eggs of Asterias glacialis, after FOL, The spermatozoa have already penetrated into the gelatinous envelope which covers the eggs. Irv A there begins to be raised up a protuberance toward the most advanced spermatozoon. In- B the protuberance and spermatozoon have met. In C the spermatozoon has penetrated into the egg. A vitelline membrane, with a crater-like orifice, has now been distinctly formed. organism. The whole procedure is therefore much more easily observ- able. The experimenter has it within his power to effect fertilisation artificially, and thus to determine precisely the point of time at which egg and semen are to meet. He needs only to collect in a watch-glass containing water ripe eggs from a female, likewise in a second watch- glass ripe semen from a male, and then to mingle the two in a suitable manner. In this way artificial fertilisation is extensively practised in fish-breeding. For the purpose of scientific investigation the selection of the particular species of animal is of the greatest importance. It is manifest that animals with large opaque eggs do not commend themselves, whereas those species are especially suit- able whose eggs are so small and transparent that one can observe them under the microscope with the highest powers, and at the same time pass in review every least speck. Many species of Echinoderms MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 39 are in this respect most excellent objects for investigation. Conse- quently it was by means of them that an accurate insight into the processes of fertilisation was first secured. They may therefore serve in the following account as the foundation of our description. If ripe eggs with egg-nucleus are removed from the ovary into a watch-glass containing sea -water, and a small quantity of seminal fluid is added, a very uniform result is obtained, since in the course of five minutes every one of many hundreds or thousands of eggs is normally fertilised, as can be accurately observed by means of high magnification. Although spermatozoa attach themselves to the gelatinous envelope Fig. 18. Fig. 19. Fig. 18.— Fertilised egg of a Sea-urchin. The head of the spermatozoon which penetrated has been converted into a sperm-nucleus (*/t> surrounded by a protoplasmic radiation, and has approached the egg-nucleus (ek). Fig. 19.— Fertilised egg of a Sea-urchin. The sperm-nucleus (sk) and the egg-nucleus (ek) have come close to each other, and both are surrounded by a protoplasmic radiation. of an egg in great numbers, — many thousands of them when con- centrated seminal fluid is employed, — still only a single one of them is concerned in fertilisation, and that is the one which by the lash- like motion of its filament first approached the egg. Where it strikes the surface of the egg with the point of its head the clear superficial expanse of the egg-protoplasm is at once elevated into a small knob that is often drawn out to a fine point, the so-called receptive promin- (Empfdngnisshiigel), or cone of attraction. At this place the jminal filament, with pendulous motions of its caudal appendage, >res its way into the egg (fig. 17 A, B). At the same time a fine tembrane (fig. 71 C) detaches itself from the yolk over the whole irface, beginning at the cone, and becomes separated from it by ever-increasing space. The space probably arises because, in msequence of fertilisation, the egg-plasma contracts and presses 40 EMBRYOLOGY. out fluid (probably the nuclear fluid which was diffused after the disappearance of the germinative vesicle). The formation of a vitelline membrane is in so far of great signi- ficance for the fertilisation, as it makes the penetration of another male element impossible. No one of the other spermatozoa swing- ing to and fro in the gelatinous envelope is able after that to get into the fertilised egg. The one which has penetrated thereupon undergoes a series of changes. The contractile filament ceases to vibrate, and soon dis- appears ; but out of the head — which, as was previously stated, is derived from the nucleus of a sperm-cell (spermatid), and consists of nuclein — there is soon developed a very small spheroidal or oval corpuscle, which afterwards becomes somewhat larger, the semen- or sperm-nucleus (fig. 18 sk). This slowly moves deeper into the yolk, whereupon it exerts an influence upon the surrounding protoplasm. For the latter is arranged radially around the sperm -nucleus («&), so that there is formed a radiate figure, which is at first small, but afterwards becomes more and more sharply expressed and more ex- tended. Now an interesting phenomenon begins to hold the attention of the observer (figs. 18, 19, 20). Egg- nucleus and sperm-nucleus mutually attract each other, as it were, and migrate through the yolk toward each other with increasing velocity. The sperm-nucleus (sk), enveloped in its protoplasmic radia- tion, changes place more rapidly than the egg -nucleus (ek). Soon the two meet, either in, or at least near, the middle of the egg (fig. 19) ; become surrounded by a common radiation, which now extends through the whole yolk-substance ; are firmly juxtaposed, and then mutually flattened at the surface of contact ; and finally fuse with each other (fig. 20 fk). The product of their fusion is the first cleavage-nucleus (fk\ which undergoes the further alterations leading to cell-division. This whole interesting process of fertilisation has consumed in the present object of investigation the short time of about ten minutes only. The phenomena of fertilisation discovered in the Echinoderms were Fig. 20. — Egg of a Sea-urchin immediately after the close of fertilisation. Egg-nucleus and sperm-nucleus are fused to form the cleavage-nucleus (fk), which occupies the centre of a protoplasmic radiation. MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 41 soon observed, either completely or at least partially, in numerous other animals also — in Ccelenterates and Worms (NUSSBAUM, VAN BENEDEN, CARNOY, ZACHARIAS, BOVERI, PLAINER), and in Molluscs and Verte- brates. As regards the last, it has been possible to follow accurately in the case of Petromyzon the penetration of a single spermatozoon into the egg through a special preformed micropyle in the vitelline lembrane (CALBERLA, KUPFFER, BENECKE, and BOHM). Likewise in Amphibia, proof has been brought forward that after fertilisation sperm-nucleus is formed at the animal pole, and that, surrounded by pigmented area, derived from the cortex of the yolk, it moves to- ward another more deeply imbedded nucleus (egg-nucleus), and fuses dth it (0. HERTWIG, BAMBEKE, BORN). In Mammals the fertilisa- tion takes place in the beginning of the oviduct. Evidence has also been produced in their case that after the liberation of the polar cells two nuclei are temporarily to be seen in the egg-cells, and that, these unite in the centre of the egg to form the cleavage-nucleus (VAN BENEDEN, TAFANI). This is the proper place in which to mention briefly the so-called micropyle. In many animals (Arthropods, Fishes, etc.) the eggs are enclosed before they are fertilised in a thick firm envelope, which is impenetrable for spermatozoa. Now, in order to make fertilisation possible, there are found in these cases at a definite place on the egg- membrane sometimes one, sometimes several, small openings (micro- pyles), at which the spermatozoa accumulate in order to glide into the interior of the egg. The egg of Nematodes has for several years rightly played an important role in the literature of the process of fertilisation. But this is especially true for the egg of the Maw-worm of the Horse (Ascaris megalocephala), which VAN BENEDEN has made the subject •of a celebrated monograph. It is an excellent object, in so far as it not only can be had for study everywhere and at all seasons of the year, but also allows one to follow step by step, in the most accurate manner, the penetration and subsequent fate of the sper- matozoon. Since, moreover, the process of fertilisation in Ascaris megalocephala presents many peculiarities in its details, an extended presentation of them is both warranted and desirable. In the case of this Worm, in which the sexes are separate individuals, there is a copulation, and the fertilisation of the egg takes place within the sexual passages of the female. In one region, which is expanded into a kind of uterus, mature spermatic bodies are met with in great numbers. The appearance of these differs greatly from that which 42 EMBRYOLOGY. the male seminal elements ordinarily present in the animal kingdom : for they are apparently motionless ; are comparable in form to a cone, a conical ball, or a thimble (fig. 21) ; and consist in part of a granular substance (b), in part of a homogeneous lustrous substance (/), and of a small spherical body of nuclear substance (&), which is imbedded in the granular substance at the base of the cone. When the small naked eggs enter into the region designated as- uterus, fertilisation takes place at once. One spermatic body, which can execute feeble amoeboid motions with its basal end (SCHNEIDER), attaches itself to the surface of the yolk (fig. 22 sk). Where contact with the egg first takes place, there is formed, exactly as in the Echinoderms, a special cone of attraction. Here the spermatic body, without essential change of form, gradually glides deeper into the yolk, until it is completely enclosed therein (fig. 23). While the two sexual products are thus externally fused, the egg itself is not yet ripe, because it still Fig. 2i.-sPermatic possesses the germinative vesicle (fig. 22 kb). but body of Ascaris ,. , megaiocephaia, jt now promptly begins to enter upon the matura- after VAN BENE- tion stage by preparing to form the polar cells. k, Nucleus ; 6, base The germinative vesicle, which is of small size in of the cone, by the case of the Maw-worm of the Horse, loses its which the attach • '•••'• • <• ment to the egg sharp delimitation trom the yolk, moves toward takes place; /, ^hat surface of the egg which is opposite to the lustrous substance resembling fat. cone of attraction (figs. 23, 24), and is gradually converted into a nuclear spindle (sp), the origin of which may be traced upon this object with considerable precision. The most important part of the process consists in the formation, out of the chromatic substance, of numerous short, rod-like pieces (figs. 23, 24, c/t), which form directly the chromatic elements of the spindle, the chromosomes (WALDEYER). As in the case of the Echinoderms, there then arise at the surface of the yolk two small polar cells (fig. 25 pz) ; as in that case, a vesicular egg-nucleus (fig. 25 ei) arises from the half of the second polar spindle which remains in the peripheral portion of the yolk. Meanwhile the spermatic body has moved farther and farther from the place of its entrance into the egg (figs. 22, 23, sk), and finally comes to lie in the middle of the yolk (fig. 24 sk), approxi- mately in the position occupied by the germinative vesicle before its migration to the surface. During this period the spermatic body has gradually lost its original form and its sharp delimitation ; out MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 43 of its nuclear substance, which was described as a small, deeply stainable spherule, there arises a vesicular nucleus (fig. 25 sk), which acquires the same size and condition as the egg-nucleus. ak — Fig. 22. Fig. 23. Fig. 22. -An egg of Ascaris megalocephala just fertilised, after VAN BENEDEN. xk, Spermatic body, with nucleus, which has entered the egg ; /, fat-like substance of the spermatic body ; kb, gerruinative vesicle. Fig. 23.— A stage of a fertilised egg of Ascaris megalocephala, somewhat older than that of fig. 22, after VAN BENEDEN. sir, Spermatic body, which has penetrated deeper into the cortex of the yolk ; sp, polar spindle which has arisen from the germinative vesicle ; rh, chromosomes of the spindle. After the rapid and continuous accomplishment of these processes, the egg of the Worm usually enters on a longer or shorter period of Fig. 24. Fig. 25. of the egg of Ascaris Fig. 24.— A still older stage of development, following that of fig. megalocephala, after BOVERI. xp, Polar spindle, which has ascended to the surface of the yolk ; ch, 2x4 chromosomes ; sk, spermatic nucleus, which has migrated into the middle of the egg. Fig. 25.— Egg of Ascaris megalocephala in preparation for the process of cleavage, after E. VAN BENEDEN. 2'>z, Two polar cells which have arisen from the polar spindle (sp) of fig. 24 by a repetition of the pi-ocess of budding ; ei, egg-nucleus ; sk, spermatic nucleus already preparing to divide ; ch, nuclear loops or chromosomes. rest. It now presents (compare fig. 25, which represents a stage already further developed) at its surface within the vitelline mem- brane two polar cells (pz), and in its interior two large vesicular nuclei, the spermatic nucleus (sk) and the egg-nucleus (ei), the 44 EMBRYOLOGY latter of which has come close up to the former, without, however, fusing with it. A union of the male and female nuclear substances into a common nuclear figure takes place in the case of the Maw- worm, when the process of egg-cleavage is beginning. The processes of fertilisation just described can be designated as typical for the animal kingdom. But they appear to recur in exactly the same manner throughout the vegetable kingdom also, as has been shown by the thorough investigations of STRASBURGER. We are therefore in a better position now than formerly to advance a theory of fertilisation based upon an important array of facts : — In fertilisation clearly demonstrable morphological processes take place. Of these the important and essential one is the union of two cell-nuclei which have arisen from different sexual cells, a female egg- nucleus and a male spermatic nucleus. These contain the fructifying nuclear substance, which is an organised body and comes into activity as such in fertilisation. Recently the attempt has been made to expand the fertilisation theory into a theory of transmission. Important reasons may be urged, as appearing to indicate that the fructifying substance is at the same time the bearer of the transmissible peculiarities. The female nuclear substance transmits the peculiarities of the mother, the male nuclear substance the peculiarities of the father, to the nascent creature. Perhaps there is in this theory a morphological basis for the fact that offspring resemble both progenitors, and in general inherit from both equally numerous peculiarities. If we accept these two theories, the nucleus, which, despite its constant presence, previously had to be described as a problematic structure of unknown significance, acquires an important role in the life of the cell. It seems to be the cell's especial organ of fertilisation and transmission, inasmuch as there is stored within it a substance (idioplasma of NAGELI) which is less subject to cell metastasis. In connection with the consideration of the process of fertilisation may be permitted a slight digression to the realm of pathological phenomena. As follows from numerous observations in both the animal and vegetable kingdoms, in the normal course of fecundation only a single spermatic filament penetrates into an egg, when the encountering sexual cells are entirely healthy. But with an impaired condition of the egg-cell, superfetation by means of two or more seminal Jilaments (polyspennia) takes place. Superfetation may be produced artificially, if by way of experiment tut 2 MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 45 one injures the egg-cell. This may be accomplished either by exposing it temporarily to a lower or a higher temperature, and thus producing cold-rigor or heat-rigor, or by affecting it with chemical reagents, — chloroforming it, or treating it with morphine,, strychnine, nicotine, quinine, etc., — or by doing violence to it in a mechanical way, such as shaking it. It is interesting to observe how, ith all of these means, the degree of superfetation is, to a certain tent, proportional to the degree of the injury ; how, for example, a small number of spermatozoa penetrate into eggs which have been slightly affected with chloral, whereas a greater number penetrate those which have been more strongly narcotised. In all unfertilised eggs the whole course of development becomes abnormal. But whether, as claimed in FOL'S hypothesis, the origin, of double and of multiple organisms is referable respectively to the penetration of two and many spermatozoa, must still be regarded as doubtful. Certainly the question suggested richly deserves to be still more thoroughly tested experimentally. HISTORY. — The facts here given concerning the theory of fecundation are acquisitions of very recent times. To omit the older hypotheses, it was generally assumed up to the year 1875 that the spermatozoa penetrate in great numbers into the substance of the egg, but that they there lose their activity and become dissolved in the yolk. J succeeded in my study of the eggs of Toxopneustes lividus in finding an object in which all the internal phenomena of fertilisation may be determined with ease and certainty, and in establishing (1) that in consequence of fertilisation the head of a spermatic filament surrounded by a stellate figure makes its appearance in the cortex of the yolk, and is metamorphosed into a small corpuscle, which I called spermatic nucleus ; (2) that within ten minuter egg-nucleus and spermatic nucleus copulate ; (3) that normally fertilisation is accomplished by only a single spermatic filament, whereas in pathologically altered eggs several spermatozoa may penetrate. I was therefore able at that time to announce the proposition, that fertilisation depends upon the fusion of two sexually differentiated cell-nuclei. A few months later, VAN BENEDEN announced that in the case of Mammals- the segmentation-nucleus arises from the fusion of two nuclei, — as had previously been observed by AUERBACHand BtiTSCHLi in the case of numerous other objects, — and expressed the conjecture that one of them, which has at first a peripheral position, might in part result from the substance of the spermatozoa, which, in great numbers, as he maintained, fuse -and become commingled with the cortical portion of the yolk. An advance was soon after this made by FOL, who investigated with the greatest detail the eggs of Echinoderms at the very moment of the penetration of a spermatic filament into the egg, and discovered the formation of a cone of attraction. Since then it has been established by means of numerous researches (those of SELENKA, FOL, HERTWIG, CALBERLA, KUPFFER, NUSSBAUM, VAN BENEDEN, EBERTH, FLEMMING, ZACHARIAS. BOVERI, PLATNER, TAFANI, BOHM, and 46 EMBRYOLOGY. others) that in other objects also, and in other branches of the animal kingdom, the processes of fertilisation take place in essentially the same manner. At the same time the comprehension of the processes of fertilisation was essentially advanced, especially by the works of VAN BENEDEN on the egg of Ascaris megalocephala, to which have been added the important investiga- tions of BOVEBI and others on the same object. STEASBUEGEE has established in a series of excellent researches the identity of the processes of fertilisation in the animal and vegetable kingdoms. Finally, the phenomena of fertilisation were utilised simultaneously by STEASBUEGEE and myself for the foundation of a theory of heredity, in our endeavor to prove— what others (KEBEE, HAECKEL, HASSE) had previously expressed as a conjecture — that the male and the female nuclear substances are the bearers of the peculiarities which are transmitted from parent to offspring. KOLLIKEB, Eoux, BAMBEKE, WEISMANN, VAN BENEDEN, BOVEEI, and others have since expressed themselves in a similar manner. SUMMARY. 1. At maturation the germinative vesicle gradually rises to the animal pole of the egg, and thereby undergoes a regressive meta- morphosis (degeneration of the nuclear membrane and the fibrous network, mingling of the nuclear fluid — Kernsaft — with the proto- plasm). 2. A nuclear spindle (polar spindle or direction-spindle) is de- veloped out of remnants of the germinative vesicle, principally, indeed, out of the substance of the germinative dot, which breaks up into chromosomes. 3. At the place where the spindle encounters the surface of the yolk with one of its ends, there are formed two polar cells or direction- bodies (Richtungskorper) by means of a process of budding, which it-- repeated. 4. At the second budding, half of the nuclear spindle remains in the cortex of the yolk, and is metamorphosed into the egg-nucleus The egg is then ripe. 5. In the case of eggs which develop parthenogenetically (Arthro- poda), ordinarily only one polar cell is formed. 6. At fertilisation only a single spermatozoon penetrates a sound egg (formation of a cone d 'attraction, detachment of a vitelline nn-m- brane). 7. The head of the spermatozoon is converted into the spermatic nucleus, around which the neighbouring protoplasmic particles aiv radially arrangt d. 8. Egg-nucleus and spermatic nucleus migrate toward each other, and in nictt instances immediately fuse to form the segmentation- LITERATURE. 47 nucleus ; in many objects they remain for a considerable time near each other, but not united, arid only later are together metamorphosed into the segmentation-spindle. 9. In some animals fertilisation of the egg takes place only after completion of its maturation, but in others it is inaugurated at the very beginning of maturation, so that the two phenomena overlap each other. 10. Fertilisation theory. Fertilisation depends on the copulation of two cell-nuclei, which are derived from a male cell and a female cell. 11. Theory of heredity. The male and female nuclear substances contained in the spermatic nucleus and the egg-nucleus are the bearers of the peculiarities which are transmissible from parents to their offspring. LITERATURE. Agassiz and Whitman. The Development of Osseous Fishes. II. The pre-embryonic Stages of Development. Pt. 2. The History of the Egg from Fertilization to Cleavage. Mem. Museum Comp. Zoology at Harvard College. Vol. XIV. No. I. Part II. 1889. Balfour. On the Phenomena accompanying the Maturation and Im- pregnation of the Ovum. Quart. Jour. Micr. Sci. Vol. XVIII. 1878, p. 109. Bambeke. Eecherches sur 1'Embryologie des Batraciens. Bull, de 1'Acad. roy. Sci. de Belgique. 2me ser. T. LXI. 1876. Beneden, Ed. van, et Charles Julin. Observations sur la maturation, la fecondation et la segmentation de 1'oeuf chez les cheiropteres. Archives de Biologic. T. I. 1880, p. 551. Beneden, E. van. La maturation de 1'oeuf, la fecondation, etc., des mammi- feres. Bull, de 1'Acad. roy. Sci. de Belgique. 2me ser. T. XL. Xr. 12. 1875. Beneden, E. van. Contributions a 1'histoire de la vesicule germinative, etc. Bull, de 1'Acad. roy. Sci. de Belgique. 2me ser. T. XLI. Nr. 1. 1876. Beneden, E. van. Eecherches sur la maturation de 1'oeuf, la fecondation et la division cellulaire. Archives de Biologic. T. IV. Paris 1883. leneden, van, et TsTeyt. Nouvelles recherches sur la fecondation et la division mitosique chez 1'Ascaride megalocephale. Leipzig 1887. And Bull, de 1'Acad. roy. Sci. de Belgique. 3me ser. T. XIV. p. 215. Blochmann. Ueber die Eichtungskorper bei den Insecteneiern. Biol. Cen- tralblatt. Bd. VII. 1887. Blochmann. Ueber die Eichtungskorper bei Insecteneiern. Morphol. Jahrb Bd. XII. 1887, p. 544. Blochmann. Ueber die Reifung der Eier bei Ameisen und Wespen. Fest- Ischrift zur Feier des SOOjiihr. Bestehens der Univ. Heidelberg. 1886. Med. Theil. 48 EMBRYOLOGY. Blochmann. Ueber die Zahl der Richtungskorper bei befruchteten un«». unbefruchteten Bieneneiern. Morpholog. Jahrb. Ed. XV. 1889. Bohm, A. Ueber Reifung und Befruchtung des Eies von Petromyzon. Archiv. f. mikr. Anat. Bd. XXXII. 1888, p. 613. Born. Ueber den Einfluss der Schwere auf das Froschei. Archiv f. mikr. Anat. Bd. XXIV. 1885, p. 475. Born. Weitere Beitrage zur Bastardirung zwischen den einheimischen Anuren. Archiv f. mikr. Anat. Bd. XXVII. 1886, p. 192. Boveri. Ueber die Bedeutung der Richtungskorper. Sitzungsb. d. Gesellsch. f. Morphol. u. Physiol. in Munchen. Sitzung vom 16. Nov. 1886, p. 101. Miinchener medic. Wochenschr. Jahrg. 33. Nr. 50. Boveri. Ueber die Befruchtung der Eier von Ascaris megalocephala. Sit- zungsb. d. Gesellsch. f. Morphol. u. Physiol. in Munchen. Sitzung vom 3. Mai, 1887, p. 71. Boveri. Ueber den Antheil des Sperraatozoons an der Theilung der Eier. Sitzungsb. d. Gesellsch. f. Morphol. u. Physiol. in Munchen. Bd. III. 1887, p. 151. Boveri. Zellenstudien. Jena. Zeitschr. Bde. XXI. XXII. XXIV. 1887. -88, -90. Butschli. Studien iiber die ersten Entwicklungsvorgiinge der Eizelle, Zell- theilung u. Conjugation der Infusorien. Abhandl. d. Senckenberg. naturf. Gesellsch. Bd. X. Frankfurt 1876. Butschli. Gedanken iiber die morphologische Bedeutung der sogenannten Richtungskorperchen. Biol. Centralblatt. Bd. IV. 1884, pp. 5-12. Butschli. Entwicklungsgeschichtliche Beitrage. Zeitschr. f. wiss. Zoologie. Bd. XXIX. 1877. Calberla. Befruchtungsvorgang beim Ei von Petromyzon Planeri. Zeitschr. f. wiss. Zoologie. Bd. XXX. 1878, p. 437. Carnoy, J. B. La cytodierese de 1'oeuf. La vesicule germinative et les globules polaires de 1'Ascaris megalocephala. 1886. And La Cellule. T. III. 1887. Dewitz. Ueber Gesetzmassigkeit in der Ortsveranderung der Spermatozoen und in der Vereinigung derselben mit dem Ei. Archiv f. d. ges. Physiol. Bd. XXXIX. 1886. Eberth. Die Befruchtung des thierischen Eies. Fortschritte der Medic. Nr. 14. 1884. Flemming, W. Ueber die Bildung von Richtungsfiguren in Siiugethiereiern beim Untergang Graaf'scher Follikel. Archiv f. Anat. u. Physiol., Anat. Abth. 1885. Flemming, W. Ueber Bauverhiiltnisse, Befruchtung u. erste Theilung der thier. Eizelle. Biol. Centralblatt. Bd. III. 1884, pp. 641, 678. Flemming, W. Beitriige zur Kenntniss der Zelle, etc. III. Theil. Arch. f. mikr. Anat Bd. XX. 1881. Fol. Sur le commencement de 1'henogenie. Archives des Sci. phys. et nut. Geneve 1877. Fol. Recherches sur la fecondation et le commencement de 1'henogenie. M£m. de la Soc. de Phys. et d'Hist. nat. (icn'n-c 1879. Frommann. Article " Befruchtung " in Real-Encyclopadie der gesammten Heilkunde. 2 A nil. Qiard, Alf. Note sur les premiers phenomenes du developpement de 1'oursin. Comptes rendus. LXXXIV. 1877. LITERATURE. 40 •G-reeff, R. Ueber den Ban und die Entwicklung der Echinodermen. Sit- zungs-b. d. Gesellsch. z. Beford. d gesammten Naturwiss. zu Marburg. Nr. 5. 1876. Hasse, C. Die Beziehungen der Morphologic zur Heilkunde Leipzig 1879. Henking. Ueber die Bildung von Kichtungskorpern in den Eiern der Insecten und dereii Schicksal. Nachr. d. kgl. Gesellsch. d. Wiss. zu Gottingen Jahrg. 1888. Hensen. Die Physiologic der Zeugung. Handbuch der Physiologie von Hermann. Bd. VI. Theil II. 1881. Hensen. Die Grundlagen der Vererbung. Landwirthsch. Jahrb. 14. 1885. Hertwig, Oscar. Beitrage zur Kenntniss der Bildung, Befruchtung u. Theilung des thier. Eies. Morphol. Jahrb. Bd. I. 1875. Hertwig, Oscar. Beitrage, etc. II. Theil. Morphol. Jahrb. Bd. III. 1877, pp. 1-86. Hertwig, Oscar. Weitere Beitrage, etc. Morphol. Jahrb. Bd. III. 1877. Hertwig, Oscar. Beitrage zur Kenntniss, etc. Morphol. Jahrb. Bd. IV. Heft 1 u. 2. 1878. Hertwig, Oscar. Welchen Einfluss iibt die Schwerkraft auf die Theilung der Zellen. Jena 1884. Hertwig, Oscar. Das Problem der Befruchtung und der Isotropie des Eies, eine Theorie der Vererbung. Jena. Zeitschr. f. Naturwiss. Bd. XVIII. Jena 1884. Hertwig, Oscar und Richard. Experimentelle Untersuchungen iiber die Bedingungen der Bastardbefruchtung. Jena 1885. Hertwig, Oscar und Richard. Ueber den Befruchtungs- und Theilungs- vorgang des thierischen Eies unter dem Einfluss ausserer Agentien. Jena 1887. Hertwig,Oscar und Richard. Experimentelle Studien am thierischen Ei. Jena. Zeitschr. Bd. XXIV. 1890. Hoffmann, C. K. Zur Ontogenie der Knochenfische. Verhandl. d. koninkl. Akad. v. Wetensch. Amsterdam. Deel XXI. 1881. Hoffmann, C. K. Ueber den Ursprung und die Bedeutung der sogenannten freien Kerne in dem Nahrungsdotter bei den Knochenfischen. Zeitschr. f. wiss. Zoologie. Bd. XLVI. 1888. Kastschenko. Zur Frage iiber die Herkunft der Dottsrkerne im Selachierei. Anat. Anzeiger. 1888. Kolliker. Die Bedeutung der Zellenkerne fiir die Vorgange der Vererbung. Zeitschr. f. wiss. Zoologie. Bd. XLIL 1885, pp. 1-46. Kolliker. Das Karyoplasma und die Vererbung. Eine Kritik der Weis- mann'schen Theorie von der Kontinuitat des Keimplasma. Zeitschr. f. wiss. Zoologie. Bd. XLIV. 1886. Kultschitzky. Ueber die Eireifung und die Befruchtungsvorgarige bei Ascaris marginata. Archiv f. mikr. Anat. Bd. XXXII. 1888. Kultschitzky. Die Befruchtungsvorgiinge bei Ascaris megalocephala. Archiv f. mikr. Anat. Bd. XXXI. 1888, p. 567. Kupffer. Betheiligung des Dotters am Befruchtungsakt bei Bufo variabilis u. vulgaris. Sitzungsb. d. math. Classe. d. Akad. d. Wissensch. zu Munchen, 1882, p. 608. fipffer, C., und B. Benecke. Der Vorgang der Befruchtung am Ei der Neunaugen. Konigsberg 1878. van, S. Beitrage zur Kenntniss der Entwicklung der Mollusca acepbala ' 50 EMBRYOLOGY. lammellibranchiata. Abhandl. d. k. schwed. Akad. der Wissensch. 1848. Im Auszuge iibersetzt. Stockholm 1879 Mark, E. L. Maturation, Fecundation and Segmentation of Limax campestris. Bull. Museum Comp. Zoology at Harvard College. Vol. VI. 1881. Massart. Sur la penetration des spermatozoides dans 1'oeuf de la grenouille. Bull, de 1'Acad. roy. Sci. de Belgique. 3me ser. T. XVIII. 1889. Minot. Proceed. Boston Soc. Nat. Hist. XIX. 1877. American Naturalist. 1880. Muller, Fr. Zur Kenntniss des Furchungsprocesses im Schneckenei. Archiv f. Naturg. 1848. Wageli, C. von. Mechanisch-physiologische Theorie der Abstammungslehre. Miinclien 1884. Nussbaum, M. Ueber die Veranderung der Geschlechtsproducte bis zur Eifurchung. Arch. f. mikr. Anat. Bd. XXIII. 1884, p. 155. Nussbaum, M. Zur Differenzirung des Geschlechts im Thierreich. Archiv f. mikr. Anat. Bd. XVIII. 1880. Wussbaum, M. Bildung und Anzahl der Richtungskorper bei Cirripedien. Zool. Anzeiger. XII. 1889. Oellacher, J. Untersuchungen iiber die Furchung und Blatterbildung im Hiihnerei. Strieker's Studien. a. d. Inst. f. exper. Pathol. 1869. Oellacher, J. Beitrage zur Geschichte des Keimbliischens im Wirbelthierei. Archiv f. mikr. Anat. Bd. VIII. 1872. Platner, G. Beitrage zur Kenntniss der Zelle und ihrer Theilung. Archiv f, mikr. Anat. Bd. XXXIII. 1889. Platner, G. Die erste Entwicklung befruchteter und parthenogenetischer Eier von Liparis dispar. Biol. Centralblatt. Bd. VIII. 1888, -89. Platner, G. Ueber die Bildung der Richtungskorperchen. Biol. Centralblatt, Bd. VIII. 1888, -89. Purkinje. Symbolae ad ovi avium historiam ante incubationem. Lipsiae 1825. Sabatier, A. Contribution & 1'etude des globules polaires et des elements elimines de 1'oeuf en ge"ne>al. (Theorie de la sexualite\) Montpellier 1884. Rev. des Sci. Nat. 1883, -84. Schneider, A. Das Ei und seine Befruchtung. Breslau 1883. Schultze, O. Untersuchungen iiber die Reifung und Befruchtung des Amphibieneies. Zeitschr. f. wiss. Zoologie. Bd. XLV. 1887. Selenka, E. Befruchtung des Eies von Toxopneustes variegatus. Lri/>:i/f 1 NTs. Strasburger, Ed, Neue Untersuchungen iiber den BefruchtungsvoriraiiL: bei den Phanerogamen als Grundlage fur eine Theorie der Zeugung. Jena 1884. Tafani. I primi momenti dollo sviluppo dei mammiferi. Publicazioni del istituto di studi superiori in Firenze. 1889. Weismann, A. Ueber die Vererbung. Jena 1883. Weismann, A. Die Continuitat des Keimplasma als Grundlage einer Theorie der Vercrbung. Jena 1885. Weismann, A. Ueber die Zahl der Richtungskorper und iiber ihre Bedeutung fur die Vererbung. Jena 1887. Weismann und Ischikawa. Ueber die Bildung der Richtungskorper bei thierischen Eiern. Berichte d. naturf. Gesellsch. zu Freiburg i. B, Bd. III. 1887, pn. 1-44. THE PROCESS OP CLEAVAGE. 51 Weismann und Ischikawa. Weitere Untersuchungen zum Zahlengesetz der Kichtungskorper. Zool. Jahrbiicher. Bd. III. Abth. f. Morph. 1889, p. 515. Weismann und Ischikawa. Ueber die Paracopulation im Daplmidenei. sowie iiber Reifung u. Befruchtung desselben. Zool. Jahrbiicher. Bd. IV. Abth. f. Morph. 1889. Whitman, C. O. The Kinetic Phenomena of the Egg during Maturation and Fecundation. Jour. Morphol. Vol. I. 1887. Zacharias, Otto. Neue Untersuchungen iiber die Copulation der Ge- schlechtsproducte und den Befruchtungsvorgang bei Ascaris megalo- cephala. Archiv f. mikr. Anat. Bd. XXX. 1887. Zacharias, Otto. Die feineren Vorgiinge bei der Befruchtung des thierischen Eies. Biol. Centralblatt. Bd. VIT. 1888, p. 659. CHAPTER III. THE PROCESS OF CLEAVAGE. FERTILISATION is in most instances immediately followed by further development, which begins with the division of the egg-cell — the simple elementary organism — into an ever-increasing number of small cells — the process of cleavage. We shall begin the study of cleavage with a very simple case, and here also choose as a foundation for the presentation of the subject the egg of an Echinoderm and the egg of the common Ascaris of the Horse. In the living egg of the Echinoderm the cleavage-nucleus (fig. 26 ;), which arose from the fusion of egg-nucleus and spermatic lucleus, is at first spheroidal, and lies exactly in the middle of the where it forms the centre of a radiation which affects the rhole yolk-mass ; but it soon begins to be slightly elongated, and the same time to become less and less distinct, so that with the /ing object one might be misled into assuming that it had been >mpletely dissolved. Before this, very regular changes in the dis- ^ibution and arrangement of the protoplasm around the. nucleus tve taken place. The monocentric radiation resulting from fer- tilisation is divided. The two newly formed radiations thereupon love to the poles of the elongated nucleus. At first small and in- jnificant, they rapidly extend, and finally each occupies a half of the egg (fig. 27), and the rays of the two systems meet at a sharp ingle in the median plane of the egg. Just in proportion as the two radiations become more distinct, >re arises, within the granular yolk, as the starting-point and 5:3 EMBRYOLOGY. centre of the radiations, a figure, which may be appropriately com- pared (fig. 27) with a dumb-bell. It arises by the accumulation of a large amount of homogeneous protoplasm around the poles of tho elongating nucleus, forming the two ends of the dumb-bell; the poles may be regarded as if they were two centres of attraction. The non-granular streak, representing the handle of the dumb-bell, is the nucleus, which has meanwhile undergone a peculiar metamor- phosis and has become indistinct. A more accurate knowledge of the nuclear metamorphosis may be got by employing suitable reagents and dyes. By means of inter- mediate stages, which may be disregarded here, thero ar'ses out of - fk Fig. 20. Fig. 27. Tig. 26 — Egg of a Sea-urchin immsdiatsly after the conclusion of fertilisation, fk, Cleavage- nucleus. Fig. 27. — Egg of a Sea-urchin in preparation for division. The nucleus is no longer to be seen ; there has arisen in its place a dumb-bell figure. Both figures are drawn from the living object. the vesicular nucleus the nuclear spindle (fig. 31 B), which is a typical structure for cell-division throughout the organic world. This (sp) consists of two substances, both of which, in my opinion, are derived from the quiescent condition of the nucleus — namely, (!) of a non- chromatic substance, which does not show affinity for any dyes, and (2) of the stainable nuclein or cliromatin. The non-chromatic substance forms extraordinarily fine, and therefore at times scarcely discernible, " spindle-fibres" which are united into a bundle, and give rise to a spindle by the convergence of their ends to points. The chromatin, on the contrary, has assumed the form of small individual granules or chromosomes, which correspond in number with the spindle-fibres, and are so arranged that each granule adjoins a spindle-fibre at its middle point. In its totality, therefore, it con- stitutes at the middle of the spindle a plate composed of individual THE PROCESS OF CLEAVAGE. 53 granules — the nuclear plate of STRASBURGER. That which in the case of the Sea-urchin ordinarily appears as a chromatic granule is found, upon the employment of the highest magnifying powers, — but especially in the study of objects (fig. 28 A) more suitable for this purpose, — to be a small Y-shaped loop. The number of the loops or chromosomes appears to be very definite, and subject to law for each species of animal. At the tips of the spindle there may be demonstrated, in addi- tion, two special and exceedingly minute bodies, one of which occupies the exact centre of each of the two previously mentioned systems of rays ; they are, in fact, to be regarded as the cause of the , Fig. 28.— Diagram of nuclear division, after RABL. Ill figure A one sees the spindle, composed of delicate non-chromatic fibres, with the protoplasmic radiations at its tips and the chromatic loops at its middle. The splitting of the filaments of the latter has already taken place. In figure B the daughter-loops resulting from the fission have moved apart in opposite directions. In figure (7 they begin to arrange themselves in a regular manner into two groups of loops. In figure D the groups of daughter-loops lie near the two poles of the spindle. diter. Inasmuch as during the elongation of the nucleus they are to be found at each of its two poles, they may be especially designated as polar corpuscles [or centrosomes\. During the whole process of the division of nucleus and cell-body, it appears as though a directing influence belongs to the two polar corpuscles. Important changes in the nuclear loops of the spindle take place iuring later stages of the process of division. Each loop is split lengthwise into two daughter-loops (fig. 28 A), as discovered by FLEMMIXG and as confirmed since then by numerous other investi- gators (STRASBURGER, HEUSER, VAN BENEDEN, RABL, and others). f These daughter-loops soon move apart toward the opposite ends of he spindle (figs. 28 JB, C'} see also the explanation of the figures), and Approach very closely to the polar corpuscles at their tips (fig. 28 D} Thus by a complicated process a division of the stainable nuclear .ubstance into similar halves is brought about. As the immediate di * 01 EMBRYOLOGY. consequence of this the protoplasmic parts of the cell also begin at this time to be divided into halves by means of the process of cleavage, which is already recognisable externally. There is formed at the surface of the egg (fig. 29 A), in a plane passing between the two groups of loops through the middle of the spindle perpendicular to its long axis, a circular furrow, which rapidly cuts deeper and deeper into the substance of the egg, and in a short time divides it into two equal parts. Each of these contains half of the spindle Fig. 29 A. — Egg of a Sea-urchin at the moment of division. A circular furrow cuts into the yolk and halves it in a plane which is perpendicular to the middle of the nuclear axis and to the long axis of the dumb-bell. B.— Egg of a Sea-urchin after its division into two cells. In each resultant of the division a vesicular daughter-nucleus has arisen. The radial arrange- ment of the protoplasm begins to become indistinct. Both figures are drawn from the living object. with half of the loops, half of the dumb-bell, and a protoplasmic radiation. The resulting halves of the egg, still surrounded in common by the vitelline membrane, then closely apply to each other the surfaces resulting from the division, and become so flattened that each one of them forms approximately a hemisphere (fig. 20 J5). Internally, however, nucleus and protoplasm enter upon a brief transitory resting stage. There is developed out of the half of the nuclear spindle with its daughter-loops a vesicular homogeneous daughter-nucleus like the first, but in the protoplasm the radial arrangement becomes less and less distinct and at last entirely disappears. The egg of the common Maw- worm of the Horse is also a very instructive object for the study of the process of cleavage, as it was for the study of fertilisation, for it allows a still deeper in.sight into this process. As has already been stated, the egg-nucleus and the THE PROCESS OF CLEAVAGE. 55 spermatic nucleus remain for a time separate, even after they have approached each other. After a brief period of rest both of them begin to exhibit simultaneously the changes which precede the for- mation of the nuclear spindle. In each the chromatic substance is metamorphosed into a fine thread, which is arranged within the nuclear membrane in numerous windings. Each filament is there- upon divided into two equally large coiled loops, the chromosomes {fig. 25 ch). Now the two vesicular nuclei lose their delimitation from the surrounding yolk, in which there arise at a little distance from each other two polar corpuscles [centrosomes], surrounded by a system of rays, which is at first faint, but subsequently becomes more distinct. Between the two centrosomes, the method of whose development no one has as yet succeeded in observing, there are formed spindle-fibres, and the four loops (chromosomes), set free by the dissolution of the two nuclear membranes, so arrange themselves that they lie upon the outside of the spindle at its equator. In the case of the egg of the Maw-worm, therefore, the union of the two sexual nuclei, which terminates the act of fertilisation, takes place only at the time of the metamorphosis to form the cleavage- spindle, in which metamorphosis they take an equal share. In conse- quence of this remarkable deviation from the ordinary course of the process of fertilisation, VAN BENEDEN has been able to establish the interesting and important fact that half of the chromosomes of the first cleavage-spindle are derived from the egg-nucleus, and half from the spermatic nucleus, and that consequently they may be distin- guished as female and male chromosomes. Since in this instance, just in nuclear division ordinarily, the four loops are split lengthwise ind then move apart toward the two polar corpuscles (centrosomes), }here are formed two groups of four daughter-loops each, of which :wo are of male origin and two of female. Each group is then meta- )rphosed into the quiescent nucleus of the daughter-cell. This furnishes incontestable proof, that to each daughter-nucleus in each 'If of the egg, which arises as the result of the first cleavage, there is ransmitted exactly the same amount of chromatic substance from the ig-nucleus as from the spermatic nucleus. The first division is followed after a brief period of rest by the >nd, this by the third, the fourth, etc., during which are repeated same series of changes in nucleus and protoplasm that have just m described. Thus in quick succession the 2 first daughter-cells ire divided into 4, these into 8, 16, 32, 64, etc. (fig. 30), until lere has resulted a large spheroidal mass, which has received the 56 EMBRYOLOGY. name morula or mulberry-sphere, because the cells protrude as small elevations at its surface. During the second and third stages of cleavage there is easily- recognisable a rigidly observed order in the direction ivhich the planes of cleavage sustain to each other. The second plane of cleavage always halves the first and cuts it perpendicularly ; the third plane, again, is perpendicular to the first two, and passes through the middle of the :ixis formed by their intersection. If one regards the ends of this axis as the poles of the egg, the first two planes of division may be designated as meridional, the third as equatorial. This uniformity is caused by the mutual relation which subsists between nucleus and protoplasm, in which connection the two follow- ing laws are to be noted : (1) The plane of division always cuts the axis of the spindle perpendicularly at its centre. (2) The position of Fig. 30.— Various stages of the process of cleavage, after QMOnnu.uk. the axis of the nuclear spindle in turn depends on the form and differ- entiation of the protoplasmic body ivhich envelops it, and in such it- manner that the two poles of the nucleus take the direction of the greatest protoplasmic masses. Thus, for example, in a sphere in which the protoplasm is uniformly distributed, the centrally situated spindle may come to lie in any radius ; but in an ovoid protoplasmic body, only in the longest diameter. In a circular protoplasmic disc the nuclear axis lies parallel to its surface in any diameter whatever of the circle, but in an oval disc, as before, in the longest diameter only. Let us return now, after these general remarks, to the case under consideration. Each daughter-cell forms at the close of the first seg- mentation a hemisphere. According to the rule, the daughter-spindle cannot assume a position perpendicular to the flat surface of the hemisphere, but must lie parallel to it, so that a division into two quadrants must result. At the next- segmentation the axis of the spindle must coincide with the long axis of the quadrant, whereby this becomes divided into two octants. THE PROCESS OF CLEAVAGE. 57 There are some important deviations from the process of division just described, which affect the form of the cleavage products, although leaving unaltered the finer processes relating to the nucleus. The deviations are induced, as we shall show more in detail in the in- dividual cases, .by the variation in the amount of deutoplasm contained in the eggs, and by the previously described variability in its distribu- tion. One may appropriately separate the various forms of the process of cleavage into two classes, and each class into two sub- classes, although the forms merge into one another by means of transitional conditions. To the first class we assign such eggs as are completely divided into segments by the process of cleavage. The cleavage itself we designate as total ; and according as the segments are of equal or un- equal size, we distinguish as subdivisions equal cleavage and unequal cleavage. With total is contrasted partial cleavage. This occurs in the case of eggs which are provided with very abundant deutoplasmr and are consequently of considerable size, and in which, at the same time, the previously described separation into formative yolk and nutritive yolk has been distinctly established. In this case the for- mative yolk alone undergoes a process of cleavage, whereas the chief mass of the egg, the nutritive yolk, remains undivided, and in general unaffected, by the processes of embryonic development ; hence the name partial cleavage. This, in turn, is resolvable into the two sub- types of discoidal and superficial cleavage, according as the forma- ive yolk rests as a disc upon the nutritive yolk, or envelops the ,tter as a thick cortical layer. REMAK has designated eggs with 1 segmentation as holoblastic, those with partial segmentation as meroblastic. We may therefore present the following scheme of cleavage : — I. TYPE- TO tal cleavage : "j (#) Equal cleavage Holoblastic eggs. (£>) Unequal cleavage j II. TYPE— Partial cleavage : "| O) Discoidal cleavage Meroblastic eggs. (&) Superficial cleavage j Ia- Equal Cleavage. In the general consideration of the process of cleavage we have y become acquainted with the phenomena of equal segmenta- 58 EMBRYOLOGY. tion. It remains to be added to what has been previously said, that this t*n^e is most frequent in the case of Invertebrates, and is to be encountered among Vertebrates only in the cases of Amphioxus and Mammals. With the latter, however, there early appears a slight difference in the size of the segments ; this has induced many investigators to designate the cleavage of Amphioxus and Mammals as unequal also. If I have not followed this suggestion, it is because the differences are of a trivial nature, because the nucleus in the egg-cell and also in its segments still occupies a central position, and because the different methods of cleavage are in general not sharply definable, but connected by transitional con- ditions. Concerning Amphioxus, HATSCHEK states that at the eight-cell stage four smaller and four larger cell are to be distinguished, and that from that time forward in all the subsequent stages there is to be observed a difference in size, and that the process of cleavage takes place in a manner similar to that which will be subsequently described for the Frog's egg. The egg of the Rabbit, concerning which we have the painstaking investigations of VAN BENEDEN, •divides at the very outset into two segments of slightly different size ; moreover, from the third stage of division onward there occurs xi difference in the rapidity with which the divisions follow each other in the different segments. After the four cleavage-spheres have been divided into eight, there is a stage with twelve spheres ; this is followed by another with sixteen, and afterwards another with twenty-four. !*>• Unequal Cleavage. As a basis for the description of unequal cleavage we may employ the Amphibian egg, the structure of which has already been con- sidered. As soon as the egg of the Frog or Triton is deposited in the water and is fertilised, and while the gelatinous envelope is swelling up, its black pigmented hemisphere or animal half becomes directed upward, because it contains more protoplasm and small yolk-spherules, and is specifically lighter. The want of uniformity in the distribution of the various components of the yolk also induces an altered position of the segmentation-nucleus. Whereas the latter assumes a central position in all cases in which the deutoplasm is uniformly distributed, it invariably alters its location whenever -one half of the egg is richer in deutoplasm and the other richer in protoplasm ; it then migrates into the more protoplasmic territory. THE PROCESS OF CLEAVAGE. 5'J In the case of the Frog's egg, consequently, we find it in the black piginented hemisphere, which is turned upward. When in this case the nucleus prepares to divide, its axis can no longer assume the position of any and every radius of the egg. In consequence of the want of uniformity in the distribution of the protoplasm, the nucleus comes under the influence of the more protoplasmic pigmented part, which rests on the more deutoplasmic portion like an inverted cup, and, on account of its less specific gravity, floats at the surface, and is spread out horizontally. But in a horizontal protoplasmic disc the nuclear spindle comes to occupy a horizontal position (fig. 31 A sp). Consequently the plane of division must be formed in a vertical direction. A small furrow now pr sp Fig. 31.— Diagram of the division of the Frog's egg. A, Stage of the first division. B, Stage of the third division. The four segments of the second stage of division are beginning to be divided by an equatorial furrow into eight segments. P, pigmented surface of the egg at the animal pole ; pr, the part of the egg which is richer in protoplasm ; d, the part which is richer in deutoplasm ; sp, nuclear spindle. begins to show itself — at the animal pole first, because the latter is more under the influence of the nuclear spindle, which lies nearer to it, and because it contains more protoplasm, from which proceed the phenomena of motion during division. The furrow gradually deepens downward, and cuts through to the vegetative pole. By the first act of division we get two hemispheres (fig. 32 2), each of which is composed of a quadrant richer in protoplasm and directed upward, and another poorer in protoplasm and directed downward. By this means both the position of the nucleus and the direction of its axis are again determined, when it prepares for the second division. According to the rule previously laid down, the nucleus is to be sought in the quadrant which contains the more protoplasm ; the axis of the spindle must .take a position parallel to the long axis of the quadrant, and must therefore come to lie horizontally GO EMBRYOLOGY. The second plane of division is consequently, like the first, vertical, and cuts the latter at right angles. After the conclusion of the second segmentation the Amphibian egg consists of four quadrants (fig. 32 4), which are separated from one another by vertical planes of division and possess two dissimilar poles, — one richer in protoplasm, lighter, and directed upwards ; tho other richer in yolk, heavier, and directed downwards. In the case of equal segmentation we saw that at the stage of the third segmentation the axis of the nuclear spindle becomes parallel to the long axis of the quadrant. The same thing occurs here also, although in a some- what modified manner. On account of the greater accumulation of protoplasm in the upper half of the quadrant, the spindle cannot, as Fig. 32.— Cleavage of Rana temporaria, after ECKER. The numbers placed above the figures indicate the number of segments present in the corre- ponding stage. in the case of equal segmentation, lie in the middle of it, but must lie nearer to the animal pole of the egg (fig. 31 B sp). Moreover, it is exactly vertical, because the four quadrants of the Amphibian egg are definitely oriented in space on account of the difference in specific gravity of their halves. In consequence of this the third plane of division must be horizontal, and must also lie above the equator of the egg-sphere more or less toward its animal pole (fig. 32 8). The segments are very unlike both in size and composition ; and this is the reason why this form of segmentation has been called unequal. The four upper segments are smaller and contain less yolk, the four lower ones are much larger and richer in yolk. They are also distinguished from each other as animal cells and vegetative cells, according to the poles near which they lie. In the course of further development, the distinction between animal and vegetative cells constantly increases, for the richer the cells are in protoplasm the more quickly and the more frequently TEE PROCESS OF CLEAVAGE. 61 <]o they divide. At the fourth stage the 4 upper segments are first divided by vertical furrows into 8, and then after an interval the 4 lower ones are divided in the same manner, so that the egg is now composed of eight smaller and eight larger cells (fig. 32 16). After a short resting stage the eight upper segments are again divided, this time by a horizontal furrow, and somewhat later a similar furrow divides the eight lower segments also (fig. 32 32). In the same manner the 32 segments are divided into 64 (fig. 32 64). In the stages which follow this, the divisions in the animal half of the egg are still more accelerated relatively to those of the vegetative half. While the 32 animal cslls are divided into 128 segments by two divisions which follow each other in quick succession, there are still found in the lower half only 32 cells which are preparing for cleavage. It thus comes to pass that, as the final result of the process of cleavage, there exists a spheroidal mass of cells with entirely dissimilar halves, — an upper, animal half with small, pigmented €ells, and a vegetative half with larger, clear cells, containing more abundant yolk. From the nature of the progress of unequal cleavage, as well as from a series of other phenomena, one may lay down a general law, first formulated by BALFOUR, that the rapidity of cleavage is pro- portional to the concentration of protoplasm in the segment. Cells which are rich in protoplasm divide more rapidly than those in which protoplasm is more scanty and deutoplasm more abundant. As we have seen, the Frog's egg, by reason of the difference in specific gravity between its animal and vegetative halves, by reason of the heterogeneous pigmentation of its surface, by reason of the unequal distribution of protoplasm and deutoplasm, and by reason or the eccentric position of its nucleus, allows us to pass fixed and easily determinable axes through its spherical body. On this account it is an especially favourable object upon which to determine the question whether the egg allows one to recognise in the position of its parts, •even before fertilisation, immediately after the same, and during the process of cleavage, fixed relations to the organs of the fully developed organism. This question has been tested by means of ingenious experiments, especially by PFLUEGER and Roux, by the latter in his •" Beitrage zur Entwicklungsmechanik des Embryo." These have resulted in determining that the first cleavage plane of the egg corresponds to the median plane of the embryo, so that it separates the material of the right half of the body from that of the left. Secondly, according to Roux, the position of the head- and tail- C2 EMBRYOLOGY. ends of the embryo may be determined in the fertilised egg. That half of the egg, namely, through which the spermatic nucleus migrates to reach the egg-nucleus, becomes the tail-end of the embryo ; the opposite half becomes the head-end. Every egg, however, can be fertilised in any meridian whatever, as was demon- strable experimentally, and thereby the tail-end of the embryo may be located at any chosen position in the egg. Thirdly, the plane in which the two sexual nuclei meet each other (copulation-plane) corresponds with the first plane of segmentation. n»- Partial Discoidal Cleavage. The Hen's egg serves us as the classical example for the description of discoidal segmentation. In this instance the whole process of •'' B C Tig. 82.— Surface view of the first stages of cleavage in the Hen's egg, after COSTE. a, Border of the germ-disc ; 6, vertical .furrow ; c, small central segment ; t/, large peripheral segment. cleavage takes place while the egg is still in the oviduct, during the period in which the yolk is being surrounded by the albuminous envelope and the calcareous shell. It results simply in a cleavage of the germ-disc of formative yolk, whereas the greater part of the egg, which contains the nutritive yolk, remains unsegmented, and becomes subsequently enclosed in an appendage to the embryo, — the so-called yolk-sac, — and is gradually consumed as nutritive material. Just as in the case of the pigmented, animal half of the Frog's egg, so also in the case of the Hen's egg, turn it in whatever direction one will, the germ- disc floats on top, because it fs the lighter part. As in the Frog's egg the first plane of cleavage is vertical and begins at the animal pole, so in the case of the Hen's egg (fig. 33 ^4) .•i .small furrow (b) makes its appearance in the middle of the din-. and advances from above downward in a vertical direction. But THE PROCESS OF CLEAVAGE. 63 whereas in the case of the Frog's egg the first plane of cleavage cuts through to the opposite pole, in the case of the Hen's egg it divides only the germ-disc into two similar segments, which like two buds rest upon the undivided yolk-mass with a broad base, by means of which they still have a physical connection with each other. Soon after this, there is formed a second vertical furrow, which crosses the first at right angles, and likewise remains limited to the germ-disc, which is now divided into four segments (fig. 33 B). Each of the four segments is again divided into halves by a radial furrow. The segments thus formed correspond to sectors, which meet in the centre of the germ-disc with pointed ends, and have et o \ 1 f. 34.— Section through the germ-disc of the Hen's egg during the later stages of segmentation after BALFOUR. > section, which represents rather more than half the breadth of the blastoderm (the middle line is at c), shows that the segments of the surface and of the centre of the disc are smallei than those below and toward the periphery. At the border they are still very large. One of the latter is indicated at a. Large peripheral cell ; b, larger cells of the lower layers ; c, middle line of the blastoderm : e, boundary between the blastoderm and the white yolk, w. their broad ends turned toward the periphery. The apex of each of the segments is then cut off by a cross furrow, i.e., by one which is parallel to the equator of the egg (fig. 33 C'), in consequence of which there are formed smaller central (c) and larger peripheral (d) seg- ments. Since from this time forward radial furrows and those that are parallel to the equator make their appearance alternately, the germ- disc is subdivided into more and more numerous segments, which are so arranged that the smaller lie at the centre of the disc, — therefore immediately around the animal pole, — the larger toward its periphery. With the advancing cleavage the smaller segments- are entirely con- stricted off from the underlying yolk, whereas the larger peripheral ones still remain at first in continuity with it (fig. 34). In this way we finally get a disc of small embryonic cells, which, toward the iddle, are arranged in several superposed layers. <34 EMBRYOLOGY. Tho layer of yolk which immediately adjoins the periphery of the cellular disc, and which is very finely granular and especially rich in protoplasm, still merits particular consideration, for in it lie isolated nuclei (fig. 35 nx), the much-discussed yolk-nuclei or parablast-nuclei {the " merocytes" of RUCKERT). In the case of the Chick they are less striking than in Teleosts and Selachians, in which they have been accurately investigated by BALFOUR, HOFFMANN, RUCKERT, and KASTSCHENKO. Formerly these were held to arise spontaneously (free formation of nuclei) in the yolk, an assumption which in itself is very improbable, since, according to our present knowledge, the free formation of nuclei does not appear to occur anywhere in fig. 35. — Section through the germ-diss of a Pristiurus embryo during segmentation, after BALFOUR. n, Nucleus ; nx, modified nucleus prior to di vision ; nx', modified nucleus in the yolk ; /, furrows which appear in the yolk adjacent to the germ-disc. either animal or vegetable kingdom. Consequently the yolk-nuclei are now rightly held to be derived from the cleavage-nuclei. They are probably produced even at an early period, when the first-formed segments, which remain, as we have seen, for a long time in connection with the yolk, begin to be constricted off from the latter. This probably takes place in the following manner : there arise in the segments nuclear spindles, the halves of which go into the completely isolated embryonic cells at the time of their separation from the yolk, while the remaining halves go into the underlying yolk-layer, and are there converted into vesicular yolk-nuclei. Their number subsequently increases by means of indirect division, as is established by the fact that in sections nuclear spindles have been observed in the yolk-layer (fig. 35 nx'). While, on the one hand, there is an increase in the number of the yolk-nuclei, so, on the other hand, there is also a diminution in their THE PROCESS OF CLEAVAGE. 65- number, as is asserted by several authors (WALDETEB, RUCKERT, BALFOUR, etc.). This takes place by the constricting off of nuclei and surrounding protoplasm, which go to enlarge the cellular disc. We may, with WALDEYER, designate these as secondary cleavage-cells^ and regard the whole process as a kind of supplementary segmentation. By means of this a part of the voluminous yolk-material continues to be gradually individualised into cells. These annex themselves to the border of the germ-disc, which with their aid increases in extent and grows over a continually increasing territory of the unsegmented yolk-sphere. In still later stages of development, long after the cellular germ-disc has been differentiated into the germ -layers, the supplementary segmentation continues to go on at the margin of the disc in the neighbouring yolk-mass, and to furnish new cell-material. Therefore the layer which encloses the yolk-nuclei forms an important connecting link between the segmented germ and the unsegmented utritive yolk; I shall come back to this subject later. The appearance of merocytes and the supplementary cleavage rhich proceeds from them are phenomena which are induced by the vast accumulation of yolk-material, and which allow the latter to be divided up into cells, even though the process is a slow one. ;The eggs of Selachians (KASTSCHENKO, RUCKERT) deviate a little m the usual method of partial cleavage in meroblastic eggs, and in a manner which recalls to a certain extent the processes of superficial cleavage, which are to be treated of later. The cleavage-nucleus, namely, is divided into two nuclei, these again into four and even a greater number, without an accompanying division of the germ -disc into a corresponding number of segments. In this case, therefore, there arises at first a multinuclear proto- plasmic mass, — a plasmodium, — in which the nuclei are distributed at regular intervals. Subsequently furrows appear, generally in great numbers and all at once, by means of which the germ-disc becomes divided into cells from the centre to the periphery. Some of the nuclei always remain in the periphery outside the territory of cleavage, here undergo further division, migrate out of the germ- disc into the surrounding nutritive yolk, and constitute the yolk- nuclei or merocytes. These cause and maintain in the yolk for Blong time the process of supplementary cleavage. When we institute a comparison between partial and unequal eavage, — for the descriptions of which we have made use of the eggs of the Hen and the Frog, — it is not difficult to derive the former m the latter, and to find a cause for the origin of the former, nu ,, ,2 ro 66 EMBRYOLOGY. It is the same as that which produced unequal cleavage from equal cleavage ; it is the great accumulation of nutritive yolk, the inequality in the distribution of the egg-substances which goes hand in hand with it, and the alteration in the position of the cleavage-nucleus. The process of differentiation, which is still in a stage of transition in the case of the Frog's egg, is carried to an extreme in the case of the Hen's egg. Protoplasmic substance was already abundantly accumulated at the animal pole in the former case, but in the latter it is still more concentrated, and at the same time has become differentiated from the nutritive yolk as a disc enclosing the segmentation-nucleus. The yolk, accumulated to an enormous extent at the opposite pole, is, in consequence of this separation, relatively poor in protoplasmic substance, which only scantily fills the interstices between the large yolk- spheres. Inasmuch as the phenomena of motion during the process of division emanate from the protoplasm and nucleus, whereas the deutoplasm remains passive, the active substance in the case of mero- blastic eggs can no longer master the passive substance and cause it to participate in the cleavage. Even in the case of the Frog's egg a preponderance of the animal pole during cleavage is observable ; within its territory the nucleus lies, the radial figures of the proto- plasm appear, and the first and second planes of division begin to arise, whereas they cut through at the vegetative pole last of all ; moreover the process of division during the later stages takes place there with greater rapidity, so that a distinction arises between the smaller animal cells and the larger vegetative ones. In the case of the Hen's egg, the preponderance of the animal pole is still further increased, and the contrast with the vegetative pole is most sharply expressed. The cleavage-furrows not only begin there, but they remain restricted to the territory immediately surrounding it. Thus we get on the one hand a disc composed of small animal cells, on the other an immense undivided yolk-mass, which corresponds to the larger vegetative cells of the Frog's egg. The yolk-nuclei enclosed in the periphery of the germ-disc are equivalent to the nuclei of the vegetative cells of the Frogs egg. IP- Partial Superficial Cleavage. The second sub-type of partial cleavage is prevalent in the phylum of Arthropods, and occurs in centrolecithal eggs, where a central yolk-mass is enclosed in a cortical layer of formative yolk. Manifold THE PROCESS OF CLEAVAGE. 67 variations are possible here, as well as transitions to equal and un- equal cleavage. When the course pursued is quite typical, the segmentation-nucleus, surrounded by a mantle of protoplasm, lies in the middle of the egg in the nutritive yolk ; here it is divided into two daughter-nuclei, without the occurrence of a corresponding division of the egg-cell. The daughter-nuclei, in turn, undergo division into 4, these into 8, 16, 32 nuclei, etc., while the egg as a whole still remains unsegmented. Subsequently the nuclei move apart, the greater number gradually migrate to the surface, and penetrate into protoplasmic cortical layer, where they arrange themselves at liform distances from each other. It is only at this stage that process of egg-segmentation takes place, for now the cortical layer divided into as many cells as there are nuclei in it, while the central remains undivided. The latter is therefore suddenly enclosed in sac formed of small cells — a blastoderm (Keimhaut). Instead of polar (telolecithal) yolk, we have a central (centrolecithal) yolk, linarily yolk-nuclei or merocytes remain behind in the yolk, as in meroblastic eggs of Vertebrates. Now that we have become acquainted with the various forms of the ocess of segmentation, it will be expedient to dwell for a moment its results. According as the process of cleavage takes place one or the other of the four methods described, there arises mass of ceils with corresponding characteristics.' From equal segmentation there arises a spherical germ with cells approximately uniform in size (Amphioxus, Mammals) (fig. 30, p. 56) ; from un- equal segmentation, as well as from discoidal, there is. produced a form of the germ with polar differentiation. This manifests itself in the first case (Cyclostomes, Amphibia) in the production of small cells at the animal pole and large yolk-laden elements at the opposite, vegetative pole (fig. 32 64, p. 60). In the other case (fig. 35, p. 64) the vegetative pole is occupied by an unsegmented yolk-mass, in which at definite regions nuclei are found (Fishes, Reptiles, and Birds). Finally there is developed from superficial cleavage a germ composed of a mantle of cells, which envelops an unsegmented yolk- Iiss in which also there are nuclei (Arthropods). The multicellular germ undergoes further changes, sometimes in e earlier stages of the cleavage-process, sometimes only in the later stages, in that a small, fluid-filled cleavage-cavity is developed in its tre, by the separation of the embryonic cells. At first small, this ' 68 EMBRYOLOGY. Fig. 36. -Blastula of Amphioxug, after HATSCHEK. A, Segmentation-cavity ; az, animal cells ; , cells with abundant yolk. cavity increases more and more in size, so that the surface of the whole germ is augmented, and the cells which were at first central come to the surface. Different names have been given to the solid and to the hollow mass of cells. A morula or mulberry -sphere is spoken of as long as the segmentation-cavity is either wanting or only slightly de- veloped. But when a larger cavity has been formed, as is almost always the case toward the end of the , nroppss the crprm )SS> is called a blastula or blas- tosphere (Keimblase). The latter in turn exhibits a four-fold variation of form, according to the abundance of yolk in the original egg and the method of the antecedent segmentation. In the simplest case (fig. 36) the wall of the blastula is only one layer thick ; the cells are of uniform size and cylindrical, and are closely united to one another to form an epithelium (many of the lower animals, Am- phioxus). In the case of lower, aquatic animals the blastulae at this stage aban- don the egg-envelopes, and, since their cylindrical cells develop cilia at the surface, swim about with rotating motion in the water as ciliate spheres or blastospheres. In eggS with Unequal Seg- mentation the blastula is ordinarily formed of several layers of cells, as in the case of the Frog and Triton, and at the same time it exhibits in different regions different thicknesses (fig. 37). At the animal pole the wall is thin ; at the vegetative pole, on the contrary, it is so much thickened that an elevation, Fig. 37. — Blastula of Triton taeniatus. fJi, Segmentation-cavity ; -/•;, marginal zone ; et, celU with abundant yolk. THE PROCESS OF CLEAVAGE. 69 composed of large yolk-cells, protrudes from this side far into the cleavage-cavity, thus considerably diminishing it. The eggs with partial discoidal segmentation (fig. 38) are modified most of all, and are therefore scarcely to be recognised as blastulse. In consequence of the immense accumulation of yolk on the ventral (vegetative) side, the cleavage-cavity (B) is extraordinarily constricted, and is still preserved only as a narrow fissure filled with albuminous fluid. Dorsally its wall consists of the small embryonic cells (kz) result- ing from the process of cleavage, which are accumulated in several superposed layers; at the surface they join each other closely, deeper they lie more loosely associated. The floor of the cleavage- cavity is formed of a yolk-mass, scattered through which are to be found the yolk-nuclei or d* kz dlc merocytes (dk), which likewise result from the cleavage-p r o c e s s. It is to be seen that they are espe- cially numerous at the place of tran- sition from the germ-disc to the yolk-mass. This nucleated yolk-mass very evidently corresponds to the large vegetative cells which constitute the floor of the cleavage-cavity in the case of the Amphibian egg (fig. 37). In the case of superficial cleavage there is formed, strictly speaking, no blastula, since the place where the segmentation -cavity should be developed is filled with nutritive yolk. The latter either remains unsegmented or is subsequently divided, as in the Insects, into in- dividual yolk-cells. Fig. 38.— Median section through a germ- disc of Pristiurus in the blastula stage, after RUCKERT. B, Cavity of the blastula ; kz, segmented germ ; dk, finely granular yolk with yolk-nuclei. HISTORY OF THE PROCESS OF CLEAVAGE. The investigation and right comprehension of the process of cleavage have been attended with manifold difficulties. A voluminous literature has arisen on this subject. We limit ourselves to pointing out the most important dis- coveries and the chief questions which have been discussed. The first observations on the process of segmentation were made on the Frog's egg. Aside from short statements by SWAMMERDAH and EO'SEL VON 70 EMBRYOLOGY. ROSENHOF, it was PREVOST ET DUMAS wlio were the first to describe, in 1824, the manner in which regular furrows arise on the Frog's egg, and how by means of these the whole surface is divided into smaller and smaller areas. According to the French investigators, the turrows were restricted to the sur- face of the egg. However, only a few years later, RUSCONI (1826) and C. E. v. BABR recognised that the furrows visible at the surface correspond to fissures which extend through the whole mass of the yolk, and divide it into separate parts. Even in his time VON BAEB rightly characterised the whole process of segmentation, in which he discerned the first impulse of life, as an automatic division of the egg-cell, but subsequently he abandoned this, the right path, since he sought for the meaning of division in the dictum : that *' all yolk -masses are subject to the influence of the fluid and volatile components of the fertilising material." In the next decennary there followed numerous discoveries of the process of segmentation in other animals. During this period acquaintance was also gained with partial segmentation. After RUSCONI and VOGT had seen it in the case of fish eggs, KO'LLIKER gave, in the year 1844, the first detailed description of it as seen in the eggs of Cephalopods, and four years later COSTE described it in the Hen's egg. The question of the significance of the cleavage-process has engaged the earnest attention of investigators, and has given rise to many controversies. The discussion first took a definite turn upon the establishment of the cell- theory. The question was, to determine whether and in what manner cleav- age was a process of cell-formation. Although there were already many observations on the division of eggs, SCHWANN himself took no definite posi- tion on this question. The views of other investigators were at variance for years. There was a difference of opinion as to whether the egg or the ger- minative vesicle was a cell, whether the segments resulting from cleavage possessed a membrane or not, and whether these segments were to be regarded as cells or not. In the earlier literature the germinative vesicle and the nuclei of the cleavage-spheres were often designated as embryonic cells, and the surrounding yolk-mass as an envelpping sphere. The difficulty of com- prehending the process of segmentation was also aggravated by the false doctrine of free cell-formation from an organic matrix— the cytoblastema — founded by SCHWANN. It remained for a long time a controverted point whether the tissue-cells of the adult organism were the direct descendants of the segmentation-spheres, or whether they arose at a later period by means of free cell-formation from cytoblastema. After NAGELI on the botanical side had adopted the right course, it was the service of KOLLIKER, REICHERT, REMAK, and LEYDIG to have paved the way to a comprehension of cleavage, and to have shown that free cell-formation does not take place, but that all cellular elements arise in uninterrupted sequence from the egg-cell. As far as regards the different kinds of cleavage, KO'LLIKER designated them as total and partial. VAN BENEDEN has given in his "Recherches sur la composition et la signification de l'oeuf"a more exhaustive review of the subject, and has also expounded in a clear way the signification of the deutoplasm for the different kinds of cleavage. Subsequently HAECKEL mate- rially simplified the categories of segmentation recognised by VAN BENEDEN, and proposed in his " Anthropogenic " and in his paper " Die Gastrula und die Eifurchung" the classification of the methods of cleavage on which is based the scheme previously given, and according to which total cleavage is divided THE PROCESS OF CLEAVAGE. 71 into equal and unequal, and partial into discoidal and superficial. At the same time HAECKEL endeavoured to derive the different methods of cleavage from one another, and apropos of this directed attention to the important role of the nutritive yolk. The processes which take place within the yolk have eluded observation and a correct interpretation even more than the external phenomena of cleav- age, so that it is only in the most recent times that we have acquired a satis- factory insight into them. It is true that the problem, as to what part the nucleus plays in segmentation, has had the uninterrupted attention of investi- gators, but without any solution having been found. For years there were in the literature two opposing views : sometimes one of them, sometimes the other, attained temporarily greater currency. According to one view — which was almost universally adopted by the botanists, and was defended on the zoological side principally by REICHERT, and even recently by AUERBACH— the nucleus disappears before every division, and is dissolved, to be afterwards formed anew in each daughter-segment; according to the other view the nucleus, on the contrary, is not dissolved, but is constricted, becomes dumb-bell-shaped, and is divided into halves, and thereby induces cell-division. This view was taught especially by such zoologists and anatomists as C. E. v. BAER, JOH. MULLER, KOLLIKER, LEYDIG, GEGENBAUB, HAECKEL, VAN BENEDEN, and others, who were supported by the observations which they made on transparent eggs of the lower animals. Light was first thrown on the disputed question at the moment when suit- able objects were studied with the aid of higher magnifications, and especially with the employment of modern methods of preparation (fixing and staining ;ents). The works of FOL, FLEMMING, SCHNEIDER, and AUERBACH on the cleavage of the eggs of various animals mark a noteworthy advance. They still main- tained, it is true, that the nucleus is dissolved at the time of cleavage, but they gave a detailed and accurate description of the striking radiation which arises in the yolk upon the disappearance of the nucleus, and which during the constriction of the egg soon becomes visible in the region of the daughter- nuclei.* SCHNEIDER observed parts of the spindle-stage. Soon after this a more exact insight into the complicated and peculiar nuclear changes was obtained by means of three investigations, which were carried out independently and simultaneously on different objects, and were published in rapid succession by BUTSCHLI, STRASBURGER, and the author. It was definitely established by these observations that there is no dissolution of the nucleus at the time of division, but a metamorphosis, such as has been described in the preceding pages. At the same time I likewise proved that the egg -nucleus is not a new formation, but is derived from parts of the germinative vesicle. From this resulted the important doctrine that, just as ill cells, so also all nuclei of the animal organism are derivatives in an uninterrupted sequence, the one from tJie egg-cell and tlie other from its nucleus. (Omnis cellula e cellula, is nucleus e nucleo.) Through these researches there was furnished for the ht able with - I Kadiating structures had already been observed in the yolk before this, t in an incomplete manner, by different authors — by GRUBE in the Hiru- dinea, by DERBES and MEISSNER in the Sea-urchin, by GEGENBAUR in Sagitta, by KROHN, KOWALEVSKY, and KUPFFER in Ascidians, by LEUCKAET in Nema- des, by BALBIANI in Spiders, and by OELLACHER in the Trout. 72 EMBRYOLOGY. first time a scheme of nuclear division and cell-division, which has since proved to be correct in all essentials, even though it has undergone important improvements and additions at the hands of FOL, FLEMMING, VAN BENEDEN, and RABL. FOL published an extended monographic investigation of the process of cleavage, which he had observed in many invertebrated animals. FLEMMING, starting with nuclear division in tissue-cells, distinguished with great acumen the non-chromatic and the chromatic parts of the nuclear figure, the non- stainable nuclear spindle-fibres, and the stainable nuclear filaments and loops, which are located upon the surface of the former. He made the interesting discovery concerning the latter, that they become split lengthwise. Ligbt was soon thrown upon this peculiar phenomenon, when HEUSER, VAN BENEDEN, and RABL, independently of each other, discovered that the halves of the split filaments moved apart toward the poles of the nucleus, and furnished the fundament for the daughter-nuclei. VAN BENEDEN at the same time made the additional and important observation on the egg of Ascaris megalocephala, that of the four chromatic loops, which are constantly to be observed in the case of the cleavage-nucleus, two are derived from the chromatic substance of .the spermatic nucleus, the other two from the chromatic substance of the egg-nucleus ; and that, in consequence of the longitudinal splitting, each daughter-nucleus receives at the time of division two male and two female nuclear loops. In addition there have appeared many other recent works of value on the process of cleavage by NUSSBAUM, RABL, CARNOY, BOVERI, PLATNBR, and others. Within the last few years PFLDGER has endeavored to prove by interesting experiments that gravitation exercises a determining influence on the position of the planes of cleavage. BORN, Roux, and the author, on the contrary, thought they were able to explain division from the organisation of the egg- cell itself. In the author's article, " Welchen Einfluss iibt die Schwerkraft auf die Theilung der Zellen ? " he recognised the causes which determine the various directions of the planes of division, (1) in the distribution of the lighter egg-plasm and the heavier deutoplasm, and (2) in the influence which the spatial arrangement of the egg-plasm exercises on the position of the nuclear spindle, and that which the position of the latter exercises upon the direction of the plane of cleavage. SUMMARY. 1. In the process of cleavage the internal and the external pheno- mena of segmentation are to be distinguished from each other. 2. The internal phenomena of cleavage find expression in changes (a) of the nucleus, (6) of the protoplasm. 3. The nucleus while in the process of division consists of a non- chromatic and a chromatic nuclear figure. The non-chromatic figure is a spindle composed of numerous fibres. The chromatic figure is formed of bent, V-shaped nuclear filaments (chromosomes), which li<- upon the surface of the middle of tlm sjiindlt'. At the two ends of the spindle there is found a special polar corpuscle [centrosome]. THE PROCESS OF CLEAVAGE 73 4. The division of the nucleus takes place in the following manner : the nuclear filaments split lengthwise, and their halves move apart in opposite directions toward the ends of the spindle, and are there converted into vesicular daughter-nuclei. 5. The protoplasm arranges itself around the ends of the spindle in filaments having the form of a stellate figure (an aster), so that double radiation or an amphiaster arises in the egg. 6. The external phenomena of cleavage consist in the division of the egg-contents into individual parts, the number of which corre- sponds to that of the daughter-nuclei. They exhibit various modifica- tions, which are dependent on the arrangement and distribution of the egg-plasm and the deutopinsm, as is to be seen from the fol- tig scheme of segmentation. Scheme of the Various Modifications of the Process of Cleavage. I. Total Cleavage. (Holoblastic eggs.) e eggs, which for the most part are small, contain a small or moderate amount of deutoplasm, and are completely divided into ughter-cells. 1. Equal Cleavage. is takes place in eggs with meagre and uniformly distributed .toplasm (alecithal). By the process of cleavage there are formed segments which, in general, are of uniform size. (Amphioxus, Mam- malia.) 2. Unequal Cleavage. This occurs in eggs in which a more abundant deutoplasm is un- equally distributed, being concentrated toward the vegetative pole, and in which the cleavage-nucleus is located nearer the animal and more protoplasmic pole. Usually the segments become unequal in size only with and after the third act of division. (Cyclostomes, Amphibia.) II. Partial Cleavage. (Meroblastic eggs.) The eggs, which are often very large, ordinarily contain con- siderable quantities of deutoplasm. In consequence of the unequal distribution of this, the egg-contents are separated into a formative yolk, in which alone the process of cleavage is manifested, and a nutritive yolk, which remains undivided, and is used up during embryonic development for the growth of the organs. 74 EMBRYOLOGY. 1. DUcoidal Cleavage. This takes place in eggs with nutritive yolk in a polar position The process of cleavage remains confined to the formative yolk accumulated at the animal pole, which has the form of a disc and contains only a small amount of deutoplasm. There is formed, con- sequently, a cellular disc. (Fishes, Reptiles, Birds.) 2. Superficial Cleavage. This occurs in the case of eggs with central yolk. In typical cases the nucleus alone, which occupies the middle of the egg, under- goes repeated division. The numerous daughter-nuclei which arise in this manner migrate into the layer of protoplasm which invests the central nutritive yolk, and the protoplasm is thereupon divided into as many segments as there are nuclei lying in it. There is formed a germ-membrane (Keimhaut). (Arthropods.) 7. Eggs with total cleavage are designated as holoblastic, eggs with partial cleavage as meroblastic. 8. The direction and position of the first cleavage-plane are strictly conformable to laws which are founded in the organisation of the cell ; they are determined by the following three factors : — First factor. The cleavage-plane always divides the axis of the nucleus which is preparing for division perpendicularly at its middle. Second factor. The position of the axis of the nucleus during division is dependent upon the form and differentiation of the en- veloping protoplasm. In a protoplasmic sphere the axis of the nuclear spindle, occupying the centre of the sphere, can lie in the direction of any radius what- ever; but in an oval protoplasmic body, only in the longest diameter. In a circular disc the nuclear axis lies parallel to its surface in any diameter of the circle, but in an oval disc only in the longest diameter. Third factor. In the case of eggs of unequal segmentation, which, in consequence of their unequally distributed, polar deutoplasm, are geocentric, and therefore assume when in equilibrium a parti- cular position, the first two planes of cleavage must be vertical, and the third must be horizontal and placed above the equator of the sphere. LITERAfURE. 75 In addition to the writings cited in the second chapter sec : — Auerbach. Organologische Stuclien. Heft I. und Heft II. Breslau 1874. Baer, C. E. von. Die Metamorphose des Eies der Batrachier. Mliller Archiv. 1834. Born, Or. Ueber die Furchung des Eies bei Doppelbildungen. Breslauer tirztl. Zeitschr. 1887. Nr. 15. Coste. Histoire generate et particuliere du developpement des corps organises. 1847—1859. Flemming. Ueber die ersten Entwicklungserscheinungen am Ei der Teich- muschel. Archiv f. mikr. Anat. Bd. X. p. 257. 1874. Flemming. Beitrage zur Kenntniss der Zelle und ihrer Lebenserscheinungen. Archiv f. mikr. Anat. Bd. XVI. p. 302. 1878. Flemming. Neue Beitrage zur Kenntniss der Zelle. Archiv f. mikr. Anat. Bd. XXIX. p. 389. 1887. Fol, H. Die ersteEntwicklungdesGeryonideneies. Jena. Zeitschr. Bd.VII. 1873. Fol, H. Sur le developpement des Pteropodes. Archives de Zoologie exper et gen. T. IV. and V. 1875-76. Gasser. Eierstocksei u. Eileiterei des Vogels. Marburger Sitzungsb. 1884. aeckel, E. Die Gastrula und Eifurchung. Jena. Zeitschr. Bd. IX. 1875. Heape, Walter. The Development of the Mole, the Ovarian Ovum, and Segmentation of the Ovum. Quart. Jour. Micr. Sci. Vol. XXVI. pp. 157 174. Vol. XXVII. pp. 123-63. 1886. olliker. Entwicklungsgeschichte der Cephalopoden. Zurich 1844. ydig, Fr. Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt und nach ihrer Bedeutung. Oken's Isis. 1848. fluger, E. Ueber den Einfluss der Schwerkraf t auf die Theilung der Zellen. Arch. f. d. ges. Physiol. Bd. XXXI. p. 311. 1883. Pfluger, E. 2. Abhandlung. Bd. XXXII. pp. 1-71. 1883. vost et Dumas. 2me Mem. sur la Generation. Ann. des sci. nat. T. II. pp. 100, 129. 1824. ,bl. Ueber Zelltheilung. Morphol. Jahrb. Bd. X. p. 214. 1885. uber, A. Furchung u. Achsenbildung bei Wirbelthieren. Zool. Anzeiger, p. 461. 1883. auber, A. Schwerkraftversuche an Forelleneiern. Berichte der naturf. Gesellsch. zu Leipzig. 1884. ichert. Der Furchungsprocess und die sogenannte Zellenbildung um Jnhaltsportionen. Miiller's Archiv. 1846. mak. Sur le developpement des animaux vertebres. Comptes rendu?. T. XXXV. p. 341. 1852. ux. Ueber die Zeit der Bestimmung der Hauptrichtungen cles Frosch- embryo. Leipzig 1883. oux. Ueber die Bedeutung der Kerntheilungsfiguren. Leipzig 1883. oux. Beitrage zur Entwicklungsmechanik des Embryo. Nr. 4. Archiv f. mikr. Anat. Bd. XXIX. p. 157. 1887. Roux. Die Entwicklungsmechanik der Organismen, eine anatomische Wis- senschaft der Zukunft. Wien 189C. Rusconi. Sur le developpement de la grenouille. Milan 1828. 76 EMBRYOLOGY. Salensky, W. Befruchtung und Furchung desSterlet-Eies. Zool. Anzeiger. Nr. 11. 1878. Sarasin, C. F. Reifung u. Furchung des Reptilieneies. Arbeiten a. d. zool.-zoot. Inst. Wurzburg. Bd. VI. p. 159. 1883. Schneider. Untersuchungen liber Plathelminthen. Jahrb. d. oberhessischen Gesellsch. f. Natur- u. Heilkunde. 1S73. Strasburger. Zcllbildnng imd Zellthcilnng. 3. Aufl. Jena 1875. CHAPTER IV, GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOP- MENT. A SIMPLE principle has exclusively controlled the embryonic pro- cesses hitherto considered. By means of the cleavage of the egg- substance, or cell-division, alone the originally simple elementary organism has been converted into a cell-colony. This presents the simplest conceivable form, inasmuch as it is a hollow sphere, the wall of which is composed of one or several layers of epithelial cells. But the principle of cell-division is not adequate for the production, out of this simple organism, of more complicated forms with dissimilar organs, such as the adult animals are ; further progress in develop- ment can be brought about from this time forward only by the supervention of two other principles, which are likewise simple ; namely, the principle of unequal growth in a cell-membrane, and the principle of the division of labour, together with the histologies 1 differentiation connected with it. Let us consider first the principle of unequal growth. When in a cell-membrane the individual elements continue to divide uniformly, the result will be either a thickening or an increase in the surface of the membrane. The former takes place when the plane of division has the same direction as the surface of the membrane, the latter when it is perpendicular to the surface. With the increase in the extent of surface the cells which were at first present are uniformly and gradually crowded apart by the introduction of the new daughter- cells, inasmuch as they are soft and plastic, and are joined together only by means of a soft cementing substance. Were we to assume that only such a growth took place in the case of the blastula during its further development, nothing else could come of it except an ever larger and tliick«-r-\v:ill«'d hoUmv sphere of cells. GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 77 The operation of an unequal growth of the surface produces quite another result. When in the middle of a membrane the cells of a single group within a short time repeatedly undergo " division " by vertical planes, they will be suddenly compelled to claim for themselves much greater surface, and they will consequently exert a vigorous pressure, due to growth, upon the cells in their vicinity, and will tend to push them apart. But in this case a separation of contiguous cells, such as takes place with gradual and uniformly distributed interstitial growth, will be impossible ; for the surrounding cells, remaining in a passive condition, will constitute, as it were, a rigid frame, as His has expressed it, around the extending part, which, in consequence of accelerated growth, demands an increased area. It must therefore secure room for itself in another manner, and increase its surface by abandoning the level of the passive part through the formation of a fold in either one direction or the other. The fold will be still further increased, and forced farther from the original level, if the increased activity of the process of cell-division in it continues. Thus by means of unequal growth there has now arisen out of the originally uniform membrane a new recognisable part, or a special organ. When the folding membrane encloses a cavity, as is the case with the blastula, there are two cases conceivable in the formation of folds. In the first place, the membrane may be folded into the interior of the body, a process which in embryology is called invagination or involution. Secondly, there may arise by evagination a fold, which projects free beyond the surface of the body. In the first case numerous variations in the details are possible, so that the most various organs, as, e.g., the glands of the animal body, parts of the sensory organs, the central nervous system, etc., are formed. In the origin of glands a small circumscribed circular part of a cellular membrane is infolded as a hollow cylinder (fig. 39 l and 4). towards the interior of the body, into the underlying tissue, and by continuous growth may attain considerable length. The invagina- tion develops into either the tubular or the alveolar form of gland (FLEMMING). If the glandular sac possesses from its mouth to its blind end nearly uniform dimensions, we have the simple tubular gland (fig. 39 !),- — the sweat glands of the skin, LIEBERKUHN'S glands of the intestine. The alveolar form of gland differs from this in that the invaginated sac does not simply increase in length, but expand* somewhat at its eud (fig. 39 5, db}, while the other part remains 78 EMBRYOLOGY. narrow and tube-like and serves as its duct (a). More complicated forms of glands arise, when the same processes to which the simple glandular sac owes its origin are repeated on the wall of the sac — , 2 3456 when on a sma11 it a more growth takes place, db .. db Tig. 39.— Diagram of the formation if glands. 1, Simple tubular gland ; 2, branched tubular gland ; 3, branched tubular gland with anastomosing branches ; 4 and 5, simple alveolar glands ; o, duct ; db, vesicular enlargement ; 6, branching alveolar gland. tract of vigorous again and a part begins to grow out from the main tube as a lateral branch (fig. 39 2 and 6). By numerous repetitions of such evaginations, the originally simple tubular gland may acquire the form of a much - branched tree, upon which we distinguish the part formed first as trunk, and the parts which have arisen by outgrowths from it as chief branches and branchlets of first, second, third, and fourth order, according to their ages and correlated sizes. According as the lateral outgrowths remain tubular or become enlarged at their tips, there arise either the compound tubular glands (fig. 39 2) (kidney, testis, liver), or the compound alveolar glands (fig. 39 6) (sebaceous glands of the skin, lungs, etc.). Again, the invaginating part of an originally flat membrane assumes other forms in the pro- duction of sense organs and the central nervous system. For example, the part of the organ of hearing which bears the nerve terminations — the membranous labyrinth — is developed out of a small tract of the surface of the body, which becomes depressed into a small pit (fig. 40) in consequence of its acquiring an extraordinary vigor in growth. The edges of the auditory pit then grow toward one another, so that this is gradually con- verted into a little sac, which still opens out at th-.- surface of the body by means of a narrow orifice only (fig. 40 a). Finally, the Tig. 40.- Diagram of the formation of the audi- tory vesicle. a, Auditory pit ; b, audi- tory vesicle, which ha*. arisen by a process of t fiction, and still i riiiains connected with the outer germ-layer by HUMUS of a solid "talk of epithelium. GENEIiAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 79 narrow orifice closes. Out of the auditory pit there has arisen a closed auditory sac (6), which then detaches itself completely from its parent tissue, the epithelium of the surface of the body. Afterwards, simply by means of the unequal growth of its different regions, by means of constrictions and various evaginations, it acquires such an extraordinarily complicated form, that it has justly received the name of membranous labyrinth, as will be shown in detail in another apter. The development of the central nervous system may serve as e last example of invagination. Spinal cord and brain take their in at an early epoch from the layer of epithelial cells which limits e outer surface of the body of the embryo. A narrow band of this epithelium lying along the axis of the back becomes thickened, and is distinguished from the thinner part of the epithelium, which produces the epidermis, as the medullary plate (fig. 41 A mp). Inasmuch as the plate grows more rapidly than its surroundings, it becomes in- folded into a gutter which is at first shallow, the medullary groove. This becomes deeper as a result of further increase of substance. At the same time the edges (fig. 41 B mf), which form the transition from the curved medullary plate to the thinner part of the cellular membrane, become slightly elevated above the surrounding parts, and constitute the so-called medullary folds. Subsequently these grow toward each other, and become so apposed that the furrow becomes a tube, which still remains temporarily open to the outside by means of a narrow longitudinal fissure. Finally, this fissure also disappears (fig. 41 C) ; the edges of the folds grow together ; the closed medullary tube (n), like the auditory vesicle, then detaches itself completely along the line of fusion (suture) of the cell-membranes of which it was originally a component part and becomes an entirely independent organ (n). Let us now examine somewhat more closely the mechanism of the fusion and detachment of the neural tube. The two medullary folds are each composed of two layers, which are continuous with each other at the edge of the fold, — the thicker medullary plate (mp), which lines the furrow or tube, and the thin- ner epidermis (ep), which has either a more lateral or a more super- ficial position. When, now, the folds come into contact, they fuse, not only along a narrow edge, but over so extensive a tract that epidermis is joined to epidermis, and that the edges of the medullary plate are joined to each other. The medullary tube thus formed, and the continuous sheet of epidermis that stretches across it, are by 80 EMBRYOLOGY. means of an intermediary cell-mass still in continuity along the suture produced by the concrescence. But a separation soon takes place mp ep jut1 Ik Ml** Fig. 41 — Cross sections through the dorsal halves of three Triton larvae. A, Cross section through an egg in which the medullary folds (HI/) begin to appear. B, Cross section through an egg whose medullary furrow is nearly closed. C, Cross section through an egg with closed neural tube and well-developed primitive segments. id/', Medullary folds; mp, medullary plate; n, neural txibe (spinal cord); ch, chorda; fj}, epidermis, or corneal layer ; mi; middle germ-layer ; mk\ parietal, mk'*, visceral sub- division of the middle germ-layer ; ik, inner germ-layer ; I'.sh, cavity of primitive segment. along this line, inasmuch as the intermediary band of substance becomes narrower and narrower, and one part of it unites with the GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 81 O plic folc whi gin par fori proti assui of s: epidermis, while the other part is annexed to the medullary tube. Thus in the formation of the suttire processes of fusion and of separation occur almost simultaneously, a condition which often recurs in the case of other invaginations, as in the constricting off of the auditory vesicle, the vesicle of the lens, etc. The neural tube having once become independent is subsequently segmented in manifold ways by the formation of foldings, in conse- uence of inequalities in the rate of surface growth, especially in its terior enlarged portion, which becomes the brain. There are med out of this by means of .four constrictions five brain-vesicles, hich lie in succession one after other ; and of these the most aii- rior, which becomes the cerebrum ith its complicated furrows and con- •lutions of first, second, and third 'der, serves as a classical example en one desires to show how a ighly differentiated organ with coni- licated morphological conditions may iginate by the simple process of biding. In addition to invagination the second thod in the formation of folds, hich depends upon a process of eva- gination, plays a no less important part in the determination of the form of animal bodies, giving rise to rotuberances of the surface of the body, which may likewise me various forms (fig. 42). As a result of exuberant growths small circular territories of a cell-membrane there arise rod- like elevations, resembling the papillae on the mucous membrane of the tongue (c), or the fine villi (a) in the small intestine (villi intestinales), which are so closely set that they give a velvety ap- pearance to the surface of the mucous membrane of the intestine. Just as the tubular glands may be abundantly branc hed, so tufted "illi are here and there developed out of simple villi, sine 3 local elerations of growth cause the budding-out of lateral branches of a second, third, and fourth order (fig. 42 b). We re call the external tufted gills of various larvae of Fishes and Amphibia, which project out from the neck-region free into the water, or the villi of the chorion in Mammals, which are characterised by still more numerous Fig. 42.— Diagram of the formation of papillae and villi. a, Simple papilla ; b, branched papilla or tufted villas; c, simple papilla, the connective-tissue core of which runs out into three points. villi ace. 82 EMBRYOLOGY. branchings. The formation of the limbs is also referable to such a process of external budding. When the growth of the membrane takes place along a line, the free edges form ridges or folds directed outward, such as the valves of KERKRING or the gill-plates on the gill-arches of Fishes. From the examples cited it is clearly to be seen how the greatest variety of forms may be attained by the simple means of invagina- tion and evagination alone. At the same time, the forms may be modified by two processes of subordinate importance, by separations and by fusions which affect the cell-layers. Vesicular and sac-like cavities acquire openings by the thinning out of the wall at a place where the vesicle or sac lies near the surface of the body, until there is a breaking through of the separating partition. Thus in the originally closed intestinal tube of Vertebrates there are formed the mouth-opening and the anal opening, as well as the gill-clefts in the neck-region; The opposite process — fusion — is still more frequently to be observed. It allows of a greater number of variations. We have already seen how the edges of an invagination may come in contact and fuse, as in the development of the auditory vesicle, the intestinal canal, and the neural tube. But concrescence may also take place over a greater extent of surface, when the facing sur- faces of an invaginated membrane come more or less completely into contact, and so unite with each other as to form a single cell-mem- brane. Such a result ensues, for example, in the closure of the embryonic gill-clefts, in the formation of the three semicircular canals of the membranous labyrinth of the ear, or, as a pathological process, in the concrescence of the surfaces of contact of serous cavities. Moreover fusions may take place between sacs which come in contact with their blind ends, as very often occurs in the com- pound tubular glands (fig. 39 3). Of the numerous lateral branches which sprout out from the tubule of a gland, some come in contact at their ends with neighboring branches, fuse with them, and establish an open communication with them by the giving way of the cells at the place of contact. It is by this means that branched forms of tubular glands pass into the net-like forms to which the testis and the liver of Man belong. In addition to the formation of folds in epithelial layers, which under a tfivat variety of modifications determine in general the organisation of the animal body, there were mentioned, as a second GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 83 developmental principle of fundamental significance, division of labor and the histological differentiation associated ivith it. In order to understand fully the significance of this principle in development, we must proceed from the thesis that the life of all organic bodies expresses itself in a series of various duties or functions. Organisms take to themselves substances from without ; they incorporate in their bodies that which is serviceable, and eliminate that which is not (function of nutrition and metastasis) ; they can alter the form of their bodies by contraction and extension (function of motion) ; they are capable of reacting upon external stimuli (function of sensibility) ; they possess the ability to bring forth new organisms of their own kind (function of reproduction). In the lowest multicellular organisms each of the individual parts discharges in the same manner as the others the enumerated functions necessary for organic life ; but the more highly an organism is developed, the more do we see that its dividual cells differentiate themselves for the duties of life, — that e assume the function of nutrition, others that of motion, others at of sensibility, and still others that of reproduction, — and that with is division of labor is likewise joined a greater degree of com- eteness in the execution of the individual functions. The evelopment of a specialised duty likewise leads invariably to an altered appearance of the cell : with the physiological division of bor there ahuays goes hand-in-hand a morphological or histological ifferentiation, Elementary parts which are especially concerned in the duties of utrition are distinguished as gland-cells ; again others, which have (veloped the power of contractility to a greater extent, have come muscle-cells, others nerve-cells, others sexual cells, etc. The Is which are concerned in one and the same duty are for the most rt associated in groups, and constitute a special tissue. Thus the study of the embryology of an organism embraces chiefly wo elements : one is the study of the development of form, the cond the study of histological differentiation. We may at the e time add that in the case of the higher organisms the morpho- ical changes are accomplished principally in the earlier stages of velopmeiit, and that the histological differentiation takes place in e final stages. A knowledge of these leading principles will materially facilitate e comprehension of the further processes of development. 84 EMBRYOLOGY. CHAPTER V. DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. ( OA STRMA - THE OR Y. ) THE advances which are brought about during the next stages in the development of the blastula depend primarily upon processes of folding. By these means there arise larval forms, which are at first composed of two, and afterwards of four epithelial membranes, or germ-layers. The larval form which is composed of two germ-layers is called the gastrula. It possesses an important developmental signification, because, as HAECKEL has shown in his celebrated Gastrsea -Theory, it is to be found in each of the six chief branches of the animal kingdom, and thus furnishes a common starting-point from which along diverging lines the separate animal forms may be derived. As with blastulse, so in the case of the gastrula four different kinds can be distinguished, according to the abundance and the method of distribution of the yolk. Starting from a simple funda- mental form, three further modifications have arisen, all of which, with the exception of a single one which is characteristic of many Arthropods, are to be encoun- tered within the phylum of Verte- brates. The simplest and most primitive formr with the considera- tion of which we have to begin, is found only in the development of Am- phioxus lanceolatus. As has been previously shown, its blastula is composed of cylin- drical cells, which are closely joined into a single-layered epithelium (fig. 43). At one place, which may be designated as the vegetative pole FP Fig. 43.- Blastula of Amphioxua lanceolatus, after HATSCHKK. fh, Cleavage-cavity ; az, animal cells ; vz, vegetative cells ; AP, animal pole ; VP, vegetative pole. : dii r, th. ik ud Fig. 44.— Gastrula of Amphioxus lanceolatus, after HATSCHEK. ak, Onter germ-layer ; ik, inner germ-layer ; u, blastopore, or mouth of archenteron (.ud). DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 85 (VP), the cells (vz) are somewhat larger and more turbid, owing to the yolk-granules lodged in them. The process of the formation of the gastrula commences at this place. The vegetative surface begins at first to be flattened, and then to be pushed in toward the middle of the sphere, the advance of the pagination the depression grows deeper and deeper, while the cleavage-cavity be- mes to the same degree diminished in size. Finally, e invaginated portion (fig. ik) comes in contact with e inner surface of the un- invaginated portion (ak) of the blastula, and completely obliterates the cleavage- cavity. As a result there has been formed out of the hollow sphere with a single wall a cup-shaped germ with double walls — te gastrula. The cavity of the gastrula, which results from the invagination and is not to be confounded with the cleavage-cavity which it has sup- planted, is the primitive intestine (archenteron) (ud), or the intestino- body cavity (coelenteron). This opens to the outside through the primitive mouth (mouth of the archenteron, blastopore) (u). Inasmuch as the names primitive intestine and primitive mouth might easily give rise to erroneous conceptions, let it be remarked, in order to preclude from the start such an event, that the cavity and its external opening which arise by this first invagination are not equivalent to the intestine and mouth of the adult animal. The archenteron of the germ, it is true, furnishes the fundament for the intestinal tube, but there are also formed out of it a number of other organs, the chief of which are the subsequently formed thoracic and .abdominal cavities. The future destination of the cavity will there- fore be better expressed by the term " ccelenteron" Finally, the primitive mouth is only an evanescent structure among vertebrated animals; later it is closed and disappears without leaving a trace, while the permanent or secondary mouth is an entirely new structure. The two cell-layers of the cup, which are continuous with each other at the edge of the blastopore, are called the two primary "• 86 EMBRYOLOGY. germ-layers, and are distinguished according to their positions as the outer (ak) and the inner (ik). Whereas in the blastula the individual cells differ only a little from one another, with the process of gastru- lation a division ot labor begins to assert itself, a fact which may be recognised in the case of the free-swimming larvae of Inver- tebrates. The outer germ-layer (ak) (also called ectoblast or ectoderm) serves as a covering for the body, is at the same time the organ of sensation, and effects locomotion when cilia are developed from the cells, as is the case with Amphioxus. The inner germ-layer (ik) (entoblast or entoderm) lines the ccelenteron and provides for nutri- tion. The cell-layers thus stand in contrast to each other both as regards position and function, since each has assumed a special duty. In view of this fact they have been designated by C. E. vox BAER as the two primitive organs of the animal body. They present us with a very instructive, because very simple, illustration of the manner in which -two organs originate from a single fundament. By invagination the undifferentiated cells of the surface of the blastula are brought into different relations to the outer world, and have consequently been compelled to follow different courses in their development, and to adapt themselves to special duties corresponding to the new relations. The separation of the embryonic cell-material into the two primi- tive organs of VON BAER is of decisive significance for the whole subsequent course of the development of the individual cells. For n very definite portion of all the ultimate organs of the body is refer- able to each of the two primitive organs. In order to put this im- portant condition in the proper light at once, let it be stated that the outer germ-layer furnishes the epithelial covering of the body, the epidermis with the glands and hair, the fundament of the nervous >ystt-m, and that part of the sense organs which is functionally most important, On this account the older embryologists imposed upon it the name of dermo-sensory layer. The inner germ-layer, on the contrary, is converted into the remaining organs of the body — into the intestine with its glands, into the body -cavity, into the muscles, etc. ; by far the greater mass of the body, therefore, is differentiated out of it, and it has to pass through the most numerous and the most trenchant metamorphoses.* * The practice of distinguishing the outer and the inner germ-layers as animal and vegetative, which was formerly in vogue and is followed even now, is not proper, and ought therefore to be given up. For the transversely striped muscu- lature of the body, which belongs to its animal organs, does not arise from DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 87 : Larval forms quite like that of Amphioxus have also been observed in the case of Invertebrates belonging to the phyla of Ccelenterata, Echinodermata, Yermes, and Brachiopoda. For the most part they quit the egg-envelope, even in the gastrula stage, to swim about in the water by means of their cilia ; and they can now take nutritive substances — small infusoria, algae, or remnants of larger animals — through the primitive mouth .to the digestive cavity, and make use of them in the fur- ther growth of their bodies. Likewise the substances which are not serviceable be- use indigestible are ejected rom the body through the me orifice. In the case the higher animals the gestion of food is not only possible at this time, but Iso superfluous, because the and the embryonic cells •ising from it still contain )lk-granules, which are [•adually consumed. The modifications which gastrulation undergoes in the Amphibia are isily referable to the simpler conditions in Amphioxus. In the case the Water- Salamander, which is to serve as an illustration in description, one half of the blastula (fig. 45), which is called le animal half, is thin-walled and composed of small cells, which lie in two or three layers one above another, and in the case of le Frog contain black pigment. The other, or vegetative half (dz), ;hibits a greatly thickened wall, composed of much larger, more leutoplasmic, polygonal cells (dz), which, loosely associated in several lyers, cause a protuberance into the cavity (fh) of the blastula, ^hich is proportionally diminished in size. Where the differentiated lalves meet, a transition is effected by means of cells, forming what JOETTE has designated marginal zone (rz). Inasmuch as the specific gravity of the animal half is much less than that of the opposite half, it is without exception directed upward in water. The former • the outer germ-layer, as, in consequence of false observations, was formerly believed, but rather from the primary inner germ-layer, as has now been esta- blished by many observations. Fig. 45.— Blastula of Triton taeniatug. fh, Cleavage-cavity ; dz, yolk-cella ; rz, marginal zone. 88 EMBRYOLOGY. Fig. 46. Egg of Triton, -which is developing into a gastrula, seen from the surface. u, Primitive mouth (blastopore). constitutes the thinner roof, the latter the highly thickened floor, of the excentrically placed cleavage- cavity. When the gastrula begins to be developed, the invagination takes place on one side in the marginal zone (fig. 46 u), and is distinguishable externally by means of a sharp, afterwards horseshoe-shaped furrow, which is bounded on one side by small cells, which in the case of the Frog contain black pigment, on V jSf the other side by large unpigmented \ < SSr elements. At the fissure-like blasto- ^*+S$j&$jpr pore there are infolded into the interior of the blastula (fig. 47 u) along its dorsal lip (dl) small cells, along its ventral lip (vl) the large deutoplasmio demerits of the vegetative half; the former constitute the roof, the latter the floor, of the ccelenteron (ud). The latter appears in the first stages of the invagination simply as a narrow fissure alongside the capacious cleavage-cavity (ffi) ; soon. however, it causes a com- plete obliteration of this cavity, the f undus of the invagination becoming enlarged into a broad sac, while the entrance always remains narrow and fissure-like. Since the ccelenteron of the Amphibia was first ob- served by the Italian investigator, RUSCONI, it is ordinarily mentioned in the older writings as RUSCONI'S digestive cavity, and the blasto- the Fig. 47.— Longitudinal [sagittal] section through an egg of Triton at the beginning of gastrulation. ak, Outer germ-layer ; ik, inner gorm-layer ; fh, cleavage- cavity ; ud, coelenteron ; u, blaatopore ; dz, yolk- cells ; •// and -cl, dorsal and ventral lips of the coelenteron. pore likewise as RUSCONIAN anus. At the close of the process of invagination the whole yolk-mass, or the vegetative half of the blastula, has b3en taken into the interior to form the lining of the coelenteron, being at the same time over- by a layer of small c"ll< Mi«r. -1-8). In the case of the Frog tlu» DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 89 Fig. 48.— Sagittal section through an egg of Triton after the end of gastrulataon. ak, ik, dz, di, vl, ud, as in fig. 47 ; d, vitelline plug ; mk, middle germ-layer. whole surface of the germ, with the exception of a small place about as large as the head of a pin, which corresponds to the blastopore, now appears black, because the small cells are deeply pigmented. At the place excepted a part of the unpigmented yolk-mass protrudes through the blastopore and closes the entrance to it as if with a stopper (d), by reason of which it bears the significant name of vitelline plug. Of the two germ-layers of the gastrula the outer subsequently becomes re- duced in thickness in the case of the Water-Sala- mander to a single layer of regularly arranged •cylindrical cells, whereas in the case of the Frog it is composed of two or three layers of small, in part cubical, dseply pigmented elements. The inner germ-layer in the roof of the ccelenteron likewise consists of small (in the Frog, pigmented) cells, but in the floor it is composed of large yolk-cells, which, heaped together in many layers, pro- duce an elevation that projects far into the co3lenteron and partly fills it. For this reason the gastrula in Amphibia is compelled to adopt in water a definite position of rest, because the yolk-mass, being the heavier part, always assumes the lowest position (fig. 48). The germ of the Amphibia is already a bilaterally symmetrical body. The thickened, yolk-containing wall of the gastrula becomes the ventral side of the adult animal ; the opposite wall, or roof of the C03lenteron, becomes the dorsum. The blastopore indicates, as the sequel shows, the posterior end, the opposite part the head-end. There may therefore be passed through the gastrula a longitudinal, a dorse-ventral, and a transverse axis, which correspond with the axes of the adult animal. This bilateral symmetry, which appears so early in the Amphibia, is solely attributable to the accumulation of yolk- material, arid to the piling up of it on the ventral side of the coalenteron. The development of Amphibia furnishes us with a transitional mdition, which is serviceable for the comprehension of the much 90 EMBRYOLOGY. more highly altered form which the gastrula acquires in the case of eggs with partial cleavage in the classes of tielachii, Teleosfs, Reptiles, and Birds. The conditions are the most readily intelligible in the case of the Selachians. That which we have described in the blastula of the Amphibia as the roof of the cleavage-cavity is in the blastula of the Selachians a v dk kz <& ft small disc of em- bryonic cells (fig. 49 kz), continuous at its margin with the extraordi- narily voluminous yolk - mass (dk), which contains nuclei, although it is not divided up into cells. This yolk-mass corre- sponds to the yolk-cells of the Amphibia, and, like the latter, forms the floor of the cleavage-cavity (Z>). Germ- disc and yolk thus together constitute a sac with an v H Fig. 49. Median section through a germ-disc of Pristiurus in the blastula stage, after RUCKERT. The posterior end of the embryo lies at the right. B, Cleavage-cavity ; dk, yolk-nuclei ; kz, germ-cells ; V and H, front and hind margins of the germ- disc. Fig. 50.— Median section through a germ-disc of Pristiurus, in which the gastrular invaginatioir has begun, after RUCKERT. vd, First rudiment of the coalenteron ; B, cleavage-cavity ; dk; yolk-nuclei ; fd, finely granular yolk ; gd, coarsely granular yolk ; V and H, front and hind margins of the germ-disc. almost obliterated cavity (B), and with walls differing in thickness and in differentiation* A very small part of the wall, the germ-disc, n insists of cells. The much larger and thicker portion is yolk-mass, which in the vicinity of the cavity contains nuclei, but is not divided into cells. As in the Amphibia, so here, the gastrulation begins at what DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 91 is subsequently the hind end (H) of the embryo, at a region in the zone of transition or margin of the germ-disc, in which the most superficial cells have assumed the cylindrical form, and are closely joined together (fig. 49). The margin of the disc is folded in (fig. 50) toward the cleavage-cavity (B), so that a small ccelen- teron (ud), shown in the accompanying section, and a fissure- like blastopore are distinctly recognisable. The neighboring yolk also participates in the invagination, since in the territory of the zone of transition the yolk-nuclei (dk), enveloped in protoplasm, become detached from the yolk, grow into the cleavage-cavity along with the invaginated cells, and contribute to the formation of the inner germ-layer in a similar manner to that in which, in the case of the Amphibia, the vegetative cells at the lower lip of the blastopore are carried in with the invagination into the cleavage-cavity. The cleavage-cavity (B) is being continually encroached upon by the in- growth of the cells originally in its roof, which form a continuous layer projecting from behind forward. Consequently in the Sela- chians also the germ-disc becomes two-layered as the result of the invagination. It lies so close upon the yolk, that the coelenteron appears at most as a fissure. Moreover, the invagination in the Selachians does not remain limited to one region of the original margin of the germ-disc, but soon stretches itself out over its whole posterior perimeter. The blastopore then appears as a large semi- circular or horseshoe-shaped fissure at the future posterior end of the embryonic fundament. The enormous volume of the yolk causes an important difference between the gastrulation of the Selachii and that of the Amphibia. In the case of the latter the mass of the yolk-cells was quite rapidly carried in with the invagination, and employed in the formation of the ventral wall of the coelenteron. In the Selachians the taking up of the yolk into the interior of the body ensues only at a slow rate (in a manner to be more accurately explained later), so that for a long time only the dorsal side of the gastrula consists of two cell- layers, whereas the ventral wall is formed by the yolk-mass. The eggs of Teleosts are very nearly related to those of Selachians in their whole method of development. The same cannot be said to be true to the same extent for the eggs of Reptiles and Birds. The latter, indeed, also belong to the meroblastic type? since they have developed a large amount of yolk, and in consequence undergo partial segmentation; but in the formation of the germ- ers, they exhibit many peculiarities, so that they require a separate laye EMBRYOLOGY. treatment. In Birds and Reptiles the investigation is accompanied with greater difficulties than in the Selachians. Particularly the development of the germ-layers in the Chick, notwithstanding the fact that the best investigators have given it their attention, has for a long time been the subject of very divergent descriptions. At the present moment, however, the main facts in the case have been established for the Bird's egg also by the very recent and excellent work of DUVAL, and upon this as a basis the gastrulation in Birds is easily to be correlated with that of the Vertebrates hitherto described. Since the Bird's egg has played such an important role in the history of embryology, and has even been called a classical object for investiga- tion, it appears necessary to go briefly into the conditions which it presents in the gastrula-stage, and in connection therewith to consider some of the important results drawn from the study of the eggs of Reptiles. The blastula arises and the germ-layers begin to be developed out of it while the Bird's egg tarries in the terminal region of the oviduct. The blastula arises in a manner which was first correctly described by DUVAL. When, by the process of segmentation a small disc of cells has been formed, there appears in the latter a narrow fissure. the cleavage-cavity (fig. 51 fh), and the cell- material is separated into an upper layer (dw) and a lower layer (vw), which are continuous with each other at the margin of the disc. The upper layer consists of fully isolated cleavage- spheres, which are flattened at their surfaces of contact and arranged into an epithelium-like layer. They correspond to the tbin-walKl half of the blastula in Triton (fig. 45), which has already been designated as the animal half. The lower layer is composed of larger cleavage-spheres, which are still in great part continuous by means of their lower halves with the whita yolk (wd), which is spread out beneath the germ-disc and is known as PANDER'S nucleus. Yolk-nuclei (merocytrs) are also found h«ro in Tig. 51.— Section through the germ-disc of a freshly laid unfeitilif ed Hen's egg, after DUVAL. fh, Cleavage-cavity ; tod, white yolk ; vw, lower cell-layer tlw, upper cell-layer of the blastula. DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 93 numbers, especially around the whole periphery of the germ-disc. Since they increase in number by nuclear division, and since some of them, enveloped in protoplasm, become detached from the yolk, they contribute to the continuous growth of the germ- disc, a process which has already (p. 65) been described as supplementary cleavage. The lower cell-layer, together with the whole yolk-mass with its free nuclei, must be compared to the vegetative half of the blastula of Triton (fig. 45 dz). The gastrulation proceeds from the posterior margin of the germ- , and begins even some time before the egg is laid. The study of it is coupled with great difficulties, and demands, most of all, that, in the investigation of the disc by means of sections, one should be accurately informed concerning the position of its anterior and posterior margins. The orientation is essentially facilitated by the fact that, in the case of every Hen's egg, with rare exceptions, the side toward which the front end of the embryo is directed can be stated accurately before opening the shell. This results from the following rule established by RUPFFER, ROLLER, GERLACH, and DUVAL. When one so places an egg in front of him that the blunt pole is turned to the left, the more pointed one to the right, then a line uniting the two poles divides the germ-disc into a half on the side toward the observer, which becomes the hind end of the embryo, and a forward half, which is developed into the head-end. By taking into account this rule, one can establish a difference on the germ- disc even during the process of cleavage. In the anterior region the cleavage takes place more slowly than in the posterior half. Con- sequently larger embryonic cells are found in front, smaller and more numerous ones behind (OELLACHER, ROLLIKER, DUVAL). The difference between anterior and posterior becomes more evident at the beginning of gastrulation. If one now examines carefully the thickened margin of the germ- disc (Eandwulst of German writers, bourrelet blastodermique of DUVAL). it is seen that the disc is limited in front and on the sides by a notched and indistinct boundary, but behind, on the contrary, by a sharper contour. The latter is caused by the fact that the marginal ridge, in consequence of a more vigorous growth of the cells, has become thickened and more opaque, and has assumed a whiter colour. It is distinctly recognisable from its surroundings as a whitish crescentic figure (fig. 52 A s). Often there is also observable in the crescent a narrow furrow, the crescentic groove (Sichelrinne, ROLLER), by means of which the germ- disc acquires a still sharper limitation behind. 94 EMBRYOLOGY. DUVAL has proved by means of sections, part of which was made in a transverse direction, and part in the sagittal, that the Bird's egg is now in the gastrula stage. Especially instructive are the two median hi vl ud ak ik v:d Tig. 52 A.— The unincubated germ-disc of a Hen's egg, after ROLLER. d, Yolk ; ksch, germ-disc ; s, crescent ; V and H, anterior and posterior margins of the germ-disc. B.— The germ-disc of a Hen's egg during the first hours of incubation, after KOLKKR. d, Yolk ; ksch, germ-disc ; Es, embryonal shield ; «, crescent ; sk, knob of the crescent ; V and //, anterior and posterior margins of the germ-disc. sections, figs. 53 and 54. As is to be seen at once in fig. 53, which re- presents the somewhat younger stage, the crescentic groove described as occupying the posterior part of the marginal ridge (vl) is contintu d in the form of a narrow fissure (ud). Whereas in the blastula stage (fig. 51) the lower cell- layer passed over con- tinuously into the white yolk, it is now sharply separated from it as far as the fissure extend-. In fig. 53 this separation has been completed only in the posterior half of the germ-disc; in the anterior half, on the con- trary, embryonic cells s&s*- m , ^»a«v— £•*"•" Fig. 53. — Longitudinal section through the germ-disc of an unincubated egg of the Siskin (Carduelis spinus). after DCVAL. ak, Outer , it, inner germ-layer ; icd, white yolk ; dk, yolk- nuclei ; ud, coalenteron ; vl, anterior lip, hi, posterior lip at the place of invagination (crescentic groove or blastopore). (dk) and yolk are still continuous. Howe\ « i . in the somewhat older stage (fig. 54) the connection is terminated in this region also, since the fissure (ud) has extended itself nearly to the anterior margin of the disc (vr). In consequence of this process the part of the white yolk which lies beneath the fissure has become destitute of and illicit i, with the exception of the marginal territory, where, DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 95 especially behind (hi) the crescentic groove, free nuclei are constantly to be found keeping up the supplementary cleavage. Owing to the appearance of the new fissure (subgerminal cavity) (fig. 53 ud), the cleavage-cavity (fig. 51 fh) is almost completely obliterated. The two cell-layers of the blast ula-st age (fig. 51 dw, vw), described as lying one above and one below the cleavage- cavity, have come close together (figs. 53 Rd 54), being separated from each other only a narrow fissure. In the upper rer (ak) the cells have assumed a cubical, d at a somewhat later stage a cylindrical, form, and constitute a compact epithelial membrane. The lower layer (ik) is composed of larger roundish and loosely arranged cells in several layers. The former is the primary outer germ -layer, the latter the inner layer. In the region of the posterior marginal ridge (vl), where the cells are at the same time engaged in more active proliferation, the two layers are continuous with each other. The highly important processes, by means which are produced the conditions repre- sented in figs. 53 and 54, present many points of comparison with the gastrulation of the Selachians and Amphibia. We can conceive that the newly appearing fissure has arisen, as in the case of the germ- disc of Pristiurus (fig. 50), by an infolding, in such a way that, us in the former case, cells grow inward from tlie posterior marginal ridge; and that at the same time, at the deep part of the in- vagination, the cells which are originally continuous with the yolk (fig. 53 dk) detach themselves from the latter, and are employed for the increase of the inner germ layer. If this explanation is correct, the fissure (ud) which now exists be- tween the inner germ-layer and the floor of the yolk corresponds to the ccelenteron, as GOETTE and EAUBER have already i\ marked, and DUVAL has for the first time demonstrated ; moreover, the cres- a I? SI * §< I f I as J 96 EMBRYOLOGY. centic groove (fig. 52 s) corresponds to the blastopore; the thickened portion of the marginal ridge (fig. 53 vl) which lies in front of the crescentic groove, within whose territory the two primary germ- layers are continuous with each other, is the anterior or dorsal lip of the blastopore ; and the yolk (hi) which lies behind the crescentic groove, and which at this early stage contains numerous free nuclei, may be designated as the posterior or ventral lip of the blastopore. The develop- v ment of the coelenteron is the cause of the gradual re- duction of the cleavage - cav- ity, and of its persisting only as a narrow fis- sure separating the primary germ:layers. The points of comparison with the gas- trula of Triton '<"'^-:- (fig. 47) arc made evidi nt as soon as we Fig. 66.— Embryonic fundament of Lacertaagilig, after KI-PFFER. , , //, Area pellucida ; df, area opaca ; u, blastopore ; s, crescent ; es, em- I'6p*AC6 bryonio shield. V, anterior, H, posterior end. mass of yolk- cells with u n- segmented yolk, and imagine nuclei imbedded in the latter in tin- region of the ventral lip of the blastopore. Through the exposition given by DUVAL, it appears to me that the contest concerning the origin of the two primary germ-layers in Birds has been happily settled. For a long time there have exist rd on this very question two irreconcilable views. According to the older view, to which many investigators still cling. the germ-disc which results from the process of cleavage is divided 1 >y fission into an upper and a lower layer (PANDER, VON BAER, REMAK, KOLLIKER, His, and others). According to the other one (HAECKEL, ODETTE, RAUBER, DUVAL, and others), the lower layer \\\\> arisi-n by — o -, DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 97 an infolding. Only by means qf the theory of infolding can be ex- plained the different conditions of the anterior and posterior margins of the germ-disc, the more active cell-growth in the territory of the crescent, the existence of a crescentic groove, and the continuity of the two primary germ-layers which is demonstrable in that region. Only by means of this theory, finally, is the relation of irds to the lower classes of the Vertebrates made possible. The discoveries which KUPFFER UND BENECKE have made in their investigations of Reptiles, which are so closely related to Birds, also contribute to the elucidation of the pending controversy. In the case of Lacerta agilis (fig. 55), Emys europsea, etc., there is found, as in the case of the Hen at a corresponding stage of development, at the boundary of the pellucid and opaque areas of the posterior end of the germ-disc, an exuberant cell-growth in the form of a crescent (s). In the middle plane and slightly in front of this crescent there is to be seen a small, transversely placed, fissure-like opening (u), which leads into a blind sac and is comparable to the crescentic groove. KUPFFER rightly interprets the opening as the blastopore, which is enclosed between an anterior and a posterior lip, and the cavity as the coelenteron. He also draws a comparison between the corre- ]nding structures in Birds and Reptiles.* Let us now direct oar attention to the succeeding developmental stages of the germ-disc of the Chick. These consist, chiefly, in constant increase of the superficial extent of the disc. In the freshly laid, unincubated egg (fig. 54) the outer germ-layer (ak) is composed of a single sheet of closely united cylindrical cells ; the inner layer (ik), on the contrary, consists of a two-layered to three-layered bed of somewhat flattened elements, which are only loosely associated. 'Under the influence of incubation the superficial extension of the germ-disc makes rapid advances (fig. 56). In this process the outer germ-layer (ak) outstrips the inner, and terminates in a region of the * In the interpretation of the manner in which the invagination takes place the case of the eggs of Reptiles and Birds, I differ from other investigators who also maintain that a gastrulation takes place (GOETTE, HAECKEL, RAUBER, BALFOUR, and others). They regard the whole margin of the germ- disc as the blastopore, at which the outer germ-layer bends over to become continuous with the inner layer. According to my interpretation, the invagina- tion occurs at a small circumscribed place of the margin. The blastopore is from the beginning surrounded by cells both on its anterior and its posterior lip. The relation of the blastopore as well as that of the germ-layers to the yolk ill be more fully dealt with hereafter. •; •w e 98 EMBRYOLOGY. yolk where the latter has not yet undergone division into entodermic cells. In the form of its cells it is, in every respect, in sharp con- trast with the inner layer. While the ecto- dermic cells (fig. 56 ak) attain their greatest height in the middle of the germ-disc, they gradually decrease in height toward the mar- gin, and undergo a transition into cubical and finally into flat- tened elements (fig. 57). The reverse is the case with the inner germ- layer ; the latter has now become converted in the middle of the germ- disc (fig. 56 ik) into a single layer of much flattened scale-like cells,, which are closely united into a thin membrane. Toward the periphery they become somewhat larger and more poly- gonal (fig. 57), and here, at some distance inside the free margin of the outer germ-layer, they become merged in the white yolk (dw), which is abundantly provided with yolk-nuclei (dk) in the region of the transi- tion. This region of the DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 99 yolk is designated as the yolk-wall (vitelline rampart). It serves for the augmentation of the inner germ-layer, in that the free nuclei increase in number by division, and keep up the process of supplementary cleavage already mentioned. During incubation the liquefaction of the yolk makes further pro- gress (fig. 56) and leads to the formation of a depression (ud), which continually increases in depth and breadth, and over which the germ- disc arches like a watch-glass. Upon examination from the surface its middle, as far as the fluid reaches under it, appears clear and translucent, whereas the marginal area, which lies upon the opaque yolk, appears dark. Such a distinction is still more observable when one detaches the whole germ-disc from the yolk, for in the region of the fluid-filled space the thin and transparent germ- layers come off' easily and clean from their substra- tum whereas it the rim Fig. 57.— Section through the margin of the germ-disc Lm> of a Hen's egg that had been incubated for six from the point where the hours, after DUVAL. i ak, Outer germ-layer ; dzt yolk-cells ; dk, yolk-nuclei : inner germ-layer merges dw J(^.waii with the yolk -wall out- ward, turbid yolk-substance remains clinging to the germ-disc. For a long time the middle, clear, circular area has been designated in embryology as the clear germinal area (area pellucida), and the more cloudy, ring-like rim as the opaque germinal area (area opaca). In the next chapter I shall treat more in extenso of the important changes which take place — up to the time when the egg is laid and during the first hours of incubation — in the vicinity of the crescentic groove and the anterior lip of the blastopore, because they are connected with the development of the middle germ-layer. It is still more difficult than in the case of the Chick to interpret in its details the development of the germ-layers in Mammals, and to refer it back to the gastrulation of the other Vertebrates. Especial service has been rendered through the painstaking investigation of these conditions : in the earlier times by BISCHOFF, in later years by HENSEN, LIEBERKUHN, VAN BENEDEN, KOLLIKER, and HEAPE. The object of investigation w^iqh has been made use of in this work, and which we shall employ "as the J basis Qf pur description, iias \isually been the Rabbit ; besides this", tKe Baifahd tK«3 l&oMiave also been employed. 100 EMBRYOLOGY. While the Mammalian egg is gradually impelled through the oviduct toward the uterus by the ciliary motion of the epithelium, it becomes converted by the cleavage process into a spherical mass of small cells (fig. 58 A). Then there arises within it, by the secretion of a fluid, a small fissure- like cleavage-cavity (fig. 58 B). The germ has consequently entered upon the vesicular or blastula stage. The wall of the blastula, or vesicula blastodermica, is composed of a single layer of polygonal cells, arranged, as has been known since BISCHOFF'S works, in mosaic, with the exception of a small region, where the wall, as in the case of the Amphibian blastula, is thickened by an accumulation of somewhat more granular and darker cells, Fig. 58.— Optical sections of a Rabbit's egg in two stages immediately following cleavage, after ED. v. BENEDEN. Copied from BALFOUR'S " Comparative Embryology." A , Solid cell-mass resulting from cleavage. B, Development of the blastula by the formation of a cleavage-cavity in the cell-mass. (According to VAN BENEDEN'S interpretation, ep is epiblast ; hy, hypoblast ; bp, blastopore.) which produce a knob-like elevation that projects far into the cleavage-cavity. A peculiarity preeminently characteristic of the further develop- ment of Mammals is that here, as in no other Vertebrate, the blastula increases enormously in size (fig. 59), by the accumulation of fluid which contains much albumen and produces a granular coagulum upon the addition of alcohol ; it soon acquires a diameter of 1*0 mm. Of course, with these processes of growth the zona pellucida is altered and distended into a thin membrane. A g< -l.-i- tinous layer (zp) already secreted by the oviduct envelops the latter. In Habits' eg£s which; a.:» .1 ni'lfim'-tne in diameter the wall of the blastula h.is become vfcry. thin ' The mosaic- like cells arranged in a single layer ha^e become very much :l.ti • T.-d. Also the knob DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 101 of cells, which projects into the cleavage-cavity, has become meta- morphosed and has spread itself out more and more in the form of a disc-like plate, which is continuous at its attenuated margins with the thin wall of the blastula. e further processes development take ,ce principally in plate. Its most perficial cells are flattened out to thin scales, such as also brm the wall of the tula elsewhere ; its aining elements, the contrary, ar- in from two three superposed layers, are larger and I her in protoplasm. dp to this time the bryo of the Mammal is in the blastula stage. It still consists everywhere of a single germ-layer. For the view which has been advanced by many persons, that the germ-disc in this Fig. 59. -Rabbits egg, 70-90 hours after fertilisation, after ED. v. BENEDEN. Copied from BALFOUR'S " Comparative Embryology." bv, Cavity of the blastula ; sp, [gelatinous layer surrounding the] zona pellucida ; ep, hy, as in Pig. 58. Fig. 60.— Cross section through the almost circular germinal area of a Rabbif a egg 6 days and 9 hours old (diameter 0-8 mm.), after BALFOUR. ak, Outer, ik, inner germ-layer. The section shows the peculiar character of the upper layer with a certain number of flattened superficial cells. Only about half of the whole breadth of the germinal area is represanted. stage of development is already in the two-layered condition, and that the outer layer of flat cells constitutes the outer germ-layer and the more protoplasmic cells lying under it the inner germ-layer, is, in my opinion, untenable. Opposed to this are, first, the fact that the flat- tened and the thicker cell-layers are firmly joined together and are not separated from each other even by the narrowest fissure, and, secondly, the further course of the development.* * Holding to this interpretation, I am of course also unable to agree with a view of VAN BENEDEN'S, according to which the gastrulation takes place at the 102 EMBRYOLOGY. Two germ-layers first appear in eggs which have already attained a diameter of more than 1 mm. and are about five clays old. At the place where the cell plate pre- viously lay, one sees by inspection from the surface a whitish spot, which is at first round, but later becomes oval or pear shaped. It is generally designated at this stage as area embryonalis, or as embryonic spot. It consists of two germ-layers (fig. 60), which are separated by a distinct fissure, and may be detached from each other. The inner germ-layer (ik) is a single sheet of greatly flattened cells. The outer germ-layer (ak), on the contrary, is considerably thicker, and shows that it is composed of two sheets of cells : (1) a deeper layer of cubical or round- ish, larger elements, and (2) a superficial layer of isolated flatter, cells, which were first accurately described by RAUBER, and which have been named after him RAUBER'S layer. Toward the margins of the embryonic spot the outer layer becomes thinner and pos- sesses only a single layer of cells ; these are continuous with the large flattened elements which, as we have seen, alone constitute the greater part of the wall of the sac in the blastula stage. The inner germ-layer is at first developed on only a small part of the wall of the sac — at the embryonic spot and its immediate vicinity; it termin-ito with a free notched margin, where there are to be found loosely associated amozboid cells, which by their increase in number and migration probably cause the further growth end of the first stages of cleavage. He interprets in the originally solid sphere of cells (fig. 58-4) the darker and larger centrally located elements (7ty) as entodenn, the layer of smaller and clearer cells (- layers are continuous with each other, and from here, as well as from the primitive streak, the middle genn-l«ij<'r takes its origin. I assume that, beginning at this place, the loiver germ-layer has in a still earlier stage been developed by an infolding of a small territory of the single-layered blastula (lig. 59). Fig. 62.— Blastula of the Rabbit 7 days old without the outer egg- membranes. Length 4*4 mm. After KOLLIKER. Magnified 10 diameters. Seen in . ( from above, in B from the side. ag, Embryonic spot (area embryonalis) ; rie, the line up to which the blastula is two-layered. DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 105 \ One circumstance is especially characteristic of the gastrulation of Mammals : that the invaginating membrane is not a closed blind sac, but possesses a free margin, with which it grows along on the inner surface of the outer germ-layer, until it has completely lined the )lastodermic vesicle. The reader ,-ill please compare with this the "atements on page 102. But the ibsence of a ventral closure becomes itelligible, when we imagine that yolk-mass, which constitutes in i€?roblastic eggs or in Amphibian s the floor of the coelenteron, s degenerated and wholly disap- aared. In this case coelenteron id cleavage-cavity become one id the same, as is the case with [animals. Moreover we are induced to as- ime that in the eggs of Mammals a >ive metamorphosis of origin- ly abundant yolk-contents must have taken place, on account of lany phenomena in their development, which would be unintelligible ps hio Fig. 63.— Pear-shaped embryonic spot of a Rabbit's egg 6 days and 18 hours old, after KOLLIKEB. ps, Short primitive streak ; hw, crescent- shaped terminal ridge ; V, anterior, H, posterior end. ik . 64. — Median section of the embryonic fundament of a Mole's egg through that part in which tke primitive streak has begun to be formed, after HEAPE. Blastopore ; ak, outer, ik, inner germ-layer ; V, anterior, //, posterior end. without this assumption. These phenomena will be considered more it length in a subsequent chapter. 106 EMBRYOLOGY. CHAPTER VI. DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. (CCELOM-THEORY.) * AFTER the completion of the gastrula stage the processes of develop- ment become more and more complicated, so that the attention of the observer from this time on must be directed to a series of changes which take place at the same time and in various parts of the embryo. For a transformation now ensues, due to the simultaneous folding of both the inner and outer germ-layers, whereby four new chief organs of the vertebrate body are called into existence. Out of the inner primary germ -layer arise (1) the two middle germ-layers, which enclose between them the body-cavity ; (2) the secondary en- toderm or entoblast (Darmdriisenblatt), which lines the secondary intestine of vertebrated animals ; and (3) the fundament of the axial skeleton, the chorda dorsalis, or notochord. At the same time there is developed from the outer germ-layer, as its only system of organs, the fundament of the central nervous system. Since these four pro- cesses in the development are in part most intimately involved in one another, they cannot be separated in their treatment. Here again we have to do with a problem which is one of the most difficult in the embryology of vertebrated animals — the history of the development of the two middle germ-layers. Not- withstanding a voluminous literature which has grown out of this theme, there are many conditions, especially among the higher classes of Vertebrata, which are not yet explained in an entirely satisfactory manner. We shall therefore enter somewhat more minutely into this topic, which, like the question as to the origin of the two primary germ-layers, possesses a fundamental significance for the comprehension of the organisation of Vertebrates. The presentation of what follows will be essentially facilitated, if we allow ourselves a short digression into the history of the develop- ment of the Invertebrata, and take under consideration a case in which the middle germ-layers and the body-cavity are established in a manner similar to that which obtains in the case of Vertebral a, but which is easier to investigate arid to understand. Such an * In figs. 66-89 the individual germ-layers are represented in different depths of shade, so as to make their relations to one another more evident. Hie middle germ -layer is darkest. DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 107 example is presented to us in the development of arrow-worms (Sagitta) or Chcetognatha, concerning which observations have been published by KOWALEVSKY, BUTSCHLI, and the author. After the process of cleavage there arises a typical blastula, which after some time is converted into a typical gastrula. While the latter elongates, two folds of the inner germ-layer arise at the bottom the ccelenteron, and grow up parallel to each other (fig. 65). ok Fig. 65. Fig. 65.— A stage in the development of Sagitta, after KOWA.LEVSKY, from BALFOUR'S " Comparative Embryology." Optical longitudinal section through a gastrula at the beginning of the formation of the body-cavity. m, Mouth ; al, alimentary cavity ; pv, body-cavity ; bl.p, blastopore. Fig. 66. —Optical cross section through a larva of Sagitta. The coelenteron is separated by means of two folds, which protrude from its ventral wall (T), • into the intestinal canal proper and the two lateral body-cavities (lh~), all of which are still in communication with one another on the dorsal side (Z>). Dorsal side ; V, ventral side ; ak, outer, ik, inner germ-layer ; mi1, parietal, ink", visceral middle layer ; lh, body-cavity. They grow larger and larger, and at the same time stretch over on to the ventral wall of the larva. From here the free edges finally grow on the one hand up to the dorsal wall, on the other up to the blastopore, and thereby completely divide the coelenteron into a middle and two lateral spaces (fig. 66 lh), which for a time communi- cate with each other near the blastopore and along the subsequent dorsum (D) of the embryo. After a short time this communication is lost ; the blastopore becomes closed, and the edges of the folds fuse with the adjacent surfaces of the ccelenteron. Of the three cavities the middle becomes that of the permanent intestinal tube, the two lateral ones (lh) become those of the two body-cavity sacs which two iat 108 EMBRYOLOGY. separate the intestine from the wall of the body. They appropri- ately take the name enterocml, since they are formed from the coelen- teron by a process of constriction, and are genetically distinguishable from other cavities which arise in other animals between the wall of the intestine and that of the body by simple splitting, and to which is given the na,me jissiccel or schizoccel. By the process of infolding the number of the germ-layers in Sagitta has been increased from two to three. The primary inner germ-layer is thereby divided into (1) a cell-layer (ik) which lines the intestinal tube, and (2) a cell-layer which serves to enclose the two body-cavities (nik1 and mk*). The first is designated as the secondary inner germ- layer or entoblast, the second as the middle germ-layer (mesoblast). One part of the latter is adjacent to the outer germ-layer, the other part to the t contact there are formed two delicate membranes, a dorsal (<1M) :iii«l a ventral (vM) mesentery, by means of which the intestinal lulu- is attached to the dorsal wall and to the ventral wall of the 1 funk. Processes very similar to those of Sagitta occur in the developing it of Vertebrata also, but in the latter case they are combined with the development of the m-unil (uln> and (he chorda dorsalis. In the presentation of these we shall proceed as in the foregoing chapter, \vliit-h treated of the formation of the gastrul.i, and consider separately m\ii»r.n\T OK 'nil-: TWO i: <;KHM I.AVKI;S 101) the processes in . \mphioxus, Amphihia, Selachians, IJird -, and Mam mills, since they diller somewhat from one another. Tin' Itisten/ ef tin' />nit'//f <>/ A /// />// it'.rns Li nc,\>lnt its is \er\' ill -Iriiclixo. The "aslrula elon.";ales, whereby t he Cteloilt eron is turned a hi I le towards the hit nro dorsal surface, ami hero terminates in the Ma .loporo, which marks ( ho future hind end of (ho worm shaped d\ . Then ( he dorsal snrlace hecomos somewhat flattened ; i ho Is iu tins region increase in height, become cylindrical, and lorm 10 medullary or neural plate (li". li!> ni/>). l>\ a h".hl infoldinv, of ic laller, there arises a. medullary i^roo\e, which forces downward •oof of the •lenleron in ic form of a, (<•/,). At „ place \\ here ie ihickened 0 d u I I a r \ lie joins (he 1 a I I celled rt of t, he iter germ- I|f (m o,,u,.ii«..Kit...i..mii, i\er, or I he Ainphioxua with flvo pninit ... I. XI.I.-.IMI //. |,,,:,l,-ii,.r ,-n.l : ./.. MIII.-I. ,-., I. II.- ,ron I. i\.-r ;,//«. W ('*•") 1 llll.v,lili:il . M. i ..ll . '. In I u 1 1 er ru | >l ion 1. 1 mi it m- Hfgmwnt; iwA. ...\ it y •>!' i>umiti\,' -.r.m< m I he conl inn no\\ lakes place, and the epulermr. ;-ro\\ oxer I he ciirxed mal plale from holh nlc . null) il halve, im el in (he middle e and fuse Thir- lliere arises aloii- I h«- hack of (he embryo i" <(>) a canal, I he lo\\er wall ol \\hlcli I. loi'llied l>\ (he cill'Ved iedullar\ plale (////'). .'Mid I he upper \\.-ill l>\ the o\ or^rowin^ epi (,//•). Il is onl\ al a la I ei s( a-jo I hat I lie medulla r\ plale in I v i ni^ under the epidormi , i i eon \crletl ml «> a nem a I I nbe J. 7'J n) I»N I he I.endin." up of its ed^os an. I I hen fusion. As (Jie lit of lhonor\oii:, s\slem hec.mii . dill'erent ialed, il extends lai toward the posterior end of the embryo, that the hlastoporo, n. -I, i located (here. III! tall vv 1 1 1 1 1 1 1 it lenilor\, a lid \\ith t ho ure o| the neural lube is included W'iUlill the end of I he laller Ih: manner il oecm . Ihal neural lulie and infest ina I lube, a:; tOWAKKVSKY lirst obs«M-\ed, are now, by means of lh<> bla lopore, linuilv (li". (i.Sc;/)al I he po: I erior end of I he bod y. Tll<^ two ier const it nle a canal c posed of I v\ o arms, I ho form of \\ Inch •ugh an •mbryo of i \ , 110 EMBRYOLOGY. Fig. 69.— Cross section of an Amphioxus embryo, in . which the first primitive segment is being formed, after HATSCHEK. ak, Out*r, ik, inner, r,ik, middle germ-layer ; hb, epidermis ; np, medullary plate ; ch, chorda ; *, evagination of the coelenteron. is comparable with a siphon. The upper arm, which is the neural tube, continues, for a time, to open to the outside world at its anterior end. The bent por- tion of the siphon, or the blastoporic region, by means of which the neural and the intestinal tube are united, is called canalis neurentericus (fig. 68 en), a structure which we shall again encounter in the development of the re- maining Yertebrata. Simultaneously with the neural tube are developed the two middle germ-layers and the chorda dorsalis (figs. 69 and 70). At the front end of the embryo there arise in the roof of the coslenteron close to each other two small evaginations, the body-sacs (mk), which grow dorsally and laterally at either side of the curved medullary groove. These are slowly enlarged, ak since the process of evagina- mp tion progresses from the an- mk - terior toward the posterior ch " end of the larva, and finally reaches the blastopore. The narrow strip of the wall of the crelenteron which is found between them and separating them (its limits marked by two stars * * in figs. 69 and 70), and which lies under the middle of the medullary groove, represents the funda- ment of the chorda (ch). The primary inner germ- layer therefore has now undergone division into four different parts : (1) the fundament of the chorda (ch), (2) and (3) the cells (mk) which line the two body-sacs (Ih) and represent the middle germ-layer, and Fig. 70.— Cross section of an Amphioxus embryo, in which the fifth primitive segment is ia process of formation, ufter HATSCHEK. ak, Outer, it, inner, mk, middle germ-layer; mp, medullary plate ; ch, chorda ; *, evagination of the ccelenteron ; tlh, intestinal cavity ; lht body-cavity. DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. Ill (4) the remaining part, which, since it is destined to form the bounding wall of the subsequent intestine (dh), is to be designated as permanent entoderm (Darmdriisenblatt) (ik). The succeeding processes of development have as their objective point the detachment from one another, by means of constriction and fusion, of the parts which are still in continuity, and the formation of discrete cavities. The processes of constriction begin at the anterior end of the embryo, and progress thence to the blastopore 70 and 71). At first the body-sacs become deeper (fig. 70 Ih), «., ik - Ih ik Fig. 71. -Cross section through an Amphioxus embryo with five well-developed primitive seg- ments, after HATSCHEK. a£, Outer, ik, inner, mk, middle germ-layer ; mp, medullary plate ; ch, chorda ; dh, intestinal cavity ; Ih, body-cavity. Fig. 72.— Cross section through the middle of the body of an Amphioxus embryo with eleven, primitive segments, after HATSCHEK. «, Neural tube; us, primitive segment. For the meaning of the other letters see Fig. 71. Fig. 71. Fig. 72. and then lose their connection with the main cavity (d/i) by the close apposition of the cells which surround the entrances to them (fig. 71). By this process the margin of the secondary entoderm (ik) comes to abut directly on the margin of the chordal fundament (ch). The latter has meanwhile also undergone changes ; the plate-like funda- ment has become so curved by the elevation of its lateral margins, that there has arisen a deep chordal groove, which is open along its ventral side. Subsequently the lateral walls of the groove come into close contact, and are thereby converted into a solid rod of cells, which temporarily shares in the closure of the roof of the secondary intestine, and appears as a ridge-like thickening of the latter. Then the cell- rod (ch) becomes detached (fig. 72) from the wall of the intestine ; the latter now, for the first time, becomes completely closed in the form of a tube. To effect this the margins of the entoderm, indicated in 112 EMBRYOLOGY. fig. 70 by stars ( * *), grow toward each other under the chorda and fuse into a median raphe. The final result of all these processes is shown in the cross section fig. 72 : the original ccelenteron has become divided into three cavities — into the ventral permanent intestine (dh), and into the two body- cavities (Ik), which are situated dorso-laterally to it, and which con- tinue to increase in size. Between these there has been interpolated the chorda (ch), upon which the intestine abuts below and the neural tube (n) above. The cells which have been cut off from the ccelen- teron by constriction — and which are more deeply shaded in figs. 69 to 72, and enclose the body-cavities (Ik) — constitute the middle germ-layer (mk). The part which lies in contact with the outer germ-layer (fig. 72) is recognisable as the parietal middle layer (ink1) ; the part which is in contact with the neural tube, chorda, and intestine as the visceral middle layer (mk2). Inasmuch as the process of differentiation just described begins, as has been already stated, at the front end of the embryo and extends slowly step by step toward the hind end, by an examina- tion of a series of sections one may follow the various stages of metamorphosis on a single object. In the description given I have presented the conditions as though in Amphioxus there arose two simple body-sacs, one on either side of the intestinal tube. The processes are, however, somewhat more •complicated, for in the case of the embryo of fig. 70 the body-sacs, while increasing in size posteriorly, undergo further changes in the anterior region, and through repeated infoldings are divided into separate compartments, the primitive segments (us), which lie one behind the other. I content myself with this statement, since for •didactic reasons I shall defer the treatment of the development of .the primitive segments until I come to a subsequent chapter. "While in the case of Amphioxus lanceolatus there is no doubt but 'that the body-cavity and the middle germ-layer are formed by an out- pocketing of the wall of the coelenteron, opinions upon the origin of the same parts in the case of the remaining Vertebrata are .still \* r\ •divergent. This results, in the first place, from the fact that the in- vestigation, which can be carried out only by means of serial sections. is coupled with greater technical difficulties, and, secondly, because the conditions are somewhat altered, owing to the greater abundance of yolk in the eggs, and furnish less clear and intelligible views. Where in the gastrula of Amphioxus a great cavity is present, we see in the •case of the remaining Vertebrates a great mass of yolk-material DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 113 collected, and the coelenteron more or less comple.ely filled with it. Consequently there are formed in these cases for the production cf the body-cavity no hollow evaginations, but solid cell-growths, iti that the parietal and the visceral lamellce of the middle germ-layer have the surfaces which in Am- phioxus bound the body- cavity pressed together at the beginning of the de- velopment and separated only at a rather late stage. In order to make easier the comprehen- sion of the somewhat dissimilar appearances furnished by an inves- tigation of the separate classes of Vertebrates, let us describe first, with the aid of two diagram- matic figures, how, according to a series of investigations which I have undertaken, the development of the middle germ-layer and the body-cavity would take place in the case of the vertebra ted animals. One of the diagrams (fig. 73) represents a cross section in front of the blastopore. kg7 ^g^gj-,dA It exhibits the inner germ- \st?\ y$5l- layer (ik) extensively thick- ^^^^^jT^rr^^\~ enec* on t^ie ventra* s^e ^7 the deposition of yolk (d), so IJjjpr that the ccelenteron is re- 74.— Cross section oi an Amphioxus embryo. duced to a Small Cavity (dh). See explanation of Fig. 70. jn th(, rQOf Qf the coelenteron ale, Outer, ik, inner, mk, middle germ-layer ; ch, -, ~ chorda. there lies a single layer cf cells (ch), the fundament of the chorda, characterised by their cylindrical form. On both sides of it the inner germ -layer has developed evaginations, the two ly-sacs (lh), which have grown dowrn some distance between 8 Fig. 73. — Diagram to show the development of the middle germ- layers and the body-cavity in Vertebrate. Cross section of an embryo in front of the blastopore. mp, Medullary plate; ch, fundament of the chorda; ale, outer, ik, inner germ-layer ; mk1, parietal, mk3, visceral lamella of the middle germ-layer; d, yolk-mass; dk, yolk-nuclei ; dh, intestinal cavity ; lh, body-cavity. -tt 114 EMBRYOLOGY. lid Ih - d — the yolk-mass and the outer germ-layer. Their wall (ink* and ink*) is composed of small cubical or polygonal elements, shaded darker in the diagram. The coelenteron is distinctly separated by means of the two coelenteric folds (* *) into a median or intestinal cavity proper (dk), lying beneath the chordal fundament, and the two narrow body-sacs (Ik), which communicate with the former only by means of narrow fissures (* *) at the right and left of the chordal funda- ment. The figure is easily reducible to the preceding (p. 113) cross section of an Amphioxus embryo (fig. 74), if we conceive the simple epithelium on the ventral side of the latter thickened by an accumula- tion of yolk, and the two small body-sacs grown down a certain distance between yolk-mass and outer germ-layer. In the second dia- grammatic cross section, which is through the blastopore (fig. 75), the ccelenteron (ud) is wholl} filled up with the yolk mass (d). The body-sacs (Ik) described in the first diagram are to be seen here also, as they crowd themselves cl< wnwa rds between yolk and outer germ-layer. Their walls are composed of small cells, and the outer or parietal layer (w/,-1) merges into the outer germ-layer at the blastopore, while the inner or visceral layer (mk2) is continuous with the yolk-mass or the inner germ -layer. Were the conditions in Vertebrates such as the two diagrams represent, there could no longer be any doubt in regard to tkn//, any more than in the case Of Amphioxus, that the body-cavity is developed out of two evaginations of the coelenteron, and that its walls constitute the two middle germ-layers. But there is not a single Vertebrate which presents such clear and convincing evidence. The distinctness is everywhere diminished, most of all by the fact that the parts which are to be interpreted as body-sacs no longer enclose cavities, because their walls are firmly pressed together, in Fig. 75. — Diagram to show the development of the middle germ-layers and the body-cavity in Vertebrata. Cross section through the blastopore of an embryo. ut Blastopore ; ud, coelenteron ; Ih, body -cavity ; d, yolk ; ak, outer germ-layer ; mk1, parietal, mk1, visceral lamella of the middle germ -layer. DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 115 consequence of the fact that the greater collection of yolk requires the space for itself. Consequently we find, in place of the body-sacs exhibited in the diagram, solid masses of cells, for which it remains to be established that they correspond to the sacs in position and development. In order to see what condition would result in consequence of a disappearance of the body-cavity, we wrill imagine that in the two diagrams the parietal and the visceral layers of the body-sacs are firmly pressed together. In the first diagram (fig. 73) we should then have a mass several cells thick, which would be everywhere dis- tinctly separated from the two germ-layers — in between which it had grown — with the exception of the place indicated by a star, which marks the entrance to the body-sac ; this is the important region whence the evagination or the outgrowth of the middle germ-layer from the inner layer has taken place. At this point the cell-mass is continuous, on the one side with the fundament of the chorda, on the other with the entoderm. In the second diagram (fig. 75) we should likewise see the thick cell-mass everywhere isolated, except in the vicinity of the blastopore, where a transition to the outer as well as to the inner germ-layer takes place. If, in addition to this, we should imagine that the two lips of the blastopore were here pressed together from right to left, we should have in the middle of the cross section a thick, many-layered cell-ma^s, which on both sides is resolved into the three germ-layers, or, in other words, at the blasto- pore all three germ-layers by their fusion meet together in a single mass of cells. By careful investigation it is, in fact, demonstrable that similar conditions to those which we have produced by changes in the diagrams are found in the investigation of the several classes of Vertebrates. For this purpose we must make sections through three different regions of the embryo : (1) through the region in front of the blastopore, (2) through the region of the blastopore itself, and (3) behind it. The agreement appears most prominent in the develop- ment of the Amphibia, among which the Tritons again furnish the nost instructive objects. When in the case of Triton the gastrulation, with the accompany- i ng obliteration of the cleavage-cavity, is fully completed, the embryo becomes slightly elongated; the future doreal surface (fig. 76 D) becomes flattened, and gives rise to a shallow furrow (r), which I Wretches from the anterior to the posterior end nearly up to the Mastopore (u). The latter has now assumed the form of a longitu- 116 EMBRYOLOGY. dinal fissure. A cross section made through the middle of the embryo in front of the blastopore (fig. 77) corresponds in every particular to our first diagram (fig. 73), if wo conceive that the Ixxly-cavity in this case has disappeared. The outer germ-layer (ak) consists of a single sheet of cells, which on the back of the embryo are cvlindrical, but become shorter toward its ventral side. The cells enclosed within the outer layer exhibit a differentiation in three ways, and therefore are subsequently converted into three different Fig . 76.— Egg of Triton with distinctly developed medullary groove, seen from the blastopore, 63 hours after artificial fertilisation. A Dorsal, V, ventral region ; u, blastopore ; h, elevation between blastopore and medullary groove (r) ; f, semicircular furrow, which encloses the blastoporal area ; dp, yolk -plug. Rf . 77.— Crow Motion of an egg of Triton with feebly expressed medullary groove. at, Outer, it, inner germ-layer ; mkl, parietal, m*a, visceral lamella of the middle germ-layer \. ch, chorda; dh, intestinal cavity ; D, dorsal, F, ventral. organs — into chorda, entoderm, and middle germ-layer. First, there is to be found on the roof of the coelenteron (dh) under the medullary groove, even close up to the blastopore, a narrow band of long cylindrical cells (ch) ; it corresponds in every respect to the funda- ment of the chorda in our diagram (fig. 73 ch), and in the cross section through Amphioxus (fig. 74 ch). Secondly, the fundament of the chorda is flanked on either side by two bands (mkl, ink2) of small oval cells, which extend downwards to about the middle of the lateral region of the embryo. They do not share in bounding the coelenteron, since a third kind of cells (ik), large and rich in yolk, lie along their inner surfaces. The latter begin at the margin of DEVELOPMENT OP THE TWO MIDDLE GERM-LAYERS. 117 s mi( cone thai the chordal fundament as a single layer, become two layers thick farther down, and thus merge into the more voluminous accumu- lation of yolk-cells, which, in all Amphibian embryos, occupy the ventral side and restrict the gastrula-cavity. They correspond, to continue with our comparison, with the entoderm, whereas the small-celled masses, which, starting from the fundament of the chorda, have crowded themselves out between the entoderm and he outer germ-layer, are comparable with the cells which in Am- ioxus and in our diagram form the wall of the body-sacs, or the middle germ-layer. The conclusion is therefore jus- ied and very obvious, ,t in Triton the two mid- dle germ-layers have arisen in the anterior territory o the embryonic body by a process of evagination at both sides of the chordal Jundament, just as in Atn- phioxus, except that in one case the evaginated cell-mass contains a cavity, in the other case none. A cross section through the blastopore of the Triton embryo (fig. 78) is to be compared with our second diagram (fig. 75). The hollow body-sacs of the latter correspond to the solid cell-bands, which are the fundament of the middle germ-layer. Near the blastopore (u) they are split into two lamellae. Of these the outer •(mA;1) merges, as in our diagram, into the inner layer of the blasto- poric lip, and becomes continuous at the edge of the blastopore with the outer germ-layer (ak) ; the inner lamella (mk2), on the contrary, is connected with the mass of yolk-cells (dz), which lies like a wall in front of the blastopore and even projects into it as the RUSCONIAN yolk-plug (dp). Posteriorly to the blastopore, the middle germ-layer stretches itself out for some distance, but here only as a single connected mass. According to the region from which the middle germ-layer is de- veloped, we may divide it into two portions, and call that part which ml-1 dp ink? ak dz . ik - dh Fig. 78.— Cross section through the blastopore of an egg of Triton with feebly expressed medullary groove. ak, Outer, ik, inner germ-layer ; mkl, parietal, mk'*, visceral lamella of the middle germ-layer; u, blastopore ; dz, yolk-cells ; dp, yolk-plug ; dh, intestinal cavity. 116 EMBRYOLOGY. is produced on both sides of the chorda the gastral mesoderm, and that which arises from the blastopore the peristomal mesoderm (RABL). ;•;; ok Ik »»*' Fij. 79.— Three cross sections from a series through an egg on which the medullary ridges begin to appear. The sections illustrate the development of the chorda out of the chordal fundament, and the constricting off of the two halves of the middle germ-layer. ak, Outer, ik, inner germ-layer ; mk\ parietal, w/fj, visceral lamella of the middle germ-layer ; mp, medullary plate; inft medullary folds; c/t, chorda; lh, body-cavity. The further development of the fundaments of mesoderm, chorda, and intestine, which subsequently become entirely separated from one another at the places where they now remain in connection , causes the agreement with the conditions found in Amphioxus to DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 119 appear in stronger relief. The process of separation is introduced by the curving of the chordal plate, and its conversion into the chordal groove (fig. 79 A ch). Inasmuch as it is continuous at its edges with the parietal lamella of the middle germ-layer (mkl), there arise in the roof of the ccelenteron the two small chordal folds, which enclose between them the chordal groove. Its free margins abut directly upon the folded edge, where the visceral lamella of the middle germ-layer (mk2) bends around into the entoderm (ik) to produce the ccelenteric fold. In the next following stage (fig. 79 B) the thickened medullary plate, consisting of long cylindrical cells, becomes distinctly marked off from the now still smaller cubical elements of the ectoderm. Meanwhile the middle germ-layer begins to detach itself from its previous connections in the vicinity of the place of evagination ; the parietal lamella becomes separated from the fundament of the chorda, the visceral lamella from the entoderm, and thereupon their detached edges become fused to each other. By means of this pro- cess the fundament of the body-sac, or of the middle germ-layer, becomes closed on all sides, and is separated from the other germ-layers. At the same time the entoderm (iJc) and the funda- ment of the chorda (ch) have come into contact along their free margins, so that the chorda appears like a thickening of the ento- derm, and for a time shares in bounding the intestinal cavity oil the dorsal side. This is changed by a second process of detachment. The fundament of the chorda, now converted into a solid rod, is gradually excluded from participation in lining the intestine (fig. 79 C), by the fact that the halves of the entoderm (ik), composed of large yolk-cells, grow toward each other underneath it, and fuse in a median raphe. The closure of the permanent intestine on the dorsal side, the con- stricting off of the two body -sacs from the inner germ-layer, and the origin of the chorda dorsalis are therefore in Amphibia, as in Amphi- oxus, processes which are most intimately related with one another. Here, too, constricting off of the parts mentioned begins at the head-end of the embryo, and advances slowly toward the posterior end, where there exists for a long time a zone of growth, by means of which the increase in the length of the body is effected. Soon after this, the moment arrives when in the embryos of Triton the body-cavity becomes visible. For after the detachment of the organs previously mentioned is completed, the two middle germ-layers at the head-end of the body, and on both sides of the chorda, separate from each 120 EMBRYOLOGY. Fig. 80 -Longitudinal [sagittal] section through an advanced em- bryo of Bombinator, after GOETTE. m, Mouth ; an, anue ; I, liver ; ne, neureuteric canal ; me, medullary tube ; ch, chorda ; pn, pineal gland. other, and thus cause to appear a right and a left body-cavity (enterocoel), which, according to my interpretation, were not pre- viously recognisable, simply on account of the intimate mutual contact of their walls. Meanwhile the medullary plate has become con- verted, by the process of folding an already described, into the neural tube (fig. 80 me), which lies beneath the epidermis. Since the neural tube subsequently encloses the blastopo^e, and is thereby in communication with the intestinal tube (as the preceding longitudinal section of an advanced embryo of Bombinator most distinctly shows), it follows that there is also in the Amphibia a structure (fig. 80 ne) corresponding to the neurenteric canal of Amphioxus (compare fig. 68 en). More fundamental differences in the development of the middle germ-layer are met with in the eggs of Fishes, Rep- tiles, and Birds, which are more abundantly provided with nutritive yolk and undergo partial cleav- age, and also Fig. 81 A and B.- Two germ-discs of Hens' eggs in the first hours of incubation, after ROLLER. (//, Area, opaca; hj\ area pellucida ; «, crescent; «fr, crescent-knob; Es, embryonic shield ; pr, primitive groove. in the eggs of Mammals. However, the variations appear in these cases to be of a subsidiary nature, whereas in the chief points the unity of the developmental processes for all vertebrated animal^ 1 .as been the more firmly established the more accurately the individual j- s have been investigated by means of improved methods. In the presentation of these difficult conditions, we shall doi;ic<>> with its lips a protruding portion of the otherwi>«- enclosed yolk-mass, — the yolk-plug,— becomes narrower, and i> continued forward into a longitudinal groove. Finally it appears (tig. 101 D ?') us a deep groove- situated at the end of the DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 137 medullary furrow, with its small circular opening filled up with a yolk-plug. In addition there are three important considerations which may be urged in support of the interpretation of the primitive groove as blastopore. First, the primitive streak, even when an open canal is wanting, is the only place in the whole germ-disc where a connection between Fig. 101. A and B.— A portion of a youn»er and of an older embryonic fundament of Emys Europaea, with the prostoma or blastopors (it), after KUPFFER. v.1, Lip of the blastopore. C and D.— Two eggs of Triton taeniatus seen from the blastopore, one 30 hours, the other 53 hours after artificial fertilisation. u, Blastopore ; h, elevation between blastopore and dorsal groove ; /, semicircular furrow, which encloses the blastoporic area ; dp, yolk-plug. all the germ-layers is constantly present, as at the Amphibian blastopore. Secondly, the chief organs of the body, such as the chorda, the neural tube, and the primitive segments, are developed in front of the primitive streak in the case of the higher Vertebrates, just as they arise in front of the blastopore in Amphioxus and the Amphibia. Both blastopore and primitive streak occupy the posterior end of the body. The so-called cephalic process of the primitive streak is nothing else than the first rudiment of the chorda. Thirdly, one may still recognise in the openings — canales neu- renterici — which have been pointed out in the primitive streak at an earlier or later stage in its development, in the case of Birds, Reptiles, and Mammals, an indication that an open communication has 138 EMBRYOLOGY. existed here from the beginning between the inner and the outer germ-layers ; further, that this communication has disappeared through the fusion of the blastoporic lips, but that it can be in part reestablished in consequence of more favorable processes of growth. At the same time the neurenteric canal, in cases where it reappears in the primitive streak, effects a very characteristic union between the posterior ends of the neural and intestinal tubes, in exactly the same manner in which the blastopore of Amphioxus, the Amphibia, and the Selachii does (compare fig. 80 with fig. 88 n.e). In the interpretation of the primitive groove as blastopore I am compelled to oppose a somewhat different view. Certain investi- gators (BALFOUR, RAUBER, and others) recognise in the primitive groove and the crescentic groove of meroblastic eggs only a small part of the blastopore ; they interpret as the major part of it the region which is encircled by the whole rim of the germ-disc and is occupied by the yolk-mass, and to which they give the name yolk- blastopore.* According to their conception, as also according to the original assumption of HAECKEL, the two-layered germ-disc is a flattened-out gastrula, — its blastoporic rim lying upon the yolk- sphere, — which gradually grows around the yolk, and finally takes the latter wholly inside itself, just as if it were a ball of food. The primitive groove is a small detached part of the blastopore, which is connected with the development of the middle germ-layer. The two parts become completely separated from each other, and are closed at different times, each for itself, the yolk-blastopore often late, at the pole of the yolk-sac which is opposite to the embryo. Such an assumption of a double blastopore appears to me to 1 >< • untenable. / propose that only that place of the germ be designated as blastopore at which, as in the gastrulation of Amphioxus and the Amphibia, there actually occurs an invagination of cells, by means of which the cleavage-cavity is obliterated. Such a process takes place in the Selachii only at the crescentic hinder part of the margin of the germ-disc, in the Reptiles and Birds at the small place designated as crescentic groove. It is also from this place alone that subse- quently the development of the middle germ-layer proceeds. The anterior margin of the germ-disc in Selachians, and, after tl»' conversion of the crescentic groove into the primitive groove, the ivhole * RAUBER has suggested for the various regions which he assumes for the blastopore the designations prostoma sulcatum lonyitudlnale (primitive groove), prostoma sulcatum falciforme (crescentic groove), and prostoma marg'uiale (yolk-blastopore). DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 139 margin of the germ-disc in Birds and Reptiles, have an entirely dif- ferent signification. This margin exhibits a very different relationship from that of the primitive streak or blastopore ; it is a peculiarity of meroblastic eggs, which is most intimately associated with the origin of partial cleavage. It indicates the place at which the segmented portion of the germ meets the unsegmented portion — the place at which there lie in the yolk free nuclei, by means of which a supple- mentary cleavage is kept up until late stages in the process of development, until, in fact, the time when the two primary germ- layers have been formed by means of the invagination which occurs at the blastopore. At the expense of the cell-material, which is constantly being augmented by supplementary cleavage, the germ- layers increase in extent at their place of transition into the yolk, and thus gradually grow over the unsegmented part. Whereas at the blastopore an invagination of cells already present takes place, there ensues at the margin of the germ- disc a formation of new cells, and thereby an increase of the marginal part and an overgrowth of the yolk. I therefore propose for it the name circumcrescence-margin of the yolk-sphere. There can be no such thing as a separate opening •or a yolk-blastopore, because the yolk is an organic part of the germ, and is in continuity with the segmented part of it by means of the layer which contains the yolk- nuclei. If we would insti- tute a comparison be- tween animals with meroblastic eggs and the Amphibia at a stage when gastrulation is not yet completed, then the blastopore of the Amphibia, which is indicated by the letter u in the accompanying section through the gastrula of a Triton (fig. 102), corresponds to the prostoma of Rep- tiles, and to the crescentic and primitive grooves of Birds ; the still •exposed mass of yolk-cells corresponds to the yolk-material which is __ ok fh - Fig. 102.~Longitudinal section through a gastrula of Triton. •cular aiid Lymph Systems. Annals and Mag. Nat. Hist. Vol. XI. 1873. DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 161 Lankester, E. Ray. Notes on the Embryology and Classification of the Animal Kingdom : comprising a Revision of Speculations Relative to the Origin and Significance of the Germ-layers. Quart, Jour. Micr Sci Vol. XVII. 1877. Xeuckart, B. Ueber die Morphologie und Verwandtschaftsverhaltnisse der wirbellosen Thiere. Braunschweig. 1848. Kowalevsky. Entwicklungsgeschichte der Sagitta. Mem. de 1'Acad. imper. des Sci. St. Petersbourg. Vile ser. T. XVI. 1871. Kowalevsky. Untersuchungen liber die Entwicklung der Brachiopoden. Nachrichten d. kaiserl. Gesellsc d. Freunde d. Naturerkenntniss, etc. Bd. XIV. Mosltau 1875. (Russia Xowalevsky. Weitere Studien uber die Entwicklungsgeschichte des Amphioxus lanceolatus, nebst einem Beitrage zur Homologie des Nerven- systems der Wiirmer und Wirbelthiere. Archiv f. mikr. Anat. Bd. XIII. 1877, p. 181. IffetschnikofF. Studien liber die Entwicklung der Echinodermen u. Ne- mertinen. Mem. de 1'Acad. imper. des Sci. St. Petersbourg. Vile ser. T. XIV. Nr. 8. 1869. Metschnikoff. Untersuchungen uber die Metamorphose einiger Seethiere. Zeitschr. f. wiss. Zoologie. Bd. XX. 1870. Metschnikoff. Studien iiber die Entwicklung der Medusen und Siphono- phoren. Zeitschr. f. wiss. Zoologie. Bd. XXIV. 1874. Wolff, Casp. Fr. Ueber die Bildung des Darmcanals iaa bebriiteten Hiihnchen. Uebersetzt von Fr. Meckel. Halle 1812. Haeckel. Ziele und Wege der heutigen Entwicklungsgeschichte. Jena 1875. His. Untersuchungen iiber die erste Anlage des Wirbelthierleibes. Leipzig 1868. His. Unsere Korperf orm und das physiol. Problem ihrer Entstehung. Leipzig 1871. Xotze. Allgemeine Physiologic. Leipzig 1851. Oken. Kritik der Dissertation von Pander. Isis 1817. Bd. II., p. 1529. Pander. Entwicklungsgeschichte des Kiichels. Oken's Isis. Jahrgang 1818. Bd. I., pp. 512-24. Rauber. Formbildung und Formstorung in der Entwicklung von Wirbel- thieren. Capitel IV. (Formbildung und Cellularmechanik.) Morphol. Jahrb. Bd. VI. 1880. Roux. Die Entwicklungsmechanik der Organismen. Eine anatomische Wissenschaft der Zukunft. 1890. Wagner, Rudolph. Lehrbuch d. Physiologic. 3. Auflage. Leipzig 1845. CHAPTER VIII. DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 'THE more one pursues the development of Vertebrates into later stages, the more numerous become the changes which simultaneously appear in the different regions of the embryonic body. We cannot here undertake to describe step by step the processes which are -simultaneously accomplished, for by that method the presentation 11 162 EMBRYOLOGY. would become fragmentary and the comprehension of the separate processes would be made more difficult ; but it is necessary, in the interest of a didactic method, to select from all the manifold pheno- mena a single process of the development, and to follow it up until it has come to a preliminary termination. After the formation of the middle germ-layer two important processes take place in the embryonic fundament. One process leads to a division of the middle germ-layers into the two lateral Fig. 103. Amphioxus embryo with five pairs of primitive segments in optical section, afte* HATSCHEK. A Seen from the side. B Seen from the dorsum. In figure B are indicated the openings of the cavities of the primitive segments into the intestinal cavity, which can be seen by deeper focussing. V, Anterior, H, posterior end ; ak, outer, ik, inner, mk, middle germ-layer ; dh, intestinal cavity ; n, neural tube ; en, neureuteric canal; us\ first primitive segment; ush, cavity of primitive segment;. ud, coelenteron. plates and into two series of cuboidal bodies, which are situated at the right and the left of the chorda, and which, under an erroneous interpretation, were formerly called protovertetoce, but for which one should now substitute exclusively the more accurate name primitive segments [mesoblastic somites]. The other process, which occurs at about the same time, at least in the case of the higher Vertebrates, leads to the origin of those cells from which the sustentative sub- stances and the blood of Vertebrates are derived. DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 163 ak ch- ill this chapter we shall take into consideration the formation of the primitive segments first in the eggs of Amphioxus and the Amphibians, and then in those of Fishes, Birds, and Mammals. In Amphioxus the formation of the primitive segments is more nearly simultaneous with the development of the middle germ- layer than in the remaining Vertebrates. As soon as the two- coelomic sacs begin to grow out from the coelenteron at the front end of the embryo, there begins a division of them into two rows of small sacs lying one behind the other (tig. 103 A, B, us), and this division proceeds from in front backwards. Here again we have to do with a process of folding, which repeats itself many times in the same manner. The wall of the groove-like coelomic evagination, composed of cylindrical cells, becomes, at a little distance from its head-end, folded transversely to the longitudinal axis of the embryo ; this fold grows from above and from the side downwards into the body-cavity; in the same manner a second trans- verse fold is soon formed on either side of the body at a little distance behind the first; behind the second a third, a fourth, and so on, at the same rate as that at which the em- bryonal body elongates and the fun- dament of the middle germ -layer increases by the progress of the evagination toward the blasto- pore. In the embryo represented in fig. 103 five sacs may be counted on either side of the body. The evagination is taking place at the region marked mk ; it advances still farther toward the blastopore and gives rise to a considerable series of primitive segments, the number of which in a larva only twenty-four hours old has already increased to about seventeen pairs. The primitive segments exhibit at first an opening, by means of which their cavities (usK) are in communication with the intestinal cavity. But these openings soon begin to be closed in succession, by their margins growing toward each other and then coalescing; this takes place in the same sequence as that in which the detachment of the parts takes place, from before Fig. 104.— Cross section through the middle of the body of an Amphioxus embryo with 11 primitive segments, after HATSCHEK. ak, Outer, ik, inner germ-layer ; mk1, parietal, mk", visceral lamella of the middle germ-layer ; us, primi- tive segment ; n, neural tube ; chr chorda ; Ih, body-cavity ; dh, intes- tinal cavity. 164 EMBRYOLOGY. backwards. At the same time the primitive segments (fig. 104} gradually spread out both dorsally and ventrally, while their cells increase in number and become altered in form. They grow upward more and more at the side of the neural tube, which has meanwhile detached itself completely from its matrix, the outer germ-layer. Ulh A Fig. 105— Two cross sections through a Triton embryo. A, Cross section through the region of the trunk in which the neural tube is not yet closed an the primitive segments begin to be constricted off from the lateral plates. B, Cross section through the region of the trunk in which the neural tube is closed and the primitive segments have been formed. TO/, Medullary folds ; mp, medullary plate ; «, neural tube ; ch, chorda ; ak, outer, ik, inner germ-layer ; ink1, parietal, m/fc3, visceral middle layer ; dh, intestinal cavity ; Ih, body-cavity uth, cavity of primitive segment ; dz, yolk-cells. Toward the ventral side they insert themselves between the secondary intestine and the outer germ-layer. Finally, it might be further mentioned here that at a still later stage, as is to be seen on the right side of fig. 104, the dorsal portions of the primitive segment are constricted off from the ventral. The former lose their lumina and furnish the transversely striped DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 165 musculature of the body, but from the cavities of the latter originates the real unsegmented body-cavity, since the partitions which at first separate them become thinner, break through, and finally disappear. Similar processes take place in a somewhat modified manner in the case of the remaining Vertebrates. , In the Tritons the middle germ-layer (fig. 105 A) becomes thickened on both sides of the chorda (ch) and of the fundament of the central nervous system (n), which is not yet closed into a tube, and at the same time there appears a cavity (usk) in its thickened part, caused by the separation of the visceral and parietal lamellae. The thickening is not produced by an increase in the number of the layers of cells, but simply by the fact that the cells increase in height and grow out into long cylinders, which are arranged around the cavity like an epithelium. We distinguish these thickened parts of the middle germ-layer, which lie on either side of the chorda and the nervous system, as the primitive-segment plates, from the lateral parts, or the lateral plates. In the territory of the latter the cells are lower, and ordinarily there is no distinctly marked cavity between visceral and parietal layer. Whereas in Amphioxus the process of forming somites extends itself over the whole of the middle germ-layer, in the case of the Amphibians, and likewise all the re- maining Vertebrates, it affects only the part which is next to the chorda and the neural tube, leaving the lateral plates, on the contrary, untouched. The segmentation begins at the head- end, and proceeds slowly toward the blastopore ; it is accomplished by fold- ing and constricting off. The epithelial lamella next to the neural tube and the chorda, being composed of cylin- drical cells, is raised up into small transverse folds, which, separated from each other by intervals of uniform size, grow into the cavity of the primitive- segment plate, and give rise to small sacs lying one behind the other (fig. 106). Soon afterwards each little sac is constricted off from the lateral plates (fig. 105 A and B). Consequently one now meets, both in ak Fig. 106. — Frontal section through the dorsum of an embryo Triton with fully developed primitive seg- ments. One sees on both sides of the chorda (ch) the primitive segments (ws) with their cavities (usK). 166 EMBRYOLOGY. transverse and frontal sections at the right and left of chorda and neural tube, cubical sacs the walls of which are formed of cylindrical cells ; these sacs are everywhere surrounded by a fissure- like space, and they enclose a small cavity (the primitive-segment cavity), which is a derivative of the body-cavity. From the front layer of the fold is produced the posterior wall of the newly formed segment, from its posterior layer the front wall of the remnant of the primitive-segment plate, or of the sac which is next to be con- stricted off, Of the Vertebrates which are developed out of meroblastic eggs, the Selachians appear to exhibit most clearly the original mode of the formation of primitive segments. A distinct body-cavity is formed on either side of the trunk by the separation of the parietal and visceral lamellse of the middle germ -layer (fig. 110). The dorsal portion of the cavity, which flanks the neural tube, acquires thickened walls (nip), and corresponds to the part previously designated as the primitive-segment plate, which at the same time with the appear- ance of the body-cavity begins to be divided into primitive segments. In the anterior part of the body a series of transverse lines of separation become visible (fig. 195 mpl), the number of which is continually increased toward the hind end of the body. For a long time the cavities of the primitive segments, which are sepa- rated from one another by these transverse furrows, remain in communication ventrally with the common body-cavity by means of narrow openings. One may therefore describe this state of affairs by saying that the body-cavity is provided toward the back of the embryo with a series of small sac-like evaginations, which lie close together one after the other. Afterwards the primitive seg- ments are entirely constricted off from the body-cavity, and then their thickened walls come into close contact, and thus cause the disappearance of the cavities of the segments (fig. Ill mp). Whereas in the Selachians it is still evident that the formation of the primitive segments depends upon folding and Constricting off, the process is obscured even to obliteration in the case of Reptiles, Birds, and Mammals; this is referable simply to the fact that the two lamellse of the middle germ-layer remain for a long time firmly pressed together, only subsequently beginning to separate, and that they are composed of several layers of small cells. The process of hiding and constricting off appears here as a splitting up of a solid cell-plate into small cubical blocks. The part of the middle germ-layer that is next to the chorda and DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 167 neural tube appears in a cross section of a Chick embryo (fig. 107) as a compact mass (Pv) consisting of many superposed small cells which, as far as it is not divided up into separate blocks, is designated as primitive-segment plate or protovertebral plate. In fig. 107 it is still connected at the side by means of a thin isthmus of cells with the lateral plates, in whose territory the middle germ- layers are thinner and sepa- rated from each other by a fissure. In observing the blasto- germ from the surface the region of the primitive-seg- ment plates, as is to be seen in the posterior part of a nine- days-old Rabbit embryo (fig. 108), appears darker than the region of the lateral plate; so that the two are dis- tinguished from each other ; •one is stem-zone (stz), the other parietal zone (pz). The development of the primitive segments is ob- servable in the Chick at the beginning of the second day of incubation, in the Rabbit at about the eighth day. Clear transverse streaks ap- pear in the stem-zone at some distance from the primi- tive streak, about in the middle of the embryonic fundament, both on the right and the left of the chorda and neural tube (fig. 108). They correspond to transverse fissures, by means of which the primitive-segment plates are divided into the small •and solid cubical primitive segments (uw). In the nine-days-old Rabbit embryo represented in fig. 108 these plates are resolved in II II II -i* 3 1 !§ •g ft |3 *"!; tj If § g -g 60 O U °t ^°" *feO »S ***.•* 168 EMBRYOLOGY. front into eight pairs of primitive segments (uw), whereas in the hind end of the embryonic area they still have the form of a con- tinuous mass of cells, the , .1 stem-zone (stz), which in sur- face-views appears darker than its surroundings. In a somewhat more ad- vanced stage the primitive i\>- j a • | segment, which probably se- cretes at the same time fluid, develops in its interior, as in the case of the Amphibia and Selachii, a cavity, around which the cells group them- selves in a radial manner. This cavity, too, is at first in communication laterally with the fissure of the body-cavityr until the primitive segment has been fully constricted off. In Vertebrates, besides the trunk-region, a part of the head-region of the embryo is also affected by this process of segmentation which we have been considering. We must therefore speak in the one case of head-segments, and in the other of trunk- segments. Up to the present time the number and condi- tion of the head-segments have been made out (by BALFOUR, MILNES MARSHALL, and VAN WLJHE) most accurately for the Selachians. In this in- stance there are nine pairs of hollow head-segments. In the higher Vertebrates such segments although fewer in number, have also been described ; however, the less sharply differentiated structures of the latter demand still further investigation. ap Fig. 108.— Rabbit embryo of the ninth day, seen from the dorsal side, after KOLLIKER. Magnified 21 diameters. The stem-zone (stz) and the parietal zone (pz) are to be distinguished. In the former 8 pairs of primitive segments have been established at the side of the chorda and neural tube. ap, Area pellucida ; r/, medullary groove ; vh, fore brain ; ab, eye-vesicle ; mh, mid brain ; hh, hind brain ; uw, primitive segment ; stz, stem-zone ; pz, parietal zone ; /*, heart ; ph, pericardial part of the body-cavity ; rd, margin of the entrance to the head-gut (vordere Darmpforte), seen through the overlying structures ; of, amniotic fold ; vo, vena omphalomesenterica. DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 169- But, in any event, the accurate study of the earliest embryonic segmentation of the body into a large number of metameres yields this result of the highest importance for the general morphology of the Vertebrate body, that the head not less than the trunk represents a segmented portion of the body and has in no wise been produced from a single primitive segment. SUMMARY. 1. In Vertebrates the middle germ-layers immediately after their origin are differentiated into several fundaments by processes of folding and constricting off. 2. The process of differentiation in the middle germ-layer exhibits- two modifications. (a) In Amphioxus the middle germ-layers are, at the time of their first appearance, completely separated into primitive- segments lying one behind the other. It is only later that each primitive segment is divided into a dorsal portion (the real primitive segment) and a ventral portion. The dorsal portion, or primitive segment proper, furnishes the- transversely striped musculature of the trunk. The ventral segments form the body-cavity, which is at first segmented, but afterwards with the disappearance of the partitions becomes a single cavity. (6) In all other Vertebrates the fundaments of the middle germ-layers are divided first into a dorsal and a ventral region — into the primitive-segment plates and the lateral plates. The lateral plate remains unsegmented. The body-cavity, which becomes visible in it by separation of the parietal and the visceral lamellae of the middle layer, is from the beginning on each side of the body a single space. The primitive-segment plate alone is divided into successive primitive segments. 3. The segmentation of the middle germ-layers also extends over the future head-region of the embryo. One therefore distinguishes — (a) Head-segments, the number of which amounts to nine ; (6) Trunk-segments, the number of which is constantly being increased during the development of the posterior trunk- region. 170 EMBRYOLOGY. CHAPTER IX. DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. ( THE MESENCHYME- THE OR Y. ) WITH the question of the origin of connective or mechanically sus- tentative substance and blood we enter a very difficult field, the cultivation of which has now been taken in hand successfully by many persons. Here also we shall acquaint ourselves with a simple case from the development of Invertebrates, before we begin with the conditions in Vertebrates, which are more difficult to comprehend. In Ccelenterates and Echinoderms there is developed between the germ-layers, which are composed of epithelial cells, a sustentative tissue. It consists of a homogeneous jelly, in which are scattered a Tig. 109. -Two stages of development of Holothuria tubulosa, in optical section (after SELEXKA), from BALFOUB. A, Blastosphere-stage at the end of cleavage. B, Gastrula-stage. mr, Micropyle ; fl, chorion ; «.c, segmentation-cavity, in which gelatinous substance is early secreted as a gelatinous core ; bl, blastoderm ; ep, outer, hy, inner germ-layer ; ms, amoeboid cells arising from the inner germ-layer ; a.e, ccelenteron (archenteron). few isolated spheroidal or stellate cells, which are capable of changing position by virtue of their amoeboid motion. It is usually developed very early ; in the Echinoderms, for example, as early as the blastula- stage (fig. 109). Into the cavity of the blastula (A ) a homogeneous soft substance, the jelly-core (s.c), is secreted by the epithelial cells. Into this jelly there migrate from the epithelium, and indeed from the particular region which at the time of gastrulation is infolded (fig. 109 B] as the DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 171 inner germ-layer (hy\ numerous cells (ms), which loose their epi- thelial character, and send out processes in the manner of lymph- corpuscles. They soon distribute themselves as migratory cells everywhere in the jelly. In the gastrula -stage and subsequently, the cell-containing jelly "between the outer and the inner germ-layers represents a third sheet, which is distinguished from the latter histologically, and, according to the definition previously given, cannot be designated as a middle germ-layer ; for by that definition we understand the term to be limited to a sheet of embryonic cells, having an epithelial arrange- ment and bounding a surface. The jelly-like sheet is a product of the germ-layers, which may be distinguished from them by the name mesenchyme or intermediate layer (Zwischenblatt). Once formed, the mesenchyme continues to grow as an independent tissue, in that the cells which at first migrated into the jelly at a definite stage of development, to which one may give the name mesenchyme-germ, continue to increase uninterruptedly by means of cell-division. In its growth it penetrates into all the interstices which arise when the germ-layers, as happens in many Coelenterates, produce the most complicated structures by the formation of folds and •evaginations ; it furnishes everywhere a support for the epithelial layers which repose upon it. At the same time some of the mesen- chyme-cells can alter their original histological character as simple trophic or nutritive cells of the intermediate substance. Thus here and there they differentiate contractile substance at their surface, .and become, as is to be seen in Ctenophores arid Echinoderms, smooth muscle-cells, the ends terminating either in one fine point, or dividing themselves into several processes, as is more frequently the •case with Invertebrates. In Vertebrates also, after the two primary germ-layers have arisen, •a process similar to that which we have just considered appears to lead to the formation of connective tissue and blood, two tissues which correspond morphologically and physiologically to the mesen- chyme of Invertebrates. In the first two editions of the " Lehrbuch " I set forth that the whole mesenchyme-question in the Vertebrates was still in a nascent •condition, that the account therefore presented nothing final, but bore in many respects the character of the provisional. Since that time an essential advance has been made in this field. Thanks to the investigations of HATSCHEK and RABL, of RUCKERT, ZIEGLER, and VAN WIJHE, we have acquired more accurate explanations concerning VAN VYIJ 172 EMBRYOLOGY. the origin of the connective substances ; the question of the origin* of the vascular endothelium and of the blood, on the contrary, is one that is less cleared up. This determines me to treat the two- questions separately in the following account. A. The Origin of the Connective Tissues. Selachian embryos appear to be the most suitable objects on which to trace the origin of the connective substances. Here the middle germ-layer serves as the matrix for the mesenchymatic tissue. At the time when the primitive segment is still connected below with the lateral plates, and when the body-cavity is visible in the latter,, there appears a cell-growth at the lower border of each primitive segment on the side which is directed toward the chorda. It is ordi- narily designated as sclerotome. It contains at first a small evagi- nation of the body-cavity (fig. 258 A sk). At the restricted place designated, which is marked off from its surroundings, and which recurs on each primitive segment, cells in large numbers (fig. 110 sk) individually detach themselves from the epithelial layer, remove by active migration from their place of origin, like the mesen- chymatic cells of Invertebrates, and distribute themselves in the space which is limited on the one side by the inner wall (mp) of the primitive segment, and on the other by the chorda (ch) and the neural tube (nr). At the time of their appearance the amosboid cells are separated by only a small amount of inter-cellular substance : they increase rapidly in number, and thereby soon crowd chorda, neural tube, and primitive segment farther apart (fig. 111). The segment-ill arrange- ment which the growths exhibit at their first appearance (fig. 195 Vr) very early ceases to exist, since by their extension they become fused together into a continuous sheet. The mesenchyme, which thus grows forth out of the middle germ- layer on both sides of the chorda, furnishes the foundation for the whole axial skeleton ; it produces the skeletogenous tissue by the growing toward each other and the fusion of the masses which are formed on the right and left sides. As fig. Ill shows, the mesen- chyme (sk) grows around the chorda (ch) both dorsally and ventrally, and envelops it with a connective-tissue sheath, which is continually becoming thicker. In the same manner it encloses the neural tube (nr) and forms the membrana reuniens superior of the older embryo- logists, the foundation out of which subsequently the connective- DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 173 tissue envelopes of the neural tube and the vertebral arches with their ligaments are differentiated. Conditions similar to those of Selachians are also to be observed, Fig. 110. Fig. 111. Tigs. 110 and 111.— Diagrams of cross sections through younger and older Selachian embryos to illustrate the development of the principal products of the middle germ-layer. After VAN WIJHE, with some changes. Tig. 110.— Cross section through the region of the pronephros of an embryo, in which the myotomes (mp) are in process of being constricted off. Tig. 111.— Cross section through a somewhat older embryo, in which the myotomes have just been detached. tir, Neural tube ; ch, chorda ; ao, aorta ; sch, subnotochordal rod ; mp, muscle-plate of the primitive segment ; w, zone of growth, at which the muscle-plate bends over into the cutis- plate (cp) ; vb, portion connecting the primitive segment with the [walls of the] body-cavity, out of which are developed, among other things, the mesonephric tubules uk (fig. Ill) ; sk, skeletogenous tissue, which arises as an outgrowth from the median wall of the con- necting portion (vb) ; vn, pronephros ; mkl, parietal, mk*, visceral middle layer, from the walls of which mesenchyme is developed ; Ih, body-cavity ; ik, entoderm ; h, cavity of the primitive segment ; uk, mesonephric tubule, arisen from the connecting portion vb of the diagram 110 ; uk1, place where the mesonephric tubule has detached itself from the primitive segment ; v.g, mesonephric duct, with which the mesonephric tubule has united on the left side ; tr, union of the mesonephric tubule with the body-cavity (nephridial funnel) ; wes1, mes", mesenchyme. which has arisen from the parietal and visceral lamellae of the middle layer respectively. although less distinctly, in Reptiles, Birds, and Mammals; they have been described by REMAK, KOLLIKER, and others, and have been brought into connection with the formation of the vertebral column. The primitive segments, which are at first solid, soon acquire a small cavity (fig. 116), around which the cells are arranged into a 174 EMBRYOLOGY. continuous epithelium. Then a part of the wall of the primitive- segment lying at its lower and median angle begins to grow with extraordinary rapidity, and to furnish a mass of embryonic connective tissue, which spreads itself around the chorda and neural tube in the manner previously described. The dorsal and lateral parts of the primitive segment (fig. 116 ms), which subsequently loses its cavity, are not involved in this -growth ; out of them arise principally the fundaments of the trunk-musculature. This part is consequently now distinguished as muscle-plate (ms). Mesenchyme arises from three other places of the middle germ- layer besides the primitive segments — from the visceral lamella, from the parietal lamella, and finally from that wall of the primitive segment which is turned toward the epidermis and has been given by RABL the name cutis-plate. Here also the conditions are best followed in Selachii. Individual cells migrate out from the visceral lamella (Darm- faserblatt), which in early stages is composed partly of cubical,, partly of cylindrical cells (fig. 110 mk2), and distribute themselves upon the surface of tfye entodermic layer ; they are found at places where no trace of a vessel is observable. They furnish the mesenchyma of the intestinal wall, which is ever becoming more abundant, and which is subsequently converted partly into connective tissue, partly into the smooth muscle-cells of the tunica muscularis (fig. Ill mes*). A similar process is repeated in the parietal lamella (Haut- faserblatt). Emigrating cells produce between the epithelium of the body-cavity and that of the epidermis an intermediate layer of mesenchyme-cells (fig. 110 ink1, fig. Ill mes1). An important region for the production of connective tissue is, finally, the cutis-plate, i.e., the epithelial layer of the original primi- tive segment which is in contact with the epidermis (fig. 110 cp). The process occurs here later than at the other places mention «M I. and begins with an active cell-growth, which gradually leads to a complete disintegration of the epithelial lamella. " The disintegra- tion," as RABL remarks, " proceeds in such a manner that the cells, which hitherto exhibited an epithelial character, separate them- selves from one another, and thereby lose their epithelial character." It is probably from this part of the mesenchyme that the corium is derived. That the mesenchyme-cells scattered between the epithelial lam- ellae are capable of executing extensive migrations, after the fashion DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 175' of migratory cells, is perhaps best shown in the investigation of transparent embryos of Bony Fishes. " One sees distinctly," thus WENKEBACH describes it, "how the cells by means of amoeboid motions, and of sometimes extraordinarily long protoplasmic pro- cesses, move themselves about independently in the body of the em- bryo and upon the yolk, which is not yet clothed with hypoblast, and creep toward definite places, as if they acted voluntarily and consciously." By virtue of this peculiarity, the mesenchyme-cells actively penetrate into all larger and smaller fissures which exist between the germ-layers and the fundaments of organs which have arisen from them. Everywhere they form a filling and connecting mass between these structures, which afterwards acquires a still greater importance as the bearer of blood- and lymph-courses as well as nerves. In comparison with the earlier editions of the " Lebrbuch," I have here given an essentially different presentation of the development of the mesen- chyme. Formerly, supported by the investigations of His, WALDEYER, KOLL- MANN, and others on meroblastic eggs, I thought it necessary to refer the chief source of the mesenchyme to a limited territory of the germ, to the area opaca, and made the cell-material arise by delamination from the entodermic layer, especially from the yolk- wall. But now I assume a manifold origin from various regions of the middle germ-layer. Thus I come back again to an in- terpretation which I had already propounded as probable in " Die Ccelomtheorie " (p. 80) and "Die Entwickelung des mittleren Keimblattes " (p. 122),— to the interpretation, namely, that mesenchyme-germs in Vertebrates are perhaps formed by an emigration of cells at several distinct places at the same time. Whether this or that be the real mode, the essence of the mesenchyma-theory is not thereby affected, for the essential part of that theory consists in this, that it establishes in the earliest development of tissue a contrast between the epithelial germ-layers and a packing tissue, produced by a dissolution of the epithelial continuity, which spreads itself out between the germ-layers,, and soon appears as an independent structure. Indeed, with this theory as a basis, it would not be surprising if the pro- duction of mesenchymatic tissue should not be limited simply to the middle germ- layer, and if the entoderm by the contribution of cell-material should participate in its formation. B. The Origin of the Vascular Endothelia and the Blood. The question of the origin of the tissues represented in the above heading is one of the most obscure in the realm of comparative embryology. The very investigators who have endeavored most recently and with the most reliable methods to elucidate this matter do not hesitate to emphasise the uncertainty in the interpretation of the conditions presented to them. Even the lowest Vertebrate, which is distinguished by the greater simplicity of its structure, and. 176 EMBRYOLOGY. by the greater ease with which all its processes of development are •understood, has failed us in this question. For HATSCHEK, who knows the development of Amphioxus better than any one else, de- signates the blood-vessels as the only system of organs concerning which he was unable to arrive at a clear understanding. Consequently in the field now to be examined there are many views and observations which in part stand in the most direct Antagonism to each other. To give a comprehensive review of them is not possible without the greatest fulness, which would be contrary to the plan of this Text-book; I therefore limit myself, first, to giving a survey of the various possibilities by which the origin of the vessels and the blood might take place, and, secondly, to present- ing a series of observations which have been made on Selachians, Birds, and Mammals ; still it is always to be kept in mind that much remains doubtful here, and that coming years may bring about many a change in our interpretations. According to one view, the vascular cavities are developed out of fissure-like spaces between the germ-layers which remain unoccupied at the time the fundament of the mesenchyme is produced. These cavities acquire a boundary in this way : the neighboring mesenchyme-cells begin to penetrate into them, and then unite into a vascular endo- thelium. " The system of blood-vessels and that of lymphatic vessels," observes ZIEGLER, " are produced in their first fundaments from remnants of the primary body-cavity (the space between the primary germ-layers), which at the general distribution of the formative tissue (mesenchyma) remain behind as vessels, lacunae, or interstices, and are enclosed by that tissue and incorporated in it." The formed elements [corpuscles] arise at separate places in the blood-courses by the growth and detachment of mesenchymatic cells. According to another view, the vessels are constructed in this manner : cells in the mesenchymatic tissue arrange themselves in rows, and these cell-cords become hollowed out; thereby the more superficial cells furnish the endothelial wall, whereas the remaining cells become blood-corpuscles. The blood-vessels are therefore nothing else than cavities which have been secondarily produced in the mesenchymatic tissues by means of their own cells. Both views agree in this, that they cause the group of sustentative substances to be brought into genetic connection with the blood, and the latter to figure as a product of the metamorphosis of the mesenchyma. Moreover, both views may present variations in the details, according as they ascribe to the mesenchyme a different origin and DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. »177 make it arise either out of the middle germ-layer alone, or out of the entoblast alone, or by the migration of cells out of both layers and their union into a single fundament. Still other variations result from the first fundament of the blood-course being some- times referred to a limited territory of the germ, sometimes to several places. Thus, for the meroblastic eggs of Birds, the area opaca is designated by some observers as the place where vessels and blood are first formed. From here they grow out as it were at first into the embryonic body proper. The opposite is reported of Bony Fishes, in which the first vessels, heart, aorta, caudal veins, and sub-intestinal veins, together with blood -corpuscles, arise earliest in the embryonic body itself, whereas they appear on the yolk only subsequently. Finally, for the Selachians a local origin of the vessels is maintained both for the area opaca and also for the embryonic body in the restricted sense. In opposition to the two views hitherto presented, a third view assumes a separate origin for the connective substances on the one hand, and for the vascular endothelium and the blood on the other. Whereas the former are produced by the emigration of cells from the middle germ-layer, the vascular endothelium is maintained to arise from cells of the entoblast. It is held that an endothelial sac is formed (perhaps by constriction) as an independent fundament, which by budding gives rise to the whole vascular system. After this brief survey of the various possibilities concerning the origin of the blood-course, I turn to a description of certain con- ditions, concerning the signification of which it must be admitted that the views are also often very divergent. The area opaca of the meroblastic eggs of Fishes, Reptiles, and Birds has always played an important role in the literature on the question of the origin of the blood. Notwithstanding the frequency with which it has been investigated, the researches concerning it cannot be regarded as concluded. It is from this standpoint that I beg the reader to judge what follows. In the case of the Chick, on which especially we shall base our account, the opaque area is composed of only the two primary germ- layers at the time when the middle germ-layer begins to be formed from the region of the blastopore by the production of folds. The outer germ-layer, as has already been described in Chapter V., has in general a simple structure, since it is composed of a single layer of small cubical cells. The inner germ-layer (fig. 56 ik and £g. 112), on the contrary, alters its condition the more we approach 12 •"&• • *•") 178 EMBRYOLOGY. the margin of the disc. In the area pellucida and in the immediately surrounding parts it appears as a single layer of greatly flattened cells, and is separated from the yolk-floor by a cavity filled with an albuminous fluid ; in the opaque area it reposes directly upon the yolk ; its cells here become higher, cubical, or polygonal, and finally it terminates with a greatly thickened marginal zone, the previously mentioned yolk- wall (dw). This is the important region of the germ with which we now have especially to deal. The yolk-wall consists in the Chick partly of embryonic cells, which are separable from one another, partly of yolk-material in which are enclosed numerous large and small nuclei enveloped in protoplasm (the me- rocytes), as at the final stages of the process of cleavage. Such free nuclei have dk dw Tig. 112.— Section through the margin of the germinal disc of a Hen's egg incubated for six hours, after DUVAL. ale, Outer germ-layer ; dz, yolk-cells ; dk, yolk-nuclei ; dw, yolk-wall. also been demonstrated with perfect certainty in the marginal terri- tory of the yolk during the course of the formation of the germ-layers in Selachians^ Teleosts, and Reptiles (KUPFFER, HOFFMANN, RUCKERT, STRAHL, SWAEN). The most accurate description of the yolk-nuclei has been given by RUCKERT for the eggs of Selachians (fig. 113). They are present in this case at the marginal portion of the germ-disc, embedded in the yolk in not inconsiderable numbers, and are remarkable for their size, sometimes reaching a diameter ten-fold as great as that of an ordinary nucleus (&1, &*). From the protoplasm enveloping the nucleus k* there proceeds a richly branched network of processes. In the interstices of the net are lodged yolk-elements (d) in great numbers, from the size of the ordinary yolk-plates down to the finest granules. The former are often in process of disintegration. One- may conclude from this, as well as from other phenomena, that a vigorous consumption of deutoplasm is taking place at the margin of the germ. This deutoplasm is taken up as nutritive material by the protoplasmic net surrounding the nucleus, and employed by means of intra-cellular digestion for its growth. Consequently one also sees the yolk-nuclei in active increase. DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 179 Toward the surface of the yolk small clusters of nuclei (fig. 113 k) arise out of the large deeper-lying yolk-nuclei. From these there are finally produced genuine cells of the germ (3), by the small nuclei surrounded by a layer of protoplasm detaching themselves from the yolk, as it were by an act of supplementary cleavage. " Since the merocytes thus on the one hand un- interruptedly take up nutritive ma- terial out of the yolk, and on the other continually surrender it in the form of cells to the germ-layers of the nascent embryo, they present an important link, between the latter and the yolk" (RUCKERT.) The views of investigators on the significance of the yolk-wall and of the merocytes enclosed in it are very divergent. Indeed there is unanimity only in this, that the yolk-wall contributes to the increase of the lower germ-layer by single cells becoming in- dependent and attaching themselves at the margin to the elements which already have an epithelial arrangement. On the other hand it appears less certain how far the yolk-wall is concerned in the formation of the blood. According to the observations of His, DISSE, RAUBER, KOLLMANN, RUCKERT, SWAEN, GENSCH, HOFFMANN, and others, it does share in this process during a limited period of development in the case of Selachians, Teleosts, Reptiles, and Birds. In the Selachians the anterior margin of the germ-disc is the first to be metamorphosed into a vascular zone. RUCKERT could find here numerous and unequivocal indications that the previously described peculiar cell-elements of the yolk (merocytes) provided with large nuclei contribute to the formation of blood-islands, in that they break up into clusters of small cells, detach themselves Fig. 113. — Yolk-nuclei (merocytes) from Pristiurus, lying underneath the germ-cavity B, after RCJCKERT. z, Embryonic cells ; k, superficial clear nuclei ; £l, deeper nuclei ; k*, marginal nuclei rich in chromatin, largely freed from th» surrounding yolk, in order to show the processes of the proto- plasmic mantle ; d, yolk-plates. 180 EMBRYOLOGY. from the yolk-containing part of the lower germ-layer, and become differentiated on the one hand into the migratory cells of the first blood-vessels, and on the other into the blood-corpuscles. RUCKERT further maintains that the material destined for the production of blood is supplemented by means of cells freshly cleft off from the yolk. SWAEN remarks with the same posit iveness, " Les premiers ilots sanguins se developpent aux depens des elements de Ihypoblaste. Ces derniers constituent a la fin de ce developpement les parois de cavites vasculaires closes et les cellules sanguines qui les remplissent." Likewise GENSCH makes the large cells in the yolk responsible for the formation of the blood in the case of the Bony Fishes. HOFF- MANN also finds in Reptiles that the blood and the endothelial wall of the vessels, as well as the spindle-shaped cells which lie between the vessels, are a product of the inner germ-layer, and that they appear at definite places of the germ-disc at a time when the middle germ-layer has not yet been formed in those regions. Finally, it is stated concerning the germ of the Chick that at the end of the first day of incubation the cells in the yolk-wall have become very numerous, through the multiplication of the nuclei enclosed in the latter, and that afterwards the abundance of the cells diminishes. For part of the cells which have been formed by the active proliferation now detach themselves from the yolk- wall, get into the space between the outer and inner germ-layers, and there produce a third independent layer, which is continually increasing in thickness, whereas the remaining part becomes modi- fied into an epithelium of large cylindrical cells containing yolk- granules. This middle layer is judged by several investigators to be an independent fundament of the germ, and has in this .sense been described by His as parablast, by DISSE and others as vascular layer, by RAUBER as desmohcemoblast, and by KOLLMANN as marginal germ or acroblast. All of these accounts need still more precise confirmation, since they have often been called in question, even up to most recent times. Thus KO'LLIKER has always defended the position that not only the connective substances, but also the vessels and the blood, are products of the middle germ-layer, and are generated by it in its peripheral regions. KASTSCHENKO, in his study of the Selachii, could not convince himself that the merocytes have special import- ance in the formation of blood and vessels, but was not, however, DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOI' 181 willing to deny it. So much the more positively do WENKEBACH and ZIEGLEE, on the strength of their investigations on Teleosts, express themselves against the mode of blood-formation given by GENSCH. According to ZIEGLER, the blood-corpuscles are developed in the blood-vessels of the embryonic body itself. The free nuclei of the yolk, the merocytes, on the contrary, it is maintained, do not share in the formation of embryonic tissues, but, in adaptation to- the function of resorbing the yolk, undergo peculiar modifications,, which " cause the frequently affirmed but never proved production of blood-corpuscles [by them] to appear improbable." Under this condition of affairs, I must regard the question of the source of the cell-layer in which, in the region of the opaque area, the formation of blood takes place as not yet ready for final judgment. So far as regards the further changes, by means of which the cell-layer under consideration is converted into connective substance and blood, on the whole I subscribe, in this difficult field of in- vestigation, to KOLLIKER'S representation. At the end of the first day of incubation, the masses of cells which lie between the inner and the outer germ-layers arrange themselves in cylindrical or irregularly limited cords, which join themselves to- gether into a close-meshed network ; they are the first fundaments both of the vessels and also of their contents, the blood. In the spaces of the net are to be found groups of indifferent cells, which afterwards become embryonic connective tissue, and which are the Substanzinseln (fig. 114) of authors. At the beginning of the second day of incubation, the solid funda- ments of the vessels become more distinct, in proportion as they become bounded superficially by a special wall, and acquire an internal cavity. The wall of the vessels is developed out of the most superficial cells of the cords, and is composed during the first days of incubation of a single layer of very much flattened polygonal elements, on account of which the first vessels of the embryo are often designated as endothelial tubes (fig. 114 and fig. 115 gw). The cavity of the vessel is probably formed by the penetration of fluid into the originally solid cord from its surroundings, thus forming the plasma of the blood, by which the cells are pressed apart and to the sides. The cells then constitute here and there thickenings of the wall, and project into the fluid-filled cavities as elevations of loosely united spherical elements (fig. 114, Blood-islands). Conse- 182 EMBRYOLOGY. fe " Blood-island Wall of blood- vessel quently the vessels which are just becoming permeable are very irregular, since narrow places and wider ones, often provided with evagina- tions, alternate (fig. 114) with one another, an d since the vessels are sometimes wholly excava- ted, fluid-filled, endothelial tubes, and sometimes re- main more or less impassable, owing to the variously formed cell ag- gregates which project from the wall. The aggrega- tions of cells themselves are simply the centres where the formed com- ponents of the blood are pro- duced. The small spherical nucleated cells, which still en- close dark yolk- granules, be- come at first homoge neons Blood-island Blood-vessel Wall of blood- vessel Substanzinseln Blood-vessel Fig. 114.— A portion of the vascular area of the germ-disc of an embryo Chick, in which 12 primitive segments are developed, after DISSE. One sees the more darkly shaded blood-coimes, in which lie the " blood- islands," the centres whence the blood-corpuscles arise- The clear spaces in the vascular network, the walls of which are formed of flat endothelial cells, are the " substance-islands " (Substanzinselu). by the dissolution of the latter, and then, owing to the formation of the coloring matter of the blood in them, they take on a slightly yellowish color, which gradually becomes more intense. DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 183 If one at this time examines a blastoderm which has been removed from the yolk, the zone in which the formation of blood takes place appears flecked with more or less intensely colored blood-red spots, some of which are roundish, others elongated, and others branched. The spots are known as the blood-points or blood-islands of the blasto- derm (fig. 114). From these formative areas the superficial cells now detach themselves and enter the blood-fluid as the isolated red blood-corpuscles. Here, as well as in the blood-islands, they multiply by means of cell-division, during which the nucleus is metamorphosed into the well-known spindle-figure. As REMAK first showed, divisions of blood-cells are to be observed in the Chick in great numbers up to the sixth day of incubation, whereas they later become more rare, and then wholly disappear. Also in the case of Mammals and of Man (FoL) the first embryonic Fig. 115.— Cross section through a portion of the vascular area, after DISSE. ak, Outer, it, inner germ-layer ; mfc1, parietal, mta, visceral lamella of the middle germ-layer ; Ih, extra-embryonic body-cavity ; gw, wall of blood-vessel formed of endothelium ; bl, blood- cells ; g, vessels. blood-corpuscles, which are at this time provided as in the other Verte- brates with a genuine cell-nucleus, possess the power of division. In proportion as blood-corpuscles still further detach themselves from the blood-points, the latter become smaller and smaller, and finally disappear altogether ; but the vessels without exception then contain, instead of a clear fluid, red blood with abundant formed elements (fig. 115 bl). Subsequently there occur changes in the Substanzinseln which lead to the formation of embryonic connective substance. The germinal cells, at first spheroidal, separate farther from one another, at the same time secreting a homogeneous inter-cellular substance ; they become stellate (fig. 116 sp), and send out processes by means of which they are united into a network, which stretches all through the gelatinous secretion ; other cells apply themselves to the endo- thelial tubes of the vessels. i 184 EMBRYOLOGY. After the formation of vessels and blood is completed, the territory of the area opaca, in which the processes just described take place, is sharply delimited at its periphery (fig. 117) in all meroblastic eggs, as well as in those of Mammals. For the close network of blood vessels ends abruptly at its periphery in a broad, circular, marginal vein (the vena or sinus terminalis, S.T.). Beyond the sinus terminalis, there is formed on the yolk neither blood nor blood-vessels. Nevertheless, the two primary germ-layers spread themselves out laterally over the yolk still farther, the- outer layer more rapidly than the inner, until they have grown entirely around it. DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 185' We must therefore now distinguish in the opaque area (Plate I.,. fig. 2, page 213) two ring-like areas, the vascular area (gh) and the yolk-area (dh), area vasculosa and area vitellina. Since, moreover r Fig 117.— Diagram of the vascular system of the yolk-sac at the end of the third day of incubation, after BALFOUR. The whole blastoderm has been removed from the egg and is represented as seen from below. Therefore what is really on the left appears on. the right, and vice versd. The part of the area opaca in which the fine vascular network has been formed is sharply limited at the periphery by the sinus terminalis, and represents the vascular area ; outside of it lies the yolk-area. The immediate vicinity of the embryo is destitute of a vascular network, and is designated now, as at an earlier stage, by the name area pellucida. H Heart; AA, aortic arches; Ao, dorsal aorta, L.Of.A, left, R.Of.A, right vitelline artery; S.T, sinus terminalis ; L.Of, left, R.Of, right vitelline vein ; S. V, sinus venosus ; D.C, ductua Cuvieri ; S.Ca.V, superior, V.Ca, inferior cardinal vein. The veins are drawn in outline, the arteries in solid black. the area pellucida is still recognisable, being traversed by only a few chief trunks of blood-vessels leading to the embryo, the body of the embryo is enclosed altogether by three zones or areas of the extra- embryonic part of the germ-layers. Up to the present we have pursued the formation of blood in the opague area. But how do the vessels in the body of the embryo- 186 EMBRYOLOGY. itself arise ? Here, too, the uncertainty of our present knowledge is to be emphasised. According to the representation of His, to which KOLLIKER also adheres, and which the author himself has made the foundation of his account in the first edition of this Text -book, blood-vessels in the embryo are not independently formed, but take their origin from those already existing in the opaque area. According to His, the germ of the blood and connective substances, originally a peripheral fundament, makes its way from the opaque area at first into the pellucid area, and from there into the body of the embryo itself, .and is distributed everywhere in the spaces between the epithelial germ-layers and the products that have arisen by constriction from them. Into the spaces migrate first of all amoeboid cells, which send out in front of them branched processes ; on the heels of these follow endothelial vascular shoots. At variance with the teachings of His are noteworthy investiga- tions of recent date, — not only the previously mentioned accounts of the manifold origin of the connective substances from the middle germ-layers, but also particularly the more recent observations con - cerning the independent origin of vessels and the endothelial sac of the heart in the body of the embryo itself. (RiJCKERT, ZIEGLER, MAYER, RABL, KASTSCHENKO, and others.) For Selachian embryos the question, whether the repository of the material for the blood-vessels of the embryo is to be sought exclusively on the nutritive yolk, is, as RUCKERT remarks, to be answered definitely in the negative. The vessels arise in the embryo itself within the territory of the mesenchyme, from cells which are sometimes loosely, sometimes compactly arranged (RtJCKERT, MAYER). RflCKERT derives the cells that form the vessels from two different sources, partly from the inner germ-layer of the yolk-wall, partly from the adjoining mesoblast, and their double origin appears to him a natural process of development, in so far as the two layers which bound the first vessels also furnish the material for their walls. To the same purport are the accounts concerning the formation of the endothelial sac of the heart. At first it consists of a rather irregular mass of cells, in which there appear separate cavities, that gradually unite to form a single cardiac space. The cell-material of the fundament of the heart is developed in situ (RtrcKERT, ZIEGLER, MAYER, RABL, and of the earlier investigators GO"TTE, BALFOUR, HOFFMANN) from the wall of the bounding germ-layers; however, DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 187 uncertainty prevails as to whether the inner germ-layer alone, or the middle, or both, are concerned in the production of the fundament. When once the first vessels have been formed, they grow further independently, and continually give rise to new lateral branches by means of a kind of budding process. It can be observed that from the walls of vessels that are already hollow, solid, slender sprouts go out, which are formed of spindle- shaped cells, and by means of cross-branches join others to form a network. The youngest and most delicate of these sprouts consist of only a few cells arranged in a row, or indeed of only a single one, which, reposing upon the endothelial tube like a knob, is drawn out into a long protoplasmic filament. Into the solid sprout there now projects from the already completed vessel a small evagination, which gradually elongates and at the same time enlarges into a tube, the wall of which is formed of the separated cells of the funda- ment. The formation of blood-corpuscles no longer takes place in this process, all the cells of the sprout being employed to form the wall of the vessel. Since out of the vessels thus produced new sprouts :are formed, and so on, the fundaments of the vessels spread them- selves out everywhere in the spaces between the germ-layers and the organs which have by constrictions been formed from them. There are, moreover, two different opinions about the manner in which the sprouting takes place. Are the solid vascular shoots formed exclusively by growth of cells in the wall of the endothelial tube, or do neighboring con- nective-tissue cells take part in their formation ? While KABL holds to the proposition that new vascular endothelia always take their origin from such as are already in existence, KOLLIKER, MAYER, and KUCKERT make statements which appear to prove that the endothelial vascular tubes both continue to grow by themselves alone, and also to elongate through the participation of the connective-tissue cells of the surrounding tissue. In the preceding pages we have endeavored to show in detail ihow in Vertebrates the material of the cleavage- cells is differen- tiated into the separate fundamental or primitive organs. As such we must designate the outer and the inner germ-layers, the two middle germ -layers, and the mesenchyme or intermediate layer. In order properly to estimate at once the significance and the rdle of these fundamental organs, we will glance at the final result of the process of development — propound the question, What organs and 188 EMBRYOLOGY. tissues take their origin in the separate germ-layers and the mesen- chyme ? A definite answer to this question is possible, except on a few points concerning which the accounts of the different observers are still contradictory, and which therefore will be indicated by a mark of interrogation. From the outer germ-layer arise : the epidermis, the epidermoidal organs, such as hair and nails, the epithelial cells of the dermal glands, the whole central nervous system with the spinal ganglia, the peripheral nervous system ('?), the epithelium of the sensory organs (eye, ear, nose), and the lens of the eye. The primary inner germ-layer is differentiated into : — 1. The secondary inner germ-layer, or entoblast ; 2. The middle germ -layers ; 3. The fundament of the chorda ; 4. The germ of the mesenchyme, which forms the intermediate layer. The entoblast (Darmdriisenblatt) furnishes the epithelial lining of the whole intestinal canal and its glandular appendages (lung, liver, pancreas), the epithelium of the urinary bladder, and the taste buds. The middle germ-layers undergo extremely various metamorphoses after having been differentiated into primitive segments and lateral plates. . From the primitive segments are derived the striated, voluntary muscles of the body and a part of the mesenchyme. From the lateral plates arise the epithelium of the pleuroperitoneal cavity ; the epithelium of ovary and testis (primitive ova, mother- cells of the spermatozoa) ; in general, the epithelial components of the sexual glands and their ducts, as well as those of the kidney and; ureter ; and finally mesenchymatic tissue. The fundament of the chorda becomes the chorda dorsalis, which in the higher Vertebrates is reduced, during later stages of development, to insignificant remnants. The mesenchyme-germs, which produce the intermediate layer, un- dergo manifold differentiations, for they spread themselves out in the body between the epithelial components as the intermediate mass. From them are derived . the multiform group of sustentative (con- nective) tissues (mucous tissue, fibrillar connective tissue, cartilage, bone), vessels (?) and blood (?), the lymphoid organs, the smooth, involuntary muscles of the vessels, of the intestine, and of various other ori:.-iii>. DEVELOPMENT OF CONNECTIVE SUBSTANCE. AND BLOOD. 189 HISTORY OF THE PAEABLAST- AND MESENCHYME-THEOEIES. The older investigators, as, for example, REMAK, grouped together all the cells which are inserted between the two primary germ-layers under the common name of the middle germ-layer, and assumed for them a common origin. To this conception His opposed in the year 1868 in " Die erste Ent- wicklung des Hiihnchens im Ei " his "pardblast-theory" in which, influenced principally by histogenetic considerations, he distinguished two fundaments of different origin, an archiblastic and aparablastic. As archiblastic fundament he designated the part of the middle germ-layer which lies in the body of the embryo itself, the axial cord (Achsenstrang) and the animal and vegetative muscle-plates, and he made them arise by de- lamination from the primary germ-layers, and therefore ultimately from the embryonic cleavage-cells. He gave the name parablast to a peripheral fundament, lying originally outside the embryo, which is the source of all the connective substances, the blood and the vascular endothelium, and which grows from the margin, or more speci- fically from the opaque area, into the body between the archiblastic tissues. The division of the middle germ-layer into archiblast (chief germ) and parablast (accessory germ), proposed by His and carried out in several of his •writings, found at the time no approbation, and encountered decided and successful opposition, especially on the part of HAECKEL, because the correct views contained in the doctrine were obscured and coyered up by peculiar conceptions about the origin of the parablast. The parablast, it was claimed, is not derived from the egg-cell, but from the white yolk, a product of the granulosa-cells, which, according to the earlier teachings of His, penetrate into the primordial ovum in great numbers and become the white yolk-cells and the yellow spherules. But the granulosa-cells in turn, it was maintained, .arise from the connective tissue (leucocytes) of the mother ; consequently after their migration into the egg they are capable of producing again only connective tissue and blood. His thought it was necessary to assume a fundamental difference between chief germ and accessory germ ; the former alone had experienced the influence of fertilisation, since it alone was descended from cleavage-cells, whereas the latter, since it issued from the white yolk (a derivative of the maternal con- nective tissue), was " purely a maternal dower." RAUBEE, in a short communication, accepted the conclusions of His, in so far as he also assumed a common origin for blood and connective tissue, a special " hsemo-desmoblast," but differed from him in that he derived them from the cleavage-cells. GOETTE (1874) is also to be mentioned in this connection, since he maintained that the blood is developed out of yolk-cells, which break up into clusters of smaller cells (Amphibia and Birds). Proceeding from other standpoints, and induced by observations on In- vertebrates, my brother and I were led in our Ccelom- Theory (1881) to a result similar to that of His, namely, that two entirely different structures had been hitherto embraced under the expression middle germ-layer, and that it was necessary to introduce in the place of the old indefinite conception two new and more precise ones, " middle germ-layer in the restricted sense " and " mesen- chyme-gerrn." But our conception, notwithstanding many points of agree- ment, took in detail a form very different from the doctrine of His. 190 EMBRYOLOGY. All fundaments of the animal body are derived from embryonic cells, which have been produced from the egg-cell by the process of cleavage. The dis- tinction between middle germ-layer and mesenchyme-germ is to be sought in another direction than in that indicated by His. The middle germ-layers are sheets of embryonic cells, having an epithelial arrangement, which arise by a process of folding from the inner germ-layer, just as the latter does by a fold- ing of the blastula (compare the historical part of Chapter VII.). The mesen- chymatic germ, on the contrary, embraces cells, which have been individually detached from epithelial union in the inner germ-layer, and furnish the founda- tion for connective substance and blood by spreading themselves out in the system of spaces between the epithelial germ-layers. After the appearance of the Ccelom-Theory, His entered again into an explanation of his parablast-theory, and modified it in his paper, " Die Lehre vom Bindesubstanzkeim," in so far as he no longer laid weight on the question whether the fundament of the connective substance was derived from the segmented or the unsegmented germ. The theory of the double origin of tlie middle germ-layers, established by His and by us in different ways, met with opposition on the part of KOLLIKER who held to the older interpretation ; but by many others it was accepted ; attempts were made further to confirm and also to modify it by KUPFFER, DISSE, WALDEYER, KOLLMANN, HEAPE, and others, who defended the existence of a special connective-tissue germ. KUPFFER and his followers furnished important observations concerning" the presence of yolk-nuclei in a definite zone of the embryonic fundament, and their relation to the formation of blood in Fishes and Reptiles. HOFFMANN and KtiCKERT showed that the yolk -nuclei do not arise by free [spontaneous] formation of nuclei, but are descendants of the cleavage-nucleus. DISSE investigated the germ-wall of the Hen's egg. KOLLMANN named the cells which migrate out between the germ-layers poreuts (Poreuten), and the whole fundament the acroblast. Finally, WALDEYER endeavored to derive the connective-tissue germ from a special part of the cleavage-material, which he divided into an archiblast and a parablast. According to WALDEYER'S theory, the cleavage of the eggs of all those animals in which there is any blood and connective substance does not take place uniformly up to the end, but one must distinguish a primary and a secondary cleavage. " The former divides the egg, so far as it is in any way capable of cleavage, into a number of cells, which are ready for the production of tissues. These then form the primary germ-layers. A remnant of im- mature cleavage -cells (in the case of holoblastic eggs), or of egg-protoplasm r which is not yet converted into the cell-form (in meroblastic eggs), is left remaining. Neither the immature cells, nor the protoplasm still unconverted into cells, enter for the present into the integrating condition of the germ- layers. On the contrary, it is only afterwards that there is effected on this material a further formation of cells, the secondary cleavage. The immature cells of the holoblastic eggs, over-loaded with nutritive yolk, divide them- selves, or, if one prefers, ' cleave ' themselves further, or the parts which are most richly provided with protoplasm constrict themselves off from the eggs, whereas the remnant of the nutritive material is consumed, — the unformed remnants of the protoplasm (germ-processes) of meroblastic eggs become divided up into cells. The cell-material thus secondarily acquired DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 19* migrates in between the primary germ-layers, and becomes blood and connec- tive substance." According to the recent investigations of RABL, ZIEGLEE, VAN WIJHE, EUCKERT, and others, the mesenchyme is produced from various regions of the middle germ-layer. A participation of the inner germ-layer in the forma- tion of the blood-vessels is rendered probable. SUMMARY. 1. Besides the four germ-layers, which have the form of epithelial lamellae, special germs are developed in the higher Vertebrates for the sustentative substances and the blood, — the mesenchyme-germs. The latter together make up the intermediate layer. 2. The mesenchyme-germs arise by cells detaching themselves from epithelial union with the germ-layers, and penetrating as migratory cells into the fissure between the four germ -layers (the remnant of the original cleavage-cavity) and spreading themselves out in this space. 3. Germ-layers and mesenchyme -germ (intermediate layer) ex- nibit a difference in the method of their origin: the former are developed by foldings of the wall of the blastula, the latter by emi- gration of isolated cells from definite territories of the germ-layers. 4. Mesenchyme-germs arise from the wall of the primitive segment,, from the cutis-plate, and at certain regions of the parietal and' visceral lamellae of the middle germ-layer. 5. Blood-vessels are developed both in the body of the embryo- itself, in a manner which still remains to be accurately determined, and also in the territory of the area opaca of meroblastic eggs. 6. The source of the cells from which the vessels and blood of the opaque area arise is at present a matter of controversy. 7. In the formation of vessels in the opaque area the following phenomena are to be regarded : — (a) The embryonic cells of the intermediate layer arrange themselves : — First into a network of cords, and Secondly into the substance-islands (Substanzinseln). (b) There are developed out of the cell-cords, at the same time with the secretion of the fluid portions of the blood, the endothelial wall of the primitive blood-vessels and their cellular contents, the blood-corpuscles (blood-islands). (c) The Substanzinseln become embryonic connective substance 192 EMBRYOLOGY. (d) The place where blood-vessels and connective substance at first arise in the opaque area is sharply limited at the periphery by a circular vessel, the sinus terminalis. {e) Since the outer and the inner germ-layers further con- tinue to spread themselves out over the yolk after the development of the intermediate layer, the body of the embryo becomes surrounded by three areas : — First by the area pellucida, Secondly by the vascular area ending in the sinus terminalis, Thirdly by the yolk-area, which is coextensive with the margin of the overgrowth. 8. The red blood-corpuscles of all Vertebrates possess in the earliest stages of development the power of increase by means of division. The red blood-corpuscles of Mammals have at this time a nucleus. 9. The following table gives a survey of the fundamental organs of the embryo, and the products of their further development: — I. Outer Germ-layer. Epidermis, hair, nails, epithelium of dermal glands, central nervous system, peripheral nervous system, epithelium of sensory organs, the lens. II. Primary Inner Germ-layer. 1. Entoblast, or secondary inner germ-layer. Epithelium of the alimentary canal and its glands, epithelium of urinary bladder. 2. Fundament of the chorda. 3. The middle germ-layers. A. Primitive Segments. Transversely striped, voluntary muscles of the body. Parts of the mesenchyme. B. Lateral Plates. Epithelium of the pleuroperitoneal cavities, the sexual cells and epithelial components of the sexual glands and their outlets, epithelium of kidney and ureters. Parts of the mesenchyme. 4. Mesenchyme- germ. Group of the connective substances, blood-vessels and blood, lymphoid organs, smooth involuntary muscles. LITERATURE. 193 LITERATURE. Afanasieff. Ueber die Entwiekelung der ersten Blutbahnen im Hiihner- embryo. Sitzungsb. d. k. Akad. d. Wissensch. Wien, matb.-nat. 01. Bd. 53. Abth. 2, p. 560. 1866. Balfour. The Development of the Blood-vessels of the Chick. Quart. Jour Micr. Sci. Vol. XIII. 1873, p. 280. Disse. Die Entstehung des Blutes und der ersten Gefasse im Huhnerei. Archiv f . mikr. Anat. Bd. XVI. 1879. Gasser. Der Parablast und der Keimwall der Vogelkeimscheibe. Sitzungsb. d. naturwiss. Gesellsch. Marburg. 1883. Gensch. Die Blutbildung auf dem Dottersack bei Knochenfischeiu Archiv f. mikr. Anat. Bd. XIX. 1881. Gensch. Das secundare Entoderm und die Blutbildung beim Ei der Knochen- fische. Inaugural-Dissertation. Konigsberg 1882. Hatschek. Ueber den Schichtenbau von Amphioxus. Anat. Anzeiger. 1888. His, W. Der Keimwall des Huhnereies und die Entstehung der parablas- tischen Zellen. Zeitschr. f. Anat. u. Entwicklungsg. 1876, p. 274. His, W. Die Lehre vom Bindesubstanzkeim (Parablast). Euckblick nebst kritischer Besprechung einiger neuerer entwicklungsgeschichtlicher Ar- beiten. Archiv f. Anat. u. Physiol. Anat. Abth. 1882. Klein. Das mittlere Keimblatt in seinen Beziehungen zur Entwicklung der ersten Blutgefasse und Blutkorperchen im Hiihnerembryo. Sitzungsb. d. k. Akad. d. Wissensch. Wien, math.-naturw. 01. Bd. 63. Abth. 2, p. 339. 1871. Kblliker, A. Ueber die Nichtexistenz eines embryonalen Bindegewebskeims (Parablast). Sitzungsb. d. phys.-med. Gesellsch. Wurzburg 1884. Kolliker, A. Kollmann's Akroblast. Zeitschr. f . wiss. Zoologie. Bd. XLI. 1885, p. 155. Kolliker, A. Die embryonalen Keimblatter und die Gewebe. Zeitschr. f. wiss. Zoologie. Bd. XL. 1884, p. 179. Kollmann, J. Der Kandwulst u. der Ursprung der Stutzsubstanz. Archiv f . Anat. u. Physiol. Anat. Abth. 1884. Kollmann, J. Bin Nachwort. Archiv f. Anat. u. Physiol. Anat. Abth 1884. Kollmann, J. Der Mesoblast und die Entwicklung der Gewebe bei Wirbel. thieren. Biol. Centralblatt. Bd. III. Nr. 24, 1884, p. 737. Kollmann, J. Gemeinsame Entwicklungsbahnen der Wirbelthiere. Archiv f. Anat. u. Physiol. Anat. Abth. 1885. Kupffer. Ueber Laichen und Entwiekelung des Ostseeherings. Jahresbericht der Comm. fiir wissensch. Untersuchung der deutschen Meere. 1878. Lankester, Ray. Connective and Vasifactive Tissues of the Leech. Quart. Jour. Micr. Sci. Vol. XX. 1880. Mayer, P. Ueber die Entwicklung des Herzens und der grossen Gefassstamme I bei den Selachiern. Mittheil. a. d. zool. Station Neapel. Bd. VII. 1887,. p. 338. Rabl, C. Ueber die Bildung des Herzens der Amphibien. Morphol. Jahrb. Bd. XII. 1886. Rabl, C. Theorie des Mesoderms. Morphol. Jahrb. Bd. XV. 1889. Rauber. Ueber den Ursprung des Blutes und der Bindesubstanzen. Sitzungsb. d. naturf. Gesellsch. Leipzig. 1877. " 194 EMBRYOLOGY. Huckert, J. Ueber den Ursprung des Herzendothels. Anat. Anzeiger. Jahrg. II. Nr. 12. 1887. Kiickert, J. Ueber die Entstehung der endothelialen Anlagen des Herzens nnd der ersten Gefassstamme bei Selachierembryonen. Biol. Centralblatt. Bd. VIII. 1888. Strahl. Die Anlage des Gefasssystems in der Keimscheibe von Lacerta agilis. Sitzungsb. d. Gesellsch. z. Beford. d. ges. Naturwiss. Marburg. 1883, p. 60. Strahl. Die Dottersackwand und der Parablast der Eidechsen. Zeitschr. f. wiss. Zoologie. Bd. XLV. 1887. Uskow. Die Blutgefasskeime und deren Entwicklung bei einem Huhnerei. M6m. de 1'Acad. imper. des Sci. St. Petersbourg. S6r. VII. T. XXXV. Nr. 4. 1887. Waldeyer. Archiblast und Parablast. Archiv f. mikr. Anat. Bd. XXII. 1883, pp. 1-77. Wenckebach. Beitrage zur Entwicklungsgeschichte der Knochenfische, Archiv f. mikr. Anat. Bd. XXVIII. 1886, p. 225. Ziegler. Der Ursprung der mesenchymatischen Gewebe bei den Selachiern. Archiv f. mikr. Anat. Bd. XXXII. 1888. Ziegler. Die Entstehung des Blutes bei Knochenfischembryonen. Archiv f. mikr. Anat. Bd. XXX. 1887. CHAPTER X. ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. AFTER having investigated in the preceding chapters the fundamental organs of the body of vertebrated animals, or the germ-layers, and their first important differentiations into neural tube, chorda, and primitive segments, as well as the origin of the blood and connective tissues, it will be our next undertaking to make ourselves acquainted with the development of the external form of the body, and with the development of the embryonic membranes, the latter being intimately connected with the former. There exists an extraordinary difference in these respects between the lower and higher Vertebrates. When the embryo of an Amphioxus has passed through the first processes of development, it elongates, becomes pointed at both ends, and already possesses in the main the worm-like or fish-like form of the adult animal. But the higher we ascend in the .series of Vertebrates, the more are the embryos, when they attain the stage of development corresponding to the Amphioxus embryo, unlike the adult animals: at this stage they assume very singular and strange forms, inasmuch as they become surrounded by peculiar envelopes and are provided with various appendages, which subsequently disappear. ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 195 The difference is referable, first of all, to the more or less extensive •accumulation of nutritive yolk, the significance of which for the nascent organism is twofold. From a physiological point of view, the nutritive yolk is a rich source of energy which alone makes it possible for the embryological processes to take place in uninterrupted sequence, until at length an organism, with an already relatively high organisation, begins its independent existence. From a morphological point of view, on the other hand, the yolk plays the role of ballast, which exerts a restrictive and modifying influence on the direct and free development of those organs which are en- trusted with the reception and elaboration of it. Even at the very beginning of development we could see how the cleavage-process and the formation of the germ-layers were retarded, altered, and to a certain extent even suppressed by the presence of yolk. In what follows we shall again have occasion to point out the same thing, — how, owing to the presence of yolk, the normal formation of the intestinal canal ;and of the body can be attained only gradually and by a circuitous [process. In the second place, the great difference which the embryos of Vertebrates present is produced by the medium in which the eggs' undergo development. Eggs which, like those of water-inhabiting Vertebrates, are deposited in the water, are developed in a more simple and direct manner than those which, provided with a firm shell, are laid upon the land, or than those which are enclosed in the womb up to the time of the birth of the embryos. In the two latter cases the growing organism attains its goal only !by very indirect ways. At the same time with the permanent organs there are also developed others which have no significance for the post-embryonic life, but which serve during the egg-stage of exist- ence either for the protection of the soft, delicate, and easily injured body, or for respiration, or for nutrition. These either undergo regressive metamorphosis at the end of embryonic life, or are cast off at birth as useless and unimportant structures. But inasmuch as they are developed out of the germ-layers, they are also properly to be regarded as belonging immediately to the nascent organism — as being its embryonic organs, and as such they too are to be treated in morphological descriptions, The extensive material which has to be mastered in this con- nection I shall present grouped into two parts. In the first part we shall inquire how the embryo overcomes the 196 EMBRYOLOGY. obstacle which it encounters in the presence of the yolk and acquires its ultimate form. In the second and likewise more extensive part we must concern ourselves more minutely with the embryonic enveloping structures and appended organs, which subserve various purposes. The collection of yolk-material disturbs the course of development least in the case of the Amphibia. The latter therefore stand, as it were, midway between Amphioxus with direct development and the remaining Verte- brates, and constitute a transition between them. In the Amphibia the yolk shares in the process of cleavage; after the close of this process it is found ac- cumulated for the most part in the large yolk-cells which form the floor of the blastula (fig. 45) ; at the time of th& differentiation into germ- layers it is taken up into the coelenteron, which it almost completely fills (fig. 47); after the formation of the body- sacs the large yolk-cells lie in a similar manner in the ventral wall of the intestine proper (fig. 118 yk). Here they are in part dissolved and employed for the growth of the remaining parts of the body, in part they share directly in the formation of the- epithelium of the ventral wall of the intestine. In consequence of the presence of the great accumulation of yolk- cells, the Amphibian embryo acquires a shapeless condition at a time when the Amphioxus larva has already become elongated and fish- like. The body, which is spherical during gastrulation, later becomes egg-shaped, owing to its elongation. Thereupon the head-end andi the tail-end begin to be established at the two poles as small eleva- tions (figs. 118 and 80). The middle or trunk-part lying between the latter becomes somewhat incurved along its dorsal region, in. Fig. 118. —Diagrammatic longitudinal section through the embryo of a Frog, after GOETTE, from BALFOUR. nc, Neural tube ; x, communication of the same with blastopore and coelenteron (al) ; yk, yolk-cells ; m, middle germ-layer. For the sake of simplicity the outer germ-layer is represented aa if composed of a single layer of cells. ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 197 -which neural tube, chorda, and primitive segments are developed, so that the cephalic and caudal elevations become joined by means of a concave line. The ventral side of the trunk-region, on the con- trary, is greatly swollen and bulges out ventrally and laterally like a hernia, since it is filled with yolk-cells. This swelling is therefore called the yolk-sac, In the further progress of development the embryo continually acquires a more fish-like shape. The anterior and the posterior ends of the body, especially the latter, increase greatly in length, find the middle of the trunk becomes thinner, for with the consump- tion of the yolk-material the yolk-sac becomes smaller and finally •disappears altogether, its walls being incorporated into the ventral wall of the intestine and that of the body. The interferences in the normal course of development become greater in the same ratio as the yolk increases in amount, as it does in the case of the meroblastic eggs of Fishes, Reptiles, and Birds. With the latter the yolk is no longer broken up into a mass of yolk-cells, as in the case of the Amphibia ; it participates in the process of cleavage, but only to a slight extent, inasmuch as nuclei make their way into the layer of yolk which is adjacent to the germ, and, sur- rounded by protoplasm, continue to increase in number by division. The gastrula-form is altered until it becomes unrecognisable; only a small part of its dorsal surface consists of cells, which are arranged into the two primary germ-layers, whereas the whole ventral side, where in the Amphibia the yolk-cells are found, is an unsegmented yolk-mass. Thus we acquire in the case of the Vertebrates mentioned a peculiar condition; the embryo, if we regard the yolk as not belonging to the body, appears to be developed from layers that are spread out flat instead of from a cup-like structure (Plate I., fig. 1, page 213). Moreover we see even a greater distinction effected between the dorsal and ventral surfaces of the egg during develop- ment than was the case with the Amphibians. The fundaments of all important organs, the nervous system, the chorda, the primitive segments (Plate I., figs. 2, 8), are at first produced exclusively on the former, whereas on the ventral side few and unimportant changes only are to be observed. These consist principally in the extension of the germ-layers, which spread out farther ventrally, grow over the yolk- mass (Plate I., figs. 2-5), and form around it a closed sac consisting of several layers. This circumcrescence of the unsegmented yolk by the germ-layers is accomplished, on the whole, very slowly, the more 198 EMBRYOLOGY. voluminous the accumulated yolk-material, the more time it requires : thus, for example, in the case of Birds it is completed at a very late stage of development, when the embryo has already attained a high state of perfection (Plate I., fig. 5). In the case of meroblastic eggs, the part of the germ-layers on which the first fundaments of the organs (neural tube, chordar primitive segments, etc.) appear has been distinguished as the- embryonic area from the remaining part, or the extra-embryonic area* The distinction is both fitting and necessary ; but the names might have been more appropriate than " embryonic and extra-embryonic,"" since obviously everything that arises from the egg-cell, and con- sequently even that •£»» which originates in the extra -embryonic area, must be rec- koned as belonging to the embryo. The differentiation into two areas persists in the course of further development, and be- comes expressed still more sharply (fig. 119). The embryonic area, by means of the folding of its flattened layers into tubes, alone forms the elongated, fish-like body which all Vertebrates at first exhibit; the extra-embryonic area, on the contrary, becomes a sac filled with yolk (ds), which, like an enormous hernia, is united to the embryo (Em) by means of a stalk (st) attached to its belly, sometimes even while the embryo is still remarkably small. We must now explain more minutely the details of the processes of development which take place in this connection : first the metamorphosis of the flattened embryonic area sBto the fish-like embryonal body, and secondly the formation of the yolk-sac. In the presentation we shall adhere chiefly to the Hen's egg, but for the time being we shall leave out of consideration the formation of the embryonic membranes. The body of the Chick is developed by a folding of the flattened layers, and by the constricting off of the tubular structures thus formed. Pig. 119. — Advanced embryo of a Shark (Pristiurus), after BALFOUR. Em, Embryo ; ds, yolk-sac ; st, stalk of the yolk-sac ; av, arteria vitellina ; vv, vena vitellina. ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. from the area pellucida. The beginning of the process of folding is recognisable upon the surface of the blastoderm by means of certain furrows, the marginal grooves (Grenzrinnen) of His. These appear earlier in the anterior than in the posterior region of the embryonic fundament, in correspondence with the law previously enunciated, according to which the anterior end of the body anticipates in development the posterior end. At first that part of the embryonic fundament which is Ifillililllil^' I I \ 7* c destined to become the head is marked off by means of a cres- centic groove (fig. 120). In the case of the Chick this is indicated during the first day of incubation, at a time when the first trace of the nervous system becomes visible. It lies immediately in front of the curved anterior end of the medullary ridges, with its concavity directed backward. At a later stage the embryonic area is marked off laterally. In the case of the embryo Seen from Fig. 120,-Surface-view of the area pellucida of ,«- . , . , a blastoderm of 18 hours, after BALFOUR. the surface in fig. 121, in which In front of the primitive groove (pr) lies th» medullary furrow (me), with the medullary ridges (A). These diverge behind and fade out on either side in front of the primitive groove ; anteriorly, on the contrary, they are continuous with each other, and form an arch behind a curved line, which represents the anterior marginal groove. The second curved line, lying in front of and concentric with tha first, is the beginning of the amniotic fold. the neural tube is already partly closed and segmented into three brain-vesicles, and in which six pairs of primitive segments are laid down, there may be re- cognised at some distance from these primitive segments two dark streaks, the two lateral marginal grooves. They become less distinct in passing from before backward, and wholly disappear at the end of the primitive groove. Finally, the tail-end of the embryo is marked off by the posterior marginal groove, which like the anterior is crescentic, but has its concavity directed toward the head. In this manner a small part of the germ-layers, which alone is required for the construction of the permanent body, is separated by a 200 EMBRYOLOGY. continuous marginal furrow from the much more extensive extra- embryonic area, which serves for the formation of evanescent organs like the yolk-sac and the em- bryonic membranes. The marginal grooves are formed by the infold- ing of the outer germ-layer and the parietal middle layer, which are together called the somatopleure, and in such a manner that the ridge of the original small fold is directed downward toward the yolk (Plate I., fig. 8 sf). The space en- closed by the two folded layers is the marginal groove (gr). As we have distinguished on the latter several regions, which are developed at different times, so must we here distinguish the corresponding folds, and we consequently speak of a headfoldj a tail- fold, and the two lateral folds. The headjold appears, first of all, even on the first, but more distinctly on the second, day of in- cubation. By means of it the head-end of the embryonal fundament is formed and separated from the extra- embryonic part of the germ-layers. At the moment of its origin it is turned directly downward toward the yolk; but the more it enlarges, — whereby the anterior marginal Fig. 121.— Blastoderm of the Chick, incubated 33 hours, after DUVAL. One sees the pellucid area, hf, surrounded by a portion of the opaque area, df. The fundament of the nervous system is closed anteriorly and segmented into three brain- vesicles, hbl, hb', hba ; behind, the medullary fold mf is still open. On either side of it lie six primitive segments, ut. The posterior end of the fundament of the embryo is occupied by the primitivi- streak with the primitive groove, pr. ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 201 groove is deepened into a pit, — the more its ridge is turned back- wards. Two diagrammatic longitudinal sections, one of which is shown in fig. 122, the other on Plate I., fig. 11, may serve to illustrate this process. In fig. 122 there is shown, projecting above the otherwise smooth flat surface of the germ-layers, a small protuberance, which encloses the anterior end of the neural tube (N.C) and the simultaneously forming intestinal tube (Z>), and which has arisen by the formation of the fold F.So. The upper sheet of the fold, by directing itself N.C Tig. 122.— Diagrammatic longitudinal section through the axis of an embryo Bird, after BALFOUB. The section represents the condition when the head-fold has begun, but the tail-fold is still wanting. -F.So, Head-fold of the somatopleure ; F.Sp, head-fold of the splanchnopleure, forming at Sp the lower wall of the front end of the mesenteron ; D, cavity of the fore gut ; pp, pleuroperitoneal cavity ; Am, fundament of the anterior fold of the amnioii ; N.C, neural tube ; Ch, chorda ; At B, C, outer, middle, inner germ-layer, everywhere distinguished by different shading; Ht, heart. backwards, furnishes the ventral wall of the cephalic elevation ; the lower sheet forms the floor of the marginal groove. In the second figure, in which there is represented a diagrammatic longitudinal section through an older embryo, the head-fold (kf1) has extended still farther backward. The head has thereby become longer, since its under surface has increased in consequence of the advance in the process of folding. Whoever desires to make this process, which is very important for the comprehension of the construction of animal forms, clearer and more intelligible, may do so with the help of an easily constructed model. Let him stretch out his left hand on a table, and spread flat •over the back of it a cloth, which is to represent the blastoderm ; then let him fold in the cloth with his right hand by tucking it a little way under the points of his left fingers. The artificially pro- duced fold corresponds to the head-fold previously described. The 202 EMBRYOLOGY. points of the fingers, which by the tucking under of the cloth have- received a covering on their lower sides, and which project above the otherwise flattened cloth, are comparable to the cephalic eleva- tion. In addition we can represent the backward growth of the- head-fold by tucking the cloth still farther under the left fingers toward the wrist. The hinder end of the embryo develops in the same manner as the front end, only somewhat later (compare fig. 11, Plate I.). Corre- sponding to the posterior marginal groove (gr), the tail-fold is so formed that its ridge is directed forward and that it grows toward the head-fold. Where in surf ace- views of the blastoderm the lateral marginal grooves are to be seen (fig. 121), one recognises on cross sections the lateral folds (Plate I., fig. 8 sf). They grow at first directly from above downwards, thus producing the lateral walls of the trunk. Afterwards their margins bend somewhat toward the median plane (Plate I., fig. 9 sf), thereby approaching each other, and in this way gradually draw together to form a tube (Plate I., fig. 10). By their infolding the trunk acquires its ventral wall. In order to avoid misconceptions, let it be further remarked that only at the beginning of their formation are head-, tail-, and lateral folds somewhat separated from one another, but that when they are more developed they are merged into one another, and thus are only parts of a single fold, which encloses the fundament of the embrya on all sides. As the separate parts of this fold increase, they grow with their bent margins from in front and from behind, from right and from left, toward one another, and finally come near together in a small territory, which corresponds approximately with the middle of the surface of the embryo's belly, and is designated on the figure of the cross section through this region (Plate I., fig. 10) by a ring-like line (hn). Thus a small tubular body is formed (Plate I., fig. 3), which lies upon the extra-embryonic area of the blastoderm and is united to it by means of a hollow stalk (hn). The stalk marks the place where the margins of the folds, growing toward one another from all sides, have met, but a complete constricting off of the embryonic territory from the extra-embryonic does not take place. We can also represent these conditions, if, in the previously men- tioned model, we in addition fold in the cloth that covers the tips of the fingers along the sides of the hand and the wrist, and thea carry the circular fold thus artificially formed still farther under,, even to the middle of the palm. Then the cloth forms around the- ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 203 hand a tubular sheath, which is continuous at one place by means of a connecting cord with the flattened, remaining portion of the cloth. A process similar to the externally visible one just described, by which the lateral and ventral walls of the body are produced from the sheet-like fundaments, takes place at the same time within the embryo in the splanchnopleure. There are developed from it, as from the somatopleure, an anterior, a posterior, and two lateral intestinal folds. First, at the time when the head is differentiated (fig. 122), the part of the splanchnopleure corresponding to it (F./Sp.) is folded together into a tube, the so-called cavity of the fore gut or head-gut (Z>). The same process repeats itself on the third day of incubation at the posterior end of the embryonal fundament, where, upon the appearance of the caudal part (Plate I., fig. 11), there is formed within it and out of the splanchnopleure the cavity of the hind gut. Both parts of the intestine at first terminate with blind ends directed toward the outer surface of the body. At the head-end the mouth-opening is still wanting, at the posterior end the anus. When, however, one raises the blastoderm with the nascent embryo from the yolk, and examines it from the under side, the anterior and posterior portions of the intestinal canal exhibit openings (vdpf and hdpf). through which one can look from the yolk-side into the blind-ending cavities. One of these is called the anterior, the other the posterior, intestinal portal or intestinal entrance (Plate I., fig. 11 vdpf and hdpf). Between the two portals the middle region of the intestinal canal remains for a long time as a leaf -like fundament. Then by its becoming somewhat bent downwards (Plate I., figs. 9 and 2) there arises under the chorda dorsalis an intestinal groove (dr), which lies between fore and hind gut. Owing to the further increase of the lateral intestinal folds (df), the groove becomes deeper and deeper, and finally, by the approximation of the edges of the folds from in front, from behind, and from both sides, becomes closed into a tube in the same manner as the wall of the body. At only one small place, which is indicated by the ring-like line dn in Plate I., figs. 3 and 10, the folding and cons tricting-ofF process is not completed, and here the intestinal tube too remains con- tinuous, by means of a hollow stalk, with the extra-embryonic part of the splanchnopleure, which encloses the yolk. The part of the germ-layers which is not employed in the formation 204 EMBRYOLOGY. of the embryo furnishes in the case of the Reptiles and Birds the yolk-sac and certain embryonic membranes. I shall speak of the development of these in the next chapter. The fate of the extra-embryonic area of the blastoderm in Fishes is more simple, since there is formed from it only a sac for the reception of the yolk. Fig. 123 exhibits the embryo (Em) of a Selachian, which has arisen by the infolding of a small area of the germ-layers in the manner described for the Chick. All the remaining part of «* the egg has become ds a great yolk-sac (ds), which is united with the middle of the belly by means of a long stalk. The Teleosts (Plate I., fig. 6) show us transitions from this condition to one in which the yolk-sac, as in Amphibians, is not separated by represents only a capacious Fig. 123.— Advanced embryo of a Shark (Pristiurus), after BALFOUR. Em, Embryo ; ds, yolk-sac ; at, stalk of the yolk-sac ; av, arteria vitellina ; w, vena vitellina. a stalk from the mesenteron, but enlargement of the latter and of the belly-wall. Let us now examine more carefully the structure of the yolk-sac. As has been remarked already, all four of the germ-layers spread themselves out one after another around the unsegmented yolk-mass of meroblastic eggs (Plate I., figs. 6 and 7). As in the embryonal body the two middle germ-layers separate from each other and allow the body-cavity to appear between them, so, too, at a later stage the same process occurs in the extra-embryonic area. Throughout the region of the middle germ-layer there is formed a narrow fissure, for which the name " extra-embryonic body-cavity," or blastospheric ccelom (cavity of the blastoderm, KOLLIKER), would be most suitable. It separates the envelope of the yolk into two layers, of which the inner is the immediate continuation of the intestinal wall (splanchnopleure), the outer, on the contrary, that of the body- wall (somatopleure). Therefore, to be exact, we have before us a double sac formed around the yolk, which we can distinguish as ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 205 intestinal yolk-sac and dermal yolk-sac. The former is simply a. hernia-like evagmation of the intestinal canal, and, like it, is composed of three layers : — (1) The intestine-glandular layer (ik), — the entoblast or secondary entoderm, which encloses the yolk ; (2) The visceral middle layer, or the pleuroperitoneal epithelium (mjfc2) ; and (3) The intermediate layer (Zwischenblatt), in which have been developed the vitelline blood-vessels, which at the beginning of the circulation of the blood have to conduct the liquefied nutritive material from the yolk-sac to the places of embryonic growth. The dermal yolk-sac is, as a continuation of the body-wall, likewise- composed of three layers — the epidermis (a% the parietal middle layer (mkl), and the connective-tissue intermediate substance (Zwischensubstanz). It has already been stated that the constricting-off of the yolk-sac from the embryonal body is quite variable in extent, and can go so- far that the connection between the two is kept up only by means of a narrow stalk. A more careful examination shows that in the latter case the stalk itself is composed of two narrow tubes one within the other (Plate I., fig. 7), of which the outer unites the dermal yolk-sac (hs) to the ventral wall of the body, and the inner the intestinal yolk-sac to the intestinal canal. The former is called the dermal stalk, the latter the intestinal stalk (dn) or vitelline duct, ductus vitello-intestinalis. The place of attachment of the dermal stalk in the middle of the ventral surface of the embryo i& called the dermal navel (hn) ; the corresponding place of attachment of the intestinal stalk to the wall of the intestine the intestinal navel (dn). The embryonic body-cavity opens out between the two, and is continuous with the fissure between dermal and intestinal yolk-sac — with the " extra-embryonic body-cavity " or the blasto- spheric ccelom (lh2). The ultimate fate of the yolk-sac in the Fishes is the same as in the Amphibia. It is still employed, even in the extreme case of the Selachians, for the formation of the wall of the intestine and that of the body. The more its contents are liquefied and absorbed, the more the yolk-sac shrivels. When the intestinal yolk-sac has become very small, it is drawn into the body-cavity and finally serves to close the intestinal navel, just as the dermal yolk-sac upon its disappearance closes up the dermal navel. With the lower Vertebrates a shedding of the embryonic parts has not yet come into* 206 EMBEYOLOOY. existence. The next chapter will explain what becomes of tho yolk-sac in the case of Reptiles and Birds. SUMMARY. 1. In the case of Vertebrates whose eggs contain little yolk, the embryo after the development of the germ-layers takes on an elongated, fish-like form. 2. In eggs with abundant yolk the body of the vertebra ted animal is produced by only a small region of the germ-layers (the embryonic fundament); the far greater extra-embryonic area is employed for the formation of a yolk-sac and of embryonic membranes (the latter only in Reptiles and Birds). 3. The separate layers of the embryonic fundament constrict them- selves off from the extra-embryonic territory, and at the same time become folded into tubes — the somatopleure into the tubular body- wall, the splanchnopleure into the intestinal tube (head-fold, tail-fold, lateral folds, intestinal groove, intestinal fold). 4. The extra -embryonic territory of the germ-layers remains in continuity with the two tubes by means of a stalk-like connection. 5. In Fishes the extra-embryonic territory of the germ-layers becomes the yolk-sac, which is composed of two sacs, the intestinal and the dermal yolk-sacs, separated from each other by a pro- longation of the embryonal body-cavity. 6. The place where the dermal yolk-sac is attached to the belly - wall of the embryo by a stalk-like prolongation is called the dermal navel or umbilicus ; the corresponding place of attachment of the intestinal yolk-sac to the middle of the intestinal canal is the intestinal navel or umbilicus. 7. In Fishes the yolk-sac after resorption of the yolk-material, accompanied by the phenomena of shrivelling, is employed for the closure of the intestinal and dermal navels. 8. In Reptiles and Birds the extra-embryonic region furnishes, in addition to the yolk-sac, several other embryonic membranes, which complicate the development. CHAPTER XI. THE FCETAL MEMBRANES OF REPTILES AND BIRDS. As has already been stated, the course of development in all animals which do not deposit their eggs in water — in Reptiles, Birds, and Mammals — is unusually complicated, owing to the appearance of THE FCETAL MEMBRANES OF REPTILES AND BIRDS. 207 •special egg-envelopes (embryonic or foetal membranes). Some of them, according to their origin, are to be referred to the extra- •embryonic area of the germ- layers, and indeed to that part which in Fishes is employed for the yolk-sac. They arise from folds, which grow around the embryo while it is still small, ;and furnish a double envelope for it. The egg- envelopes (embryonic membranes) of Reptiles and Birds, which exhibit almost identical conditions, and the consideration of which we shall take up first, are more simply •constituted than those of Mam- mals. In the case of the former there are associated with the yolk-sac, in the possession of •which they agree with the Amphibia and Fishes, three additional embryonic appen- dages, the amnion, the mem- •brana serosa (or briefly $erosa)> and the allantois. They are partly laid down at an early period, at the time when the embryonic body is converted into tubes by the infolding of the germ-layers and is thereby con- stricted off from the yolk-sac. The Chick shall again serve as a basis for our description. Fig. 124.— Surface-view of the pellucid area of a blastoderm of a Chick of 18 hours, after BALFOUB. In front of the primitive groove, pr, lies the medullary furrow surrounded by the medullary folds. Immediately in front of these one sees a curved line, the head-fold, and in front of it a second curved line running concentric with it, the anterior fold of the amnion. 1. The Amnion, the Serosa, and the Yolk-Sac. The amnwn is a structure the appearance of which is recognisable remarkably early in the Chick. At the time when one recognises the semicircular head-fold at the anterior end of the incipient embryo (fig. 124), by the growth of which the head of the embryo is marked off, there is already present, at a short distance from it, a second fold running parallel to it. This is the anterior fold of the amnion, a 208 EMBRYOLOGY. product of the extra-embryonic part of the ectoderm and of the parietal mesoderm united with it. The two infoldings, which lie near to each other, have opposite Fig. 125. -Diagrammatic longitudinal section through the axis of an embryo Bird, after BALFOUR. The section represents the condition when the head-fold is already formed, but the tail-fold i» still wanting. F.So, Head -fold of the somatopleure ; F.Sp, head-fold of the splanchnopleure, forming at Sp the floor of the anterior part of the intestine. For the remaining references see fig. 122r p. 201. directions (fig. 125). While the head-fold (F.So) advances with its margin toward the yolk, the anterior fold of the amnion (Am), sepa- rated from it by the marginal groove, rises externally above the Fig. 126.— Diagrammatic longitudinal section through the posterior end of an embryo Chick at the time of the formation of the allantois, after BALFOUR. «p, me, hy, Outer, middle, and inner germ-layers ; ch, chorda ; Sp.c, neural tube ; n.«, neurenteric canal ; p.a.g, post-anal gut ; pr, remains of the primitive streak folded toward the ventral side ; al, allantois ; an, point where the anus will be formed ; p.c, perivisceral cavity ; am, amnion ; so, aomatopleure ; sp, splanchnopleure. plane of the blastoderm. At the time when the head is being formed, the amnion enlarges rather rapidly (Plate I., fig. 1 1 vaf), andgrows over and around the head in a cap-like fold, the rim of which is directed backwards. At the end of the second day of incubation it already THE F(ETAL MEMBRANES OF REPTILES AND BIRDS. 209 covers the anterior part of the head like a thin transparent veil, and is therefore called the cephalic sheath. In like manner, but at a somewhat later stage, there arise at the tail-end and at both sides of the embryo the posterior and lateral folds of the amnion. The posterior fold is still very inconspicuous even at the time when the head is covered with the veil-like pellicle (Plate I., lig. 11 haf). It enlarges slowly, and under the name of caudal sheath covers over the posterior end of the body (fig. 126 am). The lateral folds of the amnion are elevated externally to the lateral marginal grooves (fig. 127 ow), and project in the opposite direction from those lateral folds by the bending in of which the lateral and ventral walls of the embryo are produced. By this means the rim 14 210 EMBRYOLOGY. of the fold is carried farther and farther from the s^ilanchnopleure (sp), which remains spread out flat over the yolk. In this way the extra-embryonic part of the body-cavity, or the cavity of the blasto derm (KOLLIKEB), increases in extent in the vicinity of the embryo. When the lateral folds of the amnion have grown up to the dorsal surface of the embryo (Plate I., fig. 9 saf), they begin, by the bending over of their edges inedianwards, to form the so-called lateral sheaths. Inasmuch as the folds of the amnion, which are called by special names, become, when they are in full development, continuous, and are only parts of a single ring like fold, the embryo eventually becomes surrounded on all sides as though by a high wall. With further enlargement, the amniotic sheaths then bend together over the back of the embryo from in front and behind, and from the right and the left (Plate I., figs. 2, 3, and 10, a/, vaf, haf), come together with their edges in the median plane, and then fuse with each other along a line, the amniotic suture, which closes from in front back- wards (Plate I., fig. 10), except that at one very small place near the tail-end the closing is interrupted for a considerable time, and a small opening is preserved. The fusion of the amniotic folds takes place in the same manner as the fusion of the medullary folds described on page 79. Each fold (Plate I., figs. 3 and 1 0) consists of two layers, an inner and an outer one, which are continuous at the margins of the folds, and are separated by a fissure, which is a portion of the extra-embryonic body-cavity. At the amniotic suture the corresponding layers of the folds of both sides fuse, and hand in hand with this a separa- tion of the inner from the outer layers takes place (Plate I., fig. 4). As a result of this there have now arisen two envelopes over the back of the embryo, an inner and an outer one, the amnion (A) and the serosa (S). The amnion is the product of the inner layer of the folds (Plate I., fig. 10 ifb}. It forms a sac which immediately after its origin is closely applied about the embryo, and which encloses a very small amniotic cavity filled with fluid. The serous membrane (serosa), which is derived from the outer layer of the folds (a/6, Plate I., fig. 10), lies as a very delicate trans parent membrane closely applied to the amnion, and thus encloses the embryo in still another envelope. If we now glance back at the conditions described in the previous chapter, and compare the development of Fishes with that of Reptil«s THE FCETAL MEMBRANES OF REPTILES AND BIRDS. 211 and Birds, it is to be seen that a considerable complication has arisen in the case of the latter. Whereas in Fishes the extra-embryonic area of the somatopleure becomes exclusively the dermal yolk-sac, in Reptiles and Birds two sacs have arisen out of it by a process of folding. The influences producing this folding appear to be clear. Since the egg is enclosed in firmly applied envelopes, the embryonic body, when it is formed by the folding together of the germ-layers, cannot rise from the yolk-sac ; it therefore comes to lie in a depres- sion of the latter. There is the more reason for the occurrence of this because the embryo at the beginning of development is exces- sively small in comparison with the yolk, and because the yolk-layers immediately underlying it become liquefied and absorbed. With the sinking of the body into the yolk (Plate I., figs. 2 and 3), the parts which in Fishes become the simple dermal yolk-sac (Plate I., figs. 6 and 7) fold in around it on all sides as amniotic folds, and enclose it the more completely the deeper it sinks into the yolk. The preceding account of the development of the amnion is made some- what schematic in a single point. That is to say, the anterior fold of the amnion is developed so early, that the middle germ-layer has not yet been able to spread out as far as the anterior part of the embryonic area. The in- folding, therefore, in this region involves only the outer and inner germ-layers, uhich are still closely united. This condition is changed somewhat later, when the middle germ-layer has grown into the region of the anterior fold of •fehe amnion, and has there split into a visceral and a parietal layer. The process has not yet been followed out in detail in series of longitudinal sections. But at all events we must assume that the entoblast, which is united with the visceral middle layer, retracts from the anterior fold ef the amnion and again spreads out flat, as is represented in diagrammatic figure 11 (Plate I.). In this manner the anterior amniotic fold, which in the meantime has become greatly enlarged, now consists of the outer germ-layer and the parietal middle layer, as is the case from the beginning with the subsequently arising posterior and lateral folds of the amnion. We now have to enter still more particularly upon the further relations of amnion and serosa. Up to the end of embryonic development the amniotic sac remains in continuity with a small region on the ventral side of the embryo, which is called the dermal umbilicus. In figs. 3, 4, 5, and 10 (Plate I.) this place is indicated by means of a circular line (7m). Here the primitive layers of the body-wall are continuous with the corresponding layers of the amnion, as, for instance, the epidermis of the body with an epithelial layer lining the amniotic cavity. The dermal umbilicus of Reptiles and Birds corresponds therefore with 212 EMBRYOLOGY. the structure of the same name in embryo Fishes (Plate I., fig. 7 hn), for it is at this point that the dermal yolk-sac is continuous by means of its stem-like elongation with the walls of the belly. As in the Fishes, it surrounds an opening (Plate I., figs. 7 and 5 hn) which unites the portion of the body-cavity lying within the embryo (Ik1) with the extra-embryonic part lying between the embryonic membranes (lh2). Furthermore, the stalk of the yolk-sac or vitelline duct, which is continuous with the embryonic intestine, and which is indicated in the above-mentioned figures of Plate I. by the small circle dn, passes through the opening. The amniotic sac affords an additional special advantage to the embryos of Reptiles and Birds in that an albuminous saline fluid, the liquor amnii, collects in its cavity. In it the delicate, easily injured embryo composed of plastic cells floats, as it were, and is able to move. The amniotic sac is small at the beginning of its development, but enlarges with each day of incubation, since it keeps pace with the growth of the embryo and encloses a larger and larger amount of amniotic fluid. At the same time its wall becomes contractile. Certain cells in its somatic mesoderm develop into contractile fibres, which in the Chick give rise to rhythmic movements from the fifth day of incubation onward. One can observe these while the egg-shell remains intact, if one holds the egg toward a source of blight light, and for this purpose makes use of the ooscope constructed by PREYER. In this manner it can be. determined that the amnion executes about ten contractions in a minute, which, beginning at one pole, proceed to the opposite end, like the contractions of a worm. Thus the amniotic fluid is set in motion, and the embryo oscillates or rocks regularly from one end- to the other. The rocking of the embryo, as PREYER expresses it, becomes more and more obvious in the later days of incubation, since the contractions of the amnion become more L-ii i rgetic. The serosa (X) is a \\lmlly transparent, easily ruptuml membrane, which is closely applied to the vitelline membrane. It consists of two thin cell-layers, which take their origin from the outer germ-layer and the parietal middle layer, and like them are distinguished }»y blue and red lines in (lie diagram. The serous membrane is origin- ally present as a separate structure only in the region of the amnion and of tin- • mlirvo (Plate I., fig. 4), as far as the body-cavity is formt d in the middle ir.-nn-layer. It then enlarge- to tin- same extent as the fact p. 2/5 PLATE I. S»an Soanenschein & Co.Ltrf 1 THE FCETAL MEMBRANES OF REPTILES AND BIRDS. 213 yolk becomes overgrown and as the vascular area extends farther downwards. Parietal and visceral middle layers separate more and more from each other, until finally (in the Chick toward the end of incubation) a separation results over the entire periphery of the yolk- sphere. Figs. 3, 4, and 5, Plate I., show stages in this process. In the last figure, which represents the condition on about the seventh day of incubation, the extra-embryonic part of the body-cavity has already become very considerable ; the ssrous envelope is, with the exception of a small place at the vegetative pole of the yolk, every- where formed as a separate structure. In connection with this the wall of the yolk-sac also becomes changed. Whereas at the beginning of the overgrowth it embraces for a considerable distance all the germ-layers, after the separation of the serosa it is composed exclusively of entoderm and the visceral middle layer. EXPLANATION OF THE FIGURES ON PLATE I. Figs. 1-5 are diagrammatic representations of cross and longitudinal sections through the Hen's egg at different stages of incubation. They are intended to illustrate how the body of the Chick is developed out of the embryonic funda- ment, and how the yolk-sac, the amnion, the serosa, and the allantois arise out of the extra-embryonic area of the germ-layers. For the sake of clearness the embryonic fundament, and later the embryo, are represented much too large in relation to the yolk. In order more easily to distinguish the different parts from one another different colors have been selected for them. The yolk is represented in yellow, the entoderm green, the outer germ-layer blue, and the middle germ- layer, together with the mesenchyme, red. The black clots indicate the limit to which the outer and inner germ-layers have grown over the yolk in the different stages ; the red dots mark the boundary for the time being of the middle germ-layer, which after the development of the blood-vessels ends in the sinus terminalis. The references apply to all of tlie figures. at, Outer yer m-layer (blue). I dg, Vitelline duct. iiiw, Medullary ridges or folds. N, Neural tube. ftf, Amniotic fold. vaf, haf, saf, Anterior, posterior, and lateral amniotic folds. A, Amnion. nil, Amniotic cavity. S, Serous membrane (Serosa). hn, Dermal umbilicus. #/, Lateral folds. */1, A/2, Head-fold ; afb, ifb, ai, AUantois. ds, Intestinal sac. dn, Intestinal umbilicus. mk, Middle germ -layer (red). mk ', Parietal lamella of the same or parietal middle layer. mk a, Visceral lamella of the same or visceral middle layer. st, Lateral limit of the same, sinus terminalis, marginal vein. outer and inner limbs of fold. I dm, vm, Dorsal and ventral mesenteries. ik, Inner germ-layer (green), to-, Its margin of overgrowth. ilr, Intestinal groove. th, Body-cavity. lh\ Embryonic, /A", extra- embryonic part of the same. 214 EMBRYOLOGY. Fig. I.— Cross section through a Hen's egg on the second day of incubation. The germ-layers are spread out flat over the yolk ; the middle one is less extensive than the other two. The first blood-vessels have developed, and terminate with the marginal vein (st) at the edge of the middle germ-layer. One now distinguishes therefore the vascular area, which extends to the red dotted line (.tf), and external to it the yolk-area (dh), which terminates with the black dotted line (ur~), the edge of overgrowth of the outer and inner germ-layers. Fig. 2. — Cross section through a Hen's egg on the third day of incubation. The outer and inner germ-layers are spread out over half of the yolk. The yolk-area (rf/<) terminates with the black dotted line (w), the edge of overgrowth. The middle germ-layer, with the vascular area, which is now well developed, has also grown over the yolk as far as the line st (the sinus terminals). In the middle germ-layer the body-cavity has become distinct in the embryonic region (Ik1) and in its immediate vicinity (IW), the parietal (mkl) and visceral middle layers (ink1) having separated from each other. The embryonic fundament begins to be constricted off from the extra- embryonic part by a process of folding and to constitute the trunk. The lateral folds (sf) have grown downwards for a certain distance, thus giving rise to the lateral walls of the trunk, whereas ventrally the body is still open. Corre- sponding to these lateral folds (*/), the lateral intestinal folds (i I.IKI:H figures 1 to 4 the embryo is represented in longitudinal section. U) Ovum with zona pellucida, blastula, embryonic area, and embryonic fundament. (2) Ovum in which the yolk-sac and the amnion are beginning to develop. (3) Ovum in which, by the fusion of the amniotic folds, the amniotic sac and the serous mem- brane are formed, and the allantois makes its appearance. (4) Ovum with serous membrane, which has developed villi, with a large allantois and an embryo, in which the oral and anal openings have arisen. (>} I >iagrammatic representation of a young human ovum, in which the vascular layer of the ;il lautois has become applied to the serous membrane on all sides, and has grown into its villi. The serous membrane from this time forward takes the name of chorion. The cavity THE FCETAL MEMBRANES OF MAMMALS. 227 before us in the surface-view in figure 130. It is ovate, and shows- the primitive streak (pr) in the posterior half, and in front of it a deep dorsal furrow (rj) ; the extra-embryonic part of the middle germ-layer can be designated as the vascular area (o), since the first traces of the formation of the vessels and the blood are noticeable in it. In the much further developed embryo figured in diagram 2 (at about the ninth day in the Rabbit) the middle germ-layer has spread out over about the third part of the blastula, and now encloses an easily distinguishable body-cavity, since the parietal and visceral middle layers have separated from each other in the embryonic as well as extra-embryonic regions. It extends as far as the place marked st, where the sinus terminalis is found as the outer limit of the now clearly defined vascular area. The embryonic fundament is in the act of being constricted off from the blastodermic vesicle. The head- and tail-ends of the embryo, by foldings of the separate layers, have been elevated from the area pellucida in the same way as in the Chick. As there, a cephalic and pelvic part of the intestinal tract (fore and hind gat) have arisen, with an anterior and posterior intestinal portal, which open toward the cavity of the blastodermic vesicle. At the same time occurs the development of the amnion, which was first recognised in the Mammalia by BAER and BISCHOFF. On the diagrammatic section one sees that the extra-embryonic body-cavity has become very capacious, in that the outer germ-layer with the closely applied parietal middle layer has risen up in the vicinity of the embryo and formed itself into the folds ks and ss. The anterior fold of the amnion (ks) has bent over the head, and the posterior fold (ss) over the tail. The two sheaths lie so close to the embryo in the Mammalia, that in looking from the surface they are not easily recognised, especially as they are extraordinarily transparent. On the third diagram the amniotic folds have greatly enlarged, and have grown toward each other over the back of the embryo till their of the allantow has diminished and the yolk-sac has become very small, but the amniotic cavity is in the act of increasing. d, Vitelline membrane (zona pellucida) ; d', villi of the same ; sh, serous membrane [serosa] ; ch, chorion ; ch.z, villi of the chorion ; am, amuion ; ks, ss, cephalic and caudal folds of the amnion ; a, outer germ-layer ; a', the same in the extra-embryonic region of the blastula ; HI, middle germ-layer ; TO', the same in the extra-embryonic region ; dd, inner germ-layer ; i, the same in the extra-embryonic region ; df, vascular area ; st, sinus terminalis ; kh, cavity of the blastula, which later on becomes the cavity of the yolk sac (ds) ; dg, stalk of the yolk- sac (vitelline duct) ; al, allantois ; e, embryo ; r, space between chorion and amnion. extra- embryonic part of the body-cavity, filled with albuminous fluid ; vl, ventral body-wall ; hh, pericardial cavity. 228 EMBRYOLOGY. edges are in mutual contact. The closure of the sac takes place in a somewhat different manner from that of the Chick. Instead of meeting in a longitudinal suture, the edges of the amniotic folds meet, in the Rabbit at least, approximately in the middle of the back in a small spot, where for a considerable time a circular opening in the sac is retained. The outer layer of the amniotic fold, which in diagram 3 is still in connection with the amniotic sac at the point of fusion, but which later entirely separates from it, represents, as in the Chick, the serosa. It first appears as an independent structure in the vicinity of the embryo, whereas farther downwards it is still firmly united with the entoblast, and together with it constitutes the wall of the original blastula, which is here only two-layered. In the third diagram, furthermore, we can recognise the first trace of the allantois (al), which grows out from the anterior wall of the hind gut in the manner already described (p. 217), and which in the Rabbit is seen as early as the ninth day in the form of a small, pedunculated, exceedingly vascular sac. The fourth diagram shows the development of the f oetal membranes much further advanced. The prochorion has become ruptured by the distension of the entire blastodermic vesicle, and is 110 longer recognisable as a separate membrane. What we see on the outside is the serosa, which has been changed in a striking manner. In the first place, it has become completely detached from the amnion ; however, it should be remarked in this connection that in certain Mammals, and especially in Man, a stalk uniting the two membranes is retained for a considerable time at the amniotic suture. Secondly, the serosa is everywhere separated from the yolk-sac, and loosely surrounds the embryo and its remaining membranes as a thin sac. This condition has been brought about in the following manner : the middle germ-layer, which in diagram 3 had grown over only one half of the original blastula, has now spread over the other half also, and has become divided into its two layers. By this means the extra- embryonic part of the blastula is now completely split, as in the Chick, into an outer sac, the serosa, and the yolk-sac, separated from it only by the body-cavity. Moreover, there exist in this respect differences among the Mammalia, since in some the serosa remains to a greater or less extent permanently united with the yolk-sac. This is the case, for example, in the Rabbit. In the Rabbit, in which the yolk-sac at first fills the greater part of the blastodermic vesicle, the middle germ-layer spreads out over that half of the THE FCETAL MEMBRANES OF MAMMALS. 229 yolk-sac only which is turned toward the embryo. There is developed in it a system of capillaries, which ends abruptly in a marginal vein. The other half of the yolk-sac is without vessels, and is everywhere firmly united with the serosa. When, after the resorption of its contents, the yolk-sac commences to shrivel, it begins to take on a mushroom-like form (fig. 133 ds\ owing to the folding in of the vascular half (fd) against the non. vascular part (ed"), which is fused with the serosa (*/<.). It remains united with the umbilicus of the embryo by means of an elon- gated intestinal stalk (or vitelline duct), which is com- parable to the stalk of the mushroom. The space (»•) which is produced in the blastodermic vesicle by the shrinking of the yolk-sac does not become filled out by compensating growths of the amnion (a) and allantois (al\ both of which remain small. There- fore a large amount of fluid collects between the separate foetal membranes. The space filled with fluid is none other than the extra-embryonic part of the body-cavity, which in the Babbit, as in no other Mammal, is highly developed. The allantois (al) hangs freely in this space as a stalked vesicle, a part of its surface having applied itself to that portion of the serosa (*/*) which is not united with the yolk-sac, and which is circum- scribed by the sinus termi- nalis (s£). It is gradually metamorphosed into an organ of nutrition for the embryo, the placenta inasmuch as it receives a rich supply of blood through the vessels of the allantois, the umbilical vessels. Subsequently the remaining surface of the blastodermic vesicle, over which the umbilical vessels do not extend, also becomes vascular. This is due to the fact that the albuminous fluid still contained in the mushroom-like yolk-sac becomes entirely absorbed, and that consequently its outer non-vascular and inner, invaginated vascular walls come to lie on each other and to fuse into a single membrane. In this manner the blastodermic vesicle in the Kabbit becomes provided with blood on its entire surface, but from two different sides — the placental portion from the vessels of the allantois, and the larger part of the surface from the degenerating vitelline vessels. In regard to the formation of the amnion in the Eabbit, upon which VAN BENEDEN ET JULIN have made very thorough investigations, it is to be added Pig. 133.— Diagrammatic longitudinal section through the ovum of a Rabbit at an advanced stage of pregnancy, after BISCHOFF. e, Embryo ; a, amnion ; u, urachus ; al, allantois with blood-vessels ; sh, subzonal membrane ; pi, villi of the placenta ; fd, vascular layer of the yolk-sac ; ed, ento- blast of the yolk-sac ; ed', ed", inner and outer lamellae of the entoblast which lines the flattened cavity of the yolk-sac ; ds, cavity of the yolk-sac ; st, sinus termi- nalis ; r, the space between amnion, allaiitois, and yolk-sac that is filled with fluid. 230 EMBRYOLOGY. that the middle germ-layer is wanting in the region of the anterior amniotic fold to a greater degree in this case than in the Chick. The anterior amniotic fold therefore consists during a considerable period of only the two primitive germ- layers, closely joined together. VAN BENEDEN has therefore given to the cephalic sheath, as long as the inner germ-layer takes part in its formation, the name of proamnion. Later on, however, a separation of the amnion from the entoblast takes place also in the head-region in the Rabbit. Finally, in our fourth diagram, still a third change has appeared in the serosa. By rapid growth of the epithelium large numbers of small evaginations or villi have arisen on its outer surface. On this account the name of chorion or villous layer has been applied to it when these changes have been completed. It should also be added here that in the development of the villi uniformity among all Mammals by no means prevails. In the lowest orders (Monotremes, Marsupials) the surface of the blastodermic vesicle remains almost smooth, as in Reptiles and Birds. In them, therefore, the serosa is permanently retained during embryonic life, whereas in other Mammalia it is transformed into a villous membrane. By reason of these differences KOLLIKER has divided Mammals into Mammalia achoria and Mammalia choriata. On the other embryonic membranes of fig. 132, 4, it is principally changes in size only that have been effected. The yolk-sac (ds), over the entire surface of which the vitelline vessels now spread, has become considerably smaller, and is continuous with the embryonic intestine by means of a long slender stalk, the vitelline duct (dg). The amniotic sac (am) has already enlarged and is filled with fluid, the liquor amnii Its walls are continuous at the umbilicus with the ventral wall of the embryo. The allantois (al) has become a vascular pear-shaped sac, which has grown out between the dermal stalk and umbilicus into the extra-embryonic part of the body-cavity, and soon after reaches the serosa. The accurate representation of an embryo Dog of twenty-five days (fig. 134) affords us, better than the diagram (fig. 132, 4), a view of the connection of the two vascular sacs, the allantois and yolk-sac, with the intestinal canal. The embryo is removed from the chorion and amnion. The ventral belly-wall is partly removed, and thereby the dermal um- bilicus, which about this time has become rather narrow, has been destroyed. The intestinal canal, now to be seen in its entire length, is already converted throughout into a tube (d) ; near its middle it is continuous, by means of a short vitelline duct, with the yolk-sac (ds), THE FCETAL MEMBRANES OP MAMMALS. 231 which was cut open in the process of preparation. The allantois (al) is attached to the very end of the intestinal canal by means of the attenuated stalk-like urachus. Up to this stage the correspondence in the development of the embry- onic membranes in Mammals, Birds, and Reptiles is clear. But from now on the course of development in the Mammalia becomes more and more divergent, since one portion of the embryonic membranes Fig. 134.— Embryo Dog of 25 days, extended and seen from in front. Magnified 25 diameters. After BISCHOFF. d, Intestine ; ds, yolk-sac ; al, allantois, urinary sac ; un, primitive kidney ; I, the two lobes of the liver, -with the lumen of the omphalomesenteric vein between them ; ve, fie, anterior and posterior appendages ; h, heart ; m, mouth ; au, eye ; g, olfactory pit. enters into closer relations ivith the mucous membrane of the uterus, and is thus converted into an organ of nutrition for the embryo. In this manner a compensation is provided for the loss of the yolk. The interesting adaptations for intra-uterine nutrition — they have been studied especially by the English anatomist TURNER in a series of profound comparative-embryological works — present very great differences in the separate orders of Mammalia : sometimes they are of a simple kind, at other times they are more com- 232 EMBRYOLOGY. plicated organs, which have been designated as the after-birth, or placenta. Since a knowledge of them will facilitate our compre- hension of the human placenta, we shall consider them somewhat at length. It is most expedient to distinguish three different modifications in the U'ay in ivhich the surface, of the blastodermic vesicle comes into relation with the mucous membrane of the uterus, and accordingly to divide the Mammals into three groups. In one the serosa is retained nearly in its simple primitive condition, In the second it is transformed into a villous layer or cJwrion, and In the third a placenta arises out of one or more portions of the chorion. To the first group belong, among the Mammalia, only the Mono- tremes and the Marsupials, whose embryonic membranes are in the main constituted like those of Birds and Reptiles. Ordinarily in the Marsupials the serosa retains its smooth surface. Inasmuch as it lies in close contact with the vascular mucous membrane of the uterus, it can absorb nourishment from the latter and transmit it to the deeper-lying embryonic parts. In the second group of Mammals an improvement in the intra- uterine nourishment is effected by important changes in the organisa- tion of the serosa, which is converted into a villous layer or chorion. In the first place, it is provided with blood-vessels by the allantois, which grows out into contact with it, and whose connective-tissue layer, containing the ramifications of the umbilical vessels, grows over its entire inner surface. Secondly, the epithelial membrane begins to grow out into folds and villi, into which there soon penetrate vascular outgrowths of the connective-tissue layer. By this process a larger resorbing surface is provided. Thirdly, the mucous membrane of the uterus and the chorion unite more intimately and firmly with each other, while the former also increases its surface and acquires pits and depressions into which the processes of the latter penetrate. All these changes have simply the purpose of facilitating and rendering more perfect the interchange of materials between the • •s of the mother and those of the offspring. We meet with membranes thus constituted in the Suidse, the Perissodactyla, Hippopotamidse, Tylopoda, Tragulidse, Sirenia, and Cetacea. In the Pig, which shall serve as an example, the blasto- dermic vesicle, in adaptation to the form of the uterus, is transformed into a spindle-shaped sac. The inner embryonic appendages, the- THE FCETAL MEMBRANES OF MAMMALS. 23o yolk -sac and allantois, are also drawn out in the same manner into two long tapering ends. On the entire surface of the chorion, with the exception of the two ends of the sac, there have arisen rows of very vascular pads, which radiate from separate smooth round spots of the membrane,, and are covered at their edges with small simple papillae. The mucous membrane of the uterus is exactly fitted into the elevations and depressions of the chorion. There are also found on it circular smooth places similar to those of the chorion, which are further noteworthy from the fact that it is only on them that the tubular uterine glands open out. At birth the interlocking surfaces of contact separate from each other without any loss of substance oni the part of the mucous membrane of the uterus ; for the pads and small papillae are easily withdrawn from the depressions which serva for their reception. In the third group a special organ, the placenta, or after-birth,, has been developed for the purpose of intra-uterine nutrition. Its origin was brought about by separate portions of the chorion having assumed different characters, owing to the unequal size and distri- bution of the villi. One part exhibits a condition in which the villi are entirely gone or much stunted, so that the surface of the membrane feels smooth ; moreover, it possesses few blood-vessels or is entirely destitute of them. Another part of the chorion contains, closely packed together, villi which are extremely long and covered with many ramifying lateral branches ; furthermore, it receives large blood-vessels, which approach the tufts of villi and distribute their terminal capillaries to the finest lateral ramifications of the latter; finally, it has entered into the most intimate relations with the mucous membrane of the uterus. Wherever the latter comes in contact with the tufts of villi it is much thickened, very vascular, and in a state of active growth. It encloses numerous branched cavities of varying size, into which the villi of the chorion exactly fit. The entire structure is called a placenta, in which the part of the chorion which is covered icith villi is distinguished as the placenta fcetalis, and the part of the mucous membrane of the uterus which is united with and adapted to the latter as the placenta uterina. Both parts together constitute an organ for the nutrition of the embryo. The term placenta has often been extended to the kind of chorion: which is evenly covered with small villi, such as exists in the Suidse, etc., and the designation of diffuse placenta has been created 234 for it. EMBRYOLOGY. But in the interest of a more precise definition it is advisable ^ » to use the name i f* only in the re- stricted sense in which it has been employed / Tig. 135a.— Uterus of a Cow laid open, in the middle of the period of gestation. From BALFOUR, after COLIN. V, Vagina ; U, uterus ; Ch, chorion ; C1, cotyledons of the uterus ; C3, total membrane or chorion only. The forma- tion of the pla- centa presents in its details important mo- difications. Runii- 3, in which the blastoder- mic vesicle is drawn out into two tips, as in the Pig, present a special type (fig. 135a). On their chorion (Ch) have been developed very many small foetal placentae (C2), which here are also called cotyledons. The number of the latter is ex- ceedingly vari- able in the different SDe- Fi*g 135b-~ Cotyledon of a Cow, the foetal and maternal part* half detached from each other. After COLIN, from BALFOUR. Uterus ; Cl, maternal part of the cotyledon (placenta uterina) ; cies, from sixty to one hundred in the Sheep and Cow, and only from five to six in the Doe. They are united with Ch, chorion of the embryo; C". total part of the cotyledon (chorion frondosum or placenta totalis). THE FOETAL MEMBRANES OF MAMMALS. 235 Corresponding thickenings of the uterine mucous membrane, the placentre uterinae ((71), though only in a loose manner, so that a little pulling is sufficient to produce a separation, and to draw the chorionio villi out of the depressions which serve for their reception, as one draws the hand out of a glove. In fact, in the preparation which serves as the basis of our figure 135a the cotyledons of offspring and mother (C2 and Cl) are separated from each other, since the uterus { U) has been opened by means of an incision and drawn back from the chorion (Cli) for a little distance. Figure 135b shows a single cotyledon of figure 135a somewhat larger than the natural size. The wall of the uterus (u) is drawn back a little from the choriori (Ch). As a result of this, the maternal •(Gl) and foetal parts (C2) of the cotyledon are partially separated from each other. On the placenta uterina (C1) one perceives many small pits, on the placenta fo3talis (C2) the closely packed dendritically branching chorionic villi, which have been withdrawn from the pits. As the diagrammatic section figure 136 teaches, the foetal and maternal tissues abut immediately on each other. The villi are covered with flattened cells, and the depressions of the mucous membrane are lined with cylindrical cells ; the latter develop within them granules of fat and albumen ; they disintegrate in part, and thereby contribute to the formation of a milky fluid, the so-called uterine milk, which can be pressed out of the placenta uterina and serves for the nutrition of the foetus. It is to be noticed also that in the Ruminants the uterine glands have openings on the mucous membrane only between the cotyledons. In all other Mammals that are provided with a placenta the intergrowth of the foetal and maternal tissue is still more intimate. At the same time there is formed in this way such a close union, that a separation of the chorion without injury to the mucous membrane of the uterus is now no longer possible. At birth therefore a more or less considerable superficial layer of the mucous membrane of the uterus is cast off with the foetal placenta. The part that is cast off is called the caducous membrane, or the decidua. In accordance with HUXLEY'S proposal, all Mammals in which, in consequence of the special growth of the placenta, such a membrane is formed are now grouped together as Mammalia deciduata, or briefly Deciduata, in contradistinction to the remaining Mammals — the Indnciduata, the formation of whose placentae has just been discussed. 236 EMBRYOLOGY. In the Mammalia with a decidua we must distinguish two sub- types of placenta, a ring-like and a disc-like, a placenta zonaria and a placenta discoidea. The placenta zonaria is characteristic of the Carnivora. The blastodermic vesicle in this case generally has the shape of a cask. With the exception of both poles, which retain a smooth surface, the chorion is covered with numerous villi arranged in a girdle-shaped zone ; the villi are furnished with lateral branches, like a tree. The branched villi of the chorion sink into the thickened mucous Fig. 136.— Diagrammatic representation of the finer structure of the placenta of a Cow, after TURNER. F, Foetal, M, maternal placenta ; V, villus ; e, epithelium of the chorionic villus ; e', epithelium of the maternal placenta ; d, foetal, d', maternal blood-vessels. Fig. 137.— Diagrammatic representation of the finer structure of the placenta of a Cat, after TURNER. Explanation of letters as in fig. 130. membrane of the uterus in various directions, so that in sections there arises the appearance of an iwegular interlacing (fig. 137). However, according to the concurrent accounts of TURNER and ERCOLANI, there is no penetration into the uterine glands in this case, any more than in the case of the Indeciduata. The epithelium (e') of the maternal mucous membrane (M ) persists and forms a boundary between the villi (F) and the nuiternal blood- vessels (d1), which latter have enlarged to cavities from three to four mites as ivide as the foetal capillaries (d). This enlargement of the THE FCETAL MEMBRANES OF MAMMALS. 237 maternal blood-passages is full of significance for the formation of the placenta in the Deciduata as opposed to that of the Indeciduata. The second form, the discoid placenta, is characteristic of the Rodentia, tho Insectivora, the Chiroptera and Prosimise, the Apes and Man. Here the portion of the chorion devoted to the formation of the placenta is small ; but in compensation for this the tufts of villi (fig. 138 V) are very highly developed; the union between placenta uterina (M) and placenta foetalis (F) is most in- timate • the maternal blood-spaces (d1), in the case of the Apes and Man at least, are, as no- where else, enormously distended, so that the villi of the chorion (V) appear to sink directly into them and to be bathed immediately by the maternal blood. Since we shall occupy ourselves more at length in the next chapter with the human pla- centa, which belongs to this type, these few remarks may suffice for the time being. I close this section with a reference to the high systematic signifi- cance of the embryonic accessory organs of Ver- tebrates. They present, as we have seen, such great and striking dif- ferences in the separate classes, that the utilisa- tion of them for systematic purposes which has been made by MILNE- ED WARDS, OWEN, and HUXLEY was natural. All lower Vertebrates, Amphioxus, Cyclostomes, Fishes, Dipnoi Fig. 138. — Diagrammatic representation of the finer struc- ture of the human placenta according to the hypothesis of TURNER. F, Foetal, M, maternal placenta; e', epithelium of the maternal placenta; d, foetal, d', maternal blood- vessels ; V, villus ; ds, decidua serotina of the human placenta; t, t, trabeculse of the serotina running to the foetal villi ; ca, convoluted artery which sinks into the blood-space d' ; up, one of the utero-placental veins conveying blood from the latter; x, a continuation over the villus of maternal tissue— lying outside the epithelial layer e' — which represents either the endo- thelium of the maternal blood-vessels or a delicate connective tissue pertaining to the serotina, or both together. The layer e' consists, at all events, of ma- ternal cells derived from the serotina. The foetal epithelial layer is no lonper to be seen on the villi of the completely formed human placenta. 238 EMBRYOLOGY. and Amphibia, either possess no accessory organs at all, or only an evagination of the intestinal tube, the yolk-sac. The embryos of Reptiles, Birds, and Mammals, on the contrary, are further enclosed in two fugitive membranes characteristic of embryonic life, the amnion and serosa. They have therefore been grouped together as amniotic animals or Amniota, and the classes first mentioned have been contrasted with them as non-amniotic animals or Anamnia. Among the amniotic animals a further separation into two groups can be made : on the one side are the egg-laying Reptiles and Birds, which HUXLEY unites into the Sauropsida ; on the other side Mamnjals, in which (with the exception of the Monotremes) the eggs develop in the uterus, and the young are further nourished after birth by the secretions of milk-glands. In the Mammalia the foetal membranes, inasmuch as they unite with the mucous membrane of the uterus to form an organ of nutrition, take on a still more complicated character, and present modifications which in turn can readily be utilised for systematic purposes. In Monotremes and Marsupials the outer embryonic membrane retains an almost smooth surface, as in Reptiles and Birds ; in all other Mammals there arise on the surface of the chorion villi, which grow into the maternal mucous membrane. OWEN has designated the one as Implacentalia, the other as Placentalia. The terms Achoria and Choriata introduced for these by KOLLIKER are better. In the Choriata the union of the villi witn the mucous membrane is either loose or firm ; corresponding to this there is either no detachable layer of the mucous membrane of the uterus formed, no decidua, or such a structure arises as the result of close inter- growth of the placenta uterina and placenta fcetalis. Thus we have the Mammalia indeciduata and the Mammalia deciduata. In each division there are again two sub -types in the formation of villi. In the Indeciduata the villi are either evenly distributed over the surface, or they are united into more or less numerous groups (placentae or cotyledons), which are separated from one another by smooth tracts of the chorion. In a part of the Deciduata the placenta is girdle -shaped, in another part disc-shaped. SUMMARY. 1. In the Mammalia there is developed, in the same way as in Reptiles and Birds, a yolk-sac, an amnion, a serosa, and an allantois. 2. Excepting in the Monotremes and Marsupials, the serosa is metamorphosed into a chorion, in that it puts forth villi, and in that THE FCETAL MEMBRANES OF MAMMALS 239 the connective-tissue layer of the allantois, which is provided with the umbilical blood-vessels, spreads out on its inner surface and penetrates into the villi. 3. In a part of the Mammalia certain regions of the serous membrane, where the villi grow more vigorously and put forth lateral branches, and sink into corresponding depressions of the mucous membrane of the uterus, are converted into a placenta (when many of them have arisen on one chorion they are called cotyledons).. 4. On the placenta one distinguishes : — (a) A placenta foetalis, i.e.. that part of the chorion which has: developed the tufts of villi. (b) A placenta uterina, i.e., that part of the mucous membrane of the uterus which has proliferated and is provided with, depressions for the reception of the placenta foetalis. 5. Foetal and maternal parts of the placenta can become more firmly united with each other ; the result is that at birth a larger or smaller tract of the mucous membrane of the uterus is also cast off, and is known as the caducous membrane, or the decidua. 6. According to the character of the embryonic membranes, the- following divisions of Vertebrates may be established : — I. Anamnia, animals without an amnion. (Amphioxus, Cyclostomes, Fishes, Amphibia.) II. Amniota, animals with an amnion (with yolk-sac, amnion,. serosa, and allantois). A. Sauropsida. Egg-laying, amniotic animals. (Reptiles and Birds.) B. Mammalia. In all of them, except the Monotremes, the- eggs are developed in the uterus. (a) Achoria. The serosa develops no villi, or only a few. (Monotremes, Marsupials.) (b) Choriata. The serosa becomes the villous membrane* (chorion). '(1) With evenly distributed villi. (Perissodactyla, Suidse, Hippopotamidse, Tylopoda,. MammahaJ TraguiicUe, Cetacea, etc.) n~ (2) Placentalia. The serosa is at intervals metamor- phosed into a placenta. a. Numerous cotyledons. (Ruminantia.) C ft. Placenta zonaria. (Carnivora.) Mammalia I Placenta discoidea. ([Man,] Apes,. Rodents, Ih- deciduata. I ,. -D 4. \ sectivores, Bats.) 240 EMBRYOLOGY. LITERATURE. Beneden, van, et Charles Julin. Recherches sur la formation des annexes foetales chez les Mammiferes (Lapin et Cheiropteres). Archives de Biologic. T. V. 1884. Caldwell, W. H. Eierlegen der Monotremen. Referat in Schwalbe's Jahresbericht, p. 507. 1886. Caldwell, W. H. On the Arrangement of the Embryonic Membranes in Marsupial Animals. Quart. Jour. Micr. Sci. Vol. XXIV. p. 655. 1884. Edwards, Milne. Lecons sur la physiologic et 1'anatomie comparee de 1'homme et des animaux. Paris 1870. Eschricht. De organis quae nutrition! et respiration! foetus mammalium inserviunt. Hafniae 1837. Godet. Recherches sur la structure intime du placenta du lapin. Inaugural Dissertation. Ncuveville 1877. Haacke, W. Meine Entdeckung des Eierlegens der Echidna hystrix. Zool. Anzeiger, p. 647. 1884. Hoffmann, C. K. Ueber das Amnion des zweiblatterigen Keimes. Archiv f. mikr. Anat. Bd. XXIII. p. 530. 1884. Kblliker. Entwicklungsgeschichte des Menschen und der hoheren Thiere, pp. 261-3 and 360, 361. 1879. Mauthner, Julius. Ueber den miitterlichen Kreislauf in der Kaninchen- placenta mit Rucksicht auf die in der Menschenplacenta bis jetzt vorge- f andenen anatomischen Verhaltnisse. Sitzungsb. d. k. Akad. d. Wissensch. Math.-naturw. Classe. Bd. LXVII. Abth. 3. 1873. Milne-Edwards. See Edwards, Milne. Osborn, H. F. Observations upon the Fcetal Membranes of the Opossum and other Marsupials. Quart. Jour. Micr. Sci. Vol. XVIII. 1883. Osborn, H. F. The Foetal Membranes of the Marsupials. Jour. Morphol Vol. I. 1887. Owen, R. Description of an Impregnated Uterus and of the Uterine Ova ot Echidna hystrix. Ann. and Mag. Nat. Hist. Vol. XIV. p 373. 1884. Slavjansky. Die regressiven Veranderungen der Epithelialzellen in dei serosen Hiille des Kanincheneies. Berichte iiber die Verhandl. d. k. sachsischen Gesellsch. d. Wissensch. Leipzig. Math.-phys. Classe. Bd. XXIV. pp. 247-52. 1872. Strahl, H. Die Dottersackwand u. der Parablast der Eidechse. Zeitschr. f. wiss. Zoologie. Bd. XLV. p. 282. 1887. Turner. On the Placentation of the Apes with a Comparison of the Structure of their Placenta with that of the Human Female. Philos. Trans. ];« London. Vol. CLXIX. Part I. 1878. Turner. Some General Observations on the Placenta with especial reference to the Theory of Evolution. Jour. Anat. and Physiol. 1877. Virchow, Hans. Ueber das Epithel des Dottersackes im Hiihnerei. Disser- t at ion. Ili'fHn. 1>7.~». Waldeyer, W. Die Placenta von Inuus nemestrinus. Sitzun;.r.-b. d. k. preuss. Acacl. d. Wissensch. Berlin. 1889. Numerous citations of the literature on the fostal membranes of Mammals axe to be found in Hoffmann : Grondtrekken der vergelijkende ontwikkelings- THE FCETAL MEMBRANES OF MAN. 241 CHAPTER XIII. THE FGETAL MEMBRANES OF MAN". THE investigation of the first stages in the development of man, which are accomplished during the first four weeks of pregnancy, is coupled with extraordinary difficulties. Only very exceptionally does the embryologist come into possession of young human ova, whether found in the uterus at the time of dissection, or coming into the hands of a physician as the result oFmiscarriage. In the latter case the ova have often been dead^ f or^ & long time in the uterus, and consequently are in process of decomposition. Finally, a good preservation and an accurate investigation of such small and delicate objects demand no slight degree of skill. This accounts for the fact that we do not possess in the case of Man a single observation upon the process of fertilisation or that of cleavage, upon the formation of the germ-layers, or upon the first establishment of the form of -the body, the fcetal membranes, and a large number of other organs.- Concerning this whole period we are dependent upon the conclusions which are furnished by the development of other Mammals. Thus we assume that fertilisation normally takes place in the enlarged beginning of the oviduct (Fallopian tube) ; that the seminal elements, which remain alive in the female sexual organs perhaps for days or weeks, here await the ovum as it emerges from the ovary ; that the ovum already segmented enters into the cavity of the uterus, attaches itself in the mucous membrane, and during the first weeks of pregnancy gives rise to the germ-layers, the outer form of the body, and the foetal membranes, according to the well-known rules for other Mammals. A little, although very scanty, information has been acquired, but this concerns only the second and subsequent week. A small number of ova have been described in the literature, which for the most part come from miscarriages, and the age of which has been estimated at from twelve to fifteen days. The blastodermic vesicles measured 5 to 6 mm. in diameter. Here belong two ova described by ALLEN THOMSON, and those by SCHRODER v. D. KOLK, HENNIG, REICHERT, BREUSS, BEIGEL UND LOWE, as well as the cases published by AHLFELD, KOLLMANN, FOL, and GRAF SPEE. Upon critical comparison of the discoveries, there are two facts which we can regard as established. First. At the end of the second week the blastodermic vesicle 16 242 EMBRYOLOGY. (blastula) no longer lies free in the cavity of the uterus, but is enclosed in a special capsule produced by the growth of the mucous membrane. Hitherto no one has had the opportunity to make observations concerning the formation of this capsule. Following an hypothesis of SHARPEY, which has been somewhat modified by PftCIS) Tig. 139.— Diagrammatic section through the gnvid human uterus, from WIKDEKSHEIM. U, Uterus ; UH, cavity of the same ; Tb, Fallopian tube ; Dv, decidua vera ; Dr, decidua v Pit, placenta uterina (decidua seroiina) ; Pf. p'.acenta fu'talis <>r elion,,:i fion,lo,um (Cii.t ) ; Chi, chorion laeve ; A (on black background), .-axity of the .-minion tilled with amniotie fluid ; D, yolk- (unibilical) vesicle ; in the embryo one sees the umbilii-al vessels (Al) ; t the liver traversed by the vera mnhilie.ali> ; 11. the heart; A, the aorta; ci and r«, the \>-t;a cava inferior and superior ; p, vena ix>rtarum. KEICHERT, it is now gem-mlly ns-uincd tliat the ovum upon its entrance into the uterus imbeds itself in a depression of the inm-ous membrane, which is tin-own into ridp-> and is in process of brin*,' nie(aniorp]:<^fd into tlu^ decidua. Tlie m.-ii-giiis of the depression goon grow around the hlastula on all sides, and fuse together tdt'orm i'u-tal cai Mile. The fusion lakes place at a point diametrically THE FCETAL MEMBRANES OF MAN. 243 opposite the attachment, and is described as resembling a cicatrix. It is destitute of blood-vessels, whereas these, as well as uteirne glands, are present in the remaining portion of the overgrowing mucous membrane. The blastula lies in this receptacle now, and even into the beginning of the second month, loosely enclosed ; after opening the capsule the blastula can be removed easily and without injury. Whereas in other Mammals only that part of the uterine mucous membrane which contributes to the formation of the placenta is cast off, in the case of Man there occurs a much more extensive ecdysis of the most superficial layer, namely, over the whole inner surface of the uterine cavity. Here, too, the part which is cast off is designated as deciduous membrane or decidua, and three regions are distinguish- able (fig. 139) — the part which is thrown around the blastula as decidua reflexa (Dr\ the part which forms the floor of the depression in which the ovum has established itself as decidua serotina (Pu), and the remaining portion as decidua vera (Dv). In the reflexa we become acquainted with a structure which in this complete form occurs only in the case of Man and the Apes, whereas beginnings of such a structure are also found in other groups, as, e.g., in the Carnivores. Since the foetal capsule does not at first completely fill the uterus, there remains between reflexa and vera a space filled with mucus. A second and in many respects astonishing result is, that even in very young and small blastodermic vesicles, as all discoveries agree in showing, a well-developed chorion with abundant villi is begun. The villi are either distributed over the whole surface of the ovum, or, as in REICHERT'S case (fig. 140 A and B), they leave two opposite poles of the blastula free. They attain a length of one millimetre, and in part have the form of simple cylindrical elevations ; in part they already possess lateral branches. At no place have they fused with the decidua. Like the chorion itself, they consist of two layers — of a superficial epithelial layer, derived from the serosa, concern- ing which AHLFELD and KOLLMANN have made very definite and reliable statements, and of a layer of embryonic gelatinous tissue, which extends into the axis of the villi and already appears to bear ' here and there blood-vessels. Unfortunately we have learned nothing from investigations of these youngest of nil human embryos concerning the structures within the chorion,— the remaining foetal membranes and the 244 EMBRYOLOGY. fundament of the embryo itself. Either the ova were already more or less pathologically altered, or the contents were considerably damaged in consequence of the method of preservation and by the preparation. At all events with other investigators one, I think, may conclude from the condition of the chorion that the embryo must have been in an advanced stage, in which germ-layers, yolk- sac, and amnion were already formed. This assumption is all the more reasonable, since well-developed embryos from blastodermic vesicles which were only a few milli- metres larger have been described by COSTE, ALLEN THOMSON, His, and others. In these cases the head-end of the embryo only is rather sharply differentiated from the yolk-sac, which is continuous with the fundament of the intestine throughout nearly its entire Fig. 140. — The human ovum at an early stage of development. A and B, Front and sit'e views of a human ovum of 12 to 13 days, figured by REICHERT. e, The part designated by REICHERT as embryonic spot. From QUAIN'S " Anatomy." C, An ovum of 4 to 5 weeks, show ing the general character of the villous membrane before the formation of the placenta. A part of the wall of the ovum is removed in order to show the embryo in situ. After ALLEN THOMSON, from KOLLIKER'S " Entwicklungsgeschichte des Menschen, etc." length. The neural canal is not yet closed, but the amnion never- theless is completely developed, and in fact lies almost in contact with the embryonal body; at its posterior end it is connected with the chorion by means of a short cord, which is connected with tin- fundament of the allantois and has been named the bellif-tttalk (Bauchstiel) by His. Also in the only slightly older embryo of COSTE (fig. 141) — in which the neural tube is closed, the body distinctly segmented (tif), the head provided with vixei-ai arches (vb), l:ehind the latter the- heart (/*) recognisable, and the yolk-sac (ds) further constricted off— a short belly -stalk (1st) is present. It is composed of the amnion (am1) drawn out to a point and of a connective-tissue cord, which ati-es from the ventral surface of the embryo out of the intestinal cavity of tke \ civic region, enclo.-es at it> atta< lied end a .-mall cavity am1 bit Sch - t-6 \ THE FCETAL MEMBRANES OF MAN. 245 (the allantois), and conducts the allantoic blood-vessels from the pelvic portion of the intestine to the chorion. This cord is a characteristic structure for the human embryo the significance of which is still in dispute. KOLLIKER and His have given somewhat different explanations of it. KOLLIKER brings the cord into relation with the development of the allantois. He makes the fundament of this important embryonic appendage arise, as in other Mammals, from the hind gut of the embryo, and approach the serosa as a thick vascular connective-tissue growth lined with a narrow, short epithelial tube, without previously de- veloping indde itself a large epithelial sac. He also main- tains that the connective- tissue part of the short allan- toic cord, or belly-stalk, grows around on the who^e inner side of the serosa, and into the epi- thelial villi. His regards as unwar- ranted "the assumption, in opposition to the actual state of affairs, that the human embryo at first separates itself from the part of the blasto- dermic vesicle which is employed for the chorion, and subsequently unites with it again by means of the fundament of the allantois." He does not admit that the fundament of the embryo in Man is ever wholly constricted off from the chorion, as in the remaining Mammals, and he recognises in the belly-stalk " the bridge of connection between the fundament of the embryo and the chorionic part of the original blastodermic vesicle, which has never been severed." According to him the allantois in the Tig. 141.— Human embryo with yolk-sac, amnion, and belly-stalk of 15 to 18 days, after COSTE, from His ("Menschliche Embryonen"). His has untwisted somewhat the posterior end of the body in com- parison with the original figure, in order to bring into view the right side of the end of the body, the left side being represented in COSTE'S fig. 4. The chorion is detached at am\ am, Amnion ; am1, the point of attachment of the amnion to the chorion drawn out to a tip ; bst, belly -stalk ; Sch, tail-end ; us, primitive seg- ment ; dg, vitelline blood-vessels ; ds, yolk-sac ; /*, heart ; vb, visceral arch. 246 EMBRYOLOGY. human embryo has nothing to do with the development of the belly-stalk. Neither of these two explanations seems to me entirely satisfactory. According to my view, the structure under consideration may be explained in a manner which is not only in complete harmony with the facts of the case, but also reconciles the views of KOLLIKER and His. As COSTE'S embryo appears to show, the origin of the belly-stalk is connected in the first place with a somewhat irregular formation of the amnion. It follows from the fact that the latter is drawn out posteriorly to a point (fig. 141 am1), the apex of which reaches to the chorion, that its closure in the human embryo takes place at the extreme posterior end of the body, and that at the same time a union with the chorion is retained at the place of closure. The fundament of the embryo therefore remains in connection with the chorion, not directly, as His maintains, but only indirectly by means of the amnion. In the second place, the allantois, the somewhat eccentric develop- ment of which in the case of Man is perhaps intimately connected with the above-mentioned peculiarity in the formation of the amnion, takes part in the formation of the belly-stalk. It is therefore proper in this connection to enter somewhat more fully into the allantois- question in Man, so actively discussed during the last decade. Since in other Mammals the allantois (fig. 142 al) has the form of a large stalked sac, which grows out from the navel till it comes in contact with the sero«a (sz), and carries to it, along with connective tissue, the umbilical vessels, attempts have been made ever and anon to discover such a structure in the case of human embryos also. The proof of its existence in Man appeared to be furnished by a premature embryo, on which KRAUSE described a spherical, sac-like allantois. The embryo of KRAUSE presented, however, in many respects such deviations from other known human embryos of the corre- sponding stage as to cause the statements to be accepted on the part of many persons with great reservation, and to permit the suggestion of His, that in this case it was not after all a human embryo. Upon critical examination of the facts relating to the question, I am likewise of the opinion that in the case of Man a stage of development ivith a free allantoic sac protruding out of the body-cavity is not reached. As results from the fine investigations of human embryos by His, the belly-stalk is found upon cro^s section to be composed of: — THE FCETAL MEMBRANES OF MAN. 24.7 (1) The pennant-like prolongation of the amnion ; (2) Beneath this, abundantly developed embryonic connective tissue ; (3) The fundament of the allantois, which has the form of a very narrow passage with epithelial lining ; (4) The umbilical blood-vessels, of which the arteries lie close upon the allantoic duct, while the veins run nearer to the amnion. To the question, How have these parts arisen ? that appears to me Fig. 143.— Diagram of ihe foetal membranes of a Mammal, after Tur.NER. pc, Zona pellucida with villi (prochorion) ; sz, serous membrane ; am, amnion AC, ammotic cavity; E, outer germ-layer; M, middle germ -layer ; H, inner' germ-layer ; UV, yolk-^ac (vesica umbilicalis) ; al, allantois; ALC, allantoic cavity. the most natural answer which permits of being harmonised with the known conditions in other Mammals. Now, such an agreement is possible upon the. folio wing assumption. Very early, when the hind gut begins to be formed, there arises on its ventral side as a fundament of the allantois a knob composed of many cells, and containing only a small evagination of the ento- dermic layer. The allantoic knob does not, however, grow free into the body-cavity, as in the remaining Mammals (fig. 142 al). but ex- tends along the ventral wall of the embryo, and, from the place where this is reflected off to form the amnion, along the ventral wall of the 248 EMBRYOLOGY. latter (fig. 141 am1) up to its place of attachment to the chorion. The evagination of the entodermic layer meantime becomes elongated into the narrow allantoic duct ; the more voluminous connective - tissue growth carries with it the umbilical blood-vessels to the chorion, then spreads itself out on the inner surface of the latter in the well-known manner, and penetrates into the villi of the serosa. The allantois, therefore, in its development, instead of growing out free to the serosa, makes use of the already existing connection between the latter and the embryo established by the pennant-like elongation of the amnion (am1). But this mode of development perhaps results from the fact that the posterior end of the embryo in Man, as fig. 141 shows, is closely attached to the serosa at the place of the amniotic suture, whereby the allantois has only a short distance to grow in order to reach the serosa. Finally, the early appearance of tfye allantois will become intel- ligible to us, if we remind ourselves that organs of great physiological importance have in general the tendency to an accelerated develop- ment, and that in the series of Mammals the provisions for the nutrition of the embryo by means of a placenta have become more and more complete. While there is still much obscurity about the first stages of Man's development, we possess more satisfactory insight into the changes which the embryonic membranes in Man undergo from the third week onward. From this point forward we shall examine each separate embryonic membrane by itself : first the structures that are developed from the blastodermic vesicle — (1) the chorion, (2) the amnion, (3) the yolk-sac ; then (4) the deciduse which are produced by the mucous membrane of the uterus ; and finally (5) the after-birth (placenta) and (6) the umbilical cord. 1. The Chorion. During the first weeks of pregnancy the whulf surface of the chorion is covered with villi (fig. 1325, p. 226, and fig. 140), and provided with terminal branches of the umbilical blood-vessels. After its growth has proceeded for a time uniformly, there begin to appear — from the beginning of the third month onward — differences between the part which lies directly against the wall of the uterus that is destined to become the decidua serotina and the remaining greater THE FCETAL MEMBRANES OF MAN. 249 part, which has become overgrown by the decidua reflexa (fig. 143). While on the latter the villi (»') cease to grow, on the former they increase enormously in size and take the form of long, and at the base thick, tree-like, branching structures («), which, united into tufts, project far beyond the surface of the membrane that bears them, and grow into pits of the ma- ternal mucous membrane (ds). This part, to which we shall dl^ ^^^ ^^m^ / give more par- ticular atten- tion at the time of inves- tigating the mature pla- centa, is there- fore distin- guished as chorion fron- dosum from the remaining larger part, the chorion Iceve or the smooth chorion. The expression " smooth chorion " is, strictly speaking, not quite applicable. Of the villi which are at first everywhere developed, some afterwards remain preserved on the chorion Iseve, especially in the vicinity of the placenta. They grow into the decidua reflexa, effecting a firm union with it (fig. 143 z'). At the same time a second distinction between chorion frondosum and chorion Iseve is developing. In the territory of the latter the blood-vessels arising from the umbilical arteries begin to dwindle, whereas the former becomes more and more abundantly supplied with blood-vessels, and finally alone receives the terminal distribution of the umbilical arteries. Thus the one region becomes destitute of vessels, while the other becomes extraordinarily vascular, and the nutritive organ for the embryo. Histologically the chorion Iseve, which upon examination from the Fig. 143.— Diagrammatic section through the gravid human uterus with contained embryo, after LOXGET, from BALFOUR. al, Stalk of the allantois ; nb, -umbilical vesicle ; am, amnion ; tli chorion ; ds, decidua serotina ; du, decidua vera ; dr, decidua reflexa ; I, Fallopian tube ; c, cervix uteri ; u, uterus ; z, villi of the foetal placenta ; 2', villi of the chorion Iseve. 250 EMBRYOLOGY. surface appears thin and translucent, consists of (1) a connective-tissue membrane, and (2) an epithelial covering, which is identical with the original serosa. The connective-tissue membrane possesses at first the character of embryonic mucous tissue, and exhibits therefore branched stellate cells in a homogeneous matrix. Subsequently the mucous tissue is con- verted, as at other places in the body, into fibrous connective tissue. The epithelium of the chorion consists in the first months, according to the statements of KASTSCHENKO and SEDGWICK MINOT, of two layers — a superficial one, in which no cell-boundaries are visible (protoplasmic layer), and a deeper one, in which the individual cells are distinctly separated. Additional particulars are given in the description of the placenta. The embryonic adjuncts enclosed within the chorion — the amnion and yolk-sac — undergo in Man during pregnancy the following changes. 2. The Ainnion. * The amnion (am) immediately after its origin lies close on the surface of the embryo (fig. 144), but soon becomes distended by the accumulation of fluid, the liquor amnii, in its cavity (fig. 1325). It increases to a much greater extent than in other Mammals, in which it is often found to be smaller than the allantoic sac (compare the foetal membranes of the Rabbit, fig. 133). Finally, in Man it Jills out the entire Uastodermic vesicle, since it everywhere applies itself (fig. 143 am) closely to the inner wall of the chorion (ch). Its wall is rather thin and translucent, and also consists, like the chorion, of an epithelial and a connective-tissue layer. The epithelium, derived from the outer germ-layer of the embry- onic fundament, lines the amniotic cavity within, and is continuous with the epidermis of the embryo at the dermal navel ; at the place of transition it is composed of layers ; but elsewhere it is a single sheet of pavement cells. The connective-tissue layer is thin and at the ntivcl continuous with the corium. The amniotic or fatal water is slightly alkaline, and contains about 1% solid constituents, among which are found albumen, urea, and grape-sugar. Its volume is greatest in the sixth month of pregnancy, in nl it often attains a weight of not less than a kilo [2*2 Ibs. avoir- /•. -]»ii:,'v l:i\i-r; JJf, musrulatiiiv »f tin? uterus: tr, fimni'l-^hapr.l mouths nf the m-iim- ulam's; '. iMilar^fil iv-iii'M : r.'.l'io-«l l>\ tin- wim'in- iti«.|is<)f til-' vn. \\ini; -'.iliuls. THE FCETAL MEMBRANES OF MAN. 255 months one still finds all cavities up to the mouth of the glands lined with a thin layer of cubical or flat epithelial cells. Likewise in the first stage there occurs in the inter-glandular tissue an active process of growth, especially in the upper compact layer. In this there are formed spheroidal structures, 30 to 40 /x in diameter, which have been called decidual cells by FRIEDLANDER. In many places they lie so close together that, as a consequence and because of their form, they appear very similar to an epithelium,. 256 EiMBRYOLOGY. They are also found in the spongy layer, but in the cords and septa they are more elongated and spindle-shaped. In the second stage, from the sixth month forward, in which the decidua vera becomes much thinner, and under the pressure of the growing foetus gradually diminishes from 1 cm. to 2 mm. in thickness, many regressive process.es take place in the individual parts that have just been described (fig. 147). The mouths of the glands, which caused the sieve-like condition of the inner surface of the decidua, become more and more difficult to see and finally disappear altogether. The inner compact layer (C) assumes a uniform, compact, lamellar condition, since by the pressure the cavities of the glands occupying it become wholly obliterated, and then by disappearance of the epithe- lium their walls become fused. In the spongy layer (Sp) the cavities of the glands (dh) persist. but, in consequence of the pressure, are converted into fissures, which are parallel to the wall of the uterus, and are separated by partitions which in comparison to earlier months of pregnancy have become very much thinner. The glandular cavities which are adjacent to the compact layer have lost their epithelium or exhibit cellular debris (de), swollen bodies, and a slimy mass pjrmeated with fine granules ; toward the uterine musculature, on the contrary, they possess a well- preserved epithelium of short cylindrical or cubical cells. (2) The decidua reflexa (fig. 148 Dr) exhibits close agreement in its structure with the decidua vera. That it has arisen from the latter by a process of foldinj may be inferred, as KUNDRAT has rightly maintained, especially from the circumstance that during the first months of pregnancy the mouths of uterine glands (glu}, at least at the place of transition to the vera, are found upon both its sur- faces. The mouths lead into fissures (glu) which are parallel to the surface of the reliexa and are lined with cuboidal epithelium. In the inter-glandular tissue there appear the same large, round i/criffnai- cells as in tin- veia. From the fifth month forward the space between vera and reflexa begins to disappear ; both membranes now, after loss of their epithe- lium, become firmly pressed together, and finally completely fusel with each other (fig. 147). By this process the reflexa, from which the glandular spaces disappear except in the transitional region, becomes so extraordinarily thinned that it constitutes [in sections] only a narrow band, occasionally £ mm. broad. A scpMratinu of the two membranes at the close of pregnancy THE F vena cava superior or ductus Cuvievi ; Sr' 8inus reuniens '> v^ vena umbiiicalis ; VI, left part of the ventricle ; Ho, auricle of he^t ; A diaphragm ; p.om, yeniomphaio- mesenterica ; Lb, solid fundament of the liver ; Lbg, hepatic duct. In the case of the Chick the oral pit is observable on the second day p • , ,. ,, ,. , ,, ,, of incubation, the front end of the embryonic fundament having a short time previously elevated itself as a •cephalic knob above the extra-em- bryonic part of the germ-layers. The rupture of the pharyngeal membrane takes place on the fourth day. In the case of an embryo Kabbit of nine days the pharyngeal membrane is not yet ruptured. His has studied in detail this early stage in Man on his embryo " Lg" the age of which he estimates at twelve days. In all amniotic Vertebrates the entrance to the oral pit (fig. 152 Mb) presents a very uniform condition and appears as a large five- * [It will be seen by an inspection of figure 158 that the longest straight line which can be drawn through the embryo connects the neck- and rump-regions. It is this distance which is designated as the neck, or neck-rump, measure- ment.] EMJUtTOLOCT. .sidled opening, which is surrounded by /«* ridge*. A knowledge of theae is of great importance in Studying the history of il* formation of tibef^ Of the five ridges one i* impaired, the /rwila/ or *M*0/r«fK4a/ broad, roujided projection which bounds the oral pit abor* . It* origin k connected with the development of toe cental nervous m, which reaches op to the anterior end of the cwbiyonic fiii*UnM*nf, wfaflfe it is devdopad into tbe cwcbnJ veticie* (fig. 163 ght cA. mA). yiw^ffyifM^p by ffM^ffg of a |^ffjg^t«^>n^| tarlim^ tlW frontal prooaae at tbic *t*ge, tb^refore, ntjcioaai A U/ge ca \ic y be- longing to tbe neoiml tube, and ba« tbe form of a vaade, wbieb i* oompoaed of tbree layers, tbe epidenni** a layer of meaencbyway and tbe tbkkened epitbebal vail of tbe neural tube. The primary oral cavity and tbe fundament of tbe brain are dasdy appoaed at tbe beginning of development : tbey are separated by only a tbin dbeat of t issue, witbin whose territory then is fnbmynufiUly formal, amomj other tiling*, the floor oftk* cranium. The four remaining ridges are paired Gtructurae which surround tbfr oral ^?F?ii£ upon its eidffi and below. The^e are produced by growths of tbe embryonic connective tissue, through which large blood-vasseU take their course. They are dkjtinguifihed aooording to their portions a* upper-fa* (maxillary) and /Uwr^w Xb6 former are on ^t^bfly ^Af^ in ffffff^*ffalfo contact with tbe frontal prooes^ from which they are aepa/mted by a groove only, tbe naao-optic furrow, which will be diflPiMSfld in a eubseq uent chapter, and which run* obliquely upward and outward to that region of the face in which tbe eye begins its dovnlopmnfit. The maxillary prooass is eaparated from tbe mandibular process by an tndaon which corre- sponds to the place of the future angle of the mouth. The two prommmr of either ade together form the pharyngeal arenas, or the membranous jaw-arches. Before the rupture of the pbarynga*! membrane the oral ana* bac •till deeper, but only in its upper part, whereas toward the mandibolar arch it shallow. Tbit condition is connected with cw*at*re* which in aU F well a* Selachian* aftect that part of the head which the brain-reticle* and lie* above the alimentary tvbe. For the front and of the head i* bent down toward the ventral «/i* of the embryo, and finally make* a right angle with the po*erior half of the bead (fig. 153). Gonaeqnently the place at which the ao-called «aferi«r <*pi*ht **mt*** ha* occurred, and at which tbe posterior and anterior halve* of the head bend into each other, ha* become an elefation, tbejMarMtoJ [orOTtfHfc^]rfMwN**(6che&. telhocber). SB. The latter ancloeat tbe middle brain-reticle <»A) the fml»m •nil ORGAN* in TIM IN M i, ,.i : .•III. «ne before . «s as a reM UK- :• .lembran* th&re i Of It r ill tll< ; Fig US say HI • c)i tfc« k«U of A Ohiok incubated *H*y», «f' •>f th« bvun ; v*, 11 • le ; ^M>, aqn«- .1 ...... .- KfWfl «-.-;.. . '• ix •««•< i. .., .... < (rt«noej^»Al< »i , oerebdltam ; of Am- 286 EMBRYOLOGY. aside the middle germ-layers, which extend into this region, and thus grow outward to the surface, where they unite with the epi- dermis. The latter now become depressed into furrows along the regions of contact (fig. 154), so that one can distinguish inner, deeper throat-pockets, and outer, shallower throat- or gill-furrows. The two are separated from each other for a time by a very thin clos- ing membrane, which consists of two epithelial layers, the epidermis and the epithelium lining the fore gut. The bands of substance which lie between the suc- cessive throat-pockets (figs. 154 and 157) are the mem- branous branchial, throat-, or visceral arches. They consist of an axis, which is derived from the middle germ-layer and the mesenchyma, and of an epithelial covering, which on the side toward the pharynx is furnished by the inner germ- layer, on the outside by the outer germ-layer. They are designated according to their sequence as the second, third, fourth, etc., visceral arches, inasmuch as the ridge which surrounds the mouth constitutes the first visceral arch. In all water-inhabiting Vertebrates which breathe by means of gills the thin epithelial closing plates break through between the v isceral arches, and indeed in the same sequence as that in which they arose. Currents of water therefore can now pass from the outside through the open clefts into the cavity of the fore gut and be employed for respiration, since they flow over the surface of the mucous mem- brane. There is now developed in the mucous membrane, upon both Miles of the visceral clefts, a superficial, close network of blood- capillaries, the contents of which effect an exchange of gases with tin- passing water. Moreover the mucous membrane becomes folded, for the increase of its respiratory surface, into numerous, close-set,. Fig. 154.— Frontal (reconstruction) section of the oro-pharyngeal cavity of a human embryo (Bl of His) 4*5 nun. long, neck measurement, from His " Menschliche Embryonen." Mag- nified 30 diameters. The figure shows four outer and four inner visceral furrows, with the closing plates at the bottom of them. In the visceral arches separated by furrows one sees the cross sections of the second to the fifth aortic arches. By reason of the greater development of the anterior visceral arches the posterior ones are already somewhat pressed inwards. THE ORGANS OF THE INNER GERM-LAYER. 287 parallel branchial leaflets, which are provided with the greatest abundance of capillary blood-vessels. In this manner the most anterior section of the alimentary canal, which lies immediately behind the head, has become converted into an organ of respiration adapted to life in water. The important differentiation of the alimentary canal into an anterior re- spiratory chamber and a following nutritive region is possessed by Vertebrates and Amphioxus in common with certain Invertebrates (Tunicates and Balanoglossus). Likewise in the case of the higher (amniotic) Vertebrates both inner and outer visceral furrows, together with the visceral arches separating them, are, as has already been stated, formed ; but here they are never developed into an actually functioning respiratory apparatus ; they belong consequently in the category of rudimentary organs. Upon the mucous membrane there arise no branchial leaflets; indeed the formation of open clefts is not always and everywhere achieved, since the thin epithelial closing membranes between the separate visceral arches are preserved at the bottom of the externally visible furrows. Upon this point, however, the opinions of the investigators who have been engaged in the study of the throat-region in late years are very dissimilar. Whereas His, BORN, and KOLLIKER maintain that the closing plate does not as a rule rupture, FOL, DE MEURON, KASTSCHENKO, LIESSNER, and others find that at least the first two or three visceral clefts are temporarily open. The opening takes place to a greater extent in Reptiles than in Birds and Mammals, where it remains limited to a small territory. In the most posterior visceral pockets there can be no breaking through, because they are not as deep, and the closing plate is therefore thicker and contains also a layer of connective tissue. The conditions in Reptiles and Mammals, as well as the differences in the number of visceral arches, to be mentioned directly, express separate stages in the process of regressive metamorphosis, to which the whole visceral apparatus in the vertebrate series has been subjected. The number of visceral clefts which actually appear in the separate classes of Vertebrates is variable. The greatest number is en- countered among the Selachians, where there may be as many as six (fig. 155), in a few species indeed seven or eight. In Teleosts, Amphibia, and Reptiles the number sinks to five. In Birds, Mammals, and Man (figs. 154 and 157) only four arise. We can therefore say in general that from the lower to the higher Vertebrates a reduction has taken place in the number of visceral clefts which 288 EMBRYOLOGY. make their appearance. In view of these phenomena, and guided by other comparative-anatomical considerations, many investigators have advanced the hypothesis that in the case of the ancestors of Vertebrates the fore gut has been pierced by a greater number of clefts than is now to be observed even in the Selachians, and further that degraded or metamorphosed remnants of them are still to be found in the head- and neck-regions. VAN BEMMELEN has observed in embryos of various Sharks and Skates out- pocketings of the lateral wall of the throat behind the last visceral arch, and has interpreted them as rudimentary visceral clefts, which no longer succeed in breaking through (fig. 155 nsd^). Subsequently there are developed out of them, by growth of the epithelium, glan- dular organs, the supra-pericardial bodies (BEMMELEN), which are similar in their structure to the thyroid gland. Also in the head-region, which lies in front of the first visceral arch, a reduction and a metamorphosis of clefts has, according to the opinion of various observers, taken place. DOHRN especially has propounded several hypotheses of this kind, for which, however, I do not find valid grounds : (1) that the mouth has arisen by the fusion of a pair of visceral clefts, (2) that the olfactory organs are to be referred to the metamorphosis of another pair of clefts, — a view which is also shared by M. MAR- SHALL and several others, — (3) that a dis- appearance of gill-clefts in the region of the sockets of the eye is to be assumed, and that the eye-muscles are to be inter- preted as remnants of gill-muscles. nsd Fig. 155.— Diagram of the development of the thymus, the thyroid gland, and the accessory thyroid glands, and their relations to the visceral pockets in an embryo Shark, after DE MEUROK. sch', «cA°, First and sixth visceral pockets ; th, fundament of the thymus ; sd, thyroid gland ; nad, accessory thyroid gland. In the Chick the visceral furrows become visible in the course of the third day of incubation, only three pairs at first, but, at the end of the sam« day, a fourth pair is added. In human embryos the visceral furrows are to be seen most dis- tinctly (figs. 157, 154) when the embryo has attained a length of three or four millimetres (His). Outer and inner furrows are in this case deeply excavated and separated from each other by only a thin epithelial closing plate; they diminish in length from before backward. Of the visceral arches which separate them, the first is the largest, the last the smallest ; seen in frontal section they form two rows converging below, so that the oro-pharyngeal cavity tapers funnel-like into the intestinal tube. THE ORGANS OF THE INNER GERM-LAYER. 289 From the fourth iveek of development onward the visceral arches begin to be displaced in relation to one another, owing to a more rapid growth of the first two than of the following ones (fig. 156). " They glide over one another," as His remarks, " like the tubes of a telescope, in such a way that, viewed from the outside, first the fourth arch is surrounded and covered in by the third, and this in turn by the second, whereas on the inner surface, that which is turned toward the pharynx, the fourth arch lies over the third, the third over the second." As a result the length of the oro-pharyngeal cavity is relatively less in the older than in the younger em- bryos. In consequence of this unequal growth, which moreover takes place in an entirely similar way in the embryos of Birds and Mam- mals, there is formed a deep depression of the surface at the posterior margin of the cephalo-cervical region, the neck-sinus, sinus cervicalis (EABL) or sinus prcecervi- calis (His) (figs. 156 and 158 hb). In the depths of this depression and on its front wall lie the third and fourth visceral arches, which are now no longer visible from without. The entrance to the sinus is bounded in front by the second visceral, or the hyoid, arch (zb). The latter gradually develops a small process backward, which covers over the cervical sinus and has been justly compared by RATHKE with the operculum of Fishes and Amphibia. The opercular process at last fuses with the lateral wall of the body. Thereby the sinus cervicalis, which corresponds to the cavity beneath the operculum which in Fishes and Amphibia covers in the real gill- arches, is closed up. One easily gets an accurate conception of these important processes 19 Fig. 156.— Frontal reconstruction of the oro-pharyngeal cavity of a human embryo (R, pancreas ; gn1, origin of the greater omentum (mesogastrium) at the vertebral column ; gn1, the part of the mesogastrium which is attached to the greater curvature (gc) of the stomach ; kn, lesser omenturn ; gc, greater curvature of the stomach. * Atrium and cavity of the greater omentum. THE ORGANS OF THE INNER GERM-LAYER. 301 •descends to the left pelvic region, where it is continued into the sigmoid flexure and rectum. Therefore there are distinguishable in the colon, even in the third month, the ccecum, the transverse and the descending colon. An ascending colon is still wanting. It is formed in the succeeding months (fig. 165 B) by the gradual sinking down of the coecum, which was at first under the liver, until in the seventh month it is below the right kidney, and from the eighth month onward descends past the crest of the ilium. Meanwhile the ccecum has increased in length and toward the end of pregnancy is a rather large appendage at the place of tran- sition from the small to the large intestine. It early exhibits a want of uniformity in development (fig. 165 B bid). The terminal part, which often embraces more than half its length, does not keep pace in its growth with the more rapidly enlarging proximal portion; the former is designated as the appendix vermiformis, the latter as the cwcum. At the time of birth the vermiform appendage is still not so sharply differentiated from the coecum as it is a few years later, when it has been converted into an appendage of the size of a goose- quill and 6 to 8 cm. long. Within the region embraced by the bends of the large intestine, the small intestine, which is derived from the descending arm of the loop, is disposed in more and more numerous folds owing to its extensive growth in length (fig. 165 B). At first all regions of the intestine from the stomach onward are so united to the lumbar region of the vertebral column by means of a common mesentery (mesenterium commune) that they can move freely (fig. 165 A and B). The mesentery is naturally influenced by the increase in the length of the intestine, inasmuch as its line of insertion on the intestine exceeds in length many times the line of origin at the vertebral column (radix mesenterii), and is thereby laid into folds like a frill. Such an arrangement of the mesentery is found to be the permanent condition in many Mammals, as in the Dog, the Cat, etc. But in the case of Man, from the fourth month onward, the arrangement of the mesentery is much more complicated. There occur changes which may be briefly characterised as processes of fusion and concrescence of certain portions of the mesenterial lamella with contiguous parts of the peritoneum, either of the posterior wall of the body-cavity, or of neighboring organs. They affect the mesentery of the duodenum and colon, which is always present in the first half of embryonic development. 302 EMBRYOLOGY. The duodenum, describing the well-known horseshoe-shaped curve> applies its mesentery, in which the beginning of the pancreas is en- closed, broadly to the posterior wall of the body, and fuses throughout its whole extent with the peritoneum of the latter; from being a movable it has become an immovable portion of the intestine (fig. 167 du). The large intestine (figs. 165 and 167-4 and B ct) still possesses in the third month a very broad suspensorium arising from the vertebral column, which is nothing else than a part of the common mesentery Tig. 167 A B.— Two diagrams to illustrate the development of the bursa omentalis. A, earlier, B, later stage. zf, Diaphragm ; I, liver ; p, pancreas ; mg, stomach ; gc, its greater curvature ; du, duodenum ; dd, small intestine ; ct, colon transversum ; *, bursa omentalis ; kn, lesser omentum ; gnl, posterior [dorsal] lamella of the greater omentum, arising from the vertebral column ; gin?, anterior [ventral] lamella of the same, attached to the greater curvature of the stomach (ffc) ; gn3, the part of the omentum which has grown over the small intestine ; gn*, the part of the omentum which encloses the pancreas ; mes, mesentery of the small intestine ; msc, mesocolon of the transverse colon. of the intestine, but which has received the special designation of mesocolon (msc). In consequence of the previously described twisting of the primitive loop of the intestine, not only the colon trans- versum, but also the considerable mesocolon belonging to it, has been drawn transversely across the end of the duodenum ; for a certain distance it fuses with the latter and with the posterior wall of the body, thereby acquires a new secondary line of attachment (fig. 167 msc) running from right to left, and thus appears as a part that has become detached from the common mesentery. The colon transversum (ct) with its mesocolon (msc) now divides the body-cavity into an. THE ORGANS OF THE INNER GERM-LAYER. 30$ upper [anterior] part, which contains the stomach, liver, duodenum,, and pancreas, and a lower part, holding the small intestine. Thus embryology explains the striking condition of the duodenum, which, in order to pass from the upper to the lower space and to become continuous with the small intestine, passes underneath [dorsal to] the transversely outstretched mesocolon (figs. 165 and 167 du). Also in the case of the suspensorium of the ccecum, and of the ascending and descending arms of the colon, there occurs a more or less extensive concrescence with the peritoneum of the wall of the trunk. Therefore in the adult the parts of the intestine named sometimes lie with their posterior wall broadly in contact with the body-wall ; sometimes they are supported by a broader or narrower mesentery. There still remain to be described the important changes of the bursa omentalis, the development of which during the first months of embryonic life we have already (p. 299) become acquainted with. The bursa is distinguished, first, by a very considerable growth, and, secondly, by the fact that it fuses with neighboring organs at various places. In the beginning it reaches only to the greater curvature of the stomach (figs. 165, 166), to which it is attached; but even from the third month onward it enlarges and lays itself over [ventral to] the viscera which lie below the stomach, at first over the transverse colon (fig. 167-4 gn\ gn2), then over the whole of the small intestine (fig. 167 A gn3). The bursa consists, as far as it has extended downwards, of two lamellae, which lie close to each other, separated by only a very narrow space, and are continuous at their lower margin. Of these the more superficial, the one which is nearer to the ventral wall of the belly, is attached to the greater curvature of the stomach (gc) ; the posterior [dorsal] lamella, which lies upon the intestines, is originally attached to the vertebral column and here encloses the main part of the pancreas (figs. 167 A p and 166 p). In the case of many Mammals (Dog) the bursa omentalis remains in this condition. In Man it begins as early as the fourth month to undergo fusions (fig. 167 B). On the left side of the body the posterior lamella reposes on. the posterior wall of the body over a large extent of surface, and fuses with it (gvfi), so that its line of attachment to the vertebral column moves later ad up to the origin of the diaphragm (lig. phrenico-lienale). Farther down it glides over the upper [anterior] surface of the mesocolon (msc) and over the transverse colon (ct) ; it becomes fused with both of them, with the former as early as the fourth embryonic month. At the time of 304 EMBRYOLOGY. birth the two lamellae of the portion of the bursa which has grown over the intestines are, as in many Mammals, separated by a narrow fissure (fig. 167 E yn?) ; during the first and second years after birth they ordinarily fuse into a single lamella in which fat is deposited. III. Development of the Separate Organs of the Alimentary Tube. The simple growth in length, to which is to be referred the for- mation of the convolutions just described, is only one and certainly not the chief means by which the inner surface of the intestine is increased. The latter acquires a much greater addition from the fact that the inner, originally smooth epithelial layer, which is derived from the entoblast of the germ, forms evaginations and invaginations. By invaginations toward the cavity of the intestine there arise numerous folds, small papillae and villi, which give to the mucous membrane at most places a velvety structure ; by evagina- tions toward the outer surface of the tube there are developed various kinds of larger and smaller glands. By this simple device, the formation of folds, — the great importance of which in the determination of form in animals was particularly set forth in Chapter IV. of Part I., — the mucous membrane acquires to a much greater extent the ability : (1) to secrete digestive fluids, and (2) to absorb the nutritive substances that are mechanically and chemically prepared in the intestine, and to transfer them into the body-fluids. I discuss the numerous organs which are produced by the process of folding according to the regions into which the intestinal tube is divided, beginning with the organs of the oral cavity. A. The Organs of the Oral Cavity : Tongue, Salivary Glands, and Teeth. (1) The Tongue arises, according to the investigations of His upon human embryos, out of an anterior and a posterior fundament (tig. 168). The anterior fundament appears very early as an unpaired eleva- tion (tuberculum impar, His) on the floor of the oral cavity in the space surrounded by the mandibular ridges. It grows a good deal in width, and its anterior margin projects free over the mandible, thus forming the body and tip of the tongue. Even as early as the beginning of the third month some papillae make their appearance on it (His, KOLLIKER). The posterior fundament produces the root of the tongue, which, THE ORGANS OF THE INNER GERM-LAYER. 305 NT- although free from papillae, is richly provided with follicular glands. It is developed out of two ridges in the region where the second and third visceral arches come together in the median plane. The anterior and posterior fundaments unite in a V-shaped furrow, the arms of which diverge in front. The circumvallate papillae are formed on the body of the tongue along this furrow, which persists for a long time. Where the two arms of the V meet there is a deep pit, the foramen co3cum, which His has brought into connection with the origin of the thyroid glands, which will soon be discussed. (2) The Salivary Glands are demonstrable even in the second month. The 'fundament of the submaxillary appears first in human embryos at the sixth week (CHIEVITZ), afterwards the parotid in the eighth week, and finally the sublingual. (3) From a morphological point of view, the Teeth can well be designated as the most interesting structures of the oral cavity. Their develop- ment in Man and Mammals is accomplished in a manner which is neither simple nor easily intelligible ; in the lower Vertebrates, on the con- trary, it is simpler, and for that reason I shall make use of the latter as the starting-point of the description. The teeth, which in Mammals are attached to the edges of the jaws and only bound the entrance to the alimentary tube, possess in the lower Vertebrates a very wide distribution. For in many species they not only cover the roof and the floor of the oral cavity and the inner surface of the branchial arches in immense numbers, as palatalr lingual, and pharyngeal teeth, but they are also distributed in close-set rows over the whole surface of the skin, and produce, as in the Selachians, a strong and at the same time flexible coat of mail. The teeth are originally nothing else than ossified papillae of the skin and the mucous membrane, upon the contiguous surfaces of which they are formed. The development of the dermal teeth in Selachians shows this in a very convincing manner. In young Shark embryos, by a proliferation on the part of the sub- epithelial cells, there are developed on the otherwise smooth surface 20 Fig. 168.— Tongue of a human embryo about 20 mm. long, neck measurement. After His, "Menschliche Embryonen." o06 EMBRYOLOGY. cf the dermis, which comes from the embryonic mesenchyme, small papillae composed of numerous cells (fig. 169 zp), and these pe'netrate into the thick overlying epidermis. The latter also undergoes changes on its part, which are directed toward the formation of the tooth ; for those of its cells which immediately cover the papilla grow out into very long cylindrical forms, and produce an organ the function of which is to secrete enamel, the so-called enamel- membrane (fig. 169 sni). By means of further growth the whole fundament sm ep Fig 169.- Very young fundament of a dermal tooth (a placoid scale) of a Selachian embryo. zp, Dental papilla ; sm, enamel-membrane. next assumes a form which corresponds to the future hard structure (fig. 170). Then the process of ossification begins. There is secreted by the most superficial cells of the papilla (o), the odontoblasl-layer (mem- brana eboris), a thin layer of dentine (zb), which rests upon the papilla like a cap. At the same time the enamel-membrane (5771) begins its secretive activity, and coats the outer surface of the dentinal cap (zb) with a firm, thin layer of enamel (s). The body of the tooth is developed and becomes ever firmer and larger by the subsequent continual deposition of new layers on the first-formed ones, — on the dentinal cap new dentine from within through the activity of the odontoblasts ; on the coating of enamel new layers of enamel from without, through the action of the enamel-membrane. Thus the structure projects more and more above the level of the THE ORGANS OF THE INNER GERM-LAYER. 307 skin, and the tip of the tooth finally breaks through the epidermal covering. The tooth then acquires a still firmer attachment in the dermis from the fact that, at the surface where the lower margin of the dentine occurs, salts of lime are deposited in the superficial layers of the connective tissue (lh2), and thus a kind of connective-tissue bone, the cementum of the tooth, is produced. The finished tooth therefore is constructed out of three calcified tissues, which arise from three separate fundaments. The dentine -Kh • 6 HI 'zp rig. 170.— Longitudinal section through an older fundament of a dermal tooth of a Selachian embryo. e, Epidermis ; e1, the deepest layer of epidermal cells, which are cubical ; sch, mucous cells ; lh1, the part of the dermis which is composed of connective-tissxie lamellae ; US, superficial layer of the dermis ; zp, dental papilla ; o, odontoblasts ; zb, dentine ; s, enamel ; sm, enamel- membrane. takes its origin from the odontoblast-layer of the dental papilla (mesen- ehyme), the enamel from the epithelial enamel-membrane (outer germ- layer), and the cementum from connective tissue in the vicinity by means of direct ossification. The finished tooth has, moreover, within it a cavity, which is filled with a vascular connective tissue (pulp), the remnant of the papilla. When the enamel-membrane has fulfilled its office it perishes, for in the process of secretion its •cells become shorter and shorter, and are finally reduced to flat scales, which are afterwards thrown off. In Selachians the formation of the teeth which occupy the edges of the jaws and serve for the comminution of the food differs from this simple process in one important point ; they take their origin, not on the free surface of the mucous membrane, but in its depths (fig. 171). The epithelial tract of the oral mucous membrane 308 EMBRYOLOGY. which shares in the formation of teeth has sunk deep down in the form of a ridge (zl) on the inner surface of the jaw-arches, into the under- lying loose connective tissue, and now represents a special organ, distinguishable from its surroundings. This important difference is produced by the fact that in the development of the teeth of the jaws more active processes of growth take place, first because these teeth are much larger than the dermal teeth, and, secondly, because they are more rapidly worn out and must consequently be more rapidly replaced by supplementary teeth. As we have often had the oppor- tunity of observing in the study of the production of morphological conditions in animals generally, portions of epithelial membranes that tm zp zb Fig. 171.— Cross section through the lower jaw of a Selachian embryo with fundaments of teeth. k, Mandibular cartilage ; zl, dental ridge ; zp, dental papilla ; zb, dentine ; a, enamel ; sm, enamel- membrane ; b, connective-tissue part of the mucous membrane. grow more rapidly than their surroundings emerge from the latter and become folded either outward or'inward. The process of the formation of teeth is the same on the dental ridge itself as upon the free surface of the skin. There are developed on its outer side, which is turned toward the cartilage of the jaw (&), numerous papillae (zp), lying alongside of and behind one another, which grow into the invaginated epithelium just as the dermal papillae grow into the epidermis. Thus there arise in the depths of the mucous membrane several rows of teeth, of which the most superficial anticipate in development those which lie deeper ; the former are the first to break through the mucous membrane, to become functional, and, after having been worn out, to be cast off; they are also the first to be supplanted by reserve teeth, which lie behind them, and, developing somewhat later, are consequently younger. THE ORGANS OF THE INNER GERM-LAYER. 309 Whereas in the Selachians, as well as in the lower Vertebrates generally, the replacement of teeth by new ones is throughout life an unlimited process, since new papillae are continually being formed in the depths of the dental ridge (polyphyodont), it is in the higher Vertebrates more limited, and in most Mammals occurs only once. There are formed on the ridge two fundaments (diphyodont), one behind the other, one for the milk-teeth and a second for the permanent teeth. In the case of Man the development of the teeth begins as early as the second month of embryonic life. A ridge (zl) (the enamel-germ of older authors) grows from the epithelium of the oral cavity both on the maxillary and mandibular arches — as it also does in other mammalian embryos (fig. 290) — into the richly cellular embryonic connective tissue. The region from which this growth into the depths takes place (fig. 172 A and B) is marked exteriorly by a groove, which runs parallel to the arch of the jaw, the so-called dental groove (zf). The head of the human embryo represented in figure 289 shows this groove at a little distance behind the fundament of the upper lip. At first the dental ridge is uniformly thin and separated from its surroundings by a smooth surface. There is nothing to be seen as yet of the separate fundaments of the teeth. Then the epithelial cells on the side of the ridge which is directed outwards begin at certain- places to grow and to produce at regular intervals from one another as many thickenings as there are to be teeth (fig. 172 A). In Man, who has twenty milk-teeth, the number of these is ten in each jaw. The thickenings now assume a flask-shaped form (fig. 172 B), and gradually detach themselves from the outer surface of the epithelial ridge (zl), except at the neck of the flask, which remains in connection with it at a little distance from its deep edge. Because these epithelial growths have relation to the secretion of enamel, they have received the name of enamel-organs. In the meantime the connective tissue has taken its first steps toward the formation of the tooth (fig. 172 A and B). At the bottom of each flask the connective-tissue cells exhibit active growth, and give rise to a papilla (zp) corresponding in form to the future tooth. As the papillae of the dermal teeth grow into the epidermis, so this papilla grows into the enamel-organ, which is thereby made to take the form of a cap. Then the special layers from which the formation of dentine and enamel proceed are differentiated in both fundaments so far as these are in mutual contact. At the surface of the papilla (fig. 172 B zp) 310 EMBRYOLOGY. the cells assume spindle-shaped forms and group themselves into a kind of epithelial layer, the layer of the dentine-forming cells (mem- brana eboris). On the part of the cap-like enamel-organ the cells of the deepest layer, which is in immediate contact with the papilla, are converted into very long cylinders and constitute the enamel-mem- brane (sm, membrana adamantinse). The latter becomes gradually thinner toward the base of the papilla, where it is continued as a layer of more cubical elements (se), which forms the boundary at the surface of the cap separating it from the surrounding connective tissue. Between these two cell-layers (the inner and the outer epithelium of KOLLIKER) the remaining epithelial cells of the enamel- organ undergo a peculiar metamorphosis, and produce a kind of gelatinous tissue, the enamel-pulp (sp) ; they secrete between them a Tig. 172 A B.— Two stages in the development of the teeth of Mammals. Diagrammatic sections. zf, Dental groove ; zl, dental ridge ; zl\ deepest part of the dental ridge, on which are formed the fundaments of the supplementary teeth ; zp, dental papilla ; sm, enamel-membrane : tp, enamel-pulp ; »e, outer epithelium of the enamel-organ ; zs, dental sac ; k, bony alveolus. fluid rich in mucus and albumen, and become themselves converted into stellate cells, which are united to one another by their processes. and thus form a fine network. The enamel-pulp is most highly developed in the fifth or sixth month, and then diminishes up to the time of birth in the same ratio as the teeth increase in size. The connective tissue immediately enveloping the whole fundament acquires numerous blood-vessels, from which branches also make their way into the papilla ; it becomes somewhat differentiated from the surrounding tissue, and is distinguished as dental sac (fig. 172 B zs). The soft fundaments of the teeth enlarge up to the fifth month of embryonic life, and at the same time acquire the particular forms of the teeth which are to arise from them — those of the incisors, the canines, and molars. Then the process of ossification begins (fig. 173) in the same manner as in the dermal teeth. A cap of dentine (zb) is. THE ORGANS OF THE INNER GERM-LAYER. 311 formed by the odontoblasts (o), or dentinal cells ; this cap at the same time acquires a coating of enamel (s) from the enamel-membrane (sm) ; then there are continually deposited on the first layers new ones, until the crown of the tooth is completed. Under pressure of the latter the enamel-pulp (sp) atrophies, and forms only a thin covering to the tooth at birth. The papilla (zp) is converted into a mass of connective tissue containing blood-vessels (g) and nerves, and fills the cavity of the tooth as the so- called pulp. The larger the whole structure becomes, the more it raises up the tissue of the gum, which covers the edge of the jaw, and causes it to be- come gradually thinner. Finally, it breaks through the gum soon after birth, and at the same time casts off from its sur- face the atrophied remnant of the enamel-organ. The time has now come in which the third hard sub- stance of the tooth is formed, the cementum that envelops the root. So far as the dentine has received no coating of enamel, the bounding con- nective tissue of the dental sac (zs) begins, after the eruption of the teeth, to ossify and to produce a genuine bone-tissue with numerous SHARPEY'S fibres ; this bony tissue contributes to the firmer union of the root of the tooth with its connective-tissue surroundings. The eruption of the teeth ordinarily takes place with a certain degree of uniformity in the second half of the first year after birth. First the inner incisors of the lower jaw break through in the sixth to the Fig. 173.— Section through the fundament of the tooth of a young Dog. k, Bony alveolus of the tooth ; zp, dental papilla ; g, blood-vessel ; o, odontoblast-layer(membrana eboris) ; zb, dentine ; s, enamel ; sm, enamel-membrane ; zs, dental sac ; sp, enamel-pulp. 312 EMBRYOLOGY. eighth months ; then in the course of a few weeks those of the upper jaw follow. The outer [lateral] incisors appear during the period between the seventh and ninth months, those of the lower jaw, again, somewhat earlier than those of the upper jaw. The front molars usually appear at the beginning of the second year, those of the lower jaw first ; then the gap thus left in the two rows of teeth is filled by the eruption of the canine or eye-teeth in the middle of the second year. Finally, the eruption of the back molars, which may b9 delayed into the third year, takes placa. The fundaments of the reserve teeth make their appearance at the side of those of the milk-teeth at an extra- ordinarily early period. They also take their origin from the epithelial ridge. As was previously (fig. 172 A and B) stated, the ridge extends still deeper (zl1) into the underlying tissue from the place where the enamel-organs of the milk-teeth have been differentiated from it and where they remain united to it by means of an epithelial cord, the neck. Here in a short time there again app?ar near the edge of the ridge (fig. 174 sra2, zp2) flask- shaped epithelial growths and dental papillae, which lie on the inner [median] side of the dental sacs of the milk-teeth. In addition there are developed at the ends of the epithelial ridges, in both the right and left halves of the jaw, the enamel-organs of the posterior grinders (the molar teeth of the permanent set), which are not subject to replacement, but are formed once for all. The ossification of the second generation of teeth begins a little time before birth with the first large molars, and is followed in the first and second years after birth by that of the incisors, canines, etc. As a result in the sixth year there are in both jaws forty-eight ossified teeth, — twenty milk-teeth and twenty- eight permanent crowns, — as well as four fundaments of wisdom teeth, which are still cellular. rig. 174.— Diagrammatic section to show the development of the milk-teeth and permanent teeth in Mammals. Third stage in the series of which figs. 172 A and B are the first and second. zf, Dental furrow ; zl, dental ridge ; k, bony alveolus of the tooth ; h, neck, by means of which the enamel-organ of the milk-tooth is connected with the dental ridge, zl ; zp, dental papilla ; zp', dental papilla of the permanent tooth ; zb, dentine ; «, enamel ; sni, enamel-membrane ; «/>ia, enamel-mem- brane of the permanent tooth; sp, enamel-pulp ; se, outer epithelium of the enamel-organ ; zs, dental sac. THE ORGANS OF THE INNER GERM-LAYER. 313 The shedding of the teeth ordinarily begins in the seventh year. It is initiated by the disorganisation and absorption of the roots of the milk-teeth, under the pressure of the growing new generation. One finds here exactly the same appearances as in the atrophy of osseous tissue, concerning which we have the thorough investigations of KOLLIKER. There arise on the roots of the teeth the well-known pits of HOWSHIP, in which large, multinuclear cells, the osteodasts or bone-destroyers, are imbedded. The crowns are loosened by surren- dering their union with the deeper connective-tissue layers. Finally, when the permanent teeth, owing to the growth of their roots, push forth out of the alveoli, the crowns of the milk-teeth are thereby raised up and fall off. The permanent teeth generally appear in the following order : at first, in the seventh year, the first [front] molars ; a year later the middle incisors of the lower jaw, which are followed a little later by those of the upper jaw ; in the ninth year the lateral incisors are <;ut, in the tenth year the first premolars, in the eleventh year the second premolars. Then in the twelfth and thirteenth years the canines and the second molars come through. The eruption of the third molars, or wisdom teeth, is subject to great variation : it may take place in the seventeenth year, but it may be delayed till the thirtieth. Occasionally the wisdom teeth never attain a complete development, so that they are never cut. B. The Organs arising from the Pharynx : Thymus, Thyroid Gland, Larynx, and Lung. Whereas in the water-breathing Vertebrates the visceral clefts remain throughout life and subserve respiration, they are completely closed in all Amniota as well as in a part of the Amphibia. The only exception is in the case of the first cleft, lying between the man- dibular and the hyoid arches, which is converted into the drum of the ear (tympanum) and the EUSTACHIAN tube, and thus enters into the service of the organ of hearing, in connection with which it will subsequently engage our attention. However, the remaining visceral clefts do not disappear without leaving any trace. From certain epithelial tracts of these there arises an organ of the neck-region which functionally is still proble- matic, the thymus, the morphology of which has been very essentially advanced during the last few years. 314 EMBRYOLOGY. (1) The Thymus has been for several years a favorite object of embryological investiga- tion, since the time when KOLLIKER made the interesting discovery that in mammalian embryos it takes its origin from the epithelium of a visceral cleft. This discovery has- since then been corroborated, and at the same time' extended ; for ako- in such animals as persistently breathe by means of gills the thymus is developed out of epi- thelial tracts of the open and func- tionally active gill-clefts. Let us first examine the original condition as exhibited by Fishes. As stated by DOHRN, MAURER, and DE MEURON, the thymus (t/t) of th& Selachians (fig. 175) and the Bony Fishes has a multiple origin and is derived from separate solid epithelial growths, which take place at the dorsal ends of all the gill-clefts, and, indeed, to a greater extent on the anterior than on the posterior ones. nsd Jig. 175.— Diagram to show the develop- ment of the thymus, the thyroid gland, and the accessory thyroid glands, and their relations to the visceral pockets in a Shark embryo, after DE MEURON. tch1, sch*, First and sixth visceral pockets ; th, fundaments of the thymus ; ad, hyroid gland ; nsd, accessory thyroid gland. tch th nsd Fig. 176.— Two diagrams [ventral aspect] of the development of the thymus, the thyroid gland and the accessory thyroid glands, and their relations to the visceral pockets in a Lizard embryo (A) and a Chick embryo ( //), after DE MEURON. tch1, tch'', First and second visceral pockets ; sd, thyroid gland ; nsd, accessory thyroid g!and ; th, fundament of thymus. In the Bony Fishes the separate fundaments at an early period, even before they have detached themselves from their matrix, fuse together THE ORGANS OF THE INNER GERM-LAYER. 315 into a spindle-shaped organ lying above the insertion of the gill-arches, which subsequently becomes inde- pendent, just as it does in Selachians. The originally epithelial product ac- quires a peculiar histological char- acter from being penetrated by ingrowths of connective-tissue ele- ments. In the first place lymph - cells in great quantities migrate in between the epithelial cells, in a manner similar to that described by STOHR as of frequent occurrence in the territory of mucous membranes. Secondly, the epithelial growth is traversed in all directions and cut up into small portions by connective tissue, in which lymph-follicles are formed. The thymus thereby ac- quires the appearance of a lymphoid organ, in which the epithelial rem- nants are still in part preserved, but only in the form of very small spherical portions, as the corpuscles of H ASS ALL. At a still later stage of development there arise in the organ irregular cavities filled with molecular granules. These are caused by the disintegration of lymph-cells and the melting down of the reticular connective tissue, which takes place here and there. In the higher, air-breathing Ver- tebrates the thymus is derived either from the epithelium of two or three clefts or only from the epithelium of the third visceral cleft, which becomes closed. The former is the case with Reptiles (fig. 176 A th) and Birds (fig. 176 B th), the latter with Mammals. In Reptiles and Fig. 177. — Semidiagrammatic illustra- tions to show the ultimate position of thymus, thyroid gland, and accessory thyroid gland on the neck of the Lizard (A), the Chick (B), and the Calf (C), after DE MEUBON. ad, Thyroid gland ; nsd, accessory thyroid gland ; th, thymus ; tfi1, accessory thymus ; ir, trachea ; h, heart ; vj vena jugularis ; ca, carotid vein. 316 EMBRYOLOGY. Birds the two fundaments fuse early upon either side of the trachea into a longish tract of tissue, which in the former is shorter (fig. 177 A), but in the latter very much elongated (fig. 177 J5). In Mammals it is principally the third visceral cleft which con- tributes to the formation of the thymus. According to KOLLIKER, BORN, and RABL this is the only one which comes into considera- tion, whereas DE MEURON, KASTSCHENKO, and His give an account which differs from this, but only in minor details. The further charges of the fundament of the thymus in Mammals and in Man may be briefly summarised as follows. The thymus-sac, which probably takes its origin from the third visceral pocket, encloses only a very narrow cavity, but possesses a thick wall composed of many elon- gated epithelial cells (fig. 178). It then grows downward toward the pericardium, and at the posterior end begins to form, like a botryoidal gland, numerous rounded lateral branches (c). ( KOLLIKER.) These are from the beginning of their formation solid, whereas the sac-like part (a), which occupies the neck-region, always continues to exhibit a narrow cavity. The budding continues for a long time, and meanwhile extends to the opposite end of the originally simple glandular sac, until the whole organ has assumed the lobed structure peculiar to it. At the same time an histological meta- morphosis is also taking place. Lymphoid connective tissue and blood-vessels grow into the thick epithelial walls and gradually destroy the appearance which so resembles a botryoidal gland. With the increase in the size of the organ the lymphoid elements coming from the surrounding tissue predominate more and more ; the epithelial rem- nants are finally to be found only in the concentric bodies of HASSALL, as MAURER has shown for Bony Fishes and as His has undoubtedly rightly inferred for Man and Mammals. The cavity originally present and resulting from the invagination disappears, and instead of it there arise new irregular cavities, probably the result of a breaking down of the tissue. fig. 178.— Thymua of an embryo Rabbit of 16 days, after KULI/KEK. Magnified. a, Canal of the thymus ; b, tipper, c, lower end of the organ. THE ORGANS OP THE JXNER GERM-LAYER. 317 The further history of the thymus in Man permits the recognition of two periods, one of progressive and one of regressive development. The first period extends into the second year after birth. The thymus of the right side and that of the left move in their growth close together into the median plane and here fuse into an unpaired, lobed organ, whose double origin is to be recognised only by the fact that the organ is ordinarily composed of lateral halves separated by connective tissue. It lies in front of [ventral to] the pericardium and the large blood-vessels beneath the breastbone, and is often elongated into two horns which extend upwards to the thyroid gland. The second period exhibits the organ undergoing regressive meta- morphosis, which usually leads to its total disappearance, the par- ticulars of which can be learned from the text -books of Histology. (2) The Thyroid Gland is found on the anterior surface of the neck, and appears to be developed in almost all classes of Vertebrates in a tolerably uniform, typical manner from an unpaired and a paired evaglnation of the pharyngeal epithelium. We must therefore distinguish unpaired and paired fundaments of the thyroid gland. The unpaired fundament has been longest known. There is not a single class of Vertebrates in which it is wanting, as has been established especially by the investigations of W. MULLER. It appears to be an organ of very ancient origin, which shows relation- ship to the hypobranchial furrow of Amphioxus and the Tunicates. DOHEN has opposed this hypothesis and has expressed the view, which is also shared by others, but which lacks proof, that the thyroid gland is the remnant of a lost gill-cleft of the Vertebrates. The unpaired thyroid gland arises as a small evagination of the epithelium of the front wall of the throat in the median plane and in the vicinity of the second visceral arch. Then it detaches itself completely from its place of origin, and is converted either into a solid spheroidal body (Selachians, Teleosts, Amphibia, etc.) or into an epithelial vesicle having a small cavity (Birds, Mammals, Man, etc.). The vesicle subsequently loses its cavity. In Man the development of the unpaired part of the thyroid gland is related to the formation of the root of the tongue, as His states in his investigations of human embryos. The previously described ridges lying on the floor of the throat-cavity in the vicinity of the second and third visceral arches, which unite in the median plane to form the root of the tongue, surround a deep depression,. 318 EMBRYOLOGY. which is tbe equivalent of the evagination of the pharyngeal epithelium in the remaining Vertebrates. By the further approximation of the ridges the depres- sion becomes an epithelial sac, which remains for a long time in communication with the surface of the tongue by means of a narrow passage, the ductus thyreoglossus. The paired fundaments of the thyroid gland were discovered a few years ago by STIEDA in mammalian embryos, but they have been more fully investigated by BORN, His, KASTSCHENKO, DE MEURON, and others in Mammals and other Vertebrates (excepting Cyclo- stomes). In the Amphibia, as well as in Birds and Mammals (fig. 176 J5), there are formed, a little while after the appearance of the unpaired fundament, two hollow evaginations of the ventral epithelium of the throat behind the last visceral arch and in con- nection with the last visceral cleft. They come to lie immediately on either side of the entrance to the larynx. In many Reptiles (fig. 176 A nsd) there is an interesting deviation due to the fact that an evagination is developed only on the left side of the body, while on the right it has become rudimentary. Even in the Selachians {fig. 175), as DE MEURON appears rightly to maintain, paired fundaments of thyroid glands are present. They are the previously mentioned supra-pericardial bodies discovered by v. BEMMELEN. These arise as evaginations of the epithelium of the throat behind the last pair of gill-clefts near the anterior end of the heart. In all cases the evaginated portions of the epithelium become detached from their parent tissue and enclosed on all sides by connective tissue ; they then undergo a metamorphosis similar to that of the unpaired fundament of the thyroid gland. In regard to their ultimate position there exist considerable differences between the separate classes of Vertebrates. In the Selachians the supra-pericardial bodies remain far away from the unpaired thyroid gland, being located in the vicinity of the heart ; but in the other Vertebrates they move more or less close to the gland, and have here acquired the name of accessory thyroid glands {fig. 177-4 and B nsd). Finally, in Mammals and Man the approxi- mation has led to a complete fusion of the unpaired and the lateral, paired fundaments (fig. 177 C). Together they constitute a horse- shoe-shaped body that embraces the larynx. It is, however, to be observed, that at the time of their fusion the lateral fundaments, in comparison with the median one, are only very small nodules. Consequently KASTSCHENKO, who is probably in the ri^ht, ascribes to the former an inconsiderable importance for the development of the THE ORGANS OF THE INNER GERM-LAYER. 319 whole mass of the thyroid gland, whereas His maintains that they become in Man the voluminous lateral lobes, and that the unpaired fundament becomes the small middle part of the organ. The further development of the thyroid gland is accomplished in a very similar manner in all Vertebrates. Two stages are •distinguishable. During the first stage the whole fundament grows out into numerous cylindrical cords, which in turn push out lateral buds (fig. 179). By the union of these \vith one another there is formed a network, into the interstices of which are distributed branches of the blood-ve s s e 1 s together with •embryonic con- nective tissue. In the case of the Chick it is found that the thyroid gland has reached this stage of de- velopment on the ninth day of incubation, in the Rabbit embryo when it is about six- teen days old, in Man in the second month. During the second stage the network of epithelial cords is resolved into the characteristic follicles of the thyroid gland. The cords acquire a narrow lumen, around which the cylindrical cells are regularly arranged. Then there are formed on the cords at short intervals enlargements, which are separated by slight constric- tions (fig. 180). By the deepening of the constrictions the whole network is finally subdivided into numerous, small, hollow epithelial vesicles or follicles, which are separated from one another * [The elevation caused by the mid-brain may be called the apex or crown (Scheitel). In later stages the distance between crown and rump is greater than that between neck and rump, hence the measurement is made from the crown. Compare foot-note, p. 283.] MS Fig. 179.— Right half of the thyroid gland of an embryo Pig 21 5 mm. long, crown-rump measurement,* after BORN. Magnified 80 diameters. The lateral (LS) and median (MS) thyroid glands are in process of fusion, g, Blood-vessels ; tr, trachea. 320 EMBRYOLOGY. by highly vascular embryonic tissue. Subsequently the follicles increase in size, especially in the case of Man ; this results from the epithelial cells secreting a considerable quantity of colloid substance into the cavity. A few further details concerning the thyroid gland of Man, for which we are indebted to His, may be of interest. First, it is to be noted that the lateral fundaments are considerably more voluminous than the middle part, and that the future fundamental form of the organ is thus from the beginning pre- determined. Secondly, some rare anatomical conditions (His) are explained by the development, such as the ductus lingualis, the ductus thyroideus, and the glandula supiahyoidea and praehyoidea. As was previously stated, the unpaired fundament of the thyroid gland is connected with the root of the tongue by means of the ductus tbyreoglossus. When the thyroid gland moves from its place of origin farther down, this duct becomes elon- gated into a narrow epithelial passage, whose external orifice remains permanently visible as the foramen ccecum at the base of the tongue. The remaining part usually undergoes degene- ration, but occasionally some parts of it also persist. Thus the foramen ccecum is some- times elongated into a canal (ductus lingualis) 2£ cm. long, that leads to the body of the Fig. 180. -Section through the thyroid gland of an embryo Sheep 6 cm. long, after W. M'IJLLER. sch, Sac-like fundaments of the gland ; f, glandular follicles in process of formation ; 6, interstitial connective tissue with blood-vesse hyoid bone. In other instances the middle part of the thyroid gland is prolonged upward in the form of a horn, which is continued as a tube (ductus thyroideus) to the hyoid bone. Finally, according to His, the glandular vesicles now and then to be observed in the vicinity of the hyoid bone— the accessory thyroid glands, as well as the glandula supra- and prse-hyoidea— are to be interpreted as remnants of the ductus thyreoglossus. (3) Lung and Larynx. The lung with its outlet (larynx and trachea) is developed, like a lobed gland, out of the resophagus in a tolerably uniform manner, as it appears, for all amniotic Vertebrates. Immediately behind the unpaired fundament of the thyroid gland (fig. 181 Sd) there arises on the ventral side of the oesophagus a groove (A'k), which is slightly enlarged at its proximal end. It is to be seen in the Chick at the beginning of the third day, in the Rabbit on the tenth day after fertilisation, and in the human embryo when it is 3'2 mm. long. THE ORGANS OF THE INNER GERM-LAYER. 321 Soon the groove-like evagination becomes separated from the over- lying portion of the alimentary tube by two lateral ridges; this furnishes the first indication of a differentiation into oesophagus and trachea (fig. 181). Then there grow out from the enlarged posterior onds of the groove (figs. 181, 163) two small sacs (Lj) toward the two sides of the body (in the Chick in the middle of the third day), the fundaments of the right and left lung. Enveloped in a thick layer of em- bryonic connective tissue, they are in im- mediate contact behind with the fundament of the heart; laterally they project into the anterior fissure-like prolongation of the body - cavity. With this the essential parts of the respiratory apparatus are estab- lished; at this stage in amniotic Vertebrates they resemble the simple sac-like structures which the lungs of Amphibia present permanently. In the further course of development the fun- daments of trachea and oesophagus, which com- municate by means of a fissure, become separated by a constriction which begins behind, where the pulmonary sacs have budded out, and gradually moves forward. The constricting off is here interrupted at the place which becomes the entrance to the larynx. The latter is distinguishable in the case of Man at the end of the fifth week as an enlargement at the beginning of the fundament of the trachea. It acquires its cartilages in the eighth or ninth week. Of these the thyroid cartilage arises, according to the comparative-anatomical investigations of DUBOIS, from a fusion 21 Tig, 181.— Alimentary tube of a human embryo (R of His) 5 mm. long, neck measurement. From His, ' ' Mensch- liche Embryonen." Magnified 20 diameters. RT, RATHKE'S pouch ; Uk, lower jaw ; Sd, thyroid gland ; Ch, chorda dorsalis; Kk, entrance to the larynx; Lg, lung ; Mg, stomach ; P, pancreas ; Lbg, primitive hepatic duct ; Ds, vitelline duct (stalk of the intestine) ; All, allantoic duct ; W, Wolffian duct, with kidney- duct (ureter) budding out of it ; £, bursa pelvis. 322 EMBRYOLOGY. of the fourth and fifth visceral arches, whereas the cricoid and ary- tenoid cartilages, as well as the half-rings of the trachea, are independent chondrifications in the mucous membrane. Two stages are recognisable in the metamorphosis of the primitive lung-sacs of Man and Mammals. The first stage begins with the elongation of the sac, which is attenuated at its origin from the trachea, but is enlarged at its opposite or free end. At the same time — in Man from the end of the first month (His) — it pushes out, in the manner of an alveolar gland, hollow evaginations, which grow out into the thick connective- tissue envelope and enlarge at their ends into little sacs. The first bud-like outgrowths on the two sides of the body are not symmetrical (fig. 182), because the left lung-sac produces two, the right three bud-like enlargements. An im- portant feature of the- architecture of the lungs^ is thus established from the beginning, namely, the differentiation of the right lung into three chief lobes, and of the left into two. The further budding is distinctly dichotoinous (fig. 183). It takes place in the following way : each terminal vesicle (primitive lung-vesicle), which is at first spheroidal, becomes flattened and indented on the wall (Ib) which lies opposite its attachment. Thus it becomes divided, as it were, into two new pulmonary vesicles,, each of which is then differentiated into a long stalk (lateral bronchus) and a spherical enlargement. Inasmuch as such a process of budding is kept up for a long time, — in Man until the sixth month, — there arises a complicated system of canals, the bronchial tree, which opens into the trachea by means of a single main bronchial tube from either side of the body, and the ultimate branches of which, becoming finer and finer, terminate in flask-shaped enlargements, the primitive lung-vesicles. The latter are at first confined to the surface of the lung, while the system of canals occupies its interior. During this budding the lungs as they increase in volume continue to grow downwards into the thoracic cavities, and thereby Tig. 182.— View of a reconstruction of the fundament of the lungs of a human embryo (Pr of His) 10 mm. long, neck measurement, after His. Ir, Trachea ; br, right bronchus ; sp, oesophagus ; bf, con- nective-tissue envelope and serous membrane (pleura) into which the epithelial fundament of the lung grows ; 0, M, U, fundaments of the upper, middle, and lower lobes of the right lung; 0l, V1, fundaments of the upper and lower lobes of the left lung. THE ORGANS OF THE INNER GERM -LAYER. 323 come to lie more and more at the right and left of the heart. With their ingrowth into the cavities of the chest (fig. 314 brh), they push before them the serous lining of the latter, and thus acquire their pleural covering (the pleura pulmonalis, or the visceral layer of the pleura). During the second stage the organ, which up to this time has the typical structure of a botryoidal gland, assumes the characteristic pulmonary structure. The metamorphosis begins in Man, as KOLLIKER states, in the sixth month, and comes to a close in the last month of pregnancy. There now arise close together on the fine terminal tu- bules of the bron- chial tree, on the alveolar passages, and on their ter- minal vesicular enlargem ents, very numerous small e vagi na- tions. But in dis- tinction from the earlier ones, these are not constricted off M from their source of origin, Fig. 183. — View of a reconstruction of the fundament of the lungs of a human embryo (N of His) older than that of fig. 182. After His. Magnified 50 diameters. Ap, Arteria pulmonalis ; Ir, trachea ; sp, oesophagus ; Ib, pulmonary vesicle in process of division ; 0, upper lobe of the right lung with an eparterial bronchus leading to it ; M, U, middle and lower lobes of the right lung; 0l, upper lobe .of the left lung with hyparterial bronchus leading to it ; U1, lower lobe of the left lung. but communicate with the latter by means of wide orifices, and thus constitute the air-cells or pulmonary alveoli. Their size is only a third or fourth as great in the embryo as in the adult ; from this KOLLIKER concludes that the increase in the volume of the lung from birth up to complete development of the body is to be attributed exclusively to the enlargement of the vesicular elements which exist in the embryo. The epithelial lining of the lung is variously modified in different regions during development. In the whole bronchial tree the epithelial cells increase in height, acquire in part a cylindrical, in part a cubical form, and from the fourth month onward (KOLLIKER) have their free surfaces covered with cilia. In the air-sacs, on the contrary, the cells, which are arranged in a single layer, become 324 EMBRYOLOGY. more and more flattened, and in the adult become so thin that formerly the presence of an epithelial covering was wholly denied. Then they assume a condition similar to that of endothelial cells ; as in the case of the latter, their boundaries are demonstrable only after treatment with a weak solution of silver nitrate. C. The Glands of the Small Intestine : Liver and Pancreas. (1) The Liver. In the section which treats of the liver we must enter upon a dis- cussion not only of the development of the parenchyma of the gland, but also of the various hepatic ligaments — the lesser omentum, the ligamentum suspensorium, etc. ; in fact, we must begin with the latter because they are developed out of a structure — a ventral mesentery — which is ontogenetically older than the liver itself. In view of the manner in which the body -cavity arises, as a pair of cavities, such a structure ought to be found along the whole length of the ventral side of the alimentary canal in the same manner as on its dorsal side. Instead of that, it is found only at the anterior region of the alimentary canal, along a tract which extends from the throat to the end of the duodenum. This ventral mesentery acquires a special significance, because several important organs take their origin in it; in front, the heart, together with the vessels that bring the blood back to it — the terminal parts of the venae omphalomesentericse and of the vena umbili- calis; immediately behind the latter, the liver with its outlet and its blood-vessels. The part which, during an early stage of development, encloses the heart is called mesocardium anterius and posteriw ; we shall return to it later in considering the development of that organ. The portion (fig. 184) which joins this behind [caudad] has been hitherto less regarded by embryologists. Since it stretches from the lesser curvature of the stomach and the duodenum (du) to the anterior [ventral] wall of the trunk, it may be especially designated as the ventral gastric and duodenal mesentery, or, under a more compre- fig. 184.- Diagram (view of a cross section) to •how the original re- lations of duodenum, pancreas, and liver, and of the ligamentous structures belonging to them. HR, Posterior wall of the trunk ; du, duodenum ; p, pancreas ; I, liver ; dms, dorsal mesentery ; Me/, ligamentum hepa- to-duodenale ; Is, liga- mentum suspensorium hepatis. THE ORGANS OP THE INNER GERM- LAYER. 325 umc Fig. 185.— Cross section through the anterior part of the trunk of an embryo of Scyllium, aftei BALFOUR. Between the dorsal and ventral walls of the body, where the attachment of the stalk of the yolk- sac is cut, there is stretched a broad mesentery which contains numerous eel Is and completely divides the body-cavity into a right and a left half. The duodenum (du), lying in the mesentery, is twice cut through ; dorsally it gives rise to the fundament of the pancreas (pan), ventrally to that of the liver (hp.d). Further, the place where the vitelline duct (umc) emerges from the duodenum is to be seen, sp.c, Neural tube (spinal cord) ; sp.g, ganglion of posterior root; ; ar, anterior root ; dn, dorsally directed nerve springing from the posterior root ; mp, muscle-plate ; mp1, part of muscle-plate already convei-ted into muscles ; mp. I, part of muscle-plate which gives rise to the muscles of the limbs ; nl, nervus lateralis ; ao, aorta ; ch, chorda ; sy.g, sympathetic ganglion ; ca.v, cardinal vein ; sp.n, spinal nerve ; sd, segniental duct (duct of primitive kidney) ; st , segmental tube (pronephric tubule). 326 EMBRYOLOGY. hensive title, as ventral alimentary mesentery (Ihd -f Is). It has been described by KOLLIKER on sections of Rabbit embryos as liver- ridge (Leberwulst), and by His in his "Anatomic menschlicher Embryonen " as prehepaticus (Vorleber) ; it has the form of a mass of tissue rich in cells, which inserts itself between the wall of the belly and the regions of the intestine previously mentioned. In cross sections through human and mammalian embryos there are encountered in it the capacious venae omphalomesentericse. As far as a mesocardium and a mesogastrium anterius are developed in Vertebrates, the body-cavity appears even subsequently as a paired structure. The cross section through a Selachian embryo (fig. 185) shows this distinctly. The duodenum (du) is enclosed in the connective-tissue mesentery, which reaches from the aorta (ao) to the front [ventral] wall of the trunk ; dorsally the pancreas (pan) is budded forth from its wall, ventrally the liver (hp.d). The liver begins to be developed very early in the ventral me- sentery (liver-ridge or prehepaticus), and in this exhibits, as will appear later, two modifications, which are, however, unessential ; for sometimes it appears in the form of a single, sometimes as a paired evagination of the epithelial lining of the ventral wall of the duo- denum. The first is the case, for example, in the Amphibia and Selachii. In Bombinator (fig. 159), as GOETTE has shown, the liver arises as a broad ventrally directed evagination of the intestine, which lies im- mediately in front of the accumulation of yolk-material. The liver remains permanently in this simplest form in the case of Amphioxus lanceolatus, in which it is located immediately behind the gill-region as an appendage of the intestinal canal. In the case of Birds and Mammals, on the contrary, the funda- ment of the liver is from the beginning double. As has been known since the investigations of REMAK, in the case of the Chick (fig. 186) on the third day of incubation, two sacs (I) grow out of the vent nil wall of the duodenum immediately behind the spindle-shaped stomach (St). They grow into the broad cell-mass of the ventral mesogastrium (the Leberwulst), one passing forward to the left, the other backward to the right, and thereby embrace from above the vena omphalomesenterica on its way to the heart. The process in Mammals is somewhat different. According to the observations of KOLLIKER in the case of the Rabbit, the primitive hepatic tube of the left side is forme 1 in the embryo of ten days, to which a THE ORGANS OP THE INNER GERM-LAYER. 327 00 diameters. N and Ch, the part of the cross section which is adjacent to the neural tube and the chorda ; chi, skeletogenous sheath of the chorda; ep, epidermis ; ae, outer epithelial layer of the primitive segment ; mk, nuclei of muscle-cells ; mf, muscle-fibril toe in cross section ; WZ, zone of growth —transition from the outer cell-layer to the muscle-forming layer of the primitive segment. which the musculature of the trunk is continually growing further dorsad arid ventrad. At a later stage of development, in larvoe six weeks old (fig. 191), the muscle-layers are converted into Muskelkdstchen (k), as SCHNEIDER has named these peculiar definite structural elements of the Cyclo- stomes. The facing fibrilloe-sheets of two adjacent layers (Blatter) unite with each other along their margins. Since these sheets have been produced on the two sides of one cell-plate, each formative cell is now surrounded on all sides, as though with a mantle, by the fibrillse which it has generated. THE ORGANS OF THE MIDDLE GERM-LAYER. 345 Finally, three alterations of the Muskelkastchen take place. The homogeneous cementing substance, which was indicated during the first stage by only a fine line between the two fibrillse-sheets of a muscle-layer, increases and produces the partition by means of which the individual Muskelkastchen are separated from each other, and in which afterwards connective-tissue cells and blood-vessels are also to be found. Secondly, the protoplasmic matrix of the formative cells is almost completely consumed in the continued production of numerous fine fibrillae, which finally fill the whole interior of the Kast- chen. One can now distinguish two different kinds of fibrill® — those that are centrally located, and those that are firmly attached to the partitions. Thirdly, there are to be found scattered between the fibrillse numerous small nuclei, which pro- bably are descended from the original single nucleus of the formative cell by frequently repeated division. The development of the muscle -seg- ments takes place in the remaining Ver- tebrates in a somewhat different manner from that of Amphioxus and the Cyclo- stomes. For the study of this process the tailed Amphibia furnish the most larva of Petromyzon Pianerf instructive objects. In Triton (figs. 106, ™' * 105 ush) each of the primitive segments k, Muskelkastchen; mk, nuclei . , , , . , i . , . of muscle-cells ; mf, muscle- con tains a considerable cavity, which is fibrillse cut crosswise. bounded on all sides by large cylindrical epithelial cells. In somewhat older embryos active cell-multiplication takes place in the part of the epithelium which is adjacent to the chorda and neural tube, and which, therefore, corresponds to the previously described muscle -forming layer of Amphioxus and the Cyclostomes. By this growth the cavity of a primitive segment becomes entirely filled. At the same time the cells lose their original arrangement and form ; they are converted into longitudinally ar- ranged cylinders, which correspond in length to a primitive segment and are located by the side of and above one another on both sides of, and parallel to, the spinal cord and chorda dorsalis (fig. 192). Each cylinder, which in the beginning exhibits only a single nucleus (mk), becomes surrounded with a mantle of the finest transversely striped fibrillse (mf) ; it is now comparable with a Muskelkastchen of the Cyclostomes (fig. 191). A series of further alterations also takes place in this instance as in the former. In older larvje there are 346 EMBRYOLOGY. continually being formed more fibrillae (fig. 193), which gradually fill the interior portion of the cylinder. Only in the axis of the latter are there places left free, in which the small nuclei (ink) come to lie ; these, formed by division of the single mother-nucleus, increase considerably in number. Moreover, connective tissue with blood- vessels now penetrates between the muscle-fibres or the primitive bundles (pb), as the finished elements are subsequently called. If we consider-from a general point of view the facts here presented, which have been acquired in the study of the lower Vertebrates, — dt •; vj ~ „" -*" •..„«» Fig. 102. Fig. 193. Fig. 192.— Cross section through the musculature of the trunk of a larva of Triton taeniatus 5 days old. Magnified 500 diameters. mk, Nuclei of muscle-cells ; mf, muscle-fibrillse cut crosswise ; dk, yolk -granules. Fig. 193.— Cross section through the musculature of the trunk of a larva of Triton tseniatus 10 days old. Magnified 500 diameters. pb, Primitive bundle of muscle-fibrillse (Musktlprimitivbiindel) ; mf, muscle-fibrillae cut cross- wise ; mk, nuclei of muscle cells. we arrive at two propositions of importance concerning the origin of the musculature : — (1) In Vertebrates the elements of the musculature of the trunk are developed out of epithelial cells which are derived from a circumscribed territory of the epithelium of the body-cavity, — a territory that is con- stricted off from the latter to form the primitive segments. (2) The epithelial products becorne surrounded and enveloped on all sides by connective tissue, just as do the glands and gland-ducts that bud forth from an epithelium. A comparison with the condition and development of the musculature of some classes of Invertebrates leads to a still better comprehension of the above propositions. In most of the Coelenterates the muscular elements are components of the epithelium, not only during their development, but also in the adult animal, so that the designation cjrithelio-muscular cells is suitable for them. THE ORGANS OF THE MIDDLE GERM-LAYER. 34:7 The characteristic feature of these consists in their being simple — sometimes cubical, sometimes cylindrical, sometimes thread-like — epithelial cells, the outer ends of which ordinarily reach the surface of the epithelium and are here provided with cilia, whereas their basal ends lie upon the sustentative lamella (Stiitzlamelle) of the body and are there differentiated into one or several either smooth or transversely striped muscle-fibrillge. Inasmuch as the fibrillae of numerous cells lie parallel and close to one another, muscle-lamella arise, by the activity of which the changes in the form of the body are produced. In Ccelenterates both the outer and the inner germ-layers can develop muscle-cells. When one turns to the Vermes it is seen, in those groups in which a body- cavity (an enteroccel) is formed by an infolding of the inner germ-layer, that the parietal wall of the body-cavity, or the parietal lamella of the middle germ-layer, has assumed the production of the entire musculature of the trunk. Here also, for example in the Chaetognatha, etc., the epithelial cells differentiate at their basal ends, which are directed toward the surface of the body, a lamella of muscle-fibrillae, whereas their other ends bound the body-cavity. Thus from tlie lower to the higher animals the capability of producing muscles fa, with the progressive differentiation of the body, more and more restricted to a limited special territory of the total epithelial investment of the bodi/. This process has proceeded furthest in the Vertebrates, for in them the musculature of the trunk is no longer furnished by the whole parietal lamella of the middle germ-layer, but by only a small detached part of it, the primitive segments. Consequently in Vertebrates the musculature spreads out from a email region where it originates, distributes itself first in the trunk, and then from the latter grows out into the extremities. In the Vertebrates we recognised two different forms of voluntary musculature, the muscle-layer (and the Muskelkastchen derivable from it) and the primitive bundle (Muskelprimitivbiindel). Parallels to this are found in the Inverte- brates, both in Coelenterates and in Worms. In Ccelenterates both forms are derived from the primitive smoothly outspread muscle-lamella by the forma- tion of folds, and are to be explained in the same way as the formation of those folds which in epithelial lamellae play such an important part in the origin of the most various organs. When certain tracts of a muscle-lamella are called upon to execute additional labor, this can be effected only by an increase in the number of the fibrillae lying parallel to one another. But a greater number of fibrillae can be brought into a circumscribed territory only in one or the other of two ways : either by their coming to lie in several layers one above another, or — if the more simple arrangement of lying side by side is to be retained — by the folding of the muscle-lamella. The folding exhibits two modifications. Sometimes there are produced parallel daughter-lamellae placed side by side and perpendicular to the mother-lamellae ; sometimes the folded lamellae become wholly detached from the parent-layer and converted into muscle-cylinders, which imbed themselves in the underlying sustentative lamella. With the conception here presented of the origin of the transversely striped muscle-fibres of Vertebrates, it must be assumed as very probable that subsequently an increase in their number will take place as a result of constriction and detachment into two parts, as was first maintained by WEISMANN. 348 EMBRYOLOGY. In Amphioxus, the Cyclostomes, and the Amphibia the most important function of the primitive segments is the production of the fundament of the transversely striped and voluntary musculature. On the other hand it is not very evident that the primitive segments also share, in the manner previously (p. 172) described, in the deve- lopment of the mesenchyme; this is correlated with the fact that in general the connective and sustentative substances play a slight role in the con- struction of the bodies of the lower Vertebrates, and es- pecially during larval life are developed to only a very insig- nificant amount. This is altered in the Sela- chians and the three higher classes of Vertebrates. Not only does the mesenchyme in the adult bodies of these attain a more voluminous development and a degree of differentiation that is in all directions more advanced, but it is also established earlier and likewise in greater abundance. Therefore the primitive segments here ex- hibit in their metamorphosis somewhat modified pheno- mena. At the same time with the differentiation of the muscular tissue, and in part even before that event, the development of mesen- chyme is observable. The primitive segment (fig. 194) in this case is differentiated from the start into two equally distinct fundaments, of which the one is designated as sclerotome or skeletogenous layer (sk), the other as muscle-plate (mp). While referring the reader to the ninth chapter, I add to the presentation given there a few further statements. Pig. 194.- Cros? section through the region of the pronephros of a Selachian embryo, in which the muscle-segments [myotomes] (,/«//) are in process of being constricted off. Diagram after WUHE. nr, Neural tube ; ch, chorda ; ao, aorta ; sch, sub- notochordal rod ; mp, muscle-plate of the primitive segment ; w, zone of growth, where the muscle-plate bands around into the cutis- plate (cp) ; vb, tract connecting the primitive segment with the body-cavity, out of which are developed, among other things, the meso- nephric tubules (fig. 205 uk) ; sk, skeleto- genous tissue, which arises by a proliferation from the median wall of the connecting tract vb ; rn, pronephros ; wit1, mk*, parietal and visceral middle layer, from whose walls mesenchyme is developed ; Ih, body-cavity ; ikt entob'-ast. THE ORGANS OF THE MIDDLE GERM-LAYER. 349 In the Selachians the skeletogenons layer, the origin of which has already been described, grows upward at the side of the chorda (fig. 195 Fr). Outside of this layer one finds the part of the primitive segment which serves for the formation of muscle. This consists of an inner layer (nip') and an outer layer (mp), which are separated from each other by the remnant of the cavity of the primitive segment (fig. 194 h). The inner layer (fig. 195 mp') is in contact with the skeletogenous tissue (Fr), and is composed of numerous, superposed, spindle-shaped cells, which are arranged longitudinally and give rise to transversely striped nmscle-fibrillse ; they correspond to the inner wall of the primitive segment in the larvae of Amphioxus (fig. 189) and Cyclostomes, which is in direct contact with the chorda. The outer layer lies in contact with the epidermis, and , remains for a long time composed of cubical epi- thelial cells. Dorsally and ventrally it bends around into the muscle - forming layer, and here contributes to the enlargement of the latter, as in Amphioxus Fig. 195.— Horizontal longitudinal section through the trunk of an embryo of Scyllium, after BALFOUR. The section is made at the height of the chorda, and shows the separation from the muscle-plates of the cells which form the bodies of the vertebrae. cA, Chorda ; ep, epidermis ; Vr, fundament of the bodies of the vertebrae ; mp, outer cell-layer of the primitive segment ; mp', portion of the primi- tive segment which has already been differentiated into longitudinal muscles (muscle-plate). and the Cyclostomes, by its cells becoming longer and being metamorphosed into muscle-fibres (fig. 185). The muscle - plate then spreads out farther into the wall of the trunk both above and below (figs. 185 and 205). At the same time its cavity (myoccel) gradually disappears. The muscle-forming layer (fig. 185 mp') continues to increase in thickness, since the number of muscle-fibres becomes greater ; the outer layer also loses, rather late it is true, its epithelial character, and is con- cerned on the one hand in the development of the corium (fig. 205 cp), while on the other it furnishes an additional outer, thin muscle- lamella. This observation, made by BALFOUR, has often been called in question, but has recently been confirmed by VAN WIJHE. In Reptiles, Birds, and Mammals the proliferation of the primitive segments which furnishes the skeletogenous tissue is still more extensive than in Selachians. Thereby the muscle -plate, or the dorsal plate, as it is also called, is crowded farther away from the 350 EMBRYOLOGY. chorda. The differentiation of muscle-fibres follows at a much later stage of development, in comparison with Amphioxus and the Cyclo- stomes. The inner layer of the muscle-plate is converted into longitudinal muscle-fibres, the outer contributes to the formation of the corium (fig. 202). Let us now consider somewhat more in detail the original condition of the musculature. It shows at the beginning complete uniformity in all classes of Vertebrates. Everywhere there appears as its foundation a very simple system of longitudinal contractile fibres, which first appear near the chorda and neural tube and spread themselves out thence dorsally toward the back and ventrally in the wall of the belly. The muscle-mass is divided in a very uniform manner into separate segments or myomeres by means of connective- tissue partitions (ligamenta intermuscularia), which run transversely or obliquely to the vertebral column. In the lower Vertebrates this condition persists, in the higher ones it gives place to a more complicated arrangement. We cannot recount more precisely the details of the manner in which the groups of muscles of the higher Vertebrates, so various in form and position, are derived from the original system, especially since this field of embryology has been as yet little cultivated ; let attention be here called to only two points, which come in question in the differentiation of the groups of muscles. First, a very important factor is furnished in the development of the skeleton, which with its processes affords points of attachment for muscle-fibres. Some of these find in this way opportunity to detach themselves from the remaining mass. Secondly, the development of the limbs, which arise as protuberances at the side of the trunk (figs. 157 and 158), operates toward a greater differentiation of the musculature. The limbs likewise ac- quire their musculature, which in the higher Vertebrates has a very complicated arrangement, from the primitive segments, as has been learned through the investigations of KLEINENBERG and BALFOUR, as well as recently through the very convincing accounts of DOIIRN. In the Selachians, in which the processes are most clearly recog- nisable, cell-buds sprout forth out of the still hollow primitive segments and grow into the paired and median fins, in which they become meta- morphosed into muscle- fibres. The fact that always from a large number of primitive segments buds are given off to a fin is worthy of attention, because it demonstrates that the extremity is a structure that belongs to several somites. THE ORGANS OF THE MIDDLE GERM-LAYEK. 351 B. The Segments of the Head. Important works on the development of the head have appeared in late years by GOETTE, BALFOUR, MARSHALL, WIJHE, FRORIEP. RABL, and others. They have led to the important conclusion that the head is made up of a large number of segments, in the same manner as the trunk. These conditions are most evident in the Selachians. When in these animals the middle germ-layers have grown into the fundament of the head, they here, as in the trunk, early separate from each other, and thus embrace on either side a narrow, fissure- like space, the head-cavity. This is continuous posteriorly with the general body-cavity. It follows from this that the two primitive body-sacs (ccelom-sacs) possess a greater extent in the embryo than they do sub- sequently, since they reach into the most anterior part of the embryonic fundament, the head. In the further course of development the walls of the head-cavity are differentiated, in the same rig. 196.— Cross section manner as the walls of the body-cavity, into a through the next to J Jy the last visceral arch ventral portion and a dorsal portion, the latter of an embryo of Pris- producing primitive segments. Then there arises, ^^er!^ •^w'lnner however, an important difference between head visceral pouch ; pp, and trunk ; in the trunk only the dorsal portion gS^******* is segmented, but in the head both ventral and visceral arch; aa, blood- dorsal portions are segmented, each in a manner J^^aottic^archT** peculiar to itself. The ventral part of the head-cavity is divided, in consequence of the development of the visceral clefts, into separate segments (branchiomeres AHLBORN), the first of which is situated in front of the first cleft, each of the remaining ones between two clefts. Each segment (fig. 196) consists of a wall composed of cylindrical cells and •encloses a narrow cavity. With its enveloping connective tissue it constitutes the visceral arches, which are separated from one another by the visceral clefts ; for this reason the fissures arising from the head-cavity have been designated by WIJHE as visceral-arch cavities. The latter communicate for a time under the gill-pouches with the pericardial chamber surrounding the heart. But then they begin to be closed ; their walls come into contact ; and out of the cylindrical epithelial cells are developed the transversely striped muscle-fibres which produce the muscles of the jaws and gills. Consequently there results for the head-region of Vertebrates this 352 EMBRYOLOGY. important proposition : the head- musculature is developed not only out of the primitive segments, but also out of a part of the epithelium of the head-cavity which corresponds to the lateral plates of the trunk ; whereas the, latter do not contribute to the formation of muscles. So far as regards the dorsal part of the middle germ-layer in the head-region, it is divided, as in the trunk, into primitive segments, which in the Selachians are nine in number and embrace each a cavity, with the exception of the first, which is solid. They arise first in the posterior region of the head, and increase from there forward. The segmentation of the whole body is therefore accomplished in the Selachians — and the same is likewise true for all the remaining Vertebrates — in such a manner that it begins in the neck-region, and proceeds thence on the one hand backward to the tail, on the other forward. The walls of the primitive segments of the head in part furnish muscles, in part degenerate. Out of the first three pairs arise the eye-muscles, as MARSHALL and WIJHE have demonstrated in detail. The first segment envelops the primitive eye-vesicle like a cup, and is differentiated into musculus rectus superior, rectus inferior, and obliquus inferior. The second pair gives origin to the obliquus superior, and the third pair to the rectus externus. The segments from the fourth to the sixth inclusive disappear, while out of the last three are developed muscles which extend from the skull to the pectoral girdle. In the remaining Vertebrates the metamorphosis of the middle germ-layer in the head has not been investigated in so exhaustive a manner as in the case of the Selachians. There do not appear to be any head-cavities developed, because the middle germ layers remain at all .times pressed together. However, we know that primitive segments are demonstrable even here. GOETTE describes four pairs of them in Bombinator ; FRORIEP finds in Mammals in the occipital region alone on either side four muscle-segments, of which the two most anterior are believed subsequently to degenerate. In individual cases there still remains much to be elucidated by more exhaustive investigations. RABL has recently expressed dissent in some points from the exposition of the head-segments as given by WIJHE. He divides the head-segments into two groups — four anterior or proximal, and five posterior or distal. Only the latter are according to RABL to be compared with the trunk-segments ; whereas the first, owing to their method of origin, must take a separate position. THE 01U5AXS OF THE MIDDLE GERM-LAYER. 353 II. The Development of the Urinary and Sexual Organs. The development of the urinary and sexual organs cannot be discussed separately in two chapters, because these systems of organs are most in- timately connected with each other, both anatomically and genetically. First, both take their origin at one and the same place on the epi- thelial investment of the body- cavity ; secondly, parts of the urinary system subsequently enter into the service of the sexual apparatus, for they furnish the passages or canals which are entrusted with the evacuation of the eggs and semen. In anatomy also one therefore properly embraces the two genetically united systems of organs under the common name of urogenital system or apparatus. Again in this subject we turn to one of the most interesting portions of embryology. The urogenital system claims an interest particularly from a morphological point of view, because a great number of important metamorphoses are effected in it during embryonic life. In the higher Vertebrates the pronephros and the mesonephros are formed first; they are organs of an evanescent nature, which in some cases disappear and are replaced by the permanent kidney, in other cases their ducts alone are preserved. But these transitory structures correspond to organs which are permanently functional in the lower Vertebrates. In late years, the attention of investigators having been directed to a series of entirely new and unexpected phenomena, by the excellent i-OM-arches of WALDEYER and SEMPER, the topic " urogenital organs" has been carefully worked out by very many different observers through the investigation of each separate class of Verte- brates. There has arisen a voluminous literature, and many im- portant facts have been brought to light. Nevertheless it is not to be denied that conceptions concerning many fundamental questions are still very divergent. As in several previous chapters, I shall also here give to the discussion a broader foundation by treating somewhat more ex- haustively of the lower Vertebrates in certain questions. (a) The Proucphros and the Mesonephric Duct. The first thing that becomes noticeable in the origin of the uro- pMiital apparatus is the fundament of the pronephros [head-kidney]. This is a structure which has now been demonstrated in the embryos of all Vertebrates, but which plays in some a greater part, in others a lesser one. In some Vertebrates (Myxino, Bdellostoma, Bony Fishes) it is retained permanently; in others, as the Amphibia, it 23 354 EMBRYOLOGY. Spff grows during larval life to an important organ, which disappears after the animal's metamorphosis ; finally, in the Selachians and Amniota its funda- ment is from the beginning very rudi- mentary. In the latter case it was held to be the front end of the meso- nephric duct, until through comparative embryology the right view had been at- tained. I select as types of the development of the pronephros the Selachians, Am- phibia, and Birds. In Selachians of about twenty -seven somites the prone- phros begins with the third or fourth trunk - segment and is developed from there backwards. At the place where the segmented por- tion of the middle germ - layer is con- tinuous with the lateral unsegmented portion, there grow out of its parietal lamella a number of cell -cords (fig. 197 Fig. 197. mp Fig. 198. Figs. 197 and 198.— Two cross sections through an embryo of Pristiurus, after RABL. Cross section fig. 193 lies a little farther back than section fig. 197. ch, Chorda; spg, spinal ganglion ; mp, muscle-plate of primitive segment ; W, skeletogenous tissue which has grown forth from the median wall of the primitive segment ; sch, sub- notochonlal rod ; ao, aorta ; ik, inner germ-layer ; pmb, vriib, parietal, visceral middle layer ; vn, pronephros ; vff, pronephric duct ; x, fissure in the primitive segment, which is still in communication with the body-cavity. ranged one behind which bend backwards cord. Soon afterwards vn) segment ally ar- another, in Torpedo six, in Pristiurus four, and become united into a longitudinal the fundaments acquire small cavities THE ORGANS OF THE MIDDLE GERM-LAYER. 355 through disassociation of the cells. In this manner there has now arisen between epidermis and parietal middle layer a longitudinal canal, which stretches over several segments of the trunk and com- municates with the body-cavity by means of several successive openings, the pronephric funnels (fig. 194 vri). At one place the pronephric duct comes close up to the epidermis and fuses with it (fig. 198 vg). Al- though an actual opening is never formed here, still, supported by this fact, one may express the conjecture that originally the pronephros in Ver- tebrates opened out at a point far forward on the body (VAN WIJHE, KUCKERT). A short time after its formation the fundament undergoes in its anterior half a complete degeneration ; the pos- terior half, on the contrary, is further developed and enlarges, but remains in connection with the body-cavity by means of a single funnel only (fig. 194 vri), either because, as VAN WIJHE as- serts, the several funnels are fused into n, single one, or because, in accordance with the account of RUCKERT, all the funnels except a single one become closed and degenerate. In the Amphibia, with which the Bony Fishes exactly agree in this point, the pronephros is established in the most anterior part of the trunk as an organ that is from the beginning hollow (fig. 199). Below the primitive segments, which have already been differentiated into muscle-fibres (m), there appears a groove-like evagination (u) of the parietal layer of the peritoneum, which stretches from in front backward over several somites. By detaching itself from its parent-tissue at several places, and remaining in connection with it at others, it is converted into a longitudinal canal, which in Rana and Bombinator communicates with the body-cavity by means of three pronephric Fig, 198. — Gross section through a very young Tadpole of Bombinator in the region of the anterior end of the yolk-sac, after GOETTE. a, Fold of the outer germ-layer that is continued into the dorsal fin ; is*, spinal cord ; ra, lateral muscle ; as*, outer cell-layer of the muscle- plate ; s, mesenchymatic cells ; 6, transition of the parietal into the visceral middle layer; u, pronephros; /, intestinal cavity; e, entoblast, which is continuous with the mass of yolk-cells (d) ; /', ventral ccecal pouch of the intestine, which be- comes the liver. 356 EMBRYOLOGY funnels, in Triton and Salamander by means of two. The whole fundament soon after, during the larval life, acquires ample propor- tions, owing to the fact that the nephridial funnels grow out into long and very tortuous tubes (pronephric canals). (FURBRINGER, GOETTE.) In Birds, with which Rep- tiles and Mammals agree, the pronephros appears, much as in Selachians, in a more or less rudimentary form (SEDGWICK, GASSER, EENSON, SIEMERLING, WELDON, MIHALKOVICS). It is first observable in embryo Chicks having eight primitive segments and in the region of the seventh somite; in older ~,g embryos it is developed from this place backward into the region of the twelfth somite. At the place where the primi- tive segments (fig. 200 P.v) are constricted off from the lateral plate (S.o), but still remain for some time in con- tinuity with it by means of a connecting region (the middle plate), there grows out from the parietal lamella of the middle germ -layer (somato- pleure) a ridge of cells (W.d), which is directed toward the overlying epidermis. Later, like the corresponding furrow in the Amphibia, it becomes detached in places from its parent - tissue, and when, meanwhile, the primitive seg- ments have likewise wholly detached themselves from the lateral plates, it is converted into a longitudinal cord, which is united with the epithelium of the body-cavity by means of short transverse branches. Similar conditions exist in Reptiles and Mammals. Finally, the pronephros subsequently acquires a peculiar condition THE ORGANS OF THE MIDDLE GERM-LAYER. 357 from the fact that there are developed out of the wall of the body- cavity, in the vicinity of the openings of its tubules, one or several vascular glomeruli. In the Chick for example (fig. 201), in the region from the eleventh to the fifteenth somites, there is a proliferation of connective tissue on either side of the mesentery (me), — by means of which the right and left pronephridia are separated from each other, — which grows into the body-cavity as a spheroidal body (gl). A blood-vessel from the aorta penetrates into each proliferation and is here resolved into a tuft of capillaries, which are then united again into an efferent vessel. Only in those Vertebrates in which the pronephros is functional, as in the larvae of the Amphibia, in the Oyclostomes and the Teleosts, does the glomerulus attain to a considerable development, where- as in the Selachians and Amniota it remains rudimentary. In the first case fluid or urine is pro- bably secreted by this apparatus, and then taken up by the open- ings of the pronephric tubules and conducted outside the body by means of the pronephric duct, which is to be discussed directly. There is one point in this con- nection that is noteworthy and characteristic of the structure of the pronephros: the glomerulus is developed, not in the wall of the pronephric tubule itself, as is the case in the tubules of the mesonephros, but in the wall of the body-cavity, so that the urine can be evacuated only through the agency of the latter. But in what manner does the pronephros communicate with the outside ? This communication takes place by means of a longitudinal canal, which is developed in immediate continuation with the pronephros, and, beginning in front, gradually grows backwards until it reaches the proctodseum and opens into the cloaca. It is found in all Vertebrates in the region where the primitive segments abut upon the lateral plates. At the time of its origin it is always close under the epidermis, later it is farther and farther removed from the latter Fig. 201.— Cross section through the external glomerulus of a pronephric tubule of an embryo Chick of about 100 hours, after BALFOUR. gl, Glomerulus ; ge, peritoneal epithelium ; Wd, mesonephric (Wolffian) duct ; ao, aorta ; me, mesentery. The pronephric tubule and its connection with the glo» merulus are not shown in this figure. 358 EMBRYOLOGY. by the ingrowth of embryonic connective tissue, and comes to lie very deep (fig. 202 wd and fig. 205 ug). This canal has acquired a number of different names, and is cited in the literature as pro- nephric, mesonephric, Woljfian, or segmental duct. The different designations are explainable from the fact that the canal alters its function in the course of the development of the iiephridial system serving at first as an outlet for the pronephros only, afterwards for the mesonephros. Yiews concerning the origin of the canal were for a time conflicting. According to one supposition, which a few years ago almost all investigators entertained, the longitudinal canal of the pronephros, when it had been constricted off from the parietal wall of the body- cavity, protruded with its posterior end as a free knob into the space between outer and middle germ-layers, and gradually grew out inde- pendently, by multiplication of its own cells, as far as the hind gut (proctodseum). It was said, therefore, to be constricted off from neither the outer nor the middle germ-layers, nor yet to derive from them cell-material for its increase. This interpretation has recently become untenable. As is reported in an entirely trustworthy manner concerning several different classes of Vertebrates,— for Selachians (WiJHE, KABL, BEARD), for Amphibia (PERENYI), for Reptiles (MITSUKURI), and for Mammals (HENSEN, FLEMMING, GRAF SPEE), — the posterior end of the pronephric duct in process of growth is in these cases by no means an entirely isolated structure, but is in close union with the outer germ-layer. Attention has already been called to this fact apropos of the development of the pronephros. In a Selachian embryo the condition which is repre- sented in fig. 197 is soon followed by a condition (fig. 198) in which, in a series of cross sections, the pronephric duct now appears as a ridge-like thickening of the outer germ-layer. By a study of various older embryos it can be further established, that the ridge-like thick- ening of the outer germ-layer is prolonged backwards by means of cell-proliferation in that layer, while in front it is being constricted off from the parent-tissue. The pronephric duct therefore grows at the expense of the outer germ-layer, and moves as it were along the latter, with its terminal opening behind, as far as to the hind gut. When HENSEN, FLEMMING, and GRAF SPEE made their observations on Mammals, they were thereby led to adopt the view that the mesonephric duct, as well as the whole urinary system, was derivable from the outer germ-layer. The union with the middle germ-layer they regarded as one that had arisen secondarily. But their concep- THE ORGANS OF THE MIDDLE GERM-LAYER. 359 tion cannot be brought into unison with the conditions of the pro- nephros which have been found in the remaining and especially in the lower Vertebrates (Selachians, Teleosts, Amphibia, Birds) ; on the other hand allowance is made for all observations, if we sum- marise them as follows : that the pronephros is developed from the " middle plate," and that then its posterior end comes into union with the outer germ-layer and in conjunction with the latter grows farther backward as the pronephric duct. If this explanation, which has also been expressed by WIJHE and RiiCKERT, is correct, then one can designate the pronephric duct at its first appearance as a short canal-like perforation of the wall of the body, which begins in the body-cavity with one or several inner ostia and opens out upon the skin by a single external orifice. Originally the outer and inner openings lay near together, later they moved so far apart that the outer opening of the canal united with the hind gut. It may be said, in favor of the view here presented, that in the Cyclostomes the more primitive condition, that is to say, the union with the skin, has been preserved. For in them the mesonephric duct opens to the outside at the abdominal pore. That openings should arise between the cavities of the body and its outer surface is in no way remarkable. I call to mind the intestinal tube, at various places in the territory of which there are formed openings, as mouth, anus, and branchial clefts. Still more frequent are passages through the body-wall of Invertebrates. As such, arise the openings at the tips of the hollow tentacles of the Actinia, on the ring-canal of the Medusse, and the canals (segmental organs) which in Worms lead out from the body-cavity and serve for the elimination of the sexual products and the excretions. (b) The Mesonephros. (Wolffian Body.) Following upon the origin of the pronephric system there is de- veloped in all Vertebrates, after the lapse of a longer or shorter interval of time, a still more voluminous gland, serving for the secre- tion of urine, the primitive kidney (mesonephros) or Wolffian body. It is developed earlier in those cases in which the fundament of the pronephros is from the beginning only rudimentary, as in the Sela- chians and Amniota ; it appears relatively late, on the contrary, in those Vertebrates in which the pronephros attains to a temporary functional activity, as in the Amphibia and Teleosts. The mesonephros is established on the portion of the pronephric 360 EMBRYOLOGY. duct immediately behind the pronephric tubules. The duct con- sequently serves from this time forward as an outlet for the newly formed glandular organ also, and can therefore be designated as mesonephric or Wolffian duct. When it is stated that a gland is developed on the mesonephrio duct, one at first thinks that lateral buds grow out from its wall and give forth branches, as occurs in the fundaments of g'ands formed from the outer or the inner germ-layers. Nothing of the kind takes place here. All observers — with the exception of a few earlier investigators— agree rather that the glandular tubules of the meso- nephros arise independently of the mesonephric duct. The source THE ORGANS OF THE MIDDLE GERM-LAYER. 361 of its material is either directly or indirectly the epithelium of the body-cavity, as it has been possible to prove in many cases — in Cyclostomes, Selachians, Amphibia, and Amniota. There are formed, following one another in immediate succession, short transverse tubules (fig. 202 st), which are at one end continuous with the epithelium of the body-cavity, and at the other end, which remains for a long time closed, are joined to the mesonephric duct (wd), Tig. 203.— Embryo of a Dog of 25 days, straightened out and seen from in front, after B:SCHOFF. Magnified 5 diameters. d, Intestinal tube ; ds, yolk-sac ; al, allantois ; un, mesonephros ; I, the two lobes of the liver, with the lumen of the vena omphalomesenterica between them ; ve, he, anterior and posterior extremities ; h, heart ; m, mouth ; au, eye ; g, olfactory pit. which runs close to them, but somewhat more laterad. The mesone- phros elongates from before backward and attains a great length on both sides of the mesentery, for it reaches back from the region of the liver nearly to the posterior end of the body-cavity ; it acquires a very delicate, regular condition, as the figure of an embryo Dog twenty-five days old shows (fig. 203 un), and can be designated as a comb-shaped gland, composed of a lateral collecting tube, running lengthwise of the body at a little distance from the mesentery, and, 362 EMBRYOLOGY. attached to the median side of it, short transverse branches, which we shall designate as mesonephric tubules. Whereas there can no longer exist any doubt about the origin of the mesonephric tubules from the middle germ-layer, the statements concerning the method of their formation are still at variance with one another. In accordance with the fundamental investigations of SEMPER, it was generally believed that the mesonephric tubules either were eraginated in metameric sequence along the dorsal wall of the body-cavity out of its epithelial lining, or grew forth as originally solid buds, as glandular sacs do from the outer or inner germ-layer. This view, according to the more recent investigations of SEDGWICK, WIJHE, and RticKERT for the Selachians and the three higher classes of Vertebrates, is no longer adequate. In these cases the development of the mesonephric tubules is intimately connected with that of the primitive segments. When the latter begin to be more sharply separated from the lateral plates, there arises at the place of con- striction a narrow stalk, which maintains for a time a connection between the two parts (fig. 204 vb). In the Selachians it possesses a small cavity, which unites the cavity of the primitive segment with the body-cavity. In the Amniota it is solid (fig. 200). Inasmuch as the successive cords (stalks) are here closely pressed together, they appear like a continuous cell-mass interpolated between primitive segment and lateral plate, and have been previously mentioned under the name of the middle plate. On account of its relation to the meso- nephric tubules, the middle plate is also designated as mesonephric blastema. The mesonephric duct, split off from the outer germ- layer, is to be seen taking its way on the lateral side of and close to the connecting stalks of the primitive segments. Each of the connecting stalks, which RUCKERT names at once nephrotome, — in contradistinction to the remaining parts of the primitive segment, which produce the muscle-plate (myotome) and the cell-material for the skeletogenous tissue (sclerotome), — is afterwards metamorphosed into a mesonephric tubule. Whereas one of its ends remains con- nected with the body-cavity, the other becomes separated from the primitive segment (fig. 205 ukl), then applies itself closely to the mesonephric duct, fuses with the wall of the latter, and opens into it. In the diagram (fig. 205) the detachment of the connecting stalk from the primitive segment is shown on the right, the fusion of the detached end with the mesonephric duct on the left. According to this whole process of development the mesonephros is from the very THE ORGANS OF THE MIDDLE GERM-LAYER. 363 beginning a segmentally formed organ, as can be best followed in the Selachians ; for each mesonephric canal is developed in a single segment. In Reptiles, Birds, and Mammals the connecting stalks are solid Fig. 204. Fig. 205. Figs. 204 and 205. — Diagrams of cross sections through a younger and an older embryo Selachian to show the development of the principal products of the middle germ-layer. After WIJHE, with some alterations. Fig. 204.— Cross section through the region of the pronephros of an embryo in which the muscle- segments ( 7»i/0 are in process of being constricted off. Fig. 205. — Cross section through a somewhat older embryo, in which the muscle-segments have just been constricted off. nr, Neural tube ; ch, chorda ; ao, aorta ; sch, subnotochordal rod ; mp, muscle-plate of the primitive segment ; w, zone of growth where the muscle-plate bends around into the cutis- plate (cp) ; vb, the connecting piece which unites the primitive segment to the walls of the body-cavity, and from which are developed, among other things, the mesonephric tubules (fig. 205 uk) ; sk, skeletogenous tissue, which arises by a proliferation of the median wall of the connecting piece vb ; vn, pronephros ; mkl, ink*, parietal and visceral middle layer, out of which mesenchyma is developed ; Ik, body-cavity ; ik, entoblast ; h, cavity of the primitive segment ; uk, mesonephric tubules, which have arisen from the connecting piece vb of the diagram fig. 204 ; uk1, the place where the mesonephric tubule has been detached from the primitive segment ; ug, mesonephric duct, with which, on the left side of the figure, the mesonephric tubule has united ; tr, union of the mesonephric tubule with the body-cavity (uephridial funnel) ; mes1, mes", mesenchyma that has arisen from the parietal and visceral middle layers. cords of cells (mesonephric cords). It is only when they have de- tached themselves from the primitive segment, and their blind ends have united with the mesonephric duct, that they acquire a small cavity (fig. 202 at). Now they also become more readily distin- guishable as separate canals, since they become farther removed from 364 EMBRYOLOGY. one another and are marked off from the surrounding tissue by sharper contours. Although it is often stated that in the Amniota the mesonephric tubules " are differentiated out of" the middle plate or the mesonephric blastema, it is nevertheless to be observed that this is not a case of new formation out of undifferentiated cell-material. The so-called middle plate at the time of its origin, in the manner previously described, is at once separated into segmentally arranged cords, which are afterwards metamorphosed into the mesonephric tubules. The differentiation out of a blastema is therefore here, as in most cases, to be conceived of as an increase in the distinctness of already esta- blished structures, which constitute a cell-mass that appears uiidifferentiated, but only on account of our limited means of discrimination. In the Amphibia, Teleosts, and Ganoids the origin of the m^sonephros deserves to be subjected to renewed investigation from the recently acquired points of view. Soon after their union with the mesonephric duct the individual mesonephric tubules begin to grow somewhat in length, to take 011 S-shaped curves, and to be differentiated into three regions. The middle region undergoes a vesicu'ar enlargement and is converted into a BOWMAN'S capsule. Individual transverse branches from the primitive aortse, which pass along close to the mesonephros, make their way to the capsules, and are there resolved into a tuft of capillaries. The knot of blood-vessels, or glomerulus, now grows into the epithelial vesicle, the median wall of which is pushed before it and invaginated into the interior. During this process the epithelial cells of the invaginated part of the wall become greatly flattened, whereas upon the opposite uninvaginated side they re- main tall and cuboidal. Such a structure, consisting of a vascular glomerulus and the enveloping BOWMAN'S capsule, is called a Mal- pighian corpuscle, an organ that is exceedingly characteristic of the primitive kidney (mesonephros) and the permanent kidney (meta- nephros) of Vertebrates. In addition to the enlarged middle part, there is to be distinguished on each mesonephric tubule a narrow connecting portion, which continues to increase in length, running to the mesonephric duct, and, secondly, a short portion connecting with the body-cavity. The latter is metamorphosed in different ways in the separate classes of Verte brates. In some, as in many of the Selachians, it retains its original connection with the body-cavity even in the adult animals ; it begins at the peritoneum with an opening, surrounded with ciliate cells, which was discovered by SEMPER and has been designated nephridial funnel or nephrostouie, and which in many respects recalls the THE ORGANS OF THE MIDDLE GERM-LAYER. 365 similar structures of the excretory organs of segmented Worms. In the most of the Vertebrates, however, special nephridial funnels are no longer developed, inasmuch as the mesonephric tubules soon after their origin completely detach themselves from the epithelium of the body-cavity as well as from the primitive segments, and thereby lose all relation to the body cavity. A mesonephros in the simple form in which it is at first produced develop- mentally is retained permanently only in Bdellostoma, a representative of the Cyclostomes. It here consists, as JO- HANNES MULLER has shown, of an elon- gated canal (fig. 206 A and B a) and short transverse tubules (b), which open into it at short intervals. The latter are no longer connected with the body-cavity by means of a nephridial funnel, but they enclose a vascular glomerulus at their blind end (fig. 206 B c), which is some- what set off by a constriction. In all remaining Vertebrates the meso- nephros is metamorphosed into a more voluminous and more complicated organ. For the originally short tubules, which run transversely into the mesonephric duct, begin to grow in length, and at the same time to be thrown into numerous folds (fig. 207 s.t). Moreover there are formed mesonephric tubules of a second and third order. These again are also formed independently of the mesonephric duct dorsal to the first-formed transverse tubules; their blind ends approach the primary urinary tubule and join its ter- minal part, which is thereby converted into a collecting tube. At the same time a Malpighian body is formed on each of them also. Still more exhaustive investigations concerning the formation of the second ary and tertiary mesonephric tubules, especially for the higher Vertebrates, appear to me to be desiiable. In the Selachians, according to the statements Fig. 206.— Parts of the mesone- phros of Myxine, after J. MULLER. a, Mesonephric duct ; b, mesone- phric tubules ; c, glomerulus ; d, afferent artery ; e, efferent artery. B a part of A more highly mag- nified. 366 EMBRYOLOGY. of BALPOUB, which are also confirmed by others, the epithelium of the already existing Malpighian glomeruli is the starting-point of a proliferation. Cell-buds grow out from the latter and toward the urinary tubules lying in front of them, with which their blind ends fuse. After this union has been effected they detach their other ends from the parent-tissue. Through the development of compound urinary tubules, each of the branches of which is provided with a Malpighian corpuscle, the primitive kidney (mesonephros) acquires a complicated structure. But this is not uniform in all its parts; ordinarily the condition realised in the most of the Vertebrates is this : the anterior part, which afterwards enters into relation with the sexual glands, retains simple tubules, and only the posterior part passes into a more complicated form by the production of secondary and tertiary fundaments. The more the mesonephros, with its tortuous tubules and its Fig. 207.— Diagram of the original condition of the kidney in an embryo Selachian, after BALFOUR. pd, Mesonephric duct, which opens into the body-cavity at o, and into the cloaca at the other end ; x, line along which the Mulleriau duct (lying below in the diagram) is divided off from the mesonephric (Wolffian) duct; s.t, mesonephric (segmental) tubules, which on the one hand open into the body-cavity, on the other into the mesonephric duct. further differentiation, increases in volume, the more it becomes delimited from its surroundings and emerges from the wall of the body into the body-cavity as a distinctly differentiated organ, where it forms a protruding band on either side of the mesentery (fig. 210 WK). On a cross section one can recognise in the human embryo also (NAGEL) two distinctly separated regions on each urinary tubule — (1) a larger one, which begins with the BOWMAN'S capsule and is lined with large .epithelial cells containing abundant protoplasm, and (2) a narrower region with small cubical elements. The latter is the collecting tube, which unites with other collecting tubes before it opens into the mesonephric duct ; on the other hand, probably the former region alone has the secretory function, as also it is best developed at the time of the greatest prominence of the Wolman body. The Malpighian glomeruli, likewise, attain at this time in human embryos a remarkable size (NAC;EL). THE ORGANS OF THE MIDDLE GERM-LAYER. 367 The further fate of the primitive kidney is very different in the separate classes of Vertebrates. In the Ananmia, i.e., in Fishes and Amphibia, it becomes the permanent urinary organ, through which the excretions of the body are eliminated ; but besides that, it also acquires relations to the sexual apparatus, upon which, however, I shall not enter until later. In Birds and Mammals, on the contrary, the primitive kidney is functional only a short time during embryonic life ; soon after its establishment it undergoes profound regressive changes, and at last is preserved only in part, in so far as it enters into the service of the sexual apparatus, and, as we shall likewise see later, participates in conducting away the sexual products. (c) The Kidney. (Metanephros.) The secretion of urine is assumed in the higher Vertebrates by a third gland, which is established at the posterior end of the meso- nephric duct — the permanent kidney. The method of its formation, which appears to differ at first from that of the mesonephros, presents great obstacles to its investigation. It is most accurately known from studies on the development of the Chick through the works of SEDGWICK. At the beginning of the third day of incubation in the Chick there grows out of the [posterior] end of the mesonephric duct, from its dorsal wall, an evagination — the excretory duct of the kidney or ureter. There are two conflicting views relative to its connection with the development of the kidney. According to the older view, which is still shared by many, the kidney is formed from the ureter in the manner of an ordinary glandular growth. It is maintained that paginations take place which give rise to other evaginations, and thus produce the whole parenchyma of the kidney. According to the second view, which has been formulated especially by the more recent ^mbryologists, — by SEMPER, BRAUN, FURBRINGER, SEDGWICK, and BALFOUR, — the permanent kidney is, on the contrary, developed out of two different fundaments, which come into relation with each other •only secondarily : the medullary substance with its collecting tubules out of the ureter, the cortical substance with the tortuous tubules and the loops of HENLE, on the other hand, out of a special fundament. According to this view there would be an agreement between the development of the kidney and primitive kidney, in as far as in the latter the mesonephric duct and the mesonephric tubules also arise separately, and only secondarily enter into relation with each other 368 EMBRYOLOGY. by means of fusion. The agreement here indicated is a not unim- portant ground for my giving preference to the second rather than the first view. As far as regards the details of the conditions, they are in the Chick — according to the investigations of SEDGWICK, which BALFOUR has confirmed — as follows : .the ureter, which has arisen by an evagi- nation from the end of the mesonephric duct, grows into that part of the middle plate which is located at the end of the Wolffian body in the region of the thirty-first to the thirty-fourth primitive segment. The fundament, however, is not at once and at this place converted into a kidney, but first undergoes, after the ureter has penetrated into it, a very considerable change in position; to- gether with the ureter it grows forward on the dorsal side of the mesonephric duct farther; it meanwhile gradually enlarges, and begins to show internal differentiation only when it has come into this new position. One then sees that tortuous tubules become more and more distinct in the small-celled mass and that in their walls Malpighian cor- puscles are established. One finds, in addition, that there are e vagina ted from the end of the ureter separate sacs, which grow out into collecting tubes, and probably later — certainty in regard to this has not yet been established — join the tortuous tublues which have arisen in the cortical portion of the kidney. This voluminous organ, which has soon outstripped the mesonephros in size, is originally composed of individual lobes separated by deep furrows (fig. 208). The lobation is retained permanently in Reptiles, Birds, and some of the Mammals (Cetacea). In most Mammals, however, it disappears, in Man soon after birth. The surface of the kidney acquires an entirely smooth condition ; the internal structure (Malpighian pyramids) alone points to its composition out of indi- vidual portions, originally also separated externally. For the sake of clearness the development of the three regions, pro-, meso-, and metanephros, has been treated as a whole up to this point. Consequently there have been left out of consideration for the time being other procrssis which are taking place in the vicinity Fig. 208. — Kidney and suprarenal body of a human embryo at the end of pregnancy. Tin, Suprarenal body ; n, kidney ; I, lobes of the kidney ; hi, ureter. THE ORGANS OF THE MIDDLE GERM-LAYER. 36$ of the fundament of the mesonephros at the same time. These havo to do with the evolution of the Miillerian duct and the sexual organs. (d) The Miillerian Duct. The Miillerian duct is a canal which is found lying at first parallel and close to the mesonephric duct in the embryos of most Vertebrates (Selachians, Amphibia, Reptiles, Birds, Mammals). It is a canal that is established in both sexes in the same manner, but subsequently acquires in each a different function. It takes its origin in the lower Vertebrates from the mesone- phric duct, as can be most easily followed in the Selachians (SEMPER, BALFOUR, HOFFMANN). In this case the mesonephric duct becomes enlarged, acquires in cross section (fig. 209 4) an oval form, and pre- sents a different condition in its dorsal (sd) and ventral (od) halves, the latter being at the same time in immediate con- tact with the peritoneal epithelium. The mesonephric tubules open into the dorsal half, while ventrally the wall is consider- ably thickened. Then a separation of the two parts takes place, which begins at a little distance from the anterior end (cross sections 3-1) and proceeds backward to the point of opening into the hind gut. Of the parts which result from the fission, that which lies dorsally is the permanent mesonephric duct (wd) ; it exhibits at first a broad lumen and receives the urinary tubules (fig. 207 st). Ventrally, between it and the epithelium of the body-cavity, lies the Mullerian duct (fig. 209 od and fig. 207), which is at first only a narrow passage, but later a much enlarged one. In the process of fission the anterior initial part of the primary canal (fig. 207 pd), which was described at p. 353 as pronephros and which opens into the body-cavity by means of a ciliate funnel (o),. becomes a part of the latter duct, and the ciliate funnel becomes the ostium abdominale tubae. Also in the case of the Amphibia the Mullerian duct is developed by being- split off (FtJRBRiNGER, HOFFMANN) from the mesonephric duct, with the excep- 24 Fig. 209. —Four cross sections through the anterior region of the mesonephric duct of a female embryo of Scylliun canicula, after BALFOUR. The figure shows how the Mul- lerian duct (od) is split off from the meaonephric duct (sd and wd). 370 EMBRYOLOGY. tion of the anterior end, which bears the orifices leading into the body-cavity A small territory of the epithelium oE the body-cavity immediately adjacent to the pronephros serves for the construction of this portion. The epithelium becomes thickened, owing to the fact that its cells take on a cylindrical shape ; it sinks in to constitute a groove, and then becomes constricted off from the surrounding tissue in the form of a short funnel, which in front remains in connection with the body-cavity by means of a broad opening, but posteriorly becomes continuous with the part of the Miillerian duct that is produced by fission. The pronephric tubules and the glomerulus degenerate. The fission of the single mesonephric duct into two canals lying close together is a peculiar process, which is intelligible only upon the assumption that the mesonephric duct has possessed a double function. Probably it originally served as an outlet for the secre- tions of the mesonephric tubules, and also by means of its pronephric funnel took up out of the body-cavity the sexual products (eggs or seminal filaments) eliminated into it at their maturity, and con- ducted them to the outside. Similar conditions are often observed in Invertebrates, e.g., in various divisions of the Worms, in which also the segmental canals, which break through the body-wall, transmit to the outside both secretions from the body and sexual products. In Vertebrates each of the two functions is assigned to a special canal, one of which loses its communication with the body- cavity, but remains in connection with the transverse mesonephric tubules, while the other retains as its part the ciliate funnel of the pronephros, and thus is adapted to conducting away the sexual pro- ducts (eggs). In Reptiles, Birds, and Mammals the manner of the development of the Miillerian duct is still a subject of scientific controversy. Most observers (WALDEYER, BRAUN, GASSER, JANOSIK, and others) state that at no time was a process of fission observed. According to their representation the Miillerian duct arises in Birds and Mammals quite independently as a new structure, at a time when the mesonephros is already well developed and has the form of a band- like body (the mesonephric fold) projecting into the body-cavity (fig. 210). One then sees on the lateral face of the anterior region of this body that the epithelium of the body-cavity over a limited area (a1) is thickened in a remarkable manner and composed of cylindrical cells, whereas elsewhere the cells are flattened. The thickened portion of the epithelium sinks down in the form of a funnel and applies itself closely to the mesonephric duct (?/), which is near at hand. The blind end of the funnel grows from this point backwards independently, as is usually asserted, by means of the proliferation THE ORGANS OF THE MIDDLE GEEM-LAYER. 371 of its own cells, and gives rise to a solid cord, which lies directly between the mesonephric duct and the peritoneal epithelium, which is here somewhat thickened. The funnel produced by the invagina- tion now becomes the ostium abdominale tubse, but the solid cord of cells, which is soon hollowed out and finally opens behind into the cloaca, becomes the Mullerian duct. If the representa- tion just given is cor- rect in all particulars, the Miillerian ducts in the Anamnia and the Amniota, al- though possessing the same location, form, and function, would still be non- liomologous organs, because their develop- ment is different. For 'the one is split off from the meso- nephric duct, the other is formed in- dependently by a new invagination of the epithelium. Such a surprising result appears to us, however, upon grounds of compara- tive anatomy, to be very improbable, and therefore the attempt made by some investigators to refer back the conditions found in the Amniota to such as exist in the Anamnia deserves every attention. This would be possible if the statements of BALFOUR AND SEDGWICK, which have however been called in question by others (JANOSIK), should be confirmed. As we have previously seen, there are two different regions to be distinguished on the Mullerian duct — an anterior, which is •the degenerated pronephros and bears the orifice of the tuba, Fig. 210.— Cross section through the mesonephros, the funda- ment of the Mullerian duct, and the sexual gland of a Chick of the fourth day, after WALDEYER. Magnified 160 diameters. m, Mesentery ; L, somatopleure ; a', the region of the germinal epithelium from which the Mullerian duct (2) has been invaginated ; a, thickened part of the germinal epithelium, in which the primary sexual cells, C and o, lie ; E, modi- fied mesenchyme out of which the stroma of the sexual gland is formed ; WK, mesonephros ; y, mesonephric duct 372 EMBRYOLOGY. and a posterior, which is formed by being split off from the mesonephric duct. Such a double origin BALFOUR AND SEDGWICK endeavor to establish for the Miillerian duct in the Chick also. The part produced by invagination of the peritoneum (fig. 210 z) they interpret as pronephros. A similarity with the latter they find in the fact that this part does not, according to their investigations, consist of a single invagination of the peritoneal epithelium, but of three open imaginations lying one behind the other, which are joined together by ridge-like epithelial thickenings which after- wards become hollow (fig. 211 gr 2, gr 3, r 2). From this ridge is formed a slightly curved, bhort duct, which communicates with the body-cavity through three openings. If this explanation is right, the most anterior fundament of the £ Vfd] Pig. 211 — Cross sections through two peritoneal imaginations out of which is formed the anterior region of the Miillerian duct (the pronephros) of the Chick, after BALFOUR AND SEDGWiCK. A is the llth, B the 15th, C the 18th section of the whole series. grit, 3, Second and third furrows ; r 2, second ridge ; wd, Wolffian duct. excretory system of the Chick, which was described on page 356 as pronephros, must have undergone p. change in position, and, with the appearance of the Wolffian body, have slipped backward somewhat along this organ. As long as this alteration of position is not demonstrated by the study of intermediate stages, the interpretation,, however probable it may seem to us, still lacks actual proof, As far as regards the posterior, longer region of the Miillerian duct, .SEDGWICK maintains that it arises by being split off from the mesonephric duct. One always finds, according to his researches, the pronephric part of the Miillerian duct in union at its posterior end with the ventral wall of the mesonephric duct. He maintains that it is enlarged at the expense of the latter in somewhat the same manner as the mesonephric duct grows from in front backwards by a proliferation of the outer germ-layer. The cross sections A and R THE ORGANS OP THE MIDDLE GERM-LAYER. 373 of figure 212 exhibit this condition. Figure B shows the place where the ventral wall of the mesonephric duct is thickened into a ridge (md) by an increase of the epithelial cells ; upon a cross section (A) made farther forward the thickened part has become detached as a cord (md), which subse- quently becomes still more isolated and ac- quires a cavity of its own. The condition recalls very clearly the appearances which the cross sec- tions through embryo Selachians (fig. 209) gave. According to the observations of SEDG- WICK, therefore, the anterior end of the Mullerian duct would be derived from the pronephros, but the posterior end by a splitting off of cells from the mesonephric duct. Thus an agreement with the conditions in the non-amniotic Vertebrates would be established. Fig. 212.— Two sections to show the union ox the solid terminal part of the Mullerian duct with the mesonephric duct in the Chick, after BALFOUB AND SEDGWICK. In A the terminal part of the duct is still quite distinctly separate ; in B it has united with the wall of the mesone- phric duct. md, Mullerian duct ; Wd, Wolffian duct. M.g. Fig. 213.— Cross sections through the Wolffian and Mullerian ducts of two human embryos, after NAGEL. A A female embryo 21 mm. long. £, A male embryo 22 mm. long. W.g., Wolffian duct ; M.g., end of the Miillerian duct in process of development. It still deserves to be especially mentioned that in human embryos also the Miillerian ducts (fig. 213 A and B M.g.) during their development have their posterior ends fused for a short distance with the mesonephric duct (W.g.). NAGEL, to whom we are indebted for this fine observation, expresses himself, it is true, against a splitting 374 EMBRYOLOGY. off ; however, the similarity with the conditions found in the Chick and the non-amniotic Vertebrates is not to be denied, and has indeed been emphasised by NAGEL. (e) The Germinal Epithelium. In Vertebrates, at the time when the Miillerian duct is established,, the first traces of the sexual glands are also to be recognised. The parent-tissue of these is likewise the epithelium of the body-cavity. This acquires — for example in the Chick, which is to serve as the foundation for our description — a different appearance in the various regions of the body-cavity (fig. 210). In most places the epithelia be- come extraordinarily flattened and assume the condition of the perma- nent " endothelium." Also on the mesonephros, which projects into the body-cavity as a thick, vascular fold, the epithelium is for the most part greatly flattened, but retains its original condition (1) on its lateral surface along a tract (a') from which, as we have previously seen, the Miillerian duct is formed, and (2) along a tract (a) which stretches from in front backward along the median side of the mesonephros ; the signification of the latter has been correctly estimated by BORNHAUPT and by WALDEYER, who have characterised it as germinal epithelium. From it are derived the germ-cells : in the female the primitive ova, in the male the primitive seminal cells. It is only in the very earliest stages that it is impossible to distinguish whether the germinal epithelium will be developed into testis or ovary* Differences soon appear, which allow a positive determination. We shall take up first the development of the ovary, then that of the testis. (/) The Ovary. The development of the ovary is tolerably well known both in the lower and the higher Vertebrates, except for a few controversial points. I can therefore limit myself simply to the presentation of the results which have been acquired in the case of the Chick and Mammals. At about the fifth day of incubation the germinal epithelium in the Chick increases a good deal in thickness, becoming two to three layers of cells deep. Certain elements in this thickening are promi- nent ; they are distinguishable (fig. 210 C and o) by their richness in protoplasm and by their large round nuclei. Because they stand in the closest relation to the development of eggs, they have been designated as primitive eggs by WALDEYER, who was the first to study them in detail. THE ORGANS OF THE MIDDLE GERM-LAYER. 375 Fig. 214.— Cross section through the ovary of a, Rabbit 5 days old, after BALFOUR. Highly magnified. k.e, Germinal epithelium ; u.ei, primitive (or primordial) ova ; ei.b, egg-nests ; bi, connec- tive tissue. Beneath the germinal epithelium there is to be found, even at that time, embryonic connective tissue with stellate cells (j?£), which are in an active state of proliferation. In this way there arises on the median side of the mesonephros the ovarian ridge, which is separated from the urinary tubules by a small quantity of embryonic connective substance.- Changes similar to those of the Chick occur in Mammals, with this difference, that the ger- minal epithelium appears to attain a much greater thick- ness. In older stages of develop- ment the boundaries between the germinal epithelium, which is in process of rapid prolife- ration and therefore exhibits numerous figures of nuclear division, and the underlying connective tissue become less and less distinct. This results from the simple fact that a jwocess of mutual ingrowth now occurs between the epithe- lium and the embryonic con- nective tissue (fig. 214). I purposely say a process of mutual ingrowth, for I leave it undetermined whether the germinal epithelium in con- sequence of its development grows into the embryonic con- nective tissue in the form of cords and distinct groups of cells, or whether the connective tissue penetrates with its projections into the epithelium. Probably both tissues are actively engaged in the process. In the phenomenon of intergrowth, which continues for a long time during development, two chief stages can be distinguished. At first there arise from the germinal epithelium both slender and stout cords and balls of cells (figs. 214 and 215), which have received from the name of their discoverer the designation PFLIJGER'S egg-tubes. Occasionally these are joined to one another by means of lateral Fig. 215.— Section through an egg-nest of a Rabbit 7 days old, after BALFOUR. ei, Ovum, the germinative vesicle (kb) of which exhibits a filar network ; bi, connective-tissue stroma ; /.z, follicular cells. 376 EMBRYOLOGY. branches. Together with the connective tissue separating them, they form the foundation for the cortex of the ovary. Afterwards they are covered over on the side toward the body-cavity with a thick continuous layer of connective tissue, which becomes the albuginea of the ovary ; they are thereby more sharply separated from the germinal epithelium (fig. 216 k.e), which is still preserved, even after this, as a layer of cubical cells upon the albuginea. There are two kinds of cells to be found in the Pfliigerian egg-tubes : jollicular cells and primitive ova (fig. 215 f.z and ei). Concerning the source of the former opinions are still contradictory (compare p. 382) ; according to my view both arise from the germinal epithelium. Whereas the follicular cells become by means of an uninterrupted process of division more numerous and smaller, the primitive ova increase in size continually, and their nuclei become very large and vesicular and acquire a distinctly developed filar network (kb). They rarely lie singly in the cords and balls of follicular cells, but ordi- narily in groups, which are designated as egg-nests. One frequently observes in the nests, as has been announced by BALFOUR and VAN BENEDEN, that several primitive ova become fused into a common, multinuclear mass of protoplasm — a syncytium. From this there is afterwards developed usually only a single egg. One of the numerous nuclei soon outstrips the others in size and becomes the germinative vesicle, whereas the remaining ones undergo degeneration and are dissolved. It is not to be concluded from these processes that the egg, as is occasionally asserted, corresponds to a multiple of cells ; the condition is more properly to be interpreted as follows : of the eggs contained in a nest, one outstrips the others in its growth and thereby represses them and employs them, in a certain sense as nutritive material, for its own growth. This is a process that occurs very frequently in Invertebrates, and in the phylum of the Arthropods has been studied with the greatest detail by WEISMANN. In these cases — the lower Crustacea and Insects — one can see how, step by step, out of numerous primitive ova which are originally contained in a germinal chamber of an ovariole, only one becomes the egg, whereas the others from an early period lag behind in development, then undergo degeneration, and in the form of products of degeneration are taken up as yolk-material into the persisting egg-cell. During the enlargement of the egg-cell the second stage of the process of intergrowth of epithelium and connective tissue is intro- duced : the stage of the formation of the foUick (fig. 216). At the boundary between the meduHary and cortical zones of the ovary the THE ORGANS OF THE MIDDLE GERM-LAYER. 377 surrounding connective tissue, carrying with ifc the blood-vessels, grows into the egg-tubes of PFLUGER (e.sch) and the nests (ei.b), and divides them all into spheroidal bodies, the individual follicles (/). Each such structure contains a single ovum, that is enveloped on all sides by a layer of follicular cells. The vascular connective tissue that grows around it becomes the follicular membrane or theca jfolliculi. The resolution into follicles continually advances from the me- e.sch ue e.scli' ei.b Tig. 216.— Part of a sagittal section of an ovary of a Child just born, after WALDEYER. Highly magnified. k.e, Germinal epithelium ; e.sch, PFLUGEE'S egg-tubes ; ue, primitive ova lying in the germinal epithelium ; e.sch', long PFLUGER'S tubes, in process of being converted into follicles ; ei.b, egg-balls [nests], likewise in process of being resolved into follicles ; /, youngest follicle already isolated ; gg, blood-vessels. • In the tubes and egg-nests the primordial eggs are distinguishable from the smaller epithelial cells, the future follicular epithelium. dullary substance toward the germinal epithelium ; however, there are preserved under it for a long time Pfliigerian tubes, which remain in connection with it by means of narrow epithelial cords (e.sch) and contain eggs in process of development. The formation of new Pfliigerian tubes and young ova is a process which continue in the lower Vertebrates throughout life, but in the higher appears to be limited to the period of embryonic ^development, or to the first years of life. In the first case, there 'being an unlimited capacity for the formation of new structures, 378 EMBRYOLOGY. egg-germs are found, even in the adult animal, sometimes in the most widely separated parts of the ovary, sometimes limited to definite regions of the gland. In the second case the period of forming primitive ova in the germinal epithelium bears a direct ratio to the total number of ova eliminated during the life of the- individual. Thus WALDEYER states concerning Man that in the second year after birth the formation of new ova can no longer be shown. Nevertheless in Man the number of ova contained in a single ovary is very great. They have been estimated to number in a, sexually mature girl 36,000. In other Mammals the production of new ova appears to last longer. PFLUGER'S tubes which were still connected with the germinal epithelium and contained small pri- mordial ova have been observed even in young animals (Dog, Rabbit, etc.). However, it has been questioned whether we here have really new structures or only primitive ova that in their development have remained stationary. It is maintained by VAN BENEDEN with certainty for a few Mammals, e.g., the Bat, that in the sexually mature animal PFLUGER'S tubes and primitive ova still continue to be produced from the germinal epithelium. In connection with the first formation of the follicle I will here add some statements about its further metamorphosis. This is very similar in the different Vertebrates, excepting Mammals. In most Vertebrates the follicle (fig. 216 f) consists at first of a small, centrally located egg-cell and a single layer of small follicular cells enveloping it. Soon both are more sharply separated from each other by means of a vitelline membrane. In older follicles both parts have increased in size. The follicular cells ordinarily grow out into long cylinders, and appear to play an important part in the nutrition of the egg. In many animals, e.g., in Sharks and Dipnoi, yolk-granules have been found in them, as in the egg itself, and it has been concluded from this, as well as from other phenomena, that the follicular cells take up nutritive substance from the vas- cular follicular capsule, and pass it along to the egg. Such a method of nutrition is made easier by the fact that the vitelline membrane (fig. 5 z.p) is traversed by tubules, through which the follicular cells (/.«) send protoplasmic filaments to the egg. When the egg has attained its full size, the follicular cells lose their significance as nutritive organs and become more and more flattened. In the lower Vertebrates the mature ova are generally eliminated in great numbers all at once, frequently in the course of a few days THE ORGANS OF THE MIDDLE GERM-LAYER. 379 or even hours. The discharge takes place by the rupture of the connective-tissue envelope, which causes the eggs to escape into the body-cavity, as in the Fishes and most of the Amphibia. After the elimination, the ovary, which up to tliis time was extraordinarily large and took up most of the space in the body-cavity, shrivels into a very small cord and now encloses only the young germs of ova, part of which are destined to mature during the next year. The formation of the follicle takes place in a somewhat different way in Mammals. The follicle originally contains, as in the remaining Vertebrates, only a single egg and a single layer of follicular cells, which are at first flat, then cubical, then cylindrical (fig. 216 f). For a long time these cells envelop the egg as a single layer, but Fig. 217 A and B.— Two stages ;n the development of the Graafian follicle. A with the follicular fluid beginning to be formed ; B with a greater accumulation of it. ei, Egg ; fz, follicular cells ; fzl, follicular cells which envelop the ovum and constitute the discus proligerus ; jf, follicular fluid (liquor folliculi) ; fk, follicular capsule (theca folliculi) ; :p, zona pellucida. they then grow, undergo division, and are converted into a thick envelope of many layers. But the difference from the course of development described above becomes still greater, owing to the fact that a fluid, the liquor folliculi, is secreted by the proliferated follicular cells, and collects in a small cavity at the side of the egg (fig. 217 Aff). In consequence of a considerable increase of the fluid, the originally solid follicle becomes converted finally into a large or small vesicle (fig. 217 £), which was discovered more than two hundred years ago by the Hollander REGNIER DE GRAAF and was held to be the human ovum. The structure has also been named after him the trraafian follicle. Such a follicle (fig. 217 B] now consists of (1) an outer connective- tissue, vascular envelope (fk), the theca folliculi ; • 380 EMBRYOLOGY. (2) lying on its inner surface, an epithelium composed of many layers of small follicular cells (fz), the membrana granulosa ; (3) the liquor folliculi (ff) ; and (4) the ovum (ei), which originally lay in the centre of the follicle, but which has now been crowded to the periphery. Here, enveloped in a great mass of follicular cells (fz1), it causes an elevation of the wall, — the discus proligerus, — which protrudes into the cavity. When the egg has reached complete maturity its elimination occurs by a collapse of the Graafian follicle, which has then at- tained in Man a diameter of about 5 mm. and causes an elevation at the surface of the ovary. The liquid of the follicle flows out through the rupture and at the same time carries away with it from the discus proligerus the egg, which comes first into the body- cavity, being surrounded by a small number of follicular cells, which still cling to the zona pellucida (fig. 5). The egg is then taken up by the oviduct. Into the cavity of the follicle produced by the flowing out of the liquid an effusion of blood takes place from the ruptured blood-vessels in the vicinity. The blood coagulates, and, accompanied by a prolifera- tion of the adjacent tissue, is converted into the yellow body, or corpus luteum, which is a characteristic structure of the ovary of Vertebrates. Both the follicular cells (membrana granulosa) which are left behind and the connective-tissue follicular capsule participate in this pro liferation. The follicular cells continue to multiply, penetrate into the interior of the coagulum, and after a time begin to undergo degeneration and to be dissolved into a granular mass. Vascular outgrowths from the capsule penetrate into the yellow body, and at the same time there is an extensive emigration of white blood- corpuscles or leucocytes, which likewise undergo fatty and granular degeneration at a later period. It is of great importance for the further development of the yellow body whether the egg set free is fertilised or remains unfertilised. For according as the one or the other event supervenes, the corpus luteum is distinguished as true or false. In the first case it acquires a much greater size, the maximum of which is reached in the fourth month of pregnancy. It then appears as a fleshy reddish mass. After the fourth month a process of degeneration begins. The products of degeneration, which have resulted from the granular metamorphosis of the follicular cells and leucocytes, as well as from the coagulum of blood, are absorbed by the blood-vessels. Out of the decomposed coloring mutter of the blood there have arisen hsema- THE ORGANS OF THE MIDDLE GERM-LAYER. 3811 toidin crystals, which now give to the body an orange-red color. The connective tissue, originally with an abundance of cells, begins to- shrivel, as in the formation of a scar ; as a result of these various processes of degeneration the yellow body, which projects beyond the surface of the ovary, begins to become considerably smaller, and is finally converted into a firm connective-tissue callus, which causes 4 a drawing in at the surface of the organ. When fertilisation has not occurred, the same metamorphosis and processes of growth it is true take place, but the false corpus luteum remains very much smaller. This is probably due to the fact that the afflux of blood to the sexual organs is very much less when there is no fertilisation than in case pregnancy takes place. In addition to the tubes of PFLUGER, — which arise from the germinal epithelium and produce the primitive ova, — in most classes of Vertebrates epithelial cords of anotJier kind and another origin enter into the composition of the ovary. As has been observed by various persons in Amphibia, Reptiles, Birds, and Mammals, there grow out from the Wolffian body, which lies in the immediate vicinity, epithelial shoots, the "sexual cords of the primitive kidney" and these penetrate toward the developing ovary even as early as the beginning of the intergrowth between germinal epithelium and connective tissue. They arise from the epithelium of the Malpighian corpuscles, as BRA UN has shown for Reptiles, HOFFMANN for Amphibia, . and SEMON for Birds. In Mammals, in which at present their sub- sequent fate has been most accurately traced out, they then unite with one another into a network at the base of the fundament of the ovary, which protrudes as a ridge into the body-cavity, and, pursuing tortuous courses, grow into contact with the tubes of PFLUGER. Whereas in Mammals the cortex of the ovary is de- veloped out of the latter, the former share in the composition of the future medullary substance, and are on that account designated as medullary cords. In the vicinity of the follicle they remain solid, whereas the part near the primitive kidney acquires a cavity which is surrounded by cylindrical cells. The medullary cords exhibit in different species of Mammals different degrees of development, as the comparative investigations of HARZ have established. In some animals, e.g., in the Pig and Sheep, they reach only to the base of the ovary, and therefore remain sepa- rated from the tubes of PFLUGER by a wide space ; in others they grow out into the vicinity of the latter, and in part apply themselves 382 EMBRYOLOGY. closely to them (Cat, Guinea-pig, Mouse, etc.), and take a very prominent part in the composition of the medullary substance. There are two antagonistic views relative to the significance of the sexual cords of the primitive kidney, or the medullary cords, in the formation of ova. According to KOLLIKER and ROUGET the medullary cords early fuse with the tubes of PFLUGER and furnish to them the cells which become the follicular epithelium. The cells contained in a follicle would, according to this, come from two sources — the follicular cells would arise from the primitive kidney, the eggs from the ger- minal epithelium. Most embryologists dispute this. According to their observations the medullary cords only exceptionally extend close up to a follicle, in many Mammals they do not reach it at all ; consequently not only the primitive ova but also the accompanying follicular cells must be furnished by the germinal epithelium. I also favor the latter view, which appears to me to be best supported by the facts. But what significance the medullary cords have will be better understood when we have become acquainted with the develop ment of the testis, to which we shall now proceed. (g) The Testis. I will at once state that our knowledge of the development of the testis is less complete than that of the development of the ovary. The conditions appear to me to be the clearest in the non-amniotic Vertebrata. We possess here the pioneer researches of SEMPER and BALFOUR on the Selachians, and of HOFFMANN on Amphibia. All these investigators have, with one accord, come to the conclusion that the male sexual products, as well as the female, arise from the germinal epithelium of the body-cavity. In males also there is to be recognised in the region of the primitive kidney a special thickened band of tall epithelial cells, in which are imbedded larger cells with vesicular nuclei, the primitive spermatic cells. In the Sharks, the conditions of which I shall make the basis of the further description, they form irregular cords of cells, the " Vorkeimketten " of SEMPER (fig. 218 A}. Out of these are developed small, spherical, follicular- like bodies (fig. 218 B), by the ingrowth of surrounding connective tissue into the cords, which are thereby divided up. Thus far, therefore, complete agreement exists in the development of both kinds of sexual products. But whereas in the case of the ovary one cell in each follicle increases in size and is converted into the ovum, a like process does not take place in the male ; here the THE ORGANS OF THE MIDDLE GERM-LAYER. 383 follicle-like structures become hollow and thus converted into seminal ampullae, whose epithelial cells gradually grow out into long cylinders. The greater part of these become seminal mother-cells, which by many repeated divisions are converted into sixty seminal cells, each of which is metamorphosed into a seminal filament. Since the filaments derived from each seminal mother-cell always arrange themselves parallel to one another, it is easily understood why before the attainment of complete maturity the seminal fila- ments are found united in great numbers into bundles. Whereas the testis, like the ovary, draws its specific histological components di- rectly from the germinal epithelium, it acquires its efferent ducts from the primitive kidney. As in the female, so also in the male, epithelial shoots, the sexual cords (genital canals of HOFFMANN), grow from the primitive kidney to- ward the testis ; in the Amphibia they arise as proliferations from the •cells of the wall of certain Malpighian corpuscles; in the Selachians, on the con- trary, they sprout out in a somewhat different manner from the ciliate funnels. Arrived at the base of the testicular ridge, they are joined together into a longitudinal canal, from which fine tubules are sent still farther into the substance of the testis, where they unite with the structures that take their origin in the germinal epithelium. As figure 218 B shows, the efferent tubules (sc) in Selachians at first apply their blind ends to the ampullae, and enter into open 384 EMBRYOLOGY. communication with them, but only after the maturation of the seminal filaments begins. Many differences of opinion still prevail concerning the development of the testis in the higher Vertebrates. It is true that the presence of a germinal epithelium upon the surface of the mesonephros has also been established in this case by WALDEYER for the male, but its participation in the fundament of the testis has been called in question. According to the original account of WALDEYER, which is still defended by many investigators, especially by KOLLIKER, the seminal tubules are morphological products of the primitive kidney. However, more recent researches, which it must be admitted do not yet harmonise with one another in all points, indicate that the development of the testis of Reptiles, Birds, and Mammals agrees with that of non-amniotic Vertebrates in the main outlines. la continuation of the work of BORNHAUPT and EGLI, who it is true worked with incomplete methods of investigation, BRAUN has recently maintained for Reptiles, SEMON for the Chick, MIHALKOVICS and JANOSIK for the latter and for Mammals, that in the male also the germinal epithelium begins to proliferate, penetrates into the depths of the testis, and furnishes the primitive seminal cells. The tubules, which according to KOLLIKER and WALDEYER grow into the funda- ment of the testis from the primitive kidney, — the sexual cords, — serve only for carrying away tne semen. As stated by BRAUN for Reptiles, and by SEMON for the Chick, they sprout out from the epithelium of Malpighian corpuscles, as in the case of the Amphibia. Although according to these accounts the double origin of the substance of the testis, on the one hand from the germinal epithelium, on the other from the primitive kidney, can no longer be well called in question, nevertheless in the details many conditions, which are still differently described in the higher Vertebrates, demand renewed investigation. Before all else this point should be still further explained : In what proportion do the epithelial cell* furnished by the germinal epithelium and those by the primitive kidney share in the formation of the testicular substance ? Are the tubules which produce the semen formed exclusively from germinal epithelium, or is it only the seminal mother-cells which have this origin, while there are associated with the latter indifferent cells from the " sexual cords of the primitive kidney"? I hold it to be the more probable that the tubules producing the semen, the tubuli seminiferi, are derived from the germinal epithelium; the tubuli recti and the rete testis, on the contrary, from the pi'imitive kidney. THE ORGANS OF THE MIDDLE GERM-LAYER. 385 NAGEL has studied the development of the testis in human embryos. Accord- ing to his description also, there arise from the actively proliferating germinal epithelium numerous cords, in which large primitive seminal cells are imbedded. The cords afterwards become the seminal tubules. In Man there prevails from the beginning, as NAGEL remarks, such a great difference between the two sexes, both in the form of the original germinal ridge and in the whole process of its differentiation, that one can recognise in the anatomical structure of the sexual glands from a very early stage whether one has before him a male or a female. (h) Metamorphosis of the Different Fundaments of the Urogenital System into the Adult Condition. We have become acquainted in the preceding pages with the first development of the various parts which constitute the foundations of the urogenital system. These are (fig. 219) three pairs of canals — the mesonephric ducts (ug), the Miillerian ducts (mg), and the ureters (hi) — and in addition a great number of glandular structures — pronephros, mesonephros (un), metanephros (n), and the sexual glands (kd), ovary and testis. It will be my task in what follows to indicate how the ultimate condition is derived from these embryonic fundaments. In this I shall limit myself, in the main, to Man, because we now have to do with more easily investigated, and in general well-known conditions. In a human embryo eight weeks old (fig. 220) the fundaments, if we neglect differences which are recognisable only by the aid of the microscope, are so similar in male and female as to be i ndistinguishable . All the glands lie at the sides of the lumbar vertebras : farthest forward the kidney (n), which is a small bean-shaped body ; upon this lies the suprarenal body (nn), that at this time is dispropor- tionately large and is to be seen only on the left half of the figure. Somewhat lateral to the kidney one sees the primitive kidney (un) as an elongated, narrow tract of tissue. It is attached to the wall of the trunk by a connective-tissue lamella, a fold of the peritoneum, the so-called mesentery of the primitive kidney. In the middle of the gland it is rather broad, but above, toward the diaphragm, it is elongated into a narrow band, which KOLLIKER has described as the diaphragmatic ligament of the primitive kidney. Upon careful examination one also observes at the lower end of the primitive kidney a second fold of the peritoneum, which runs from it to the inguinal region (figs. 219 and 220 gh). It encloses a firm strand of connective tissue, a kind of ligament, that is destined to play a 25 386 EMBRYOLOGY. part in the development of the female and male sexual organs — the inguinal ligament of the primitive kidney. It subsequently becomes in man the gubernaculum Hunteri, in woman the round ligament of the uterus (ligamentum teres uteri}. On the median side of the primitive kidney is found either the testis or the ovary (kd), according to the sex of the embryo, both sexual organs still being at this time small oval bodies. They also possess me- senteries of their own, a mesorchium or meso- varium, by means of which they are con- nected with the root of the primitive kidney. As long as the sexual organs retain their posi- tions on each side of the lumbar vertebrae, the blood-vessels that supply them run in an exactly transverse direc- tion : the arteria sper- matica from the aorta to the ovary or the testis, the vena sperma- tica from the gland to the vena cava inferior. The various efferent ducts lie at this time close together at the margin of the mesone- phric fold (fig. 219), the most anterior [ventral] being the Miillerian duct (mg). Farther back- wards toward the pelvis the ducts of both sides approach the median plane (fig. 219), whereby the Miillerian duct (mg) comes to lie for a certain distance on the median side of and then behind [dorsal of] the mesonephric duct (ug), so that altogether it describes around the latter a kind of spiral course. When they reach the lesser pelvis, cl' Tig. 219.— Diagram of the indifferent fundament of the urogenital system of a Mammal at an early stage. n, Kidney ; kd, sexual gland ; un, primitive kidney ; ug, mesonephric duct ; mg, Miillerian duct ; ntf, its an- terior end ; gh, gubernaculum Hunteri (mesonephric inguinal ligament) ; hi, ureter ; hi', its opening into the urinary bladder ; ug", mg", openings of the mesone- phric and Mullerian ducts into the sinus urogenitalis (sug) ; md, rectum ; cl, cloaca ; ghS, sexual eminence ; gw, sexual ridges ; cl', external orifice of the cloaca ; hbl, urinary bladder ; hbl', its elongation into the urachus (the future lig. vesico-umbilicale). THE ORGANS OF THE MIDDLE GERM-LAYER. 387 the four ducts are united behind the bladder (hU) into a fascicle, the genital cord • this union is due to their becoming surrounded by the umbilical arteries — which have at this time attained a large size, and which run from the aorta on both sides of the bladder up to the umbilicus — and to their being, as it were, tied up into a bundle by them. In a cross section through the genital cord (fig. 228) we find the mesonephric ducts (ug) some- what more anterior [ventral] and at the same time farther apart than the Miillerian ducts (mg), which are a little behind them and pressed quite close together in the median plane. In older embryos there arise in the evolution of the urogenital system differences between the two sexes which are visible even •externally and which become more distinct from month to month. These result from fundamental metamorphoses, which the whole apparatus continually undergoes in its separate parts. In connection with this some originally quite large fundaments undergo almost complete degeneration; of those which remain some are serviceable only in the female, others only in the male ; when not employed, they disappear. Moreover the conditions which were referred to at the beginning of the description are extensively altered by the fact that the sexual organs surrender their original position, on either side of the lumbar vertebrae, and move farther downward into the pelvic cavity. I describe first the changes in the male, then those in the female. Fig. 220. — Urinary and sexual organs of a human embryo 8 weeks old, after KOL- LIKER. Magnified about 3 diameters, and seen from the ventral side. nn, Right suprarenal body ; un, primitive kidney; n, kidney; ung, mesonephric duct ; gJi, HUNTER'S directive or inguinal ligament (gubernaculum Hunteri or liga- mentum uteri rotundum) ; m, rectum ; b, bladder ; kd, sexual gland. (A) The Metamorphosis in the Male. Descensus testiculorum. Whereas the testis (figs. 221 and 222) by conglomeration of the seminal tubules becomes a bulky organ (h), the mesonephros (nh+pa) is retarded in its development more and more, and is at the same time differently metamorphosed in its anterior and its posterior portions. The anterior or sexual part of the primitive kidney (nh), 388 EMBRYOLOGY. which has come into communication with the seminal tubules by means of individual canals, in the manner previously described, and has thereby furnished the rete testis and the tubuli recti, is converted into the head of the epididymis. It exhibits in the tenth to the twelfth week from ten to twenty short transverse canals, which are now to be designated as vasa efferentia testis. They unite in the- mesonephric duct (fig. 222), which continues to have a straight course, and has now become the seminal duct (si, vas deferens). During the fourth and fifth months the individual canals begin to grow in length and thereby to become tortuous. The vasa efferentia in this way produce the coni vasculosir which are at once the initial part of the vas deferens and the tail of the epididymis. - h Incidentally let it be stated that near the external opening of the vas deferens, as it passes along the posterior surface of the bladder, there arises in the third month a small e vagi nation, which becomes the seminal vesicle (sbl), The posterior region of the primitive kidney (pa) degenerates into very in- significant remnants. In older embryos one still finds for a time, between vas deferens and testis, small, tortuous canals, usually blind at both ends, be- tween which degenerated Malpighian corpuscles also occur. The whole forms a small yellow body. In the adult these remnants are still further reduced; they produce on the one hand the vasa aberrantia of the epididymis, and on the other the organ discovered by GIRALDES, the paradidymis. The latter consists,, according to HENLE'S description, of a small number of flat, white bodies, lying in contact with the blood-vessels of the seminal cord, each of which is a knotted tubule blind at both ends ; each tubule is lined with an epithelium containing fat, and is enlarged at its blind ends into irregularly lobed vesicles. The Miillerian ducts (fig. 222 mg) do not acquire in the male any function, and therefore, as useless structures, undergo degeneration ; the middle region in fact usually disappears without leaving a trace although it has been for a time during embryonic life demonstrable as Fig. 221.— The internal sexual organs of a male human embryo 9 cm. long, after WALDEYER. Magnified 8 diameters. h, Testis ; nh, epididymis (sexual part of the primitive kidney) ; pa, paradidymis (remnant of the primitive kidney); si, vas deferens (duct of the primitive kidney); which extends downward to the cloaca, of which it is, as it were, the continuation. 400 EMBRYOLOGY. In the following weeks of development the eminence protrudes still more, and thereby becomes converted into the genital member, which is at first possessed by both sexes in the same condition ; meanwhile the groove (gr) on its under surface becomes deeper, and surrounded, at the right and left, by projecting folds of the skin, the genital folds (gf). (Compare also the diagrams fig. 219 gho, gw, cl' and fig. 229 cp, Is, cl.) Alterations follow (fig. 231 M and TF) by which the cloaca is differentiated into two openings, one lying behind the other, the anus (a) and the separate urogenital opening (ug). The deep partition (fig. 229) by which the sinus urogenitalis and the rectum are separated from each other begins to grow outward, and at the same time folds also arise on the lateral walls of the cloaca and unite with it. Thus a membrane (fig. 231 d) is developed which separates a posterior opening (a), the anus, from an anterior opening, the entrance to the sinus urogenitalis (ug). Inasmuch as this partition continues to become thicker up to the end of embryonic life, it finally crowds the two openings far apart and forms between them the perinaeum (fig. 231 M* and W* d). In this way the anus (a) moves entirely out of the territory of the previously mentioned genital ridge (fig. 230 gw). From the fourth month onward great differences arise in the develop- ment of the external sexual parts in male and female embryos. In the female (fig. 231 W and W*) the metamorphoses of the originally common embryonic foundations are on the whole only slight; the genital eminence grows only slowly and becomes the female member, the clitoris (cl). Its anterior end begins to thicken and to be marked off from the remaining part of the body as the glans. By a process of folding in the integument there is developed around it (fig. 231 W* vh) a kind of foreskin (the praeputium clitoridis). The two genital folds (W gf), which have bounded the groove on the under surface of the genital knob, take on a more vigorous develop- ment in the female than in the male, and are converted into the labia minora ( W* ksch). The space between them (W ug), or the sinus urogenitalis, which receives the outlet of the urinary bladder and the vagina developed by the fusion of the MUllerian ducts, is called the vestibulum vaginae (W* vv). In the female the genital ridges ( W gw), owing to the deposition of fatty tissue, become very volu- minous, and are thus converted into the labia majora (W* gsch). The corresponding fundaments pass through much more essential metamorphoses in the male (fig. 231 3f and M*). By an extra- ordinarily vigorous growth in length the genital eminence is THE ORGANS OF THE MIDDLE GERM-LAYER. Fig. 230. 401 Fig. 231. d&r Figs. 230 and 231.— Six stages in the development of the external sexual organs in the male and the female, after the ECKER-ZIEGLEK wax models. Fig. 230 A and B. —Two stages in which a difference of the sexes is not yet to be recognised, B from an embryo 8 weeks old. Fig. 231.— The two stages 3/and M* exhibit the metamorphosis of the original fundament in the male in embryos 2£ and 3 months old respectively. The stages W and W* present the metamorphosis in the female (2% and 4£ months). The same designations are used for all of the figures. he, Posterior paired extremity ; do, cloaca ; gh, genital eminence ; gf, genital fold ; gr, genital groove ; gio, genital ridges ; gp, glans penis ; cl, clitoris ; d, perinaeum ; a, anus ; ug, entrance to sinus urogenitalis or vestibulum vaginae ; ve, vestibulum vaginae ; vh, foreskin (prepuce) ; hs, scrotum ; d & r, raphe perinei and scroti ; gsch, labia majora ; ksch, labia minora. 26 402 EMBRYOLOGY. converted into the male member, or the penis, which corresponds to the clitoris of the female. Like the latter, it possesses an anterior knob-like enlargement, the glans (M gp), which is embraced by a fold of the skin, the pneputium (M* vh). The sinus urogenitalis, which in the female remains short and broad as the vestibulum vaginae, is in the male converted by a process of fusion into a long narrow canal, the urinary tube or urethra. This results from the fact that the furrow on the under surface of the genital protuberance (M gr) becomes elongated during the development of the latter and at the same time deeper, and that the sexual folds (gf) bordering it protrude farther, coming into immediate contact along their edges (M*) as early as the fourth month, and begin to fuse together. The posterior end of the urethra early (second month) undergoes changes by which the prostata (fig. 222 pr) is formed. The walls become greatly thickened, acquire non-striate muscular tissue, and constitute a ring-like ridge, into which evaginations from the epi- thelium of the tube penetrate, and by their branchings furnish the glandular portions of the organ. On its posterior wall are found, as is well known, the openings (dej) of the vasa deferentia, and between them the sinus prostaticus or uterus masculinus (um), produced by the fusion of the Miillerian ducts. The genital ridges (fig. 231 M gw), which in woman become the labia majora, also undergo a fusion in man. They surround the root of the penis and then fuse in the median plane, where the place of union is indicated afterwards by the so-called raphe scroti (M*r). Into the scrotum (M* hs) thus formed the testes, toward the end of embryonic life, migrate, as previously described. From the fact that originally the external sexual parts are con- stituted exactly alike in both sexes, it is evident why, with a derangement of the normal course of development, forms come into existence in which it is sometimes extremely difficult to determine whether one has to do with male or female external parts. These are cases which in earlier times were erroneously designated as hermaphroditism. There are two ways in which they may arise. They are either to be referred to the fact that in a female the process of development has proceeded further than normally (i.e., as in the male), or that in a male the process of development has suffered an early interruption, and thereby led to formations which are similar to the female genital parts. As far as regards the first kind of malformations, the genital eminence in the female occasionally assumes such a size and form THE ORGANS OF THE MIDDLE GERM-LATER. 403 that it resembles in every particular the male organ. The resem- blance may become even greater, when the ovaries migrate into the inguinal region instead of the true pelvis, pass through the wall of the abdomen, and become imbedded in the labia majora. In con- sequence of this the latter lie upon the root of the large clitoris and simulate a kind of scrotum. The malformations which have given occasion for the assumption of hermaphroditism are of more frequent occurrence in the male. They are attributable to the fact that the processes of fusion which normally take place are interrupted. We then have a genital member, which ordinarily is rudimentary, along the under side of which there runs only a furrow instead of the urethra, a malforma- tion which is designated as hypospadias. With this morphological deficiency may be united, secondly, an arrest of the normal descent of the testes. The latter remain in the body-cavity, and the genital ridges thus acquire a great similarity to the labia majora of the female. III. The Development of the Suprarenal Bodies. The discussion of the suprarenal bodies best follows that of the urogenital system. For, aside from the fact that the suprarenal bodies and the genito-urinary organs are in all Vertebrates very closely connected spatially, they also appear to stand in very close relation to each other in the history of their development. At least the recent investigations of WELDON, JANOSIK, and MIHALKOVICS point that way, and are perhaps also sufficient to suggest the direction of the physiological research by which one can acquire an explanation concerning the ever problematic function of these bodies. As is well known, there are to be distinguished in the suprarenal bodies two different substances, which in Mammals are described, according to their mutual relations, as medulla and cortex. Most investigators ascribe to them a double origin. BALFOUR, BRAUN, KOLLIKER, and MITSUK'URI make the medulla arise from the gang- lionic fundaments of the sympathetic nerve-trunk (Grenzstrang), — it is for this reason that in many text-books the suprarenal bodies are treated of in connection with the sympathetic, — but GOTTSCHAU and JANOSIK controvert this ; they maintain that only certain ganglionic cells and nerve-fibres grow in from the sympathetic, but that the real medullary cells arise by a metamorphosis of cortical cells. It appears to me from the existing investigations that the question is not ready for discussion. 404 EMBRYOLOGY. There are also two different interpretations concerning the develop- ment of the cortical substance. BALFOUR, BRAUN, BRUNN, and MIT- SUKURI derive it from accumulations of connective-tissue cells, which are formed at the anterior portion of the primitive kidney along the course of the inferior vena cava and the cardinal veins. According to JANOSIK, WELDON, and MIHALKOVICS, on the contrary, the cell- accumulations are either directly or indirectly formative products of the epithelium of the body-cavity. I say " direct or indirect " because in details the results of the three investigators named differ somewhat. According to JANOSIK and MIHALKOVICS, it is the germinal epithelium in the anterior portion of the genital ridge that furnishes by its proliferation the material for the suprarenal body. MIHALKOVICS therefore calls it " a detached part of the sexually undifferentiated genital gland, which consequently remains at a primitive stage of development." WELDON, on the contrary, brings the suprarenal body into relation with the most anterior part of the primitive kidney. According to his representation, which appears to me to deserve especial consideration, and from which indeed other researches will have to begin, the sexual cords of the primitive kidney are concerned in the formation of the suprarenal bodies. When, at the head-end of the kidney, they sprout out of the epithelium of the Malpighian glomerulus in the manner previously (p. 383) described, they divide into two branches. One of these grows ventrally into the fundament of the sexual gland, the other turns dorsally and spreads out in the vicinity of the vena cava. Moreover, even MIHALKOVICS describes a connection of the sexual cords with the fundament of the suprarenal body at certain places, but makes both arise from proliferations of the epithelium of the body-cavity. The connection is subsequently destroyed by the inter- polation of blood-vessels. For the solution of the still pending questions most is to be expected from the investigation of non-amniotic animals. During its development the suprarenal body is for a time of very considerable size. In Mammals it temporarily covers the much smaller kidney, as in the human embryo of the eighth week repre- sented in fig. 220, in which at the left the supra n -luil body (nn) is to be seen in its normal position, whereas on the right it has been removed to disclose the kidney (n). Afterwards its growth does not keep pace with that of the kidney ; however at birth (fig. 208), when it already rests upon the latter (n) as a crescentic body (nn), it still is larger in comparison with the kidney than it is in the adult. THE ORGANS OP THE MIDDLE GERM-LAYER. 405 During its development some small portions of the fundament of the suprarenal cortex appear sometimes to detach themselves and to remain in the vicinity of the sexual organs, in whose migrations they participate. Thus, indeed, are to be explained the accessory supra- renal bodies observed by MARCHAND at the margin of the broad ligament. SUMMARY. 1. The following structures are to be interpreted as formative products of the middle germ-layer : the epithelium of the body-cavity (of the pericardium, of the thoracic and abdominal cavities, of the cavity of the scrotum), the whole of the transversely striped, voluntary musculature, the seminal cells and ova, the epithelium of the sexual glands, of the kidneys and their outlets, and the cortical cords of the suprarenal bodies. The Development of the Musculature. 2. The musculature of the trunk is developed exclusively from the cell-layer of the primitive segments that abuts upon the chorda and neural tube, which by the formation of muscle-fibrillse is con- verted into a muscle-plate. 3. The muscle-plate enlarges dorsally and ventrally, where it becomes continuous (zone of growth) with the outer (lateral) epi- thelial layer of the primitive segment, and spreads itself out over the neural tube above and into the walls of the abdomen below. 4. The original musculature consists of segments of longitudinal fibres (myomeres), which are separated from one another by connec- tive-tissue partitions (ligamenta intermuscularia). 5. The musculature causes the first segmentation of the body of Vertebrates into equivalent successive parts or metamera. 6. Buds grow out from the muscle-plates (Selachians) into the fundaments of the limbs, and thus furnish the foundation for the whole musculature of the extremities. 7. In the head-region of Vertebrates the musculature is developed not only out of the primitive segments, the number of which in Selachians amounts to nine, but also out of that part of the middle germ-layer which corresponds to the lateral plates of the trunk, and which is divided up by the formation of the visceral clefts into sepa- rate visceral-arch cords, which in the Selachians are provided with cavities. 406 EMBRYOLOGY. 8. From the primitive segments of the head are formed the muscles of the eyes, and from the visceral-arch cords the masticatory muscles, the muscles of the hyoid arch and also those of the small bones of the ear (?). ' The Development of the Urogenital System. 9. The first fundament of the urogenital system is the same in both sexes: it consists of (1) threa pairs of canals — the mesonephric duct, the Miillerian duct, and the ureter; (2) four pairs of glands — the pro-, meso-, and metanephros and the sexual gland, which at first is indifferent. 10. The mesonephric duct arises in its most anterior part out of a groove-like evagination or a ridge-like thickening of the parietal middle layer ; posteriorly it detaches itself from its parental tissues, fuses with the neighboring outer germ-layer, and thereby forms at first a short, tubular communication between the crelom and the surface of the body. 11. The mesonephric duct is gradually converted into a long canal, inasmuch as it grows backward on the outer germ-layer, which forms a thickened ridge, until it opens out into the cloaca (terminal part of the hind intestine). 12. The pronephros (head-kidney) is developed at the anterior part of the mesonephric duct in the following manner : the duct, upon being constricted off from the parietal middle layer, remains in connection with the latter at several places, and the resulting cords of connection grow out into long pronephric tubules, at the inner openings of which an in tra peritoneal vascular glomerulus is estab- lished out of the wall of the body-cavity. 13. Behind the pronephros the mesonephros (primitive kidney) arises thus : when the primitive segments are constricted off from the lateral plates, segmentally arranged cellular tubes or cords (nephrotome) are formed, which communicate at one of their ends with the body-cavity and at their other ends put themselves into connection with the laterally situated mesonephric duct and become the mesonephric tubules. (Development of Malpighian corpuscles, of secondary and tertiary mesonephric tubules and the glomeration of the latter.) 14. In the higher Vertebrates the development of the primitive kidney is to a certain extent abbreviated, in so far as the separate cords of cells which arise at the constricting off of the primitive segments lie very close together and constitute an apparently THE ORGANS OF THE MIDDLE GERM-LAYER. 407 undifferentiated cell-mass (the middle plate or the mesonephric blastema), out of which the mesonephric tubules subsequently — when they become clearly distinguishable — appear to have been differentiated. 15. In a part of the non-amniotic Vertebrates (some Selachians, Amphibians) the primitive kidney remains in open communication with the body-cavity by means of numerous ciliate funnels (nephro- stomes), whereas in all Amniota the mesonephric tubules early surrender their genetically established connection with the body- cavity through the disappearance of the ciliate funnels. 16. The permanent kidney (metanephros) is the latest to be formed and takes its origin from two separate patts : — (a) From an evagination of the end of the mesonephric duct, which furnishes the ureters, the pelvis of the kidney, and the straight urinary tubules (in other words, the efferent apparatus) ; (b) From a renal blastema, which represents a backward pro- longation of the mesonephric blastema, has the same origin as the latter, and is converted into the tortuous urinary tubules with the Malpighian corpuscles (therefore the secretory part of the kidney). 17. The fundaments of the kidney, which have arisen far back in the body, rapidly increase in size and undergo a change of position by moving farther forward by the side of the primitive kidneys, whereby the ureter becomes wholly detached from the mesonephric duct and moves to the posterior [dorsal] surface of the allantois, the future urinary bladder. 18. In the non-amniotic Vertebrates the mesonephros also gives rise by a process of fission to the Miillerian duct, which runs parallel with it. 19. In the Amniota the relation of the Miillerian duct to the mesonephric duct is still uncertain, because the front end of the former is established by a groove-like depression of the epithelial invest- ment on the lateral face of the mesonephros, while concerning the remaining part it is still undetermined whether it grows backwards independently or is constricted off from the mesonephric duct. 20. The sexual glands proceed from two fundaments : — (a) From a germinal epithelium, a modified part of the epithelium of the body-cavity, located on the median face of the primitive kidney ; (b) From the sexual cords, which grow out toward the germinal 408 EMBRYOLOGY. epithelium from the adjacent part of the primitive kidney (in Reptiles and Birds from the epithelium of Malpighian glomeruli). 21. The specific components of the sexual glands, the eggs and seminal cells, arise from the germinal epithelium (with its primitive ova and primitive seminal cells). 22. In the female there arise, in consequence of a process of mutual intergrowth on the part of the germinal epithelium and the subjacent stroma, the tubes of PFLUGER and egg-balls (or nests), and out of these finally egg-follicles, containing each a single ovum ; in the male there are formed, in consequence of a similar process, seminal ampullae (Selachians, some Amphibia) or seminal tubules (tubuli seminiferi) with their seminal mother-cells. 23. The sexual cords of the primitive kidney participate in the composition of the medullary substance of the ovary as medullary cords ; in the testis they unite with the seminal ampullae or seminal tubules and furnish the tubuli recti and the rete testis, consequently the initial part of the outlet for the semen. 24. The ovarian follicles are composed of a centrally located ovum, an envelope of follicular cells, and a vascular connective-tissue capsule (theca folliculi). 25. In Mammals the ovarian follicle is converted into a Graafian follicle by an increase in the number of follicular cells and by their secreting between them a follicular fluid. (Discus proligerus, mem- brana granulosa.) 26. The Graafian follicles, after the elimination of the mature ova into the abdominal cavity, become the so-called yellow bodies in the following manner : blood flows out of the ruptured blood-vessels into their cavities, and both the follicular cells left behind and the connective-tissue capsule undergo proliferation accompanied by an emigration of white blood-corpuscles (true and false corpora lutea). 27. The yellow bodies subsequently cause by their scar-like shrivel- ling the cicatriculae and callosities on the surface of old ovaries. 28. The canals and glands of the urogenital system, which are at first established in the same form in both sexes, are afterwards differently employed in the male and female and undergo a partial degeneration. 29. In the male the mesonephric duct becomes the vas deferens, in the female it becomes rudimentary (GARTNER'S duct, in many Mammals). 30. The Mullerian duct assumes in the male no function, and THE ORGANS OF THE MIDDLE GERM-LAYER. 409 only inconspicuous remnants of it are left at its ends (hydatid of the epididymis and sinus prostaticus or uterus masculinus) ; in the female it- becomes the efferent apparatus of the ovary, — the anterior part the oviduct, the posterior part the uterus and vagina, the latter resulting from the fusion of the ducts of the opposite sides of the body as far as they are enclosed in the genital cord. 31. In the male the anterior portion of the primitive kidney (mesonephros) — having united with the seminal tubules by means of the sexual cords — persists as the epididymis ; the remainder de- generates into the paradidymis. In the female both parts degenerate into epoophoron and paroophoron, which correspond respectively to the epididymis and paradidymis of the male. 32. The sexual glands, which are originally established in the lumbar region, gradually move with their outlets downward toward the pelvic cavity. (Descensus testiculorum et ovariorum. Oblique course of the spermatic arteries and veins.) 33. In the migration of the sexual glands a role appears to be played by the inguinal ligament, which passes from the primitive kidney underneath the peritoneum to the inguinal region, penetrates through the wall of the abdomen, and ends in the skin of the genital ridges that surround the cloaca. (Gubernaculum Hunteri in the male ; round ligament and ligamentum ovarii of the female.) 34. The testis is received some time before birth into the scrotum, an appendage of the body-cavity ; the scrotum owes its origin to the fact that the peritoneum forms an evagination (processus vaginalis peritonei) through the wall of the abdomen into the genital ridges, and that afterwards the evagination is completely cut off from the body-cavity by the closure of the inguinal canal. 35. The layers of the scrotum or the envelopes of the testes corre- spond, in accordance with their development, to the separate layers of the body-wall, as is shown in the following comparative summary : — Envelopes of the Testes. Wall of the Abdomen. Scrotum with tunica dartos. Skin of the abdomen. COOPER'S fascia. Superficial abdominal fascia. Tunica vaginalis communis with Muscle-layer and fascia trans- cremaster. versa abdominis. Tunica vaginalis propria (parietal Peritoneum. and visceral layers). 36. The external sexual organs are developed in man and woman from the same kinds of fundaments in the neighborhood of the cloaca. 410 EMBRYOLOGY. 37. The term cloaca is applied to a depression at the hinder end of the embryo, into which open the hind gut and the allantois, after the latter has received — on the posterior face of its attenuated terminal part, the sinus urogenitalis — the closely approximated Miillerian and mesonephric ducts. 38. The cloaca becomes divided by projecting folds, which unite to form the perinseum, into an anterior [ventral] and posterior [dorsal] portion, of which the former is the prolongation of the sinus urogenitalis, the latter the prolongation of the intestine (anus). 39. At the anterior margin of the cloaca, or, after completed separation, at the anterior rim of the sinus urogenitalis, there is found in both sexes the genital eminence, which bears along its under surface a groove flanked by the two genital folds ; the eminence, together with the opening lying under it (cloaca or sinus urogeni- talis), is embraced by the genital ridges. 40. In the female the genital eminence remains small and becomes the clitoris, the genital folds become the labia minora, the genital ridges the labia majora; the sinus urogenitalis remains short and broad and represents the vestibulum, which receives the vagina (the end of the Miillerian ducts) and the external orifice of the allantois or urinary bladder, the female urethra. 41. In the male the genital eminence grows out to a great length as the male organ ; the genital folds close on their under surface to form a narrow canal, which appears as a prolongation of the narrow sinus urogenitalis, together with the latter is designated as the male urethra, and receives at its beginning the vas deferens and the uterus masculinus ; the two genital ridges, which increase in size for the reception of the testes, surround the roots of the male organ and unite to form the scrotum. 42. The following table gives a brief survey (1) of the compar- able parts of the outer and inner sexual organs of the male and female, and (2) of their derivation from indifferent fundaments of the urogenital system in Mammals : — Male texual parti. Seminal anipullw and semi- nal tubules. testis and tubuli rccti. (6) Paradidytuia. The common, form from />•!< icfi both arise. Germinal epithelium. 1'ri native kidney. (a) Anterior part with the sexual coris (sexual part). (6) Posterior pari (the real mesonephric part). Female texual parts. Ovarian follicle, Graafian follicle. (a) Epoophoron with medul- lary con's of the ovary. (b) Paroc-phoion. LITERATURE. 411 Male sexual parts. The common form from ichich both arise. Female sexual parts. Vas deferens with seminal vesicles. Mesonephric duct. GARTNER'S canal, in some Mammals. Kidney and ureter. Kidney and ureter. Kidney and ureter. Hydatid of epididymis. Sinus prostaticus. (Uterus masculinus.) !• Mvillerian duct. j Oviduct and fimbrise. Uterus and vagina. Gubernaculum Hunteri. Inguinal ligament of primi- tive kidney. Round ligament and lig. ovarii. Male urethra (pars prostatica et membranacea). Sinus urogenitalis. Vestibulum vaginaa. Penis. Pars cavernosa urethra. Scrotum. Genital eminence. „ folds. „ ridges. Clitoris. Labia minora. „ majora. The Development of the Suprarenal bodies. 43. The most anterior part of the mesonephros appears to share in the development of the suprarenal bodies, since lateral branches sprout out from the sexual cords, become detached, and are converted into the peculiar cellular cords of the cortical substance. 44. The suprarenal bodies in the embryo for a time exceed in size the kidneys. LITERATURE. (1) Development of the Musculature. Ahlborn. Ueber die Segmentation des Wirbelthierkorpers. Zeitschr. f. wiss. Zoologie. Bd. XL. 1884. Grenadier. Muskulatur der Cyclostomen und Leptocardier. Zeitschr. f. wiss. Zoologie. Bd. XVII. 1867, p. 577. Hertwig, Oscar. Ueber die Muskulatur der Coelenteraten. Sitzungsb. d. Gesellsch. f. Medicin u. Naturwiss. Jena. Jahrg. 1879. Marshall, A. Milnes. On the Head-cavities and Associated Nerves of Elasmobranchs. Quart. Jour. Micr. Sci. Vol. XXI. 1881, p. 72. Schneider, Anton. Beitrage zur vergleichenden Anatomic und Entwick- lungsgeschichte der Wirbelthiere. Berlin 1879. Sedgwick. On the Origin of Metameric Segmentation and some other Morphological Questions. Quart. Jour. Micr. Sci. Vol. XXIV. 1884, p. 43. "Wijhe. Ueber die Mesodermsegmente und die Entwicklung der Nerven des Selachierkopfes. Verhandel. d. k. Akad. van Wetensch. Amsterdam 1883. "Wijhe. Ueber Somiten und Nerven im Kopfe von Vogel- und Reptilien- embryonen. Zool. Anzeiger. Jahrg. IX. Nr. 237, 1886, p. 657. Wijhe. Ueber die Kopfsegmente und die Phylogenie des Geruchsorgans der Wirbelthiere. Zool. Anzeiger. Jahrg. IX. Nr. 238, 1886, p. 678. 412 EMBRYOLOGY. (2) Development of the Urogenltal System. Balbiani. Lecons sur la generation des vertebres. Paris 1879. Balfour, P. M. On the Origin and History of the Urogenital Organs of Vertebrates. Jour. Anat. and Physiol. Vol. X. 1876. Balfour, F. M. On the Structure and Development of the Vertebrate Ovary. Quart. Jour. Micr. Sci. Vol. XVIII. 1878. Balfour, F. M. Ueber die Entwicklung und die Morphologie der Suprarenal - korper (Nebennieren). Biol. Centralblatt. 1881. Nr. 5. Balfour, F. M., and Adam Sedgwick. On the Existence of a Head-kidney in the Embryo Chick and on Certain Points in the Development of the Miillerian Duct. Quart. Jour. Micr. Sci. Vol. XIX. 1879. Beard, J. The Origin of the Segmental Duct in Elasmobranchs. AnaU Anzeiger. Jahrg. II. Nr. 21. 1887. Beneden, van. Contribution a la connaissance de 1'ovaire des mammiferes Archives de Biologic. T. I. 1880. Born. Ueber die Entwicklung des Eierstocks des Pferdes. Archiv f. Anat. u. Physiol. 1874. Bornhaupt, T. Untersuchungen iiber die Entwicklung des Urogenital- systems beim Huhnchen. Dissertation. Dorpat 1867. Bramann, F. Beitrag zur Lehre von dem Descensus testiculorum und dem Gubernaculum Hunteri des Menschen. Archiv f. Anat. u. Physiol. Anat. Abth. Jahrg. 1884. Braun. Das Urogenitalsystem der einheimischen Reptilien. Arbeiten a. d. zool.-zoot. Inst. Wiirzburg. Bd. IV. 1877. Braun. Bau und Entwicklung der Nebennieren bei Reptilien. Arbeiten a. d. zool.-zoot. Inst. Wiirzburg. Bd. V. 1879. Brook, Q. Note on the Epibiastic Origin of the Segmental Duct in Tele- ostean Fishes and in Birds. Proceed. Roy. Soc. Edinburgh. Vol. XIV. 1888. Brunn, A. v. Ein Beitrag zur Kenntniss des f einern Baues und der Entwick- lung der Nebennieren. Archiv f. mikr. Anat. Bd. VIII. 1872. Cadiat. Memoire sur 1'uterus et les trompes. Jour, de 1'Anat. et de la Physiol. T. XX. 1884, p. 409. Cadiat. Du developpement du canal de 1'urethre et des organes genitaux de 1'embryon. Jour, de 1'Anat. et de la Physiol. T XX. 1884, p. 242. Clarke, S. F. The Early Development of the Wolffian Body in Amblystoma punctatum. Studies Biol. Lab. Johns Hopkins Univ. Vol. II. 1883, p. 39. Dansky und Kostenitsch. Ueber die Entwicklung der Keimblatter und des Wolff 'schen Gauges im Hiihnerei. Mem. de 1'Acad. des Sci. St. Peters- bourg. Ser. VII. T. XXVII. 1880. Dohrn. Ueber die Gartner'schen Canale beim Weibe. Archiv f. Gynakologie. Bd.XXI. 1883. Egli. Beitrage zur Anatomic u. Entwicklungsgeschichte der Geschlechts- organe. Zur Entwicklung des Urogenitalsystems beim Kaninchen. Dissertation der Universitat Basel. 1876. Emery, C. Recherches embryologiques sur le rein des mammiferes. Archives ital. de Biologic. T. IV. Flemming, "W. Die ectoblastische Anlage des Urogenitalsystems beim Kaninchen. Archiv f. Anat. u. Physiol. Anat. Abth. 1886. LITERATURE. 413 Foulis. The Development of the Ova, etc. (1874). Trans. Roy. Soc. Edin- burgh. Vol. XXVII. 1876, p. 345. Furbringer, Max. Zur vergl. Anatomic und Entwicklungsgeschichte der Excretionsorgane der Vertebraten. Morphol. Jahrb. Bd. IV. 1878. Gasser. Beitr. zur Entwicklungsgeschichte der Allantois, der Miiller'schen Gange und des Afters. Habilitationsschrift. Frankfurt a M. 1874. Also Abhandl. Senekenb. Naturf. Gesellsch. Bd. IX. p. 293. Gasser. Beobachtungen liber die Entstehung des WolfE'schen Ganges bei Embryonen von Hiihnern und Gansen. Archiv f. mikr. Anat. Bd. XIV. 1877. Gasser. Embryonalreste am mannlichen Genitalapparat. Sitzungsb. d. Marburger naturf. Gesellschaft. 1882. Gasser. Einige Entwicklungszustande der mannlichen Sexualorgane beim Menschen. Sitzungsb. d. Marburger naturf. Gesellschaft. 1884. Gottschau, M. Structur und embryonale Entwicklung der Nebennieren bei Saugethieren. Archiv f. Anat. u. Physiol. Anat. Abth. 1883. Haddon. Suggestion Respecting the Epiblastic Origin of the Segmental Duct. Sci. Proceed. Roy. Dublin Soc. N. S. Vol. V. 1887, p. 463. Harz, "N. Beitrage zur Histologie des Ovariums der Saugethiere. Archiv f. mikr. Anat. Bd. XXII. 1883, p. 374. Hensen. Beobachtungen iiber die Befruchtung und Entwicklung des Kaninchens und Meerschweinchens. Zeitschr. f . Anat. u. Entwg. Bd. I. Archiv f. Anat. u. Physiol. Anat. Abth. 1875, p. 213. Hoffmann, C. K. Zur Entwicklungsgeschichte der Urogenitalorgane bei den Anamnia. Zeitschr. f. wiss. Zoologie. Bd. XLIV. 1886. Janoaik. Histologisch-embryologische Untersuchungen iiber das Urogenital- system. Sitzungsb. d. k. Akad. d. Wissensch. Wien, math.-naturw. 01. Bd. XCI. Abth. 3, 1885, p. 97. Janosik. Bemerkungen iiber die Entwicklung der Nebenniere. Archiv f. mikr. Anat. Bd. XXII. 1883. Kapff. Untersuchungen iiber das Ovarium und dessen Beziehungen zum Peritoneum. Archiv f. Anat. u. Physiol. Anat. Abth. 1872. Kocks. Ueber die Gartner schen Gange beim Weibe. Archiv f . Gynakologie. XX. 1882. Kollmann. Ueber die Verbindung zwischen Coelom u. Nephridium. Fest- schrift zur Feier des .SOOjahrigen Bestehens der Universitat Wiirzburg, gewidmet von der Uuiversitat Basel 1882. Kupffer. Untersuchungen iiber die Entwicklung des Harn- und Geschlechts- systems. Archiv f. mikr. Anat. Bd. I. u. II. 1865, 1866. Leod, Jules Mac. Contributions a 1'etude de la structure de 1'ovaire des rnammiferes. Archives de Biologic. Vol. I. 1880. Marchand. Ueber accessorische Nebennieren im Ligamentum latum. Archiv f. path. Anat. u. Physiol. Bd. XCII. 1883. Martin. Ueber die Anlage der Urniere beim Kaninchen. Archiv f. Anat. u. Physiol. Anat. Abth. 1888. Mihalkovics, G. v. Untersuchungen iiber die Entwicklung des Harn- und Geschlechtsapparates der Amnioten. Internationale Monatsschr. f. Anat. u. Histol. Bd. II. 1885. Mitsukuri. On the Development of the Suprarenal Bodies in Mammalia. Quart. Jour. Micr. Sci. Vol. XXII. Also in Studies Morphol. Lab University of Cambridge. Vol. II. 1882. 414 EMBRYOLOGY. Mitsukuri. The Ectoblastic Origin of the Wolffian Duct in Chelonia. Zool. Anzeiger. Jahrg. XI. 1888, p. 111. Miiller, Johannes. Bildungsgeschichte der Genitalien. Dus.vldorf 1830. Miiller, Wilhelm. Ueber das Urogenitalsystem des AmpLioxus u. der Cyclostomen. Jena. Zeitschr. Bd. II. 1875. Nagel, "W. Ueber die Entwicklung des Urogenitalsystems des Menschen. Archiv f. mikr. Anat. Bd. XXXIV. 1889, p. 269. Neumann. Die Beziehungen des Flimmerepithels der Bauchhohle zum Eileiterepithel. Archiv f. mikr. Anat. Bd. XI. 1875. Perenyi, J. Die ektoblastische Anlage des Urogenitalsystems bei Rana esculenta und Lacerta viridis. Zool. Anzeiger. Jahrg. X. 1887. Xr. 243, p. 66. Perenyi, J. Amnion und Wolff'scher Gang d. Eidechsen. Math. u. natur- wiss. Berichte aus Ungarn. Bd. VI. 1887-8. Berlin u. Budapest 1889 and Zool. Anzeiger. Jahrg. XI. 1888, p. 138. Pniiger, E. Die Eierstb'cke der Saugethiere und des Menschen. Leipzig 1863. Rathke, H. Beobachtungen und Betrachtungen iiber die Entwicklung der Geschlechtswerkzeuge bei den Wirbelthieren. Xeue Schriften d. naturf. Gesellsch. Danzig. Bd. I. 1825. Renson. Contributions a I'embryologie des organes d'excretions des oiseaux et des mammiferes. These. Bruxelles 1883. Abstract in Archiv f. mikr. Anat. Bd. XXII. 1883. Romiti, W. Ueber den Bau und die Entwicklung des Eierstockes und des Wolff'schen Ganges. Archiv f. mikr. Anat. Bd. X. 1874. Rosenberg, A. Untersuch. iiber die Entwickl. der Teleostierniere. Disserta- tion. Dor pat 1867. Roth. Ueber einige Urnierenreste beim Menschen. Baseler Festschr, z. Wiirzburger Jubilaum. 1882. Rouget. Evolution comparee des glandes genitales m&le et femelle chez les embryons des mammiferes. Compt. rend. T. 88. 1879. Riickert. Entstehung des Vornierensystems. Munchener medic. Wochenschr. Jahn?. 36. 1889. Schafer, E. A. On the Structure of the Immature Ovarian Ovum in the Common Fowl and in the Rabbit, etc. Proceed. Roy. Soc. London. 1880 Nr. 202. Schmiegelow, E. Studier over Testis og Epididymis Udviklings-historie. Afhandling for Doctorsgraden. Kjcibenhavn 1881. Schmiegelow, E. Studien iiber die Entwickelung des Hodens und Xeben hodens. Archiv f. Anat. u. Physiol. Anat. Abth. 1882. Bedgwick, Adam. Development of the Kidney in its Relation to the Wolffian Body in the Chick. Quart. Jour. Micr. Sci. Vol. XX. 1880. Sedgwick, Adam. On the Development of the Structure known as the "Glomerulus of the Head-kidney' in the Chick. Quart. Jour. Micr. Sci. Vol. XX. 1880. Sedgwick, Adam. On the Early Development of the Anterior Part of the Wolffian Duct anil Body in the Chick, together with some Remarks on the Excretory S\>icmof the Vertebrata. Studies Morphol. Lab. Univer- sity of Cambridge. 1882. Also in Quart. J,,ur. Micr. Sci. Vol. XXI. 1881. LITERATURE. 415 Semon, Richard. Die indifferente Anlage der Keimdriisen beim Hiihn- chen und ihre Differenzirung zum Hoden. Habilitationsschrift. Jena 1887. Semper, C. Das Urogenitalsystem der Plagiostomen und seine Bedeutung fiir das der iibrigen Wirbelthiere. Wurzburg 1875. Siemerling. Beit rage zur Embryologie der Excretionsorgane des Vogels, Inaug.-Diss. Marburg 1882. Spee, G-raf Ferdinand. Ueber directe Betheiligung des Ektoderms an der Bildung der Urnierenanlage des Meerschweinchens. Archiv f. Anat. u. Physiol. Anat. Abth. 1884. Spengel. Das Urogenitalsystem der AmpMbien. Arbeiten a. d. zool.-zoot. Inst. Wilrzburg. Bd. III. 1876. Toldt. Untersuchungen liber das Wachsthum der Nieren des Menschen und der Saugethiere. Sitzungsb. d. k. Akad. der Wissensch. Wien, math.- naturw. Cl. Bd. LXIX. Abth. 3, p. 123. Tourneux et Legay. Memoire sur le developpement de 1'uterus et du vagin. Jour, de 1'Anat. et de la Physiol. 1884. Tourneux. Sur les premiers developpements du cloaque, du tubercule genital et de 1'anus chez 1'embryon de mouton. Jour, de 1'Anat. et de la Physiol. T. XXIV. 1888. Tourneux. Sur le developpement et 1'evolution du tubercule genital chez le foetus humain dans les deux sexes. Jour, de 1'Anat. et de la Physiol. T. XXV. 1889. Waldeyer. Ueber die sogenannte ungestielte Hydatide der Hoden. Archiv f. mikr. Anat. Bd. XIII. 1877. Waldeyer. Eierstock und Ei. Ein Beitrag zur Anatomic u. Entwicklungs- geschichte der Sexualorgane. Leipzig 1870. Weldon. Note on the Early Development of Lacerta muralis. Quart. Jour. Micr. Sci. Vol. XXIII. 1883. Weldon. On the Head-kidney of Bdellostoma, with a Suggestion as to the Origin of the Suprarenal Bodies. Quart. Jour. Micr. Sci. Vol. XXIV. 1884. Weldon. Note on the Origin of the Suprarenal Bodies of Vertebrates. Proceed. Roy. Soc. London. Vol. XXXVII. 1884, p. 422. Weldon. On the Suprarenal Bodies of Vertebrata. Quart. Jour. Micr. Sci. Vol. XXV. 1 885. Wieger, G. Ueber die Entstehung und Entwicklung der Bander des weib- lichen Genitalapparates beim Menschen. Ein Beitrag zur Lehre des Descensus o variorum. Archiv f. Anat. u. Physiol. Anat. Abtheil. 1885. Wijhe, J. W. van. Die Betheiligung des Ektoderms an der Entwicklung des Vornierenganges. Zool. Anzeiger. Nr. 236, 1886, p. 633. Wijhe, J. W. van. Ueber die Mesodermsegmente des Rumpfes und die Entwicklung des Excretionssystems bei Selachiern. Archiv f . mikr. Anat. Bd. XXXIII. 1889, p. 461. 416 EMBRYOLOGY. CHAPTER XVI. THE ORGANS OF THE OUTER GERM-LAYER. THE outer germ-layer has for a long time also borne the name dermo-sensory layer. By this its two most important functions are both indicated. For in the first place it forms the epidermis together with its various products, such as hair, nails, scales, horns, and feathers ; and in addition various kinds of glands : the sebaceous, sweat- and milk-glands. Secondly, it is the matrix out of which the nervous system and the most important functional parts of the sensory organs, the optic, auditory, and olfactory cells, are derived. I begin with the most important function of the outer germ-layer, the development of the nervous system, then proceed to the develop- ment of the organs of sense (eye, ear, and organ of smell), 'and finally discuss the development of the epidermis and its products. I. The Development of the Nervous System. A. The Development of the Central Nervous System. The central nervous system of Vertebrates is one of the organs first established after the separation of the germ into the four primary germ-layers. As has already been stated, it is developed (fig. 41 A) out of a broad band of the outer germ-layer (mp), which stretches from the anterior to the posterior end of the embryonic fundament and lies in the median plane directly above the chorda dorsalis (ch). In this region the cells of the outer germ-layer grow out into long cylindrical or spindle-shaped structures, whereas the elements occurring in the surrounding parts (ep) flatten out and under certain conditions become altogether scale-like. Consequently the outer germ-layer is now divided into two regions — into the attenuated primitive epidermis (Hornblatt) (ep) and the thicker median neural or medullary plate (mp). Both regions are soon sharply separated from each other, since the neural plate bends in a little (fig. 41 B) and its edges rise above the surface of the germ. In this way there arise the two medullary or dorsal folds (mf), which enclose between them the originally broad and shallow medullary or dorsal furrow. They are simply folds of the outer germ-layer, formed at the place where the neural plate is continuous with the primitive epidermis. They are therefore com- posed of an outer and an inner layer, of which the inner belongs to THE ORGANS OP THE OUTER GERM-LAYER. 417 the marginal part of the neural plate, the outer, on the contrary, to the adjacent epidermis. In all the classes of Vertebrates the medullary plate is transformed into a neural tube at a very early period. This process can be accomplished in three different ways. In most of the classes of Vertebrates, namely Reptiles, Birds, and Mammals, the tube is formed by a typical process of folding. The medullary folds rise still higher above the surface of the germ, then bend together toward the median plane, and grow toward each other until their edges meet, along which they then begin to fuse. The neural tube, thus formed, still continues to remain in connection with the over- lying epidermis along the line of fusion, a connection which soon disappears, since the connecting cells become loosened and separated from one another (fig. 41 0). The closure begins in all Vertebrates at the place which corresponds approximately to the future mid-brain — in the Chick (fig. 87 hb2) on the second and in the Rabbit on the ninth day of development — and from there proceeds slowly both backwards and forwards. There is retained for a long time, especially behind, a place where the neural tube is open to the exterior. A connection with the intestinal tube by means of the neurenteric canal also exists at the posterior end, as has been already mentioned (p. 126) in the discussion of the germ-layers. It is only at a later period that this connection is interrupted by the closing of the blastopore. The second type in the development of the central nervous system is met with in Cyclostomes and Teleosts. In them the neural plate is transformed into a solid cord of cells instead of a tube. Instead of the folds rising up over the surface of the germ, the neural plate grows downward in the form of a wedge. In this way the right and left halves of the plate come to lie immediately in contact with each other, so that one cannot find the slightest trace of a space between them ; only after the cord of cells has been constricted off from the primitive epidermis do the halves separate and allow a small cavity, the central canal, to appear between them. Probably this modification in the Bony Fishes and Cyclostomes is connected with the fact that the egg with its abundant yolk is very closely enveloped by the vitelline membrane, as a result of which the medullary folds cannot rise toward the surface. The third modification occurs only in Amphioxus lanceolatus. It- has already been described briefly in another place (p. 109). The neural tube retains an undifferentiated condition in Amphioxus 27 418 EMBRYOLOGY. lanceolatus only ; in all other Vertebrates, on the contrary, it is differentiated into spinal cord and brain. (a) The Development of the Spinal Cord. The part of the neural tube which is converted into the spinal cord is oval in cross section (fig. 200). At an early period a separa- tion into a right and left half can be recognised (fig. 232). For Fig. 232. — Cross section of an embryo Lizard with completely closed intestinal tube, after SAOEMEHL. he, Posterior, vc, anterior commissure of the spinal cord ; vw, anterior root of nerve ; nf, nerve- fibrillae ; *pk, spinal ganglion ; nip1, muscle-plate, muscle-forming layer ; tnp", outer layer of the muscle-plate; mp3, transition of the outer into the muscle-forming layer. the lateral walls are greatly thickened and consist of several layers of long, cylindrical cells, whereas the upper and lower walls are thin and can be distinguished respectively as posterior [dorsal] and anterior commissure (he and vc), or as roof -plate and floor-plate. The further development, of which I shall mention only the most important points, takes place in such a manner that the lateral halves become thicker and thicker (fig. 233). The cells continue to increase in number by division, and at the same time to be differ- entiated into two histological groups — (1) into elements which provide the sustentative framework, the epithelium surrounding the central THE ORGANS OF THE OUTER GERM-LAYER. 419 canal and the spongiosa (spongioblasts of His), and (2) into elements which are transformed into ganglionic cells and nerve-fibres (neuro- blasts of His). The thickening of the lateral walls depends partly upon the multiplication of cells, but mainly upon the fact that nerve- fibres apply themselves to the cell-mass from the outside. In time these fibres are separated into the anterior, lateral, and posterior columns of the spinal cord (fig. 233 pew, lew, acw). At their first appearance the nerve- fibres are non-medul- lated (fig. 232 nf), and only subse- quently, sometimes earlier, sometimes later, acquire a me- dullary sheath. In this manner the al- ready considerably thickened halves of the spinal cord be- come differentiated into the central gray substance containing the ganglionic cells, and into the white substance, which en- velops the surface of the former like a mantle. Since, meanwhile, the roof- and floor- plates grow only a little and are not differentiated into ganglionic cells, they come to lie deeper and deeper at the bottom of anterior and posterior longitudinal furrows (c and af). Finally, the completely formed spinal cord is composed of large lateral halves, which are separated from each other by deep anterior and posterior longitudinal fissures, being united only deep down by a thin trans- verse bridge. The latter is derived from the roof- and floor-plates, which have been retarded in their growth, and encloses in its middle the central canal, which has also remained small. Fig. 233.— Cross section through the spinal cord of an embryo Chick of seven days, after BALFOUR. pew, Posterior white column ; lew, lateral white column ; acw, anterior white column ; c, dorsal tissue filling up the place where the dorsal fissure will be formed ; pc, posterior horn of the gray substance ; ac, anterior horn ; ep, epithelial cells ; age, anterior gray commissure ; pf, posterior [dorsal] part of the spinal canal ; spc, anterior [ventral] part of the spinal canal ; af, anterior fissure. 420 EMBRYOLOGY. At the beginning — in Man up to the foarth month of embryonic development — the spinal cord occupies the entire length of the body. Therefore, at the time when the axial skeleton is divided up into separate vertebral regions, it reaches from the first cervical down to the last coccygeal vertebra. The end of the spinal cord, however, does not even begin to develop ganglionic cells and nerve-fibres, but remains throughout life as a small epithelial tube. It is united to the larger anterior portion, which has developed nerve-fibres and ganglionic cells, by means of a conically tapering rogion, which is spoken of in descriptive anatomy as the conus medullaris. As long as the spinal cord keeps pace with the vertebral column in its growth, the pairs of nerves arising from it, in leaving the vertebral canal, pass out at right angles directly to the intervertebral foramina. In Man, beginning, with the fourth month, this arrange- ment is changed ; from that time forward the growth of the spinal cord does not equal that of the spinal column, and therefore the cord can no longer occupy the entire length of the vertebral canal. Since it is attached above to the medulla oblongata, and this together with the brain is firmly held in the cranial capsule, it must assume a higher and higher position in the vertebral canal. In the sixth month the conus medullaris is found in the upper end of the sacral canal, at birth in the region of the third lumbar vertebra, and some years later at the lower edge of the first lumbar vertebra, where it terminates even in the adult. In the ascent (ascensus medullje spinalis) the lower end of the spinal cord, the small epithelial tube which is attached to the coccyx, is drawn out into a long, fine filament, which persists even in the adult as the Jilum terminate intemum and externum. At first it presents a small cavity, which is lined by ciliated cylindrical cells, and which forms a continuation of the central canal of the spinal cord. Further downward it is continued in the form of a cord of connective tissue as far as the coccyx. A second consequence of the ascent of the spinal cord is a change in the course of the roots of the peripheral nerve-stems. Since, together with the spinal cord, their points of origin come to lie in the spinal canal relatively nearer and nearer the head, and since the places where they pass through the intervertebral foramina do not change, they are compelled to pass from a transverse to a more and more oblique course. The obliquity, moreover, is greater the farther down the nerve leaves the vertebral canal. In the neck-region their direction is still transverse, in the thoracic region it begins to be more and THE ORGANS OF THE OUTER GERM-LAYER. 421 more oblique, and finally, in the lumbar region, and still more so in the sacral, it is more sharply downward. On this account the nerve- stems arising from the last part of the spinal cord come to lie for a considerable distance in the vertebral canal before they reach the sacral foramina serving for their exit ; they therefore surround the conus medullaris and filum terminale, forming the structure known as the horse-tail or cauda equina. Finally the spinal cord undergoes some changes in its form also. Even in the third and fourth months there appear differences of calibre in different regions. The places in the cervical and lumbar regions of the spinal cord at which the peripheral nerves depart to the anterior and posterior extremities, grow vigorously by the abundant formation of ganglionic cells ; they become considerably thicker than the adjoin- ing portions of the cord, on account of which they are distinguished as cervical and lumbar enlargements (intumescentia cervicalis et lumbalis). (b) The Development of the Brain. By the study of embryology knowledge of the anatomy of the brain has been greatly promoted. Justly, therefore, in all recent text books of human anatomy, the embryonic condition serves as the starting-point in the description of the intricate structure of the brain, the aim being to derive the complicated ultimate conditions from the more simple embryonic ones, and to explain them by means of the latter. The initial form of the brain as well as of the spinal cord is a simple tube. At an early period, even before it is everywhere closed, it becomes metameric, on account of its growth being greater in some regions than in others. By means of two constrictions of its lateral walls it is divided into the three primary brain-vesicles (fig. 87 hb1, hb2, A63), which remain united with one another by means of wide openings, and are designated as the fore-, mid-, and hind-brain. The posterior of these divisions is the longest, gradually tapering and becoming continuous with the tubular spinal cord. The first stage is quickly followed by a second, and that by a third, since the primary brain-vesicles soon separate into four, and finally five divisions. During the second stage (fig. 234) the lateral walls of the primary fore-brain (pvh) begin to grow outward more vigorously and to evaginate to form the two optic vesicles (au). At the same time the 422 EMBRYOLOGY. jn-h lateral walls of the hind-brain, which from the beginning has been the longest portion, acquire a constriction which divides the hind- brain into two vesicles, that of the cere- bellum (kh) and the medulla (nh), or after-brain. The five-fold segmentation of the neural tube (fig. 235) soon succeeds the four-fold condition; by means of it the fore-brain vesicle undergoes fundamental transformations. First, the primary optic vesicles (au) begin to be constricted off from the fore- brain vesicle, until they remain at- tached by only slender, hollow stalks. Since the constriction takes place mainly from above downward, the stalks remain in connection with the base of the fore-brain vesicle. The front wall of the vesicle then begins to protrude anteriorly, and to be marked off by means of a lateral furrow, which runs from above and behind obliquely downward and for- ward. In this manner the primary vesicle of the fore-brain, like the hind-brain vesicle, is secondarily di- vided into two portions, which we can now distinguish as the vesicles of the cerebrum and the between-brain (gh, zh). The optic nerves remain united with the base of the latter. The vesicle of the cerebrum is distinguished by a very rapid growth, and soon begins to surpass all the other parts of the brain in size. But it becomes divided before this into right and left halves. From the connective tissue enveloping the neural tube there grows down in the median plane a process, the future falx cerebri. This growth advances from above and in front against the cerebral vesicle and deeply infolds its upper wall. The halves (fig. 236 hms] that have thus arisen are united at their bases ; they present a more flat median and a convex outer surface, and are called the two vesicles of the hemi- spheres, since they furnish the foundation for the cerebral hemispheres. The separate regions of the brain-tube produced by constrictions Fig. 234.- Dorsal aspect, by trans- mitted light, of the head of a Chick incubated 58 hours, after MIHALKOVICS. Magnified 40 diameters. x, Anterior wall of the primary fore- brain vesicle, which afterwards evaginates to form the cerebrum ; prh, primary fore-brain vesicle ; au, optic vesicle ; mh, mid-brain vesicle ; kh, vesicle of the cere- bellum ; nh, after-brain vesicle ; h, heart ; vo, omphalomesenteric vein ; nn, spinal cord ; us, primitive segment. THE ORGANS OF THE OUTER GERM-LAYER. 423 and evaginations subsequently become still more sharply marked off from one another, owing to the alteration of their positions. Fig. 235.— Brain of a human embryo of the third week (Lg). Profile reconstruction. After His. gh, Cerebral vesicle ; zh, between-brain vesicle ; mh, mid-brain vesicle ; kh, nh, vesicles of the cerebellum and medulla oblongata ; au, optic vesicle ; gb, auditory vesicle ; tr, infundibulum ; rf, area rhomboidalis ; nb, nuchal flexure ; kb, cephalic flexure. At the beginning the three brain-vesicles formed by the first constrictions lie in a straight line one behind the other (fig. 87) and above the chorda dorsalis ; the latter extends only as far as to the anterior end of the mid- brain vesicle, where it tapers to a point. But from the moment when the optic vesicles begin to be constricted off, the three primary vesicles shift their positions in such a way that the longitudinal axis uniting them undergoes sharp, characteristic folds, which are distinguished as the cephalic, pontal, and nuchal flexures (fig. 235 kb, nb). The cause of the formation of the curvatures, which are of fundamental importance in the anatomy of the brain, is to be sought princi- pally in the more vigorous longitudinal growth which distinguishes the cerebral tube, and more especially its dorsal wall, from the surrounding parts. As His has established by means of measurements, the fundament of the brain more than doubles its length, while the spinal cord increases by only about one-sixth of its length. The cephalic flexure (fig. 235 kb) is developed first. The floor of the fore-brain sinks downward a little around the anterior end of the chorda.dorsalis (fig. 237 ch), and forms at first a right angle with Fig. 236. — Brain of a human embryo seven weeks old, parietal (Scheitel) aspect, after M;HALKOVICS. msp, Longitudinal or in- terpallial fissure (Man- telspalte), at the bottom of which is seen the embryonic lamina ter- minaHs (Schlussplatte) ; hms, left hemisphere ; zh, betweeu-brain ; mh, mid-brain ; hh, hind- brain and after-brain. 424 EMBRYOLOGY. the part of the base of the brain lying behind it, but afterwards an acute angle (figs. 235, 238). In consequence of this, the vesicle of the mid -brain (fig. 235 mh) comes to lie highest, and forms a promi- nence, which causes a great protrusion of the surface of the embryo and is known as the parietal prominence (fig. 158 «). The nuchal flexure, which makes its appearance at the boundary between medulla oblongata and spinal cord, is less prominent (fig. 235 nb). It produces in the embryos of the higher Ver- dk Fig. 237.— Median section through the head of a Rabbit embryo 6 nun. long, after MIHALKOVICS. rh, Pharyngeal membrane ; hp, place whence the hypophysis develops ; h, heart ; kd, cavity of the head-gut ; ch, chorda ; v, ventricle of the cere- brum ; v', third ventricle, that of the between- brain ; v*, fourth ventricle, that of the hind- and after-brain ; ck, central canal of the spinal cord.* tebrates a curvature which also projects outward, the so-called nuchal prominence (fig. 158). The third curva- ture, which has been designated by KOL- LIKER as the pontal flexure (fig. 239 bb), because it arises in the neighborhood of the future pons Varolii, is, on the contrary, very marked. It is further distinguished from the two other curvatures described, by the fact that its convexity is not di- rected toward the back of the embryo, Fig. 238.— Median sagittal section through the head of a Chick incubated four and a-half days, after MIHALKOVICS. SH, Parietal prominence ; sr, lateral ventricle ; v3, third ventricle ; v1, fourth ventricle ; Sio, aqueduct of SYLVIUS ; yh, vesicle of the cerebrum ; zh, between-brain ; mh, mid- brain ; kh, cerebellum ; z/, pineal process (epiphysis) ; hp, i KH-krt of the hypophysis (pouch of RATHKE) ; cA, chorda ; ba, basilar artery. but toward its ventral side. It is formed between the floor of the * [For terminology of the regions of the brain, see footnote, p. 282.] THE ORGANS OF THE OUTER GERM-LAYER. 425 vesicle of the cerebellum and that of the after-brain, and has the form of a ridge which projects ventrally for a considerable distance, where subsequently the transverse fibres of the pons Varolii are established. The extent of these curvatures is very different in the various classes of Vertebrates. Thus the cephalic flexure is only s'ightly emphasised in the lower Vertebrates (Cyclostomes, Fishes, Amphibia) ; it is, on the contrary, much greater in Reptiles, Birds, and Mammals ; but in Man especially, whose brain is the most voluminous, all of the flexures are developed to a very high degree. The five brain-vesicles furnish the foundation for a natural sub- division of the brain, whose various chief divisions can be referred back to them. As the study of the further development teaches, there are formed from the after-brain vesicle the medulla oblongata, from the vesicle of the cerebellum the vermi- form process with the ML hemispheres of the cere- bellum and the pons . Fig. 239.— Brain of a Rabbit embryo 16 mm. long, viewed V arolu, trom the mid- from the left side. The outer wall of the left cerebrum brain vesicle the crura is removed. After MIHALKOVICS. sn, Optic nerve ; ML, foramen of MONBO ; agf, fold of the Cerebri and Corpora choroid plexus ; amf, fold of the cormi Ammonis ; miadrip-pmina from the zh> between-brain : mh, mid-brain (cephalic or parietal Qa' " flexure) ; kh, cerebellum ; Dp, roof-plate of the fourth between - brain vesicle rentricle ; bb, pontal flexure ; mo, medulla oblongata. the between-brain [thalamencephalon] with the inf undibulum, the pineal gland, and the optic thalami, and finally from the vesicle of the cerebrum the cerebral hemispheres. Tn this metamorphosis the cavities of the primitive cerebral tube become the so-called ventricles of the brain : from the cavities of the fourth and fifth vesicles is derived the fourth ventricle or fossa rhomboidalis ; from the cavity of the mid-brain vesicle, the aque- duct of SYLVIUS ; from tke between-brain, the third ventricle ; and finally from the cavities of the hemispheres, the two lateral ventricles, which are also designated as the first and second ventricles. A brief sketch will suffice to show in what manner the most important parts of the brain develop out of the five vesicular fundaments, and that at the same time histological and morphological differentiations are most intimately associated. 4:26 EMBRYOLOGY. Histoloyicaliy considered the warlls of the vesicles originally consist everywhere of closely crowded spindle-shaped cells, just as in the spinal cord. These cells undergo in different places unlike modifica- tions. In some places they retain their epithelial character, and furnish (1) the epithelial covering of the choroid plexus in the roof of the between-brain and after-brain, (2) the ependyma lining the ventricles of the brain, and (3) follicular structures such as the epiphysis (fig. 246). On the greater part of the wall of the five brain-vesicles the cells multiply to an extraordinary extent, and are transformed into more or less extensive layers of ganglionic cells and nerve-fibres. The distribution of the gray and white substances thus formed no longer presents in the brain-vesicles the same uniform condition that it does in the spinal cord. The only uniformity is found in the fact that in every part of the brain there occur gray " nuclei," which, like the anterior and posterior gray columns of the spinal cord, are enveloped with a mantle of white substance. How- ever, there are added to the two parts of the brain that have attained the greatest development layers containing ganglionic cells, which furnish a superficial covering, the gray cortex of the cerebrum and cerebellum. By this means the white substance in certain parts of the brain becomes the core (nucleus medullaris), whereas the gray portion becomes the cortex, a condition differing in an important manner from the structure of the spinal cord. The morphological differentiation of the toain depends upon the very unequal growth both of the five separate vesicles and of different tracts of their walls. For example, the other four vesicles remain in their development far behind that of the cerebral vesicle, in comparison with which they constitute only a small fraction of the entire mass of the brain (figs. 240, 241). They become overgrown by the cerebral vesicle from above and on the sides, and enveloped as by a mantle, so that they remain uncovered and visible only at the base of the brain. Therefore they, together with a small part of the basal portion of the cerebrum, are grouped together as the stalk of the brain, in contradistinction to the remaining chief part of the cere- brum, which constitutes the cerebral mantle. The unequal growth of the walls of the brain manifests itself in the appearance of thickened and attenuated places, in the development of special nerve-columns (pedunculi cerebri, cerebelli, etc.), and in the formation of more or less extensive layers of ganglionic cells (thalamus opticus, corpus striatum). By these means the principle of the formation of folds, which was fully described in the fourth THE ORGANS OF THE OUTER GERM-LAYER. 427 chapter, is shown to be carried out in a special manner on the hemispheres of the cerebrum and cerebellum inclusive of the vermiform process, — that is to say, on the two parts of the brain which are covered with a gray cortex. That the functional capacity of the cerebrum and cerebellum depends upon the extent of the gray cortex and the regularly arranged ganglionic cells in it, is to be concluded from a large number of phenomena. In this way is explained the very extensive increase of surface which is brought about in the cerebrum and cerebellum by means of somewhat different processes of folding. In the cerebrum broad ridges (gyri) arise from the medullary layer of the hemispheres (centrum semi- ovale), which, running in meandering convolutions, produce the characteristic relief of the surface (fig. 256). In the cerebellum the - schei.l ft I Fig. 240.— Lateral view of the brain of a human embryo from the first half of the fifth month, after MIHALKOVICS. Natural size. stl, Frontal lobe ; schei.l, parietal lobe ; hi, occipital lobe ; schl.l, temporal lobe ; Sy.g, fissure of SYLVIUS ; m, olfactory nerve ; kh, cerebellum ; br, pons; mob, medulla oblongata. numerous ridges proceeding from the medullary nucleus are narrow, arranged parallel to one another, and provided with smaller accessory (secondary and tertiary) ridges, so that the cross section of the cerebellum presents an arborescent figure (arbor vitse). If, after these preliminary remarks, we take under consideration the metamorphoses of the five vesicles, we may distinguish on each, as MIHALKOVICS has done in his monograph of the development of the brain, four regions : floor, roof, and two lateral parts. We shall begin our description with the fifth vesicle, because in its structure it approaches most closely to the spinal cord. (1) Metamorphosis of the Fifth Brain-Vesicle. The fifth brain-vesicle exhibits in different Vertebrates at the beginning of development (in the Chick on the second and third 428 EMBRYOLOGY. days) faint, regular infoldings of its lateral walls, by means of which it becomes separated into several smaller parts, lying one behind the other. Inasmuch as these afterward disappear without leaving any trace, no great importance was ascribed to them by the earlier investigators (REMAK). Recently, however, several persons have maintained for them a real significance. RABL and BERANECK cmg kh Fig. 241.— Brain of a human embryo from the first half of the fifth month, divided in the median plane ; view of the median surface of the right half, after M.HALKOVICS. Natural size. rn, Olfactory nerve ; tr, infundibulum of the between-brain ; ana, commissura anterior; ML, foramen of MONRO ; frx, fornix ; apt, septum pellucidum ; bal, corpus callosum, which be'.ow, at the germ, is continuous with the embryonic lamina terminalis ; cmg, sulcus calloso- marginalis ; fo, fissura occipitalis ; zw, cuneus ; fc, fissura calcarina ; z, epiphysis ; vh, corpora quadrigemina ; kh, cerebellum. msp Fig. 242. Brain of a human embryo from the second half of the third month, seen from behind, after MIHALKOVICS. Natural size. msp, Longitudinal (interjMillial) fissure; vh, corpora quadrigemiua ; vma, velum medullare anterius; kh, hemispheres of the cerebellum; v*, fourth ventricle (fossa rhomboidalis) ; , medulla oblongata. recognise in them a segmentation of the brain-tube which is related to the exit of certain cranial nerves and is of importance in regard to the question of the metamerism of the entire head-region. The circumstance that the folds are so transitory appears to me to favor the older view. In the further development of the vesicle of the after-brain a distinction arises between the floor and side walls on the one hand THE ORGANS OP THE OUTER GERM-LAYER. 429 and the roof on the other. The former (figs. 241, 242) are con- siderably thickened by the addition of nervous substance and become separated on either side of the body (in Man in the third to the sixth months) into columns, which are recognisable from the outside because they are separated by grooves ; these are the extensions with certain modifications of the three familiar columns of the spinal cord. The roof of the vesicle (fig. 235 rf and fig. 243 Dp), on the contrary, produces no nerve -substance, retains its epithelial structure, becomes still thinner, and in the adult consists of a single layer of flat cells. This forms the only covering to the cavity of the dorso- ventrally compressed vesicle of the after- brain — the fourth ventricle or fossa rhomboidalis. It is firmly applied to the under surface of the pia mater, and with it produces the posterior choroid plexus (tela choroidea inferior). The name choroid plexus has been chosen because the pia mater in this region becomes very vascular and in the form of two rows of branched villi grows into the cavity of the after-brain vesicle, always carrying before it, and thus infolding, the thin epithelial roof. Laterally the roof -plate or the epithelium of the choroid plexus is continuous with the parts of the brain-vesicle that have been meta- morphosed into nervous matter. The transition is effected by means of thin bands of white nervous substance, which, as obex, tsenia sinus rhomboidalis, velum medullare posterius, and pedunculus flocculi, surround the edge of the fossa rhomboidalis. If with the pia mater one strips off from the medulla oblongata the posterior medullary velum, the epithelial covering of the fourth ventricle adhering to the latter will naturally be removed with it. In this way is produced the posterior brain-fissure of the older authors, through which one can penetrate into the system of cavities in the brain and spinal cord. (2) Metamorphosis of the Fourth Brain-Vesicle. The wall of the fourth brain-vesicle undergoes a considerable thick- ening in all its parts, and surrounds its cavity in the form of a ring differentiated into several regions ; the cavity becomes the anterior part of the fossa rhomboidalis (figs. 243, 242, 241). The floor furnishes the pons (bb), the cross fibres of which become evident in the fourth month. From the lateral walls arise the pedunculi cerebelli ad pontem. But it is the roof that grows to an extraordinary extent and gives to the cerebellum its characteristic stamp. At first 430 EMBRYOLOGY. it appears as a thick transverse ridge (figs. 242, 243 kh), which over- hangs the attenuated roof of the medulla. In the third month the middle portion of the ridge acquires four deep trans- verse folds by the sinking in of the pia mater (fig. 242), and in this way is distinguished as the vermiform process from the lateral parts, which still appear smooth (kh). From this time forward the lateral parts outstrip the middle part in growth, bulge out at the sides as two hemispheres, and, ac- quiring transverse folds, in the fourth month be- come the voluminous hemispheres of the cerebellum. Only a little nerve-substance is developed where the roof of the fourth brain- vesicle, which has become thickened to constitute the vermiform process and hemispheres, is continuous with the roof of the third and fifth vesicles (fig. 241). Consequently there arise here thin medullary lamellae, which serve as a transition on the one hand to the posterior choroid plexus, and on the other to the lamina quadrigemina (vh) — the posterior and the anterior velum medullare. Fig. 243- —Brain of an embryo Calf 5 cm. long, seen from the side. The lateral wall of the hemisphere is removed. After MIHALKOVICS. Magnified 3 diameters. at, Corpus Btriatum ; ML, foramen of MONRO ; agf, fold of the choroid plexus (plexus choroideus lateralis) ; am/, fold of the cornu Ammonis ; kh, cerebellum ; Dp, roof -plate of the fourth ventricle ; 66, pontal flexure; mo, medulla oblongata; mh, mid-brain (cephalic flexure). (3) Metamorphosis of the Third or Mid-brain Vesicle. (Figs. 235, 243, 242, 241.) The mid-brain vesicle is the most conservative portion of the embry- onic neural tube, the part which is changed least of all ; in Man a small portion only of the brain is derived from it. Its walls become rather uniformly thickened on all sides of the cavity, which is narrow and becomes the aqueduct of SYLVIUS. The base and lateral walls together supply the crura cerebri and substantia perforata posterior. The roof -plate (fig. 242 vh) becomes the corpora quadrigemina, owing to the appearance, in the third month, of a median furrow, and, in the fifth month, of a transverse one crossing it at right angles. THE ORGANS OF THE OUTER GERM-LAYER. 431 Whereas at the beginning of the development the mid-brain vesicle (figs. 235, 243 mh), as a consequence of the curvature of the neural tube, occupies the highest position and produces the parietal prominence of the head (fig. 158 s), it is afterwards covered in from above by the other parts of the brain, which are becoming more voluminous, — the cerebellum and cerebrum, — and is crowded down to the base of the brain (compare fig. 235 mh with fig. 241 vh). (4) Metamorphosis of the Second or Between-brain Vesicle. The between-brain vesicle also remains small, but undergoes a series of interesting changes, since, apart from the optic vesicles, which grow out from its walls, two other appendages, of proble- matical meaning, are developed from it — the pineal gland and the hypophysis. In the case of the between-brain vesicle, it is only in the lateral walls that a considerable amount of nerve-substance is formed. By this means the walls thicken into the optic thalami with their ganglionic layers. Between them the cavity of the vesicle is retained as a narrow vertical fissure, known as the third ventricle ; it is united with the fossa rhomboidalis by means of the aqueduct of SYLVIUS. The floor remains thin and at an early period becomes evaginated downwards ; it thus acquires the form (figs. 235, 241 tr) of a short funnel (infundibulum), with the apex of which is united the hypophysis, soon to be fully described. The roof presents in its metamorphosis a striking similarity to the corresponding part of the after -brain vesicle (fig. 241). It persists as a simple, thin epithelial layer, unites with the very vascular pia mater, — which sends out in this case also villous outgrowths with capillary loops which pass into the third ventricle, — and together with it constitutes the anterior choroid plexus (tela choroidea anterior or superior). When in withdrawing the pia mater the choroid plexus is also removed, the third ventricle is opened ; thus is produced the anterior great fissure of the brain through which, as through the structure of the same name in the medulla oblongata, one can penetrate into the cavities of the brain. The agreement with the medulla oblongata is expressed in still another point. As in the case of the latter the edges of the roof- plate develop into thin medullary bands, by means of which the attachment to the sides of the fossa rhomboidalis is accomplished, so 432 EMBRYOLOGY. here also the epithelium of the choroid plexus attaches itself to the surface of the optic thalamus by means of thin bands consisting of ruedullated nerve-fibres ftsenise thalami optici). Finally, out of the hindermost portion of the roof of the between- brain vesicle a peculiar organ, the pineal gland (fig. 241 z), takes its origin at a very early period, in Man in the course of the second month. Since in recent years numerous interesting works have appeared concerning it, and since many striking discoveries have been brought to light both in the case of the Selachians and more especially in that of the Reptiles, I will describe it at some- what greater length. The Development of the Pineal Gland (Epiphysis cerebri). First it is to be mentioned that, with the exception of Amphioxus lanceolatus, the pineal gland (glandula pinealis s. conarium) is not wanting in any Vertebrate. It is in all cases formed in exactly the same way. On the roof of the between-brain, where it is continuous with the roof of the mid-brain or the lamina quadrigemina, there arises an evagination (figs. 238 and 241 z) which has the shape of the finger of a glove, the processus pinealis [epiphysis cerebri], the apex of which is at first directed forward, but subsequently backward. In its further metamorphosis there appear, as far as our knowledge at present extends, differences of considerable importance. According to the investigations of EHLERS, the pineal process attains in adult Selachians an unusual length ; its closed end swells into a vesicle, which penetrates the .cranial capsule and extends out to the dermal surface. In many Selachians, such as Acanthias and Raja, the vesicular end is enclosed in a canal of the cranial capsule itself ; in others it lies outside between the cranial capsule and the corium. The [proximal] end of the vesicle is united to the between- brain by means of a long slender canal. Manifold conditions are met with in Reptiles, as the recent investi- gations of SPENCER have taught. These conditions permit in part a direct comparison with the Selachians, but in part they are widely altered. Here, too, the pineal gland is a structure of considerable length, the peripheral end of which lies far away from the between- brain under the epidermis ; it passes out through an opening in the roof of the skull which is situated in the parietal bone and is known as the foramen parietale. The position of the latter can easily be determined on the head of the living animal, because at this place THE ORGANS OF THE OUTER GERM-LAYER. 433 the dermal scutes acquire a special condition and form, and, above all, are transparent. In regard to the particular form of the organ, there are essentially three types to be distinguished. In many Reptiles, e.g., in Platydactylus, the pineal gland has the same structure as in Sharks : a small peripheral vesicle, which is «c/i& p et bl x Fig. 244.— Diagrammatic longitudinal section through the brain of Chameleo vulgaris with the pineal organ, which is separated into three portions, a vesicular, a cord-like, and a tube-like portion, after BALDWIN SPENCER. schb, Parietal bone with the foramen parietale ; p, pigment of the integument ; st, cord-like middle portion of the epiphysis ; bl, its vesicular terminal portion ; x, transparent region of the integument ; grh, cerebrum ; sh, optic thalamus ; v*, third ventricle, which is continued upwards into the tube-like initial portion (A) of the epiphysis. enclosed in the parietal foramen, is lined with ciliated cylindrical cells, and is connected with the roof of the between-brain by means of a long, hollow stalk. In other Reptiles, as in the Chameleon, the organ is differentiated into three portions (fig. 244) : first into a small closed vesicle (6Q, which lies under a transparent scale (x) in the foramen parietale and is lined with ciliated epithelium ; secondly into a solid cord 28 434 EMBRYOLOGY. (st), which consists of fibres and spindle-shaped cells, and bears a certain resemblance to the embryonic optic nerve ; and thirdly into a hollow, funnel-shaped projection (A) of the roof of the between- brain, which still exhibits here and there sac-like enlargements. In a third division of the Hep tiles, in Hatteria, Monitor, the Blind-w o r m s, and Lizards, the vesicular distal portion of the pineal gland under- goes a striking metamo r p h o- sis, by means of which it ac- quires a certain resemblance to the eye of many In vertebra t e s (fig. 245). The portion of its wall which lies next to the sur- face of the body has been trans- formed into a lens-like struc- ture (I) ; the part of the wall lying opposite the latter and continuous with the fibrous cord (St) has, on the contrary, been converted into a retina-like structure (r). The formation of the lens (I) is due to the fact that the epithelial cells of the anterior wall of the vesicle have become elongated into cylindrical cells and uninucleate fibres, and have thereby produced an elevation, the convex surface of which ft Fig. 246.— Longitudinal vertical section through the pineal eye of Hatteria punotata and its connective-tissue capsule, after BALDWIN SPENCER. Slightly enlarged. The anterior part of the capsule fills up the parietal foramen. K, Connective-tissue capsule ; I, lens ; h, cavity of the eye filled with fluid ; r, retina-like portion of the optic vesicle ; M, molecular layer of the retina ; g, blood-vessels ; x, cells in the stalk of the pineal eye ; St, stalk of the pineal eye, comparable with the optic nerve. THE ORGANS OF THE OUTER GERM-LAYER. 435 projects into the cavity of the vesicle. In the posterior portion the epithelial cells are separated into different layers, the innermost of which is distinguished by the abundance of its pigment. Between the pigmented cells there are imbedded others, which can be compared to the rods of the visual cells in the paired eyes of Vertebrates, and which appear to be in connection below with nerve- fibres. Those investigators who, like RABL-RUCKHARD, AHLBORN, SPENCER, and others, have studied the pineal gland, are of opinion that the pineal body 'must be considered as an unpaired parietal eye, which in many classes, for example in Reptiles, appears to be tolerably well preserved, but in most Vertebrates is in process of degeneration. That we have to do in Reptiles with an organ which reacts under the influence of light, does not appear improbable, when one takes into consideration that, owing to the transparency of the dermal scutes at the place in the skull where the parietal foramen is located, rays of light are here able to penetrate through the integu- ment. The presence of a lens-like body and pigment is also favorable to this view. But whether the organ serves for sight, or only for the transmission of sensations of warmth, — whether, consequently, it is more an organ for the perception of warmth than an eye, — must for the present remain undecided. It is still more an open question whether this organ of warmth is a structure which has been developed as a special modification of the epiphysis of Reptiles alone, — as the auditory sac, for example, has been developed in the tail of the Crustacean Mysis, — or whether it represents a structure originally common to all Vertebrates. In the latter case processes of degeneration must be assumed to be wide- spread, for up to the present time nothing like the condition in Reptiles has been found in other Vertebrates. In Birds and Mammals the pineal process undergoes metamor- phoses which give rise to an organ of a glandular, follicular structure. In Birds (fig. 246) it never attains such great length as in Selachians and Reptiles. At a certain stage it sends out from its surface into the surrounding vascular connective tissue cellular out- growths, which increase in number by means of budding and finally break up into numerous small follicles (fig. 246 f). These consist of several layers of cells, the outermost being small, spherical elements, the innermost cylindrical ciliated cells. The proximal portion of the pineal process does not become involved in the follicular metamor- phosis and persists as a funnel-shaped outfolding of the roof of the between-brain ; the individual follicular vesicles constricted off from 436 EMBRYOLOGY. the parental tissue are united with its upper end by means of connective tissue. In Mammals the development takes place in a manner similar to that of the Chick. In the Rabbit there also arise follicles, each of which at first encloses a small cavity, but later becomes solid. They are then entirely filled with spherical cells, which possess a certain resemblance to lymph- corpuscles. The opinion has therefore been expressed by many (HENLE) that the pineal body is a lymphoid organ, an opinion, Fig. 246.^sectio7through the pineal gland however, which is refuted by the of a Turkey, after MIHALKOVICS. Mag- study of the development, for nified ISO diameters. ,. ,, , r ir i /.Follicle of the pineal gland with ite cavities; genetically the follicles are ex- 6, connective tissue with blood-vessels. clusively epithelial structures. In the adult there are formed within the individual follicles concretions, the brain-sand (acervulus cerebri). In Man the pineal body, which begins to appear in the sixth week (His), exhibits -a peculiarity as regards its position. Whereas the free end of the epiphysis is at first directed forward, and in other Vertebrates is also retained in this position, it acquires in Man an opposite direction, inasmuch as it bends backward on to the surface of the lamina quadrigemina. Probably this is connected with the fact that the gland is crowded back by the excessive development of the corpus callosum. As the signification of the pineal gland is still doubtful, so is that of the pituitary body or hypophysis cerebri, which, as has been previously mentioned, is united with the floor of the between-brain at the apex of the infundibular process. The Development of the Hypophysis (Pituitary Body). Tlut hypophysis is an organ which lias a double origin. This is expressed in its entire structure, since it is composed of a larger, anterior and a smaller, posterior lobe, which in their histological characters are fundamentally different from each other. In order to observe the beginning of its formation, it is necessary to go back to a very early stage (fig. 237), in which the oral sinus THE ORGANS OP THE OUTER GERM-LAYER. 437 has just arisen and is still separated from the cavity of the head-gut by means of the pharyngeal membrane (rh). At this time the cephalic flexure of the brain-vesicles has already appeared, and the anterior end of the chorda dorsalis (ch) terminates immediately behind the attachment of the pharyngeal membrane. In front of this is located the important place where the hypophysis is developed, as was first established by GOETTE and MIHALKOVICS. The hypo- physis is therefore a product of the outer germ-layer and not a growth from the cavity of the head-gut, as had always been maintained previous to this time. The first steps introductory to the formation of the hypophysis take place soon after the rupture of the pharyngeal membrane (figs. 238, 247), some unimportant remnants of which are retained at the base of the skull as the so-called primitive velum palatinum. Anterior to these there is now developed (in the Chick on the fourth day of incubation, in Man during the fourth week, His) a small evagination, the pouch of KATHKE or the pocket of the hypophysis (hy), which grows ' to- ward the base of the b e t w e e n-b rain (tr). Then it becomes deeper, begins to be constricted off from its parent tissue, and to be metamor- phosed into a small sac, whose wall is composed of several layers of cylin- drical cells (fig. 248). The sac of the hypo- physis (hy) remains for a long time in connec- tion with the oral cavity by means of a narrow duct (hyg). In later stages, however, the connection in the higher Vertebrates ch nli Fig. 247.— Median sagittal section through the hypophysis of a Rabbit embryo 12 mm. long, after MIHALKOVICS. Magnified 50 diameters. tr, Floor of the between-brain with the infundibulum ; nh, floor of the after-brain j ch, chorda ; hy, pocket of the hypophysis. IS interrupted, because the embryonic connective tissue, which supplies the foundation for the development of the skeleton of the head, becomes thickened and crowds the sac farther away from the oral cavity (figs. 248, 249). When, later on, the process of chondrification 438 EMBRYOLOGY. - tr takes place in the connective tissue, by means of which the carti- laginous base of the skull (schb) is established, the sac of the hypo- physis (hy) comes to lie above the latter at the under surface of the be- tween-brain (tr). At this time also the duct of the hypophysis (hyg), which meanwhile has lost its lumen, begins to shrivel and degenerate (fig. 249). In many Vertebrates, however, as in the schb hyg schb Fig. 248.— Sagittal section through the hypophysis of a Rabbit embryo 20 mm. long, after MIHALKOVICS. Magnified 55 diameters. tr, Floor of the between-brain with inf undibulum ; hy, hypophysis ; hy", part of the hypophysis in which the formation of the glandular tubules begins; hyg, duct of the hypophysis; schb, base of the skull ; ch, chorda; si, dorsum sellse. Selachians, it is retained throughout life and forms a hollow canal, which perfo- rates the carti- laginous base of the skull and is united with the epithelium of the mucous membrane of the mouth. In extremely rare cases there retained Man also IS iii a canal in the basi-sphenoid, which leads from the sella turcica to the base of the skull (SUCHANNEK). schb _ Fig. 249.— Sagittal section through the hypophysis of a Rabbit embry* 30 mm. long, after MIHALKOVICS. Magnified 40 diameters. tr, Floor of the between-brain with inf undibulum ; hy, original pouch- like part of the hypophysis ; htf, the glandular tubules which have budded out from the sac of the hypophysis; si, dorsum sell*; ba, basilar artery; ch, chorda ; sckb, cartilaginous base of the skull; em, epithelium of oral cavity. and receives a prolongation of the hypophysis THE ORGANS OF THE OUTER GERM-LAYER. 439 At an early period an evagination from the between-brain (figs. 247, 249), called the infunclibulum (tr\ has grown out toward the sac of the hypophysis and applied itself to the posterior wall of the latter, which it has folded in toward the anterior or opposite wall. This first stage is followed by a second, in which the sac and the adjoining end of the infundibulum are metamorphosed into the two lobes of the complete organ already mentioned. The sac begins (in Man in the second half of the second month, His) to send out from its surface into the surrounding very vascular connective tissue hollow tubules (the tubules of the hypophysis) (figs. 248, 249 hy'\ These are then detached from the walls of the sac, by becoming enclosed on all sides by vascular connective tissue. In this respect the process of development agrees in the main with that of the thyroid gland, only that the spherical follicles are here represented by tubular structures. After the entire sac has been resolved into a large number of small, tortuous tubules provided with narrow lumina, the lobe thus produced applies itself closely to the lower end of the infundibulum, with which it becomes united by means of connective tissue. The end of the infundibulum itself is transformed in the lower Vertebrates into a small lobe of the brain, in which, moreover, ganglionic cells and nerve-fibres can be identified. In the higher Vertebrates, on the contrary, no trace of such histological elements can be detected in the posterior lobe of the hypophysis, which in these forms consists of closely packed spindle-cells, and thus acquires a close resemblance to a spindle-cell sarcoma. (5) Development of the First or Fore-Brain Vesicle. The most important changes, the comprehension of which is in part attended with serious difficulties, take place in the vesicle of the fore-brain or cerebrum. It is divided (fig. 250), even at the time of its formation, as has already been mentioned, into a right and a left portion, owing to the fact that its wall becomes infolded from in front and from above by means of a vertical process of the connective- tissue envelope of the brain, the primitive falx. The two portions, the vesicles of the hemispheres (hms), come close together, being separated by only the narrow longitudinal or interpallial fissure (msp), which is filled up by the falx, so that their median surfaces become mutually flattened, whereas their lateral and under surfaces are 440 EMBRYOLOGY. convex. Where the plane and convex surfaces are continuous with each other there is a sharp bend in the mantle (Mantelkante). The vesicles of the hemispheres at first have thin walls formed of several layers of spindle- shaped cells (fig. 251, i) and each encloses a large cavity, the lateral ventricle (fig. 251), which is derived from the central canal of the neural tube. Inasmuch as these have been reckoned by the earlier authors as the first and second ventricles, it is plain why the cavities of the between-brain and medulla oblongata are respectively designated as the third and fourth ventricles. In Man, during the earlier months, each lateral ventricle is in communication with the third ventricle by means of a wide opening, the primitive foramen of MONRO (figs. 239 ML and 254 m). Tig. 250. -Brain of a human embryo seven weeks old, parietal (Soheitel) aspect, after MIHALKOVICS. msp, Interpallial (longi- tudinal) fissure, at the bottom of which is seen the embryonic lamina terminalis (Schluss- platte) ; hms, left hemi- sphere ; zh, between- brain ; mk, mid-brain ; hh, hind - brain and after-brain. Anterior to the foramen of MONEO lies the part of the wall of the cerebrum which was infolded by the development of the great interpallial fissure : on the one hand it effects the anterior union of the walls of the two hemispheres ; on the other it bounds the third ventricle in front, and is therefore called the anterior closing plate (lamina terminalis). It is continuous below with the anterior wall ^ £. of the infundibulum of the between-brain. In the further develop- ment of each vesicle of the hemispheres four processes are intimately associated : ( 1 ) an extraordinary growth and an enlargement in all directions resulting from it ; (2) an infolding of the wall of the vesicle, so that externally there arise deep clefts (the fissures), and internally projections into the lateral ventricles; (3) the development of a system of commissures, by means of which the right and left hemispheres are brought into closer union (corpus callosum and fornix) ; (4) the formation of Fig. 251.— Brain of a human embryo of three months, after KO'LLIKER. Natural size. 1. From above with the hemispheres removed and the mid-brain opened. 2. The same from below. /, Anterior part of the marginal arch (Randbogen) of the cerebrum cut through ; /', posterior part (hippocampus) of the marginal arch ; tho, optio thalamus ; cst, corpus striatum ; to, tractus opticus ; OH, corpora raammillaria ; p, pons Varolii. THE ORGANS OF THE OUTEE GERM-LAYER. 441 furrows that cut into the cortex of the cerebrum more or less deeply from the outside, but cause no corresponding internal projections in the wall of the ventricle. As regards its general features, the embryonic growth of the cerebral vesicles is especially characterised by an enlargement backward. In the third month the posterior lobe already completely overlies the optic thalamus (fig. 242) ; in the fifth month it begins to extend over the corpora quadrigemina (fig. 241), which it entirely covers up in the sixth month. From there it spreads over the cerebellum (fig. 256). The cerebrum is not characterised in all Mammals by such an extraordinary growth as in Man; comparative anatomy teaches rather that the stages of development of the human brain in different months here described, are met with in other Mammals as permanent conditions. In some animals the posterior margins of the hemispheres extend as far as the corpora quadrigemina ; in others they cover these more or less completely ; in others, finally, they have grown over the cerebellum more or less. On the whole, the increase in the volume of the cerebrum, which is so varied in Mammals, goes hand in hand with an increase in intelligence. The vesicles of the hemispheres undergo additional complication (in Man in the course of the second and third months), owing to infoldings of their thin walls, which still enclose a large cavity. As a result of this there arise on the outer surface deep furrows, which separate large areas from one another and which have been designated as total furrows or fissures by His, who has rightly estimated their importance in the architecture of the brain. Corresponding to the furrows which are visible on the outer surface, there are more or less prominent elevations on the inner surface of the lateral ventricles, by means of which the latter become narrowed and reduced in size. The total furrows of the cerebral hemispheres are the fissure of SYLVIUS (fossa Sylvii), the arcuate fissure, embracing the hippo- campal fissure (fissura hippocampi), the fissura choroidea, the fissura calcarina, and the fissura parieto-occipitalis. The elevations produced by them are called the corpus striatum, fornix and pes hippocampi, tela choroidea and calcar avis. A prominence which in the embryo corresponds to the fissura parieto-occipitalis, becomes obliterated in the adult by a considerable thickening of the wall of the brain, so that no permanent structure results from it. 'Yhs fissure of SYLVIUS (fig. 252 Sy.g} is the first one formed. It appears as a shallow depression of the con rex outer surface at about 442 EMBRYOLOGY. the middle of the lower margin of each hemisphere. The part of the wall which is thus depressed- becomes considerably thickened (figs. 243, 251 cst, and 254 st), and forms an elevation on the floor of the cerebrum projecting into its cavity, the corpus striatum, in which several nuclei of gray matter are developed (the nucleus caudatus, the nucleus lentiformis, and the claustrum). Inasmuch as the elevation lies at the base of the brain and forms the direct forward and lateral continuation of the optic thalamus, it is regarded as belonging to the brain-stalk, and is distinguished as the stalk part of the cerebral hemispheres in distinction from the remaining portion or mantle part. The outer surface of the stalk part can be seen from the outside for a time, — as long as the Sylvian fissure is still shallow (fig. 252 Sy.y), Kltd.1 Fig. 262.— Lateral view of the brain of a human embryo during the first half of the fifth month, after MIHALKOVICS. Natural size. *«, Frontal lobe ; schei.l, parietal lobe ; hi, occipital lobe ; schl.l, temporal lobe ; Sy.g, fissure of SYLVIUS ; rn, olfactory nerve ; kh, cerebellum ; 6)-, pons ; 71106, medulla oblongata. — but it then becomes entirely overgrown and hidden by the edges of the gradually deepening fissure. Later this surface acquires in the embryo several cortical furrows and becomes the island of REIL (insula Reilii), or the central lobe (Stammlappen). The mantle portion, as it enlarges, spreads out uniformly around the island of REIL, as though about a fixed point, and surrounds it in the form of a half-ring open below (fig. 252) ; on this account it nas received the name ring-lobe. Even now the regions of the four chief lobes into which the convex surface of each hemisphere is subsequently divided can readily be distinguished, although they are not yet sharply limited. The end of the half ring which is directed forward and lies above the fissure of SYLVIUS (Sy.g) is the frontal lobe (stl) ; the opposite end, which embraces the fissure behind and THE ORGANS OF THE OUTER GERM-LAYER. 443 below, is the temporal lobe (schl.l) ; the region lying above and connecting the two is the parietal lobe (schei.l). A prominence which is developed from the ring-lobe backward becomes the occipital lobe (hi). The lateral ventricle has also become altered and corresponds to the external form of each hemisphere (fig. 253). It also assumes the shape of a half-ring, which lies above and surrounds the corpus striatum (cst) — that part of the wall of the vesicle which is forced inward by the fissure of SYLVIUS. Subsequently, when the individual lobes of the hemispheres are more sharply differentiated from one another, the lateral ventricle also undergoes a subdivision correspond- ing to the lobes. It becomes slightly enlarged at both ends, in front into the anterior cornu occupying the frontal lobe, behind and below into the inferior cornu of the temporal lobe. Finally, from the half- ring there is developed a small evagination, the posterior cornu, which extends backward into the occipital lobe. The region lying between the horns is narrowed and becomes the cella media. All the fissures hitherto mentioned, except that of SYLVIUS, are developed on the plane [median] surface of the vesicle of the hemisphere. At a very early stage — in Man in the fifth week (His) — there arise on this wall of the hemisphere two furrows running almost parallel with the edge or bend of the mantle, the arcuate or hippocampal fissure and the fissure of the choroid plexus (fissura hippocampi snidfissura choroidea) ; both conform very closely in their direction to the ring- lobe, and, like it, with crescentic form embrace from above the stalk part of the cerebrum, the corpus striatum. They begin at the foramen of MONRO and extend from there to the tip of the temporal lobe, forming the boundaries of a region known as the marginal arch (Randbogen) ; this projects as a thickening on the median surface of the hemisphere, and takes part in the development of the commissural system. The invaginations of the median wall of the ventricle, caused by the fissures, the hippocampal fold and the fold of the lateral choroid plexus, are best understood by removing in an embryo the lateral wall of the hemisphere, so that one can survey the inner surface of the median wall of the still very spacious and ring-like lateral ventricle (fig. 253). The cavity is then seen to be partly filled with a reddish frilled fold (agf), which lies in the form of a crescent on the upper surface of the corpus striatum (cst). In the region of the fold the wall of the brain undergoes changes similar to those in the roof of the medulla oblongata and of the vesicle of the between-brain 444 KMllKYOLOGY. Fif . 853. —Lateral visw of the brain of an embryo Calf 6 cm. Ion;. The lateral vail of the hemisphere has been removed. After MIHAI.KOVICS. Magni- fied 8 diameters. «*, Corpus stria twin ; ML, foramen of MOXRO ; agf, plexus ohoroidens lateral is ; an\f, hippocampal fold ; !•*, cerebellum ; Dp, roof of the fourth ventricle; 66, pontal flexure; mo, medulla ob- longata ; mh, mid-brain (parietal flexure). (figs. 254 pi and 255 aaf). Instead of thickening and developing mh nerve-substance, it becomes attenuated, and is trans- formed into a single layer of flat epithelial cells, which are firmly united with the pia mater. The latter then becomes very vascular along the entire fold, and grows into the lateral ventricle in the form of tufts, which carry the epithelium before them. In this way the lateral choroid plexus arise* (fig. 254 pi), which afterwards, in the adult, fills a part of the cella media and in- ferior cornu. \ It begins at p( the foramen of MONRO (fig. 253JHX),where it is continuous with the an- terior unpaired choroid plexus which has ari>eii in the roof of the be- tween-brain Y«>iclo. If the delicate \as- eularpia mater is drawn out from the cho- roid fissure, the wall of the brain, which is reduced to a thin epithe- lium, is at the median wall of Fif. 954.— Transverse section through the brain of an embryo Sheep 27 cm. in length, after Koi IIKVI;. The section passes through the region of the foramen of MONRO. st, Corpus striatum ; m, foramen of MOXRO ; (, third ventricle ; pi, plexus choroideus of the lateral % outride :/. falx cerebri ; tk, deepest anterior part of the optic thalamus ; c*. chiasma : o, optiv I • bres of the cms cerebri; A, hippcH-ampal fold : ;', pharynx ; •a, preephenoid; a. orlnto sphenoid ; .«, part of the ux>f of the brain at the junction of the roof of the third ventricle with the lamina tciininalis; I, lateral ventricle. same time destroyed, and there is prodm-ed in the the hemisphere a gaping tisMire. which extends from THE ORGANS OF THE OUTER GERM-LAYER. 445 the foramen of MONRO to the tip of the temporal lobe and leads from the outside into the lateral ventricle. This is the lateral cerebral fis- sure, or the great fissure of the hemispheres (fissura cerebri transversa). In a preparation made in the manner described the hippocampal fold is to be seen at a short distance from the choroid plexus and parallel to it (figs. 253 and 255 am/ and fig. 254 h). This increases in size toward the apex of the inferior cornu, and in the completely formed brain produces the cornu Ammonis or pes hippocampi. Consequently that part of the lateral ventricle enclosed in the tem- poral lobe becomes (as the result of two infoldings of its median wall) restricted by two pro- jections, the choroid plexus and the cornu Ammonis. As in the between- brain and me- dulla o b 1 o n- gata, the epi- thelial covering of the choroid plexus is con- tinuous with the thicker nerve-sub- stance of the cornu A m- monis. The transition is effected by means of a thin medullary plate, which in anatomy is described as the fimbria. Inasmuch as the occipital lobe with its cavity develops as an evagination of the ring-lobe, the fissura calcarina belonging to it is therefore developed somewhat later than the arcuate fissure (fig. 241 fc). It appears at the end of the third month as a fissure branching off from the latter, and runs in a horizontal direction until near the apex of the occipital lobe. It invaginates the median wall of the lobe and produces the calcar avis, which invades the posterior cornu in the same way as the hippocampus major (cornu Ammonis) does the inferior cornu. At the beginning of the fourth month the fissura occipitalis (fig. 241/0) is added to it. The latter rises from Fig. 255. — Transverse section through the brain of a Rabbit embryo 3'8 cm. in length, after MIHALKOVICS. Magnified 9 diameters. The section passes through the foramina of MONRO. hs, Great falx cerebri which fills up the interpallial fissure ; A1, h2, plane inner [median] and convex outer wall of the cerebral hemisphere ; agf, fold of the choroid plexus ; amj\ hippocampal fold ; /, fornix ; tv, lateral ventricle ; ML, foramen of MONRO ; v3, third ventricle ; ch, optic chiasma ; frx1, descending root of the fornix. 446 EMBRYOLOGY. the anterior end of the fissura calcarina in a vertical direction to the bent rim of the mantle (Mantelkante), and sharply separates the occipital and parietal lobes from each other. A third factor of great importance in the development of the cerebrum is the formation of a system of commissures, which sup- plements the connection of the two cerebral vesicles, at first effected by the embryonic lamina terminalis only. Those investigators who have occupied themselves with these difficult matters assert that in the third embryonic month fusions take place between the facing median walls of the hemispheres. These fusions begin in front of the foramen of MONRO within a triangular area. The fusions in this region occur only at the periphery, not in the middle of the area. Three parts of the brain of the adult are thus produced : in front, the genu of the corpus callosum, behind, the columns of the fornix, and between them, the septum pellucidum ; the latter contains a fissure- like cavity, in the region of which the contiguous walls of the hemi- spheres, here very much attenuated, have remained separated from each other. Consequently the cavity just mentioned — the ventriculus septi pellucidi [or fifth ventricle] — ought not to be placed in the same category with the other cavities of the brain ; for while the latter are derived from the central canal of the embryonic neural tube, the former is a new production, which has arisen by the enclosure of a portion of the space lying outside the brain between the two hemi- spheres— the narrow interpallial fissure. A further enlargement of the commissural system is accomplished in the fifth and sixth months. The fusion now proceeds still farther, advancing from in front backwards, and involves that region of the median walls of the hemispheres which, situated between the arcuate fissure [above] and the fissure of the choroid plexus [below], has already been described as the marginal arch (Randbogen). By fusion of the anterior part of the marginal arch with its fellow of the opposite side, — which process takes place as far as the posterior limit of the between-brain, — there arise the body of the corpus callosum and the splenium, as well as the underlying fornix. The furrow bounding the corpus callosum above (sulcus corporis callosi) is there- fore the anterior part of the arcuate furrow, whereas the posterior portion, that of the temporal lobe, is subsequently known as the iissura hippocampi. The structure of the cerebrum is completed by the appearance of numerous cortical furrows. These differ in rank from the total furrows already described, because they are confined to the outer surface of the THE ORGANS OF THE OUTER GERM-LAYER. 447 brain and do not cause corresponding projections into the ventricles. Their formation begins as soon as the wall of the brain becomes greatly increased in thickness by the development of white medullary substance, which occurs during and after the fifth month. This is due to the fact that the gray cortex with its ganglionic cells increases more rapidly in superficial extent than the white substance and is therefore raised into folds, the cerebral convolutions or gyri, into which only thin processes of white substance penetrate. At first, therefore, the furrows are quite shallow; they become deeper in proportion as the hemispheres become thicker and the cortical folds project farther out- ward. Of the numerous fur- rows which the completely formed brain presents, some appear during the develop- ment earlier, others later. Thus they acquire different values in the architecture of the cerebral surface. For " the earlier a furrow appears the deeper it le- comes, the later it ap- pears the shallower it is " (PANSCH). The first are therefore the more impor- tant and constant ones, and are fittingly to be distin- guished as chief or primary furrows from the subse- quently formed and more variable secondary and tertiary furrows. They begin to appear at the commencement of the sixth month. The first of them to appear is the central furrow (fig. 256 cf), which is one of the most important, since it separates the frontal and parietal lobes from each other. " In the ninth month all of the chief sulci and convolutions are formed, and since at this time the secondary sulci are still wanting, the brain during the ninth month presents a typical illustration of the sulci and convolutions " (MIHALKOVICS). Very great differences exist between the different divisions of Mammals in the extent to which the sulci of the cerebrum are developed. On the one hand are the Monotremes, Insectivores, and many Rodents, whose cerebrum— also- fo Fig. 256.— Brain of a human embryo at the beginning of the eighth month, after MIHALKOVICS. Three- fourths natural size. tif, Central furrow ; veto, hew, anterior and posterior central convolutions ; fo, fissura occipitalis. 448 EMBRYOLOGY. usually less developed in other features — possesses a smooth surface, and thus, as it were, remains permanently in the foetal condition of the human brain. On the other hand the brains of the Carnivores and Primates, owing to the great number of their convolutions, approach more closely to the human brain. Finally, in treating of the development of the cerebrum there is still to be considered an appendage to it, the olfactory nerve. This part, as well as the optic nerve, is distinguished from the peripheral nerves by its entire development, and must be considered as a specially modified portion of the cerebral vesicle. The older de- signation of nerve is therefore now more frequently replaced by the more appropriate name of olfactory lobe (lobus olfactorius, rhinencephalon). Even at an early stage — in the Chick on the seventh day of incubation, in Man during the fifth week (His) — there is formed on the floor of each frontal lobe at its anterior end a small evagination, which is directed forward (figs. 240, 241 m). This gradually assumes the form of a club, the enlarged end of which, the part lying on the cribriform plate of the ethmoid bone, is designated as the bulbus olfactorius. The bul- bus encloses a cavity which is in communication with the lateral ventricle. During the first month of development the olfactory lobe, even in Man, is relatively large and provided with a central cavity. Later it begins to diminish somewhat, the sense of smell being only slightly developed in Man; its growth is arrested and at the' same time its cavity also disappears. In most Mammals, on the contrary, — whose sense of smell, as is well known, is more acute than that of Man, — the olfactory lobe attains a greater size in the adult animal and exhibits more clearly the character of a part of the brain, for it permanently encloses in its bulb a cavity, which Fig. 257.— Brain of Galeus canis in situ, dorsal aspect, after ROHON. Lol, Lobus olfactorius ; Tro, tractus nervi olfactorii ; VH, fore-brain, provided at fn with a vascular foramen (foramen nutritium); ZH, between -brain ; MH, mid-brain ; Hff, hind-brain ; NH, after- brain ; S, spinal cord ; //, n. opticus ; ///, n. oculomotorius ; IV, n. trochlearis ; V, n. trigeminus ; L, Trig, lobus trigemini ; C,rest, corpus resti forme ; IX, glosso- pharyngeus; X, vagus; £,t, eminentise teretes. THE ORGANS OF THE OUTER GERM-LAYER. 44D" frequently (Horse) is even in connection with the anterior cornu by means of a narrow canal in the tractus olfactorius. The olfactory lobe (Lol+Tro) attains an extraordinary develop- ment (fig. 257) in the Selachia, in which it exceeds in size the between-brain (ZH) and mid-brain (MH . In the Selachians two long hollow processes (tractus olfactorius, Tro) extend out from the anterior end of the little-developed cerebrum and terminate at a considerable distance from the fore-brain in two large hollow lobes,, that are sometimes provided with furrows (Lol). B. The Development of the Peripheral Nervous System. Although it is easy to follow the development of the brain and spinal cord, the investigation of the origin of the peripheral nervous system is very difficult, for it requires the study of histological processes of the most subtle nature — the first appearance of non-medullated nerve-fibres and the method of their termination in embryos composed of more or less undifferentiated cells. One who knows how difficult it is even in the adult animal to follow non-medullated nerve-fibrillse in epithelial layers or in non-striate mussle-tissue, and to get a clear idea of their method of termination, will understand that many, and indeed the most interesting, questions in regard to the development of the peripheral nerves are not yet ripe for discussion, because the observations necessary for their settlement are still wanting. There is only one point which is entirely clear. That concerns the development of the spinal ganglia, which His and BALFOUR independently of each other were the first to recognise, the- one in the Chick, the other in Selachians. Since then numerous- investigations embracing different groups of Vertebrates have been published on this subject by HENSEN, MILNES MARSHALL, KOLLIKER, SAGEMEHL, VAN WIJHE, BEDOT, ONODI, BERANECK, RABL, BEARD,. KASTSCHENKO, and others. (a) The Development of the Spinal Ganglia. The development of the spinal ganglia in the spinal cord is very easily followed. It begins just at the time the medullary groove closes to form a tube (fig. 258 A and B). At this time a thin ridge of cells (spgr, spy) one or two layers deep grows out of the neural tube on either side of the line of fusion, and, passing outward 29 450 EMBRYOLOGY. and downward, inserts itself between the tube and the closely investing primitive epidermis. In this way it reaches the dorsal angle of the primitive somites (its), which are by this time well developed. Then the neural crest, as BAL- FOUR names it, or the ganglionic ridge, as SAGEMEHL calls it, is divided up into successive regions. For the tracts which alternate with the primitive segments lag behind in their growth, while the parts lying opposite the middle of seg- ments grow more vigorously, become thickened, and at the same time ad- vance farther ven- trad, penetrating be- tween primitive seg- ment and neural tube. Frontal sections furnish very instruc- tive views of this stage. Fig. 259 ex- hibits such a section, taken from SAGE- MEHL'S work. Inas- much as the longi- tudinal axis of the Lizard embryo em- ployed for the sec- tions was greatly curved, the five segments seen in the section are cut at different heights, the middle one deeper than the two preceding and the two following. In the middle segment the fundament of the ganglion (spk) is differentiated and it is bounded by blood-vessels Fig. 258. .4, Cross section through an embryo of Pristiurus, after RABL. The primitive segments are still connected with the remaining portion of the middle germ-layer. At the region of tran- sition there ia to be seen an outfolding, sk, from which the skeletogenous tissue is developed, ch, Chorda ; spg, spinal ganglion ; mp, muscle-plate of the primitive segment ; ich, subchordal rod ; ao, aorta ; -ik, inner germ-layer ; pmb, parietal, vmb, visceral middle layer. £, Cross section through a Lizard embryo, after SAGEMEHL. rm, Spinal cord ; spy, lower thickened part of the neural ridge ; sptf, its upper attenuated part, which is continuous with the roof of the neural tube ; us, primitive segment. THE ORGANS OF THE OUTER GERM-LAYER. 451 in front and behind, whereas in the segments that are cut more •dorsally, near the origin of the ganglia from the neural tube, the fundaments are still connected with one another. This connection appears to be most conspicuously developed and most per- sistent in the case of the Selachians ; it has been called the longitudinal commis- sure by BALFOUR. Outside the ganglia are found the primitive segments (mp, mp'), each of which at this time still exhibits within it a narrow fissure. In a monographic treatment of the peripheral nervous system BEARD differs from the preceding account, in which BALFOUR, KOLLIKER, BABL, HENSEN, SAGEMEHL, KASTSCHENKO, and others agree. He believes that the fundaments of the ganglia arise, not out of the neural tube, but out of the deeper cell-layers of the adjacent part of the outer germ-layer. He finds that they are from the beginning separated from each other and seg- mentally arranged. According to him, moreover, they make their appearance earlier than is stated in the preceding account ; for they are already recognisable as especially thickened places in the outer germ-layer at the light and left of the neural plate when the latter first begins to be bent inward. Subsequently, upon the closure of the neural tube, the ganglionic cells come to lie between the raphe and the primitive epidermis. From here they grow down ventrally at the sides of the brain and spinal cord. BEARD approximates in his results the conception first expressed and subsequently maintained by His. For His derives the ganglionic ridge, not from the raphe of the neural tube, but from a neighboring part of the outer germ-layer, which he names intermediate cord (Zwischenstrang). The accuracy of BEARD'S description is, however, positively denied by RABL and KASTSCHENKO. Different views are entertained concerning the further changes which take place in the fundaments of the spinal ganglia : — According to His and SAGEMEHL the separate ganglionic funda- ments are completely detached from the neural tube, and for a time lie at the side of it without any connection with it whatever. Secondarily a union is again established, through the development of the dorsal nerve-roots, by the formation of nerve-fibrillse, which either grow out from the spinal cord into the ganglion, or from the .ganglion into the spinal cord, or in both directions. SAGEMEHL Fig. 259. — Frontal section of a Lizard embryo, after SAGEMEHL. rm, Spinal cord ; spk, neural ridge with thickenings that are converted into the spinal ganglia; mp', the part of the primitive segment that produces the muscle-plate ; mp, outer layer of the primitive segment. 452 EMBRYOLOGY. favors the first view, His the last. All other investigators main- tain that the fundaments of the ganglia, while they increase in size and become spindle-shaped, are permanently united with the neural tube by means of slender cords of cells which are metamorphosed1 into the dorsal roots. If the latter view is correct, the dorsal roots of the nerves must in time alter their place of attachment to the neural tube by moving from the raphe laterally and ventrally. The discrepancy of these accounts is connected with the different interpretations which exist concerning the development of the peri- pheral nerves in general. (b) The Development of the Peripheral Nerves. When one reviews the various opinions which have been expressed concerning the development of the peripheral nerves, it is found that there are in the literature two chief opposing views. The greater number of investigators assume that the peripheral nervous system is developed out of the central, — that the nerves grow forth from the brain and spinal cord uninterruptedly until they reach the periphery, where for thefrst time they effect a union with their specific terminal organs. The outgrowth of the nerves from the spinal cord was first asserted for the ventral roots and conjectured for the dorsal ones by BIDDER UND KUPFFER. Their conclusions have since been adopted by KOLLIKER, His, BALFOUR, MARSHALL, SAGEMEHL, and others. However, views concerning the 'method of the formation of the nerve-fibres are not in agreement. According to KUPFFER, His, KOLLIKER, SAGEMEHL, and others the outgrowing nerve-fibres are processes of ganglionic cells located in the central organ, which must grow out to an enormous length in order to reach their terminal apparatus. There are at first no cells or nuclei among them. These are furnished secondarily by the surrounding connective tissue. According to the accounts of KO'LLIKER and His, cellular elements from the mesenchyme approach the bundles of nerve-fibrillse, surround them, and then penetrate into the interior of the nervous stem, at first sparingly, afterwards more abundantly, and form around the axis-cylinders the sheaths of SciIWANN. On the other hand, BALFOUR defends most positively the doctrine that cells which migrate out of the spinal cord along with the nerves share in the development. In his " Treatise on Comparative Embry- ology " [vol. ii., p. 372] he remarks upon this subject : " The cellular THE ORGANS OF THE OUTER GERM-LAYER. 453 structure of embryonic nerves is a point oil which I should have anticipated that a difference of opinion was impossible, had it not been for the fact that His and KOLLIKER, following REMAK and other older embryologists, absolutely deny the fact. I feel quite sure that no one studying the development of the nerves in Elasmo- branchii with well-preserved specimens could for a moment be doubtful on this point." Of the more recent investigators VAN WIJHE, DOHRN, and BEARD side with BALFOUR. HENSEN has taken an entirely different view on the question of the origin of the peripheral nervous system, one which differs from that of KUPFFER, His, and K'OLLIKER, as well as from that of BALFOUR. He opposes the doctrine of the outgrowth of nerve-fibres •chiefly from physiological considerations. He can think of no motive which is capable of conducting the nerves that grow out from the spinal cord to their proper terminations — which shall cause, for example, the ventral roots always to go to muscles, the •dorsal roots to organs that are not muscular, and shall prevent confusion taking place between the nerves of the iris and those of the eye-muscles, between the branches of the trigeminus and the acusticus or facialis, etc. Therefore HENSEN maintains on theoretical grounds that it is necessary to assume that " the nerves never grow out to their terminations, but are always in connection with them." According to his view, which he endeavors to support by observations, the •embryonic cells are for the most part united with one another by means of fine connecting filaments. He maintains that when a •cell divides the connecting thread also splits, and in this manner there arises " an endless network of fibres." Out of these the nerve- tracts are developed, while other parts of the network degenerate. The reasons given by HENSEN are certainly worthy of great attention. With further reflection on the subject they are easily added to. If the nerves grow out to their terminal apparatus, why do they not take the most direct course to their destination, why •are they often compelled to pursue circuitous paths, and why do they enter into the formation of complicated plexuses of the greatest variety? whence are the ganglionic cells that are found to be xfeveloped in considerable numbers even in the peripheral nervous system in different organs, especially in the sympathetic nerve ? In order to make progress in this difficult field the peripheral nervous .system of Invertebrates must be taken into account more than it is .at present, and in the investigation of embryos not only series of sections but also other histological methods (surface-preparations of 454 EMBRYOLOGY. suitable objects together with staining of the nerve-fibrillse, isolation of the elements preceded by maceration arid staining) must be- employed. Having thus sketched out the various standpoints taken by numer- ous investigators on the question of the source of the peripheral nervous system, I give a number of observations that have been made upon the development of certain nerves. These relate to the development of : — (1) The ventral and dorsal roots of the nerves ; (2) Certain large peripheral nerve-trunks, as the nervus lateralis; and (3) The nerves of the head and their relation to the spinal nerves. (1) Of the roots of the nerves the anterior [ventral] are de- monstrable earlier. There may be distinguished three stages in their development. The first stage has been observed by DOHRN and VAN WIJHE in- Selachian embryos. At a time when the neural tube has not yet developed any mantle of nervous substance, and the muscle-segment still lies very close to it, there arises between the two a connection in the form of a very short protoplasmic cord. The fundament of the nerve is therefore, as VAN WIJHE remarks, ab origine near its muscle-complex, from which it never separates. Soon after this it is elongated by the removal of the muscle-segment farther from the neural tube ; it increases in thickness and now encloses numerous nuclei, and possesses therefore a cellular composition, a condition which I shall designate as second stage. There is a difference of opinion as to the cells which make their appearance in the fundament of the nerve. Whereas KOLLIKER His, and SAGEMEHL recognise in them immigrated connective-tissue elements, which are destined to form simply the envelopes of the nerves, BALFOUR, MARSHALL, VAN WIJHE, DOHRN, and BEARD main- tain that they migrate out from the spinal cord and share in the development of the nerves themselves. BEARD even derives the- motor terminal plate from them. Soon after, as is asserted, connective-tissue cells from the surrounding mesenchyme become associated with the nerve-cells derived from the spinal cord and ordinarily become indistinguishable from them. Finally, in the third stage the cellular fundament of the motor root acquires a fibrillar condition (fig. 260 vw), and it now becomes- possible to trace the origin of the nerve-fibrillse in the spinal cord- from groups of embryonal ganglionic cells or neuroblasts (His), THE ORGANS OF THE OUTER GERM-LAYER. 455 The formation of the nerve-fibrillae is also a subject of controversy, as has already been stated and as will be further explained in this connection. According to the view of most observers, the nerve- fibrillse — the future axis -cylinders — are formed as processes of gang- 1 ionic cells of the spinal cord, the free ends of which grow out from the surface of the latter until they reach their terminal organs (KUPFFER UND BlDDER, KoLLIKER, HlS, SAGEMEHL). Such accounts Fig. 260.— Cross section of a Lizard embryo with completely closed intestinal canal, after SAGEMEHL. he, Posterior [dorsal], vc, anterior commissure of the spinal cord; vw, ventral nerve-root; nf, nerve-fibrillas ; spk, spinal ganglion; nip1, muscle-plate, muscle-prodiicing layer; nip*, outer layer of the muscle-plate ; mpa, transition from the outer to the muscle- forming layer. are given especially for the development of the motor roots in the higher Vertebrates. According to the opinion of DOHRN and VAN WIJHE, on the contrary, the nerve-fibrillae arise in situ, as products of differentiation, from the protoplasm of the cords of cells by means of which muscle- segment and spinal cord are already united. They do not need to seek out the terminal organ, since there exists already a protoplasmic union with it. They arise in a manner similar to that in which the muscle-fibrillae do from the plasma of their muscle-cells. 456 EMBRYOLOGY. I desire to lay particular stress upon the observations of Don EN and VAN WIJHB, because they harmonise with the theoretical views which I have formed as the result of investigations on Invertebrates. As I have in several articles endeavored to establish, protoplasmic connections of the cells are the foundation out of which the nerve-fibrillae are developed. The formation of a specific nervous system is preceded by a protoplasmic union of cells, which is effected at a time when the central and terminal nervous organs are still in the immediate vicinity of each other. The dorsal roots become visible somewhat later than the ventral roots ; there are formed fibrillse which unite the upper [dorsal] end of the spinal ganglion with the side of the spinal cord. (2) GOTTE, SEMPER, WIJHE, HOFFMANN, and BEARD have made concerning certain nerves the noteworthy statement — which has been called in quastion by some observers (BALFOUR, SAGEMEHL) — that the epidermis participates in their formation. In Amphibian larvae and Selachian embryos the posterior end of the nervus lateralis vagi in process of development is completely fused with the primitive epidermis, which is thickened in the lateral line (fig. 262 nl). Some- what farther forward the nerve is detached but still lies in close contact with the primitive epidermis, whereas in the vicinity of the head it is situated deeper and lies between the muscles. At the places where the nerve has become separated from the primitive epidermis, it remains in connection with the fundaments of the lateral organs by means of fine accessory branches only. Similar observations have also been made in the case of many of the branches of other cranial nerves in Selachian embryos. WIJHE sees, for example, a short branch of the n. facialis near its emergence from the brain so fused with a thickened portion of the epidermis composed of cylindrical cells, that it is impossible to say whether at the place of transition the cell-nuclei belong to the nerve or to its terminal organ. During a more advanced stage the older part of the nerve is detached from the terminal organ, sinks into the depths, becoming separated from the skin by ingrowing connective tissue, and remains united with the terminal organ only through fine accessory branches. The persistently growing younger end of the nerve still continues to be connected with the epidermis. Also in the case of tho higher Vertebrates similar conditions have oeen observed by BEARD, FRORIEP, and KASTSCHENKO. They find the ganglionic fundaments of the facialis, glossopharyngeus, and vagus at the dorsal margin of the corresponding visceral clefts for a long time broadly fused with the epithelium, which is thickened and has become depressed into a pit. In these connections they discern THE ORGANS OF THE OUTER GERM-LAYER. 457 the fundaments of branchial sensory organs, which no longer attain to complete development. Also FRORIEP, on the strength of his own •observations, holds as admissible the interpretation that at those places where fusion occurs formative material passes out of the epidermis into deeper parts to share in the formation of nervous tracts. BEARD expresses himself still more precisely to the effect that the sensory nervous elements of the whole peripheral nervous system arise as differentiations from the outer germ-layer, independ- ently of the central nervous system. The accounts here given concerning a connection, in early stages of develop- ment, of certain nerve-trunks with the outer germ-layer, appear to me to afford an indication in favor of the hypothesis expressed by my brother and me, that the sensory nerves of the Vertebrates may have originally been formed -out of a sub-epithelial nervous plexus, such as still exists in the epidermis of many Invertebrates. (3) The investigations of the last few years, which have been •carried out especially by BALFOUR, MARSHALL, KOLLIKER, WIJHE, FRORIEP, KABL, and KASTSCHENKO, have produced important results concerning the development of the cranial nerves, their relations to the head-segments and their value as compared with spinal nerves. •On the brain, as well as on the spinal cord, there arise roots, some of which are dorsal, some ventral. Even at the time when the brain-plate is not yet fully closed into a tube (fig. 261), there is formed on either side, at the place of its bending over into the primitive epidermis, a neural ridge (vg\ which begins rather far forward and may be traced on serial sections uninterruptedly in a posterior direction, where it is continuous with the neural ridge of the spinal cord. When, somewhat later, the closure and the detachment of the brain-vesicles from the primitive epidermis has taken place, the ridge lies on the roof of the vesicles and is fused with them in the median plane. The most of the cranial nerves — namely, the trigeminus with the Gasserian ganglion, the acusticus and facialis with the ganglion acusticum and probably also the ganglion geniculi, and the glossopharyngeus and vagus with the related ganglion jugulare and g. nodosum — are differentiated out of this fundament in the same manner as the dorsal roots of the .spinal nerves. The nerves, which emerge dorsally, afterwards shift their origin downward along the lateral walls of the brain -vesicles toward the base of the latter. All the remaining unenumerated cranial nerves — oculomotorius, trochlearis, abducens, hypoglossus, and accessorius — are developed 458 EMBRYOLOGY. independently of the neural ridge, as individual outgrowths of the brain-vesicles nearer their base, and are comparable with the ventral roots from the spinal cord. FRORIEP finds that the hypoglossus in Mammals possesses dorsal roots, with small ganglionic fundaments, in addition to ventral roots. The latter subsequently undergo degeneration. The agreement between cranial and spinal nerves which is ex- pressed in this method of development, becomes still greater and ..--*/ Fig. 261.-Cross section through the hind part of the head of a Chick embryo of 30 hours, after BALFOUR. hb, Hind-brain ; vg, vagus ; ep, epiblast ; ch, chorda ; x, thickening of hypoblast (possibly a rudiment of the subchordal rod) ; al, throat ; fit, heart ; pp, body-cavity ; so, somatic mesoblast ; «/, splanchnic mesoblasb (Darmseitenplatte) ; Ay, hypoblast. acquires a further significance from the fact that in the liead also the nerves can be assigned to separate segments in much the same manner as in the trunk. In this particular the conditions are clearest in the Selachians, where, in fact, the head-segments have been most thoroughly investigated, so that 1 limit myself to a statement of the results acquired in this field by WIJHE. According to WIJHE nine * segments are distinguishable in the head of Selachians. To the first segment belongs the ramus * [Recent investigations indicate that the head-segments in Selachians are much more numerous.— TRANSLATOR.] THE ORGANS OF THE OUTER GERM-LAYER. 459 ophthalmicus of the trigeminus and, as motor root, the oculo- inotorius. The second segment is supplied by the remaining part of the trigeminus and the trochlearis, the latter having a ventral origin. The dorsal roots of the third (and fourth?) segments are represented by the acustico-facialis, the ventral roots by the abducens. The fifth segment possesses only the exclusively sensory glossopharyngeus, which arises from the neural ridge. The segments from the sixth to the ninth inclusive are innervated by the vagus and the hypoglossus, the former of which represents a series of dorsal roots, the latter a series of ventral ones. According to WIJHE'S account, notwithstanding the general agree- ment, there still exists a considerable difference between the innervation of the cephalic segments and that of the trunk-segments. For in the head the ventral, motor roots (oculomotorius, trochlearis, abducens, hypoglossus) supply only a part of the musculature — the eye- muscles and certain muscles that run from the skull to the pectoral girdle ; that is to say, muscles which, as has already been stated, are developed out of the cephalic segments. Other groups of muscles, which aiise from the lateral plates of the head, are innervated by the trigeminus and facialis, which have a dorsal origin. Thus the dorsal roots of the nerves in the head would be distinguished from those in the trunk by the important fact that they contain motor as well as sensory fibres. BELL'S law would consequently possess a very limited application for the head-region of Vertebrates, and would have to be replaced by the following law, formulated by WIJHE : — " The dorsal roots of the head-nerves are not exclusively sensory, but also innervate the muscles that arise from the lateral plates, not, however, those from the primitive segments (somites)." " The ventral roots are motor, but innervate only the muscles of the primitive segments (somites), not those of the lateral plates." In view of this fundamental difference, I desire to express a doubt whether there are not after all enclosed in the facialis and trigeminus parts which are established as ventral roots, but have hitherto been overlooked, as in the beginning all the ventral roots in the brain (see BALFOUR) were overlooked. According to KABL the nerves of the posterior part of the head only — glossopharyngeus, vagus, accessovius, and hypoglossus — can be compared with the type of spinal nerves ; the nerves of the anterior part of the head, on the contrary,— the olfactorius, opticus, trigeminus, together with those of the eye- muscles and the acustico-facialis, — belong in a separate category, just as the four most anterior head- segments do. 460 EMBRYOLOGY. As is evident from this brief survey, there still exist many unsolved problems in the difficult subject of the development of the peripheral nervous system. Without permitting myself to enter upon a further discussion of the contradictory opinions entertained on this subject, I close this topic with a comparative-anatomical proposition, which appears to me sufficient to furnish the morphological explanation of BELL'S law, or the separate origin of the sensory and motor nerve- roots. In Amphioxus and the Cyclostomes the motor and sensory nerve- fibres are completely separated, not only at their origin from the spinal cord, but also throughout their whole peripheral distribution. The former pass at once from their origin in the spinal cord to the muscle-segments ; the latter ascend to the surface to be distributed to all parts of the skin to supply its sensory cells and sensory organs. The separation of the peripheral nervous system into a sensory and a motor portion, which is rigorously carried out in Amphioxus and the Cyclostomes, is explained by the fact that the territories to which their ends are distributed are spatially distinct in their origin, since the sensory cells arise from the outer germ-layer, the voluntary muscles from a tract of the middle germ-layer. Therefore the sensory nerve- fibres have been developed from the spinal cord in connection with the outer germ-layer, the motor fibres in relation with the muscle- segments. I regard the sub-epithelial position of the sensory nerve-fibres as the original one, just as we find in many Invertebrates the whole peripheral sensory nervous system developed as a plexus in the •deepest portion of the epidermis. The important conditions above described — according to which many dermal nerves (nervus lateralis, etc., fig. 262 nl) are fused with the epidermis at the time of their origin, and only subsequently become detached from it and sink deeper into the underlying mesenchyme — appear to me to indicate that such a position was the primitive one in the case of Vertebrates also. I look upon the union of the sensory and motor nerve-fibres into mixed trunks (which occurs soon after their separate origin from the spinal cord, in the case of all Vertebrates except Amphioxus and the Cyclostomes) as a secondary condition, and maintain that it is caused especially by the following embryological influences : by the change in the position of the spinal cord and the muscular masses, and by the great increase in the amount of the connective substances. THE ORGANS OF THE OUTER GERM-LAYER. 461 Fig. 262. — Cross section through the anterior part of tie trunk of an embryo of Scyllium, after BALFOUR. Between the dorsal wall of the trunk and its ventral wall, where the attachment of the stalk of the yolk-sac is cut, there is stretched a thick richly cellular mesentery, which completely divides the body-cavity into right and left halves. Within the mesentery the duodenum (du), from which the fundament of the pancreas (pan) is given off dorsally and the funda- ment of the liver (hp.d) ventrally, is twice cut through. In addition, the place where the vitelline duct [umbilical canal] (umc) joins the duodenum is visible. sp.c, Spinal cord ; s.pg, ganglion of posterior root ; ar, anterior root ; dn, dorsally directed nerve springing from the posterior root ; mp, muscle-plate ; -nip', part of the muscle-plate already converted into muscles ; mp.l, part of the muscle-plate which gives rise to the muscles of the limbs ; nl, nervus lateralis ; ao, aorta ; c/t, chorda ; sy.g, sympathetic ganglion; ca.v, cardinal vein ; sp.n, spinal nerve ; sd, segmeutal (archiuephric) duct ; st, ssgmental tube. 462 EMBRYOLOGY. Since the spinal cord comes to lie in deeper layers of the body far away from its place of origin, the dermal nerves must follow it, and therefore their origins are correspondingly farther separated from their terminations. Since also, on the other hand, the muscle- plates grow around the neural tube, certain motor and sensory nerve-cords are brought near to each other in their passage to their peripheral distribution. And this will occur especially in all cases where the motor and sensory peripheral terminations lie at a great distance from the origin of the nerves out of the spinal cord, as, for example, in the case of the limbs. The mutual approximation of sensory and motor nerve-tracts thus brought about will finally lead to the formation of common tracts, according to the same principle of simplified organisation in accordance with which the blood-vessels also adapt themselves closely to the course of the nerves. (c) The Development of the Sympathetic System The development of the sympathetic nervous system has as yet been investigated by only a few observers. BALFOUR first announced that it arose in connection with the cranial and spinal nerves, and therefore was, like the latter, really derived from the outer germ- layer, In the Selachians he found the sympathetic ganglia (fig. 262 sy.g) as small enlargements of the chief trunks of the spinal nerves (sp.n) a little below their ganglia (sp.g). In older embryos, according to BALFOUR'S account, they recede from the spinal ganglia, and then at a later period unite with one another, by the development of a longitudinal commissure, into a continuous cord (Grenzstrang). The origin of the sympathetic system has been the most thoroughly :studied by ONODI in researches covering several classes of Verte- brates. According to him the sympathetic ganglia arise directly, as BALFOUR suggested and as BEARD has also lately reiterated, from the -piiial ganglia. The ventral ends of the spinal ganglia undergo proliferation, as is best seen in Fishes. The proliferated part de- taches itself, and, as fundament of a sympathetic ganglion, moves ventrally. The fundaments of the individual segments are at first separate from one another. The cord (Grenzstrang) is a secondary product, produced by the growing out of the individual ganglia toward each other and the union of the outgrowths. Afterwards the sympathetic ganglia and plexuses of the body-cavity are derived from this part. THE ORGANS OF THE OUTER GERM-LAYER. 463 SUMMARY. Central Nervous System. 1. The central nervous system is developed out of the thickened region of the outer germ-layer which is designated as the medullary plate. 2. The medullary plate is folded together to form the medullary tube (medullary ridges, medullary groove). 3. The formation of the neural tube exhibits three principal modifications : (a) Amphioxus, (6) Petromyzon, Teleosts, (c) the re- maining Vertebrates. 4. The lateral walls of the medullary tube become thickened, whereas the dorsal and ventral walls remain thin ; the latter come to occupy the depths of the anterior and posterior longitudinal fissures, and constitute the commissures of the lateral halves of the spinal cord. 5. The spinal cord at first fills the whole length of the vertebral canal, but it grows more slowly than the latter, and finally terminates at the second lumbar vertebra (explanation of the oblique course of the lumbar and sacral nerves). 6. The part of the neural tube which forms the brain becomes segmented into the three primary cerebral vesicles (primary fore- brain vesicle, mid-brain vesicle, hind-brain vesicle). 7. The lateral walls of the fore-brain vesicle are evaginated to form the optic vesicles, the anterior wall to form the vesicles of the cerebrum. 8. The hind-brain vesicle is divided by constriction into the vesicles of the cerebellum and the medulla. 9. Thus from the three primary brain- vesicles there finally arise five secondary ones arranged in a single series one after the other — (a) cerebral vesicle (that of the hemispheres), (b) between-brain vesicle with the laterally attached optic vesicles, (c) mid-brain vesicle, (d) vesicle of the cerebellum, (e) vesicle of the medulla oblongata. 10. The originally straight axis uniting the brain- vesicles to one another later becomes at certain places sharply bent, in consequence of which the mutual relations of the vesicles are changed (cephalic flexure, pontal flexure, nuchal flexure). The cephalic or parietal protuberance at the surface of the embryo corresponds to the cephalic flexure, the nuchal protuberance to the nuchal flexure. 464 EMBRYOLOGY. 11. The separate parts of the brain are derivable from the five- brain-vesicles ; the accompanying table (MIHALKOVICS, SCHWALBE) gives a survey of the subject. 12. In the metamorphoses of the vesicles the following processes take place: (a) certain regions of the walls become more or less thickened, whereas other regions undergo a diminution in thickness and do not develop nervous substance (roof -plates of the third and fourth ventricles) ; (b) the walls of the vesicles are infolded mr (c) some of the vesicles (first and fourth) greatly exceed in their growth the remaining ones (bet ween- brain, mid-brain, after-brain, 01 medulla oblongata). 13. The four ventricles of the brain and the aqueductus Sylvii are derived from the cavities of the vesicles. 14. Of the five vesicles that of the mid-brain is the most conser- vative and undergoes the least metamorphosis. 15. The vesicles of the between-brain and after-brain exhibit similar alterations : their upper walls or roof -plates are reduced in thickness to a single layer of epithelial cells, and in conjunction with the growing pia mater produce the choroid plexuses (anterior, lateral, posterior choroid plexus ; anterior, posterior brain-fissure). 16. The cerebral vesicle is divided by the development of the longitudinal (interpallial) fissure and the falx cerebri into lateral halves, the two vesicles of the cerebral hemispheres. 17. In Man the cerebral hemispheres finally exceed in volume all the remaining parts of the brain, and grow from above and from the sides as cerebral mantle over the other brain-vesicles (from the second to the fifth inclusive) or the brain-stalk. 18. In the folding of the walls of the hemispheres there are to be- distinguished fissures and sulci. 19. The fissures (fossa Sylvii, fissura hippocampi, fissura choroidea, fissura calcarina, fissura occipitalis) are complete folds of the wall of the brain, by means of which there are produced deep incisions in the surface and corresponding projections into the lateral ventricles (corpus striatum, cornu Ammonis, fold of the choroid plexus, calcar avis). 20. The sulci are incisions limited to the cortical portion of the wall of the brain, and are deeper or shallower according to the time of their formation (primary, secondary, tertiary sulci). 21. In general the fissures appear earlier than the sulci. 22. The olfactory nerve is not equivalent to a peripheral nerve- trunk, but, like the optic vesicle and optic nerve, a special part of THE ORGANS OF THE OUTER GERM-LAYER. 465 1 1 •4 J ri a M * m P • s s . " i S j3 O ;« r^ CO o £ .2 ^ E s s "T2 r^ 'C '-+2 5 S S S cc* f i c| o o*1 * t^45 ^ •v * 08 g ! 1 8 lei I s 3 P oS i ' o fH Q 3 •i-] i«s-s CT1 8 ^ ^ CO E3 "§ * i • p O C- ^ j_, ^ b^ ^ 5 ^ C^ ^ E c 1 2 s 8 i-l .S g S M ^ rj *0 ^ 1 g i o 2 wo 1 lit 111 ^ •U 'C •"H 03 § 0 0 . c3 •r« co T1 ._ • 5 a ® a i Membrana tecto ventriculi quar1 (obex, ligula) Velum medulla] posterius cerebell Velum medullai anterius. Corpora quadrigemina. Commissura poste: Glandula pineali Membrana tectoi ventriculi terti: (taenia thalami] Mantle-part of Corpus callosu Fornix ; i i Medulla oblongata. 1 3 s e Pedunculi cerebri. Lamina perforata posterior. Corpora candicautia. Tuber cinereum cum infunclibulo. Chiasma nervorum opticorum. Lamina pertorata ant. Lobus alfactorius. Insula (with nucleus candatus and n. lenti- formis) is comprised in brain-stalk, § N ^ d di g ^ 3 i Hi Hal "o III liij t> 5 co o . * £; '-> w ° ® 8"i 1 3 g i| IS £ g ^^ H *.s 1 ^ .S 0 S *-§ s h-J ^ _o *^5 HH P *rH H !a H- 1 HH ^ A ^ 30 466 EMBRYOLOGY. the brain produced by an evagination of the frontal lobe of the cerebral hemisphere (lobus or bulbus olfactorius with tracttis olfac- torius). (Enormous development of the olfactory lobes in lower Vertebrates, — Sharks, — degeneration in Man.) Peripheral Nervous System. 23. The spinal ganglia are developed out of a neural ridge (crest), which grows outward and downward from the raphe of the neural tube on either side between the tube and the primitive epidermis, and becomes thickened in the middle of each primitive segment into a ganglion. 24. The spinal ganglia therefore arise, like the neural tube itself, from the outer germ-layer. 25. The sympathetic ganglia of the longitudinal cord (Grenz- strang) are probably detached parts of the spinal ganglia. 26. Concerning the development of the peripheral nerve-fibres there are different hypotheses : — First hypothesis. The peripheral nerve-fibres grow out from the central nervous system and only secondarily unite with their peripheral terminal apparatus. Second hypothesis. The fundaments of the peripheral terminal apparatus (muscles, sensory organs) and the central nervous system are connected from early stages of • development by means of filaments which become nerve- fibres (HENSEN). 27. Anterior and posterior nerve-roots are developed on the spinal cord separately from each other, one ventrally, the other dorsally. 28. The cranial nerves arise in part like posterior, in part like anterior roots of spinal nerves. 29. The following cranial nerves with their ganglia, which are comparable with spinal ganglia, are developed out of a neural ridge which grows out from the raphe of the brain-vesicles : the trigeminus with the ganglion Gasseri, the acusticus and facialis with the gang- lion acusticum and g. .geniculi, the glossopharyngeus and vagus with the ganglion jugulare and g. nodosum. 30. The oculomotorius, trochlearis, abducens, hypoglossus, and accessorius are developed like ventral roots of spinal nerves. 31. The olfactory and optic nerves are metamorphosed parts of the brain. THE ORGANS OF THE OUTER GERM-LAYER. 467 II. The Development of the Sensory Organs, Eye, Ear, and Organ of Smell. As the outer germ-layer is the parental tissue of the central nervous system, so also does it form the substratum for the higher sensory organs, the eye, the ear, and the organ of smell. For it furnishes the sensory epithelium, a component which, in comparison with the remaining parts, derived from the mesenchyma, is, it is true, of very small volume, but, notwithstanding, by far the most important both from a functional and a morphological point of view. Whether a sensory organ is adapted for seeing, hearing, smelling, or tasting depends primarily upon the character of its sensory epithelium, i.e., upon whether it is composed of optic, auditory, olfactory, or gustatory cells. But also morphologically the epithelial part is preeminent, because it is chiefly this which determines the fundamental form of the sensory organs and affords the fixed centre around which the remaining accessory components are arranged. The genetic connection with the outer germ-layer may be most clearly recognised in many Invertebrates, inasmuch as here the sensory organs are permanently located in the epidermis, whereas in Vertebrates, as is well known, they are, for the sake of protection, embedded in deep-lying tissues. I begin with the eye, and then proceed to the organ of hearing and that of smell. A. The Development of the Eye. As has already been stated in the description of the brain, the lateral walls of the primary fore-brain (figs. 234, 263) are evaginated rf gb nh rib ffh Tig. 263.— Brain of a human embryo of the third week (Lg). Profile reconstruction, after His. equator of the lens, and run toward the middle of the anterior surface, where they form terminal loops, and also unite with blood-vessels of the choroid membrane (fig. 267 x). Separate parts of the nourishing membrane of the lens, having been discovered at different times by various investigators, have received special names, as membrana pupillaris, m. capsulo-pupillaris, m. capsularis. The first to be observed was the membrana pupillaris, the part of the vascular membrane which is situated behind the pupil on the anterior surface of the lens. It was the most easily found, because occasionally it persists even after birth as a fine membrane closing the pupil, and producing atresia pupillce congenita. Later it was found that the membrana pupillaris is also continued* laterally from the pupil on the anterior face of the lens, and this part was called membrana capsulo-pupillaris. Finally it was dis- covered that the blood-vessels are spread out on the posterior wall of the lens — the membrana capsularis. It is superfluous to retain all these names, and most suitable to speak of a nutritive membrane of the lens, or a membrana vasculosa lentis. This vascular membrane attains its greatest development in the seventh month, after which it begins to degenerate. Ordinarily it has entirely disappeared before birth ; only in exceptional cases do some parts of it persist. Toward the end of embryonic life, more- over, the chief growth of the lens itself has ceased. For according to weighings carried on by the anatomist HUSCHKE, it has a weight of 123 milligrammes in the new-born child, and 190 milligrammes in the adult, so that the total increase which the organ undergoes during life amounts to only 67 milligrammes. (b) TJie Development of the Vitreous Body. The question of the development of the vascular membrane of the- lens leads to that of the vitreous body. As was previously men- tioned, there grows out from the embryonic connective tissue a THE ORGANS OF THE OUTER GERM-LAYER. 475 process with a vascular loop, which makes its way into the primary optic vesicle and its stalk (fig. 265). The vascular loop then begins to send out new lateral branches; likewise the connective-tissue matrix, which is at first scanty, increases greatly and is characterised by its extraordinarily slight consistency and its large proportion of water (figs. 266, 267 g). There are also to be found in it here and there isolated stellate connective -tissue cells ; but these disappear later, and in their place occur migratory cells (leucocytes), which are assumed to be immigrated white blood-corpuscles. There are two opposing views regarding the nature and develop- ment of the vitreous body. According to KESSLER we have to do, not with a genuine connective substance, but with a transudation, — a fluid, — which has been secreted from the vascular loops ; the cells are from the beginning simply immigrated white blood-corpuscles. KOLLIKER, SCHWALBE, and other investigators, on the contrary, regard the vitreous body as a genuine connective substance. Accord- ing to SCHWALBE'S definition, to which I adhere, it consists of an exceedingly watery connective tissue, whose fixed cells have early disappeared, but whose interfibrillar substance infiltrated with water is traversed by migratory cells. The vitreous body is afterwards surrounded by a structureless membrane, the membrana hyaloidea, which, according to some investigators, belongs to the retina, al- though, according to the researches of SCHWALBE, this view is not admissible. The vitreous body, which in the adult is quite destitute of blood- vessels, is bountifully supplied with them in the embryo. They come from the arteria centralis retince, the branch of the ophthalmic artery that runs along the axis of the optic nerve. The arteria centralis retinae is prolonged from the papilla of the optic nerve as a branch which is designated as the arteria hyaloidea. This, resolved into several branches, runs forward through the vitreous body to the posterior surface of the lens, where its numerous terminal ramifications spread out in the tunica vasculosa, and at the equator pass over on to the anterior face of the lens. During the last months of embryonic life the vessels of the vitreous body, to- gether with the nutritive membrane of the lens, undergo degenera- tion ; they entirely disappear, with the exception of a rudiment of the chief stem, which runs forward from the entrance of the optic nerve to the anterior surface of the vitreous body, and during the degeneration is converted into a canal filled with fluid, the canalis hyaloideus. 476 EMBRYOLOGY. (c) The Development of the Secondary Optic Cup and the Coats of the Eye. The optic cup is further metamorphosed at the same time with the layer of mesenchyma which en- velops it, and which furnishes the middle and outer tunics of the eye, so that it seems to be desirable to treat of both at the same time. I begin with the stage represented in figures 266 and 269. The optic cup still possesses at this time a broad opening, in which the lens (le) is em- braced. The latter is either separated from the epidermis by only an ex- ceedingly thin sheet of mesenchyma, as in the Mammals (fig. 266), or its anterior face is in immediate contact with the epidermis, as in the Chick (fig. 269). In the beginning, therefore, there is no separate fundament for the cornea between lens and epidermis ; moreover, both the anterior chamber of the eye and the iris are wanting. The fundament of the cornea is de- rived from the surrounding mesen- chyma, which, as a richly cellular tissue, envelops the eyeball. In the Chick (fig. 269), as early as the fourth d.-iy, it grows in between the epidermis and the front surface of the lens as a thin sheet (bi). At first this sheet is struc- tureless, then numerous mesenchyrnatic cells migrate into it from the margin and become the corneal corpuscles. These produce the corneal fibres in the same way that embryonic con- nective-tissue cells do the connective- tissue fibres, while the structureless sheet in part goes to form the cement- ing substance between them, and in part is preserved on the anterior and posterior walls as thin layers Fig. 269.— Section through the an- terior portion of the fundament of the eye in an embryo Chick on the fifth day of incubation, after KESSLER. he, Corneal epithelium ; le, lens-epi- thelium ; h, structureless sheet of the corneal fundament ; bi, em- bryonic connective substance, which envelops the optic cup and, penetrating between lens- epithelium (le) and corneal epi- thelium (he), furnishes the funda- ment of the cornea; ab, outer, ib, inner layer of the secondary optic cup. THE ORGANS OF THE OUTER GERM-LAYER. 477 destitute of cells : these layers, undergoing chemical metamorphosis, become respectively the membrana elastica anterior and the mem- brane of DESCEMET. The internal endothelium of the cornea is developed at an extra- ordinarily early epoch in the Chick. For as soon as the structureless sheet previously mentioned (fig. 269 h) has attained a certain thick- ness, mesenchymatic cells proceeding from the margin spread them- selves out on its inner surface as a single-layered thin cell-membrane.. With this begins also the formation of the anterior chamber of the eye. For the thin fundament of the cornea, which at first lay in immediate contact with the front surface of the lens, now becomes somewhat elevated from the latter, and separated from it by a fissure-like space- filled with fluid (humor aqueus). The fissure is first observable at the margin of the secondary optic cup, and spreads out from this region toward the anterior pole of the lens. The anterior chamber of the eye does not, however, acquire a greater size and its definite form until the development of the iris. Two opposing views exist concerning the origin of the structureless sheet- winch has been described as constituting the first fundament of the cornea in the Chick. According to KESSLER it is a [product of the secretion of the epidermis, whereas the corneal corpuscles migrate in from the mesenchyma. In his opinion, therefore, the cornea is composed of two entirely different fundaments. According to KOLLIKER, on the contrary, all its parts are developed out of the mesenchyma, and the homogeneous matrix simply outstrips the cells in its growth and extension. In Mammals (fig. 266) the conditions differ somewhat from those of the Chick ; for as soon as the lens- vesicle in Mammals is fully constricted off, it is already enveloped by a thin sheet of mesenchyma, (h) with few cells, which separates it from the epidermis. The thin layer is rapidly thickened by the immigration of cells from the vicinity. Then it is separated into two layers (fig. 267), the pupillar membrane (tv) and the fundament of the cornea (h}. The former is a thin, very vascular membrane lying on the anterior surface of the lens; its network of blood-vessels communicates on the one hand posteriorly with the vessels of the vitreous body, together with which it constitutes the tunica vasculosa lentis, and on the other anastomoses at the margin of the optic cup with the vascular network of the latter. The fundament of the cornea is first sharply delimited from the pupillary membrane at the time when the anterior chamber of the eye (&) is formed as a narrow fissure, which gradually increases in extent with the appearance of the iris. 478 EMBRYOLOGY. r pi li •18 ck 1.2.8. lp tchD Tig. £70.— Section through the margin of the optic cup of an embryo Turdus musicun, after KISSIEK. r, Retina ; pi, pigmented epithe- lium of the retina (outer lamella of the optic cup) ; !>,, connective-tissue envelope of the optic cup (choroidea HI id sclera) ; * ora serrata (boundary between the mar- ginal zone and the fundus of the optic cup) ; ck, ciliary body; 1, 2, 8, iris; 1 and 2, inner and outer lamellae of the para iridia retina; ; 8, con- i r, ti\r-ti.sMif plate of the ins ; /;>, ligamcntum pecti- iiatiim iridi.s ; .srA, canal of S( HI.EMM J D, IMS' membrane; A, cornea; he, oorneal fpithelium. During these processes the condition of the optic cup itself has also changed. Its outer and inner lamellae continually be- come more and more unlike. The former (figs. 266, 267 pi) remains thin and com- posed of a single layer of cubical epi- thelial cells. Black pigment granules are deposited in this in increasing abundance, until finally the whole lamella appears upon sections as a black streak. The inner layer (r), on the contrary, remains entirely free from pigment, with the ex- ception of a part of the marginal zone ; the cells, as in the wall of the brain- vesicles, become elongated and spindle- shaped, and lie in many superposed layers. Moreover the bottom of the cup and its rim assume different conditions, and hasten to fulfil different destinies; the former is converted into the retina, the latter is principally concerned in the production of the ciliary body and the iris. The edge of the cup (fig. 267 rz, fig. 270 *, and fig. 271) becomes very much reduced in thickness by the cells of its inner layer arranging themselves in a single sheet, remaining for a time cylindrical, and then assuming a cubical form. But with its reduction in thickness there goes hand in hand an increase in its superficial extent. Consequently the margin of the optic cup now grows into the anterior chamber of the eye between cornea and the anterior surface of the lens, until it has nearly reached the middle of the latter. Then it at last bounds only a small orifice which leads into the cavity of the optic cup — the pupil. The pigment layer of the iris is derived from the mar- ginal region of the cup, as KESSLER first THE ORGANS OF THE OUTER GERM-LAYER. 479 showed (fig. 270 * and 2). Pigment granules are now deposited in the inner epithelial layer, just as in the outer lamella, so that at last the two are no longer distinguishable as separate layers. The mesenchymatic layer which envelops the two epithelial lamellae keeps pace with them in their superficial extension. It becomes thickened and furnishes the stroma of the iris with its abundant non-striated muscles and blood-vessels (fig. 2703). In Mammals (fig. 267 x) this is for a time continuous with the tunica vasculosa lentis (tv), in consequence of which the pupil in embryos is closed by a thin vascular connective - tissue membrane, as has already been stated. The part of the optic cup which is adjacent to the pig- ment layer of the iris and surrounds the equator of the lens, and which likewise be- longs to the attenuated mar- ginal zone of the cup (fig. 270 ck), undergoes an inter- esting alteration. In con- junction with the neighboring layer of connective substance, it is converted into the ciliary body of the eye. This process begins in the Chick on the ninth or tenth day of incubation (KESSLER), in Man at the end of the second or beginning of the third month (KOLLIKER). The attenuated «piti elial double lamella of the cup, in consequence of an especially vigorous growth in area, is laid into numerous, [nearly] parallel short folds, which are arranged radially around the equator of the lens. As in the iris, so here, the adjacent mesenchymatic layer participates in the growth and penetrates between the folds in the form of fine processes. A cross section through the folded part of the optic cup of a Cat embryo 10 cm. long (fig. 271) affords informa- tion concerning the original form of these processes in Mammals. It shows that the individual folds are very thin and enclose within them only a very small amount of embryonic connective tissue (bi ') with fine capillaries, and that, unlike the pigment epithelium of the iris, only the outer of the two epithelial layers (ab) is pigmented, Fig. 271.— Cross section through the ciliary par of the eye of an embryo Cat 10 cm. long, after KESSLER. Three ciliary processes formed by the folding of the optic cup are shown, bi, Connective-tissue part of the ciliary body ; ibt inner layer, ab, outer pigmented layer of the optic cup; bi', sheet of connective tissue that has pene- trated into the epithelial fold. 480 EMBRYOLOGY. whereas the inner (ib) remains unpigmented even later and is composed of cylindrical cells. Subsequently the ciliary processes become greatly thickened through increase of the very vascular connective-tissue framework, and acquire a firm union with the capsule of the lens through the formation of the zonula Zinnii. In Man the latter is formed,, according to KOLLIKER'S account, during the fourth month, in a manner that here, as well as in other Mammals, is still incompletely explained. LiEBERKttKN remarks that the zonula is distinctly recognisable in eyes which have attained half their definite size. If one takes out of an eye the vitreous body together with the lens, and then removes the latter by opening the capsule on the front side, the margin of the capsule appears surrounded oy blood-vessels which pass from the posterior over on to the anterior surface. "At the places where the processus ciliares are entirely removed, tufts of fine fibres are to be seen which correspond to, and fill up, the depressions between the ciliary processes ; but between these tufts is also to be seen a thin layer of the same kind of finely striate masses, which must have lain at the same level as the ciliary processes." Furthermore LlEBERKtrKN states that " there lie within this striated tissue numerous cell-bodies of the same appearance as those that are found elsewhere in the embryonic vitreous body at a later period." ANGELUCCI believes that the zonula arises from the anterior part of the vitreous body ; at the time when iris and ciliary processes are developed he finds the vitreous body traversed by fine fibres, which extend from the ora serrata to the margin of the lens. He describes as lying between the fibres sparse migratory cells, which are maintained, however, to have no share in the formation of the fibres. The fundus of the optic cup (figs. 266, 267, 270) furnishes the most important part of the eye — the retina. The inner lamella of the cup (r) becomes greatly thickened, and, in consequence of its cells being elongated into spindles and overlapping one another in several layers, acquires an appearance similar to that of the wall of the embryonic brain. Subsequently it becomes marked off by an indented line, the ora serrata (at the place indicated by a star in fig. 270), from the adjoining attenuated part of the optic vesicle, which furnishes the ciliary folds. It also early acquires at its two Mirf.-ices a sharp limitation through the secretion of two delicate membranes : on the side toward the fundament of the vitreous body it is bounded by the membrana liinitans interna ; on that toward the outer lamella, which becomes pigmented epithelium, by the membriina limitans externa. In the course of development its cells, all of which are at first THE ORGANS OF THE OUTER GERM-LAYER. 481 alike, become specialised in very different ways, as a result of which there are produced the well-known layers distinguished by MAX SCHULTZE. I shall not go into the details of this histological differentiation, but shall mention some further points of general importance. As WILHELM MULLER in his " Stammesentwicklung des Sehorgans der Wirbelthiere " has clearly shown, the development of the originally similar epithelial cells of the retina takes place in all Vertebrates in two chief directions : a part of them become sensory epithelium and the specific structures of the central nervous system — ganglionic cells and nerve-fibres; another part are metamorphosed into supporting and isolating elements — into MULLER'S radial fibres and the granular [reticular or molecular] layers, which can be grouped together as epithelial sustentative tissue (fulcrum). Finally, with the descendants of the epithelium are associated connective-tissue elements, which grow from the surrounding connective tissue into the epithelial layer for its better nutrition, in the same manner as in the central nervous system. These ingrowths are branches of the arteria centralis retina with their extremely thin connective-tissue sheaths. The Lampreys alone form an exception, their retina remaining free from blood-vessels. In all other Vertebrates blood- vessels are present, but they are limited to the inner layers of the retina, leaving the outer granular (Korner) layer and that of the rods and cones free ; the latter have been distinguished as sensory epithelium from the remaining portions with their nerve-fibres and ganglionic cells — the brain-part of the retina. Of all the parts of the retina the layer of rods and cones is the last to be developed. According to the investigations of KOLLIKER, BABUCHIN, MAX SCHULTZE, and W. MULLER, it arises as a product of the outer granular (Korner) layer, which, composed of fine spindle-shaped elements, is held to be, as has been stated, the essential sensory epithelium of the eye. In the Chick the development of the rods and cones can be made out on the tenth day of incubation. MAX SCHULTZE states concerning young Cats and Rabbits, which are born blind, that the fundament of the rods and cones can be distinguished for the first time in the early days after birth ; in other Mammals and in Man, on the contrary, they are formed before birth. In all Vertebrates, as long as rods and cones are not present, the inner layer of the optic cup is bounded on the side toward the outer layer by an entirely smooth contour, due to the membrana limitans 31 482 EMBRYOLOGY. externa. Then there appear upon the latter numerous, small, lustrous elevations, which have been secreted by the outer granules or visual cells. The elevations, which consist of a protoplasmic substance and are stained red in carmine, become elongated and acquire the form of the inner limb of the retinal element. Finally there is formed at their outer ends the outer limb, which MAX SCHULTZE and W. MILLER compare to a cuticular product, on account of its lamellate structure. Inasmuch as the rods and cones of the retinal cells grow out in this way beyond the membrana limitans externa, they penetrate into the closely applied outer lamella of the optic cup, which becomes the pigmented epithelium of the retina (figs. 266, 267, 270 pi); their outer limbs come to lie in minute niches of the large, hexagonal pigment-cells, so that the individual elements are separated from one another by pigmented partitions. A few additional words concerning the connective tissue enveloping the fundament of the optic cup. It acquires here, .as on the ciliary body and the iris, a special, and for this region characteristic, stamp. It is differentiated into vascular [choroid] and fibrous [sclerotic] membranes, which in Man are distinguishable in the sixth week (KOLLIKER). The former is characterised by its vascularity at an early period, and develops on the side toward the optic cup a special layer, provided with a fine network of capillary vessels, the mem- brana choriocapillaris, for the nourishment of the pigment-layer and the layer of rods and cones, which have no blood-vessels of their own. It further differs from the ciliary body in the fact that at the fundament of the optic cup the choroid membrane is easily separable from the adjoining membranes of the eye, whereas in the ciliary body a firm union exists between all the membranes. If we now glance back at the processes of development last described, one thing will appear clear to us from this short sketch : that the changes in the form of the secondary optic cup are of preeminent importance for the origin of the individual regions of the eye. Through different processes of growth, which have received a general discussion in Chapter IV., there have been formed in the cup three distinct portions. By means of an increase in thickness and various differentiations of the numerous cell-layers, there is formed the retina ; by an increase of surface, on the contrary, is produced an anterior, thinner part, which bounds the pupil and is subdivided into two regions by the formation of folds in the vicinity of the lens. From the folded part, which joins the retina at the ora serrata, is THE ORGANS OF THE OUTER GERM-LAYER. 483 formed the epithelial lining of the ciliary body ; from the thin portion which surrounds the pupil and which remains smooth, the pigmented epithelium (uvea) of the iris. Consequently there are now to be distin- guished on the secondary optic cup three regions, as retinal, ciliary, and iridal parts. To each of these territories the contiguous connective tissue, and especially the part which becomes the middle tunic of the eye, is adapted in a particular manner ; here it furnishes the connective- tissue plate of the iris with its non-striated muscu- lature, there the connective -tissue framework of the ciliary body •with the ciliary muscle, and in the third region the vascular choroidea with the choriocapillaris and lamina fusca. In the development of the optic cup there arose on its lower wall a fissure (fig. 265 aits), which marks the place at which the funda- ment of the vitreous body grew into the interior of the cup. What is the ultimate fate of this fissure, which is usually referred to in the literature as choroid fissure ? It is for a time easily recognisable, after pigment has been deposited in the outer lamella of the optic cup. It then appears on the lower median side of the eyeball as a clear, unpigmented streak, which reaches forward from the entrance of the optic nerve to the margin of the pupil. The name choroid fissure takes its origin from this phenomenon. It was given at a time when the formation of the optic cup was not adequately known, when the pigmented epithelium was still referred to the choroidea. Therefore in the absence of pigment along a clear streak on the under side of the eyeball it was supposed that a defect of the choroidea — a choroid fissure — had been observed. The clear streak afterwards disappears. The fissure of the eye is closed by the fusion of its edges and the deposition of pigment in the raphe. In the Chick this takes place on the ninth day, in Man during the sixth or seventh week. In still another respect is the choroid fissure noteworthy. In many Vertebrates (Fishes, Reptiles, Birds) a highly vascular process of the choroidea grows through the fissure, before its closure, into the vitreous body and there forms a lamellar projection, which extends from the optic nerve to the lens. In Birds it has received the name " pecten," because it is folded into numerous parallel ridges. It consists almost entirely of the walls of blood-vessels, which are held together by a small amount of a black pigmented connective tissue. In Mammals such a growth into the vitreous body is wanting 484 EMBRYOLOGY. The closure of the choroid fissure takes place at an early period and completely. Occasionally in Man the normal course of development is inter- rupted, so that the margins of the choroid fissure remain apart. The iiMial consequence of this is a defective development of the vascular tunic of the eye at the corresponding place — an indication of the extent to which the development of the connective-tissue envelope is dependent on the formative processes of the two epithelial layei has already been stated. Both retinal and choroidal pigment are therefore wanting along a streak which begins at the optic nerve, so that the white sclera of the eye shows through to the inside and ran be recognised in examinations with the ophthalmoscope. When the defect reaches forward to the margin of the pupil, a fissure is formed in the iris which is easily recognised upon external observation of the The two structures resulting from this interrupted develop- ment are distinguished from each other as choroidal and iridalfosure* (coloboma choroideae and coloboma iridis). (d) The Development of the Optic Nerve. The stalk of the optic vesicle (fig, 272), by which the vesicle is united with the between-brain, is in direct connection with both lamellae of the optic cup, the primary optic vesicle having been infolded from below by the fundament of the vitreous- body to form the cup. Its dorsal wall is continuous with the outer lamella or pigment-epithelium of the retina ; its ventral wall is prolonged into the inner lamella, which becomes the retina. aside from the formation of tht> vitreous body, the development of a choroid fasure afao has a siynincttnc? in view of the persistence of the direct connection between retina and optic nerve. For if we conceive the optie vesicle invaginated merely at its an- terior face by the lens, the wall of the optic nerve would be continued into- the outer, uninvaginated lamella only; direct connection with the- retina itself, or the invaginated part, would be wanting. Fig. 272.— Plastic representation of the optic cup with lens and vitreous body. 06, Outer wall of the cap; ib, its inner wall ; A, space between the two walls, which afterwards en- tirely disappears ; Sn, fundament of the optic nerve (stalk of the optic vesicle with groove-for- mation along its lower face); au$, choroid fissure ; pi, vitreous body ; /, lens. THE ORGANS OF THE OUTER GERM-LAYER. 485 Originally the optic nerve is a tube with a small lumen, which unites the cavity of the optic vesicle with the third ventricle (fig. 264 A). It is gradually converted into a solid cord. In the case of most Vertebrates this is produced simply by a thickening of the walls of the stalk, due to cell-proliferation, until the cavity is obliterated. In Mammals only the larger portion, that which adjoins the brain, is metamorphosed in this manner ; the smaller part, that which is united with the optic vesicle, is, on the contrary, infolded by the prolongation of the choroid fissure backward for some distance, whereby the ventral wall is pressed in against the dorsal. Con- sequently the optic nerve here assumes the form of a groove, in which is imbedded a connective-tissue cord with a blood-vessel that becomes the arteria centralis retinae. By the growing together of the edges of the groove, the cord afterwards becomes completely enclosed. For a time the optic nerve consists exclusively of spindle-shaped, radially arranged cells in layers, and resembles in its finer structure the wall of the brain and the optic vesicle. Different views are held concerning its further metamorphoses, and especially concerning the origin of nerve-fibres in it. Differences similar to those concerning the origin of the peripheral nerve-fibres are maintained. Upon this point three theories have been brought forward. According to the older view, which LIEBERKCHN shares, the optic fibres are developed in loco by the elongation of the spindle-shaped cells. According to His, KOLLIKER, and W. M(JLLER, on the con- trary, the wall of the optic vesicle furnishes the sustentative tissue only, whereas, the nerve-fibres grow into it from outside, either from the brain toward the retina (His, KOLLIKER), or in the reverse direction (MULLER). The stalk of the optic vesicle would constitute, according to this view, only a guiding structure as it were — would predeter- mine the way for its growth. When the ingrowth has taken place, the sustentative cells are, as KOLLIKER describes them, arranged radially and so united with one another that they constitute a delicate framework with longitudinally elongated spaces. In the latter are lodged the small bundles of very fine non-nuclear nerve- fibres and numerous cells, arranged in longitudinal rows, which likewise belong to the epithelial sustentative tissue and help to complete the trestle-work. The embryonic optic nerve is enveloped in a connective-tissue sheath, which is separated, as in the case of the brain and secondary optic cup, into an inner, soft, vascular and an outer compact 486 EMBRYOLOGY. fibrous layer. The former, or the pial sheath, unites the pia mater of the brain and the choroid membrane of the eye ; the latter, or the dural sheath, is a continuation of the dura mater and at the eye- ball becomes continuous with the sclerotica. Later the optic nerve acquires a still more complicated structure, owing to the fact that vascular processes of the pial sheath grow into it and provide the nerve-bundles and the epithelial sustentative cells belonging to them with connective-tissue investments. As has been previously stated, the direction in which optic fibres grow into the stalk of the optic vesicle is still a subject of controversy. His, with whom KOLLIKER is in agreement, maintains that they grow out from groups of gang- lionic cells (thalamus opticus, corpora quadrigemina), and are only secondarily distributed in the retina. He supports his view on the one hand by the agree- ment in this particular which exists with the development of the remaining peripheral nerves, and on the other by the circumstance that the nerve-fibres are first distinctly recognisable in the vicinity of the brain. W. MCLLER, on the contrary, believes that the outgrowth takes place in the opposite direction ; he maintains that the nerve-fibres arise as prolongations of the ganglionic cells located in the retina, and that they enter into union with the central nervous apparatus only secondarily. He is strengthened in his opinion by the conditions in Petromyzon, which he declares to be one of the most valuable objects for the solution of the controversy concerning the origin of the optic nerve. I refer, moreover, in connection with this controversy, to the section which treats of the development of the peripheral nervous system (p. 452). (e) The Development of the Accessory Apparatus of the Eye. There are associated with the eyeball auxiliary apparatus, which serve in different ways for the protection of the cornea : the eyelids with the Meibomian glands and the eyelashes, the lachrymal glands and the lachrymal ducts. The eyelids, the upper and under, are developed at an early period by the formation, at some distance from the margin of the cornea, of two folds of the skin, which protrude beyond the surface. The folds grow over the cornea from above and below until their edges, meet and thus produce in front of the eyeball the conjunctival sac, which opens out through the fissure between the lids. The sac derives its name from the fact that the innermost layer of the lid-fold, which is reflected on to the anterior surface of the eyeball at the fornix con- junctives, is of the nature of a mucous membrane, and is designated as the conjunctiva, or connecting membrane, of the eye. In many Mammals and likewise in Man there is during embryonic- life a temporary closure of tlie conjunctival sac. The edges of the lids THE ORGANS OF THE OUTER GERM-LAYER. 487 become united throughout their whole extent, their epithelial invest- ments fusing with each other. In Man the concrescence begins in the third month, and usually undergoes retrogression a short time before birth. But in many Reptiles (Snakes) the closure is perma- nent. Thus a thin transparent membrane is formed in front of the cornea. In Man during the concrescence of the eyelids there are developed at their margins the Meibomian glands. The cells of the rete Malpighii begin to proliferate and to send into the middle connective- tissue plate of the eyelid solid rods, which afterwards become covered with lateral buds. The glands, at first entirely solid, acquire a lumen by the fatty degeneration and dissolution of the axial cells. At about the time of the development of the Meibomian glands, the formation of the eyelashes takes place ; this corresponds with the development of the ordinary hair, and therefore will be considered along with the latter in a subsequent section of this chapter. In most of the Vertebrates there is associated with the upper and under lids still a third, the nictitating membrane or membrana nictitans, which is formed at the inner [median] side of the eye as a vertical fold of the conjunctiva. In Man it is present only in a rudimentary condition as plica semilunaris. A number of small glands which are developed in it produce a small reddish nodule, the caruncula lacrymalis. The lachrymal gland is an additional auxiliary organ of the eye, which is destined to keep the sac of the conjunctiva moist and the anterior surface of the cornea clean. In Man it is developed in the third month through the formation of buds from the epithelium of the conjunctival sac on the outer side of the eye, at the place where the conjunctiva of the upper lid is continuous with that of the eye- ball. The buds form numerous branches, and are at first solid, like the Meibomian glands, but gradually become hollow, the cavity beginning with the chief outlet and extending toward the finer branches. A special efferent lachrymal apparatus, which leads from the inner angle of the eye into the nasal cavity, has been developed for the removal of the secretions of the various glands collected in the conjunctival sac, but particularly the lachrymal fluid. Such an apparatus is present in all classes of Vertebrates from the Amphibia upward ; its development has been especially investigated by BORN in a series of researches. In the Amphibia it begins to be formed at the time the process of 488 EMBRYOLOGY. chondrification becomes observable in the membranous nasal capsule. At that time the mucous layer of the epidermis, along a line that extends from the median side of the eye directly to the nasal cavity, undergoes proliferation and sinks into the underlying connective- tissue layer as a solid ridge. Then from the nose to the eye the ridge becomes constricted off, subsequently acquires a lumen, whereby it is converted into a canal lined with epithelium, and opens out into the nasal cavity. Toward the eye-end the canal is divided into two tubules, which at the time of detachment from the epidermis remain in connection with the conjunctival sac and suck up out of it the lachrymal fluid. In Birds and Mammals, including Man (fig. 273), the place where the lachrymal duct is located is early marked externally by a furrow which runs from the inner angle of the eye to the nasal chamber. By means of this furrow two ridges, which play an important part in the for- mation of the face, — the maxillary process and the outer nasal process, — are sharply marked off from each other; these will engage our atten- tion later. According to COSTE and KOLUKER the lachrymal duct arises Fig 273.-Head of a human embryo, b ^ rf j approximation and COT1- from which the mandibular pro- * cesses have been removed to crescence of the edges of the lachrymal groove. These older conclusions have been contradicted by BORN and LEGAL, one of whom has investigated Reptiles and Birds, the other Mammals. According to them there arises, in nearly the same manner as in Amphibia, through proliferation of the mucous epithelium, at the bottom of the lachrymal groove an epithelial ridge, which becomes detached but is not converted into a canal until a rather late period. When we raise the question, how phylogenetically the lachrymal duct may have first originated, we shall doubtless find that it has been derived from a groove, by means of which the sac of the con- junctiva and the nasal chamber are first put into connection. When, therefore, we see the lachrymal duct established from the very begin- ning simply as a solid ridge, as for example in the Amphibia, we must call to mind how* in other cases also originally groove-like fundaments, such as the medullary furrow, make their appearance, under special circumstances, as solid riders. THE ORGANS OF THE OUTER GERM-LAYER. 489 Finally, as far as regards the development of the lachrymal tubules in Birds and Mammals, BORN and LEGAL refer the upper tubule to the proximal part of the epithelial ridge, and maintain that the lower one buds out from the upper. EWETSKY, on the contrary, declares that the proximal end of the -epithelial ridge expands at the inner angle of the eye. and becomes divided by the ingrowth of underlying connective tissue, and metamorphosed into the two tubules, so that both arise from a common fundament. SUMMARY. 1. The lateral walls of the primary fore-brain vesicle are evaginated to form the optic vesicles. 2. The optic vesicles remain united by means of a stalk, the future optic nerve, with that part of the primary fore-brain vesicle which becomes the between-brain. 3. The optic vesicle is converted into the optic cup through the invagination of its lateral and lower walls by the fundaments of the lens and vitreous body. 4. At the place where the lateral wall of the primary optic vesicle •encounters the outer germ-layer, the latter becomes thickened, then depressed into a pit, and finally constricted off as a lens-vesicle. 5. The cells of the posterior wall of the lens- vesicle grow out into lens-fibres, those of the anterior wall become the lens-epithelium. 6. The fundament of the lens is enveloped at the time of its principal growth by a vascular capsule (tunica vasculosa lentis), which afterwards entirely disappears. 7. The membrana capsulo-pupillaris is the anterior part of the tunica vasculosa lentis and lies behind the pupil. 8. The development of the vitreous body causes the choroid fissure. 9. The optic cup has double walls ; it consists of an inner and an outer epithelium, which are continuous with each other at the open- ing of the cup, which embraces the lens, and at the choroid fissure. 10. Mesenchymatic cells from the vicinity grow in between the lens and the somewhat closely applied epidermis to form the cornea -and DESCEMET'S membrane, the latter being separated from the tunica vasculosa lentis by a fissure, the anterior chamber of the eye. 1 1 . The optic cup is differentiated into a posterior portion, within the territory of which its inner layer becomes thickened and con- stitutes the retina, and an anterior portion, which begins at the ora 490 EMBRYOLOGY. serrata. becomes very much reduced in thickness, and extends over the front surface of the lens, growing into the anterior chamber of the eye until the originally wide opening of the cup i» reduced to the size of the pupil. 12. The anteripr attenuated portion of the cup is, in turn, divided into two zones, of which the posterior becomes folded at the periphery of the equator of the lens to form the ciliary processes, whereas in front it remains smooth ; so that in the whole cup three parta may now be distinguished, as retina, pars ciliaris, and pars iridia retinae. 13. Corresponding to the three portions of the epithelial optic cup, the adjoining connective -tissue envelope takes on somewhat different conditions as the choroid proper, and as the connective-tissue frame- work of the ciliary body and that of the iris. 14. The skin surrounding the cornea becomes infolded to form the upper and lower eyelids and the nictitating membrane, of which the last is rudimentary in Man, persisting only as the plica semilunaris. 15. The epithelial layers of the edges of the two eyelids grow together in the last months of development, but become separated again before birth. 16. The lachrymal groove in Mammals passes from the inner angle of the eye, between the maxillary and outer nasal processes, to the nasal chamber. 17. The lachrymal duct for carrying away the lachrymal fluid is formed by the downgrowth and constricting off of an epithelial ridge from the bottom of .the lachrymal groove, the ridge becoming hollow. 18. The two lachrymal tubules are developed by the division of the epithelial ridge at the angle of the eye. B. The Development of the Organ of Hearing. In the case of the ear numerous parts of quite different origin unite, in much the same manner as in the case of the eye, to form a single very complicated apparatus ; of these, too, it is the portion to which the auditory nerve is distributed — the membranous labyrinth with its auditory epithelium— that is by far the most important, out- st ripping as it does all the remaining parts in its development: it must consequently be considered first. THE ORGANS OF THE OUTER GERM-LAYER 491 (a) The Development of the Otocyst into the Labyrinth. The membranous labyrinth is preeminently a product of the outer germ-layer. However great its complication in the adult is, — a complication that has given it the name labyrinth, — its earliest fundament is exceedingly simple. It arises on the dorsal surface of the embryo in the region of the medulla oblongata (fig. 263 gb), above the first visceral cleft and the attachment of the second visceral arch (fig. 274 above the numeral 3). Here over a circular territory the outer germ-layer becomes thickened and soon sinks down into an auditory pit. This process can be traced very easily in the embryo Chick on and after the end of the second day of incubation, and in the embryo Rabbit fifteen days old. The auditory nerve makes its way from the brain, near at hand, to the fundus of the pit, where it terminates in a ganglionic enlargement. The Bony Fishes alone ex- hibit a deviation from these conditions. Just as the central nervous system was in their case formed not as a tube, but as a solid body, and the eye not as a vesicle, but as an epithelial ball, so we see here that instead of an auditory pit there is formed by means of the proliferation of the outer germ-layer a solid epithelial plug. This, like the brain-tube and the eye-vesicle, acquires an internal chamber at a later period only — namely, after being constricted off. The next stage shows the pit converted into an auditory vesicle. In the Chick this takes place in the course of the third day. The invagination of the outer germ-layer grows deeper and deeper, and by the approximation of its margins becomes pear-shaped ; soon the connection with the outer germ-layer becomes entirely lost, as is shown by a section through the head of an embryo Sheep (fig. 275 Ib). In nearly all Vertebrates the auditory vesicle is constricted off from the ectoderm in the same manner. The Selachians are an exception : here the auditory vesicle which is metamorphosed into the labyrinth retains permanently its connection with the surface of the Fig. 274. — Head of a human embryo 7'5 mm. long, neck measurement, From His, " Menschliche- Embryonen." The auditory vesicle lies above the first visceral cleft. In the circumference of the visceral cleft there are to be seen six elevations, de- signated by numerals, from which the external ear is developed. 492 EMBRYOLOGY. body in the form of a long narrow tube, which traverses the cartila- ginous primordial cranium and is in union dorsally with the epidermis at the surface of the body, where it possesses an external opening. In its first fundament the organ of hearing in Vertebrates resembles in the highest degree those structures which in the Invertebrates are interpreted as organs of hearing. These are lymph-filled vesicles lying under the skin, which are likewise developed out of the epidermis. Either they are wholly constricted off from the epidermis, or they remain connected with it by means of a long, ciliate, epithelial canal, as in the Cephalopods, even after they have become surrounded by connective tissue. In both cases the vesicles are lined with epithelium which con- sists of two kinds of cell : first of low, flat elements, which ordinarily exhibit ciliary movements and thereby put in motion the fluid within the vesicle, and secondly of longer cylindrical, or thread-like, au- ditory cells with stiff hairs, which project into the endo- lymph. The auditory cells are either distributed individually over the inner surface of the auditory vesicle or arranged in groups, or they are united at a particular place into an auditory epithelium, — the au- ditory patch (macula acustica) or the auditory ridge (crista acustica), — which may be either single or double. To all the auditory vesicles of the Invertebrates there is sent, moreover, a nerve which ends at the sensory cells in fine fibrillae. Finally, there is present as a characteristic structure a firm, crystalline body, the otolith, which is suspended in the midst of the endolymph and is ordinarily set in vibration by the motion of the cilia. It consists of crystals of phosphate or carbonate of lime. Sometimes there is only a single large, in most cases concentrically laminated, spherical body, sometimes a number of small calcareous crystals, which are held together by means of a soft pulpy substance. Fig. 875.— Vertical [cross] section through the vesicle of the labyrinth of an embryo Sheep 1 -3 cm. long, after BOETTCHER. Magnified 80 diameters. nh, Wall of the after-brain ; rl, recessus labyrinth! ; Ib, vesicle of the labyrinth ; yc, ganglion coch- leare, which is in contact with a part of the labyrinth- vesicle (dc) that grows out into the ductus cochlearis. THE ORGANS OF THE OUTER GERM-LAYER. 493 It is difficult to follow the formation of the otoliths within the otocyst. In one case, which FOL was able to follow, they were developed by an epithelial cell in the wall of the vesicle. The cell secretes small calcareous concretions in its protoplasm, becomes enlarged in consequence, and protrudes as an elevation into the endolymph. When it has become more heavily loaded with calcic salts, it is connected with the wall by means of a stalk only, and finally it becomes entirely detached from the wall and falls into the cavity of the vesicle, in which it is kept float- ing and rotating by the ciliate cells. In Vertebrates the otocyst, which, as we have seen, agrees in its first fundament with the organ of hearing in Invertebrates, is con- verted into a very com- plicated structure, — the membranous labyrinth, — the evolution of which in Mammals I shall de- scribe in some detail. It undergoes metamor- phoses, in which the formation of folds and constrictions plays the principal part (fig. 276). The auditory sao de- tached from the epi- dermis, and lying at the side of the after-brain, soon exhibits a small, dorsally directed pro- jection, the recessus labyrinth* or ductus endolymphaticus (fig. 275 rl). Probably we have to do in this with the remnant of the original stalk by means of which the auditory vesicle was connected with the epidermis. According to some investigators, on the contrary, the stalk disappears entirely and this evagination is a new structure. The first assumption is favored especially by the previously mentioned condition in the Selachians — the presence of a long tube, which maintains a permanent connection between labyrinth and epidermis. dc Fig. 276 Membranous labyrinth of the left side of a [human] embryo, after a wax model by KRAUSE. rl, Recessus labyrinth! ; dc, ductus cochlearis ; lib, pocket from which the horizontal semicircular canal is formed ; am', enlargement of the pocket which becomes the ampulla of the horizontal canal ; am (vb), vb', * com- mon pocket from which the two vertical semicircular canals are developed; am (vb), enlargement of the common pocket from which the ampulla of the an- terior vertical canal arises. An opening (0) has beea formed in the pocket, through which one sees the recessus labyrinthi. * Region of the pocket which becomes the common arm of the two vertical canals (sinus superior) ; vb', part of the common pocket which furnishes the posterior vertical canal. 494 EMBRYOLOGY. Later this appendage of the labyrinth (figs. 276-9 rl) grows out dorsally to a great length, during which its walls come into close contact with each other, excepting at the blind end, which is enlarged into a small sac (fig. 279 rl *). Meanwhile the auditory sac itself (figs. 275-7) begins to be elongated and to be formed into a ventrally directed conical process (dc), the first fundament of the ductus cochlearis, which is curved inward .a little toward the brain (fig. 277 nh), and the concave side of which Pig. 277.— Cross section through the head of a Sheep embryo 1*6 cm. long, in the region of the labyrinth-sac. On the right aide is represented a section which passes through the middle of the sao ; on the left, one that is situated somewhat farther forward. After BOETTCHER. hn, Auditory nerve ; vb, vertical semicircular canal ; .-< iil>» - to ZANDER'S view of the terminal position of the fundament of the nail, but, supported by the investigations of BOAS, opposes ZANDER'S assumption of a migration of the funda- ment of the nail dorsally. He distinguishes in the development of nails and claws two parts (fig. 293), the dorsally located firm nail- THE ORGANS OF THE OUTER GERM-LAYER. 527 plate (np) and the plantar horn (Sohlenhorn, sh) connected with it ventrally. Of these the latter arises from the smaller ventral surface of the primary basis of the nail. In unguiculate and ungulate Vertebrates it (fig. 294 sh) is developed to a great extent ; in Man it atrophies, and is recognisable only in an exceedingly reduced condition as nail-welt. By this term is meant the welt-like thickening of the epidermis which forms the transition from the bed of the nail to the corrugated skin of the ball of the finger. The nail-plate, on the contrary, is from the beginning exclusively a product of the dorsal surface of the basis of the nail. There is therefore neither in Man nor in other Mammals a dorsal migration of the terminal fundament of the nail, but only a degeneration of nw sh np np • nw np sh Fig. 293. Fig. 294. Fig. 293. — A, Longitudinal section through the toe of a Cercopithecus B, Longitudinal section through the second finger of Macacus ater. After GEGENBACJR. np, Nail-plate ; sh, plantar horn (Sohlenhorn) ; me. nail-wall. Fig 294. — Section through a Dog's toe. Af ter GEGEXBAUR. «v<, Nail-plate ; sh, plantar horn : 6, ball of toe. its ventral portion, which otherwise furnishes a more complete plantar horn. So far as regards the particular events in the development of the nail-plate, the structure is demonstrable in human embryos four months old as a thin flat layer of cornified, closely united cells on the dorsal surface of the primary basis of the nail or the bed of the nail. It is produced by the mucous layer upon which it im- mediately lies, but continues for a time to be covered by the thin corneous layer that is present at all points of the epidermis. This ih vestment — UNNA'S eponychium — is not lost until the fifth month. However, notwithstanding their investment, the nails are easily recognisable some weeks before this from their whiteness, in dis- tinction from the reddish or dark red color of the surrounding skin. 528 EMBRYOLOGY. Owing to the addition of new cells from the mucous membrane, both from below and from the posterior margin, the nail-plate grows — it becomes thickened and increased in surface extent. It is now pushed forward from behind over the bed of the nail, and at the seventh month its free margin begins to project beyond the latter. With this the nail has acquired essentially the appearance and con- dition which it has in the adult. In new-born infants it possesses a margin which projects far over the ball of the finger, and which — because it was formed at an early embryonic period — is both much thinner and also narrower than the part formed later, which rests on the bed of the nail. This margin is therefore detached soon after birth. (d) The Glands of the Skin. The glandular structures of the epidermis, which are established by invagination, are of three kinds : sebaceous, sweat-, and milk- glands. They all arise as proliferations of the mucous layer which grow down as solid plugs into the derma, and then undergo further development either according to the tubular or the alveolar type. The sweat-glands and the ear-wax glands are developed on the tubular plan. They begin in the fifth month to penetrate from the mucous membrane into the corium ; in the seventh month they acquire a small lumen, take a winding course in consequence of increased growth in length, and become coiled especially at their deep ends, thereby giving rise to the first fundament of the glomerulus. Sebaceous glands and milk-glands are alveolar structures. The former are either developed directly from the epidermis, as, for example, at the edges of the lips, on the prepuce and on the glans penis, or they are in close connection with the hairs, which is the ordinary condi- tion. In the latter case they are formed as solid thickenings of the outer sheath of the root of the hair near the orifice of the follicle, even before the hairs are completely developed (fig. 292 (7, #, td) ; at first they have the form of a flask, then they send out a few lateral buds, which develop club-shaped enlargements at their ends. The glands acquire cavities by the fatty degeneration and disintegration of the interior cells, which are eliminated as a secretion. The development of the milk-glands, which are more voluminous organs entrusted with an important function and peculiar to tlfe class Mammalia, is of greater interest. Of the numerous works that have appeared concerning them, the comparative-anatomic-nl investigations of GEGENBAUR especially have led to valuable results. THE ORGANS OF THE OUTER GERM-LAYER. 529 df g I present at the very beginning of the discussion the following proposition, which is of importance in interpreting the conditions found : each milk-gland in Man is not a simple organ, like an ear- gland or a submaxillary salivary gland, with a simple outlet, but a great glandular complex. Its earliest fundament has been observed in the human embryo at the end of the second month as a considerable thickening of the epidermis (fig. 295) upon the right and left sides of the breast. It has arisen as the result of a special proliferation of the mucous layer, which has sunk into the derma in the form of a hemispherical knob (df). But modifications arise afterwards in the corneous layer also, by its becoming thickened and projecting as a corneous plug into the proliferation of the mucous layer. Ordinarily there is found a small depression (g) at the middle of the whole epithelial fundament. The proliferation of the epi- dermis that first appears is not precisely, as assumed by REIN, the first fundament of the glandular parenchyma ; it therefore does not correspond to the epithelial plugs which sink into the derma in the development of the sweat and sebaceous glands, because the further course of develop- ment and especially comparative- anatomical studies show, that by the thickening of the epidermis there is only an early delimitation of a tract of the skin, which is subsequently metamorphosed into the nipple-area and papilla, and from the floor of which the separate milk-producing glands at length sprout forth. The correctness of this view is 'shown by the following changes: In older embryos the lens-shaped thickening produced by the proliferation of the epidermis has increased at the periphery and has thereby become flattened (fig. 296 df). At the same time it is more sharply defined at the surface, owing to the derma becoming thickened and elevated into a wall (div) — the cutis-wall. Therefore the whole fundament now has the form of a shallow depression (df) of the skin, for which the name glandular area is very appropriate. For there early grow out from its mucous layer into the derma solid 34 Fig. 295.— Section through the fundament of the milk-gland of a female human embryo 10 cm. long, after Huss. df, Fundament of the glandular area ; g, small depression at its surface. 530 EMBRYOLOGY. buds (dg\ just as at other places the sebaceous glands arise from the epidermis. In the seventh month they are already well developed, and radiate out below and laterally from the pit-like depression. Their number increases up to the time of birth, and the larger ones become covered with solid lateral buds (db). Each sprout is the fundament of a Luilk-producing gland, which opens out on the glandular area (df) by means of a special orifice ; each is morpho- logically comparable with a sebaceous gland, although its function has become different. The name glandular area is also a happily selected one because it presents a point of comparison with the primitive conditions of the Monotremes. For in these animals one does Fig. 296. — Section through the fundament of the milk-gland of a female human e.nbryo 32 cm, long, after Huss. <#", Glandular area ; die, gland-wall ; dg, duct of gland ; db, vesicle of gland. not find, as in the higher Mammals, a sharply differentiated single complex of milk-glands, but instead a somewhat depressed area of the skin, even provided with small hairs, over which are distributed single small glands, the secretion of which is licked up with the tongue by the young, which are born in a very immature state. In the remaining Mammals the glands, in the former case opening separately upon the area, are united into a single organ, which better serves the young in sucking, namely a pcqrillc [nipple] or teat, which encloses all the outlets of the glands and is grasped by the mouth of the suckling. In Man their development begins after birth. The glandular area, which is encircled l»y the cutis-wall and which before birth was depressed into a pit, THE ORGANS OF THE OUTER GERM-LAYER. 531 now becomes flattened until it lies in the same niveau with the surrounding skin. It is distinguished from the latter by its redder color, which is due to its greater vascularity and the thinner condition of its epidermis. Then during the first years after birth the middle of the glandular area, together with the outlets (ductus lactiferi), which there open out close to one another, is raised up and becomes the nipple, in the derma of which non- striate muscle-fibres are formed in great numbers; the remaining part of the area as far as the cutis-wall becomes the areola mammae. The metamorphosis takes place somewhat earlier in the female than in the male. * Soon after birth alterations take place in the still feebly developed glandular tissue. There occurs a transitory swelling of the pectoral glands accompanied with increased blood-pressure, and it becomes possible to press out of the gland a small quantity of a milky fluid, the so-called witches' milk. According to KOLLIKER its formation is due to the originally solid ducts of the glands acquiring at this time a lumen by the fatty degeneration of the central cells, which are dissolved, and, suspended in a fluid, are discharged from the ducts. According to the investigations of BARFURTH, on the contrary, the so-called witches' milk of infants is the product of a genuine tran- sitory secretion, and is like the real milk of the mother both in its morphological and chemical components. After birth great differences arise between the two sexes in the condition of the milk-glands. Whereas in the male the glandular parenchyma remains stationary in its development, in the female it begins to increase, especially at the time of sexual maturity and still more after the beginning of pregnancy, From the first-formed ducts of the glands there grow out numerous lateral, hollow branches, which become covered with hollow vesicular glands (alveoli) lined with a single layer of cylindrical epithelium. At the same time there are developed in the connective tissue, between the separate lobules of the gland, numerous islands of fat-cells. In consequence the region at which the complex of milk-glands has been formed swells into a more or less prominent elevation, the mamma. SUMMARY. 1. The development of the hair is inaugurated in human embryos by the growing down of processes of the mucous layer of the epidermis — the hair-germs — into the underlying derma. 533 BMBRYOLOQY. 2. At the deep end of the hair-germ the vascular hair-papilla is begun by a growth of connective tissue. 3. The epithelial hair-germ is differentiated into : — (a) A young hair, by the cornification of a part of the cells ; (6) An actively growing cell-layer situated between the shaft of the hair and the papilla, — the bulb, — which fur- nishes the material for the growth of the hair ; (c) The outer and the inner sheaths of the root. 4. Around the epithelial part of the fundament of the hair there is formed from the surrounding connective tissue the hair- follicle. 5. The nails in Man and the claws in other Mammals are de- veloped from a dorsal fundament — the nail-plate — and a ventral fundament — the plantar horn. 6. The plantar horn in Man is reduced to the nail-welt. 7. The thin nail-plate which is formed at first is for a time covered -with a layer of cornified cells, the eponychium, which in Man is shed in the fifth month. 8. The milk-gland is a complex of alveolar glands. 9. At first there arises a thickening of the mucous layer of the epidermis, which is converted into the glandular area that is after- wards marked off from the surrounding parts by a wall and becomes somewhat depressed. 10. From the bottom of the glandular area there grow forth in great numbers the fundaments of alveolar glands. 11. After birth the glandular area, embracing the excretory ducts of the glands, is elevated above the surface of the skin, and converted into the nipple and the areola mammae. 12. 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Wissensch. Bd. XV. 1889. His. Die Entvvicklung der ersten Nerve nbahnen beim menschlichen Embryo. Archiv f. Anat. u. Physiol. Anat. Abth. 1887. His, "W., jun. Zur Entwicklungsgeschichte des Acustico-facialis-Gebietes beim Menschen. Archiv f. Anat. u. Physiol. Anat. Abth. 1889. Suppl.- Bd. pp. 1-28. Julin, Ch. De la signification morphologique de 1'epiphyse des vertebres. Bull. sci. du depart, du Nord. Ser. II. T. X. 1888. Kollmann, J. Die Entwicklung der Adergeflechte. Ein Beitrag zur Entwicklungsgesch. des Gehirns. Leipzig 1861. Krause, W. Ueber die Doppelnatur des Ganglion ciliare. Morphol. Jahrb. Bd. VII. 1882, p. 43. Kraushaar, Richard. Die Entwicklung der Hypophysis u. Epiphysis bei Nagethieren. Zeitschr. f. wiss. Zoologie. Bd. XLI. 1884, p. 79. (Com- plete catalogue of the literature.) Kupffer. Primare Metamerie des Neuralrohrs der Vertebraten. Sitzungsb. d. k. bair. Akad. Munchen. Bd. XV. 1886, p. 469. Lowe, L. Beitrage zur Anatomic und Entwicklung des Nervensystems der Saugethiere u. des Menschen. Berlin 1880. Marshall, Milnes. The Development of the Cranial Nerves in the Chick. Quart. Jour. Micr. Sci. Vol. XVIII. 1878. Marshall, Milnes. On the Early Stages of Development of the Nerves in Birds. Jour. Anat. and Physiol. Vol. XI. 1877. Marshall, Milnes. On the Head Cavities and Assqciated Nerves of Elasmo- branchs. Quart. Jour. Micr. Sci. Vol. XXI. 1881. Mihalkovics, v. Wirbelsaite und Hirnanhang. Archiv f. mikr. Anat. Bd. XI. 1875. Mihalkovics, v. Entwicklungsgeschichte des Gehirns. Nach Untersuch- ungen an hoheren Wirbelthieren und dem Menschen dargestellt. Leipzig 1877. (Catalogue of the older literature.) Miiller, W. Ueber Entwicklung und Ban der Hypophysis und des Processus infundibuli cerebri. Jena. Zeitschr. Bd. VI. 1871. Onodi. Ueber die Entwicklung des sympath. Nervensystems. Archiv f. mikr. Anat. Bd. XXVI. 1886. Onodi. Ueber die Entwicklung der Spinalganglien und der Nervenwurzeln. Internat. Monatsschr. f. Anat. u. Histol. Bd. I. 1884. Osborn, H. F. The Origin of the Corpus Callosum, a Contribution upon the Cerebral Commissures of the Vertebrata. Morphol. Jahrb. Bd. XIL 1887. Rabl. Bemerkung iiber die Segmentirung des Hirns. Zool. Anzeiger. Jahrg. VIII. 1885, p. 192. LITERATURE. 535 Rabl-Riickhard. Das gegenseitige Verhaltniss der Chorda, Hypophysis und des mittleren Schadelbalkens bei Haifischembryonen etc. Morphol Jahrb. Bd. VI. 1880. Rabl-Riickhard. Zur Deutung und Entwicklung des Gehirns der Knochen- fische. Archiv f. Anat. u. Physiol. Anat. Abth. 1882. Rabl-Riickhard. Das Grosshirn der Knochenfische und seine Anbangs- gebilde. Archiv f. Anat. u. Physiol. Anat. Abth. 1883. Rathke, H. Ueber die Entstehung der Glandula pituitaria. Archiv f. Anat. u. Physiol. Bd. V. 1838. Reichert. Der Bau des menschlichen Gehirns. Leipzig 1859 and 1861. Sagemehl. Untersuchungen iiber die Entwicklung der Spinalnerven. Dorpat 1882. Schmidt, F. Beitrage zur Entwicklungsgeschichte des Gehirns. Zeitschr. f. wiss. Zoologie. Bd. XI. 1862. Schultze, O. Ueber die Entwicklung der Medullarplatte des Froscheies. Verhandl. der phys.-med. Gesellsch. Wiirzburg. N. F. Bd. XXIII. 1889. Schwalbe, G-. Das Ganglion oculomotorii. Jena. Zeitschr. Bd. XIII. 1879. Schwalbe, G-. Lehrbuch der Neurologic. Erlatigen 1880. Spencer, W. Baldwin. On the Presence and Structure of the Pineal Eye in Lacertilia. Quart. Jour. Micr. Sci. Vol. XXVII. 18S6. Suchannek. Ein Fall von Persistenz des Hypophysenganges. Anat. Anzeiger. Jahrg.II. Nr.16. 1887. Tiedemann, Fr. Anatomic und Bildungsgeschichte des Gehirns im Foetus des Menschen. Number g 1816. Wijhe, J. W. v. Ueber die Mesodermsegmente und die Entwicklung der Nerven des Selachierkopfes. Verhandl. d. koninkl. Akad. d. Wetenschapperi Amsterdam. 1882. Deel XXII. (2) Development of the Eye. Angelucci, A. Ueber Entwicklung und Bau des vorderen Uvealtractus der Vertebraten. Archiv f. mikr. Anat. Bd. XIX. 1881, p. 152. Arnold, Jul. Beitrage zur Entwicklungsgeschichte des Auges. Heidelberg 1874. Babuchin. Beitrage zur Entwicklungsgesch. des Auges. Wiirzburger Naturwiss. Zeitschr. Bd. IV. 1863, p. 71. Bambeke. Contribution a 1'histoire du developpement de 1'ceil humain. Ann. de la Soc. de med. de Gand. 1879. Ewetsky, v. Beitrage zur Entwicklungsgeschichte des Auges. Archiv f. Augenheilkunde. Bd. VIII. 1879. Gottschau. Zur Entwickluiig der Saugethierlinse. Anat. Anzeiger. Jahrg. I. 1886. Keibel, Fr. Zur Entwicklung des Glaskorpers. Archiv f. Anat. u. Physiol. Anat. Abth. 1886. Kessler. Untersuchungen iiber die Entwicklung des Auges, angestellt am Hiihnchen und Triton. Dissertation. Dorpat 1871. Kessler. Zur Entwicklung des Auges der Wirbelthiere. Leipzig 1877. Kolliker. Ueber die Entwicklung der Linse. Zeitschr. f. wiss. Zoologie. Bd. VI. 1855. 536 EMBRYOLOGY. Kolliker. Zur Entwicklung des Auges und Gerucbsorganes menschlicher Embryonen. Zum Jubilaum der Universitat ZUrich. Wiirzburg 1883. Koranyi, Alexander. Beitrage zur Entwicklung der Krystalllinse bei den \Virbelthieren. Internat. Monatsschr. f. Anat. u. Histol. Bd. III. 1886. Kupffer. Untersuchungen iiber die Entwicklung des Augenstiels. Sitzungsb. d. Gesellsch. f. Morphol. u. Physiol. Miinchen. Bd. I. 1885, p. 174. Iiieberkiihn, N. Ueber das Auge des Wirbelthierembryos. Schriften d. Gesellsch. z. Beford. d. ges. Naturwiss. Marburg. Bd. X. 1872, p. 299. Lieberkiihn, N. Zur Anatomic des embryonalen Auges. Sitzungsb. d. Gesellsch. z. Beford. d. ges. Naturwiss. Marburg. 1877, p. 125. Lieberktihn, N. Beitrage zur Anatomic des embryonalen Auges. Archiv f. Anat. u. Entwicklungsg. Anat. Abth. Jahrg. 1879, pp. 1-29. Manz. Entwicklungsgeschichte des menschlichen Auges. Graefe u. Saemisch. Handbuch d. Augenheilkunde. Bd. II. Leipzig 1875, pp. 1-57. Mihalkovics, v. Ein Beitrag zur ersten Anlage der Augenlinse. Archiv f. mikr. Anat. Bd. XI. 1875. Miiller, W. Ueber die Stammesentwicklung des Sehorgans der Wirbelthiere. Festgabe an Carl Ludwig. Leipzig 1874. Rumschewitsch. Zur Lehre von der Entwicklung des Auges. Schriften d. Gesellsch. d. Naturf. Kiew. Bd. V. Heft 2, 1878, p. 144. (Russian.) "Wurzburg, A. Zur Entwicklungsgeschichte des Saugethierauges. In- auguraldissertation der Berliner Universitat. 1876. (3) Development of the Ear. Boettcher, A. Ueber Entwicklung u. Bau des Gehb'rlabyiinths. Nach Untersuchungen an Saugethieren. Yerhandl. d. Kaiserl. Leop.-Carol. Acad. Bd. XXXV. 1869. Gradenigo, G. Die embryonale Anlage der Gehorknbchelchen und des tubo- tympanalen Eaumes. Centralbl. f. d. med. Wiss. 1886. Nr. 35. Gradenigo, G. Die embryonale Anlage des Mittelohres. Die morpholog. Bedeutung der Gehorknochelchen. Mitth. a. cl. embryol. Inst. d. Univ. Wien. Heft 1887, p. 85. Hasse. Die vergleich. Morphologie u. Histologie d. hautigen Gehororgans der Wirbelthiere. Leipzig 1873. Hensen. Zur Morphologie der Schnecke. Zeitschr. f. wiss. Zoologie. Bd. XIII. 1863. His, W. Anatonr.le menschlicher Embryonen. Leipzig 1880, 1882, 1885. Hoffmann, C. K. Ueber die Beziehung der ersten Kiementasche zu der Anlage der Tuba Eustachh u. des Cavum tympani. Archiv f. mikr. Anat. Bd. XXIII. 1884. Huschke. Ueber die erste Bildungsgesch. d. Auges u. Ohres beim bcbriiteten HUhnchen. Oken's Isis, 1831, p. 950. Huschke. Ueber die erste Entwicklung des Auges. Meckel's Archiv. 1832. Moldenhauer. Zur Entwicklung des mittleren und au.-si-ivn Ohres. Morphol. Jahrb. r.d. III. 1877. Noorden, C. v. Die Entwicklung des Labyrinths bei Knochenfischen. Archiv f. Anat. u. Physiol. Anat Abth. 1883. Reissner. De Anris internae formatione. Inaug.-])i<>. LITERATURE. 537 Rosenberg, E. Untersuchungen iiber die Entwickl. des Canalis coclilearis d. Saugethiere. Diss. Dorpat 1868. Riidinger Zur Entwicklung der hautigen Bogengange des inneren Ohres. Sitzungsb. d. math.-physik. 01. d. Acad. d. Wissensch. Munchen. 1888. Tuttle. The Relation of the External Meatus, Tympanum and Eustachian Tube to the First Visceral Cleft. Proceed. Amer. Acad. Arts a. Sci. 1883-4- TTrbantschitsch. Ueber die erste Anlage des Mittelohres u. d. TrommelfeUes. Mitth. a. d. embryol. Inst. Wien. Heft I. 1877. (4) Development of the Organ of Smell. Blaue, J. Untersuchungen iiber den Bau der Nasenschleimhaut bei Fischen. u. Amphibien etc. Archiv f. Anat. u. Physiol. Anat. Abth. 1884. Born, G. Die Nasenhohlen und der Thranennasengang der Amphibien. Morphol. Jahrb. Bd. II. 1876. Born, G. Die Nasenhohle u. d. Thranennasengang der amnioten Wirbelthiere. Morphol. Jahrb. Bd. V. 1879 u. Bd. VIII. 1883. Diirsy. Zur Entwicklungsgeschichte des Kopfes. Tubingen 1869. Fleischer, R. Beitrage zur Entwicklungsgeschichte des Jacobson'schen Organs u. zur Anat. der Nase. Sitzungsb. d. physic.-med. Soc. Erlangen. 1877. Herzfeld. Ueber das Jacobson'sche Organ des Menschen u. d. Saugethiere. Zool. Jahrbiicher. Bd. III. 1888, p. 551. Kb'lliker, A. Ueber die Jacobson'schen Organe des Menschen. Gratula- tionsschrift d. Wtirzb. Medic. Facultat fur Rinecker. 1877. Kolliker, A. Zur Entwicklung des Auges und Geruchsorgans menschlicher Embryonen. Festschrift der Schweizerischen Universitat Zurich zur Feier ihres SOjahr. Jubilaums gewidmet. Wiirzburg 1883. Kolliker, Th. Ueber das Os interrnaxillare des Menschen etc. Nova acta L.-C. Acad. Bd. XLII. p. 325. Halle 1881. Legal. Die Nasenhohle und der Thranennasengang der amnioten Wirbelthiere. Morphol. Jahrb. Bd. VIII. 1883. Legal. Zur Entwickluugsgeschichte des Thranennasengangs bei Saugethieren. Inaug.-Diss. Breslau 1882 (?). Marshall, Milnes. The Morphology of the Vertebrate Olfactory Organ. Quart. Jour. Micr. Sci. Vol. XIX. 1879. (5) Development of the Skin and its Or yam. Barfurth. Zur Entwicklung der Milchdruse. Bonn 1882. Boas, J. E. V. Ein Beitrag zur Morphol. der Nagel, Krallen, Hufe und Klauen d. Saugethiere. Morphol. Jahrb. Bd. IX. 1884. Creighton, C. On the Development of the Manama and of the Mammary Function. Jour. Anat, and Physiol. Vol. XI. 1877, pp. 1-32. Feiertag. Ueber die Bildung der Haare. Inaug.-Diss. Dorpat 1875. Gegenbaur, C. Zur Morphologic des Nagels. Morphol. Jahrb. Bd. X. 1885. Gegenbaur, C. Bemerkungen iiber die Milchdriisenpapillen der Sangethiere. Jena. Zeitschr. Bd. VII. 1873. Gegenbaur, C. Zur genaueren Kenntniss der Zitzen der Saugethiere. Morphol. Jahrb. Bd. I. 1875. 538 EMBRYOLOGY. Gotte. Zur Morphologic der Haare. Archiv f. mikr. Anat. Bd. IV. 1868, p. 27:),. Hensen. Beitrag zur Morphologic der Korperfonn und des Gehirns des menschl. Embryos. Archiv f. Anat. u. Entwicklungsg. Anat. Abth. Jahrg. 1877. HUBS, M. Beitrage zur Entwicklung der MilchdrUsen bei Menschen und bei Wiederkauern. Jena. Zeitschr. Bd. VII. 1873. Klaatsch, Hermann. Zur Morphologic der Saugethier-Zitzen. Morphol. Jahrb. Bd. IX. 1884. Kolliker, A. Zur Entwicklungsgeschichte der aussern Haut. Zeitschr. f . wiss. Zoologie. Bd. II. 1850, p. 67. Kblliker, Th. Beitrage zur Kenntniss der Brustdrlise. Verhandl. Wurzburg. physical.-med. Gesellsch. Bd. XIV. 1879. Langer, C. Ueber den Bau und die Entwicklung der MilchdrUsen. Denkschr, d. k. Acad. d. Wissensch. Wien. Bd. HI. 1851. Rein, Q-. Untersuchungen iiber die embryonale Entwicklungsgeschichte der Milchdruse. Archiv f. mikr. Anat. Bde. XX. u. XXI. 1882. Reissner. Beitrage zur Kenntniss der Haare des Menschen und der Thiere. Breslau 1854. Toldt, C. Ueber die Altersbestimmung menschlicher Embryonen. Prager med. Wochenschr. 1879. TTnna, P. Z. Beitrage zur Histologie und Entwicklungsgeschichte der menschlichen Oberhaut und ihrer Anhangsgebilde. Archiv f. mikr. Anat. Bd. XII. 1876. Zander, R. Die friihesten Stadien der Nagelentwicklung und ihre Beziehungen zu den Digitalnerven. Archiv f. Anat. u. Entwicklungsg. Jahrg. 1884. CHAPTER XVII. THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. THE grounds which made it appear necessary to distinguish in addition to the four epithelial germ-layers a special intermediate layer or mesenchyme have already been given in the first part of this text-book. This distinction is also warranted by the further progress of development. For all the various tissues and organs which are derived in many ways from the intermediate layer allow, even subsequently, a recognition of their close relationship. Histo- logically the various kinds of connective substance have been for a long time considered as constituting a single family of tissues. It will be my endeavor to emphasise the relationship of the organs of the intermediate layer, and whatever is characteristic of them from a morphological point of view, more than has been hitherto customary in text-books, and to do the same in a formal THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 539 way by embracing these organs in a chapter by themselves and discussing them apart from the organs of the inner, middle, and outer germ-layers, It is the original province of the intermediate layer to form a packing and sustentative substance between the epithelial layers, a fact which stands out with the greatest distinctness particularly in the lower groups, as for example in the Ccelenterates. It is there- fore closely dependent upon the epithelial layers in the matter of its distribution. When the germ-layers are raised up into folds, it penetrates between the layers of the fold as a sustentative lamella ; when the germ-layers are folded inwards, it receives the parts that are being differentiated — as for example in the Vertebrates, the neural tube, the masses of the transversely striped muscles, the secretory parenchyma of glands, the optic cups, and the auditory vesicles — and provides them with a special envelopment that adjusts itself to them (the membranes of the brain, the perimysium, and the connective-tissue substance of the glands). In consequence of this the intermediate layer, in the same proportion as the germ-layers become more fully organised, becomes itself converted into an extra- ordinarily complicated framework, and resolved into the most diver- gent organs, by the formation of evaginations and imaginations and the constricting off of parts. The form of the intermediate layer thus produced is of a second- ary nature, for it is dependent upon the metamorphosis of the germ- layers, with which it is most intimately connected. But in addition, the intermediate layer, owing to its own great power of metamor- phosis, acquires in all higher organisms, particularly in the Verte- brates, an intricate structure, especially in the way of histological differentiation or metamorphosis. In this way it gives rise to a long series of various organs — the cartilaginous and bony skeletal parts, the fasciae, aponeuroses, and tendons, the blood-vessels and lymphatic glands, etc. It is therefore fitting to enter here somewhat more particularly upon a discussion of the principle of histological differentiation, and especially to inquire in what manner it is concerned in the origin of organs differentiated in the mesenchyme. The most primitive and simplest form of mesenchyme is gelatinous tissue. Not only does it predominate in the lower groups of animals, but it is also the first to be developed in all Vertebrates, out of the em- bryonic cells of the intermediate layer, and is here the forerunner and the foundation of all the remaining forms of sustentative substance. 540 EMBRYOLOGY. In a homogeneous, soft, quite transparent matrix, which chemically considered contains mucous substance or inucin, and therefore does not swell in warm water or acetic acid, there lie at short and regular intervals from one another numerous cells, which send out in all directions abundantly branched protoplasmic processes and by means of these are joined to each other in a network. In the lower Vertebrates the gelatinous tissue persists at many places, even when the animals are fully grown ; in Man and other Mammals it early disappears, being converted into two higher forms of connective substance, either into fibrillar connective tissue or into cartilaginous tissue. The first-named arises in the gelatinous matrix by the differentiation of connective-tissue fibres on the part of the cells, which are sometimes close together, sometimes widely scattered. These fibres consist of collagen and upon boiling produce glue. At first sparsely represented, these glue-producing fibres continually increase in volume in older animals. Thus transitional forms, which are designated as foetal or immature connective tissue, lead from gelatinous tissue to mature connective tissue, which consists almost exclusively of fibres and the cells which have produced them. This is capable of a great variety of uses in the organism, according as its fibres cross one another in various directions without order, or are arranged parallel to one another and grouped into special cords and strands. Thus, in connection with other parts derived from the germ- layers, it gives rise to a great variety of organs. In some places it forms the foundation for epithelial layers of great superficial extent ; together with them it produces the integument, composed of epidermis, cerium, and subcutaneous connective tissue, and the various mucous and serous membranes ; in others it unites with masses of transversely striped muscle, and arranges itself under the influence of their traction into parallel bundles of tense fibres, furnishing tendons and aponeuroses. Again at other places it takes the form of firm sheets of connective tissue, which serve for the separation or enveloping of masses of muscle, the intermuscular ligaments and the fasciae of muscles. The second metamorphic product of the primary mesenchyine, cartilage, is developed in the following manner : At certain places the embryonic gelatinous tissue acquires as a result of proliferation a greater number of cells, and the cells secrete between them a carti- laginous matrix, chondrin. The parts which have resulted from the process of chondrification exceed in rigidity to a considerable extent the remaining kinds of sustentative substance, the gelatinous THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 54J and the glue-producing intermediate tissue; they are sharply differentiated from their softer surroundings, and become adapted, in consequence of their peculiar physical properties, to the as- sumption of special functions. Cartilage serves in part to keep canals open (cartilage of the larynx and bronchial tree), in part for the protection of vital organs, around which they form a firm envelope (cartilaginous cranium, capsule of the labyrinth, vertebral canal, etc.), and in part for the support of structures projecting from the surface of the body (cartilage of the limbs, branchial rays, etc.). At the same time they afford firm points of attachment for the masses of muscle imbedded in the mesenchyme, neighboring parts of the muscles entering into firm union with them. In this manner there has arisen through histological metamorphosis a differentiated skeletal apparatus, which increases in complication in the same proportion as it acquires more manifold relations with the muscu- lature. Cartilaginous and connective tissues, finally, are capable of a further histological metamorphosis, since the last form of sustenta- tive substance, osseous tissue, is developed from them in connection with the secretion of salts of lime. There are therefore bones that have arisen from a cartilaginous matrix and others from one of con- nective tissue. With the appearance of bone, the skeletal apparatus of Vertebrates has reached its highest perfection. Even if the mesenchyme has by these processes experienced an extraordinarily high degree of differentiation and a great diversity of form, the histological processes of differentiation which take place in it are nevertheless not yet exhausted. In the gelatinous or connective-tissue matrix canals and spaces arise in which blood and lymph move in accomplishing their function of intermediating in the metastasis of the organism, not only conveying the nutritive fluids to the individual organs, but also conducting away both the substances which — owing to the chemical processes in the tissues — have become useless and the superfluous fluids. Out of these first beginnings arises a very complicated organic apparatus. The larger cavities constitute arteries and veins, and acquire peculiarly constructed thick walls, provided with non-striate muscle-cells and elastic fibres, in which three different layers can be dis- tinguished as tunica intima, media, and adventitia. A small part of the blood-passages, especially distinguished by an abundance of muscle-cells, is converted into a propulsive apparatus for the fluid — the heart. The elementary corpuscles that circulate in the 542 EMBRYOLOGY. currents of the fluid, the blood-cells and lymph-cells, demand renewal the more frequently the more complex the metastasis becomes. This leads to the formation of special breeding places, as it were, for the lymph-corpuscles. In the course of the lymphatic vessels and spaces there takes place at certain points in the con- nective tissue an especially active proliferation of cells. The substance of the connective-tissue framework assumes here the special modification of reticular or adenoid tissue. The surplus of cells produced enters into the lymphatic current as it sweeps past. According as these lymphoid organs exhibit a simple or a complicated structure, they are distinguished as solitary or aggregated follicles, as lymphatic ganglia and spleen. Finally there are formed at very many places in the intermediate layer, as especially in the whole course of the intestinal canal, organic [non-striate] muscles. After this brief survey of the processes of differentiation in the intermediate layer, which are primarily of an histological nature, I turn to the special history of the development of the organs which arise from it, the blood-vessel and skeletal systems. I. The Development of the Blood-vessel System. The very first fundament of the blood-vessels and the blood has already been treated of in the first part of this text-book. We will therefore here concern ourselves with the special conditions of the vascular system, with the origin of the heart and chief blood -vessels, and with the special forms which the circulation presents in the various stages of development, and which are dependent on the formation of the foetal membranes. In this I shall treat separately, both for the heart and for the rest of the vascular system, the first fundamental processes of development and the succeeding altera- tions, from which the ultimate condition is finally evolved. A. The first Developmental Conditions of the Vascular System, (a) Of the Heart. The vascular system of Vertebrates can be referred back to a very simple fundamental form— namely, to two blood-vessel trunks — of which the one runs above and the other below the intestine in the direction of the longitudinal ;>xis of the body. The dorsal trunk, the THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 543 aorta, lies in the attachment of the dorsal mesentery, by means of which the intestine is connected to the vertebral column ; the other trunk, on the contrary, is imbedded in the ventral mesentery, as far, at least, as such a structure is ever established in the Vertebrates ; it is almost completely metamorphosed into the heart. The latter is therefore nothing else than a peculiarly developed part of a main blood-vessel provided with especially strong muscular walls. In the first fundament of the heart there are two different types to be distinguished, one of which is present in Selachians, Ganoids, Amphibia, and Cyclostomes. the other in Bony Fishes and the higher Vertebrates — Reptiles. Birds, arid Mammals. In the description of the first type I select as an example the Pig. 297.— Cross section through the region of the heart of an embryo of Salamandra maculosa, in which the fourth visceral arch is indicated, after RABL. d, Epithelium of the intestine ; vm, visceral middle layer ; ep, epidermis ; Ih, anterior part of the body-cavity (pericardio-thoracic cavity) ; end, endocardium ; p, pericardium ; vhg, meso- cardium anterius. development of the heart in the Amphibia, concerning which a detailed account has very recently been published by RABL. In Amphibia the heart is established very far forward in the embryonic body, underneath the pharynx or cavity of the head-gut (figs. 297, 298). The embryonic body-cavity (Ih) reaches into this region, and in cross sections appears upon both sides of the median plane as a narrow fissure. The lateral halves of the body-cavity are separated from each other by a ventral mesentery (vhy), by means of which the under surface of the pharynx is united with the wall of the body. If we examine the ventral mesentery more closely, we observe that in its middle the two mesodermic layers from which it has been developed separate from each other and allow a small cavity (h) to appear, the primitive cardiac cavity. This is sur- 544 EMBRYOLOGY. rounded by a single layer of cells, which is afterwards developed into the endocardium (end).* Outside of the latter the adjacent cells of the middle germ-layer are thickened; they furnish the material out of which the cardiac musculature (the myocardium) and the superficial membrane of the heart (pericardium viscerale) arise. The fundament of the heart is attached above [dorsally] to the pharynx (d) and below to the body-wall by the remnant of the mesentery, which persists as a thin membrane. We designate these two parts as the suspensory ligaments of the heart, as back [dorsal] and front [ventral] cardiac mesenteries (hhg, vhg), or as mesocardium posterius and anterins. At this time there is nothing to be seen of a pericardial sac, unless we should designate as such the anterior [ventral] region of the body- cavity, from which, as the further course of development will show, the pericardium is chiefly derived. In the second type the heart arises from distinct and widely separated halves, as the con- ditions in the Chick and the Rabbit most distinctly teach. In the Chick the first traces of the fundament may be de- monstrated at an early period, in embryos with four to six primitive segments. They appear here at a time when the various germ-layers are still spread out flat, at a time when the front part of the embryonic fundament first begins to be elevated as the small cephalic protuberance, and the cephalic portion of the intes- tine is still in the first phases of development. As has already been stated, the intestinal cavity in the Chick is developed by the folding together and fusion of the intestinal plates [splanchnopleure]. If one examines carefully the ridge of an intestinal fold in the very process of being formed (fig. 299 A df), one observes that its visceral middle layer is somewhat thickened, composed of large cells, and separated from the entoblast by a space filled with a jelly-like matrix. In the latter there lie a few isolated cells, which subsequently * Relative to the origin of the endothelial sac of the heart, compare th& observations given on pnire 186. ep Fig. 298.— Cross section from the same series as that from which fig. 297 was drawn, after RABL. d, Epithelium of the intestine ; vm, visceral, pm, parietal middle layer ; hhg, posterior, vhg, anterior mesocarJium ; end, endocardium ; h, cavity of the heart ; Ih, ventral part of the body-cavity ; ?j>, epidermis. THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 545 mk* ak iklhmk'hdf Fig. 299. — Three diagrams to illustrate the formation of the heart in the Chick. n, Neural tube ; m, mesen- chyma of the head ; d, in- testinal cavity ; df, folds of the intestinal plate [splanchnopleure], in which the endothelial sacs of the heart are established ; h, en- dothelial sac of the heart ; ch, chorda ; Ih, body- cavity ; ak, outer, ik. inner germ -layer ; mkl, parietal middle layer ; mk", visceral middle layer, from the thickened portion of which the musculature of the heart is developed ; dn, in- testinal suture, in which the two intestinal folds are fused ; db, part of the ento- blast which has become de- tached from the epithelium of the cephalic portion of the intestine at the intes- tinal suture and lies on the yolk ; + dorsal meso- cardium ; * ventral meso- cardium. A, The youngest stage shows the infolding of the splanch- nopleure, by means of which the cephalic part of the in- testine is formed. In the angles of the intestinal folds the two endothelial sacs of the heart have been esta- blished between the inner germ-layer and the visceral middle layer. B, Somewhat older stage. The two folds (A df) have met in the intestinal suture (dn), so that the two endo- thelial sacs of the heart lie close together in the median plane below the head-gut. C, Oldest stage. The part of the entoblast which lines the head-gut (d) has become separated at the intestinal suture (B dn) from the re- maining part of the ento- blast, which (db) lies upon the yolk, so that the two eudothelial sacs of the heart are in contact ; they subsequently fuse. They lie in a cardiac suspensorium formed by the visceral middle layers, the mesocardium, on, which one can distinguish an upper [dorsal] and an under part — mesocardium superius(+)and inferius (*). By means of this mesocardium the primitive body-cavity is temporarily divided into two portions. h db h mk* Ih 35 546 EMBRYOLOGY. surround a small cavity, the primitive cardiac cavity (Ji). These cells assume more of an endothelial character. While the intestinal folds grow toward each other, the two endothelial tubes become enlarged and push the thickened part of the visceral middle layer before them, so that the latter forms a low, ridge-like elevation into the primitive body-cavity. In the embryos of higher Vertebrates also, just as in the Amphibia, this stretches forward into the embryonic fundament as far as the last visceral arch, and has here received the special name of neck-cavity or parietal cavity. In older embryos (fig. 299 B) the edges of the two folds have met in the median plane, and consequently the two cardiac tubes have moved close together. A process of fusion then takes place between the corresponding parts of the two intestinal folds. First the entoblastic layers fuse, and in this way is produced (fig. 2995) beneath the chorda dorsalis (ch) the cavity of the head-gut (d), which then detaches itself from the remaining part of the ento blast (fig. 299 C db) ; the latter is left lying on the yolk and becomes the yolk-sac. Under the cavity of the head-gut the two cardiac sacs have come close together, so that their cavities are separated from each other by their own endothelial walls only. By the break- ing through of these there soon arises from them (h) a single cardiac tube. On the side toward the body-cavity this is covered by the visceral middle layer (mk2), the cells of which are distinguished in the region of the fundament of the heart by their great length and furnish the material for the cardiac musculature, while the inner endothelial membrane becomes only the endocardium. The whole fundament of the heart lies, as in the Amphibia, in a ventral mesentery, the upper [dorsal] part of which, extending from the heart to the head-gut (fig. 299 G + ), can here also be called the dorsal suspensory of the heart or mesocardium posterius, and the lower [ventral] part (*) mesocardium anterius. In the Chick, when the cardiac tube begins to be elongated and bent into an S- shaped form, the mesocardium anterius quickly disappears. Similar conditions are furnished by cross sections through Rabbit embryos 8 or 9 days old. In the latter the paired fundaments of the heart are indeed developed still earlier and more distinctly than in the Chick, even at a time when the entoderm is still spread out flat and has not yet begun to be infolded. Upon cross sections one sees (fig. 301), in a small region at some distance from the median plane, the splanchnopleure separated from the somatopleure by a small fissure (ph), which is the front end of the primitive body-cavity. At THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 547 this place the visceral middle layer (ahh) is also raised up somewhat from the entoderm (sw), so that it causes a projection into the body- cavity (ph). Here there is developed between the two layers a small cavity, which is surrounded by an endothelial membrane (ihh), the primitive cardiac sac. At their first appearance the halves of the heart lie very far apart. They are to be seen both in the very slightly magnified cross section (fig. 300) and also in the surface view of an embryo Rabbit (fig. 302) at the place indicated by h. They iff Fig. 301. Figs. 300, 301. — Cross section through the head of an embryo Raboit of the same age as that shown in fig. 302. From KOLLIKER. Fig. 301 is a part of fig. 300 more highly magnified. Fig. 300. — h, h', Fundaments of the heart ; sr, cesophageal groove. Fig. 301. — rf, Dorsal groove ; mp, medullary plate ; rw, medullary ridge ; h, outer germ-layer ; dd, inner germ-layer ; dd', its chordal thickening ; sp, undivided middle layer ; hp, parietal, dfp, visceral middle layer ; ph, pericardial part of the body -cavity ; ahh, muscular wall of the heart; ihh, endothelial layer of the heart; mes, lateral undivided part of the middle layer ; sw, intestinal fold, from which the ventral wall of the pharynx is formed. afterwards move toward each other in the same manner as in the Chick by the infolding of the splanchnopleure, and come to lie on the under side of the head-gut, where they fuse and are temporarily attached above and below by means of a dorsal and ventral mesentery. Concerning the processes of development just sketched the question may be raised : What relation do the paired and the unpaired funda- ments of the heart sustain to each other ? It is to be answered to this, that the unpaired fundament of the heart, which is present in the lower Vertebrates, is to be regarded as the original form. The double 648 EMBRYOLOGY. heart-formation, however abeivant it at first sight appears, can be easily referred back to this. A single cardiac tube cannot be developed in the higher Vertebrates, because at the time of its formation a head- gut does not yet exist, but only the fundament of it is formed in the still flat ento- derm. The parts which will subsequently form the ventral wall of the head-gut, and in which the heart is developed, are still two separated terri- tories; they still lie at some distance from the median plane at the right and at the left. If therefore it is necessary for the heart to be formed at this early period, it must arise in the separated regions, which by the process of infolding are joined into a single ventral tract. The vessel must arise as two parts, which, like the two intestinal folds, subsequently fuse. Whether the heart is formed in one way or the other, in either case it has for a time the form of a straight sac lying ventral to the head -gut and composed of two tubes one within the other, which are separated by a large space assumably filled with a gela- tinous matrix. The inner, endothelial tube becomes the endocardium ; the outer tube, which is derived from the visceral middle layer, furnishes the foundation for the myocardium and the pericardial membrane that immediately invests the surface of the heart. ap Fig. 302.— Embryo Rabbit of the ninth day, seen from the dorsal side, after Kiii.i.iKr.R. Mag- nified 21 diameters. The axial (stem-) zone (stz) and the parietal zone (j>z) are to be distinguished. In the former 8 pairs of primitive segments have been formed at the side of the chorda and neural tube. ap, Area pellucida ; rf, dorsal groove; vh, fore brain ; ab, optic vesicle ; mh, mid-brain; hh, hind-brain ; uic, primitive segment ; stz, axial zone ; pz, parietal zone ; h, heart ; ph, pericar- dial part of the body-cavity ; rd, margin of the anterior intestinal portal showing through the overlying struct in t-> ; «/. fuhl «.f tin- nmninn ; vo, vena onphaloiiMMnteiioft, THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 549 (b) The First Developmental Conditions of the Large Vessels. Vitelline Circulation, Allantoic and Placental Circulation. At both ends, in front and behind, the heart is continuous with the trunks of blood-vessels, which have been established at the same time with it. The anterior or arterial end of the cardiac tube is elongated into an unpaired vessel, the truncus arteriosus, which con- tinues the forward course under the head-gut, and is divided in the region of the first visceral arch into two arms, which embrace the head-gut on the right and left and ascend within the arch to the dorsal surface of the embryo. Here they bend around and run back- ward in the longitudinal axis of the body to the tail-end. These two vessels are the primitive aortce (figs. 107, 116 ao); they take their course on either side of the chorda dorsalis, above the entoderm and below the primitive segments. They give off lateral branches, among which the arterice omphalomesentericce are in the Amniota distinguished by their great size. These betake themselves to the yolk-sac and conduct the greatest portion of the blood from the two primitive aortae into the area vasculosa, where it goes through the vitelline circulation. In the Chick, the conditions of which form the basis of the following account (fig. 303), the two vitelline arteries (R.Of.A, L.Of.A) quit the aortas at some distance from their tail-ends, and pass out laterally from the embryonic fundament between entoderm and visceral middle layer into the area pellucida, traverse the latter, and distribute them- selves in the vascular area. They are here resolved into a fine net- work of vessels, which lie, as a cross section (fig. 116) shows, in the mesenchyme between the entoderm and the visceral middle layer, and which are sharply bounded at their outer edge (toward the vitelline area) by a large marginal vessel (fig. 303 8.T), the sinus ter- minalis. The latter forms a ring which is everywhere closed, with the exception of a small region which lies in front, at the place where the anterior amniotic sheath has been developed. From the vascular area the blood is collected into several large venous trunks, by means of which it is conducted back to the heart. From the front part of the marginal sinus it returns in the two vence vitettince anteriores, which run in a straight line from in front backwards and also receive lateral branches from the vascular network. From the hind part of the sinus terminalis the blood is taken up by the venae vitellinas posteriores, of which the one of the left side is larger than the one of the right ; the latter afterwards 550 EMBRYOLOGY. degenerates more and more. From the sides likewise there come still larger collecting vessels, the venae vitellinse laterales. All the vitelline veins of either side now unite in the middle of the embryonic body to form a single large trunk, the vena omphalo- Vitelline area. sx. Fig. 303.— Diagram of the vascular system of the yolk-sac at the end of the third day of incubation, after BALFOUR. The whole blastoderm has been removed from the egg and is represented as seen from below. Hence what is really at the right appears at the left, and vice vcrsd. The part of the area opaca in which the close vascular network has been formed is sharply terminated at its periphery by the sinus terminalis, and forms the vascular area ; outside of the latter lies the vitelline area. The immediate neighborhood of the embryo is free from a vascular net- work, and now, as previously, is distinguished by the name area pellucida. H, Heart; A A, aortic arches; Ao, dorsal aorta; L.Of.A, left, R.Of.A, right vitelline artery; S.T, sinus terminalis ; L.Of, left, K.Of, right vitelline vein ; S. V, sinus venosus ; D.C, ductus Cuvieri ; S.Ca. V, superior, T'.Ca, inferior cardinal vein. The veins are left in outline; the arteries are black. mesenterica (7?.0/*and L.Of), which enters the posterior end of the heart (H). The motion of the blood begins to be visible in the case of the Chick as early as the second day of incubation. At this time the blood is still a clear fluid, which contains only few formed THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 551 components. For the most of the blood-corpuscles still continue to lie in groups on the walls of the tubes, where they constitute the previously described Hood-islands (fig. 114), which cause the red- besprinkled appearance of the vascular area. The contractions of the heart, by which the blood is set in motion, are at first slow and then become more and more rapid. On the average, according to PREYER, the strokes then amount to 130 — 150 per minute. How- ever, the frequency of pulsations is largely dependent upon external influences; it increases with the elevation of the temperature of incubation and diminishes at every depression of it, as well as when the egg is opened for study. At the time when the heart begins to pulsate, no muscle-fibrillse have been demonstrated in the myocar- dium ; from this results the interesting fact that purely proto- plasmic, still undifferentiated cells are in a condition to make strong rhythmical contractions. At the end of the third or fourth day the vitelline circulation in the Chick is at its highest development ; it has undergone some slight changes. We find instead of a single vascular network a double one, an arterial and a venous. The arterial network, which receives the blood from the vitelline arteries, lies deeper, nearer to the yolk, while the venous spreads itself out above the former and is adjacent to the visceral middle layer. The circulating blood is distinguished by the abundance of its blood-corpuscles, the blood- islands having entirely disappeared. The function of the vitelline circulation is twofold. First it serves to provide the blood with oxygen, opportunity for acquiring which is afforded by the whole vascular network being spread out at the surface of the egg. Secondly it serves to bring nutritive substances to the embryo. The yolk-elements below the entoblast are disassociated, liquefied, and taken up into the blood-vessels, by which they are carried to the embryo, where they serve as nutrition for the rapidly dividing cells. Thus far the embryonic body increases in size at the expense of the yolk-material in the yolk- sac, which becomes liquefied and absorbed. The system of vitelline blood-vessels in Mammals agrees in general with that of the Chick, and is distinguished from the latter only in some unimportant points, which do not need to be discussed. How- ever, this question certainly arises • What signification has a vitelline circulation in Mammals (fig. 134 ds) in which the egg is furnished with only a small amount of yolk-material ? Two things are here to be kept in mind ; first, that the eggs of 552 EMBRYOLOGY. Mammals were originally provided with abundant yolk-material, like those of Reptiles (compare p. 222), and, secondly, that the blasto- dermic vesicle, which arises after the process of cleavage, becomes greatly distended by the accumulation within it of a fluid very rich in albumen, furnished by the walls of the uterus. Out of this vesicle likewise the vitclline blood-vessels undoubtedly take up nutritive material and convey it to the embryo, until a more ample nutrition is provided by means of the placenta. In addition to the vitelline blood-vessels there arises in the higher Vertebrates a second system of vessels, which is distributed in the foetal membranes outside the embryo and for a time is more developed than the remaining vessels of the embryo. It serves for the allantoic circulation of Birds and Reptiles and the placental circu- lation of Mammals. When in the Chick the allantois (PI. I., fig. 5 al) is evaginated from the front [ventral] wall of the hind-gut, and as an ever increasing sac soon grows out of the body-cavity through the dermal umbilicus into the ccelom of the blastodermic vesicle between the serosa and the yolk-sac, there appear in its walls two blood-vessels, which grow forth from the ends of the two primitive aortas — the umbilical vessels, or arterice umbilicales. The blood is again collected from the fine capillary network, into which these vessels have been resolved, into the two umbilical veins (venae umbilicales), which, after having arrived at the navel, pass on to the two Cuvierian ducts (see p. 577) and pour their blood into these near the entrance of the latter into the sinus venosus. The terminal part of the right vein soon atrophies, whereas the left receives the lateral branches of the right side and is correspondingly developed into a larger trunk. This now also loses its original connection with the ductus Cuvieri, since it effects with the left hepatic vein (vena hepatica revehens) an anastomosis, which continually becomes larger and finally carries the whole stream of blood. Together with the left hepatic vein the left umbilical vein then empties directly into the sinus venosus at the posterior margin of the liver (HOCHSTETTER). The umbilical and vitelline veins undergo opposite changes in calibre during development: while the vitelline circulation is well developed, the umbilical veins are inconspicuous stems ; afterwards, however, with the increase in the size of the allantois they enlarge, whereas the vense omphalomesentericae undergo degeneration and in the same proportion as the yolk-sac by the absorption of the yolk becomes smaller and loses in significance. THU ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 553 So far as regards the purpose of the umbilical circulation, it subserves in Reptiles and Birds the function of respiration. For the allantois, when it has become larger, in the Chick for example, applies itself closely to the serosa and spreads itself out in the vicinity of the air-chamber and underneath the shell, so that the blood circulating in it can enter into an exchange of gases with the atmospheric air. It loses its importance for respiration in the egg only at the moment when the Chick with its beak breaks through the surrounding embryonic membranes, and breathes directly the air contained in the air-chamber. For the conditions of the circulation are now altered throughout the whole body, since with the begin- ning of the process of respiration the lungs are in a condition to take up a greater quantity of blood, resulting in a degeneration of the umbilical vessels (compare also p. 584). The umbilical or placental circulation in Mammals (fig. 139 A I) plays a still more important role ; for here the two umbilical arteries convey the blood to the placenta. After the blood has been laden in this organ with oxygen and nutritive substances, it flows back again to the heart, at first through two, afterwards through a single umbilical vein (p. 584). B. The further Development of the Vascular System up to the Mature Condition. (a) The Metamoi'phosis of the Tubular Heart into a Heart with Chambers. As has been shown in a preceding section, the heart of a Verte- brate originally has for a short time the form of a straight sac, which sends off" at its anterior end the two primitive aortic arches, while it receives at its posterior end the two omphalomesenteric veins. The sac lies far forward immediately behind the head on the ventral side of the neck (fig. 304 h), in a prolongation of the body-cavity (the parietal or cervical cavity). It is here attached by means of a mesentery of only brief duration, which stretches from the alimentary canal to the ventral wrall of the throat, and which is divided by the cardiac sac itself into an upper [dorsal] and an under part, or mesocardium posterius and anterius. During the first period of embryonic development the heart is distinguished by a very considerable growth, especially in the longi- tudinal direction ; consequently it soon ceases to find the necessary 554 EMBRYOLOGY. room for itself as a straight sac, and is therefore compelled to bend itself into an S-shaped loop (fig. 304). It then takes such a position in the neck that one of the bends of the S, which receives the vitelline veins or, let us say briefly, the venous portion, comes to lie behind and at the left ; the other or arterial portion, which sends off the aortic arches, in front and at the right (fig. 305). But this initial position is soon altered (figs. 305, 313) by the two curves of the S assuming another relation to each other. The venous portion moves headwards, the arterial, on the contrary, in the opposite direc- tion, until both lie approximately in the same transverse plane. At the same time they become turned around the longitudinal axis of the embryo, the venous loop moving dorsally, the arterial, on the contrary, ventrally. Seen from in front [ventral aspect] one hides the other, so that it is only in a side view that the S-shaped cur- vature of the cardiac sac is distinctly recognisable. By the increase in the size of this viscus the anterior part of the body- cavity is already greatly distended, and becomes still more so in later stages, when there is produced a very thin- walled elevation, that projects out to a great distance (figs. 157 ^,314). Inasmuch as the heart completely fills the cavity, and is covered in by only the thin, transparent, and closely applied wall of the trunk, — the membrana reunions inferior of RATHKE, — it appears as though at this time the heart were located entirely outside of the body of the embryo. After the completion of the twisting, there is effected a division of the S-shaped sac into several successive compartments (figs. 306, 308). The venous portion, which has become broader, and the arterial part are separated from each other by a deep constriction (ok) andean now be distinguished as atrium (vh) and ventricle, while the constricted region between the two may be indicated, by a designation introduced Fig 304.— Head of a Chick incubated 58 hours, seen from the dorsal face, after MIHALKOVICS. Mag- nified 40 diameters. The brain is divided into 4 vesicles : pvh, primary fore-brain vesicle ; mh, mid-brain vesicle ; kh, hind- brain vesicle ; nh, after-brain vesicle; au, optic vesicle ; h, heart (seen through the last brain- vesicle) ; vo, vena omphalomesen- terica; us, primitive segment; rm, spinal cord ; x. anterior wall of brain, which is evaginated to form the cerebrum. THE OKGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 555 by HALLER, as auricular canal (ok). The atrium thereby acquires a striking form, since its two lateral walls develop large out-pocketings (ho\ the auricles of the heart (auriculae cordis) ; the free edges of the latter, which in addition soon acquire notches, are turned for- ward, and subsequently enfold more and more the arterial part of the heart, the truncus arteriosus (Ta), and a part of the surface of the ventricle. The auricular canal (fig. 308 ok) is in embryos a well-distinguished narrowed place in the cardiac tube. Owing to the great flattening of its endothelial tube in the sagittal direction, — its walls almost Fig. 306. Fig. 305.— Heart of a human embryo, the body of which was 2-15 mm. long (embryo Lg), after His. [Compare fig. 313.] K, Ventricle ; Ta, truncus cirteriosus ; V, venous end of the S-shapel cardiac sac. Fig. 306. — Heart of a human embryo that was 43 mm. long, neck measurement (embryo £1), after His. k, Ventricle ; Ta, truncus arteriosus ; ok, canalis auricularis ; vh, atihim with the heart-auricles ho (auriciilse cordis). coming into contact, — the passage between atrium and ventricle is reduced to a narrow transverse fissure. It is here that the atrio- ventricular valves are afterwards developed. The fundament of the ventricle at first presents the form of a curved tube (figs. 305, 306 &), which however soon changes its form. For at a very early period there is observable on its anterior [ventral] and posterior surfaces a shallow furrow running from above down- ward, the sulcus interventricularis (fig. 307 si), which allows a left and a right half of the ventricle to be distinguished externally. The latter is the narrower, and is continued upward into the truncus arteriosus (Ta\ the beginning of which is somewhat enlarged and 556 EMBRYOLOGY. designated as bulbus. Between bulbus and ventricle lies a place that is only slightly constricted, called the /return Halleri ; it was recognised even by the older anatomists, then remained for a time little regarded, and now has been again described as noteworthy 'by His. For it marks the place at which subsequently the semilunar valves are established. During the externally visible changes of form, some alterations are also progressing in the finer structure of the walls of the heart. As previously remarked, the fundament of the heart1 consists in the beginning of two sacs, one within the other — an inner endothelial tube lined with flat cells, and an outer muscular sac consisting of cells with abundant protoplasm and derived from the middle germ-layer. The two are completely sepa- rated from each other by a considerable space, which is probably filled with gela- tinous substance. The endothelial tube is in general a tolerably faithful copy of the muscular sac, yet the narrower and wider regions are more sharply marked off from one an- other in the former than in the latter ; " as regards its form, it sustains such a relation to the whole heart as it would if it were a greatly shrivelled, internal cast of it " (His). In the muscular sac distinct traces of muscle-fibres can be recog- nised even at the time when the S-shaped curvature makes its appearance. At later stages in the development differences appear between atrium and ventricle. In the atrium the muscular wall is uniformly thickened into a compact plate, with the inside of which the endothelial tube is in immediate contact. In the ventricle, on the contrary, there occurs a loosening, as it were, of the muscular wall. There are formed numerous small trabeculae of muscular cells, which project into the previously mentioned space between the two sacs and become united to one another, forming a large-meshed network (fig. 31 1 A). The endothelial tube of the heart, by forming out-pocketings, Fig. 307.— Heart of a human embryo of the fifth week, after His. rk, Right, Ik, left ventricle ; si, sulcus interventricu- laris ; Ta, truncus arteriosus ; Iho, left, rho, right auricle of the heart. THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 557 soon comes into intimate contact with the trabeculse, and envelops each one of them with a special covering (His). Thus there arise in the spongy wall of the ventricle numerous spaces lined with endothe.ium, which toward the surface of the heart end blindly, but which communicate with the central cavity and like this receive into them the stream of blood. The embryonic heart of Man and Mammals resembles in its first condition — that which has been described up to this point — the heart of the lowest Vertebrates, the Fishes. In the former as in the latter it consists of a region, the atrium, which receives the venous blood from the body, and of another, the ventricle, which drives the blood into the arterial vessels. Corresponding to this condition of the heart, the whole circulation in embryos of this stage and in Fishes is still a single and a single one. This becomes changed in the evolution of Vertebrates, as in the embryonic life of the individual, with the development of the lungs, upon the, appearance of which a doubling of the heart and of the blood-circulation is introduced. The cause of such a change is clear, from the topographical relation of the two lungs to the heart, the former arising in the immediate vicinity of the heart by evagination of the fore-gut (fig. 314 lg}. The lungs therefore receive their blood from an arterial stem lying very near the heart, from the fifth [sixth] pair of aortic arches that arise from the trtmcus arteriosus. Similarly they give back again the venous pulmonary blood directly to the heart through short stems, the pulmonary veins, which, originally united into a single collecting trunk (BoRN, ROSE), open into the atrium at the left of the great venous trunks. Therefore the blood that flows directly out of the heart into the lungs also flows directly back again to the heart. Herein is furnished the prerequisite for a double circulation. This comes into existence when the pulmonary and the body currents are separated from each other by means of partitions throughout the short course of the vascular system which both traverse in common (viz., atrium, ventricle, and truncus arteriosus). The process of separation begins in the vertebrate phylum with the Dipnoi and Amphibia, in which pulmonary respiration appears for the first time and supplants bronchial respiration. In the amniotic Vertebrates it is accomplished during their embryonic development. Therefore we now have to follow out further the manner in which, in the case of Mammals and especially of Man, according to the recent investigations of His, BORN, and ROSE, the partitions are formed — how atrium and ventricle are each divided into right and 558 EMBRYOLOGY. left compartments, and the truncus arteriosus into arteria pul- monalis and aorta, and how in this way the heart attains its definite form. The partitions arise independently in each of the three divisions of the heart mentioned. Let us first take into consideration the atrium, which is for a time the largest and most capacious region of the cardiac sac (fig. 308). In Man a separation into left and right halves (lv and rv) is observable even in the fourth week, since there is then formed on its hinder [dorsal] and upper wall a perpendicular projection inward, the first trace of the atrial partition (vs) or septum atriorum, The halves are even now distinguished by the fact that they receive different venous trunks. The vitel- line and umbilical veins, as well as the Cuvieriuii ducts to be discussed later, empty their blood into the right compartment, not directly, however, and by means of separate orifices, but after they have united with one another in the vicinity of the heart to form a large venous sinus (sr] — the sinus venosus or s. reunions. This is imme- diately adjacent to the atrium and communicates with it by means of a large opening in its posterior [dorsal] wall, which is flanked on the right and on the left by a large venous valve (*). Only one small vessel, which traverses the musculature of the heart obliquely, opens, near the atrial partition, into the left compartment; it is the previously mentioned unpaired pulmonary vein, which is formed immediately outside the atrium by the union of four branches, two of which come from each of the two wings of the lung now being established. In the further course of development the atrial partition grows Fig-. 308.— Heart of a human embryo 10 mm. long, neck measurement ; posterior [dorsal] half of the heart, the front walls of which have been removed. After II;>. tt, Partition of the ventricle ; Ik, left, rk, right ven- tricle ; ok, auricular canal ; lv, left, rv, right atrium ; .', later condition. After GEOENBAUR. ink, Membranous valve ; mk\ the primitive part of the same ; cht, chordae tendinese ; v, cavity of the ventricle ; b, trabecular network of cardiac musculature ; pm, papillary muscles ; tc, trabeculse carneae. of muscular trabeculse, which are invested by the endocardium and the interstices of which communicate with the small central cavity (fig. 311 A). Such a spongy condition of the wall of the heart persists permanently in Fishes and Amphibia ; in the higher Verte- brates and Man, on the contrary, metamorphoses occur. Toward its external surface the wall of the heart becomes more compact, in that the muscular trabeculse become thicker and the spaces between them narrower, in some parts even disappearing entirely (fig. 311 B tc). The reverse of this process takes place toward the inside. In the vicinity of the atrioventricular opening the trabeculse become thinner and the interstices larger. In this way a part of the thick wall of the ventricle, which looks toward the atrium and encloses the opening, is undermined, as it were, by the blood-current. In this part the muscle-fibres afterwards become entirely rudimentary; THE ORGANS OF THE INTERMEDIATE LAYER OR MESEN CHYME. 563 there are formed from the interstitial connective-tissue substance tendinous plates, which with the endocardial cushions attached to their margins become the permanent atrioventricular valves {tig. 311 B mk). The latter therefore arise from a part of the spongy wall of the ventricle. The remnants of the shrivelled muscular trabecula? (fig. 311 B cht), which are attached to the valve from below, become still more rudimentary in the immediate vicinity of the attachment : here also a part of the muscular fibres disappears entirely ; the connective tissue, on the contrary, is preserved, and is converted into the tendinous cords which, known under the name of ckordce tendinece, serve to hold in place the valves. At some distance from the latter the trabeculse projecting into the ventricle preserve their fleshy con- dition and become the papillary muscles (pm), from the apices of which the chordae tendinese arise. " Whatever of the primitive trabecular network still persists on the inner surface of the ventricle forms a more or less stout meshwork of muscles, the fleshy pillars of the heart (tc), or trabeculse carneEe." In consequence of all these alterations the originally small cavity of the ventricle has become considerably enlarged at the expense of a part of its spongy wall. For the whole of the space which in fig. 311 B lies below the valves has been produced from the system of originally narrow spaces (fig. 311 A), and has been employed for the enlargement of the central cavity by the degeneration of the fleshy •columns into slender tendinous cords. It still remains for us to investigate the division of the truncus arteriosus and the final metamorphosis of the atrium. At about the time when the formation of the partition in the ventricle takes place, the truncus arteriosus, which arises from it, "becomes somewhat flattened, and thus acquires a fissure-like lumen. 'On the flat sides two ridge-like thickenings make their appearance (fig. 310 A and B s), grow toward each other, and by their fusion divide the cavity into two passages which are triangular in cross section. Now, too, the beginning of the internal separation makes itself visible externally as two longitudinal furrows, in the same way that the formation of a partition in the ventricle is indicated by the sulcus interventricularis. The two canals resulting from the division are the aorta and the pulmonary artery (Ao and Pu). For ti time they continue to be surrounded by a common adventitia, then they become widely separated and also externally detached from each other. The whole process of separation in the truncus arteriosus 564 EMBRYOLOGY. takes place independently of the development of a partition in the ventricle, beginning as it does at first above and advancing from there downwards. Finally the aortic septum penetrates also into the cavity of the ventricle itself (tig. 310 B s and ks), there unites with the independently developed ventricular partition, furnishes the part known as pars membranacea (Oi), and thus completes the separation of the vessels leading out from the heart, the aorta falling to the lot of the left ventricle, the art. pulmonalis to the right. The pars membranacea indicates therefore in the finished heart the place at which the separation between the right and left halves of the heart is completed (fig. 310 B Oi}. " It is, as it were, the keystone in the final separation of the primitive simple cardiac sac into the four secondary cardiac cavities, as they are formed in Birds- and Mammals" (ROSE). From a comparative-anatomical point of view this place presents a special interest from the fact that in Reptiles there exists here a permanent opening between the two ventricles, the foramen Pannizzae. •Even before the division of the truncus Fig. 312.— Diagram of the ar- arteriosus, the semiluiiar valves have become rangement of the arterial 77-77 /* -7 • .- r valves. From GEGENBAUR. established as four ridges, consisting of A, Undivided tmncus arteriosus gelatinous tissue with a covering of endo- with four fundaments of 7 ' . . valves, a, Division into pui- thelium, at the contracted place which is monaiis (p) and aorta (a), designated as the /return Halleri. Two of each of which possesses three 11-1*1 • PIT- valves. them are halved at the time or the divi- sion of the truncus into aorta and art- pulmonalis. For each vessel, therefore, there are now three ridges, which, owing to a shrivelling of the gelatinous tissue, assume the form of pockets. Their arrangement, to which GEGENBAUR has called attention, is intelligible from their method of development, as the accompanying diagram (fi£. 312) shows. "By the division of the originally single bulbus arteriosus (A) into two canals (j5), the nodule-like fundaments of the four original valves are distributed in such a manner that the anterior [ventral] one and the anterior halves of the two lateral ones fall to the anterior arterial trunk (pulmonalis), the posterior and the posterior halves of the lateral ones to the posterior arterial trunk (aorta)." Finally, as regards the atrium, it is to be said that the sinus venosus, mentioned at p. 558, the mouth of the pulmonary vein, and the foramen ovale undergo important alterations. The sinus venosus disappears as an independent structure, since it THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 565 is gradually merged into the wall of the atrium. In consequence of this the great venous trunks, which originally emptied their blood into it and which have meanwhile been converted into the superior and inferior venae cavse and into the sinus coronarius (the details of which are given in section d), empty directly into the right half of the atrium, and here gradually separate farther and farther from one another. Of the two valves which surround, as was previously stated, the mouth of the sinus venosus, the left becomes rudimentary (figs. 308, 309) ; the right (*), on the contrary, persists at the mouth of the inferior vena cava and of the sinus coronarius, and is divided, corresponding to these, into a larger and a smaller portion, of which the former becomes the valvula Eustachii, the latter the valvula Thebesii. The four pulmonary veins are united for a time into a common short trunk, which empties into the left half of the atrium. Sub- sequently the common terminal portion becomes greatly enlarged and merged with the wall of the heart, in the same way as the sinus venosus does. In consequence the four pulmonary veins then open .separately and directly into the atrium. The foramen ovale, the formation of which was previously described, maintains a broad communication between the two sides of the atrium during the entire embryonic life. It is bounded behind and below by the at rial partition, a connective-tissue mem- brane that subsequently receives the name of valvula foramini.s ovalis (fig. 309 si). Also from above and in front there is formed a -sharp limitation, since a muscular ridge projects inward from the atrial partition, the anterior atrial crescent or the limbus Vieussenii (vi). Even in the third month all of these parts are distinctly developed; the valvula foraminis ovalis already reaches nearly to the thickened margin of the anterior muscular crescent, but is deflected obliquely into the left half of the atrium, so that a broad fissure remains open and permits the blood of the inferior. vena cava to enter into the left part of the atrium. After birth the margins of the anterior and posterior folds come into contact, and, with occasional exceptions, fuse completely. The posterior fold furnishes the membranous partition of the foramen ovale ; the anterior, with its thickened muscular margin, produces above and in front the limbus Vieussenii. With this the heart has attained its permanent structure. While the cardiac sac undergoes these complicated differentiations, it changes its position in the body of the embryo and acquires at an 566 EMBRYOLOGY. early period a special investment, the pericardium. In connection with the latter the diaphragm is formed as a partition between the thoracic and abdominal cavities. This is consequently the most suitable place at which to acquaint ourselves better with these important processes, a part of which are not easily understood. The most of the discoveries in this field we owe to the investigations of CADIAT, His, BALFOUR, USKOW, and others. M l> (b) The Development of the Pericardial Sac and the Diaphragm. The Differentiation of the Primary Body-cavity into Pericardial, Thoracic, and Abdominal Cavities. Originally the body-cavity is widely extended in the body of the embryo, for it can be traced in the lower Vertebrates into the fun- dament of the head, where it furnishes the cavities of the visceral arches. After the latter have become closed, during which muscles arise from the cells composing their walls, the body-cavity extends forward as far as the last visceral arch and constitutes a large space (fig. 313), in which the heart is developed within the ventral mesentery (mesocardium anterius and posterius). REMAK and KO'L- LIKER named this space throat- cavity] His introduced the name parietal cavity. But it will be most appropriate if one designates it, after the permanent organs which are derived from it, as the peri- cardio - thoracic cavity. The more the cardiac tube i> thrown into curves, the more extensive this cavity becomes, and it soon acquires in the embryo a comparatively enormous size. By this its front wall is protruded ventrally like a hernia between the head and the navel of the embryo (figs. 314, 157). Fig. 313.- Human embryo (L