OWN [$303 ‘@ & i 4 9 Gornell University Library Dthaca, Nem York Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031490521 THE ELEMENTS OF EMBRYOLOGY. THE ELEMENTS OF EMBRYOLOGY BY M. FOSTER, M.A., M.D., LL.D., F.R.S. PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE, FELLOW OF TRINITY COLLEGE, CAMBRIDGE, AND THE LATE FRANCIS M. BALFOUR, M.A., LL.D. F.RS., LATE FELLOW OF TRINITY COLLEGE, CAMBRIDGE, AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY. EDITED BY ADAM SEDGWICK, M.A., FELLOW AND ASSISTANT LECTURER OF TRINITY COLLEGE, CAMBRIDGE, AND WALTER HEAPE, LATE DEMONSTRATOR IN THE MORPHOLOGICAL LABORATORY OF THE UNIVERSITY OF CAMBRIDGE. London: MACMILLAN AND CO. AND NEW YORK. 1889 [4 Rights reserved.] x PUY Ese A.9 56 First Edition 1874. Second Edition revised 1883. Reprinted 1889. PREFACE TO THE SECOND EDITION. Wuen this little work first appeared, it was put for- ward as a Part I, to be followed by other Parts. That plan was however soon abandoned. Nevertheless the volume seemed to have a place of its own; and my dear lost friend undertook to prepare a second edition, in- tending to add some account of the development of the Mammal with a view of making the work an elementary introduction to vertebrate embryology more particularly suited for medical students. He was occu- pied with the task at the time of his sad death; and indeed a melancholy interest is attached to. some of the sheets, by the fact that he had taken them to Switzer- land with him, on that fatal journey. All the first part up to p. 160 he had passed for press; and he had further revised up to about p. 202. The whole of the rest of the volume has been under- vi PREFACE. taken by Mr Adam Sedgwick and Mr Walter Heape. They have attempted to carry out as far as possible what we believe to have been Balfour’s views, and trust that the public will judge leniently of their efforts to perform a difficult task. I have myself been able to do no more than offer general advice from time to time; and though it has not been thought advisable to change the title, the merits as well as the responsi- bilities of the latter part of the work must rest with them. M. FOSTER. TRINITY CoLLEGE, CamBRIDGE, March, 1883. TABLE OF CONTENTS. PART I. THE HISTORY OF THE CHICK. CHAPTER I. Tue Srructure or THE Hen’s Eac, aND THE CHANGES WHICH TAKE PLACE UP TO THE BEGINNING oF INCUBATION + pp. I—24. The shell and shell-membrane, 1—3. The albumen, 3. The vitelline membrane, 4. The yolk, s—7. Area opaca, 7. Area pellucida, 8. The structure of the blastoderm, 7—10. Recapitu- lation, 10. The ovarian ovum, 11—15. The descent of the ovum along the oviduct, 15—17. Impregnation, 17. Segmentation, * 18—24. CHAPTER II. Brier SumMMaRyY oF THE WHOLE History or IncuBaTion, PP. 25—47- The embryo is formed in the area pellucida, 25. The germinal layers, 25, 26. The extension of the blastoderm over the yolk, 26. The vascular area, 27. The head-fold, 27—36. The tail-fold, 37. The lateral folds, 37. The yolk-sac, 37. The alimentary canal, 39. The neural tube, 39, 40. The body-cavity, 41. The somatopleure, 41. The splanchnopleure, 42. The stalk of the yolk-sac, 42, 43. The amnion, 43—46. The allantois, 46, 47. ; Vill TABLE OF CONTENTS. OHAPTER III. Tue CHANGES WHICH TAKE PLACE DURING THE First Day or Incvu- BATION : . . pp- 48—76. Variations in the progress of development, 48, 49. The embryonic shield, 49. Formation of hypoblast, 51. The germinal wall, 52. The primitive streak, 52—54. Formation of primitive streak meso- blast, 54, 55. Hypoblastic mesoblast, 55. Primitive groove, 56, 57. The notochord, 597—62. The medullary groove, 62, 63. Amnion, 63. The changes taking place in the three layers, 63—66. The germinal wall, 65, 66. The increase of the head-fold, 66. The closure of the medullary canal, 66, 67. The cleavage of the mesoblast ; formation of spanchnopleure and somatopleure, 68. The vertebral and lateral plates, 69. The mesoblastic somites, 70. The sinus rhomboidalis, 71. The neurenteric passage, 71—74. Formation of the vascular area, 74,75. Recapitulation, 75, 76. CHAPTER IV. Tur CHANGES WHICH TAKE PLACE DURING THE First Hanr or THE : Sreconp Day . 7 - pp. 77—95. Increasing distinctness and prominence of embryo, 77. The first cerebral vesicle, 78, 79. The auditory pits, 81. Increase in number of mesoblastic somites, 81. The fore-gut, 82. The heart, 82—89. The vascular system, 89—94. Formation of blood-vessels, 92—94. The rudiment of the Wolffian duct, 94. Summary, 94, 95. CHAPTER V. Tae CHANGES WHICH TAKE PLACE DURING THE SEconD Har or THE Szconp Day . Fs 7 pp. 96—108. Increasing prominence of the embryo; the tail-fold and lateral folds, 96. Continued closure of medullary canal, 96—98. The brain, 98—101. The optic vesicles, 98. The second and third cerebral vesicles, roo. The cerebral hemispheres, 100. First appearance of cranial nerves, 100, 1o1. The notochord, ror. The cranial flexure, 101. The auditory vesicle, 101. Increase of curvature of heart, ror, TABLE OF CONTENTS. 1x 102. PP. 423—471. Incubators, 423—425. Hardening reagents, 425—428. Staining reagents, 428—432. Imbedding, 432—434. Cutting sections, 434, 433° Mounting sections, 436. Preservation of embryos as a whole, 430. 437: X1V TABLE OF CONTENTS. Practical directions for obtaining and studying chick embryos, 437 —460. Examination of a 36—48 hours embryo, 437—444. Of an embryo of about 48—so hours, 444—447- Of an embryo at the end of the 3rd day, 447—451. Of an embryo of the 4th day, 451—453- Of a blastoderm of 20 hours, 453—456. Of an unincubated blasto- derm, 457. Of the process of segmentation, 458. Of the later changes of the embryo, 459. Of the development of blood-vessels, 459, 460. Practical directions for obtaining and studying Mammalian em- bryos, 460—471. Animals and breeding, 460, 461. Examination and treatment of segmenting ova, 461—464. Of the blastodermic vesicle, 72—90 hours, 465. Of the blastodermic vesicle of 7 days, 465, 466. Of an 8 days embryo, 466—468. Of an embryo of 8 days 12 hours, 468, 469. Of the foetal membranes of an embryo of 14 days, 469, 470. Note A. Automatic microtome, 471. Nore B. New method of mounting sections, 471. PART I. THE HISTORY OF THE CHICK. CHAPTER I. THE STRUCTURE OF THE HEN’S EGG, AND THE CHANGES WHICH TAKE PLACE UP TO THE BEGINNING OF IN- CUBATION. In a hen’s egg quite newly laid we meet with the following structures. Most external is the shell (Fig. 1, s.), composed of an organic basis, impregnated with calcic salts. It is sufficiently porous to allow of the interchange of gases between its interior and the exter- nal air, and thus the chemical processes of respiration, feeble at first, but gradually increasing in intensity, are carried on during the whole period of incubation. It is formed of two layers, both of which may contain pigment. The inner layer is by far the thickest, and is perforated by vertical canals which open freely on its inner aspect. Superficially these canals appear to be closed by the extremely thin outer layer. They are probably of some importance in facilitating the pene- tration of air through the shell. Lining the shell, is the shell-membrane, which is double, being made up of two layers: an outer thicker BF. & B. 1 2 THE HEN’S EGG. [CHAP. (Fig. 1, s. m.), and an inner thinner one (7. s.m.). Both of these layers consist of several laminz of felted fibres of various sizes, intermediate in nature between connec- tive and elastic fibres. DiacramMMatic SECTION OF AN UNINCUBATED Fowl’s Eee ° (modified from Allen Thomson). bi. blastoderm. w. y. white yolk. This consists of a central flask-shaped mass and a number of layers arranged con- centrically around this. y. y. yellow yolk. ». ¢. vitelline membrane. w. layer of more fluid albumen immediately surrounding the yolk. w. albumen consisting of alternate denser and more fluid layers. ch. 2. chalaza. a, ch. air- chamber at the broad end of the egg. This chamber is merely a space left between the two layers of the shell-mem- brane, 7. s. m. internal layer of shell-membrane. s. m. external layer of shell-membrane. s. shell. 1.] THE WHITE OF THE EGG. 3 ‘Over the greater part of the egg the two layers of the shell-membrane remain permanently in close appo- sition ; but at the broad end they tend to separate, and thus to develope between them a space into which air finds its way. This air-chamber, as it is called, is not to be found in perfectly fresh eggs, but makes its appearance in eggs which have been kept for some time, whether incubated or not, and gradually increases in size, as the white of the egg shrinks in bulk from evaporation. Immediately beneath the shell-membrane is the white of the egg or albumen (Fig. 1, w.), which is, chemi- cally speaking, a mixture of various forms of proteid material, with fatty, extractive, and saline bodies. The outer part of the white, especially in eggs which are not perfectly fresh, is more fluid than that nearer the yolk. Its average composition may be taken as ‘12-0 p. c. proteid matter, 15 p.c. fat and extractives, 5 p. c. saline matter, chiefly sodic and potassic chlorides, with phosphates and sulphates, 86:0 p. c. water. The white of the egg when boiled shews in section alter- nate concentric layers of a transparent and of a finely granular opaque material. In the natural condition, the layers corre- sponding to these opaque layers are composed of more fluid albumen, while those corresponding to the transparent layers are less fluid, and consist of networks of fibres, containing fluid in their meshes. The innermost layer, however, immediately surrounding the yolk (Fig. 1, 2.), is of the more fluid finely granular kind. In eggs which have been hardened a spiral arrange- ment of the white may be observed, and it is possible to 1—2 4 THE HEN’S EGG. [CHAP. tear off laminz in a spiral direction from left to right, from the broad to the narrow end of the egg. Two twisted cords called the chalazw (Fig. 1, ch. 1), composed of coiled membranous layers of denser albu- men, run from the two extremities of the egg to the opposite portions of the yolk. Their inner extremities expand and merge into a layer of denser albumen sur- rounding the fluid layer next the yolk. Their outer extremities are free, and do not quite reach the outer layer of the white. Thus they cannot serve to suspend the yolk, although they may help to keep it in position, by acting as elastic pads. The interior of each chalaza presents the appearance of a succession of opaque white knots; hence the name chalaze (hailstones). The yolk is enclosed in the vitelline membrane (Fig. 1, v.¢.), a transparent somewhat elastic membrane easily thrown into creases and wrinkles. It might almost be called structureless, but under a high power a fine fibrillation is visible, and a transverse section has a dotted or punctuated appearance ; it is probably there- fore composed of fibrils.‘ Its affinities are with elastic connective tissue. The whole space within the vitelline membrane is occupied by the yolk. To the naked eye this appears ‘tolerably uniform throughout, except at one particular point of its surface, at which may be seen, lying imme- diately under the vitelline membrane, a small white disc, about 4 mm. in diameter. This is the blastoderm, or cicatricula, A tolerably typical cicatricula in a fecundated egg will shew an outer white rim of some little breadth, and within that a circular transparent area, in the centre of 1.] THE WHITE YOLK. 5 which, again, there is an opacity, varying in appearance, sometimes homogeneous, and sometimes dotted. The disc is always found to be uppermost whatever be the position of the egg, provided there is no restraint to the rotation of the yolk. The explanation of this is to be sought for in the lighter specific gravity of that portion of the yolk which is in the neighbourhood of the disc, and the phenomenon is not in any way due to the action of the chalaze. A section of the yolk of a hard-boiled egg will shew that it is not perfectly uniform throughout, but that there is a portion of it having the form of a flask, with a funnel-shaped neck, which, when the egg is boiled, does not become so solid as the rest of the yolk, but remains more or less fluid. The expanded neck of this flask-shaped space is situated immediately underneath the disc, while its bulbous enlargement is about in the middle of the yolk. We shall return to it directly. The great mass of the yolk is composed of what is known as the yellow yolk (Fig. 1, y. y.). This consists of spheres (Fig. 2, A.) of from 25 to 100 in diameter filled with numerous minute highly refractive granules ; these spheres are very delicate and easily destroyed by crushing. When boiled or otherwise hardened in situ, they assume a polyhedral form, from mutual pressure. The granules they contain seem to be of an albuminous nature, as they are insoluble in ether or alcohol. Chemically speaking the yolk is characterized by the presence in large quantities of a proteid matter, having many affinities with globulin, and called vitellin. This exists in peculiar associa- 1 »=:001 mm. 6 THE HEN’S EGG. (CHAP. tion with the remarkable body Lecithin. (Compare Hoppe- Seyler, Hab. Phys. Chem. Anal.) Other fatty bodies, colouring matters, extractives (and, according to Dareste, starch in small quantities), &c. are also present. Miescher (Hoppe-Seyler, Chem. Untersuch. p. 502) states that a considerable quantity of nuclein may be obtained from the yolk, probably from the spherules of the white yolk. Fie, 2. A. Yellow yolk-sphere filled with fine granules. The outline of the sphere has been rendered too bold. B. ‘White yolk-spheres and spherules of various sizes and pre- senting different appearances. (It is very difficult in a woodcut to give a satisfactory representation of these pe- culiar structures.) The yellow yolk, thus forming the great mass of the entire yolk, is clothed externally by a thin layer of a different material, known as the white yolk, which at the edge of the blastoderm passes underneath the disc, and becoming thicker at this spot forms, as it were, a bed on which the blastoderm rests. Immediately under the middle of the blastoderm this bed of white yolk is connected, by a narrow neck, with a central mass of similar material, lying in the middle of the yolk (Fig. 1, w. y.). When boiled, or otherwise hardened, the white yolk does not become so solid as the yellow yolk; hence the appearances to be seen in sections of the hardened yolk. The upper expanded extremity of this neck of 1] THE YELLOW YOLK. 7 white yolk is generally known as the “nucleus of Pander.” Concentric to the outer enveloping layer of white yolk there are within the yolk other inner layers of the same substance, which cause sections of the hardened yolk to appear to be composed of alternate concentric thicker lamine of darker (yellow) yolk, and thinner lamin of lighter (white) yolk (Fig. 1, w, y.). The microscopical characters of the white yolk elements are very different from those of the yellow yolk. Itis composed of vesicles (Fig. 2, B.) for the most part smaller than those of the yellow yolk (4u—75y), with a highly refractive body, often as small as ly, in the interior of each; and also of larger spheres, each of which contains a number of spherules, similar to the smaller spheres. Another feature of the white yolk, according to His, is that in the region of the blastoderm it contains numerous large vacuoles filled with fluid; they are sufficiently large to be seen with the naked eye, but do not seem to be present in the ripe ovarian ovum. It is now necessary to return to the blastoderm. In this, as we have already said, the naked eye can distin- guish an opaque white rim surrounding a more trans- parent central area, in the middle of which again is a white spot of variable appearance. In an unfecundated cicatricula the white disc is simply marked with a number of irregular clear spaces, there being no proper division into a transparent centre and an opaque rim. The opaque rim is the commencement of what we shall henceforward speak of as the area opaca; the central transparent portion is in the same way the 8 THE HEN’S EGG. [CHAP. beginning of the area pellucida. In the part corre- sponding to the area opaca the blastoderm rests imme- diately on, the white yolk; underneath the area pellu- cida is a shallow space containing a nearly clear fluid, to the presence of which the central transparency seems to be due. The white spot in the middle of the area pellucida appears to be the nucleus of Pander shining through. Vertical sections of the blastoderm shew that it is formed of two layers. The upper of these two layers is composed, see Fig. 3, ep, of a single row of cells, with their long axes arranged vertically, adhering together so as to form a distinct membrane, the edge of which rests upon the white yolk. After staining with silver nitrate, this membrane viewed from above shews a mosaic of uniform polygonal cells. Each cell is composed of granular protoplasm filled with highly refractive globules; and in each an oval nu- cleus may be distinguished. They are of a nearly uniform size (about Y x) over the opaque and the pellucid areas. The under layer (Fig. 3, ), is composed of cells which vary considerably in diameter; but even the smaller cells of this layer are larger than the cells of the upper layer. They are spherical, and so filled with granules and highly refractive globules, that a nucleus can rarely be seen in them: in the larger cells these globules are identical with the smaller white yolk spheres. The cells of this layer do not form a distinct mem- brane like the cells of the upper layer, but lie as a somewhat irregular network of cells between the upper layer and the bed of white yolk on which the blastoderm 1] THE BLASTODERM. rests. The lowest are generally the largest. The layer is thicker at the peri- phery than at the centre: and rests on a bed of white yolk, from which it is in parts separated by a more or less de- veloped cavity, containing probably fluid yolk matter about to be absorbed. In the bed of white yolk nuclei are present, which are destined to become the nuclei of cells about to join the lower layer of the blastoderm. These nuclei are gene- rally more numerous in the neighbour- hood of the thickened periphery of the blastoderm than elsewhere. Amongst the lower layer cells are to be found Fia. 3, SEction of A BLASTODERM OF A Fow.’s Eae AT THE COMMENCEMENT OF INCUBATION. The thin but complete upper layer ep com- posed of columnar cells rests on the in- complete lower layer 7, composed of larger and more granular cells. The lower layer is thicker in some places than in others, and is especially thick at the periphery. The line below the under layer marks the upper surface of the white yolk. The larger so-called formative cells are seen at }, lying on the white yolk. The figure does not take in quite the whole breadth of the blastoderm ; but the reader must under- stand that both to the right hand and the left ep is continued farther than /, so that at the extreme edge it rests directly on the white yolk. 10 THE HEN’S EGG. [cHaP. peculiar large spherical bodies, which superficially re- semble the larger cells around them, and have been called formative cells. Their real nature is still very doubtful, and though some are no doubt true célls, others are perhaps only nutritive masses of yolk. The opacity of the peripheral part of the blastoderm is in a large measure due to the collection of the lower layer cells in this region, and the thickening, so caused, appears to be more pronounced for a small are which subsequently constitutes the hinder border of the area pellucida. Over nearly the whole of the blastoderm the upper layer rests on the under layer. At the circumference however the upper layer stretches for a short distance beyond the under layer, and here consequently rests directly on the white yolk. To recapitulate:—In the normal unincubated hen’s egg we recognize the blastoderm, consisting of a com- plete upper layer of smaller nucleated granular cells and a more or less incomplete under layer of larger cells, filled with larger granules; in these lower cells nuclei are rarely visible. The thin flat disc so formed rests, at the uppermost part of the entire yolk, on a bed of white yolk, and a peripheral thickening of the lower layer causes the appearance in the blastodermic disc of an area opaca and an area pellucida. The great mass of the entire yolk consists of the so-called yellow yolk composed of granular spheres. The white yolk is composed of smaller spheres of pecu- liar structure, and exists, in small part, as a thin coating around, and as thin concentric lamine in the substance of the yellow yolk, but chiefly in the 1] THE OVARIAN OVUM. ll form of a flask-shaped mass in the interior of the yolk, the upper somewhat expanded top of the neck of which forms the bed on which the blastoderm rests. The whole yolk is invested with the vitelline mem- brane, this again with the white; and the whole is covered with two shell-membranes and a shell. Such an egg has however undergone most important changes while still within the body of the hen; and in order to understand the nature of the structures which have just been described, it will be necessary to trace briefly the history of the egg from the stage when it exists as a so-called ovarian ovum in the ovary of a hen up to the time when it is laid. In birds the left ovary alone is found in the adult ; and is attached by the mesovariwm to the dorsal wall of the abdominal cavity, on the left side of the vertebral column. It consists of a mass of vascular stroma in which the ova are imbedded, is covered superficially by a layer of epithelium, continuous with the epithelial lining of the peritoneal cavity. The appearance of the ovary varies greatly according to the age of the indi- vidual. In the mature and sexually active females it is almost wholly formed of pedunculated and highly vascular capsules of various sizes, each containing a more or less developed ovum; in the young animal however it is much more compact, owing to the absence of advanced ova. If one of the largest capsules of the ovary of a hen which is laying regularly be opened, it will be found to contain a nearly spherical (or more correctly, ellipsoidal with but slightly unequal axes) yellow body enclosed in a delicate membrane. This is the ovarian ovuin or egg. 12 THE HEN’S EGG. [CHAP. Examined with care the ovum, which is tolerably uni- form in appearance, will be found to be marked at one spot (generally facing the stalk of the capsule and form- ing the pole of the shorter axis of the ovum) by a small dise differing in appearance from the rest of the ovum. This disc which is known as the germinal disc or discus Fic. 4. Oe Ww, Yy- SECTION THROUGH THE GERMINAL Disc OF THE RIPE OVARIAN Ovum oF a Fowl WHILE YET ENCLOSED IN ITS CAPSULE. a. Connective-tissue capsule of the ovum. 8. follicular epithe- lium, at the surface of which nearest the ovum lies the vitelline membrane. c¢. granular material of the germinal disc, which becomes converted into the blastoderm. (This is not very well represented in the woodcut. In sections which have been hardened in chromic acid it consists of fine granules.) w. y, white yolk, which passes insensibly into the fine granular material of the disc. «, germinal vesicle enclosed in a distinct membrane, but shrivelled up by the action of the chromic acid. y, space originally completely filled up by the germinal vesicle, before the latter was shrivelled up by the action of the chromic acid. proligerus, consists of a lenticular mass of protoplasm (Fig. 4, c), imbedded in which is a globular or ellipsoidal body (Fig. 4, 7), about 310% in diameter, called the germinal vesicle. This has a delicate wall, and its con- tents are clear and fluid in the fresh state, but become granular upon the addition of reagents. 1] THE OVARIAN OVUM. 13 The rest of the ovum is known as the yolk. This consists of two elements, the white yolk- and the yellow yolk-spheres, which are distributed respectively very much in the same way as in the laid egg, the yellow yolk forming the main mass of the ovum, and the white yolk being gathered underneath and around the disc (Fig. 4, w. y), and also forming a flask-shaped mass in the interior. The delicate membrane surrounding the whole is the vitelline membrane. The youngest ova in the ovary of a fowl, in common with those of all other animals, present the characters of a simple cell, Such a cell is diagrammatically repre- sented in Fig. 5. It is seen to consist of a naked protoplasmic body containing in its interior a nucleus—-the germinal vesi- cle—which in its turn envelopes Fie. 5, a nucleolus—constituting what is known as the germinal spot. Such young ova are enclosed in a capsule of epithelium, named the follicle or follicular mem- brane, and are irregularly scat- tered in the stroma of the ovary. DIAGRAM OF THE The difference between such Ovom. (From Gegen- an immature ovum and the ripe baur.) ovum just described is very great, a Gren Pre Chet throughout its growth the plasm. b. Nucleus (ger- : minal vesicle). ¢. Nu- ovum retains the characters of a cleolus (germinalspot). cell, so that the mature ova- rian ovum, equally with the youngest ovum in the ovary, is a single cell. The most striking changes which takes place in the 14 THE HEN’S EGG. [CHAP. course of the maturation of the ovum concern the body of the cell rather than the germinal vesicle. As the body grows in size a number of granules make their appearance in its interior. These granules are formed by the inherent activity of the protoplasm, which is itself nourished, in a large measure at any rate, by the cells of the follicle. The outermost layer of the proto- plasm remains free from these granules. As the ovum grows older the granules become larger, first of all in the centre, and subsequently at the periphery, and take the form of white yolk-spherules. The greater part of them become at a later stage converted into yellow yolk-spheres, while a portion of them, situated in the position of the white yolk of the ripe ovum, retain their original characters. The germinal vesicle, which in the youngest ova is situated centrally or subcentrally, travels in the course of the growth of the ovum towards the periphery, and the protoplasm immediately surrounding it remains relatively free from yolk granules, and so constitutes the germinal disc. In the younger ova there is but a single germinal spot in the germinal vesicle, but as the ova enlarge several accessory germinal spots make their appearance, while in the ripe ovum it seems doubtful whether there is any longer a trace of a germinal spot. The cells of the follicular epithelium are at first arranged in a single row, but at a later stage become two or more rows deep: they undergo however a nearly complete atrophy in the ripe ovum. Around the follicular epithelium there is present a membrana propria, and in the later stages of the growth of the 1] THE OVARIAN OVUM. 15 ovum this is in its turn embraced by a highly vascular connective-tissue capsule. The youngest ova are, as has already been stated, quite naked. In ova of about 1°5 mm. the superficial layer of the ovum becomes converted into a radiately striated membrane called the zona radiata. At a later period a second membrane, placed between the zona radiata and the cells of the follicle, makes its appearance, but its mode of origin is still unknown. As the ovum approaches maturity the zona radiata disappears, and in the ripe ovum the second membrane, which has already been spoken of as the vitelline membrane, alone remains. From what has just been stated it follows that in an egg which has been laid the yolk alone constitutes the true ovum. The white and the shell are in fact accessory structures formed during the passage of the ovum down the oviduct. When the ovarian ovum is ripe and about to be discharged from the ovary, its capsule is clasped by the open infundibulum of the oviduct. The capsule then bursts, and the ovum escapes into the oviduct, its longer axis corresponding with the long axis of the oviduct, the germinal disc therefore being to one side. In describing the changes which take place in the oviduct, it will be convenient, following the order pre- viously adopted, to treat first of all of the formation of the accessory parts of the egg. These are secreted by the glandular walls of the oviduct. This organ therefore requires some description. It may be said to consist of four parts:—Ist. The dilated infundibulum 16 THE HEN’S EGG. [CHAP. with an abdominal opening. 2nd. A long tubular portion—the oviduct proper—opening by a narrow neck or isthmus into the 8rd portion, which is much dilated, and has been called the uterus; the 4th part is some- what narrow, and leads from the uterus into the cloaca, The whole of the mucous membrane lining the oviduct is largely ciliated. The accessory parts of the egg are entirely formed in the 2nd and 3rd portions. The layer of albumen which immediately surrounds the yolk is first de- posited; the chalaze are next formed. Their spiral character and the less distinctly marked spiral arrange- ment of the whole albumen is brought about by the motion of the egg along the spiral ridges into which the interior of the second or tubular portion of the oviduct is thrown. The spirals of the two chalazw are in different directions. This is probably produced by their peripheral ends remaining fixed while the yolk to which their central ends are attached is caused to rotate by the contractions of the oviduct. During the formation of the chalaze the rest of the albumen is also deposited ; and finally the shell-membrane is formed in the narrow neck of the 2nd portion, by the fibrilla- tion of the most external layer of albumen. The egg passes through the 2nd portion in little more than 3 hours. In the 3rd portion the shell is formed. The mucous membrane of this part is raised into nume- rous flattened folds, like large villi, containing follicu- lar glands. From these a thick white fluid is poured out, which soon forms a kind of covering to the egg, in which the inorganic particles are deposited. In this portion of the oviduct the egg remains from 12 to 18 L] IMPREGNATION. 17 hours, during which time the shell acquires its normal consistency. At the time of laying it is expelled from the uterus by violent muscular contractions, and passes with its narrow end downwards along the remainder of the oviduct, to reach the exterior. Impregnation. This process occurs in the upper portion of the oviduct; the spermatozoa being found actively moving in a fluid which is there contained. We have as yet, as far asthe fowl is concerned, no direct observations concerning the changes preceding and following upon impregnation ; nor indeed concern- ing the actual nature of the act of impregnation. In other types however these processes have been followed with considerable care, and the result has been to shew that prior to impregnation a division of the ovum takes place into two very unequal parts. The smaller of these parts is known as the polar body, and plays no further part in the development. In the course of the division of the ovum into these two parts the germinal vesicle also divides, and one part of it enters the polar body, while a portion remains in the larger segment which continues to be called the ovum, and is there known as the female pronucleus. Im- pregnation has been found to consist essentially in the entrance of a single spermatozoon into the ovum, followed by the fusion of the two. The spermatozoon itself is to be regarded as a cell, the head of which corresponds to the nucleus. When the spermatozoon enters the ovum the substance forming its tail becomes mingled with the protoplasm of the latter, but the head enlarges and constitutes a distinct body called the male pronucleus, which travels towards and finally fuses with F. & B. 2 18 THE HEN’S EGG. [CHAP. the female pronucleus to constitute the nucleus of the impregnated ovum. Segmentation. There follows upon the impregna- tion a remarkable process known as the segmentation. The process consists essentially in the division of the impregnated ovum by a series of successive segmenta- tions into a number of cells, of which the whole of the cells of the future animal are the direct descendants. In the majority of instances this process results in the division of the whole ovum into cells; but in cases of ova where there is a large amount of food yolk, only that part of the ovum in which the protoplasm is but slightly loaded with food material, and which we have already described as the germinal disc, becomes so divided. The remainder of the ovum constitutes a food reservoir for the use of the developing embryo and is known as the food yolk. The segmentation in such ova, of which that of the fowl is one of the best known examples, is described as being partial or meroblastic’, Tn order to understand the process of segmentation in the fowl’s ovum it must be borne in mind that the germinal disc is not sharply separated from the re- mainder of the ovum, but that the two graduate insen- sibly into each other. The segmentation commences in the lower part of the oviduct, shortly before the shell has begun to be formed. Viewed from above, a furrow is seen to make its 1 For a fuller account of the relation between holoblastic and meroblastic segmentation the reader is referred to the treatise on Comparative Embryology by Balfour, Vol. 1. chapter iii. 1] SEGMENTATION. 19 Fie. 6. Surrace VIEWS OF THE EARLY STAGES OF THE SEGMENTATION In A Fowt’s Eaa. (A and C after Coste.) A represents the earliest stage. The first furrow (b) has begun to make its appearance in the centre of the germinal disc, whose periphery is marked by the line a. In B, the first furrow is completed nearly across the disc, and a second similar furrow at right angles to the first has appeared. The disc thus becomes divided somewhat irregularly into quadrants by four (half) furrows. In a later stage (C) the meridian furrows b have increased in number, from four, as in ZB, to nine, and cross furrows have also made their appearance. The disc is thus cut up into small central (c) and larger peripheral (d) segments. Several new cross furrows are seen just beginning, as ex. gr. close to the end of the line of reference d. appearance, runnin’, across the germinal disc, though not for the whole breadth, and dividing it into two halves (Fig. 6, A). This primary furrow is succeeded by a second at right angles to itself. The surface thus becomes divided into four segments or quadrants (Fig. 6, B). 2-2 20 THE HEN’S EGG. [CHAP. The second furrow cuts the first somewhat excen- trically. The first four furrows do not extend through the whole thickness of the germinal disc, and the four seg- ments marked out by them are not separated from the dise on their lower aspect. Each of these is again bisected by radiating furrows, and thus the number of segments is increased from four to eight (it may be seven or nine). The central portion of each segment is then, by a cross furrow, cut off from the peripheral portion, giving rise to the appearance of a number of central smaller segments, surrounded by more external elongated segments (Fig. 6, C). The excentricity in the arrangement of the segments is moreover still preserved, the smaller segments being situated nearer one side of the germinal disc. The excentricity of the segmentation gives to the segmenting germinal disc a bilateral symmetry, but the relation between the axis of symmetry of the segmenting germinal disc and the long axis of the embryo is not known. Division of the segments now proceeds rapidly by means of furrows running in various directions. And it is important to note that the central segments divide more rapidly than the peripheral, and con- sequently become at once smaller and more numerous (Fig. 7). Meanwhile sections of the hardened blastoderm teach us that segmentation is not confined to the sur- face, but extends through the mass of the blastoderm; they shew us moreover that division takes place by means of not only vertical, but also horizontal furrows, i.e. furrows parallel to the surface of the disc (Fig. 8). 1] SEGMENTATION. 21 Fia. 7, Surrace VIEW oF THE GERMINAL Disc oF a HeEn’s Eco DURING THE LATER STAGES OF SEGMENTATION. (Chromic Acid Preparation.) Atcin the centre of the disc the segmentation masses are very small and numerous. At 06, nearer the edge, they are larger and fewer; while those at the extreme margin a are largest and fewest of all. It will be noticed that the radiating furrows marking off the segments a do not reach to the extreme margin e of the disc. The drawing is completed in one quadrant only ; it will of course be understood that the whole circle ought to be filled up in a precisely similar manner. In this way, by repeated division or segmentation, the original germinal disc is cut up into a large number of small rounded masses of protoplasm, which are small- est in the centre, and increase in size towards the peri- phery. The segments lying uppermost are moreover smaller than those beneath, and thus the establishment of the two layers of the blastoderm is foreshadowed. a 22 THE HEN’S EGG. [CHAP. Fie. 8. SECTION OF THE GERMINAL Disc oF A FowL DURING THE LaTER STAGES OF SEGMENTATION. The section, which. represents rather more than half the breadth of the blastoderm (the middle line being shewn at c), shews that the upper and central parts of the disc segment faster than those below and towards the periphery. At the periphery the segments are still very large. One of the larger segments is shewn at a. In the majority of segments a nucleus can be seen; and it seems probable that a nucleus is present in all. Most of the segments are filled with highly refracting spherules, but these are more numerous in some cells (especially the larger cells near the yolk) than in others. In the central part of the blastoderm the upper cells have commenced to form a distinct layer. a, large peripheral cell. 6. larger cells of the lower parts of the blastoderm. c. middle line of blastoderm. e. edge of the blastoderm adjoining the white yolk. w. white yolk. In the later stages of segmentation not only do the first-formed segments become further divided, but seg mentation also extends into the remainder of the germi- nal disc. The behaviour of the nucleus during the segmenta- tion has not been satisfactorily followed, but there is, 1] SEGMENTATION. 23 from the analogy of other forms, no doubt that in the formation of the first two segments the original nucleus, formed by the fusion of the male and female pronuclei, becomes divided, and that a fresh division of the nucleus takes place with the formation of each fresh segment. Nuclei make their appearance moreover in the part of the ovum immediately below that in which the segmen- tation has already taken place; these are in all proba- bility also derived from the primitive nucleus. The substance round some of these nuclei rises up in the form of papille, which are subsequently constricted off and set free as supplementary segmentation masses ; while some of the nuclei remain and form the nuclei already spoken of as existing in the bed of white yolk below the blastoderm in the unincubated egg. Between the segmented germinal disc, which we may now call the blastoderm, and the bed of white yolk on which it rests, a space containing fluid makes its appearance. As development proceeds, segmentation reaches its limits in the centre, but continues at the periphery, and thus eventually the masses at the periphery become of the same size as those in the centre. The distinction however between an upper and a lower layer becomes more and more obvious. The masses of the upper layer arrange themselves, side by side, with their long axes vertical; their nuclei become very distinct. In fact they form a membrane of columnar nucleated cells. The masses of the lower layer, remaining larger than those of the upper layer, continue markedly granular and round, and form rather a close irregular network 24 THE HEN’S EGG. (CHAP. I. than a distinct membrane. Their nuclei are not readily visible. At the time when the segmentation-spheres in the centre are smaller than those at the periphery, and those above are also smaller than those below, a few large spherical masses, probably containing each one of the nuclei already spoken of, arise by a process of seg- mentation from the bed of white yolk, and rest directly on the white yolk at the bottom of the shallow cavity below the mass of segmentation-spheres. They contain either numerous small spherules, or fine granules; the spherules precisely resembling the smaller spheres of white yolk. These loose spherical masses form the majority of the formative cells already spoken of. Thus the original germinal disc of the ovarian ovum becomes, by the process of segmentation, converted into the blastoderm of the laid egg with its upper layer of columnar nucleated cells, and its lower layer of irregu- larly disposed cells, accompanied by a few stray “forma- tive” cells lying loose in the cavity below. CHAPTER II. A BRIEF SUMMARY OF THE WHOLE HISTORY OF INCUBATION. Step by step the simple two-layered blastoderm _ described in the previous chapter is converted into the complex organism of the chick. The details of the many changes through which this end is reached will perhaps be rendered more intelligible if we prefix to the special history of them a brief summary of the general course of events from the beginning to the end of incu- bation. In the first place, it is to be borne in mind that the embryo itself is formed in the area pellucida, and in the area pellucida alone. The area opaca in no part enters directly into the body of the chick; the structures to which it gives rise are to be regarded as appendages, which sooner or later disappear. Germinal layers. The blastoderm at starting con- sists of two layers. Very soon a third layer makes its appearance between the other two. These three layers, known as the germinal layers, the establishment of which is a fact of fundamental importance in the history of the embryo, are called respectively the upper, middle and lower layers, or epiblast, mesoblast and hypoblast. Of 26 PRELIMINARY ACCOUNT. [CHAP. these the epiblast and hypoblast constitute the primary layers. Three similar germinal layers are found in the embryos of all vertebrate and most invertebrate forms, and their history is one of the most important parts of comparative embryology. The epiblast gives rise to the epidermis, the central and peripheral parts of the nervous system, and to the most important parts of the organs of special sense. The hypoblast is essentially the secretory layer, and furnishes the whole epithelial lining of the alimentary tract and its glands, with the exception of part of the mouth and anus which are lined by the epiblast and are spoken of by embryologists as the stomodeuwm and proctodewm. Finally the mesoblast is a source from which the whole of the vascular system, the muscular and skeletal system, and the connective tissue of all parts of the body, are developed. It gives in fact origin to the connective-tissue basis both of the skin and of the mucous membrane of the alimentary tract, and to all the structures lying between these two with the exceptions already indicated. It is more especially to be noted that it gives rise to the excretory organs and generative glands. Formation of the embryo. The blastoderm which at first, as we have seen, lies like a watch-glass over the cavity below, its margin resting on the circular germinal wall of white yolk, spreads, as a thin circular sheet, over the yolk, immediately under the vitelline membrane. Increasing uniformly at all points of its circumference, the blastodermic expansion covers more and more of the _, yolk, and at last, reaching the opposite pole, completely envelopes it. Thus the whole yolk, instead of being 11] THE HEAD-FOLD. 27 enclosed as formerly by the vitelline membrane alone, comes to be also enclosed in a bag formed by the blasto- derm. It is not however until quite a late period that the complete closing in at the opposite pole takes place; in fact the extension of the blastoderm must be thought of as going on during the first seven days of incubation. Both the area opaca and the area pellucida share in this enlargement, but the area opaca increases much more rapidly than the area pellucida, and plays the principal part in encompassing the yolk. The mesoblast, in that part of the area opaca which is nearest to the area pellucida, becomes the seat of peculiar changes, which result in the formation of blood- vessels. Hence this part of the area opaca is called the vascular area. The embryo itself may be said to be formed by a folding off the central portion of the area pellucida from the rest of the blastoderm. At first the area pellucida is quite flat, or, inasmuch as it forms part of the circum- ference of the yolk, slightly but uniformly curved. Very soon, however, there appears at a certain spot a semi- lunar groove, at first small, but gradually increasing in depth and extent; this groove, which is represented in section in the diagram (Fig. 9, A), breaks the uni- formity of the level of the area pellucida. It may be spoken of as a tucking in of a small portion of the blastoderm in the form of a crescent. When viewed from above, it presents itseif as a curved line (the hinder of the two concentric curved lines in front of A in Fig. 22), which marks the hind margin of the groove, the depression itself being hidden. 28 PRELIMINARY ACCOUNT. [CHAP. Fia. 9. Fig. 9, A to W forms a series of purely diagrammatic repre- sentations introduced to facilitate the comprehension of the manner in which the body of the embryo is formed, and of the various relations of the yolk-sac, amnion and allantois. In all vt is the vitelline membrane, placed, for convenience sake, at some distance from its contents, and represented as per- sisting in the later stages; in the actual egg it is in direct contact with the blastoderm (or yolk), and early ceases to have a separate existence. In all e indicates the embryo, pp the general pleuro- peritoneal space, af the folds of the amnion proper ; ae or ac the cavity holding the liquor amnii; al the allantois; a the ali- mentary canal; y or ys the yolk or yolk-sac. A, which may be considered as a vertical section taken longi- tudinally along the axis of the embryo, represents the relations of the parts of the egg at the time of the first appearance of the head-fold, seen on the right-hand side of the blastoderm e. The I1.] THE EMBRYONIC APPENDAGES. 29 blastoderm is spreading both behind (to the left hand in the figure), and in front (to right hand) of the head-fold, its limits being indicated by the shading and thickening for a certain dis- tance of the margin of the yolk y. As yet there is no fold on the left side of e corresponding to the head-fold on the right. B isa vertical transverse section of the same period drawn for convenience sake on a larger scale (it should have been made flatter and less curved). It shews that the blastoderm (vertically shaded) is extending laterally as well as fore and aft, in fact in all directions ; but there are no lateral folds, and therefore no lateral limits to the body of the embryo as distinguished from the blastoderm. ; Incidentally it shews the formation of the medullary grcove by the rising up of the lamine dorsales. Beneath the section of the groove is seen the rudiment of the notochord. On either side a line indicates the cleavage of the mesoblast just commencing. In C, which represents a vertical longitudinal section of later date, both head-fold (on the right) and tail-fold (on the left) have advanced considerably. The alimentary canal is therefore closed in, both in front and behind, but is in the middle still widely open to the yolk y below. Though the axial parts of the embryo have become thickened by growth, the body-walls are still thin ; in them however is seen the cleavage of the mesoblast, and the divergence of the somatopleure and splanchnopleure. The splanchnopleure both at the head and at the tail is folded in to a greater extent than the somatopleure, and forms the still wide splanchnic stalk. At the end of the stalk, which is as yet short, it bends outwards again and spreads over the surface of the yolk. The somatopleure, folded in less than the splanchnopleure to form the wider somatic stalk, sooner bends round and runs out- wards again. At a little distance from both the head and the tail it is raised up into a fold, af, af, that in front of the head being the highest. These are the amniotic folds. Descending from either fold, it speedily joins the splanchnopleure again, and the two, once more united into an uncleft membrane, extend some way downwards over the yolk, the limit or outer margin of the opaque area not being shewn. All the space between the soma- topleure and the splanchnopleure, pp, is shaded with dots. Close 30 PRELIMINARY ACCOUNT. [CHAP, to the body this space may be called the pleuroperitoneal cavity ; but outside the body it runs up into either amniotic fold, and also extends some little way over the yolk. D represents the tail end at about the same stage on a more enlarged scale, in order to illustrate the position of the allantois al (which was for the sake of simplicity omitted in C), shewnas a bud from the splanchnopleure, stretching downwards into the pleu- roperitoneal cavity pp. The dotted area representing as before the Stas I1.] THE EMBRYONIC APPENDAGES. 31 whole space between the splanchnopleure and the somatopleure, it is evident that a way is open for the allantois to extend from its present position into the space between the two limbs of the amniotic fold af. &, also a longitudinal section, represents a stage still farther advanced. Both splanchnic and somatic stalks are much nar- rowed, especially the former, the cavity of the alimentary canal being now connected with the cavity of the yolk-sack by a mere canal. The folds of the amnion are spreading over the top of the embryo and nearly meet. Each fold consists of two walls or limbs, the space between which (dotted) is as before merely a part of the space between the somatopleure and splanchno- pleure. Between these arched amniotic folds and the body of the embryo is a space not as yet entirely closed in. ¥F represents on a different scale a transverse section of taken through the middle of the splanchnic stalk. The dark ring in the body of the embryo shews the position of the neural canal, below which is a black spot, marking the notochord. On either side of the notochord the divergence of somatopleure and splanch- nopleure is obvious. The splanchnopleure, more or less thick- ened, is somewhat bent in towards the middle line, but the two sides do not unite, the alimentary canal being as yet open below at this spot ; after converging somewhat they diverge again and run outwards over the yolk. The somatopleure, folded in to some extent to form the body-walls, soon bends outwards again, and is almost immediately raised up into the lateral folds of the amnion af. The continuity of the pleuroperitoneal cavity within the body with the interior of the amniotic fold outside the body is evident; both cavities are dotted. G, which corresponds to D at a later stage, is introduced to shew the manner in which the allantois, now a distinctly hollow body, whose cavity is continuous with that of the alimentary canal, becomes directed towards the amniotic fold. In # a longitudinal, and J a transverse section of later date, great changes have taken place. The several folds of the amnion have met and coalesced above the body of the embryo. The inner limbs of the several folds have united into a single membrane (a), which encloses a space (aé or ac) round the embryo. This mem- 32 PRELIMINARY ACCOUNT. [CHAP, brane (a) isthe amnion proper, and the cavity within it, z.c. between it and the embryo, is the cavity of the amnion containing the liquor amnii. The allantois is omitted for the sake of sim- plicity. It will be seen that the amnion @ now forms in every direc- tion the termination of the somatopleure ; the peripheral portions of the somatopleure, the united outer or descending limbs of the folds af in C, D, F, @ having been cut adrift, and now forming an independent continuous membrane, the serous membrane, immediately underneath the vitelline membrane. In J the splanchnopleure is seen converging to complete the closure of the alimentary canal a’ even at the stalk (elsewhere the canal has of course long been closed in), and then spreading outwards as before over the yolk. The point at which it unites with the somatopleure, marking the extreme limit of the cleavage of the mesoblast, is now much nearer the lower pole of the diminished yolk. 11.] THE EMBRYONIC APPENDAGES. 33 As a result of these several changes, a great increase in the dotted space has taken place. It is now possible to pass from the actual peritoneal cavity within the body, on the one hand round a great portion of the circumference of the yolk, and on the other hand above the amnion a, in the space between it and the serous envelope. Into this space the allantois is seen spreading in X at al. In Z the splanchnopleure has completely invested the yolk- sac, but at the lower pole of the yolk is still continuous with that peripheral remnant of the somatopleure now called the serous membrane. In other words, the cleavage of the mesoblast has been carried all round the yolk (ys) except just at the lower pole. In & the cleavage has been carried through the pole itself; the peripheral portion of the splanchnopleure forms a complete investment of the yolk, quite unconnected with the peripheral portion of the somatopleure, which now exists as a continuous membrane lining the interior of the shell. The yolk-sac (ys) is therefore quite loose in the pleuroperitoneal cavity, being con- nected only with the alimentary canal (a’) by a solid pedicle. Lastly, in V the yolk-sac (ys) is shewn being withdrawn into the cavity of the body of the embryo. The allantois isas before, for the sake of simplicity, omitted ; its pedicle would of course lie by the side of ys in the somatic stalk marked by the usual dotted shading. It may be repeated that the above are diagrams, the various spaces being shewn distended, whereas in many of them in the actual egg the walls have collapsed, and are in near juxta- position. In a vertical longitudinal section carried through the middle line, we may recognize the following parts (Fig. 9, A, or on a larger scale Fig. 10, which also shews details which need not be considered now). Beginning at what will become the posterior extremity of the embryo (the left-hand side of the figure in each case), and following the surface of the blastoderm forwards (to the right in the F. & B, 3 34 PRELIMINARY ACCOUNT. [CHAP. Fie. 10. Diagrammatic LoNGITUDINAL SECTION THROUGH THE AXIS OF AN EMBRYO. The section is supposed to be made at a time when the head- fold has commenced but the tail-fold has not yet appeared. F. So. fold of the somatopleure. fF. Sp. fold of the splanchnopleure. The line of reference /. So. is placed in the lower bay, outside. the embryo. The line of Dis placed in the upper bay inside the embryo; this will remain as the alimentary canal. Both folds (Ff. So., F. Sp.) are parts of the head-fold, and are to be thought of as continually travelling onwards (to the left) as de- velopment proceeds. pp. space between somatopleure and splanchnopleure: pleuro- peritoneal cavity. Am. commencing (head) fold of the amnion. A fuller explanation is given under Fig. 29. figures), the level is maintained for some distance, and then there is a sudden descent, the blastoderm bending round and pursuing a precisely opposite direction to its previous one, running backwards instead of forwards, for some distance. It soon, however, turns round again, and once more running forward, with a gentle ascent, regains the original level. As seen in section, then, the blasto- derm at this spot may be said to be folded up in the 01.] THE HEAD-FOLD. 35 form of the letter @. This fold we shall always speak of as the head-fold. In it we may recognize two limbs: an upper limb in which the curve is directed forwards, and its bay, opening backwards, is underneath the blas- toderm, 2.¢. as we shall see, inside the embryo (Fig. 10. D); and an under limb in which the curve is directed backwards, and its bay, opening forwards, is above the blastoderm,?.¢. outside the embryo. Ifan @ like the above, made of some elastic material, were stretched laterally, the effect would be to make both limbs longer and proportionally narrower, and their bays, instead of being shallow cups, would become more tubular. Such a result is in part arrived at by the growth of the blasto- derm; the upper limb of the @ is continually growing forward (but, unlike the stretched elastic model, in- creases in all its dimensions at the same time), and the lower limb is as continually lengthening backwards; and thus both upper and lower bays become longer and longer. This we shall hereafter speak of as the travel- ling backwards of the head-fold. The two bays do not however both become tubular. The section we have been speaking of is supposed to be taken vertically along a line; which will afterwards be- come the axis of the embryo; and the lower bay of the @ is a section of the crescentic groove mentioned above, in its middle or deepest part. On either side of the middle line the groove gradually becomes shallower. Hence in sections taken on either side of the middle line or axis of the embryo (above or below the plane of the figures), the groove would appear the less marked the farther the section from the middle line, and at a certain distance would disappear altogether. It must be 3—2 36 PRELIMINARY ACCOUNT. [CHAP. remembered that the groove is at first crescent-shaped, with the concavity of the crescent turned towards what will be the hind end of the embryo (Fig. 22). As the whole head-fold is carried farther and farther back, the horns of the crescent are more and more drawn in towards the middle line, the groove becoming first semicircular, then horse-shoe-shaped. In other words, the head-fold, instead of being a simple fold running straight back- wards, becomes a curved fold with a central portion in front running backwards, and two side portions running in towards the middle line. The effect of this is that the upper bay of the @ (that within the embryo) gets closed in at the sides as well as in the front, and thus speedily becomes tubular. The under bay of the A (that outside the embryo) remains of course open at the sides as in front, and forms a sort of horse-shoe-shaped ditch surrounding the front end of the embryo. We have dwelt thus at length on the formation of the head-fold, because, unless its characters are fairly grasped, much difficulty may be found in understanding many events in the history of the chick. The reader will perhaps find the matter easier to comprehend if he makes for himself a rough model, which he easily can do by spreading a cloth out flat to represent the blasto- derm, placing one hand underneath it, to mark the axis of the embryo; and then tucking in the cloth from above under the tips of his fingers. The fingers, coveted with the cloth and slightly projecting from the level of the rest of the cloth, will represent the head, in front of which will be the semicircular or horse-shoe-shaped groove of the head-fold. At its first appearance the whole @ may be spoken 11] THE TAIL-FOLD. 37 of as the head-fold, but later on it will be found con- venient to restrict the name chiefly to the lower limb of the 2. Some time after the appearance of the head-fold, an altogether similar but at first less conspicuous fold makes its appearance, at a point which will become the posterior end of the embryo. This fold, which travels forwards just as the head-fold travels backwards, is the tarl-fold (Fig. 9, C). In addition, between the head- and the tail-fold two lateral folds appear, one on either side. These are simpler in character than either head-fold or tail-fold, inasmuch as they are nearly straight folds directed inwards towards the axis of the body (Fig. 8, F’), and not complicated by being crescentic in form. Otherwise they are exactly similar, and in fact are formed by the con- tinuations of the head- and tail-folds respectively. As these several folds become more and more de- veloped, the head-fold travelling backwards, the tail- fold forwards, and the lateral folds inwards, they tend to unite in the middle point; and thus give rise more and more distinctly to the appearance of a small tubular sac seated upon, and connected, by a continually-nar- rowing hollow stalk, with that larger sac which is formed by the extension of the rest of the blastoderm over the whole yolk. The smaller sac we may call the “embryonic sac,” the larger one “ the yolk-sac.” As incubation proceeds, the smaller sac (Fig. 9) gets larger and larger at the expense of the yolk-sac (the contents of the latter being gradually assimilated by nutritive processes into the tissues forming the growing walls of the former, not 38 PRELIMINARY ACCOUNT. [CHAP. directly transferred from one cavity into the other). Within a day or two, of the hatching of the chick, at a time when the yolk-sac is still of some considerable size, or at least has not yet dwindled away altogether, and the development of the embryonic sac is nearly com- plete, the yolk-sac (Fig. 9, N’) is slipped into the body of the embryo, so that ultimately the embryonic sac alone remains. The embryo, then, is formed by a folding-off of a portion of the blastoderm from the yolk-sac. The general outline of the embryo is due to the direction and shape of the several folds which share in its forma- tion; these, while preserving a nearly perfect bilateral symmetry, present marked differences at the two ends of the embryo. Hence from the very first there is no difficulty in distinguishing the end which will be the head from that which will be the tail. In addition to this, the tubular sac of the embryo, while everywhere gradually acquiring thicker and thicker walls, undergoes at various points, through local activities of growth in the form of thickenings, ridges, buds or other processes, many modifications of the outline conferred upon it by the constituent folds. Thus bud-like processes start out from the trunk to form the rudiments of the limbs, and similar thickenings and ridges give rise to the jaws and other parts of the face. By the unequal development of these outgrowths the body of the chick is gradually moulded into its proper outward shape. Were the changes which take place of this class only, the result would be a tubular sac of somewhat com- plicated outline, but still a simple tubular sac. Such II.] THE MEDULLARY CANAL. 39 a simple sac might perhaps be roughly taken to repre- sent the body of many an invertebrate animal ; but the typical structure of a bird or other vertebrate animal is widely different. It may very briefly be described as follows. First there is, above, a canal running lengthways along the body, in which are lodged the brain and spinal cord. Below this neural tube is an axis repre- sented by the bodies of the vertebre and their con- tinuation forwards in the structures which form the base of the skull. Underneath’ this, again, is another tube closed in above by the axis, and on the sides and below by the body-walls. Enclosed in this second tube, and suspended from the axis, is a third tube, consisting of the alimentary canal with its appendages (liver, pan- creas, lungs, &c., which are fundamentally mere diver- ticula from one simple canal). The cavity of the outer tube, which also contains the heart and other parts of the vascular system, is the general body cavity; it con- sists of a thoracic or pleural, and an abdominal or peri- toneal section; these two parts are, however, from their mode of origin, portions of one and the same tube. Thus a transverse section of a vertebrate animal always shews the same fundamental structure: above a single tube, below a double tube, the latter consisting of one tube enclosed within another, the inner being the ali- mentary canal, the outer the general cavity of the body. Into such a triple tube the simple tubular embryonic sac of the chick is converted by a series of changes of a remarkable character. The upper or neural tube is formed in the following way. At a very early period the upper layer of the 40 PRELIMINARY ACCOUNT. [cHar. blastoderm or epiblast in the region which will become the embryo, is raised up into two ridges or folds, which run parallel to each other at a short distance on either side of what will be the long axis of the embryo, and thus leave between them a shallow longitudinal groove (Fig. 9, B, also Figs. 21, mc). As these ridges, which bear the name of medullary folds, increase in height they arch over towards each other, and eventually meet and coalesce in the middle line, thus converting the groove into a canal, which at the same time becomes closed at either end (Fig. 8, & J, also Fig. 34. Mc.). The cavity so formed is the cavity of the neural tube, and eventually becomes the cerebro-spinal canal. Its walls are wholly formed of epiblast. The lower double tube, that of the alimentary canal, and of the general cavity of the body, is formed in an entirely different way. It is, broadly speaking, the result of the junction and coalescence of the funda- mental embryonic folds, the head-fold, tail-fold, and lateral folds; in a certain sense the cavity of the body is the cavity of the tubular sac described in the last paragraph. But it is obvious that a tubular sac formed by the folding-in of a single sheet of tissue, such as we have hitherto considered the blastoderm to be, must be a simple tubular sac possessing a single cavity only. The blastoderm however does not long remain a single sheet, but speedily becomes a double sheet of such a kind that, when folded in, it gives rise to a double tube. Very early the blastoderm becomes thickened in the region of the embryo, the thickening being chiefly due .] THE BODY CAVITY. 41 to an increase in the middle layer or mesoblast, while at the same time it becomes split or cleft horizontally over the greater part of its extent into two leaves, an upper leaf and a lower leaf. In the neighbourhood of the axis of the body, beneath the neural tube, this cleavage is absent (Fig. 9, B; also Figs. 24, 34), in fact, it begins at some little distance on either side of the axis and spreads thence into the periphery in all direc- tions. It is along the mesoblast that the cleavage takes place, the upper part of the mesoblast uniting with epiblast to form the upper leaf, and the lower part with the hypoblast to form the lower leaf. In the fundamental folds both leaves are involved, both leaves are folded downwards and inwards, both leaves tend to meet in the middle below; but the lower leaf is folded in more rapidly, and thus diverges from the upper leaf, a space being gradually developed between them (Fig. 9). In course of time the several folds of the lower leaf meet and unite to form an inner tube quite independently of the upper leaf, whose own folds in turn meet and unite to form an outer tube separated from the inner one by an intervening space. The inner tube which from its mode of formation is clearly lined by hypoblast is the alimentary canal which is subsequently perforated at both ends to form the mouth and anus; the walls of the outer tube are the walls of the body; and the space between the two tubes is the general body or pleuroperitoneal cavity. Hence the upper (or outer) leaf of the blastoderm, from its giving rise to the body-walls, is called the somatopleure*; the lower (or inner) leaf, from its form- 1 Soma, body, pleuron, side. 42 PRELIMINARY ACCOUNT. (CHAP. ing the alimentary canal and its tributary viscera, the splanchnopleure *. This horizontal splitting of the blastoderm into a somatopleure and a splanchnopleure, which we shall hereafter speak of as the cleavage of the mesoblast, is not confined to the region of the embryo, but gradually extends over the whole of the yolk-sac. Hence in the later days of incubation the yolk-sac comes to have two distinct coats, an inner splanchnopleuric and an outer somatopleuric, separable from each other all over the sac. We have seen that, owing to the manner of its formation, the ‘embryonic sac’ is con- nected with the ‘yolk-sac’ by a continually narrowing hollow stalk; but this stalk must, like the embryonic sac itself, be a double stalk, and consist of a smaller inner stalk within a larger outer one, Fig. 9, £, H. The folds of the splanchnopleure, as they tend to meet and unite in the middle line below, give rise to a continually narrowing hollow stalk of their own, a splanchnic stalk, by means of which the walls of the alimentary canal are continuous with the splanch- nopleuric investment of the yolk-sac, and the interior of that canal is continuous with the cavity inside the yolk-sac. In the same way the folds of the somato- pleure form a similar stalk of their own, a somatic stalk, by means of which the body-walls of the chick are continuous (for some time; the continuity, as we shall see, being eventually broken by the development of the amnion) with the somatopleuric investment of the yolk-sac; and the pleuroperitoneal cavity of the 1 Splanchnon, viscus, pleuron, side. II.] THE AMNION. 43 body of the chick is continuous with the narrow space between the two investments of the yolk-sac. At a comparatively early period the canal of the splanchnic stalk becomes obliterated, so that the material of the yolk can no longer pass directly into the alimentary cavity, but has to find its way into the body of the chick by absorption through the blood- vessels. The somatic stalk, on the other hand, remains widely open for a much longer time; but the somatic shell of the yolk-sac never undergoes that thickening which takes place in the somatic walls of the embryo itself; on the contrary, it remains thin and insignificant. When, accordingly, in the last days of incubation the greatly diminished yolk-sac with its splanchnic invest- ment is withdrawn into the rapidly enlarging abdominal cavity of the embryo, the walls of the abdomen close in and unite, without any regard to the shrivelled, emptied somatopleuric investment of the yolk-sac, which is cast off as no longer of any use. (Fig. 9. Com- pare the series.) The Amnion. Very closely connected with the cleavage of the mesoblast and the division into soma- topleure and splanchnopleure, is the formation of the amnion, all mention of which was, for the sake of simplicity, purposely omitted in the description just given, The amnion is a peculiar membrane enveloping the embryo, which takes its origin from certain folds of the somatopleure, and of the somatopleure only, in the following way. At atime when the cleavage of the mesoblast has somewhat advanced, there appears, a little way in front 44 PRELIMINARY ACCOUNT. [CHAP. of the semilunar head-fold, a second fold (Fig. 22, also Fig. 9, C.), running more or less parallel or rather con- centric with the first, and not unlike it in general appearance, though differing widely from it in nature. In the head-fold the whole thickness of the blastoderm is involved; in it both somatopleure and splanchno- pleure (where they exist, i.e. where the mesoblast is cleft) take part. This second fold, on the contrary, is limited entirely to the somatopleure. Compare Figs. 9 and 10. In front of the head-fold, and therefore alto- gether in front of the body of the embryo, the somato- pleure is a very thin membrane, consisting only of epiblast and a very thin layer of mesoblast; and the fold we are speaking of is, in consequence, itself thin and delicate. Rising up as a semilunar fold with its concavity directed towards the embryo (Fig. 9, C, af), as it increases in height it is gradually drawn back- wards over the developing head of the embryo. The fold thus covering the head is in due time accompanied by similar folds of the somatopleure starting at some little distance behind the tail, and at some little dis- tance from the sides (Fig. 9, C, D, £, F, and Fig. 11 am.). In this way the embryo becomes surrounded by a series of folds of thin somatopleure, which form a con- tinuous wall all round it. All are drawn gradually over the body of the embryo, and at last meet and completely coalesce (Fig. 9, H, J), all traces of their junction being removed. Beneath these united folds there is therefore a cavity, within which the embryo lies (Fig. 9, H, ae). This cavity is the cavity of the amnion. The folds which we have been describing are those which form the amnion. ° 11] THE AMNION. 45 Fig. 11. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POS- TERIOR END OF AN EMBRYO Birp, AT THE TIME OF THE FORMATION OF THE ALLANTOIS. ep. epiblast ; Sp.c. spinal canal; ch. notochord ; n.c. neurenteric canal; hy. hypoblast ; p.a.g. postanal gut; pr. remains of primitive streak folded in on the ventral side; J. allantois ; me. mesoblast; an. point where anus will be formed ; p.c. perivisceral cavity; am. amnion; so. somatopleure; sp. splanchnopleure. Each fold, of course, necessarily consists of two limbs, both limbs consisting of epiblast and a very thin layer of mesoblast; but in one limb the epibiast looks towards the embryo, while in the other it looks away from it. The space between the two limbs of the fold, as can easily be seen in Figs. 9 and 11, is really part of the space between the somatopleure and splanch- nopleure ; it is therefore continuous with the general space, part of which afterwards becomes the pleuro- peritoneal cavity of the body, shaded with dots in figure 9 and marked (pp). It is thus possible to pass from the cavity between the two limbs of each 46 PRELIMINARY ACCOUNT. [CHAaP. fold of the amnion into the cavity which surrounds the alimentary canal. When the several folds meet and coalesce together above the embryo, they unite in such a way that all their inner limbs go to form a continuous inner membrane or sac, and all their outer limbs a similarly continuous outer membrane or sac. The inner membrane thus built up forms a completely closed sac round the body of the embryo, and is called the amniotic sac, or amnion proper (Fig. 9, H, I, &c. a.), and the fluid which it afterwards contains is called the amniotic fluid, or liquor amni. The space between the inner and outer sac, being formed by the united cavities of the several folds, is, from the mode of its formation, simply a part of the general cavity found everywhere between somatopleure and splanchnopleure. The outer sac over the embryo lies close under the vitellime membrane, while its periphery is gradually extended over the yolk as the somatopleuric invest- ment of the yolk-sac described in the preceding para- graph. It constitutes the false amnion while the mem- brane of which it forms a part is frequently known as the serous membrane. The Allantois. If the mode of origin of these two sacs (the inner or true amnion, and the outer or false amnion, as Baer called it) and their relations to the embryo be borne in mind, the reader will have no diffi- culty in understanding the course taken in its growth by an important organ, the allantois, of which we shall hereafter have to speak more in detail. The allantois is essentially a diverticulum of the alimentary tract, into which it opens immediately in front of the anus. It at first (Fig. 11, al) forms a IL] THE ALLANTOIS. 47 flattened sac projecting into the pleuroperitoneal cavity, the walls of the sac being formed of a layer of splanchnic mesoblast lined by hypoblast. It grows forwards in the peritoneal cavity until it reaches the stalk connecting the embryo with the yolk- sac, and thence very rapidly pushes its way into the space between the true and false amniotic sacs (Fig. 9, G, K). Curving over the embryo, it comes to lie above the embryo and the amnion proper, separated from the shell (and vitelline membrane) by nothing more than the thin false amnion. In this position it becomes highly vascular, and performs the functions of a respi- ratory organ. It is evident that though now placed quite outside the embryo, the space in which it lies is a continuation of that peritoneal cavity in which it took its origin. It is only necessary to add, that the serous mem- brane, including the false amnion, either coalesces with the vitelline membrane, in contact with which it lies, or else replaces it; and in the later days of incubation was called by the older embryologists the chorion—a name however which we shall not adopt. CHAPTER III. THE CHANGES WHICH TAKE PLACE DURING THE FIRST DAY OF INCUBATION. Durine the descent of the egg along the oviduct, where it is exposed to a temperature of about 40° C., the gerthinal disc, as we have seen, undergoes important changes. When the egg is laid and becomes cold these changes all but entirely cease, and the blastoderm remains inactive until, under the influence of the higher temperature of natural or artificial incubation, the vital activities of the germ are brought back into play, the arrested changes go on again, and usher in the series of events which we have now to describe in detail. The condition of the blastoderm at the time when the egg is laid is not exactly the same im all eggs; in some the changes being farther advanced than in others, though the differences of course are slight. In some eggs, especially in warm weather, changes of the same kind as those caused by actual incubation may take place, to a certain extent, in the interval between laying and incubation ;. lastly, in all eggs, both under patural and especially under artificial incubation, the CHAP. III.] THE EMBRYONIC SHIELD. 49 dates of the several changes are, within the limits of some hours, very uncertain, particularly in the first few days; one egg being found, for example, at 36 hours in the same stage as another at 24 or 30 hours, or a third at 40 or 48 hours. When we speak therefore of any event as taking place at any given hour or part of any given day, we are to be understood as meaning that such an event will generally be found to have taken place at about that time. We introduce exact dates for the convenience of description. The changes which take place during the first day will be most easily considered under several periods. From the 1st to about the 8th hour.—During this period the blastoderm, when viewed from above, is found to have increased in size. The pellucid area, which at the best is but obscurely marked in the unin- cubated egg, becomes very distinct (the central opacity having disappeared), and contrasts strongly with the opaque area, which has even still more increased both in distinctness and size. For the first few hours both the pellucid and opaque areas remain approximately circular, and the most im- portant change, besides increase in size and greater distinctness which can be observed in them, is a slight ill-defined opacity or loss of transparency, which makes its appearance in the hinder half of the pellucid area. This is known as the embryonic shield. Slight as are the changes which can at this stage be seen from surface views, sections taken from hardened specimens bring to light many most important changes in the nature and arrangement of the constituent cells, F.& B, 4 50 EXER, o 82: ee, THE FIRST DAY. [CHAP, Fie. 12. Section or A BLASTODERM OF A Fowt’s Eee AT THE COMMENCEMENT OF INCUBATION. The thin but complete upper layer ep composed of columnar cells rests on the in- complete lower layer 7, composed of larger and more granular cells. The lower layer is thicker in some places than in others, and is especially thick at the periphery. The line below the under layer marks the upper sur- face of the white yolk. The larger so-called formative cells are seen at 0, lying on the white yolk. The figure does not take in quite the whole breadth of the blastoderm; but the reader must understand that both to the right hand and the left ep is continued farther than 1, so that at the extreme edge it rests directly on the white yolk. It will be remembered that the blastoderm in the unincubated egg is composed of two layers, an upper (Fig. 12, ep) and an under layer; that the upper is a coherent membrane of colum- nar nucleated cells, but that the lower one (Fig. 12, 7) is formed of an irregular network of larger cells in which the nuclei are with difficulty visible; and that in addition to this there are certain still larger cells, called ‘formative cells’ (Fig. 12,5), lying at the bottom of the segmentation-cavity. Under the influence of incubation changes take place very rapidly, which I11.] THE HYPOBLAST. 51 result in the formation of the three layers of the blasto- derm. The upper layer, which is the epiblast already spoken of (Fig. 13), takes at first but little share in these changes. In the lower layer, however, certain of the cells begin to get flattened horizontally, their granules become less numerous, and the nucleus becomes distinct; the cells so altered cohere together and form a membrane. The membrane thus formed, which is first completed in Fie, 13. TRANSVERSE SECTION THROUGH THE BLASTODERM OF A CHICK BEFORE THE APPEARANCE OF THE PRIMITIVE STREAK. The epiblast is represented somewhat diagrammatically. The hyphens shew the points of junction of the two halves of the section. The hypoblast is already constituted as a membrane of flattened cells, and a number of scattered cells are seen between it and the epiblast. the centre of the pellucid area, constitutes the hypoblast. Between the hypoblastic membrane and the epiblast there remain a number of scattered cells (Fig. 13) which cannot however be said tu form a definite layer altogether distinct from the hypoblast. They are almost entirely confined to the posterior part of the area pellucida, and 4—2 52 THE FIRST DAY. [CHAP. give rise to the opacity of that part, which we have spoken of as the embryonic shield. At the edge of the area pellucida the hypoblast becomes continuous with a thickened rim of material, underlying the epiblast, and derived from the original thickened edge of the blastoderm and the subjacent yolk. It is mainly formed of yolk granules, with a varying number of cells and nuclei imbedded in it. It is known as the germinal wall, and is spoken of more in detail on pp. 65 and 66. The epiblast is the Hornbdlatt (corneal layer), and the hypo- blast the Darmdrisenblatt (epithelial glandular layer) of the Germans, while those parts of the mesoblast which take part in the formation of the somatopleure and splanchnopleure cor- respond respectively to the Haut-muskel-platte and Darm-faser- platte. All blood-vessels arise in the mesoblast. Hence the vascular layer of the older writers falls entirely within the mesoblast. The serous layer of the old authors includes the whole of the epiblast, but also comprises a certain portion of mesoblast ; for they speak of all the organs of animal life (skin, bones, muscle, &c.) as being formed out of the serous layer, whereas the epiblast proper gives rise only to the epidermis and to certain parts of the nervous system. In the same way their mucous layer corresponds to the hypoblast with so much of the mesoblast as takes part in the formation of the organs of organic life. Their vascular layer therefore answers to a part only of the mesoblast viz. that part in which blood-vessels are especially developed. From the 8th to the 12th hour. The changes which next take place result in the complete differen- tiation of the embryonic layers, a process which is inti- mately connected with the formation of a structure known as the primitive streak. The full meaning of the I11.] THE PRIMITIVE STREAK. 53 latter structure, and its relation to the embryo, can how- ever only be understood by comparison with the develop- ment of the lower forms of vertebrate life. It will be remembered that in surface views of the unincubated blastoderm a small arc, at what we stated to be the posterior end, close to the junction between the area opaca and the area pellucida is distinguished by its more opaque appearance. In the surface view the primitive streak appears as a linear opacity, which gradually grows forwards from the middle of this arc till it reaches about one-third of the diameter of the Fie, 14. Prs ‘AREA PELLUCIDA OF A VERY YOUNG BLASTODERM oF A CHICK, SHEWING THE PRIMITIVE STREAK SHORTLY AFTER ITS FIRST APPEARANCE, pr.s. primitive streak ; ap. area pellucida; a.op. area opaca. area pellucida. During the formation of the primitive streak the embryonic shield grows fainter and finally vanishes. When definitely established the primitive streak has the appearance diagrammatically represented in Fig. 14. 54 THE FIRST DAY. [CHAP. Sections at this stage throw a very important light on the nature and mode of origin of the primitive streak. In the region in front of it the blastoderm is still formed of two layers only, but in the region of the streak itself the structure of the blastoderm is greatly altered. The most important features in it are repre- sented in Fig. 15. This figure shews that the median Fre, 15, TRANSVERSE SECTION THROUGH A BLASTODERM OF ABOUT THE AGE REPRESENTED IN Fic. 14, SHEWING THE First Dir- FERENTIATION OF THE PRIMITIVE STREAK. The section passes through about the middle of the primitive streak. pvs. primitive streak ; ep. epiblast; hy. hypoblast ; yé. yolk of the germinal wall. portion of the blastoderm has become very much thick- ened (thus producing the opacity of the primitive streak), and that this thickening is caused by a proliferation of rounded cells from the epiblast. In the very young primitive streak, of which Fig. 15 is a section, the rounded cells are still continuous throughout with the epiblast, but they form nevertheless the rudiment of the greater part of a sheet of mesoblast, which will soon arise in this region. III. ] THE PRIMITIVE STREAK. 55 In addition to the cells clearly derived from the epiblast, there are certain otber cells (Fig. 15), closely adjoining the hypoblast; these are derivatives of the cells, interposed between the epiblast and hypoblast, which gave rise to the appearance of the embryonic shield during the previous stage. In our opinion these cells also have a share in forming the future meso- blast. It thus appears that the primitive streak is essen- tially a linear proliferation of epiblast cells; the cells produced being destined to give rise to the mesoblast. This proliferation first commences at the hinder end of the area pellucida, and thence proceeds forwards. While the primitive streak is being established, the epiblast becomes two or more rows-of cells deep in the region of the area pellucida. Soon after this, the hitherto circular pellucid area becomes oval (the opaque area remaining circular), The oval is, with remarkable regularity, so placed that its long axis forms a right angle, or very nearly a right angle, with the long axis of the egg itself. Its narrow end corresponds with the future hind end of the embryo. If an egg be placed with its broad end to the right hand of the observer, the head of the embryo will in nearly all cases be found pointing away from him. The 12th to the 16th hour. The primitive streak at its first appearance is shadowy and ill-defined; gradu- ally however it becomes more distinct; and during the same period the pellucid area rapidly increases in size, and from being oval becomes pear-shaped (Fig. 16). The primitive streak grows even more rapidly than the pellucid area; so that by the 16th hour it is not only 56 THE FIRST DAY. [CHaAP. absolutely, but also relatively to the pellucid area, longer than it was at the 12th hour. It finally occupies about two-thirds of the length of the area pellucida; but its hinder end in many instances appears to stop short of the posterior border of the area pellucida (Fig. 16). The median line of the Fig. 16. Sorrace View oF THE AREA PELLUCIDA OF A CHICK’s BLASTODERM SHORTLY AFTER THE FORMATION OF THE PRIMITIVE GROOVE. pr. primitive streak with primitive groove ; af. amuiotic fold. The darker shading round the primitive streak shews the extension of the mesoblast. primitive streak becomes marked by a shallow furrow running along its axis. In fresh specimens, viewed with transmitted light, this furrow appears as a linear trans- parency, but in hardened specimens seen under reflected light may be distinctly recognized as a narrow groove, 111, ] THE PRIMITIVE GROOVE. 57 the bottom of which, being thinner than the sides, appears more transparent when viewed with transmitted light. It is known as the primitive groove. Its depth and the extent of its development are subject to great variations. During these changes in external appearance there grow from the edges of the cord of cells constituting the primitive streak two lateral wings of mesoblast cells, which gradually extend till they reach the sides of the area pellucida (Fig. 17). The two wings of mesoblast meet along the line of the primitive streak, where they still remain attached to the epiblast. During this period many sections through the primitive streak give an impression of the mesoblast being involuted along the lips of a groove. The hypoblast below the primitive streak is always quite independent of the mesoblast above, though much more closely attached to it in the median line than at the sides. The part of the mesoblast, which we believe to be derived from the primitive lower layer cells, can generally be distinctly traced. In many cases, especially at the front end of the primitive streak, it forms, as in Fig. 17, a distinct layer of stellate cells, quite unlike the rounded cells of the mesoblastic involution of the primitive streak. In the region in front of the primitive streak, where the first trace of the embryo will shortly appear, the layers at first undergo no important changes, except that the hypoblast becomes somewhat thicker. Soon, however, as shewn in longitudinal section in Fig. 18, the hypoblast along the axial line becomes continuous be- hind with the front end of the primitive streak. Thus at this point, which is the future hind end of the THE FIRST DAY. [cHAP. 58 Fig. 18. Fig. 17 gw is Ob} 111. } FORMATION OF THE EMBRYO. 59 Fig. 17, TRANSVERSE SECTION THROUGH THE Front END or THE PRE MITIVE STREAK OF A BLASTODERM OF THE SAME AGE AS Fie. 16. pv. primitive groove; m. mesoblast; ep. epiblast; dy. hypo- blast ; yA. yolk of germinal wall. Fie. 18. LoneitupInaL SECTION THROUGH THE AXIAL LINE OF THE PRIMITIVE STREAK, AND THE Part oF THE BLASTODERM IN FRONT OF IT, OF THE BLASTODERM OF A CHICK SOME- WHAT YOUNGER THAN Fig. 19. pr.s. primitive streak; ep. epiblast ; hy. hypoblast of region in front of primitive streak; n. nuclei; yé. yolk of germinal wall. embryo, the mesoblast, the epiblast, and the hypoblast all unite together. From the 16th to the 20th hours. At about the 16th hour, in blastoderms of the stage represented in Fig.16,animportantchange takes place in the constitution of the primitive hypoblast in front of the primitive streak. The rounded cells, of which it is at first composed (Fig. 18), break up into (1) a layer formed of a single row of more or less flattened elements below—the hypoblast proper—and (2) into a layer formed of several rows of stellate elements, between the hypoblast and the epiblast —the mesoblast (Fig. 19 m). A separation between these two layers is at first hardly apparent, and before it has become at all well marked, especially in the median line, an axial opaque line makes its appearance in surface views, continued forwards from the front end of the primitive streak, but stopping short at a semicircular 60 THE FIRST DAY. [cHAP. Fie, 19. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE BLASTODERM OF A CHICK SHORTLY PRIOR TO THE ForMATION oF THE MEDULLARY GROOVE AND Noro- CHORD. m. median line of the section ; ep. epiblast ; 7.2. lower layer cells (primitive hypoblast) not yet completely differentiated into mesoblast and hypoblast ; n. nuclei. fold—the future head-fold—near the front end of the area pellucida. In section (Fig. 20) this opaque line is seen to be due to a special concentration of cells in the form of a cord. This cord is the commencement of an extremely important structure found in all vertebrate embryos, which is known as the notochord (ch). In most instances the commencing notochord remains attached to the hypoblast, after the mesoblast has at the sides become quite detached (vide Fig. 20), but in other cases the notochord appears to become differentiated in the already separated layer of mesoblast. In all cases the notochord and the hypoblast below it unite with the front end of the primitive streak; with which also the two lateral plates of mesoblast become continuous. From what has just been said it is clear that in the region of the embryo the mesoblast originates as two lateral plates split off from the primitive hypoblast, and I. | THE NOTOCHORD. 61 Fie. 20, TAO One (THO iu 4 nO my ray LON h TO. ‘ einen ee TRANSVERSE SECTION THROUGH THE Empryonic REGION OF THE BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION or THE NoTOCHORD, BUT BEFORE THE APPEARANCE OF THE MEDULLARY GROOVE. ep. epiblast; Ay. hypoblast; ch. notochord; me. mesoblast ; yk. yolk of germinal wall. Fie. 21, A BN c 2 COO mS B22299.2.L99 © OBOOLO LOGB.OOGS ara ga ce SOSece a ® ee Os Oa Ger GOOG SPSS,“ 905498 6300 06 SO0oqD OBsossooguouTE 8S Gana’ B \ aiRenea pasa re Meee HIP 8 TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS. The section passes through the medullary groove me., at some distance behind its front end. A. Epiblast. B. Mesoblast. C. Hypoblast. m.c. medullary groove ; mf. medullary fold; ch. notochord. 62 THE FIRST DAY. [CHAP. that the notochord originates simultaneously with the mesoblast, with which it is at first continuous, as a median plate similarly of hypoblastic origin. Kélliker! holds that the mesoblast of the region of the em- bryo is derived from a forward growth from the primitive streak. There is no theoretical objection to this view, and we think it would be impossible to shew for certain by sections whether or no there is a growth such as he describes ; but such sections as that represented in Fig. 19 (and we have series of such sections from several embryos) appear to us to be conclusive in favour of the view that the mesoblast of the region of the embryo is to a large , extent derived from a differentiation of the primitive hypoblast. The mesoblast of the primitive streak forms in part the vascular structures found in the area pellucida, and probably also in part ‘the mesoblast of the allantois. The differentiation of the embryo may be said to commence with the formation of the notochord and the lateral plates of mesoblast. Very shortly after the for- mation of these parts, the axial part of the epiblast above the notochord and in front of the primitive streak, being here somewhat thicker than in the lateral parts, becomes differentiated into a distinct medullary plate, the sides of which form two folds known as the medullary folds, enclosing between them a groove known as the medullary groove. The medullary plate itself consti- tutes that portion of the epiblast which gives rise to the central nervous system. Between the 18th to the 20th hour the medullary groove, with its medullary folds or laminz dorsales, is fully established. It then presents the appearance, to- wards the hinder extremity of the embryo, of a shallow } Entwick. d. Menschen u. héheren Thiere. Leipzig, 1879. III] THE NOTOCHORD. 63 groove with sloping diverging walls, which embrace be- tween them the front end of the primitive streak. Passing forwards towards what will become the head of the embryo the groove becomes narrower and deeper with steeper walls. On reaching the head-fold (Fig. 22), which continually becomes more and more prominent, the medullary folds curve round and meet each other in the middle line, so as to form a somewhat rounded end to the groove. In front therefore the canal does not become lost by the gradual flattening and divergence of its walls, as is the case behind, but has a definite termi- nation, the limit being marked by the head-fold. In front of the head-fold, quite out of the region of the medullary folds, there is usually another small fold formed earlier than the head-fold, which is the begin- ning of the amnion (Fig. 22). The appearance of the embryo and its relation to the surrounding parts are somewhat diagrammatically represented in Fig. 22. The primitive streak now ends with an anterior swelling (not represented in the figure), and is usually somewhat unsymmetrical. In most cases its axis is more nearly continuous with the left, or rarely the right, medullary fold than with the medullary groove. In sections its front end appears as a ridge on one side or rarely in the middle of the floor of the wide medullary groove. The general structure of the developing embryo at the present stage is best understood from such a section as that represented in Fig. 21. The medullary groove (m. c.) lined by thickened epiblast is seen in the median line of the section. Below it is placed the notochord (ch), which at this stage is a mere rod of cells, and on each 64 THE FIRST DAY. [CHAP. Surrace VIEW OF THE PELLUCID AREA OF A BLASTODERM OF 18 HOURS. None of the opaque area is shewn, the pear-shaped outline indicating the limits of the pellucid area. At the hinder part of the area isseen the primitive groove pr., with its nearly parallel walls, fading away behind, but curv- ing round and meeting in front so as to form a distinct anterior termination to the groove, about half way up the pellucid area. Above the primitive groove is seen the medullary groove m.c., with the medullary folds A. These diverging behind, slope away on either side of the primitive groove, while in front they curve round and meet each other close upon a curved line which repre- sents the head-fold. The second curved line in front of and concentric with the first is the commencing fold of the amnion. IIL. THE GERMINAL WALL. 65 side are situated the mesoblastic plates (B). The hypo- blast forms a continuous and nearly flat layer below. While the changes just described have been occur- ring in the area pellucida, the growth of the area opaca has also progressed actively. The epiblast has greatly extended itself, and important changes have taken place in the constitution of the germinal wall already spoken of. The mesoblast and hypoblast of the area opaca do not arise by simple extension of the corresponding layers of the area pellucida; but the whole of the hypoblast of the area opaca, and a large portion of the meso- blast, and possibly even some of the epiblast, take their origin from the peculiar material which forms the germinal wall and which is continuous with the hypo- blast at the edge of the area opaca (vide figs. 15, 17, 18, 19, 20). The exact nature of this material has been the subject of many controversies. Into these controversies it isnot our purpose to enter, but subjoined are the results of our own examination. The germinal wall first consists, as already mentioned, of the lower cells of the thickened edge of the blastoderm, and of the subjacent yolk material with nuclei. During the period before the formation of the primitive streak the epiblast appears to extend itself over the yolk, partly at the expense of the cells of the germinal wall, and possibly even of cells formed around the nuclei in this part. The cells of the germinal wall, which are at first well separated from the yolk below, become gradually ab- sorbed in the growth of the hypoblast, and the remaining cells and yolk then become mingled together, and constitute a com- pound structure, continuous at its inner border with the hypo- blast. This structure is the germinal wall usually so described. It is mainly formed of yolk granules with numerous nuclei, and a somewhat variable number of rather large cells imbedded F, &B. 5 66 THE FIRST DAY. [CHAP amongst them. The nuclei, some of which are probably enclosed by a definite cell body, typically form a special layer immedi- ately below the epiblast. A special mass of nuclei (zzde Figs. 18 and 20, n) is usually present at the junction of the hypoblast with the germinal wall. The germinal wall retains the characters just enumerated till near the close of the first day of incubation. One function of its cells appears to be the absorption of yolk material for the growth ‘of the embryo. , The chief events then of the second period of the first day are the appearance of the medullary folds and groove, the formation of the notochord and lateral plates of mesoblast, the beginning of the head-fold and amnion, and the histological changes taking place in the several layers. From the 20th to the 24th hour. A view of the embryo during this period is given in Fig. 23. The head-fold enlarges rapidly, the crescentic groove becoming deeper, while at the same time the over- hanging margin of the groove (the upper limb of the Q), rises up above the level of the blastoderm ; in fact, the formation of the head of the embryo may now be said to have definitely begun. The medullary folds, increasing in size in every dimension, but especially in height, lean over from either side towards the middle line, and thus tend more and more to roof in the medullary canal, espe- _cially near the head. About the end of the first day they come into direct contact in the region which will afterwards become the brain, though they do not as yet coalesce. In this way a tubular canal is formed. This is the medullary or neural canal (Fig. 23, Fig. 24, (iL.] THE MEDULLARY CANAL, 67 Fic. 23, DorsaL VIEW OF THE HARDENED AREA PELLUCIDA OF A CHICK with Five Mesosiastic Somitrs. THE MEDULLARY FoLps HAVE MET FOR PART OF THEIR EXTENT, BUT HAVE NOT UNITED. apr. anterior part of the primitive streak; p.pr. posterior part of the primitive streak. Mc.). It is not completely closed in till a period con- siderably later than the one we are considering. Meanwhile important changes are taking place in the axial portions of the mesoblast, which lie on each side of the notochord beneath the medullary folds. In an embryo of the middle period of this day, examined with transmitted light, the notochord is seen at the bottom of the medullary groove between . the medullary folds, as a transparent line shining through the floor of the groove when the embryo is viewed from above. On either side of the notochord the body of the embryo appears somewhat opaque, 5—2 68 THE FIRST DAY. [ CHAP. owing to the thickness of the medullary folds; as these folds slope away outwards on either side, so the opacity gradually fades away in the pellucid area. There is present at the sides no sharp line of demarca- tion between the body of the embryo and the rest of the area; nor will there be any till the lateral folds ‘make their appearance ; and transverse vertical sections shew (Fig. 21) that there is no break in the mesoblast, from the notochord to the margin of the pellucid area, but only a gradual thinning. During the latter period of the day, however, the plates of mesoblast on either side of the notochord begin to be split horizontally into two layers, the one of which attaching itself to the epiblast, forms with it the somatopleure (shewn for a somewhat later stage in Fig. 24), while the other, attaching itself to the hypoblast, forms with it the splanchnopleure. By the separation of these two layers from each other, a cavity (Pp), containing fluid only, and more con- spicuous in certain parts of the embryo than in others, is developed. This cavity is the beginning of that great serous cavity of the body which afterwards becomes divided into separate cavities. We shall speak of it as the plewro-peritoneal cavity. This cleavage into somatopleure and splanchno- pleure extends close up to the walls of the medullary canal, but close to the medullary canal a central or axial portion of each plate becomes marked off by a slight constriction from the peripheral (Fig. 24), and receives the name of vertebral plate, the more external mesoblast being called the lateral plate. The cavity between the two layers of the lateral plate rapidly Itl. | VERTEBRAL PLATE. 69 enlarges, while that in the vertebral plate remains in the condition of a mere split. Fie. 24. TRANSVERSE SECTION THROUGH THE DoRsAL REGION OF AN Empryo oF THE SeconD Day (copied from His), intro- duced here to illustrate the formation of the mesoblastic somitis, and the cleavage of the mesoblast. M. medullary canal; Pv. mesoblastic somite; w. rudiment of Wolffian duct ; A. epiblast; C. hypoblast; Ch. notochord ; Ao. aorta ; BC. splanchnopleure. At first each vertebral plate is not only unbroken along its length, but also continuous at its outer edge with the upper and lower layers of the lateral plate of the same side. Very soon, however, clear trans- verse lines are seen, in surface views (Fig. 28), stretch- ing inwards across each vertebral plate from the edge of the lateral plate towards the notochord; while a transparent longitudinal line makes its appearance on either side of the notochord along the line of junction of the lateral with the vertebral plate. The transverse lines are caused by the formation of vertical clefts, that is to say, narrow spaces containing nothing but clear fluid; and sections shew that they 70 THE FIRST DAY. [CHAP. are due to breaches of continuity in the mesoblast only, the epiblast and hypoblast having no share in the matter. Thus each vertebral plate appears in surface views to be cut up into a series of square plots, bounded by transparent lines (Fig. 23). Each square plot is the surface of a corresponding cubical mass (Fig. 24, Pv.). The two such cubical masses first formed, lying one on each side of the notochord, beneath and a little to the outside of the medullary folds, are the first pair of mesoblastic somites’. The mesoblastic somites form the basis out of which the voluntary muscles of the trunk and the bodies of the vertebra are formed. The first somite rises close to the anterior ex- tremity of the primitive streak, but the next is stated to arise in front of this, so that the first-formed sv- mite corresponds to the second permanent vertebra. The region of the embryo in front of the second formed somite—at first the largest part of the whole embryo—is the cephalic region (Fig. 23). The somites following the second are formed in regular succession from before backwards, out of the unsegmented mesoblast of the posterior end of the embryo, which rapidly grows in length to supply the necessary material. With the growth of the embryo the primitive streak is con- tinually carried back, the lengthening of the embryo always taking place between the front end of the primitive streak and the last somite; and during this 1 These bodies are frequently called protovertebre, but we shall employ for them the term mesoblastic somites. IIL] THE NEURENTERIC PASSAGE. 71 process the primitive streak undergoes important changes both in itself and in its relation to the embryo. Its anterior thicker part, which is embraced by the diverging medullary folds, soon becomes distinguished in structure from the posterior part, and is placed symmetrically in relation to the axis of the embryo, (Fig. 23 a.pr); at the same time the medullary folds, which at first simply diverge on each side of the primitive streak, bend in again and meet behind so as completely to enclose this front part of the primi- tive streak. The region, where the medullary folds diverge, is known as the sinus rhomboidalis of the embryo bird, though it has no connection with the similarly named structure in the adult. This is a convenient place to notice remarkable appearances which present themselves close to the junction of the neural plate and the primitive streak. These are temporary passages leading from the hinder end of the neural groove or tube into the alimen- tary canal. They vary somewhat in different species of birds, and it is possible that in some species there may be several openings of the kind, which appear one after the other and then close again. They were first discovered by Gasser, and are spoken of as the neurenteric passages or canals}, In all cases, with some doubtful exceptions, they lead round the posterior end of the notochord, or through the point where the notochord falls into the primitive streak. The largest of these passages is present in the embryo duck with twenty-six mesoblastic somites, and is represented in the series of sections (Fig. 25). The passage leads obliquely back- wards and ventralwards from the hind end of the neural tube 1 “Die Primitivstreifen bei Vogelembryonen.” Schrift. d. Gesell. z. Beford d. Gesammten Naturwiss, zu Marburg. Vol. 1. Supple- ment 1. 1879. 72 THE FIRST DAY. [CHAP. Fig. 25. Four TRANSVERSE SECTIONS THROUGH THE NEURENTERIC PassaGE AND ADJormnIna Parts In A Duck Empryo WITH TWENTY-six MEsospLastc SomITEs. A. Section in front of the neurenteric canal, shewing a lumen in the notochord. B. Section through the passage from the medullary canal into the notochord. : C. Section shewing the hypoblastic opening of the neuren- teric canal, and the groove on the surface of the primitive streak, which opens in front into the medullary canal. D. Primitive streak immediately behind the opening of the neurenteric passage. me. medullary canal; ep. epiblast ; hy. hypoblast ; ch. noto- chord ; pr. primitive streak. I11.] THE NEURENTERIC PASSAGE. 73 into the notochord, -where the latter joins the primitive streak (B). A narrow diverticulum from this passage is continued for- wards for a short distance along the axis of the notochord (A, ch). After traversing the notochord, the passage is continued into a hypoblastic diverticulum, ayiiiel opens ventrally into the future lumen of the alimentary tract (C). Shortly behind the point where the neurenteric passage communicates with the neural tube the latter structure opens dorsally, and a groove on the surface of the primitive streak is continued backwards from it for a short distance (C). The first part of this passage to appear is the hypoblastic diverticulum above mentioned. Fic. 26, Diagrammatic LoneirupiInaL SECTION THROUGH THE Pos- TERIOR END OF AN Embryo BIRD AT THE TIME OF THE ForRMATION OF THE ALLANTOIS. ep. epiblast; Sp.c. spinal canal ; ch, notochord ; n.e. neurenteric canal; Ay. hypoblast; p.a.g. post-anal gut; pr. remains of primitive streak folded in on the ventral side; a. allantois ; me. mesoblast ; an. point where anus will be formed; p.c. perivisceral cavity; am. amnion; so. somatopleure; sp. splanchnopleure. In the chick we have found in some cases an incomplete pas- sage prior to the formation of the first somite. Ata later stage 74 THE FIRST DAY. [cHAP. there is a perforation on the floor of the neural canal, which is not so marked as those in the goose or duck, and never results in a complete continuity between the neural and alimentary tracts ; but simply leads from the floor of the neural canal into the tissues of the tail-swelling, and thence into a cavity in the posterior part of the notochord. The hinder diverticulum of the neural canal along the line of the primitive groove is, moreover, very considerable in the chick, and is not so soon obliterated as in the goose. The incomplete passage in the chick arises at a period when about twelve somites are present. The third passage is formed in the chick during the third day of incuba- tion. The anterior part of the primitive streak becomes con- verted into the tail-swelling; the groove of the posterior part gradually shallows and finally disappears. The hinder part itself atrophies from behind forwards, and in the course of the folding off of the embryo from the yolk the part of the blastoderm where it was placed becomes folded in, so as to form part of the ventral wall of the embryo. The apparent hinder part of the primitive streak is therefore in reality ventral and anterior in relation to the embryo. Since the commencement of incubation the area opaca has been spreading outwards over the surface of the yolk, and by the end of the first day has reached about the diameter of a sixpence. It appears more or less mottled over the greater part of its extent, but this is more particularly the case with the portion lying next to the pellucid area; so much so, that around the pel- lucid area an inner ring of the opaque area may be distinguished from the rest by the difference of its aspect. The mottled appearance of this inner ring is due to changes taking place in the mesoblast above the germi- nal wall—changes which eventually result in the forma- IIL] SUMMARY. 75 tion of what is called the vascular area, the outer border of which marks the extreme limit to which the meso- blast extends. The changes then which occur during the first day may thus be briefly summarized : (1) The hypoblast is formed as a continuous layer of plate-like cells from the lower layer of the segmenta- tion spheres. (2) The primitive streak is formed in the hinder part of the area pellucida as a linear proliferation of epiblast cells. These cells spread out as a layer on each side of the primitive streak, and form part of the mesoblast. (3) The primitive groove is formed along the axis of the primitive streak. (4) The pellucid area becomes pear-shaped, the broad end corresponding with the future head of the embryo. Its long axis lies at right angles to the long axis of the egg. (5) The medullary plate with the medullary groove makes its appearance in front of the primitive groove. (6) The primitive hypoblast in the region of the medullary plate gives rise to an axial rod of cells forming the notochord, and to two lateral plates of mesoblast. The innermost stratum of the primitive layer forms the permanent hypoblast. (7) The development of the head-fold gives rise to the first definite appearance of the head. (8) The medullary folds rise up and meet first in the region of the mid-brain to form the neural tube. (9) By the cleavage of the mesoblast, the somato- pleure separates from the splanchnopleure. 76 THE FIRST DAY. [CHAP. III. (10) One or more pairs of mesoblastic somites make their appearance in the vertebral portion of the meso- blastic plates. (11) ‘he first trace of the amnion appears in front of the head-fold. (12) The vascular area begins to be distinguished from the rest of the opaque area. CHAPTER IV. THE CHANGES WHICH TAKE PLACE DURING THE FIRST HALF OF THE SECOND DAY. General development, In attempting to remove the blastoderm from an egg which has undergone from 30 to 36 hours’ incubation, the observer can- not fail to notice a marked change in the consist- ency of the blastodermic structures. The excessive delicacy and softness of texture which rendered the extraction of an 18 or 20 hours’ blastoderm so difficult, has given place to a considerable amount of firmness; the outlines of the embryo and its appendages are much bolder and more distinct; and the whole blastoderm can be removed from the egg with much greater ease. In the embryo itself viewed from above one of the features which first attracts attention is the progress in the head-fold (Fig. 27). The upper limb or head has become much more prominent, while the lower groove is not only proportionately deeper, but is also being carried back beneath the body of the embryo. The medullary folds are closing rapidly. In the region of the head they have quite coalesced, a slight notch in the middle line at the extreme front marking 78 THE SECOND DAY. [cHaP. for some little time their line of junction (Fig. 28). The open medullary groove of the first day has thus become converted into a tube, the neural canal, closed in front, but as yet open behind. Even before the Fie. 27. EMBRYO OF THE CHICK BETWEEN THIRTY AND THIRTY-SIX HOURS, VIEWED FROM ABOVE AS AN OPAQUE OBJECT. (Chromic acid preparation.) fb. front-brain : mb. mid-brain ; 2.6. hind-brain ; op.v. optic vesi- cle; au.p. auditory pit; o,f. vitelline vein ; p.v. mesoblastic somite; mf. line of junction of the medullary folds above the Iv.] THE BRAIN, 79 medullary canal; s.r, sinus rhomboidalis ; ¢. tail-fold ; p.r. remains of primitive groove (not satisfactorily represented) ; a.p. area pellucida. The line to the side between p.v. and mf. represents the true length of the embryo. The fiddle-shaped outline indicates the margin of the pellucid area. The head, which reaches as far back as 0.f,, is dis- tinctly marked off; but neither the somatopleuric nor splanchnopleuric folds are shewn in the figure ; the latter diverge at the level of o,f, the former considerably nearer the front, somewhere between the lines m.b. and 4.b, The optic vesicles op.v. are seen bulging out beneath the superfi- cial epiblast. The heart lying underneath the opaque body cannot be seen. The tail-fold ¢. is just indicated ; no dis- tinct lateral folds are as yet visible in the region midway between head and tail. At mf. the line of junction between the medullary folds is still visible, being lost forwards over the cerebral vesicles, while behind may be seen the remains of the sinus rhomboidalis, s.7. medullary folds coalesce completely in the cephalic region, the front end of the neural canal dilates into a small bulb, whose cavity remains continuous with the rest of the canal, and whose walls are similarly formed of epiblast. This bulb is known as the first cerebral vesicle, Fig. 27, fb, and makes its appearance in the early hours of the second day. From its sides two lateral processes almost at once grow out: they are known as the optic vesicles (Fig. 27, op.v.), and their history will be dealt with at length somewhat later. Behind the first cerebral vesicle a second and a third soon make their appearance; they are successively formed very shortly after the first vesicle; but the consideration of them may be conveniently reserved to a later period. At the level of the hind end of the 80 THE SECOND DAY. [CHAP. An Empryo CHIcK oF aBout THIRTY-sIx Hours. VIEWED FROM BELOW AS A TRANSPARENT OBJECT. FB. the fore-brain or first cerebral vesicle, projecting from the sides of which are seen the optic vesicles, op. A definite head is now constituted, the backward limit of the somato- pleure fold being indicated by the faint line 8.0. Around the head are seen the two limbs of the amniotic head-fold : one, the true amnion a, closely enveloping the head, the other, the false amnion a’, at some distance from it. The head is seen to project beyond the anterior limit of the pellucid area. The splanchnopleure folds extend as far back as sp. Along its diverging limbs are seen the conspicuous venous roots of Iv.] THE MESOBLASTIC SOMITES. 81 the vitelline veins, uniting to form the heart h, already established by the coalescence of two lateral halves which, continuing forward as the bulbus arteriosus b.a., is lost in the substance of the head just in front of the somatopleure fold. HB. hind-brain; MB. mid-brain; p.v. and v.pl. mesoblastic somites ; ch, front end of notochord ; mc. posterior part of notochord ; ¢. parietal mesoblast ; p/. outline of area pellu- cida ; pv. primitive streak. head two shallow pits are visible. They constitute the first rudiments of the organ of hearing, and are known as the auditory pits (Fig. 27, au.p.). The number of mesoblastic somites increases rapidiy by a continued segmentation of the vertebral plates of mesoblast. The four or five pairs formed during the first day have by the middle of the second increased to as many as fifteen. The addition takes place from before backwards; and the hindermost one is for some time placed nearly on a level with the boundary be- tween the hind end of the trunk of the embryo, and the front end of the primitive streak. For some time the already formed somites do not increase in size,. so that at first the embryo clearly elongates by addi- tions to its hinder end. Immediately behind the level of the last meso- blastic somite there is placed an enlargement of the unclosed portion of the medullary canal. This enlarge- ment is the sinus rhomboidalis already spoken of. It is shewn in Fig. 23. On its floor is placed the front end of the primitive streak. It is a purely embryonic structure which disappears during the second day. In a former chapter it was pointed out (p. 27) that the embryo is virtually formed by a folding F.& B. 6 82 THE SECOND DAY. [CHAP. or tucking in of a limited portion of the blastoderm, first at the anterior extremity, and afterwards at the posterior extremity and at the sides. One of the results of this doubling up of. the blastoderm to form the head is the appearance, below the anterior extremity of the medullary tube, of a short canal, ending blindly in front, but open widely behind (Fig. 29, D), a cul de sac, in fact, lined with hypoblast and reaching from the extreme front of the embryo to the point where the splanchnopleuric leaf of the head-fold (Fig. 29, F. Sp) turns back on itself. This cul de sac, which of course be- comes longer and longer the farther back the head-fold is carried, is the rudiment of the front end of the alimen- tary canal, the fore-gut, as it might be called. In trans- verse section it appears to be flattened horizontally, and also bent, so as to have its convex surface looking downwards (Fig. 30, al). At first the anterior end is quite blind, there being no mouth as yet; the formation of this at a subsequent date will be described later on. At the end of the first half of the second day the head-fold has not proceeded very far backwards, and its limits can easily be seen in the fresh embryo both from above and from below (Fig. 28). The heart. It is in the head-fold that the forma- tion of the heart takes place, its mode of origin being connected with that cleavage of the mesoblast and con- sequent formation of splanchnopleure and somatopleure of which we have already spoken. At the extreme end of the embryo (Fig. 29), where the blastoderm begins to be folded back, the mesoblast is never cleft, and here consequently there is neither somatopleure nor splanchnopleure; but at a point a Iv.] THE HEART. 83 Fic, 29. Me. —_—— aan = So ee DiacRAMMaTIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN Empryo. The section is supposed to be made at a time when the head- fold has commenced but the tail-fold has not yet appeared. N.C. neural canal, closed in front but as yet oven behind. Ch. notochord. The section being taken in the middle line, the protovertebre are of course not shewn. In front of the notochord is seen a mass of uncleft mesoblast, which will eventually form part of the skull. JD. the commencing foregut or front part of the alimentary canal. F. So. Somatopleure, raised up in its peripheral portion into the amniotic fold Am. Sp. Splanchnopleure. At Sp. it forms the under wall of the foregut; at F. Sp. it is turning round and about to run forward. Just at its turning point the cavity of the heart Ht. is being developed in its mesoblast. pp. pleuroperitoneal cavity. A epiblast, B mesoblast, C hypoblast, indicated in the rest of the figure by differences in the shading. At the part where these three lines of reference end the mesoblast is as yet uncleft. very little further back, close under the blind end of the foregut, the cleavage (at the stage of which we are speaking) begins, and the somatopleure, F.So, and splanchnopleure, F. Sp. diverge from each other. They 6—2 84 THE SECOND DAY. [CHaP. thus enclose between them a cavity, pp, which rapidly increases behind by reason of the fact that the fold of the splanchnopleure is carried on towards the hinder extremity of the embryo considerably in advance of that of the somatopleure. Both folds, after running a certain distance towards the hind end of the embryo, are turned round again, and then course once more for- wards over the yolk-sac. As they thus return (the somatopleure having meanwhile given off the fold of the amnion, Am.), they are united again to form the uncleft blastodermic investment of the yolk-sac. In this way the cavity arising from their separation is closed below. It is in this cavity, which from its mode of forma- tion the reader will recognise as a part (and indeed at this epoch it constitutes the greater part) of the general pleuroperitoneal cavity, that the heart is formed. This makes its appearance at the under surface and hind end of the foregut, just where the splanchnopleure folds turn round to pursue a forward course (Fig. 29, Ht.) ; and by the end of the first half of the second day (Fig. 28, h) has acquired somewhat the form of a flask with a slight bend to the right. At its anterior end a slight swelling marks the future bulbus arteriosus ; and a bulging behind indicates the position of the auricles. It is hollow, and its cavity opens behind into two vessels called the witelline veins (Figs. 27, o,f. and 28 sp.), which pass outwards in the folds of the splanchno- pleure at nearly right angles to the axis of the embryo. The anterior extremity of the heart is connected with the two aorte. The heart, including both its muscular wall and its Iv.] THE HEART, 85 epitheloid lining, is developed out of the splanchnic mesoblast on the ventral side of the throat. But since the first commencements of the heart make their appearance prior to the formation of the throat, the development of this organ is somewhat complicated; and in order to gain a clear conception of the manner in which it takes place the topography of the region where it is formed needs to be very distinctly under- stood. In the region where the heart is about to appear, the splanchnopleure is continually being folded in on either side, and these lateral folds are progressively meeting and uniting in the middle line to form the under or ventral wall of the foregut. At any given moment these folds will be found to have completely united in the middle line along a certain distance measured from the point in front where the cleavage of the mesoblast (i.e. the separation into somatopleure and splanch- nopleure) begins, to a particular point farther back. They will here be found to be diverging from the point where they were united, and not only diverging late- rally each from the middle line, but also both turning so as to run in a forward direction to regain the surface of the yolk and rejoin the somatopleure, Fig. 29. Ina transverse section taken behind this extreme point of union, or point of divergence, as we may call it, the splanchnopleure on either side when traced downwards from the axis of the embryo may be seen to bend in towards the middle so as to approach its fellow, and then to run rapidly outwards, Fig. 31, B. A longitudinal section shews that it runs forwards also at the same time, Fig. 29. A section through the very point of 86 THE SECOND DAY. [cHaP. divergence shews the two folds meeting in the middle line and then separating again, so as to form something like the letter 2, with the upper limbs converging, and the lower limbs diverging. In a section taken in front of the point of divergence, the lower diverging limbs of the # have disappeared altogether; nothing is left but the upper limbs, which, completely united in the middle line, form the under-wall of the fore- gut. As development proceeds, what we have called the point of divergence is continually being carried farther and farther back, so that the distance between it and the point where the somatopleure and splanchnopleure separate from each other in front, 7. ¢. the length of the foregut, is continually increasing. In the chick, as we have already stated, the heart commences to be formed in a region where the folds of the splanchnopleure have not yet united to form the ventral wall of the throat, and appears in the form of two thickenings of the mesoblast of the splanchno- pleure, along the diverging folds, ze. along the lower limbs of the #, just behind the point of divergence. These thickenings are continued into each other by a similar thickening of the mesoblast extending through the point of divergence itself. The heart has thus at first the form of an inverted V, and consists of two independent cords of splanchnic mesoblast which meet in front, without however uniting. As the folding-in of the splanchnopleure is continued backwards the two diverging halves of the heart are gradually brought together. Thus very soon the develop- ing heart has the form of an inverted Y, consisting of an Iv.] THE HEART. 87 unpaired portion in front and two diverging limbs be- hind. The unpaired portion is the true heart, while the diverging limbs are the vitelline veins already spoken of (Fig. 28, sp). While the changes just spoken of have been taking place in the external form of the heart, its internal parts have also become differentiated. A cavity is formed in each of the halves of the heart before even they have coalesced. Each of these cavities has at first the form of an irregular space Fic. 30. Al So SS ag7 BA Sp (g. On Para OLOY ch-~__ 4 ¢ BHA it e) of sad Lio gnise a TP ef GAC wo WF TRANSVERSE SECTION THROUGH THE PosTERIOR PaRT OF THE HEAD oF AN Empryo CuHick or THIRTY Hours. hb. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochord ; «. thickening of hypoblast (possibly a rudiment of the sub- notochordal rod); al. throat; At. heart; pp. body cavity ; so. somatic mesoblast ; sf splanchnic mesoblast ; Ay. hypo- blast. 88 THE SECOND DAY. [cHAP. between the splanchnic mesoblast and the wall of the throat (Fig. 30, At.). During their formation (Fig. 30), a thin layer of mesoblast remains in contact with the hypoblast, but connected with the main mass of the mesoblast of the heart by protoplasmic processes. A second layer next becomes split from the main mass of mesoblast, being still connected with the first layer by the above-mentioned protoplasmic processes. These two layers unite to form a tube which constitutes the epithe- lioid lining of the heart; the lumen of this tube is the cavity of the heart, and soon loses the protoplasmic trabecule which at first traverse it. The cavity of the heart may thus be described as being formed by a hollowing out of the splanchnic mesoblast. Some of the central cells of the original thickenings probably become blood-corpuscles. The thick outer part of the cords of splanchnic meso- blast which form the heart become the muscular walls and peritoneal covering of this organ. The muscular wall of each division of the heart has at first the form of a half tube widely open on its dorsal aspect, that is towards the hypoblast of the gut (Fig. 30 and 32). After the two halves of the heart have coalesced in the manner already explained, the muscular walls grow in towards the middle line on the dorsal side until they meet each other and coalesce, thus forming a complete tube as shewn diagrammatically in Fig. 31,A. They remain, however, at first continuous with the splanchnic mesoblast surrounding the throat, thus forming a pro- visional mesentery—the mesocardium—attaching the heart to the ventral wall of the throat. The epithelioid tubes formed in the two halves of the heart remain for Iv.] THE VASCULAR SYSTEM. 89 some time separate, and cause the cavity of the heart to be divided into two tubes even after its two halves have to all appearance completely coalesced’. Soon after its formation the heart begins to beat; its at first slow and rare pulsations beginning at the venous and passing on to the arterial end. It is of some interest to note that its functional activity commences long before the cells of which it is composed shew any distinct differentiation into muscular or nervous ele- ments. Vascular system. To provide channels for the fluid thus pressed by the contractions of the heart, a system of tubes has made its appearance in the meso- blast both of the embryo itself and of the vascular and pellucid areas. In front the single tube of the bulbus arteriosus bifurcates into two primitive aortew, each of which bending round the front end of the foregut, passes from its under to its upper side, the two forming together a sort of incomplete arterial collar imbedded in the mesoblast of the gut. Arrived at the upper side of the gut, they turn sharply round, and run separate but parallel to each other backwards towards the tail, in the mesoblast on each side of the notochord immediately under the mesoblastic somites (Figs. 32, Ao, 34, ao). About half way to the hinder extremity each gives off at right angles to the axis of the embryo a large branch, the vitelline artery (Fig. 36, Of, A.), which, passing outwards, is distributed over the pellucid and vascular areas, the main trunk of each aorta passing on with greatly diminished calibre towards the tail, in which it becomes lost. 1 This is not shewn in the diagram, Fig. 31, A. 90 THE SECOND DAY. [CHAP. Fie. 31. Two D1IaGRAMMATIC SECTIONS OF A THIRTY-SIX HOURS’ EmBryo ILLUSTRATING THE STRUCTURE OF THE HEART SHORTLY AFTER ITS FORMATION. A IS THE ANTERIOR SECTION. hb. hind brain; ne. notochord; £. epiblast ; so. somatopleure sp. splanchnopleure; d. alimentary canal; hy. hypoblast hz. (in A) heart ; of: vitelline vein. In A the two halves of the heart have coalesced to form an unpaired tube suspended from the ventral wall of the throat. Iv.] THE VASCULAR SYSTEM. 91 In B are seen in the diverging folds of the splanchnopleure the two vitelline veins (of) which will shortly unite to form the ductus venosus. Fie. 32. TRANSVERSE SECTION OF AN EMBRYO AT THE END OF THE Seconp Day PASSING THROUGH THE REGION oF THE BuLBus ARTERIOSUS. (Copied from His.) 4M. medullary canal in the region of the hind brain ; V. anterior cardinal vein; do. Aorta; Ch. Notochord; al. alimentary canal; H. Heart (bulbus arteriosus); Pp. Pleuroperitoneal cavity; am. amnion. In the vascular and pellucid areas, the formation of vascular channels with a subsequent differentiation into arteries, capillaries and veins, is proceeding rapidly. Blood-corpuscles too are being formed in considerable numbers. The mottled yellow vascular area becomes covered with red patches consisting of aggregations of blood-corpuscles, often spoken of as blood-islands. Round the extreme margin of the vascular area and nearly completely encircling it, is seen a thin red line, the sinus or vena terminalis (Fig. 36, Sv.). This will soon increase in size and importance. From the vascular and pellucid area several large channels are seen to unite and form two large trunks, 92 THE SECOND DAY. [cHAP. one on either side, which running along the splanch- nopleure folds at nearly right angles to the axis of the embryo, unite at the “point of divergence” to join the venous end of the heart. These are the vitelline veins spoken of above. Both vessels and corpuscles are formed entirely from the cells of the mesoblast; and in the regions where the mesoblast is cleft, are at first observed ex- clusively in the splanchnopleure. Ultimately of course they are found in the mesoblast everywhere. In the pellucid area, where the formation of the blood-vessels may be most easily observed, a number of mesoblastic cells are seen to send out processes (Fig. 33). These processes unite, and by their union a protoplasmic network is formed containing nuclei at the points from which the processes started. The nuclei, which as a rule are much elongated and contain large oval nucleoli, increase very rapidly by division, and thus form groups of nuclei at the, so to speak, nodal points of the network. Several nuclei may also be seen here and there in the processes themselves. The network being completed, these groups, by continued division of the nuclei, increase rapidly in'size; the protoplasm around them acquires a red colour, and the whole mass breaks up into blood-corpuscles (Fig. 33, b.c.) The proto- plasm. on the outside of each group, as well as that of the uniting processes, remains granular, and together with the nuclei in it forms the walls of the blood-vessels. A plasma is secreted by the walls, and in this the blood-corpuscles float freely. Each nodal point is thus transformed into a more or less rounded mass of blood-corpuscles floating in plasma but en- veloped by a layer of nucleated protoplasm, the several groups being united by strands of nucleated protoplasm. These uniting strands rapidly increase in thickness; new processes are also continually being formed; and thus the network is kept close and thickset while the area is increasing in size. By changes similar to those which took place in the nodal Iv.] THE VASCULAR SYSTEM. 93 points, blood-corpuscles make their appearance in the pro- cesses also,.the central portions of which become at the same time liquefied. By the continued widening of the connecting processes and solution of their central portions, accompanied by a corresponding increase in the enveloping nucleated cells, the original proto- Fia. 33. SurFaceE VIEW FROM BELOW OF A SMALL PORTION OF THE Postrriok Enp oF THE PELLUCID AREA OF A THIRTY-SIX Hours’ Cuick. To illustrate the formation of the blood- capillaries and blood-corpuscles, magnified 400 diameters. b.c. Blood-corpuscles at a nodal point, already beginning to acquire a red colour. They are enclosed in a layer of proto- plasm, in the outermost part of which are found nuclei, a. These nuclei subsequently become the nuclei of the cells forming the walls of the vessels. The nodal groups are united by protoplasmic processes (p.pr), also containing nuclei with large nucleoli (n). 94 THE SECOND DAY. (CHAP. plasmic network is converted into a system of communicating tubes, the canals of which contain blood-corpuscles and plasma, and the walls of which are formed of flattened nucleated cells. The blood-corpuscles pass freely from the nodal points into the hollow processes, and thus the network of protoplasm be- comes a network of blood-vessels, the nuclei of the corpuscles and of the walls of which have been, by separate paths of development, derived from the nuclei of the original protoplasm. The formation of the corpuscles does not proceed equally rapidly or to the same extent in all parts of the blastoderm. By far the greater part are formed in the vascular area, but some arise in the pellucid area, especially in the hinder part. In the front of the pellucid area the processes are longer and the network accordingly more open; the corpuscles also are both later in appearing and less numerous when formed. Assuming the truth of the above account, it is evident that the blood-vessels of the yolk-sack of the chick do not arise as spaces or channels between adjacent cells of the mesoblast, but are hollowed out in the communicating protoplasmic substance of the cells themselves. The larger vessels of the trunk are however probably formed as spaces between the cells, much as is the case with the heart. Wolffian duct. About this period there may be seen in transverse sections, taken through the embryo in the region of the seventh to the eleventh somite a small group of cells (Fig. 34, W. d) projecting on either side from the mass of uncleft mesoblast on the outside of the mesoblastic somites, into the somewhat triangular space bounded by the epiblast above, the upper and outer angle of the mesoblastic somite on the inside, and the somatic mesoblast on the outside. This group of cells is the section of a longitudinal ridge, the rudiment of the Wolffian duct or primitive duct of the excretory system; while the mass of cells Iv.] SUMMARY. 95 from which it springs is known as the intermediate cell mass. We shall return to them immediately. Summary. The most important changes then which take place during the first half of the second day are, the closure of the medullary folds, especially in the anterior part, and the dilatation of the canal so formed into the first cerebral vesicle; the establishment of a certain number of mesoblastic somites; the elevation of the head from the plane of the blastoderm; the forma- tion of the tubular heart and of the great blood-vessels ; and the appearance of the rudiment of the Wolffian duct. It is important to remember that the embryo of which we are now speaking is simply a part of the whole germinal membrane, which is gradually spreading over the surface of the yolk. It is important also to bear in mind that all that part of the embryo which is in front of the foremost somite corresponds to the future head, and the rest to the neck, body and tail. During this period the head occupies about a third of the whole length of the embryo. CHAPTER V. THE CHANGES WHICH TAKE PLACE DURING THE SECOND HALF OF THE SECOND DAY. ONE important feature of this stage is the rapid increase in the process of the folding-off of the embryo from the plane of the germ, and its consequent con- version into a distinct tubular cavity. At the begin- ning of the second day, the head alone projected from the rest of the germ, the remainder of the embryo being simply a part of a flat blastoderm, nearly com- pletely level from the front mesoblastic somite to the hind edge of the pellucid area. At this epoch, however, a tail-fold makes its appearance, elevating the tail above the level of the blastoderm in the same way that the head was elevated. Lateral folds also, one on either side, soon begin to be very obvious. By the progress of these, together with the rapid backward extension of the head-fold and the slower forward extension of the tail-fold, the body of the embryo becomes more and more distinctly raised up and marked off from the rest of the blastoderm. The medullary canal closes up rapidly. The wide sinus rhomboidalis becomes a narrow fusiform space, CHAP. V.]| THE BRAIN. 97 and at the end of this period is entirely roofed over. The conversion of the original medullary groove into a closed tube is thus completed. The brain. In the region of the head most im- portant changes now take place. We saw that at the beginning of this day the front end of the medullary canal was dilated into a bulb, the first cerebral vesicle, which by budding off two lateral vesicles became con- verted into three vesicles: a median one connected * by short hollow stalks with a lateral one on either side. The lateral vesicles known as the optic vesicles (Fig. 27, op. v, Fig. 35, a), become converted into parts of the eyes; the median one still retains the name of the first cerebral vesicle. The original vesicle being primarily an involution of the epiblast, the walls of all three vesicles are formed of epiblast; all three vesicles are in addition covered over with the common epiblastic investment which will eventually become the epidermis of the skin of the head. Between this superficial epiblast and the invo- luted epiblast of the vesicles, there exists a certain quantity of mesoblast to serve as the material out of which will be formed the dermis of the scalp, the skull, and other parts of the head. At this epoch, however, the mesoblast is found chiefly underneath the several vesicles (Fig. 30). A small quantity may in section be seen at the sides; but at the top the epidermic epiblast is either in close contact with the involuted epiblast of the cerebral and optic vesicles or separated from it by fluid alone, there being as yet in this region between the two no cellular elements representing the mesoblast. The constrictions marking off the optic vesicles also F. & B. 7 [CHAP. THE SECOND DAY. 98 Fia. 34, rs a ee EO ae ee TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN Empryo oF 45 HOURS. v.] THE BRAIN. 99 A. epiblast. B. mesoblast. C. hypoblast consisting of a single row of flattened cells. J/.c. medullary canal. P. v. meso- blastic somite. W.d. Wolffian duct. 8.0. Somatopleure. S.p. Splanchnopleure. . p. pleuroperitoneal cavity. ¢. h. notochord. a.o. dorsal aorta. ». blovd-vessels of the yolk- sac. o.p. line of junction between opaque and pellucid areas ; w, palisade-like yolk spheres which constitute the ger- minal wall. Only one-half of the section is represented in the figure—if completed it would be bilaterally symmetrical about the line of the medullary canal. take place of course beneath the common epiblastic investment, which is not involved in them. As a con- sequence, though easily seen in the transparent fresh Fic. 35. Heap oF A CHICK aT THE END OF THE SEcoND Day VIEWED FROM BELOW AS A TRANSPARENT OBJECT. (Copied from Huxley). I. first cerebral vesicle. a. optic vesicle. d. infundibulum. The specimen shews the formation of the optic vesicles (a), as outgrowths from the Ist cerebral vesicle or vesicle of the 3rd ventricle, so that the optic vesicles and vesicle of the 3rd ven- tricle at first freely communicated with each other, and also the growth of the lower wall of the vesicle of the 3rd ventricle into a process which becomes the infundibulum (@). 7—2 100 THE SECOND DAY. [CHAP, embryo (Fig. 28), they are but slightly indicated in hardened specimens (Fig. 27). When an embryo of the early part of the second day is examined as a transparent object, that portion of the medullary canal which lies immediately behind the first cerebral vesicle is seen to be conical in shape, witha its walls thrown into a number of wrinkles. These wrinkles may vary a good deal in appearance, and shift from time to time, but eventually, before the close of the second day, after the formation of the optical vesicles, settle down into two constrictions, one separat- ing the first cerebral vesicle from that part of the medullary canal which is immediately behind it, and the other separating this second portion from a third. So that instead of there being one cerebral vesicle only, as at the commencement of the second day, there is now, in addition to the optic vesicles, a series of three, one behind the other: a second and third cerebral vesicle have been added to the first (Fig. 27, mb, hb). They may be also called the “fore brain,” the “mid brain,” and the “hind brain,” for into these parts will they eventually be developed. The optic vesicles, lying underneath the epiblast, towards the end of the day are turned back and pressed somewhat backwards and downwards against the sides of the first cerebral vesicle or fore brain, an elongation of their stalks permitting this movement to take place. The whole head becomes in consequence somewhat thicker and rounder. Before the end of the day the fore brain elongates anteriorly. The part so established is not at first sepa- rate from that behind, but it is nevertheless the first v.] THE CRANIAL FLEXURE. 101 unpaired commencement of two vesicles which develop into the cerebral hemispheres ; but up to the end of the day it is still very small and inconspicuous. Early on the second day the commencements of several of the cranial nerves make their appearance as outgrowths of the (Fig. 30, vg) roof of the mid and hind brains, but their development, together with that of the spinal nerves, will be dealt with in the next chapter. The notochord. The notochord, whose origin was described in the account of the first day, is during the whole of the second day a very conspicuous object. It is seen as a transparent rod, somewhat elliptical in section (Fig. 34, ch), lying immediately underneath the medullary canal for the greater part of its length, and reaching forward in front as far as below the hind border of the first cerebral vesicle. Cranial flexure. Round the anterior termination of the notochord, the medullary canal, which up to the present time has remained perfectly straight, towards the end of the day begins to curve. The front portion of the canal, ze. the fore-brain with its optic and cere- bral vesicles, becomes slightly bent downwards, so as to form a rounded obtuse angle with the rest of the embryo. This is the commencement of the so-called cranial flecure and is, mechanically speaking, a con- sequence of the more rapid growth of the dorsal wall of the anterior part of the brain as compared with that of the ventral. Auditory vesicle. Lastly, as far as the head is concerned, the epiblastic plates forming the rudiments of the auditory vesicles become converted into deep pits 102 THE SECOND DAY. [ CHAP. opening one on each side of the hind-brain (Fig. 27, au. p). Heart. We left the heart as a fusiform body slightly bent to the right, attached to the under wall of the foregut by the mesocardium. The curvature now increases so much that the heart becomes almost w-shaped, the venous portion being drawn up towards the head so as to lie somewhat above (dorsal to) and behind the arterial portion. (It would perhaps be more correct to say that the free intermediate portion is by its own growth bent downwards, backwards, and some- what to the right, while the venous root of the heart is at the same time continually being lengthened by the carrying back of that “point of divergence” of the splanchnopleure folds which marks the union of the vitelline veins into a single venous trunk.) The heart then has at this time two bends, the one, the venous bend, the right-hand curve of the m; the other, the arterial bend, the left-hand curve of the m. The venous bend which, as we have said, is placed above and somewhat behind the arterial bend, becomes marked by two bulgings, one on either side. These are the rudiments of the auricles, or rather of the auricular appendages. The ascending limb of the arterial bend soon becomes conspicuous as the bulbus arteriosus, while the rounded point of the bend itself will here- after grow into the ventricles. Vascular system. The blood-vessels, whose origin during the first half of this day has been already described, become during the latter part of the day so connected as to form a complete system, through which a definite circulation of the blood is now for the first v.] THE VASCULAR SYSTEM. 103 time (consequently some little while after the com- mencement of the heart’s pulsation) carried on. The two primitive aorte have already been de- scribed as encircling the foregut, and then passing along the body of the embryo immediately beneath the mesoblastic somites on each side of the notochord. They are shewn in Figs. 32 A.o. and 34 a. in section as two large rounded spaces lined with flattened cells. At first they run as two distinct canals along the whole length of the embryo; but, after a short time, unite at some little distance behind the head into a single trunk, which lies in the middle line of the body immediately below the notochord (Fig. 57). Lower down, nearer the tail, this single primitive trunk again divides into two aorte, which, getting smaller and smaller, are finally lost in the small blood-vessels of the tail. At this epoch, therefore, there are two aortic arches springing from the bulbus arteriosus, and uniting above the ali- mentary canal in the back of the embryo to form the single dorsal aorta, which travelling backwards in the median line divides near the tail into two main branches. From each of the two primitive aorte, or from each of the two branches into which the single aorta, divides, there is given off on either side a large branch. These have been already spoken of as the vitelline arteries. At this stage they are so large that by far the greater part of the blood passing down the aorta finds its way into them, and a small remnant only pursues a straight course into the continuations of the aorta towards the tail. Each vitelline artery leaving the aorta at nearly right angles (at a point some little way behind the 104 THE SECOND DAY. [cHap, backward limit of the splanchnopleure fold which is forming the alimentary canal), runs outwards beneath the mesoblastic somites in the lower range of the meso- blast, close to the hypoblast. Consequently, when in its course outwards it reaches the point where the meso- blast is cleft to form the somatopleure and splanchno- pleure, it attaches itself to the latter. Travelling along this, and dividing rapidly into branches, it reaches the vascular area in whose network of small vessels (and also to a certain extent in the similar small vessels of the pellucid area) it finally loses itself. The terminations of the vitelline arteries in the vascular and pellucid areas are further connected with the heart in two different ways. From the network of capillaries, as we may call them, a number of veins take their origin, and finally unite into two main trunks, the vitelline veins. These have already been described as running along the folds of the splanchnopleure to form the venous roots of the heart. Their course is conse- quently more or less parallel to that of the vitelline arteries, but at some little distance nearer the head, inasmuch as the arteries run in that part of the splanch- nopleure which has not yet been folded in to form the ali- mentary canal. Besides forming the direct roots of the vitelline veins, the terminations of the vitelline arteries in the vascular area are also connected with the sinus terminalis spoken of above as running almost completely round, and forming the outer margin of the vascular area. This (Fig. 36, ST.), may be best described as composed of two semicircular canals, which nearly meet at points opposite the head and opposite the tail, thus all but encircling the vascular area between them. At the v.] THE VASCULAR SYSTEM. 105 point opposite the head the end of each semicircle is connected with vessels (Fig. 36), which run straight in towards the heart along the fold of the splanchnopleure, and join the right and left vitelline veins. At the point opposite the tail there is at this stage no such definite connection. At the two sides, midway between their head and tail ends, the two semicircles are espe- cially connected with the vitelline arteries. The circulation of the blood then during the latter half of the second day may be described as follows. The blood brought by the vitelline veins falls into the twisted cavity of the heart, and is driven thence through the bulbus arteriosus and aortic arches into the aorta. From the aorta, by far the greater part of the blood flows into the vitelline arteries, only a small remnant passing on into the caudal terminations. From the capillary net-work of the vascular and pellucid areas into which the vitelline arteries discharge their contents, part of the blood is gathered up at once into the lateral or direct trunks of the vitelline veins. Part however goes into the middle region of each lateral half of the sinus terminalis, and there divides on each side into two streams. One stream, and that the larger one, flows in a forward direction until it reaches the point opposite the head, thence it returns by the veins spoken of above, straight to the vitelline trunks. The other stream flows backward, and becomes lost at the point opposite to the tail. This is the condition of things during the second day; it becomes considerably changed on the succeeding day. At the time that the heart first begins to beat the capillary system of the vascular and pellucid areas is 106 THE SECOND DAY. (CHAP. not yet completed; and the fluid which is at first driven by the heart contains, according to most observers, very few corpuscles. At the close of the second day the single pair of aortic arches into which the bulbus arteriosus divides is found to be accompanied by a second pair, formed in the same way as the first, and occupying a position a little behind it. Sometimes even a third pair is added. Of these aortic arches we shall have to speak more fully later on. Wolffian duct. During the latter half of the second day the Wolffian duct to which we have already alluded becomes fully established, while the first traces of the embryonic excretory organs or kidneys, known as the Wolffian bodies, make their appearance. The develop- ment of the latter will be dealt with in the history of the third day, but the history of the duct itself may conveniently be completed here. The first trace of it is visible in an embryo Chick with eight somites, as a ridge projecting from the inter- mediate cell mass towards the epiblast in the region of the seventh somite. In the course of further develop- ment it continues to constitute such a ridge as far as the eleventh somite (Fig. 34 Wd.), but from this point it grows backwards by the division of its cells, as a free column in the space between the epiblast and mesoblast. In an embryo with fourteen somites of about the stage represented in fig. 28 a small lumen has appeared in its middle part, and in front it is connected with rudimentary Wolffian tubules, which develop in con- tinuity with it. In the succeeding stages the lumen of the duct gradually extends backwards and forwards, v.] THE AMNION. 107 and the duct itself also passes inwards relatively to the epiblast (fig. 43 wd). Its hind end elongates till it comes into connection with, and opens on the fourth day into the cloacal section of the hind-gut. The amnion and allantois. The amnion, especially the anterior or head fold, advances in growth very rapidly during the second day, and at the close of the day completely covers the head and neck of the embryo; so much so that it is necessary to tear or remove it when the head has to be examined in hardened opaque speci- mens. The tail and lateral folds of the amnion, though still progressing, lag considerably behind the head-fold. The side-folds eventually meet in the median dorsal line, and their coalescence proceeds backwards from the head-fold in a linear direction, till there is only a small opening left over the tail of the embryo. This finally becomes closed early on the third day. In Figs. 32 and 43 am. the folds of the amnion are shewn before they have coalesced. After the coalescence of the folds of the amnion above the embryo the two limbs of which each is formed become, as already ex- plained in chapter IL, separate from each other: the inner, forming a special investment of the embryo, and constituting the amnion proper (Fig. 65), the outer at- taching itself to the vitelline membrane and becoming the serous envelope. The development of the allantois commences during the second day, but since it is mainly completed during the third day we need not dwell upon it further in this place. Summary. The chief events, then, which occur during the second half of the second day are as follow:— 108 THE SECOND DAY. [CHAP. V. 1. The second and third cerebral vesicles make their appearance behind the first. .2. The optic vesicles spring as hollow buds from the lateral, and the unpaired commencement of the cere- bral hemispheres from the front, portions of the first cerebral vesicle. 3. The auditory plate becomes converted into a pit, opening at the side of the hind-brain or third cere- bral vesicle. 4. The first indications of the cranial flextire be- come visible. 5. The head-fold, and especially the splanchno- pleure moiety, advances rapidly backwards; the head of the embryo is in consequence more definitely formed. The tail-fold also becomes distinct. 6. The curvature of the heart increases; the first rudiments of the auricles appear. 7. The circulation of the yolk-sac is established. 8. The amnion grows rapidly, and the allantois commences to be formed. CHAPTER VI. THE CHANGES WHICH TAKE PLACE DURING THE THIRD DAY. Or all days in the history of the chick within the egg this perhaps is the most eventful; the rudi- ments of so many important organs now first make their appearance. In many instances we shall trace the history of these organs beyond the third day of incubation, in order to give the reader a complete view of their development. On opening an egg on the third day the first thing which attracts notice is the diminution of the white of the egg. This seems to be one of the consequences of the functional activity of the newly-established vascular area whose blood-vessels are engaged either in directly absorbing the white or, as is more probable, in absorbing the yolk, which is in turn replenished at the expense of the white. The absorption, once begun, goes on so actively that, by the end of the day, the decrease of the white is very striking. The blastoderm has now spread over about half the yolk, the extreme margin of the opaque area reach- 110 THE THIRD DAY. [cHAP. ing about half-way towards the pole of the yolk opposite to the embryo, The vascular area, though still increasing, is much smaller than the total opaque area, being in average- sized eggs about as large as a florin. Still smaller than the vascular area is the pellucid area in the centre of which lies the rapidly growing embryo. During the third day the vascular area is not only a means for providing the embryo with nourish- ment from the yolk, but also, inasmuch as by the dimi- nution of the white it is brought close under the shell and therefore fully exposed to the influence of the atmosphere, serves as the chief organ of respiration. This in fact is the period at which the vascular area may be said to be in the stage of its most complete de- velopment; for though it will afterwards become larger, it will at the same time become less definite and rela- tively less important. We may therefore, before we proceed, add a few words to the description of it given in the last chapter. The blood leaving the body of the embryo by the vitelline arteries (Fig. 36, R. Of. A., L. Of A) is carried to the small vessels and capillaries of the vascu- lar area, a small portion only being appropriated by the pellucid area. From the vascular area part of the blood returns directly to the heart by the main lateral trunks of the vitelline veins, R. Of, L. Of. During the second day these venous trunks joined the body of the embryo considerably in front of, that is, nearer the head than, the corresponding arterial ones. Towards the end of the third day, owing to the continued lengthening of v1.] THE VASCULAR AREA 111 AX} PAY SIE ‘4 (ns 4 \ a Zi 7 f} \f eietecaep Ae Ny we x \y AW ROPA DiaGRaM OF THE CIRCULATION OF THE YOLK-SACK AT THE END OF THE THIRD Day oF INCUBATION. H. heart. 4A. the second, third and fourth aortic arches ; the first has become obliterated in its median portion, but is continued at its proximal end as the external carotid, and at its distal end as the internal carotid. AQ. dorsal aorta. ZL. Of. A. left vitelline artery. R. Of. A. right vitelline artery. S. 7. sinus terminalis. JZ. Of left vitelline vein. R. Of right vitelline vein. §&. V. sinus venosus. D. C. ductus Cuvieri. S. Ca. V. superior cardinal or jugular vein. V. Ca. inferior cardinal vein. The veins are marked in 112 THE THIRD DAY. [ CHAP. outline and the arteries are made black. The whole blasto- derm has been removed from the egg and is supposed to be viewed from below. Hence the left is seen on the right, and vice versd. the heart, the veins and arteries run not only parallel to each other, but almost in the same line, the points at which they respectively join and leave the body being nearly at the same distance from the head. The rest of the blood brought by the vitelline arteries finds its way into the lateral portions of the sinus terminalis, S.7., and there divides on each side into two streams. Of these, the two which, one on each side, flow backward, meet at a point about oppo- site to the tail of the embryo, and are conveyed along a distinct vein which, running straight forward parallel to the axis of the embryo, empties itself into the left vitel- line vein. The two forward streams reaching the gap in the front part of the sinus terminalis fall into either one, or in some cases two veins, which run straight backward parallel to the axis of the embryo, and so reach the roots of the heart. When one such vein only is present, it joins the left vitelline trunk; where there are two they join the left and right vitelline trunks respectively. The left vein is always considerably larger than the right; and the latter when present rapidly gets smaller and speedily disappears. The chief differences, then, between the peripheral circulation of the second and of the third day are due to the greater prominence of the sinus terminalis and the more complete arrangements for returning the blood from it to the heart. After this day, although the vas- cular area will go on increasing in size until it finally VI.] CHANGE OF POSITION OF THE EMBRYO. 113 all but encompasses the yolk, the prominence of the sinus terminalis will become less and less in proportion as the respiratory work of the vascular area is shifted on to the allantois, and its activities confined to absorb- ing nutritive matter from the yolk. The folding-in of the embryo makes great pro- gress during this day. Both head and tail have become most distinct, and the side folds which are to constitute the lateral walls have advanced so rapidly that the embryo is now a bond fide tubular sac, connected with the rest of the yolk by a broad stalk. This stalk, as was explained in Chap. II, is double, and consists of an inner splanchnic stalk continuous with the alimen- tary canal, which is now a tube closed at both ends and open to the stalk along its middle third only, and an outer somatic stalk continuous with the body-walls of the embryo, which have not closed nearly to the same extent as the walls of the alimentary canal. (Compare Fig. 9, 4 and B, which may be taken as diagrammatic representations of longitudinal and transverse sections of an embryo of this period.) The embryo is almost completely covered by the amnion. Early in this day the several amniotic folds will have met and completely coalesced along a line over the back of the embryo in the manner already explained in the last chapter. During this day a most remarkable change takes place in the position of the embryo. Up to this time it has been lying symmetrically upon the yolk with the part which will be its mouth directed straight downwards. It now turns round so as to lie on its left side. BF. & B. . 8 114 THE THIRD DAY. (cHaP Fie. 37. CHIcK or THe THtrD Day (Firty-rour Hours) VIEWED FROM UNDERNEATH AS A TRANSPARENT OBJECT. a’. the outer amniotic fold or false amnion. This is very con- spicuous around the head, but may also be seen at the tail. a. the true amnion, very closely enveloping the head, and here seen only between the projections of the several cerebral vesicles.. It may also be traced at the tail. In the embryo of which this is a drawing, the head-fold of the amnion reached a little farther backward than the reference w, v1] GENERAL VIEW OF EMBRYO. 115 but its limit could not be distinctly seen through the body of the embryo. The prominence of the false amnion at the head is apt to puzzle the student; but if he bears in mind the fact, which could not well be shewn in Fig. 9, that the whole amniotic fold, both the true and the false limb, is tucked in underneath the head, the matter will on reflection become intelligible. C. H. cerebral hemisphere. /”. B. thalamencephalon or vesicle of the third ventricle. MU. B. mid-brain. H. B. hind-brain. Op. optic vesicle. Ot. otic vesicle. Of V. vitelline veins forming the venous roots of the heart. The trunk on the right hand (left trunk when the embryo is viewed in its natural position from: above) receives a large branch, shewn by dotted lines, coming from the anterior portion of the sinus terminalis. Hi. the heart, now completely twisted on itself. Ao. the bulbus arteriosus, the three aortic arches being dimly seen stretching from it across the throat, and uniting into the aorta, still more dimly seen as a curved dark line running along the body. The other curved dark line by its side, ending near the reference y, is the notochord ch. About opposite the line of reference xz the aorta divides into two trunks, which, running in the line of the somewhat opaque mesoblastic somites on either side, are not clearly seen. Their branches however, Ofa, the vitelline arteries, are conspicuous and are seen to curve round the commencing side folds. Pv. mesoblastic somites. Below the level of the vitelline arteries the vertebral plates are but imperfectly cut up into meso- blastic somites, and lower down still, not at all. a is placed at the “point of divergence” of the splanchnopleure folds. The blind foregut begins here and extends about up to y. w# therefore marks the present hind limit of the splanchnopleure folds. The limit of the more transparent somatopleure folds is not shewn. 1t will be of course understood that all the body of the embryo above the level of the reference x, is seen through the portion of the yolk-sac (vascular and pellucid area), which has been removed 8—2 116 THE THIRD DAY. [CHAP, with the embryo from the egg, as well as through the double amniotic fold. We may repeat that, the view being from below, whatever is described in the natural position as being to the right here appears to be left, and vice versd. This important change of position at first affects only the head (Fig. 37), but subsequently extends also to the trunk. It is not usually completed till the fourth day. Atthe same time the left vitelline vein, the one on the side on which the embryo comes to lie, grows very much larger than the right, which henceforward gradu- ally dwindles and finally disappears. Coincidently with the change of position the whole embryo begins to be curved on itself in a slightly spiral manner. This curvature of the body becomes still more marked on the fourth day, Fig. 67. In the head very important changes take place. One of these is the cramal flexure, Figs. 37, 38. This (which must not be confounded with the curvature of the body just referred to) we have already seen was commenced in the course of the second day, by the bending downwards of the head round a point which may be considered as the extreme end either of the notochord or of the alimentary canal. The flexure progresses rapidly, the front-brain being more and more folded down till, at the end of the third day, it is no longer the first vesicle or fore-brain, but the second cerebral vesicle or mid-brain, which occupies the extreme front of the long axis of the embryo. In fact a straight line through the long axis of the embryo would now pass through the mid-brain instead of, as at the beginning of the second day, through the fore-brain, vi.] THE BRAIN, 117 so completely has the front end of the neural canal been folded over the end of the notochord. The com- mencement of this cranial flexure gives the body of an embryo of the third day somewhat the appearance of a retort, the head of the embryo corresponding to the bulb. On the fourth day the flexure is still greater than on the third, but on the fifth and succeeding days it becomes less obvious, owing to the filling up of the parts of the skull. The brain. The vesicle of the cerebral hemispheres, which on the second day began to grow out from the front of the fore-brain, increases rapidly in size during the third day, growing out laterally, so as to form two vesicles, so much so that by the end of the day it (Fig. 37, CH, Fig. 38) is as large or larger than the original vesicle from which it sprang, and forms the most con- spicuous part of the brain. In its growth it pushes aside the optic vesicles, and thus contributes largely to the roundness which the head is now acquiring. Hach lateral vesicle possesses a cavity, which afterwards becomes one of the lateral ventricles. These cavities are continuous behind with the cavity of the fore-brain. Owing to the development of the cerebral vesicle the original fore-brain no longer occupies the front position (Fig. 37, FB, Fig. 38, Ib), and ceases to be the con- spicuous object that it was. Inasmuch as its walls will hereafter be developed into the parts surrounding the so-called third ventricle of the brain, we shall hence- forward speak of it as the vesicle of the third ventricle, or thalamencephalon. On the summit of the thalamencephalon there may now be seen a small conical projection, the rudiment of 118 THE THIRD DAY. (CHAP. Fie, 38. Heap oF A OHICK OF THE THIRD Day VIEWED SIDEWAYS AS A TRANSPARENT OBJECT. (From Huxley.) Ta. the vesicle of the cerebral hemisphere. I0. the vesicle of the third ventricle (the original fore-brain) ; at its summit is seen the projection of the pineal gland e. Below this portion of the brain is seen, in optical section, the optic vesicle a already involuted with its thick inner and thinner outer wall (the letter a is placed on the junction of the two, the primary cavity being almost obliterated). In the centre of the vesicle lies the lens, the shaded portion being the expression of its cavity. Below the lens between the two limbs of the horse- shoe is the choroidal fissure. II. the mid-brain. IIT. the hind-brain. V. the rudiments of the fifth cranial nerve, VII. of the seventh. Below the seventh nerve is seen the auditory vesicle 6. The head having been subjected to pressure, the vesicle appears somewhat distorted as if squeezed out of place. The orifice is not yet quite closed up. 1, the inferior maxillary process of the first visceral or man- dibular fold, Below, and to the right of this, is seen the first visceral cleft, below that again the second visceral fold (2), and lower down the third (3) and fourth (4) visceral folds. In front of the folds (¢.e. to’ the left) is seen the arterial end of the heart, the aortic arches being buried in their respective visceral folds. f. represents the mesoblast of the base of the brain and spinal cord, vI.] THE PITUITARY BODY. 119 the pineal gland (Fig. 38, e), while the centre of the floor is produced into a funnel-shaped process, the infun- dibulum (Fig. 39, In), which, stretching towards the Fie. 39. LonerrupinaL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EMBRYO. cer, commencement of cerebral hemisphere; jn. pineal gland ; Zn. infundibulum ; pt. ingrowth of mouth to form the pituitary body; mb. mid-brain ; cb. cerebellum; ch. noto- chord ; al. alimentary tract ; Jaa. artery of mandibular arch. extreme end of the oral invagination or stomodeum, joins a diverticulum of this which becomes the pituitary body. The development of the pituitary body or hypophysis cerebri has been the subject of considerable controversy amongst embryo- logists, and it is only within the last few years that its origin from the oral epithelium has been satisfactorily established. In the course of cranial flexure the epiblast on the under side of the head becomes tucked in between the blind end of the throat and the base of the brain. The part so tucked in constitutes a kind of bay, and forms the stomodeum or primitive buccal cavity already spoken of. The blind end of this bay becomes produced as a papilliform diverticulum which may be called the pituitary diverticulum. It is represented as it appears in a 120 THE THIRD DAY. [CHAP, lower vertebrate embryo (Elasmobranch) in Fig. 39, but is in all important respects exactly similar in the chick. Very shortly after the pituitary diverticulum becomes first established the boundary wall between the stomodeum and the throat becomes perforated, and the limits of the stomodeum obliterated, so that the pituitary diverticulum looks as if it had arisen from the hypoblast. During the third day of incubation the front part of the notochord becomes bent downward, and, ending in a somewhat enlarged extremity, comes in contact with the termination of the pituitary diverticulum. The mesoblast around increases and grows up, in front of the notochord and behind the vesicle of the third ventricle, to form the posterior clinoid process. The base of the vesicle of the third ventricle at the same time grows downwards towards the pituitary diverticulum, and forms what is known as the infundibulum. On the fourth day the mesoblastic tissue around the notochord increases in quantity, and the end of the notochord, though still bent downwards, recedes a little from the termination of the pituitary diverticulum, which is still a triangular space with a wide opening into the alimentary canal. On the fifth day, the opening of the pituitary diverticulum into the alimentary canal has become narrowed, and around the whole diverticulum an investment of mesoblast-cells has appeared. Behind it the clinoid process has become cartilaginous, while to the sides and in front it is enclosed by the trabecule. At this stage, in fact, we have a diverticulum from the alimentary canal passing through the base of skull to the infundibulum. On the seventh day the communication between the cavity of the diverticulum and that of the throat has become still narrower. The diverticulum is all but converted into a vesicle, and its epiblastic walls have commenced to send out into the mesoblastic investment solid processes. The infundibulum now appears aS a narrow process from the base of the vesicle of the third ventricle, which approaches, but does not unite with, the pituitary vesicle. By the tenth day the opening of the pituitary vesicle into the throat becomes almost obliterated, and the lumen of the vesicle itself very much diminished. The body consists of anastomosing cords of epiblast-cells, the mesoblast between VI] THE PITUITARY BODY. 121 which has already commenced to become vascular. The cords or masses of epiblast cells are surrounded by a delicate mem- brana propria, and a few of them possess a small lumen. The infundibulum has increased in length. The relative positions of the pituitary body and infundibulum are shewn in the figure of the brain in Chapter vit. On the twelfth day the communication between the pituitary vesicle and the throat is entirely obliterated, but a solid cord of cells still connects the two. The vessels of the pia mater of the vesicle of the third ventricle have become connected with the pituitary body, and the infundibulum has grown down along its posterior border. In the later stages all connection is lost between the pituitary body and the throat, and the former becomes attached to the elongated processus infundibult. The real nature of the pituitary body is still extremely obscure, but it is not improbably the remnant of a glandular structure which may have opened into the mouth in primitive vertebrate forms, but which has ceased to have a function in existing vertebrates}. Beyond an increase in size, which it shares with nearly all parts of the embryo, and the change of position to which we have already referred, the mid- brain undergoes no great alteration during the third day. Its roof will ultimately become developed into the corpora bigemina or optic lobes, its floor will form the crura cerebri, and its cavity will be reduced to the narrow canal known as the ter a tertio ad quartum ventriculum. In the hind-brain, or third cerebral vesicle, that part which lies nearest to the mid-brain, is during ‘ 1 Wilhelm Miiller Ueber die Entwicklung und Bau der Hypophysis wnd des Processus Infundibuli Cerebri, Jenaische Zeitschrift, Bd. v1. 1871, and V. von Mihalkovics, Wirbelsaite u. Hirnanhang, Archiv f. mikr. Anat, Vol. x1. 1875. 122 THE THIRD DAY. [CHAP. the third day marked off from the rest by a slight constriction. This distinction, which becomes much more evident later on by a thickening of the walls and roof of the front portion, separates the hind-brain into the cerebellum in front, and the medulla oblongata behind (Figs. 38 and 39). While the walls of the cerebellar portion of the hind-brain become very much thickened as well at the roof as at the floor and sides, the roof of the posterior or medulla oblongata portion thins out into a mere membrane, forming a delicate covering to the cavity of the vesicle (Fig. 40, Iv), which here becoming broad and shallow with greatly thick- ened floor and sides, is known as the fourth ventricle, subsequently overhung by the largely developed pos- terior portion of the cerebellum. The third day, therefore, marks the differentiation of the brain into five distinct parts: the cerebral hemispheres, the central masses round the third ventricle, the corpora bigemina or optic lobes, the cerebellum and the medulla oblongata; the original cavity of the neural canal at the same time passing from its temporary division of three single cavities into the permanent arrangement of a series of connected ventricles, viz. the lateral ventricles, the third ventricle, the iter (with a prolongation into the optic lobe on each side), and the fourth ventricle. At the same time that the outward external shape of the brain is thus being moulded, internal changes are taking place in the whole neural canal. These are best seen in sections. At its first formation, the section of the cavity of the neural canal is round, or nearly so. VI] THE CRANIAL AND SPINAL NERVES. 123 About this time, however, the lining of involuted epiblast along the length of the whole spinal cord becomes very much thickened at each side, while increasing but little at the mid-points above and below. The result of this is that the cavity as seen in section (Figs. 64 and 65), instead of being circular, has become a narrow vertical slit, almost completely filled in on each side. In the region of the brain the thickening of the lining epiblast follows a somewhat different course. While almost everywhere the sides and floor of the canal are greatly thickened, the roof in the region of the various ventricles, especially of the third and fourth, becomes excessively thin, so as to form a membrane reduced to almost a single layer of cells. (Fig. 40, Iv.) Cranial and spinal nerves. A most important event which takes place during the second and third days, is the formation of the cranial and spinal nerves. Till within a comparatively recent period embryologists were nearly unanimous in believing that the peripheral nerves originated from the mesoblast at the sides of the brain and spinal cord. This view has now however been definitely disproved, and it has been established that both the cranial and spinal nerves take their origin as outgrowths of the central nervous system. The cranial nerves are the first to be developed and arise before the complete closure of the neural groove. They are formed as paired outgrowths of a continuous band known as the neural band, composed of two laminz, which connects the dorsal edges of the incom- pletely closed neural canal with the external epiblast. This mode of development will best be understood by 124 THE THIRD DAY. [cHar. Fie. 40. AOA SECTION THROUGH THE HInD-BRAIN OF A CHICK AT THE END oF THE TurrD Day or INCUBATION. JV. Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle. Ch. Notochord—(diagrammatic shading). CY. Anterior cardinal or jugular vein. CC. Involuted auditory vesicle. CC points to the end which will form the cochlear canal. AZ. Recessus labyrinthi. hy. hypoblast lining the alimentary canal. fy is itself placed in the cavity of the alimentary canal, in that part of the canal which will become the throat. The ventral (anterior) wall of the canal is not shewn in the section, but on each side are seen portions of a pair of visceral arches. In each arch is seen the section of the aortic arch AOA belonging to the visceral arch. The vessel thus cut through is running upwards towards the head, being about to join the dorsal aorta AO, Had the section been nearer the head, and carried through the plane at which the aortic arch curves VL] THE CRANIAL AND SPINAL NERVES. 125 round the alimentary canal to reach the mesoblast above it, AOA and AO would have formed one continuous curved space. In sections lower down in the back the two aorte, AO, one on each side, would be found fused into one median canal. an examination of Fig. 41, where the two roots of the vagus nerve (vg) are shewn growing out from the neural band. Shortly after this stage the neural band becomes separated from the external epiblast, and constitutes Fic, 41. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE Heap oF AN Empryo Cuick oF Turrty Hours. Ab. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochord ; a. thickening of hypoblast (possibly a rudiment of the sub- notochordal rod) ; al. throat ; At. heart ; pp. body cavity ; so. somatic mesoblast ; sf splanchnic mesoblast ; hy. hypo- blast. 126 THE THIRD DAY. (CHAP. a crest attached to the roof of the brain, while its two laminz become fused. Anteriorly, the neural crest extends as far as the roof of the mid-brain. The pairs of nerves which undoubtedly grow out from it are the fifth pair, the seventh and auditory (as a single root), the glosso- pharyngeal and the various elements of the vagus (as a single root). After the roots of these nerves have become estab- lished, the crest connecting them becomes partially obliterated. The roots themselves grow centrifugally, and eventually give rise to the whole of each of the cranial nerves. Each complete root develops a gan- glionic enlargement near its base, and (with the ex- ception of the third nerve) is distributed to one of the visceral arches, of which we shall say more hereafter. The. primitive attachment of the nerves is to the roof of the brain, but in most instances this attachment is replaced by a secondary attachment to the sides or floor. The rudiments of four cranial nerves, of which two lie in front of and two behind the auditory vesicle, are easily seen during the third day at the sides of the hind-brain. They form a series of four small opaque masses, somewhat pearshaped, with the stalk directed away from the middle line. The most anterior of these is the rudiment of the fifth nerve (Figs. 42 and 67, V). Its narrowed outer portion or stalk divides into two bands or nerves. Of these one passing towards the eye terminates at present in the immediate neighbourhood of that organ. The other branch (the rudiment of the inferior maxillary v1] THE CRANIAL NERVES. 127 Fie. 42. Heap oF an Empryo CHICK oF THE THIRD Day (SEVENTY- Five Hours) VIEWED SIDEWAYS AS A TRANSPARENT OBJECT. (From Huxley.) Ia, cerebral hemispheres. 0. vesicle of the third ventricle. IT. mid-brain, III. hind-brain. g. nasal pit. w. optic vesicle. 6. otic vesicle. d. infundibulum. e. pineal body. A. noto- chord. V. fifth nerve. VII. seventh nerve. VIII. united glossopharyngeal and pneumogastric nerves. I, 2, 3, 4, 5 the five visceral folds. branch of tne fifth nerve) is distributed to the first visceral arch. The second mass (Figs. 42 and 67, VII) is the rudi- ment of the seventh, or facial nerve, and of the audi- tory nerve. It is the nerve of the second visceral arch. The two masses behind the auditory vesicle repre- sent the glossopharyngeal and pneumogastric nerves (Fig. 42, VIII, Fig. 67, G. Ph. and Pq.). At first united, they subsequently become separate. The glosso- pharyngeal supplies the third arch, and the pneumo- gastric the fourth and succeeding arches. The later development of the cranial nerves has only been partially worked out, and we will confine ourselves here to a very 128 THE THIRD DAY. [ CHAP. brief statement of some of the main results arrived at. The outgrowth for the vagus nerve supplies in the embryo the fourth and succeeding visceral arches, and from what we know of it in the lower vertebrate types, we may conclude that it is a compound nerve, composed of as many primitively distinct nerves as there are branches to the visceral arches. The glossopharyngeal nerve is the nerve supplying the third visceral arch, the homologue of the first branchial arch of Fishes. The development of the hypoglossal nerve is not known, but it is perhaps the anterior root of a spinal nerve. The spinal accessory nerve has still smaller claims than the hypoglossal to be regarded as a true cranial nerve. The primitively single root of the seventh auditory nerves divides almost at once into two branches. The anterior of these pursues a straight course to the hyoid arch and forms the rudiment of the facial nerve, Fig. 67, vi1; the second of the two, which is the rudiment of the auditory nerve, develops a ganglionic enlargement, and, turning backwards, closely hugs the ventral wall of the auditory involution. The sixth nerve appears to arise later than the seventh nerve from the ventral part of the hind-brain, and has no ganglion near its root. Shortly after its development the root of the fifth nerve shifts so as to be attached about half-way down the side of the brain. A large ganglion is developed close to the root, which becomes the Gasserian ganglion. The main branch of the nerve grows into the mandibular arch (Fig. 67), maintaining towards it similar relations to those of the nerves behind it to their respective arches. An important branch becomes early developed which is directed straight towards the eye (Fig. 67), near which it meets and unites with the third nerve, where the ciliary ganglion is developed. This branch is usually called the ophthalmic branch of the fifth nerve, and may perhaps represent an inde- pendent nerve. Later than these two branches there is developed a third branch, passing the upper process of the first visceral arch. It forms the superior maxillary branch of the adult. Nothing is known with reference to the development of the fourth nerve. VI] THE SPINAL NERVES. 129 The history of the third nerve is still imperfectly known. There is developed early on the second day from the neural crest, on the roof of the mid-brain, an outgrowth on each side, very similar to the rudiment of the posterior nerves. This out- growth is believed by Marshall to be the third nerve, but it must be borne in mind that there is no direct evidence on the point, the fate of the outgrowth in question not having been satisfac- torily followed. At a very considerably later period a nerve may be found springing from the floor of the mid-brain, which is undoubtedly the third nerve. If identical with the outgrowth just spoken of, it must have shifted its attachment from the roof to the floor of the brain. The nerve when it springs from the floor of the brain runs directly backwards till it terminates in the ciliary ganglion, from which two branches to the eye-muscles are given off. [A. Marshall, “The development of the cranial nerves in the Chick.” Quart. Journal of Microscop. Science, Vol. xvutr.] In the case of the spinal nerves the posterior roots originate as outgrowths of a series of median processes of cells, which make their appearance on the dorsal side of the spinal cord. The outgrowths, symmetrically placed on each side, soon take a pyriform aspect, and apply themselves to the walls of the spinal cord. They are represented as they appear in birds in Fig. 43, sp. g., and as they appear in a lower vertebrate form in Fig. 44. The original attachment of the nerve-rudiment to the medullary wall is not permanent. It becomes, in fact, very soon either extremely delicate or absolutely interrupted. The nerve-rudiment now becomes divided into three parts, (1) a proximal rounded portion; (2) an enlarged middle portion, forming the rudiment of a ganglion ; (8) a distal portion, forming the commencement of the nerve. The proximal portion may very soon be observed to be F. & B. 9 130 THE THIRD DAY. [CHAP. Fie, 43, apg an eu BOSSs —, ee ai TRANSVERSE SECTION THROUGH THE TRUNK OF A Duck EMBRYO WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES. am. amnion; so.somatopleure; sp. splanchnopleure; wd. Wolffian duct ; st. segmental tube ; ca.v. cardinal vein; ms. muscle- plate ; sp.g. spinal ganglion; sp.c. spinal cord; ch. notochord ; ao. aorta ; hy. hypoblast. united with the side of the spinal cord at a very con- siderable distance from its original point of origin. It is moreover attached, not by its extremity, but by its side. The above points, which are much more easily studied in some of the lower vertebrate forms than in Birds, are illustrated by the subjoined section of an Elasmobranch embryo, Fig. 45. VLJ THE SPINAL NERVES. 131 Fie, 44, TRANSVERSE SECTION THROUGH THE TRUNK OF A YOUNG EMBRYO oF a Doa-FisH. ne. neural canal; pr. posterior root of spinal nerve; 2. sub- notochordal rod; ao. aorta; sc. somatic mesoblast; sp. splanchnic mesoblast ; mp. muscle-plate ; mp’. portion of muscle-plate converted into muscle; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract. It is extremely difficult to decide whether the per- manent attachment of the posterior nerve-roots to the spinal cord is entirely a new formation, or merely due to the shifting of the original point of attachment. We are inclined to adopt the former view. The origin of the anterior roots of the spinal nerves has not as yet been satisfactorily made out in Birds; but it appears probable that they grow from the ventral corner of the spinal cord, considerably later than the posterior roots, as a number of strands for each nerve, 9—2 132 THE THIRD DAY. [CHAP. mp SECTION THROUGH THE DORSAL REGION OF AN EMBRYO Doc-FisH. pr. posterior root; sp.g. spinal ganglion; m. nerve; x. attach- ment of ganglion to spinal cord; ne. neural canal; mp. muscle-plate ; ch. notochord ; 7. investment of spinal cord. which subsequently join the posterior roots below the ganglia. The shape of the root of a completely formed spinal nerve, as it appears in an embryo of the fourth day, is represented in Fig. 68. The Eye. In the preceding chapter we saw how the first cerebral vesicle, by means of lateral outgrowths followed by constrictions, gave rise to the optic vesicles. These and the parts surrounding them undergo on the third day changes which result in the formation of the eyeball. At their first appearance the optic vesicles stand out at nearly right angles to the long axis of the embryo (Fig. 27), and the stalks which connect them VL] THE EYE. 133 with the fore-brain are short and wide. The con- strictions which give rise to the stalks take place chiefly from above downwards, and also somewhat inwards and backwards. Thus from the first the vesicles appear to spring from the under part of the fore-brain. These stalks soon become comparatively narrow, and constitute the rudiments of the optic nerves (Fig. 466). The constriction to which the stalk or optic Fie. 46. SECTION THROUGH THE HEAD OF AN EMBRYO TELEOSTEAN, TO SHEW THE FORMATION OF THE OPTIC VESICLES, ETC. (From Gegenbaur ; after Schenk.) ce. fore-brain; a. optic vesicle; }. stalk of optic vesicle; d. epidermis. nerve is due takes place obliquely downwards and backwards, so that the optic nerves open into the base of the front part of the thalamencephalon (Fig. 46 5). While these changes have been going on in the optic stalks, development has also proceeded in the region of the vesicles themselves, and given rise to the rudiments of the retina, lens, vitreous humour, and other parts of the eye. 134 THE THIRD DAY. (CHAP. { Towards the end of the second day the external or superficial epiblast which covers, and is in all but immediate contact with, the most projecting portion of the optic vesicle, becomes thickened. This thickened portion is then driven inwards in the form of a shallow open pit with thick walls (Fig. 47 A, 0), carrying before it the front wall (r) of the optic vesicle. To such an extent does this involution of the superficial epiblast take place, that the front wall of the optic vesicle is pushed close up to the hind wall, and the cavity of the vesicle becomes almost obliterated (Fig. 47, B). The bulb of the optic vesicle is thus converted into a cup with double walls, containing in its cavity the portion of involuted epiblast. This cup, in order to distinguish its cavity from that of the original optic vesicle, is generally called the secondary optic vesicle. We may, for the sake of brevity, speak of it as the optic cup; in reality it never is a vesicle, since it always remains widely open in front. Of its double walls the inner or anterior (Fig. 47 B, r) is formed from the front portion, the outer or posterior (Fig. 47 B, u) from the hind portion of the wall of the primary optic vesicle. The inner or anterior (r), which very speedily becomes thicker than the other, is converted into the retina; in the outer or posterior (w), which remains thin, pigment is eventually deposited, and it ultimately becomes the tesselated pigment-layer of the choroid. By the closure of its mouth the pit of involuted epiblast becomes a completely closed sac with thick walls and a small central cavity (Fig. 47 B, 1). At the same time it breaks away from the external epi- VI.) THE EYE. 1385 Fia. 47. DiagRaMMaTic SECTIONS ILLUSTRATING THE FoRMATION OF THE Ey. (After Remak.) In A, the thin superficial epiblast A is seen to be thickened at x, in front of the optic vesicle, and involuted so as to form a pit 0, the mouth of which has already begun to close in. Owing to this involution, which forms the rudiment of the lens, the optic vesicle is doubled in, its front portion r being pushed against the back portion w, and the original cavity of the vesicle thus reduced in size. The stalk of the vesicle is shewn as still broad. In B, the optic Vesicle is still further doubled in so as to form a cup with a posterior wall « and an anterior wall r. In the hollow of this cup lies the lens 7, now completely detached from the superficial epiblast 2 Its cavity is still shewn. The cavity of the stalk of the optic vesicle is already much narrowed. blast, which forms a continuous layer in front of it, all traces of the original opening being lost. There is thus left lying in the cup of the secondary optic vesicle, an isolated elliptical mass of epiblast. This is the rudiment of the lens. The small cavity within it speedily becomes still less by the thickening of the walls, especially of the hinder one. At its first appearance the lens is in immediate contact with the anterior wall of the secondary optic vesicle (Fig. 47 B). In a short time, however, the lens 136 THE THIRD DAY. [CHAP. is seen to lie in the mouth of the cup (Fig. 50 A), a space (vh) (which is occupied by the vitreous humour) making its appearance between the lens and anterior wall of the vesicle, In order to understand how this space is developed, the position of the optic vesicle and the relations of its stalk must be borne in mind. The vesicle lies at the side of the head, and its stalk is directed downwards, inwards and backwards. The stalk in fact slants away from the vesicle. Hence when the involution of the lens takes place, the direc- tion in which the front wall of the vesicle is pushed in is not in a line with the axis of the stalk, as for simplicity’s sake has been represented in the diagram Fig. 47, but forms an obtuse angle with that axis, after the manner of Fig. 48, where s’ represents the cavity Fic, 48. DracRAMMATIC SECTION OF THE EYE AND THE Optic NERVE AT AN EARLY staGE (from Lieberktihn), to shew the lens Z occupying the whole hollow of the optic cup, the inclination of the stalk s to the optic cup, and the continuity of the cavity of the stalk s’ with that of the primary vesicle ¢; 7, anterior, « posterior wall of the optic cup. vi] THE EYE, 137 of the stalk leading away from the almost obliterated cavity of the primary vesicle. Fig. 48 represents the early stage at which the lens fills the whole cup of the secondary vesicle. The subsequent state of affairs is brought about through the growth of the walls of the cup taking place more rapidly than that of the lens. But this growth or this dilatation does not take place equally in all parts of the cup. The walls of the cup rise up all round except that part of the circumference of the cup which adjoins the stalk. While elsewhere the walls increase rapidly in height, carrying so to speak the lens with them, at this spot, which in the natural position of the eye is on its under surface, there is no growth: the wall is here imperfect, and a gap is left. Through this gap, which afterwards receives the name of the cho- roidal fissure, a way is open from the mesoblastic tissue surrounding the optic vesicle and stalk into the interior of the cavity of the cup. From the manner of its formation the gap or fissure is evidently in a line with the axis of the optic stalk, and in order to be seen must be looked for on the under surface of the optic vesicle. In this position it is readily recognized in the transparent embryo of the third day, Figs. 37 and 48, Bearing in mind these relations of the gap to the optic stalk, the reader will understand how sections of the optic vesicle at this stage present very different appearances according to the plane in which the sections are taken. When the head of the chick is viewed from under- neath as a transparent object the eye presents very 138 THE THIRD DAY. [CHAP. much the appearance represented in the diagram Fig. 49. A section of such an eye taken along the line y, perpendicular to the plane of the paper, would give a figure corresponding to that of Fig. 50 A. The lens, the cavity and double walls of the secondary vesicle, and the remains of the primary cavity, would all be repre- Fie. 49. Diagrammatic REPRESENTATION OF THE EYE OF THE CHICK OF ABOUT THE THIRD Day AS SEEN WHEN THE HEAD IS VIEWED FROM UNDERNEATH AS A TRANSPARENT OBJECT. Z the lens, 7’ the cavity of the lens, lying in the hollow of the optic cup. r the anterior, wu the posterior wall of the optic cup, ¢ the cavity of the primary optic vesicle, now nearly obliterated. By inadvertence u has been drawn thicker than 7, it should have been thinner throughout. s the stalk of the optic cup with s' its cavity, at a lower level than the cup itself and therefore out of focus ; the dotted line indicates the continuity of the cavity of the stalk with that of the primary vesicle. The line z, 2, through which the section shewn in Fig. 50 C is supposed to be taken, passes through the choroidal fissure. v1] THE EYE. 139 Diagrammatic section taken perpendicular to the plane of the paper, along the line y, y, Fig. 49. The stalk is not seen, the section falling quite out of its region. vh, hollow of optic cup filled with vitreous humour ; other letters as in Fig. 47 B. Section taken parallel to the plane of paper through Fig. 49, so far behind the front surface of the eye as to shave off a small portion of the posterior surface of the lens /, but so far in front as not to be carried at all through the stalk. Letters as before ; f, the choroidal fissure. Section along the line z, z, perpendicular to the plane of the paper, to shew the choroidal fissure f, and the continuity of the cavity of the optic stalk with that of the primary optic vesicle. Had this section been taken a little to either side of the line z, z, the wall of the optic cup would have extended up to the lens below as well as above. Letters as above. sented (the superficial epiblast of the head would also be shewn); but there would be nothing seen of either the stalk or the fissure. If on the other hand the section were taken in a plane parallel to the plane of the paper, at some distance above the level of the stalk, some such figure would be gained as that shewn in Fig. 50 B. Here the fissure f is obvious, and the communication of the cavity vh of the secondary vesicle with the outside of the eye evident; the section of course would not go through the superficial epiblast. 140 THE THIRD DAY. [CHAP. Lastly, a section, taken perpendicular to the plane of the paper along the line z, 7.e. through the fissure itself, would present the appearances of Fig. 50 C, where the wall of the vesicle is entirely wanting in the region of the fissure marked by the position of the letter f. The external epiblast has been omitted in the figure. The fissure such as we have described it exists for a short time only. Its lips come into contact, and unite (in the neighbourhood of the lens, directly, but in the neighbourhood of the stalk, by the intervention of a structure which we shall describe presently), and thus the cup-like cavity of the secondary optic vesicle is furnished with a complete wall all round. The interior of the cavity is filled by the vitreous humour, a clear fluid in which are a few scattered cells. With reference to the above description, two points require to be noticed. Firstly it is extremely doubtful whether the invagination of the secondary optic vesicle is to be viewed as an actual mechanical result of the ingrowth of the lens. Secondly it seems probable that the choroid fissure is not simply due to a deficiency in the growth of part of the walls of the secondary optic cup, but is partly due to a more complicated inequality of growth resulting in a doubling up of the primary vesicle from the side along the line of the fissure, at the same time that the lens is being thrust in in front. In Mammalia, the doubling up involves the optic stalk, which becomes flattened (whereby its original cavity is obliterated) and then folded in on itself, so as to embrace a new central cavity continuous with the cavity of the vitreous humour. During the changes in the optic vesicle just de- scribed, the surrounding mesoblast takes on the cha- racters of a distinct investment, whereby the outline of v1] THE EYE. 141 the eyeball is definitely formed. The internal portions of this investment, nearest to the retina, become the choroid (.e. the chorio-capillaris, and the lamina fusca, the pigment epithelium, as we have seen, being derived from the epiblastic optic cup), and pigment is subsequently deposited in it. The remaining external portion of the investment forms the sclerotic. The complete differentiation of these two coats of the eye does not however take place till a late period. In front of the optic cup the mesoblastic invest- ment grows forwards, between the lens and the super- ficial epiblast, and so gives rise to the substance of the cornea; the epiblast supplying only the anterior epithelium. We may now proceed to give some further details with reference to the histological differentiation of the parts, whose general development has been dealt with in the preceding pages. The histological condition of the eye in its earliest stages is very simple. Both the epiblast forming the walls of the optic vesicle, and the superficial layer which is thickened to become the lens, are composed of simple columnar cells. The surrounding mesoblast is made up of cells whose protoplasm is more or less branched and irregular. These simple elements are gradually modified into the complicated tissues of the adult eye, the changes undergone being most marked in the cases of the retina, the optic nerve, and the lens with its appendages. The optic vesicle. We left the original cavity of the primary optic vesicle as a nearly obliterated space 142 THE THIRD DAY. [ CHAP. between the two walls of the optic cup. By the end of the third day the obliteration is complete, and the two walls are in immediate contact. The inner or anterior wall is, from the first, thicker than the outer or posterior; and over the greater part of the cup this contrast increases with the growth of the eye, the anterior wall becoming markedly thicker and undergoing changes of which we shall have to speak directly (Fig. 51). In the front portion however, along, so to speak, the lip of the cup, anterior to a line which afterwards be- comes the ora serrata, both layers not only cease to take part in the increased thickening, accompanied by peculiar histological changes, which the rest of the cup is undergoing, but also completely coalesce together.- Thus a hind portion or true retina is marked off from a front portion. The front portion, accompanied by the choroid which immediately overlays it, is, behind the lens, thrown into folds, the ciliary ridges; while further for- ward it bends in between the lens and the cornea to form the iris. The original wide opening of the optic cup is thus narrowed to a smaller orifice, the pupil; and the lens, which before lay in the open mouth, is now inclosed in the cavity of the cup. While in the hind portion of the cup, or retina proper, no deposit of black pigment takes place in the layer formed out of the inner or anterior wall of the vesicle, in the front portion we are speaking of, pigment is largely deposited throughout both layers, so that eventually this portion seems to become nothing more than a forward pro- longation of the pigment-epithelium of the choroid. Vi.| THE OPTIC VESICLE, 143 Fie, 51. SEctTIon oF THE Eye oF CHICK aT THE FourtH Day. ep. superficial epiblast of the side of the head. &. true retina: anterior wall of the optic cup. p. Ch. pigment- epithelium of the choroid: posterior wall of the optic cup. 6 is placed at the extreme lip of the optic cup at what will become the margin of the iris. 7. the lens. The hind wall, the nuclei of whose elongated cells are shewn at nl, now forms nearly the whole mass of the lens, the front wall being reduced to a layer of flattened cells ed. m. the mesoblast surrounding the optic cup and about to form the choroid and sclerotic. It is seen to pass forward between the lip of the optic cup and the superficial epiblast. 144 THE THIRD DAY. [CHAP. Filling up a large part of the hollow of the optic cup is seen a hyaline mass forming the hyaloid membrane and the coagulum of the vitreous humour. In the neighbourhood of the lens it seems to be continuous as at cl with the tissue a, which in turn is continuous with the mesoblast m, and appears to be the rudiment of the capsule of the lens and suspensory ligament. Thus while the hind moiety of the optic cup be- comes the retina proper, including the choroid-pigment in which the rods and cones are imbedded, the front moiety is converted into the ciliary portion of the retina, covering the ciliary processes, and into the uvea of the iris; the bodies of the ciliary processes and the substance of the iris, their vessels, muscles, connective tissue and ramified pigment, being derived from the mesoblastic choroid. The margin of the pupil marks the extreme lip of the optic vesicle, where the outer or posterior wall turns round to join the inner or anterior. The ciliary muscle and the ligamentum pectinatum are both derived from the mesoblast between the cornea and the iris. The retina. At first, as we have said, the two walls of the optic cup do not greatly differ in thickness. On the third day the outer or posterio. becomes much thinner than the inner or anterior, and by the middle of the fourth day is reduced to a single layer of flat- tened cells (Fig. 51, p. Ch.). At about the 80th hour its cells commence to receive a deposit of pigment, and eventually form the so-called pigmentary epithelium of the choroid; from them no part of the true retina (or no other part of the retina, if the pigment-layer in question be supposed to belong more truly to the retina than to the choroid) is derived. v1] THE RETINA. 145 On the fourth day, the inner (anterior) wall of the optic cup (Fig. 51, #) is perfectly uniform in structure, being composed of elongated somewhat spindle-shaped cells, with distinct nuclei. On its external (posterior) surface a distinct cuticular membrane, the membrana limitans externa, early appears. As the wall increases in thickness, its cells multiply rapidly, so that it soon appears to be several cells thick : each cell being however probably continued through the whole thickness of the layer. The wall at this stage corresponds closely in its structure with the brain, of which it may properly be looked upon as part. Ac- cording to the usual view, which is not however fully supported by recent observations, the retina becomes divided in its subsequent growth into (1) an outer part, corresponding morphologically to the epithelial lming of the cerebro-spinal canal, composed of what may be called the visual cells of the eye, 7.¢. the cells forming the outer granular (nuclear) layer and the rods and cones attached to them; and (2) an inner portion consisting of the inner granular (nuclear) layer, the inner molecular layer, the ganglionic layer and the layer of nerve-fibres corresponding morphologically to the substance of the brain and spinal cord. The actual development of the retina is not thoroughly understood. According to the usual statements (Kélliker) the layer of ganglion cells and the inner molecular layer are first differentiated, while the remaining cells give rise to the rest of the retina proper, and are bounded externally by the membrana limitans externa. On the inner side of the ganglionic layer the stratum of nerve-fibres is also very early established. The rods 1 Entwick. d. Menschen, etc., 1879. F. &B. 10 146 THE THIRD DAY. [ CHAP. and cones are formed as prolongations or cuticularizations of the cells which eventually form the outer granular layer. The layer of cells external to the molecular layer is not divided till comparatively late into the inner and outer granular (nuclear) layers, and the interposed outer molecular layer. Léwe? has recently written an elaborate paper on this subject in which he arrives at very different results from Kélliker and other observers. According to him only the outer limbs of the rods and cones, which he holds to be metamorphosed cells, correspond to the epithelial layer of the brain. The changes described above are confined to that portion of the retina which lies behind the ora serrata. In front of this both walls of the cup coalesce as we have said into a cellular layer in which a deposit of pigment takes place. At a very early period a membrane appears on the side of the retina adjoining the vitreous humour. This membrane is the hyaloid membrane. It is formed at a time when there is no trace of mesoblastic structures in the cavity of the vitreous humour, and must therefore be regarded as a cuticular deposit of the cells of the optic cup. The optic nerve. The optic nerves are derived, as we have said, from the at first hollow stalks of the optic vesicles. ‘Their cavities gradually become oblite- rated by a thickening of the walls, the obliteration proceeding from the retinal end inwards towards the brain. While the proximal ends of the optic stalks are still hollow, the rudiments of the optic chiasma are formed at the roots of the stalks, the fibres of the one stalk growing over into the attachment of the other. The decussation of the fibres would appear 1 Archiv fiir mikr. Anat. Vol. xv. v1.) THE CHOROID FISSURE. 147 to be complete. The fibres arise in the remainder of the nerves somewhat later. At first the optic nerve is equally continuous with both walls of the optic cup; as must of necessity be the case, since the interval which primarily exists between the two walls is con- tinuous with the cavity of the stalk. When the cavity within the optic nerve vanishes, and the fibres of the optic nerve appear, all connection between the outer wall of the optic cup and the optic nerve disappears, and the optic nerve simply perforates the outer wall, remaining continuous with the inner one. The choroid fissure, During the third day of incu- bation there passes in through the choroid slit a vas- cular loop, which no doubt supplies the transuded material for the growth of the vitreous humour. Up to the fifth day this vascular loop is the only structure passing through the choroid slit. On this day however a new structure appears, which remains permanently through life, and is known as the pecten. It consists of a lamellar process of the mesoblast cells round the eye, passing through the choroid slit near the optic nerve, and enveloping part of the afferent branch of the vascular loop above mentioned. The proximal part of the free edge of the pecten is somewhat swollen, and sections through this part have a club-shaped form. On the sixth day the choroid slit becomes rapidly closed, so that at the end of the sixth day it is reduced toa mere seam. There are however two parts of this seam where the edges of the optic cup have not coalesced. The proximal of these adjoins the optic nerve, and permits the passage of the pecten, and at a later period of the optic nerve; and the second or distal 10—2 148 THE THIRD DAY. [CHAP. one is placed near the ciliary edge of the slit, and is traversed by the efferent branch of the above-men- tioned vascular loop. This vessel soon atrophies, and with it the distal opening in the choroid slit completely vanishes. In some varieties of domestic Fowl (Lieber- kihn) the opening however persists. The seam which marks the original site of the choroid slit is at first con- spicuous by the absence of pigment, and at a later period by the deep colour of its pigment. Finally, a little after the ninth day, no trace of it is to be seen. Up to the eighth day the pecten remains as a simple lamina; by the tenth or twelfth day it begins to be folded or rather puckered, and by the seventeenth or eighteenth day it is richly pigmented, and the pucker- ings have become nearly as numerous as in the adult, there being in all seventeen or eighteen. The pecten is now almost entirely composed of vascular coils, which are supported by a sparse pigmented connective tissue ; and in the adult the pecten is still extremely vascular. The original artery which became enveloped at the formation of the pecten continues, when the latter be- comes vascular, to supply it with blood. The vein is practically a fresh development after the atrophy of the distal portion of the primitive vascular loop of the vitreous humour. There are no true retinal blood-vessels. The permanent opening in the choroid fissure for the pecten is intimately related to the entrance of the optic nerve into the eyeball; the fibres of the optic nerve passing in at the inner border of the pecten, coursing along its sides to its outer border, and radi- vi.| THE LENS. 149 ating from it as from a centre to all parts of the retina. The lens, This when first formed is somewhat elliptical in section with a small central cavity of a similar shape, the front and hind walls being of nearly equal thickness, each consisting of a single layer of elongated columnar cells. In the subsequent growth of the lens, the develop- ment of the hind wall is of a precisely opposite cha- racter to that of the front wall. The hind wall becomes much thicker, and tends to obliterate the central cavity by becoming convex on its front surface. At the same time its cells, still remaining as a single layer, become elongated and fibre-like. The front wall on the con- trary becomes thinner and thinner and its cells more and more flattened and pavement-like. These ‘modes of growth continue until at the end of the fourth day, as shewn in Fig. 51, the convex hind wall 7 comes into absolute contact with the front wall el and the cavity is thus entirely obliterated. The cells of the hind wall have by this time become veritable fibres, which, when seen in section, appear to be arranged nearly parallel to the optic axis, their nuclei nl being seen in arow along their middle. The front wall, some- what thickened at either side where it becomes continu- ous with the hind wall, is now a single layer of flattened cells separating the hind wall of the lens, or as we may now say the lens itself, from the front limb of the lens-capsule ; of this it becomes the epithelium. The subsequent changes undergone consist chiefly in the continued elongation and multiplication of the lens- fibres, with the partial disappearance of their nuclei. 150 THE THIRD DAY. [CHAP. During their multiplication they become arranged in the manner characteristic of the adult lens. The lens capsule is probably formed as a cuticular membrane deposited by the epithelial cells of the lens. But it should be stated that many embryologists regard it as a product of the mesoblast. The vitreous humour, The vitreous humour is a mesoblastic product, entering the cavity of the optic cup by the choroid slit just spoken of. It is nourished by the vascular ingrowths through the choroid slit. Its exact nature has been much disputed. It arises as a kind of transudation, but frequently however contains blood-corpuscles and embryonic mesoblastic cells. It is therefore intermediate in its character between or- dinary intercellular substance, and the fluids contained in serous cavities. The integral parts of the eye in front of the lens are the cornea, the aqueous humour, and the iris, The development of the latter has already been sufficiently described in connection with the retina, and there re- main to be dealt with the cornea, and the cavity con- taining the aqueous humour. The cornea. The cornea is formed by the coales- cence of two structures, viz. the epithelium of the cornea and the cornea proper. The former is directly derived from the external epiblast, which covers the eye after the invagination of the lens. The latter is formed in a somewhat remarkable manner, first. clearly made out by Kessler. When the lens is completely separated from the epi- dermis the central part of its outer wall remains directly vi] THE CORNEA. 151 in contact with the epidermis (future corneal epithelium). At its edge there is a small ring-shaped space bounded by the outer skin, the lens and the edge of the optic cup. There appears, at about the time when the cavity of the lens is completely obliterated, a structureless layer external to the above ring-like space and immediately adjoining the inner face of the epidermis. This layer, which forms the commencement of the cornea proper, at first only forms a ring at the border of the lens, thickest at its outer edge, and gradually thinning away towards the centre. It soon however becomes broader, and finally forms a continuous stratum of con- siderable thickness, interposed between the external skin and the lens. As soon as this stratum has reached a certain thickness, a layer of flattened cells grows in along its inner side from the mesoblast sur- rounding the optic cup (Fig. 52,dm). This layer is the epithelioid layer of the membrane of Descemet’. After it has become completely established, the meso- blast around the edge of the cornea becomes divided into two strata; an inner one (Fig. 52 cb) destined to form the mesoblastic tissue of the iris already described, and an outer one (Fig. 52 cc) adjoining the epidermis. The outer stratum gives rise to the corneal corpuscles, which are the only constituents of the cornea not yet developed. ‘The corneal corpuscles make their way 1 It appears possible that Lieberkiihn may be right in stating that the epithelium of Descemet’s membrane grows in between the lens and the epiblast before the formation of the cornea proper, and that Kessler’s account, given above, may on this point require correc- tion. From the structure of the eye in some of the lower forms it seems probable that Descemet’s membrane is continuous with the choroid. 152 ; THE THIRD DAY. [CHAP. Fie. 52. SECTION THROUGH THE EYE OF A FOWL ON THE EIGHTH DAY OF DEVELOPMENT, TO SHEW THE IRIS AND CORNEA IN THE PROCESS OF FORMATION. (After Kessler.) ep. epiblastic epithelium of cornea ; ec. corneal corpuscles growing into the structureless matrix of the cornea; dm. Descemet’s membrane; 77. iris; cb. mesoblast of the iris (this reference letter points a little too high). The space between the layers dm. and ep. is filled with the structureless matrix of the cornea. through the structureless corneal layer, and divide it into two strata, one adjoining the epiblast, and the other adjoining the inner epithelium. The two strata become gradually thinner as the corpuscles invade a larger and larger portion of their substance, and finally the outermost portion of each alone remains to form above and below the membrana elastica anterior and posterior (Descemet’s membrane) of the cornea. The corneal corpuscles, which have grown in from the sides, thus form a layer which becomes continually thicker, and gives rise to the main substance of the cornea. Whether the increase in the thickness of the layer is due to the immigration of fresh corpuscles, or to the division of those already there, is not clear. After the VL] THE AQUEOUS HUMOUR. 1538 cellular elements have made their way into the cornea, the latter becomes continuous at its edge with the meso- blast which forms the sclerotic. The derivation of the original structureless layer of the cornea is still uncertain. Kessler derives it from the epiblast, but it appears more probable that Kolliker! is right in regarding it as derived from the mesoblast. The grounds for this view are, (1) the fact of its growth inwards from the border of the meso- blast round the edge of the eye, (2) the peculiar relations between it and the corneal corpuscles at a later period. This view would receive still further support if a layer of mesoblast between the lens and the epiblast were really present as believed by Lieber- kiihn, It must however be admitted that the objections to Kessler’s view of its epiblastic nature are rather @ priori than founded on definite observation. The observations of Kessler, which have been mainly followed in the above account, are strongly opposed by Lieberkiihn and other observers, and are not entirely accepted by Kdlliker. It is however especially on the development of these parts in Mam- malia (to be spoken of in the sequel) that the above authors found their objections. The aqueous humour. The cavity for the aqueous humour has its origin in the ring-shaped space round the front of the lens, which, as already mentioned, is bounded by the external skin, the edge of the optic cup, and the lens. By the formation of the cornea this space is shut off from the external skin, and on the appearance of the epithelioid layer of Descemet’s membrane a continuous cavity is developed between the cornea and the lens. This cavity enlarges and 1 ZL, Kessler, Zur Entwick. d. Auges d. Wirbelthiere. Leipzig, 1874. N. Lieberkiihn, “ Beitriige z. Anat. d. embryonalen Auges,” Archiv f. Anat. u. Phys., 1879. Kélliker, Entwick. d. Menschen, etc. Leipzig, 1879. 154 THE THIRD DAY. [CHAP. receives its final form upon the full development of the iris. Summary. We may briefly recapitulate the main facts in the development of the eye as follows. The eye commences as a lateral outgrowth of the fore-brain, in the form of a stalked vesicle. The stalk, becoming narrowed and subsequently solid, is converted into the optic nerve. An involution of the superficial epiblast over the front of the optic vesicle, in the form first of a pit, then of a closed sac with thick walls, and lastly, of a solid rounded mass (the small central cavity being entirely obliterated by the thickening of the hind wall), gives rise to the lens, Coincidently with this involution of the lens, the optic vesicle is doubled up on itself, and its cavity obliterated; thus a secondary optic vesicle or optic cup with a thick anterior and a thin posterior wall is produced. Asa result of the manner in which the doubling up takes place, or of the mode of growth afterwards, the cup of the secondary optic vesicle is at first imperfect along its under surface, where a gap, the choroidal fissure, exists for some little time, but subse- quently closes up. The mesoblast in which the eye is imbedded gathers itself together around the optic cup into a distinct in- vestment, of which the internal layers become the choroid, the external the sclerotic. An ingrowth of this investment between the front surface of the lens and the superficial epiblast furnishes the body of the cornea, the epiblast itself remaining as the anterior corneal epithelium. The mesoblast entering on the under side through VI] THE LACRYMAL DUCT. 155 the choroidal fissure gives rise to the vitreous humour, while at a later stage a definite process of this meso- blast becomes the pecten. Of the walls of the optic cup, the thinner outer (posterior) wall becomes, behind the line of the ora serrata, the pigment-epithelium of the choroid, while the thicker inner (anterior) wall supplies all the ele- ments of the retina, including the rods and cones which grow out from it into the pigment-epithelium. In front of the line of the ora serrata, both walls of the optic cup, quite thin and wholly fused together, give rise to the pigment-epithelium of the ciliary processes and iris, the bodies of both these organs being formed from the mesoblastic investment. Accessory Organs connected with the Eye. Eyelids. The most important accessory structures connected with the eye are the eyelids. They are developed as simple folds of the integument with a mesoblastic prolongation between their two laminz. They are three in number, viz. an upper and lower, and a lateral one—the nictitating membrane—springing from the inner or anterior border of the eye. Their inner face is lined by a prolongation of conjunctiva, which is the modified epiblast covering the cornea and part of the sclerotic. The Lacrymal glands and Lacrymal duct. The lacrymal glands are formed as solid ingrowths of the conjunctival epithelium. They appear on the eighth day of incubation. The lacrymal duct begins as a solid ridge of the epidermis, projecting inwards along the line of the so-called lacrymal groove, from the eye to the nasal pit. At the end of the sixth day this ridge begins to be separated from the epidermis, remaining however united with it on the inner side of the lower eyelid. 156 THE THIRD DAY. [CHAP. After it has become free, it forms a solid cord, the lower end of which unites with the wall of the nasal cavity. The cord so formed gives rise directly to the whole of the duct proper and to the lower branch of the collecting tube. The upper branch of the collecting tube is formed as an outgrowth from it. A lumen begins to be formed in it on the twelfth day of incubation, and first appears at the nasal end. It arises as a space amongst the cells of the cord, but is not due to an absorption of the central cells}. Organ of hearing. During the second day the ear first made its appearance on either side of the hind- brain as an involution of the external epiblast, thrust down into the mass of mesoblast rapidly developing between the epiblast of the skin and that of the neural Fia, 53. Pp SECTION THROUGH THE HEapD oF aN ELasmMoBRrancH Empryo, AT THE LeveL or tHE AUDITORY INVOLUTION. aup. auditory pit; aun. ganglion of auditory nerve; zv.v. roof of fourth ventricle ; a.c.v. anterior cardinal vein; aa. aorta; 1G. Born: ‘‘Die Nasenhéhlen u. Thrinennasengang d. amnioten Wirbelthiere, 1, Lacertilia 1. Aves.” Morphologisches Jahrbuch, Vol. v., 1879. VI.] THE EAR. 157 J.aa. aortic trunk of mandibular arch ; pp. head cavity of mandibular arch ; Jve. alimentary pouch which will form the first visceral cleft ; 7. rudiment of thyroid body. canal (Fig. 27, au. p.). It then had the form of a shallow pit with a widely open mouth, similar in form to that shewn for an embryo dog-fish in Fig. 53, au. p. Before the end of the third day, its mouth closes up and all signs of the opening are obliterated. The pit thus becomes converted into a closed vesicle, lined with fepiblast, and surrounded by mesoblast. This vesicle is the otic vesicle, whose cavity rapidly enlarges while its walls become thickened (Fig. 54, CC). Fie, 54. AOA SEcTION THROUGH THE HinD-BRAIN OF A CHICK AT THE END oF THE THIRD Day oF INCUBATION. IV. Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle. 158 THE THIRD DAY. [cHaP. Ch. Notochord—(diagrammatic shading). CV. Anterior cardinal or jugular vein. CC. Involuted auditory vesicle. CC points to the end which will form the cochlear canal. AZ. Recessus labyrinthi. hy. hypoblast lining the alimentary canal. fy is itself placed in the cavity of the alimentary canal, in that part of the canal which will become the throat. The lower (anterior) wall of the canal is not shewn in the section, but on each side are seen portions of a pair of visceral arches. In each arch is seen the section of the aortic arch AOA belonging to the visceral arch. The vessel thus cut through is running upwards towards the head, being about to join the dorsal aorta AO. Had the section been nearer the head, and carried through the plane at which the aortic arch curves round the alimentary canal to reach the mesoblast above it, AOA and AO would have formed one continuous curved space. In sections lower down in the back the two aorta, AO, one on either side, would be found fused into one median canal. The changes by which this simple otic vesicle is converted into the complicated system of parts known as the internal ear, have been much more completely worked out for Mammals than for Birds. We shall therefore reserve a full account of them for a later portion of this work. Meanwhile a brief statement of the essential nature of the changes may be useful; and will be most conveniently introduced here. The internal ear consists essentially of an inner membranous labyrinth lying loosely in and only partially attached to an outer osseous labyrinth. The membranous labyrinth (Fig. 55) consists of two _ parts: (1) the vestzbule, with which are connected three pairs of semicircular canals, pag’, fr’, hor’, and a long narrow hollow process, the aqueductus or recessus vesti- Two VIEWS OF THE MEMBRANOUS LABYRINTH OF COLUMBA Domestica (copied from Hasse). A, from the exterior, B, from the interior. hor’. horizontal semicircular canal, hor. ampulla of ditto, pag’. pos- terior vertical semicircular canal, pag. ampulla of ditto, jr’. anterior vertical semicircular canal, fr. ampulla of ditto, u. utriculus, rw. recessus utriculi, v. the connecting tube between the ampulla of the anterior vertical semicircular canal and the utriculus, de. ductus endolymphaticus (recessus vestibuli), s. sacculus hemisphericus, er. canalis reuniens, lag. lagena, mr. membrane of Reissner, »b. Basilar membrane. buli, and (2) the ductus cochlearis, which in birds is a flask-shaped cavity slightly bent on itself, the dilated end of which is called the lagena. The several parts of each of these cavities freely communicate, and the two are joined together by a narrow canal, the canalis re- uniens, cr. The osseous labyrinth has a corresponding form, and may be similarly divided into parts: into a bony vestibule, with its bony semicircular canals and recessus 160 THE THIRD DAY. [CHAP. vestibuli, and into a bony cochlea; but the junction between the cochlea and the bony vestibule is much wider than the membranous canalis reuniens. The cavity of the osseous cochlea is partially divided lengthways by the ductus cochlearis into a scala tym- pani and a scala vestibuli, which do not however extend to the lagena. The auditory nerve, piercing the osseous labyrinth in various points, is distributed in the walls of the mem- branous labyrinth. All these complicated structures are derived from the simple primary otic vesicle and the surrounding mesoblast by changes in its form and differentiation of its walls. All the epiblast of the vesicle goes to form the epithelium of the membranous labyrinth, whose cavity, filled with endolymph, represents the original cavity which was first open to the surface but subse- quently covered in. It gradually attains its curiously twisted form by a series of peculiar processes of unequal growth in the, at first, simple walls of the vesicle. The corium of the membranous labyrinth, and all the tissues of the osseous labyrinth, are developed out of the meso- blastic investment of the vesicle. The space between the osseous and membranous labyrinths, including the scala vestibuli and scala tympani, may be regarded as essentially a series of lymphatic cavities hollowed out in the mesoblast. It will be seen then that the ear, while resembling the eye in so far as the peculiar structures in which the sensory nerve in each case terminates are formed of involuted epiblast, differs from it inasmuch as it arises by an independent involution of the superficial epiblast, vi.] THE OLFACTORY ORGAN, 161 whereas the eye is a constricted portion of the general involution which gives rise to the central nervous system. The origin of the auditory nerve has already been described. It is shewn in close contact with the walls of the auditory pit in Fig. 53. Organ of Smell. The organ of smell makes its ap- pearance during the third day, as two depressions or pits, on the under surface of the head, a little in front of the eye (Fig. 56, 1”). Fie. 56, Heap or an Empryo CHIck oF THE THIRD Day VIEWzD SIDEWAYS AS AN Opaque OBJECT. (Chromic acid preparation.) C.H. Cerebral hemispheres. .B. Vesicle of third ventricle. MB. Mid-brain. 0d. Cerebellum. 4.B. Medulla ob- longata. NV. Nasal pit. of. otic vesicle in the stage of a pit with the open- ing not yet closed up. op. Optic vesicle, with /. lens and ch.f. choroidal fissure. The superficial epiblast moulds itself to the form of the optic vesicle and the lens ; hence the choroidal fissure, though formed entirely underneath the superficial epiblast, is distinctly visible from the outside. 1 Ff. The first visceral fold; above itis seen a slight indication of the superior maxillary process. 2,3,4F/. Second, third and fourth visceral folds, with the vis- ceral clefts between them. F. & B. 11 162 THE THIRD DAY. [CHAP, Like the lens and the labyrinth of the ear, they are formed from the external epiblast; unlike them they are never closed up. The olfactory nerves arise as outgrowths of the front end of the cerebral hemispheres, before any trace of a special division of the brain, forming an olfactory lobe, has become established. Their peripheral extremities unite with the walls of the olfactory pits during the third day. The olfactory lobes arise as outgrowths of the cerebral hemispheres on the seventh day of incuba- tion. Visceral Arches and Visceral Clefts. It must be borne in mind that, especially in the early stages of development, owing to the very unequal growth of different parts, the relative position of the various structures is continually shifting. This is very well seen in the instance of the heart. At its first appear- ance, the heart is lodged immediately beneath the extreme front of the alimentary canal, so far forwards as to underlie that portion of the medullary canal which will form the brain. It is, in fact, at that epoch a part of the head. From that early position it gradually recedes farther and farther backward, until, at the end of the third day, a considerable interval is observed between it and the actual head. In other words, a distinct neck has been formed, in which most important changes take place. The neck is distinguished from the trunk in which the heart now lies by the important feature that in it there is no cleavage of the mesoblast into somatopleure and splanchnopleure, and consequently no pleuroperito- neal cavity. In passing from the exterior into the ali- vI.] THE VISCERAL CLEFTS. 163 mentary canal, the three layers of the blastoderm are successively traversed, without any breach of continuity, save such as is caused by the cavities of the blood- vessels. In this neck, so constituted, there appear on the third day certain fissures or clefts, the visceral or branchial clefts. These are real clefts or slits passing right through the walls of the throat, and are placed in series on either side across the axis of the alimentary canal, lying not quite at right angles to that axis and parallel to each other, but converging somewhat to the middle of the throat in front (Fig. 56). Viewed from the outside in either fresh or preserved embryos they are not very distinctly seen to be clefts; but when they are seen from within, after laying open the throat, their characters as elongated oval slits can easily be recog- nised. Four in number on either side, the most anterior is the first to be formed, the other three following in suc- cession. Their formation takes place from within out- wards. The hypoblast is pushed outwards as a pouch, which grows till it meets the epiblast, which is then broken through, while the hypoblast forms a junction with the epiblast at the outside of the throat. No sooner has a cleft been formed than its anterior border (2.e. the border nearer the head) becomes raised into a thick lip or fold, the vesceral or branchial fold. Each cleft has its own fold on its anterior border, and in addition the posterior border of the fourth or last visceral cleft is raised into a similar fold. There are thus five visceral folds to four visceral clefts (Fig. 56). The last two folds however, and especially the last, are not nearly so thick and prominent as the other three, the second 11—2 164 THE THIRD DAY. [CHAP. being the broadest and most conspicuous of all. The first fold meets, or nearly meets, its fellow in the middle line in front, but the second falls short of reaching the middle line, and the third, fourth and fifth do so in an increasing degree. Thus in front views of the neck a triangular space with its apex directed towards the head is observed between the ends of the several folds. Into this space the pleuroperitoneal cavity extends, the somatopleure separating from the splanchnopleure along the ends of the folds; and it is here that the aorta plunges into the mesoblast of the body. The visceral clefts and arches to a large extent dis- appear in the adult, and constitute examples of an intc- resting class of embryonic organs, whose presence is only to be explained by the fact that, in the ancestors of the types in which they are now developed in the embryo, they performed an important function in the adult. The visceral arches and clefts are in fact the homologues of the branchial arches and branchial clefts of Fishes, which continue to be formed in the embryos of the higher vertebrate types, although they have ceased to serve as organs of respiration. The skeletal structures developed in the visceral arches persist as the jaw-bones and hyoid bone, but the clefts, with the exception of the first, become obliterated. Of the history of the skeletal elements we shall speak in detail hereafter; meanwhile we may briefly deal with the general history of these parts. The first fold on either side, increasing rapidly in size and prominence, does not, like the others, remain single, but sends off in the course of the third day a branch or bud-like process from its anterior edge. This VIL] THE VISCERAL ARCHES. 165 branch, starting from near the dorsal beginning of the fold, runs ventralwards and forwards, tending to meet the corresponding branch from the fold on the other side, at a point in the middle line nearer the front of the head than the junction of the main folds. The two branches do not quite meet, being separated by a median process, which at the same time grows down from the extreme front of the head, and against which they abut. Between the main folds, which are directed somewhat backwards and the branches which slant forwards, a somewhat lozenge-shaped space is developed which, as the folds become more and more prominent, grows deeper and deeper. In the main folds are developed the man- dibles, and in the branches the superior masille: the lozenge-shaped cavity between them is the cavity of the mouth, and the descending process which helps to complete the upper margin of this cavity is called, from the parts which will be formed out of it, the fronto- nasal process. Part of the mesoblast of the two succeeding pairs of visceral folds is transformed into the hyoid bone, which will be best considered in connection with the develop- ment of the skull. The two last arches disappear with- out giving rise to any permanent structures. With the exception of the first the visceral clefts become obliterated at an early stage of embryonic life ; but the first persists, although it loses all trace of its original branchial function and becomes intimately con- nected with the organ of hearing, of which in fact it forms a most essential part, becoming converted into the Eustachian tube and tympanic cavity. The outer opening and the outer part also of the cleft become 166 THE THIRD DAY. [CHaAP. obliterated at an early date, but from the inner part of the cleft a diverticulum is given off towards the ex- terior, which becomes the tympanic cavity. The inner part of the cleft itself forms the Eustachian tube, while its mouth forms the oral aperture of this tube. The meatus auditorius externus first appears as a shallow depression at the region where the closure of the first visceral cleft takes place. It is in part formed by the tissue surrounding this depression growing up in the form ‘of a wall, but the blind end of the meatus also becomes actually pushed in towards the tympanic cavity. The tympanic membrane is derived from the tissue which separates the meatus auditorius externus from the tympanic cavity. This tissue is obviously consti- tuted of an hypoblastic epithelium on its inner aspect, an epiblastic epithelium on its outer aspect, and a layer of mesoblast between them, and these three layers give rise to the three layers of which this membrane is formed in the adult. During the greater part of foetal life it is relatively very thick, and presents a structure bearing but little resemblance to that in the adult. The tympanic cavity is bounded on its inner aspect by the osseous investment of the internal ear, but at two points, known as the fenestra ovalis and fenestra rotunda, the bone is deficient and its place is taken by a membrane. These two fenestre appear early, and are probably formed by the nonchrondrification of a small area of the embryonic cartilage. The upper of the two, or fenestra ovalis, contains the base of a bone, known as the columella. The main part of the columella is vi] THE AORTIC ARCHES. 167 formed of a stalk which is held by Parker to be derived from part of the skeleton of the visceral arches, while the base, forming the stapes, appears to be an inde- pendent formation. The stalk of the columella extends to the tympanic membrane; its outer end becoming imbedded in this membrane, and serving to transmit the vibrations of the membrane to the fluid in the internal ear. Vascular system. By the end of the second day three pairs of aortic arches had been established in connection with the heart. When the visceral folds and clefts are formed, a definite arrangement between them and the aortic arches is always observed. The first visceral cleft runs between the first and second aortic arches. Consequently the first aortic arch runs in the first visceral fold, and the second in the second. In the same way, the second visceral cleft lies between the second and third aortic arches, the third aortic arch running in the third visceral fold. Each aortic arch runs in the thickened mesoblast of the corresponding fold. Arrived at the dorsal surface of the alimentary canal, these arches unite at acute angles to form a common trunk, the dorsal aorta (Fig. 57, A.O), which runs along the back immediately under the notochord. The length of this common single trunk is not great, as it soon divides into two main branches, each of which, after giving off the large vitelline artery, Of-A., pursues its course with diminished calibre to the tail, where it is finally lost in the capillaries of that part. The heart is now completely doubled up on itself. Its mode of curvature is apparently somewhat compli- cated. Starting from the point of junction of the vitel- 168 THE THIRD DAY. [cHapP. DiacGRaM OF THE ARTERIAL CIRCULATION ON THE TuirD Day. 1, 2,3. The first three pairs of aortic arches. A. The vessel formed by the junction of the three pairs of arches. A.0. Dorsal aorta formed by the junction of the two branches A and A; it quickly divides again into two branches which pass down one on each side of the notochord. From each of these is given off a large branch Of.A., the vitelline artery. ECA, I.CA, external and internal carotid arteries. line veins (Fig. 37, Ht), there is first a slight curvature towards the left; this is followed by a turn to the right, and then the heart is completely bent on itself, so that afterwards it pursues a course directed from behind quite straight forwards (except perhaps for a little incli- nation to the left) to the point where the aortic arches branch off. In this way, as shewn in section in Fig. 59, A, the end of the bulbus arteriosus (v) comes to lie just underneath (or in front of according to the position of vi] THE HEART. 169 the embryo) that part which has already been marked off by the lateral bulgings as the auricular portion (aw). That part of the heart which is turned to the right, including the point of doubling up, is the ventricular portion, and is even at this stage separated from the auricular portion by a slight neck. This external con- striction corresponds to an internal narrowing of the lumen of the heart, and marks the position of the future canals auricularis. The ventricular portion is, on the other hand, like- wise separated by a fainter constriction from the ante- rior continuation of the heart which forms the bulbus arteriosus. The projecting part where the doubling takes place is at this stage still quite round; we shall see that later on it becomes pointed and forms the apex of the heart. The whole venous portion of the heart (if we may so speak of it, though of course at this stage blood of the same quality passes right along the whole cardiac canal) lies in a plane which is more dorsal than the arterial por- tion. The point at which the venous roots of the heart, i.e. the two vitelline trunks, unite into a single canal, is on this day carried farther and farther away from the heart itself By the end of the day there is a consider- able distance between the auricular portion of the actual heart and the point where the venous roots separate, each to pursue its course along the splanchnopleure-fold of its own side. This distance is traversed by a single venous trunk, of which the portion close to the auricles is called the sinus venosus, and the more distant the ductus venosus. We shall give to the whole trunk the name used by the older observers, the meatus venosus. 170 THE THIRD DAY. [cHaAP Small arteries to various parts of the body are now being given off by the aorta and its branches. The capillaries in which these end are gathered into veins which unite to form two main trunks on either side, the cardinal veins, anterior and posterior (Fig. 36, Fig. 58 Fie. 58. DIAGRAM OF THE VENOUS CIRCULATION ON THE Tairp Day. HT. Heart. J. Jugular or anterior cardinal vein. C. Inferior or posterior cardinal vein. Of Vitelline vein. de. Ductus Cuvieri. J and C), which run parallel to the long axis of the body in the upper part of the mesoblast, a little external to the mesoblastic somites. These veins, which do not attain to any great importance till well on in the third day, unite opposite to the heart, on each side, into a short common trunk at right angles to themselves. The two short trunks thus formed, which bear the name of ductus Cuviert (Fig. 36, Fig. 58, dc), running ventralwards and then transversely straight inwards towards the middle line fall into the sinus venosus. The two ductus Cuvieri pass from the heart to the body walls in a special horizontal mesentery, whose for- mation and function we shall return to in speaking of the formation of the pericardial cavity. The position of one of them is shewn in section in Fig. 59 B, de. VI] THE TAIL-FOLD. 171 Fie. 59. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TweEnty-onE Mrsopuastic SoMITES TO SHEW THE For- MATION OF THE PERICARDIAL Cavity, A. BEING THE ANTERIOR SECTION. pp. body cavity ; pe. pericardial cavity ; al. alimentary cavity ; au. auricle; v. ventricle; sv. sinus venosus; de. ductus Cuvieri ; ao. aorta ; mp. muscle-plate ; me. medullary cord. The alimentary canal. As we stated above, the folding in of the splanchnopleure to form the alimentary canal is proceeding with great rapidity, the tail-fold as well as the head-fold contributing largely to this result. The formation of the tail-fold is very similar to that of the head-fold. The tail is a solid, somewhat curved, blunt cone of mesoblast, immediately coated with the 172 THE THIRD DAY. [CHAP. superficial epiblast except at the upper surface (corre- sponding to the back of the embryo), where lies the pointed termination of the neural tube. So rapid is the closure of the splanchnopleure both in front and behind, that two of the three parts into which the digestive tract may be divided, are brought, on this day, to the condition of complete tubes. The first division, including the region from the mouth to the duodenum, is completely folded in by the end of the day; so likewise is the third division com- prising the large intestine and the cloaca. The middle division, corresponding to the future small intestine, still remains quite open to the yolk-sac below. The attachment of the newly formed alimentary canal to the body above is at first very broad, and only a thin stratum of mesoblast separates the hypoblast of the canal from the notochord and mesoblastic somites; even that.maybe absent under the notochord. During the third day, however, along such portions of the canal as have become regularly enclosed, 2.¢. the hinder division and the posterior moiety of the anterior division, the mesoblastic attachment becomes narrower and (in a ver- tical direction) longer, the canal appearing to be drawn more ventralwards (or according to the position of the embryo forwards), away from the vertebral column. In what may be regarded as the pleural division of the general pleuroperitoneal space, along that part of the alimentary canal which will form the cesophagus, this withdrawal is very slight (Fig. 59), but it is very marked in the peritoneal space. Here such parts of the digestive canal as are formed come to be suspended from the body above by a narrow flattened band of mesoblas- VI] THE GiSOPHAGUS, ima tic tissue which reaches from the neighbourhood of the notochord, and becomes continuous with the mesoblas- tic coating which wraps round the hypoblast of the canal. This flattened band is the mesentery, shewn commencing in Fig. 65, and much more advanced in Fig. 68, M. It is covered on either side by a layer of flat cells forming the epithelioid lining of the peritoneal membrane, while its interior is composed of indifferent tissue. The front division of the digestive tract consists of three parts. The most anterior part, the esophagus, still ending blindly in front reaches back as far as the level of the hind end of the heart; and is divided into two regions, viz. an anterior region, characterized by the presence of the visceral clefts, whose development has already been dealt with, and a posterior region without such clefts. Its transverse section, which up to the end of the second day was somewhat crescent-shaped, with the convexity downwards, becomes on this day more nearly circular. Close to its hinder limit, the lungs (Fig. 60, lg), of whose formation we shall speak directly, take their origin. The portion of the digestive canal which succeeds the cesophagus, becomes towards the close of the third day somewhat dilated (Fig. 60, St); the region of the stomach is thus indicated. The hinder or pyloric end of the stomach is separated by a very small interval from the point where the com- plete closing in of the alimentary canal ceases, and where the splanchnopleure-folds spread out over the yolk, This short tract is nevertheless clearly marked out as 174 THE THIRD DAY. [CHAP. . Fie. 60. DiaGRaM OF A PORTION OF THE DIGESTIVE TRACT OF A CuHIck UPON THE FourtH Day. (Copied from Gitte.) The black inner line represents the hypoblast, the outer shading the mesoblast. dg. lung-diverticulum with expanded termi- nation, forming the primary lung-vesicle. S¢. stomach. 1. two hepatic diverticula with their terminations united by cords of hypoblast cells. . diverticulum of the pancreas with the vesicular diverticula coming from it. the duodenum by the fact that from it, as we shall presently point out, the rudiments of the ducts of the liver and pancreas are beginning to be formed. The posterior division of the digestive tract, cor- responding to the great intestine and cloaca, is from its very first formation nearly circular in section and of a larger bore than the cesophagus. During part of the third day the hinder end of this section of the gut is in communication with the neural tube by the neurenteric canal already spoken of (Fig: 61, ne). The communication between the two tubes VL] THE PROCTODEUM. 175 Fie. 61. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POS8- TERIOR END OF AN EmMBRyo Birp, AT THE TIME OF THE FoRMATION ON THE ALLANTOIS. ep. epiblast; Sp.c. spinal canal; ch. notochord ; n.e. neurenteric canal ; hy. hypoblast; p.a.g. postanal gut; pr. remains of primitive streak folded in on the ventral side; al. allantois ; me. mesoblast ; an. point where anus will be formed; p.c. perivisceral cavity; am. amnion; so. somatopleure; sp. splanchnopleure. does not last long, but even after its rupture there re- mains a portion of the canal continuous with the gut; this, however, constitutes a purely embryonic and tran- sient section of the alimentary canal, and is known as the postanal gut. Immediately in front of it is a deep infolding of the epiblast, which nearly meets the hypoblast (Fig. 61, an) and forms the rudiment of the anus and of the outer section of the cloaca into which the bursa Fabricii opens in the adult. It is known to embryologists as the proctodeum, but does not open into the alimentary tract till considerably later. The 176 THE THIRD DAY. [CHAP. section of the alimentary tract immediately in front of the postanal gut is somewhat enlarged, and becomes the inner section of the adult cloaca receiving the urinary and genital ducts. The allantois, to whose develop- ment we shall return directly, opens into it ventrally. It is to be noted that the two sections of the cloaca of adult birds have a different origin. The inner section being part of the primitive alimentary tract and lined by hypoblast; the outer being part of an involution of the outer skin and lined by epiblast. The lungs are in their origin essentially buds or processes from the primitive cesophagus. At a point immediately behind the region of the visceral clefts the cavity of the alimentary canal be- comes compressed laterally, and at the same time con- stricted in the middle so that its transverse section (Fig. 62,1) is somewhat hourglass-shaped, and shews an upper or dorsal chamber d, joining on to a lower or ventral chamber J by a short narrow neck. The hinder end of the lower tube enlarges (Fig. 62, 2),and then becomes partially divided into two lobes (Fig. 62,3). All these parts at first freely communicate, but the two lobes behind, partly by their own growth, and partly by a process of constriction, soon become isolated posteriorly (Fig. 60, fg); while in front they open into the lower chamber of the cesophagus. By a continuation forwards of the process of con- striction the lower chamber of the cesophagus, carrying with it the two lobes above mentioned, becomes gradu- ally transformed into an independent tube, opening in front by a narrow slit-like aperture into the cesophagus. The single tube in front is the rudiment of the trachea v1] THE LUNGS, 177 and larynx, while the two diverticula behind (Fig. 60, lg) become the bronchial tubes and lungs, While the above changes are taking place in the hypoblastic walls of the alimentary tract, the splanchnic Fig. 62, 1 2 a a Z v4 3 4 d a> ad a . Vo 3 Py a \ : V 7 t Four DIAGRAMS ILLUSTRATING THE FoRMATION OF THE Lunes. (After Gétte.) a. mesoblast; 6. hypoblast; d. cavity of digestive canal; J. cavity of the pulmonary diverticulum. In (1) the digestive canal has commenced to be constricted into a dorsal and ventral canal; the former the true alimentary canal, the latter the pulmonary tube; the two tubes communi- cate with each other in the centre. In (2) the ventral (pulmonary) tube has become expanded. In (3) the expanded portion of the tube has become con- stricted into two tubes, still communicating with each other and with the digestive canal. In (4) these are completely separated from each other and from the digestive canal, and the mesoblast has also begun to exhibit externally changes corresponding to the internal changes which have been going on. F. & B. 12 178 THE THIRD DAY. [CHAP. mesoblast surrounding these structures becomes very much thickened; but otherwise bears no marks of the internal changes which are going on, so that the above formation of the lungs and trachea cannot be seen from the surface. As the paired diverticula of the lungs grow backwards, the mesoblast around them takes however the form of two lobes, into which they gradually bore their way. The further development of the lungs is, at first, essentially similar to that of a racemose gland. From each primitive diverticulum numerous branches are given off. These branches, which are mainly confined to the dorsal and lateral parts, penetrate into the sur- rounding mesoblast and continue to give rise to second- ary and tertiary branches. At right angles to the finest of these the arborescent branches so charac- teristic of the avian lung are given off. In the meso- blast around them numerous capillaries make their appearance. The air sacs, which form such important adjuncts of the avian lungs, are the dilated extremities of the primary pulmonary diverticula and of their main branches. The whole pulmonary structure is therefore the result of the growth by budding of a system of branched hypoblastic tubes in the midst of a mass of mesoblastic tissue, the hypoblastic elements giving rise to the epi- thelium of the tubes and the mesoblast providing the elastic, muscular, cartilaginous, connective and other tissues of the tracheal and bronchial walls. The liver is the first formed chylopoietic appendage of the digestive canal, and arises between the 55th and VI] THE LIVER. 179 60th hour as a couple of diverticula one from either side of the duodenum immediately behind the stomach (Fig. 60, 2). These diverticula are of course lined by hypoblast. The right’ one is, in all cases, from the first longer, but of smaller diameter than the left. Situated a little behind the heart, they embrace between them the two vitelline veins forming the roots of the meatus venosus. The diverticula soon give rise to numerous hollow branches or processes, which extend into the surround- ing mesoblast. Towards the end of the third day there may further be observed in the greatly thickened mesoblastic invest- ment of either diverticulum a number of cylindrical solid cords of hypoblast which are apparently out- growths from the hypoblast of the branches of the di- verticula. These cylinders rapidly increase in number, apparently by a process of sprouting, and their some- what swollen peripheral extremities come into contact and unite. And thus, about the ninetieth hour, a sort of network of solid thick strings of hypoblastic cells is formed, the mesoblast in the meshes of the network becoming at the same time largely converted into blood-vessels. Each diverticulum becomes in this way surrounded by a thick mass composed partly of solid cylinders, and to a less extent of hollow processes, con- tinuous with the cylinders on the one hand, and the main diverticulum on the other, all knit together with commencing blood-vessels and unchanged mesoblastic tissue. Between the two masses runs the now fused roots of the meatus venosus with which the blood- vessels in each mass are connected. 12—2 180 THE THIRD DAY. [CHaP. Early on the fourth day each mass sends out ventral to the meatus venosus a solid projection of hypoblas- tic cylinders towards its fellow, that from the left side being much the longest. The two projections unite and form a long solid wedge, which passes obliquely down from the right (or from the embryo lying on its left side, the upper) mass to the left (or lower) one. In this new wedge may be seen the same arrangement of a network of hypoblastic cylinders filled in with vascular mesoblast as in the rest of the liver. The two original diverticula with their investing masses represent respec- tively the right and left lobes of the liver, and the wedge- like bridge connecting them is the middle lobe. During the fourth and fifth days the growth of the solid, lobed liver thus formed is very considerable; the hypoblastic cylinders multiply rapidly, and the network formed by them becomes very close, the meshes contain- ing little more than blood-vessels. The hollow processes of the diverticula also ramify widely, each branch being composed of a lining of hypoblast enveloped in a coating of spindle-shaped mesoblastic cells. The blood-vessels are in direct connection with the meatus venosus—have become, in fact, branches of it. It may soon be observed, that in those vessels which are connected with the pos- terior part of the liver (Fig. 74), the stream of blood is directed from the meatus venosus into the network of the liver. In those connected with the anterior part the reverse is the case; here the blood flows from the liver into the meatus venosus. The thick network of solid cylinders represents the hepatic parenchyma of the adult liver, while the hollow processes of the diverticula are the rudiments of the biliary ducts; and we may suppose v1] THE PANCREAS. 181 each solid cylinder to represent a duct with its lumen almost, but perhaps not quite, completely obliterated. During the fifth day, a special sac or pouch is deve- loped from the right primary diverticulum. This pouch, consisting of an inner coat of hypoblast, and an outer of mesoblast, is the rudiment of the gall-bladder. The Pancreas arises nearly at the same time as the liver in the form of an almost solid outgrowth from the dorsal side of the intestine nearly opposite but slightly behind the hepatic outgrowths (Fig. 60, p). Its blind end becomes somewhat enlarged and from it numerous diverticula grow out into the passive splanchnic meso- blast. As the ductules grow longer and become branched, vascular processes grow in between them, and the whole forms a compact glandular body in the mesentery on the dorsal side of the alimentary tract. The primitive outgrowth elongates and assumes the character of a duct. On the sixth day a new similar outgrowth from the duodenum takes place between the primary diver- ticulum and the stomach. This, which ultimately coalesces with its predecessor, gives rise to the second duct, and forms a considerable part of the adult pan- creas. A third duct is formed at a much later period. The Thyroid body. The thyroid body arises at the end of the second or beginning of the third day as an outgrowth from the hypoblast of the ventral wall of the throat opposite the point of origin of the anterior aortic arch. It has at first the form of a groove extending forwards up to the future mouth, and in its front part extending ventrally to the epiblast. It has not been made out whether the whole groove becomes converted into the permanent thyroid. By the fourth day it becomes a solid mass of cells, and by the fifth ceases to be connected 182 THE THIRD DAY. [CHAP. with the epithelium of the throat, becoming at the same time bilobed. By the seventh day it has travelled somewhat back- wards, and the two lobes have completely separated from each other. By-the ninth day the whole is invested by a capsule of connective tissue, which sends in septa dividing it into a number of lobes or solid masses of cells, and by the sixteenth day its two lobes are composed of a number of follicles, each with a ‘mem- brana propria,’ and separated from each other by septa of con- nective tissue, much as in the adult}, The spleen, Although the spleen cannot be reckoned amongst the glands of the alimentary tract its development may conveniently be dealt with here. It is formed shortly after the first appearance of the pancreas, as a thickening of the me- sentery of the stomach (mesogastrium) and is therefore entirely a mesoblastic structure. The mass of mesoblast which forms the spleen becomes early separated by a groove on the one side from the pancreas and on the other from the mesentery. Some of its cells become elongated, and send out processes which, uniting with like processes from other cells, form the trabecular system. From the remainder of the tissue are derived the cells of the spleen pulp, which frequently contain more than one nucleus. Especial accumulations of these take place at a later period to form the so-called Malpighian corpuscles of the spleen. The Allantois. We have already had occasion to point out that the allantois is essentially a diverticulum of the alimentary tract into which it opens immediately in front of the anus. Its walls are formed of vascular splanchnic mesoblast, within which is a lining of hypo- blast. It becomes a conspicuous object on the third day of incubation, but its first development takes place at an earlier period, and is intimately connected with the formation of the posterior section of the gut. At the time of the folding in of the hinder end of 1 Miller Ueber die Entwickelung der Schilddriise. Jenaische Zeitschrift, 1871. VIL] THE ALLANTOIS. 183 the gut the splitting of the mesoblast into somatopleure and splanchnopleure has extended up to the border of the hinder division of the primitive streak. The ventral wall of what we have already termed the postanal section of the alimentary tract is formed by the primi- tive streak. Immediately in front of this is the involu- tion which forms the proctodeum; while the wall of the hindgut in front of the proctodeeum owes its origin to a folding in of the splanchnopleure. The allantois first appears as a narrow diverticulum formed by a special fold of the splanchnopleure just in front of the proctodeum. This protuberance arises, how- ever, before the splanchnopleure has begun to be tucked in so as to form the ventral wall of the hindgut; and it then forms a diverticulum (Fig. 63 A, All) the open end of which is directed forward, while its blind end points somewhat dorsalwards and towards the peritoneal space behind the embryo. As the hindgut becomes folded in the allantois shifts its position, and forms (Figs. 63 B and 61) a rather wide vesicle lying immediately ventral to the hind end of the digestive canal, with which it communicates freely by a still considerable opening; its blind end projects into the pleuroperitoneal cavity below. Still later the allantois grows forward, and becomes a large spherical vesicle, still however remaining con- nected with the cloaca by a narrow canal which? forms its neck or stalk (Fig. 9 G, al). From the first the allantois lies in the pleuroperitoneal cavity. In this cavity it grows forwards till it reaches the front limit of the hindgut, where the splanchnopleure turns back to enclose the yolk-sac. It does not during the third 184 THE THIRD DAY. [CHAP. Fie. 63. ‘Two LoneirupinaL SECTIONS oF THE TAIL-END oF aN Em- BRYO CHICK TO SHEW THE ORIGIN OF THE ALLANTOIS. A AT THE BEGINNING OF THE THIRD Day; B av THE MIDDLE OF THE THIRD Day. (After Dobrynin.) t. the tail; m. the mesoblast; 2’. the epiblast; «”. the neural -eanal; Dd. the dorsal wall of the hindgut; SO. somato- pleure; Spl. splanchnopleure; w. the mesoblast of the splanchnopleure carrying the vessels of the yolk-sac; pp. pleuroperitoneal cavity; Df. the epithelium lining the pleuroperitoneal cavity; AlZ. the commencing allantois; w. projection formed by anterior and posterior divisions of the primitive streak; y. hypoblast which will form the ventral wall of the hindgut; v. anal invagination (procto- deeum); G. cloaca. day project beyond this point; but on the fourth day begins te pass out beyond the body of the chick, along the as yet wide space between the splanchnic and soma- tic stalks of the embryo, on its way to the space between the external and internal folds of the amnion, which it will be remembered, is directly continuous with the pleuroperitoneal cavity (Fig. 9 K). In this space it VIL] THE MESOBLASTIC SOMITES. 185 eventually spreads out over the whole body of the chick. On the first half of the fourth day the vesicle is still very small, and its growth is not very rapid. Its mesoblast wall still remains very thick. In the latter half of the day its growth becomes very rapid, and it forms a very conspicuous object in a chick of that date (Fig. 67, Al). At the same time its blood-vessels be- come important. It receives its supply of blood from two branches of the aorta known as the allantoic arte- ries, and the blood is brought back from it by two allan- toic veins which run along in the body walls, and after uniting into a single trunk fall into the vitelline vein close behind the liver. Mesoblast of the trunk. Coincidently with the appearance of these several rudiments of important organs in the more or less modified splanchnopleure- folds, the solid trunk of the embryo is undergoing marked changes. When we compare a transverse section taken through say the middle of the trunk at the end of the third day (Fig. 65), with a, similar one of the second day (Fig. 34), or even the commencement of the third day (Fig. 64), we are struck with the great increase of depth (from dorsal to ventral surface) in proportion to breadth. This is partly due to the slope of the side walls of the body having become much steeper, as a direct result of the rapidly progressing folding off of the embryo from the yolk-sac. But it is also brought about by the great changes both of shape and structure which are taking place in the mesoblastic somites, as well as by the development of a mass of tissue between the notochord and the hypoblast of the alimentary canal. 186 THE THIRD DAY. [CHAP. It will be remembered that the horizontal splitting of the mesoblast into somatic and splanchnic layers extends at first to the dorsal summit of the vertebral plates, but after the formation of the somites the split Fie. 64. ane spe YP a 6 ys Pre es NS SOO LSC IO, fk . Ly 2 steac(ecey A: ae 1 ESS et IA LS Sp. TRANSVERSE SECTION THROUGH THE TRUNK OF A Duck EmBryo WITH ABOUT TWENTY-FOUR MEsosuastic So- MITES. am. amnion ; so. somatopleure; sp. splanchnopleure ; wd. Wolf- fian duct; sé. segmental tube; ca.v. cardinal vein; ms. muscle-plate; sp.g. spinal ganglion; sp.c. spinal cord ; ch, notochord ; ao. aorta ; hy. hypoblast. between the somatic and splanchnic layers becomes to a large extent obliterated, though in the anterior somites VL] THE MUSCLE-PLATES. 187 it appears in part to persist. The somites on the second day, as seen in a transverse section (Fig. 34, P.v), are somewhat quadrilateral in form but broader than they are deep. Each at that time consists of a somewhat thick cortex of radiating rather granular columnar cells, enclosing a small kernel of spherical cells. They are not, as may be seen in the above figure, completely separated from the ventral (or rather at this period lateral) parts of the mesoblastic plate, and the dorsal and outer layer of the cortex of the somites is continuous with the somatic layer of mesoblast, the remainder of the cortex, with the central kernel, being continuous with the splanchnic layer. Towards the end of the second and beginning of the third day the dorsal and outer layer of the cortex, together probably with some of the central cells of the kernel, becomes separated off as a special plate. From this plate, which is shewn in the act of being formed in Fig. 64, ms, the greater part of the voluntary muscular system of the trunk is developed. When once formed the muscle- plates have in surface views a somewhat oblong form, and consist of two layers, an inner and an outer, which enclose between them an almost obliterated central cavity (Fig. 65, mp). No sooner is the muscle-plate formed than the middle portion of the inner layer be- comes converted into longitudinal muscles. The central space in the muscle-plates is clearly a remnant of the vertebral portion of the body cavity, which, though it wholly or partially disappears in a previous stage, re- appears again on the formation of the muscle-plate. It is especially interesting to note that the first 188 THE THIRD DAY. [cHaP Fie. 65. mC cy. ep _ ———— “Oy . SECTION THROUGH THE Dorsal REGION OF AN Empryo CHICK AT THE END OF THE THIRD Day. Am. amnion, m.p. muscle-plate. C. V. cardinal vein. Ao. dorsal aorta. The section passes through the point where the dorsal aorta is just commencing to divide into two branches. Ch. notochord. W. d. Wolian duct. W. 6. commencing differentiation of the mesoblast cells to form the Wolffian body. ep. epiblast. SO. somatopleure. Sp. splanchno- pleure. fy. hypoblast. The section passes through the point where the digestive canal communicates with the yolk- sac, and is consequently still open below. This section should be compared with the section through the dorsal region of an embryo at the commencement of the third VI] THE INTERMEDIATE CELL-MASS. 189 day (Fig. 64). The chief differences between them arise from the great increase in the space (now filled with mesoblast-cells) between the notochord and the hypoblast. In addition to this we have in the later section the completely formed amnion, the separation of the muscle-plate from the mesoblastic somites, the formation of the Wolffian body, etc. The mesoblast including the Wolffian body and the muscle- plate (mp.) is represented in a purely diagrammatic manner. The amnion, of which only the inner limb or true amnion is represented in the figure, is seen to be composed of epiblast and a layer of mesoblast ; though in contact with the body above the top of the medullary canal, it does not in any way coalesce with it, as might be concluded from the figure. formed muscles in embryo birds have an arrangement like that which is permanent in fishes; being longi- tudinal in direction, and divided into segments. The remainder of the somites, after the formation of the muscle-plates, is of very considerable bulk ; the cells of the cortex belonging to them lose their distinctive characters, and their major part becomes converted, in a manner which will be more particularly described in a future chapter, into the bodies of the permanent ver- tebree. We may merely add here that each of these bodies sends a process inwards ventral to the medullary cord, and that the processes from each pair of these bodies envelope between them the notochord. The intermediate cell-mass and the Wolffian body. In a transverse section of a 45 hours’ embryo a consider- able mass of cells may be seen collected between the meso- blastic somites and the point where the divergence into somatopleure and splanchnopleure begins (Fig. 34, just below Wd). This mass of cells, which we have already 190 THE THIRD DAY. [cHAP. spoken of'’as the intermediate cell-mass, is at first indis- tinguishable from the cells lining the inner end of the body cavity; but on the third day, a special peritoneal lining of epithelioid cells is developed which is more or less sharply marked off from the adjoming part of the intermediate cell-mass. This latter now also passes without any very sharp line of demarcation into the mesoblastic somite itself; and as the folding in of the side wall progressés, the mass of cells in this position increases in size and grows in between the notochord and the hypoblast, but does not accumulate to a suffi- cient extent to separate them widely until the end of the third or beginning of the fourth day. The fusion between the intermediate cell-mass and the inner portions of the somites becomes so complete on the third day that it is almost impossible to say which of the cells in the neighbourhood of the notochord are derived from the somites and which form the intermediate cell-mass. It seems almost certain however that the cells which form the immediate invest- ment of the notochord really belong to the somites. The intermediate cell-mass is of special importance to the embryologist, in that the excretory and generative systems are developed from it. We have already described (p. 106) the development of the Wolffian duct, and we have now to deal with the Wolffian body which is, as the reader has no doubt gathered, the embryonic excretory organ. The structure of the fully developed Wolffian body is fundamentally similar to that of the permanent kid- neys, and consists essentially of convoluted tubules, commencing in Malpighian bodies with vascular glome- ruli, and opening into the duct. vi. ] THE WOLFFIAN BODY. 191 The tubules of the Wolffian body are developed independently of the Wolffian duct, and are derived from the intermediate cell-mass, shewn in Fig. 34, between the upper end of the body-cavity and the meso- blastic somite. Jn the chick the mode of development of this mass into the segmental tubules is different in the regions in front of and behind about the sixteenth segment. In front of about the sixteenth segment special parts of the intermediate cell-mass remain attached to the peritoneal epithelium, on this layer becoming differentiated ; there being several such parts to each segment. The parts of the intermediate cell- mass attached to the peritoneal epithelium become converted into S-shaped cords (Fig. 64 st) which soon unite with the Wolffian duct (wd), and constitute the primitive Wolffian tubules. Into the commencement of each of these cords the lumen of the body-cavity is for a short distance prolonged, so that this part con- stitutes a rudimentary peritoneal funnel leading from the body-cavity into the lumen of the Wolffian tubule. In the foremost Wolffian tubules, which never reach a very complete development, the peritoneal funnels widen considerably. The section of the tube adjoining the wide peritoneal funnel becomes partially invaginated by the formation of a vascular ingrowth known as a glomerulus, and this glomerulus soon grows to such an extent as to project through the peritoneal funnel, the neck of which it completely fills, into the body-cavity (Fig. 66, gl). There is thus formed a series of glomeruli belonging to the anterior Wolffian tubuli projecting freely into the body-cavity. These glomeruli with their tubuli become however early aborted. 192 THE THIRD DAY. [CHAP. Fic. 66. SECTION THROUGH THE EXTERNAL GLOMERULUS OF ONE OF THE ANTERIOR SEGMENTAL TUBES OF AN EmBryo CHICK OF ABOUT 100 HOURS, gt. glomerulus ; ge. peritoneal epithelium ; Wd. Wolffian duct ; ao. aorta ; me. mesentery. The Wolffian tubule, and the connection between the external and internal parts of the glomerulus are not shewn in this figure. In the case of the remaining tubules developed from the S-shaped cords, the attachment to the peritoneal epithelium is very soon lost. The cords acquire a lumen, and open into the Wolffian duct. Their blind extremities constitute the commencements of Malpi- ghian bodies. In the posterior part of the Wolffian body of the chick the intermediate cell-mass becomes very early ‘detached from the peritoneal epithelium, and at a con- siderably later period breaks up into oval vesicles, which elongate into Wolffian tubules. In addition to the primary tubules, whose development has just been described, secondary and tertiary tubules are formed on the dorsal side of the primary tubules. They are VI] THE WOLFFIAN BODY. 193 differentiated out of the mesoblast of the intermediate cell-mass and open independently into the Wolffian duct. A tubule of the Wolffian body typically consists of the follow- ing parts, (1) a section carrying the peritoneal opening, and known as the peritoneal funnel, (2) a dilated vesicle into which this opens, (3) a coiled tubulus proceeding from (2), and termi- nating in (4) a wider portion opening into the Wolffian duct. In the chick, the peritoneal funnel is only found in the most anterior tubules and soon atrophies; it is not developed in the tubules of the posterior part of the Wolffian body. Region No. 4 also is not clearly marked off from region No. 3. One part of the wall of the dilated vesicle (2) is invaginated by a bunch of capillaries and gives rise to the Malpighian body. In consequence of the continual folding in of the somatopleure and especially of the splanchnopleure, as well as owing to the changes taking place in the meso- blastic somites, the Wolffian duct undergoes on the third day a remarkable change of position. Instead of lying, as on the second day, immediately under the epiblast (Fig. 34, W.d), it is soon found to have appa- rently descended into the middle of the intermediate cell-mass (Fig. 64, w.d) and at the end of the third day occupies a still lower position and even projects some- what towards the pleuroperitoneal cavity. (Fig. 65, W.d.) The chief events then which take place on the third day are as follows: 1. The turning over of the embryo so that it now lies on its left side. 2. The cranial flexure round the anterior extremity of the notochord. F. & B, 13 194 THE THIRD DAY. (CHAP. VI. 3. The completion of the circulation of the yolk- sac; the increased curvature of the heart, and the demarcation of its several parts; the appearance of new aortic arches, and of the cardinal veins. 4. The formation of four visceral clefts and five visceral arches. 5. The involution to form the lens, and the forma- tion of the secondary optic vesicle. 6. The closing in of the otic vesicle. 7. The formation of the nasal pits. 8. The appearance of the vesicles of the cerebral hemispheres ; the separation of the hind-brain into cere- bellum and medulla oblongata. 9. The definite establishment of the cranial and spinal nerves as outgrowths of the central nervous system. 10. The completion of the fore-gut and of the hind-gut; the division of the former into cesophagus, stomach and duodenum, of the latter into large intestine and cloaca. 11. The formation of the lungs from a diverticulum of the alimentary canal immediately in front of the stomach. 12. The formation of the liver and pancreas: the former as two diverticula from the duodenum, which subsequently become united by nearly solid outgrowths ; the latter as a single diverticulum also from the duo- denum. 13. The changes in the mesoblastic somites and the appearance of the muscle-plates. 14. The definite formation of the Wolffian bodies and the change in position of the Wolffian duct. CHAPTER VII. THE CHANGES WHICH TAKE PLACE DURING THE FOURTH DAY. ON opening an egg in the middle or towards the end of the fourth day, a number of points in which progress has been made since the third day are at once apparent. In the first place, the general growth of the embryo has been very rapid, so that its size is very much greater than on the previous day. In the second place, the white of the egg has still further diminished, the em- bryo lying almost in immediate contact with the shell membrane. The germinal membrane embraces more than half the yolk, and the vascular area is about as large as a halfpenny. Corresponding to the increased size of the embryo, there is a great increase in the quantity of blood circu- lating in the vascular area as a whole, though the sinus terminalis is already less distinct than it was. The amnion becomes increasingly conspicuous. It is now seen as a distinct covering obscuring to a certain extent the view of the body of the chick beneath, and 13—2 196 THE FOURTH DAY. [CHAP. all traces of the junction of its folds are by this time lost. As yet there is very little fluid in the amniotic sac proper, so that the true amnion lies close upon the embryo. The folding off of the embryo from the yolk sac has made great progress. The splanchnic stalk, which on the third day was still tolerably wide, inasmuch as about one third of the total length of the alimentary canal was as yet quite open to the yolk sac below, now be- comes so much constricted by the progressive closing in of the splanchnopleure folds, that the alimentary canal may be said to be connected with the yolk sac by a very narrow neck only. This remnant of the splanchnic stalk we may now call the vitelline duct; though narrow, it is as yet quite open, affording still free communica- tion between the inside of the yolk sac and the interior of the alimentary canal. The somatic stalk, though narrowing somewhat, is much wider than the splanchnic’ stalk, so that a con- siderable ring-shaped space exists between the two. Another very prominent feature is the increase in the cranial flexure. During the third day, the axis of the front part of the head was about at right angles to the long axis of the body; the whole embryo being still somewhat retort-shaped. On this day, however, the flexure has so much increased that the angle between the long axis of the body and that of the front segment of the head is an acute one. The tail-fold, which commenced to be noticeable during the third day, has during this day increased very much, and the somewhat curved tail (Fig. 67) forms. quite a conspicuous feature of the embryo. The general vit] THE TAIL FOLD, 197 S Ht Ly poe . as EmBryYo AT THE END OF THE FourRTH Day SEEN AS A TRANSPARENT OBJECT. The amnion has been completely removed, the cut end of the somatic stalk is shewn at S.S. with the allantois (4/.) protruding from it. C.H. cerebral hemisphere. /.B. fore-brain or vesicle of the third ventricle (thalamencephalon) with the pineal gland (Pn.) projecting from its summit. M.B. mid-brain. Cb. cerebellum. 1V.YV. fourth ventricle. Z. lens. ch.s. choroid slit. Owing to the growth of the optic cup the two layers of which it is com- posed cannot any longer be seen from the surface; the pos- terior surface of the choroid layer alone is visible. Cen. V. auditory vesicle. s.m. superior maxillary process. 1/, 2F, etc. first, second, third and fourth visceral folds. V. fifth nerve sending one branch to the eye, the ophthalmic branch, and 198 THE FOURTH DAY. [CHAP. another to the first visceral arch. VJ. seventh nerve passing to the second visceral arch. G.Ph. glossopharyngeal nerve passing towards the third visceral arch. Pg. pneumogastric nerve passing towards the fourth visceral arch. iv. investing mass (basilar plate). No attempt has been made in the figure to indicate the position of the dorsal wall of the throat, which cannot be easily made out in the living embryo. ch. noto- chord. The front end of this cannot be seen in the living embryo. It does not end however as shewn in the figure, but takes a sudden bend downwards and then terminates in apoint. Ht. heart seen through the walls of the chest. J.P. muscle-plates. W. wing. AZ. hind limb. Beneath the hind limb is seen the curved tail. curvature of the body has also gone on increasing, and as the result of these various flexures, the embryo has become somewhat spirally curled up on itself (Fig. 67). The distinct appearance of the limbs must be reckoned as one of the most important events of the fourth day. Owing to the continued greater increase of depth than of breadth, the body of the embryo appears in section (Fig. 68) higher and relatively narrower than even on the third day, and the muscle-plates, instead of simply slanting downwards, come to be nearly vertical in position. Not far from the line which marks their lower ends, the somatopleure, almost immediately after it diverges from the splanchnopleure, is raised up (Fig. 68, W.R.) into a low rounded ridge which runs along nearly the whole length of the embryo from the neck to the tail. It is on this ridge, which is known as the Wolffian ridge, that the limbs first appear as flattened conical buds projecting outwards. They seem to be local de- VII.] THE LIMBS. 199 Fie. 68. SECTION THROUGH THE LUMBAR REGION OF AN EMBRYO aT THE END OF THE FourtTH Day. mc. neural canal. y.r. posterior root of spinal nerve with gan- glion. ar. anterior root of spinal nerve. A.G.C. anterior grey column of spinal cord. A. W.C. anterior white column of spinal cord just commencing to be formed, and not very distinctly marked in the figure. mp. muscle-plate. ch. notochord. W.R. Wolffian ridge. A.O. dorsal aorta. Vic, posterior cardinal vein. W.d. Wolffian duct. W.6. Wolffian body, consisting of tubules and Malpighian corpuscles. One of the latter is represented on each side. g.e. germinal 200 THE FOURTH DAY. [CHAP. epithelium. d. alimentary canal. Jf commencing me- sentery. 4.0. somatopleure. §.P. splanchnopleure. V. blood-vessels. pp. pleuroperitoneal cavity. velopments of the ridge, the rest of which becomes less and less prominent as they increase in size. Each bud, roughly triangular in section, consists of somewhat dense mesoblast covered by epiblast which on the sum- mit is thickened into a sort of cap. The front limbs or wings (Fig. 67) arise just behind the level of the heart, and the hind limbs in the immediate vicinity of the tail. The first traces of them can be seen towards the end of the third, but they do not become conspicuous till the fourth day, by the end of which the two pairs may be already distinguished by their different shapes. The front limbs are the narrowest and longest, the hind limbs being comparatively short and broad. Both are flattened from above downwards and become more so as their growth continues. In the head, the vesicles of the cerebral hemispheres are rapidly increasing in size, their growth being enor- mous as compared with that of the thalamencephalon or vesicle of the third ventricle. The mid-brain is now, as compared to the other parts of the brain, larger than at any other epoch, and an indistinct median furrow on its upper surface indicates its division into two lateral halves. The great increase of the mesoblastic contents of the secondary optic vesicle or involuted retinal cup causes the two eyeballs to project largely from the sides of the head (Fig. 69, Op). The mass of mesoblast which invests all the various parts of the brain, is not only growing rapidly below and at the sides, but is also undergoing developments which result in the formation vit.] THE HEAD. 201 Fig, 69. A. Heap or an Empryo Cuick or THE FourrH Dar VIEWED FROM BELOW AS AN OPAQUE OBJECT. (Chromic acid preparation.) C.H. cerebral hemispheres. F.B. vesicle of the third ventricle or thalamencephalon. Op. eyeball. nf. naso-frontal process. M. cavity of mouth. SI. superior maxillary process of F. 1, the first visceral fold (mandibular arch). /. 2, /. 3 second and third visceral arches. JV. nasal pit. In order to gain the view here given the neck was cut across between the third and fourth visceral folds. In the section e thus made are seen the alimentary canal al, the neural canal n.c., the notochord ch., the dorsal aorta AO., and the jugular veins V. Ao. bulbus arteriosus. In the drawing the nasal groove has been rather exaggerated in its upper part. On the other hand the lower and shallower part of the groove where it runs between the superior maxillary process S./. and the broad naso-frontal process has not been satisfactorily rendered. Hence the end of the superior maxillary process seems to join the inner and not, as described in the text, the onter margin of the nasal groove. A few hours later the separation of the two would have been very visible. B. The same seen sideways, to shew the visceral folds. ot. otic vesicle. Remaining letters as before. 202 THE FOURTH DAY. [CHAP. of the primitive skull. All these events, added to the cranial flexure spoken of above, give to the anterior extremity of the embryo a shape which it becomes more and more easy to recognize as that of a head. Meanwhile the face is also being changed. The two nasal pits were on the third day shallow depressions com- plete all round. As the pits deepen on the fourth day by the growth upwards of a rim round them, a deficiency or break in the ridge may be observed on that side of it turned towards the mouth; which constitutes a kind of shallow groove (Fig. 69 NV) directed obliquely downwards towards the cavity of the mouth. The fronto-nasal process or median ridge (Fig. 69, nf), which on the third day rose up between the superficial projections caused by the bulging anterior extremities of the vesicles of the cerebral hemispheres, and on the fourth day becomes increasingly prominent, separates the two grooves from each other, and helps to form the inner wall of each of them, while the depth of the groove also becomes in- creased by the prolongation along its inner side of the rim surrounding the nasal pit. Abutting on the outer side of each groove near the mouth and so helping to form the outer wall of each, lie the ends of the superior maxillary processes of the first visceral arch (Fig. 69 B, SM), which like the fronto-nasal process are increasing in size. By their continued growth, the groove is more and more deepened, and leading as it does from the nasal pit to the cavity of the mouth, may already be recognized as the rudiment of the passage of the pos- terlor nares. During the latter half of the fourth day there ap- pears at the bottom of the deep lozenge-shaped cavity VIL] THE CRANIAL NERVES. 203 of the stomodeum or primitive buccal cavity, in the now thin wall dividing it from the alimentary canal, a longi- tudinal, or according to Gotte a vertical slit which, soon becoming a wide opening, places the two cavities in complete communication. The cavity of the mouth, being, it will be remember- ed, formed partly by depression, partly by the growth: of the surrounding folds, is lined entirely with epiblast, from which the epithelium of its surface and of its various glands is derived. In this respect, as Remak pointed out, it differs from all the rest of the alimentary canal, whose whole epithelium is formed out of hypoblast. By the side of the hind-brain the cerebellum, through the increased thickening of its upper walls, is becoming more and more distinct from the medulla oblongata; while both in front and behind the auditory vesicle, in which the rudiments of the cochlea and recessus ves- tibuli are already visible, the cranial ganglia and nerves are acquiring increased distinctness and size. They may be very plainly seen when the head of the fresh embryo is subjected to pressure. The foremost is the fifth cranial nerve (Fig. 67, V.) with its Gasserian ganglion; it lies a little way behind the anterior extremity of the notochord immediately below the cerebellum. Next to this comes the seventh nerve (Fig. 67, VIJ.), starting just in front of the ear- vesicle, and extending far downwards towards the second visceral arch. The two nerves which lie behind the ear- vesicle are now obviously separate from each other; the front one is the glossopharyngeal (Fig. 67, G.Ph.), and the hinder one already shews itself to be the pneumo- gastric (Fig. 67, P9.). 204 THE FOURTH DAY. [ CHAP. The mesoblastic somites, which by the continued differentiation of the axial mesoblast at the tail end of the embryo have increased in number from thirty to forty, undergo during this day changes of great import- ance. Since these changes are intimately connected with the subsequent development of the vertebral column, it will perhaps be more convenient to describe briefly here the whole series of events through which the somites become converted into the permanent structures to which they give rise, though many of the changes do not take place till a much later date than the fourth day. The separation of the muscle-plates (p. 187) left the remainder of each somite as a somewhat triangular mass lying between the neural canal and notochord on the inside, and the muscle-plate and intermediate cell- mass on the outside (Fig. 64). Already on the third day (Fig. 65) the upper angle of this triangle grows upwards, between its muscle-plate and the neural canal, and meeting its fellow in the middle line above, forms a roof of mesoblast over the neural canal, between it and the superficial epiblast. At about the same time, the inner and lower angle of the triangle grows inwards to- wards the notochord, and passing both below it (between it and the aorta) and above it (between it and the neural canal), meets a similar growth from its fellow somite of the other side, and thus completely invests the notochord with a coat of mesoblast, which, as seen in Fig. 68, is at first much thicker on the under than on the upper side. Both neural canal and notochord are thus furnished from neck to tail with a complete investment of meso- VII. ] THE PERMANENT VERTEBR&. 205 blast, still marked, however, by the transparent lines indicating the fore and aft limits of the several somites. This mesoblastic investment is sometimes spoken of as the “membranous” vertebral column. The portions of the somites thus forming the primary vertebree or membranous vertebral column are converted into the permanent vertebre; but their conversion is complicated by a remarkable new or secondary segmen- tation of the whole vertebral column. On the fourth day, the transparent lines marking the fore and aft limits of the somites are still distinctly visible. On the fifth day, however, they disappear, so that the whole vertebral column becomes fused into a homogeneous mass whose division into vertebre is only indicated by the series of ganglia. This fusion, which does not extend to the muscle-plates in which the primary lines of division still remain visible, is quickly followed by a fresh segmentation, the resulting segments being the rudiments of the permanent vertebre. The new segmentation, however, does not follow the lines of the segmentation of the muscle-plates, but is so effected that the centres of the vertebral bodies are opposite the septa between the muscle-plates. The explanation of this character in the segmentation is not difficult to find. The primary segmentation of the body is that of the muscle-plates, which were present in the primitive forms in which vertebre had not appeared. As soon however as the notochordal sheath was required to be strong as well as flexible, it necessarily became divided into a series of segments. The condition under which the lateral muscles can best cause the flexure of the vertebral column is clearly that each muscle- plate shall be capable of acting on two vertebre ; and this con- dition can only be fulfilled when the muscle-segments are oppo- 206 THE FOURTH DAY. (CHAP. site the intervals between the vertebre. For this reason, when the vertebrae became formed, their centres were opposite not the middle of the muscle-plates but the inter-muscular septa. These considerations fully explain the characters of the secondary segmentation of the vertebral column. On the other hand the primary segmentation of the vertebral rudiments is clearly a remnant of a condition when no vertebral bodies were present ; and has no greater morphological significance than the fact that the cells of the vertebree were derived from the seg- mented muscle-plates, and then became fused into a continuous sheath around the notochord and nervous axis; till finally they became in still higher forms differentiated into vertebrae: and their arches. By these changes this remarkable result is brought about, that each permanent vertebra is formed out of portions of two consecutive mesoblastic somites. Thus, for instance, the tenth permanent vertebra is formed out of the hind portion of the tenth somite, and the front portion of the eleventh somite. The new segmentation is associated with or rather is caused by histological changes. At the time when the fusion takes place, the mesoblast, which in the form of processes from the somites surrounds and invests the notochord, has not only increased in mass but also has become cartilaginous, so that, as Gegenbaur* points out, there is present for a short period on the fifth day a continuous and unsegmented cartilaginous investment of the notochord. This cartilaginous tube does not however long re- main uniform. At a series of points corresponding in number to the original somites it becomes connected 1 Untersuchung zur vergleichenden Anatomie der Wirbelstule bei Amphibien und Reptilien, Leipzig, 1862. Vil. | THE PERMANENT VERTEBR2. 207 with a number of cartilaginous arches which appear in the mesoblastic investment of the neural canal. These arches, which thus roof in the neural canal, are the cartilaginous precursors of the osseous vertebral arches. We further find that the portions of the cartilaginous tube from which the arches spring come to differ histo- logically from the portions between them not connected with arches: they are clearer and their cells are less closely packed. There is however at this period no distinct segmentation of the cartilaginous tube, but merely a want of uniformity in its composition. The clearer portions, from which the arches spring, form the bodies of the vertebre, the segments between them the intervertebral regions of the column. On the fifth day a division takes place of each of the intervertebral segments into two parts, which respec- tively attach themselves to the contiguous vertebral regions. A part of each intervertebral region, immedi- ately adjoining the notochord, does not however undergo this division, and afterwards gives rise to the ligamen- tum suspensorium. This fresh segmentation is not well marked, if in- deed it takes place at all in the sacral region. To recapitulate:—the original somites lying side by side along the notochord, after giving off the muscle- plates, grow around, and by fusing together completely invest, with mesoblast, both neural canal and notochord. This investment, of which by reason of its greater growth the original bodies of the somites now seem to be only an outlying part, becomes cartilaginous in such a way that while the notochord becomes surrounded with a thick tube of cartilage bearing no signs of segmenta- 208 THE FOURTH DAY. [cHapP. tion, but having the ganglia lodged on its exterior at intervals, the neural canal is covered in with a series of cartilaginous arches, connected to each other by ordinary mesoblastic tissue only, but passing at their bases di- rectly into the cartilaginous tube around the notochord. By a process of histological differentiation the carti- laginous tube is divided into vertebral and interverte- bral portions, the vertebral portions corresponding to the arches over the neural canal. Fresh lines of seg- mentation then appear in the intervertebral portions, dividing each of them into two parts, of which one at- taches itself to the vertebra in front and the other to the vertebra behind. The notochord. Meanwhile from the fourth to the sixth day important changes take place in the notochord itself. On its first appearance the notochord was, as we have seen, composed of somewhat radiately arranged but otherwise perfectly typical mesoblast-cells. On the third day some of the central cells become vacuolated, while the peripheral cells are still normal. The vacuolated cells exhibit around the vacuole a peri- pheral layer of granular protoplasm in which the nucleus lies embedded, whilst the vacuoles themselves are filled with a perfectly clear and transparent material, which in an unaltered condition is probably fluid. Towards the end of the day the notochord acquires a delicate structureless sheath which is no doubt a product of its peripheral cells. On the fourth day all the cells become vacuolated with the exception of a single layer of flattened cells at the periphery. The vacuoles go on enlarging until VII.] THE NOTOCHORD. 209 on the sixth day the vacuoles in each cell have so much increased at the expense of the protoplasm that only a very thin layer of the latter is left at the circumference of the cell, at one part of which, where there is gene- rally more protoplasm than elsewhere, the starved re- mains of a nucleus may generally be detected. Thus the whole notochord becomes transformed into a spongy reticulum, the meshes of which correspond to the vacu- oles of the cells and the septa to the remains of their cell-walls. The notochord is on the sixth day at the maximum of its development, the change which it henceforward undergoes being of a retrograde character. From the seventh day onward, it is at various points encroached upon by its investment. Constrictions are thus produced which first make their appearance in the intervertebral portions of the sacral region. In the cer- vical region, according to Gegenbaur, the intervertebral portions are not constricted till the ninth day, though in the vertebral portions of the lower cervical vertebre con- strictions are visible as early as the seventh day. By the ninth and tenth days, however, all the interverte- bral portions have become distinctly constricted, and at the same time in each vertebral portion there have also appeared two constrictions giving rise to a central and to two terminal enlargements. In the space therefore corresponding to each vertebra and its appropriate in- tervertebral portion, there are in all three constrictions and three enlargements. On the twelfth day the ossification of the bodies of the vertebree commences. The first vertebra to ossify is the second or third cervical, and the ossification gradu- F. & B. 14 210 THE FOURTH DAY. [CHAP. ally extends backwards. It does not commence in the arches till somewhat later. For each arch there are two centres of ossification, one on each side. The notochord persists for the greater part of foetal life and even into post-foetal life. The larger vertebral portions are often the first completely to vanish. They would seem in many cases at any rate (Gegenbaur) to be converted into cartilage and so form an integral part of the permanent vertebra. Rudiments of the inter- vertebral portions of the notochord may long be detected in the ligamenta suspensoria. We may remind the reader that in the adult bird we find between each of the vertebrae of a neck and back a cartilaginous disc—the meniscus—which is pierced in the centre. These discs are thick at the circumference but thin off to a fine edge round the central hole. Owing to the shape of these discs there are left between each pair of vertebre two cavities, which only commu- nicate through the central aperture of the meniscus. Through this central aperture there passes a band called the ‘ligamen- tum suspensorium,’ connecting the two vertebre. In the tail the menisci are replaced by bodies known as the ‘annuli fibrosi, which precisely resemble the similarly named bodies in mammals. They differ from the menisci in being attached over their whole surface to the ends of the vertebral bodies, so that the cavities between the menisci and the vertebrae do not exist. They are pierced however by a body corre- sponding with the ligamentum suspensorium and known as the ‘nucleus pulposus.’ In the intervertebral regions the chorda, soon after the com- mencement of ossification, entirely disappears. The cartilage around it however becomes converted (in the region of the neck) into the ligamentum suspensorium, which unites the two ver- tebrze between which it is placed. In the tail the cartilage becomes the nucleus pulposus, which corresponds exactly to the ‘ligamentum suspensorium’ of the neck and back. VIL] THE MUSCLE-PLATES. 211 Shortly after the formation of the ligamentum suspensorium the remaining cartilage of the intervertebral segments is con- verted into the meniscus between each two vertebre, and in the tail into the annulus fibrosus. Both are absent in the sacrum. Muscle-plates. We shall conclude our account of the mesoblastic somites by describing the changes which take place in the muscle-plates. In the chick these are somewhat complicated, and have not been fully worked out. On the third day the muscle-plates end opposite the point where the mesoblast becomes split into somato- pleure and splanchnopleure. On the fourth day how- ever (Fig. 68 mp.) they extend a certain distance into the side walls of the body beyond the point of the division into somatopleure and splanchnopleure. Into what muscles of the trunk they become con- verted has been somewhat disputed. Some embryolo- gists have stated that they only form the muscles of the back. We have, however, little doubt that all the episkeletal muscles, to use Professor Huxley’s term (Vertebrates, p. 46), are their products; a view also adopted by Professors Huxley and Kolliker. The development of the subvertebral system of muscles (hyposkeletal of Huxley) has not been worked out, but on the whole there is reason to believe that it is derived from the muscle-plates. Kélliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin. Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out. It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kélliker however believes that they have an independent origin. 14—2 212 THE FOURTH DAY. [CHaP. The limb-muscles, both extrinsic and intrinsic, are in certain fishes (Elasmobranchii), derived from the muscle-plates, and a similar origin has been observed in Lacertilia and Amphibia. In the Chick and other higher Vertebrata on the other hand the entrance of the muscle-plates into the limbs has not been made out (Kélliker). It seems therefore probable that by an embryological modification, of which instances are so frequent, the cells which give rise to the muscles of the limbs in the higher Vertobrata can no longer be traced into a direct connection with the muscle-plates. At first, as is clear from their mode of origin, the muscle-plates correspond in number with the protover- tebrz, and this condition is permanent in the lower vertebrates, such as fishes, where we find that the lateral muscle is divided by septa into a series of segments corresponding in number with the vertebree. Wolffian body or mesonephros. Of all the events of the fourth day, none perhaps are more important than those by which the rudiments of the complex urinary and generative systems are added to the simple Wolffian duct and body, which up to that time are the sole repre- sentatives of both systems. We saw that the duct arose on the second day (pp. 94, 106) as a solid ridge which subsequently became a tube, lying immediately underneath the epiblast: above the intermediate cell-mass, close against the upper and outer angles of the somites, and reaching from about opposite to the seventh somite away to the hinder end of the embryo. At first the duct occupies a position immediately underneath the superficial epiblast, but very soon after its formation the growth of the somites and the changes which take place in the intermediate cell-mass, together VIL] THE WOLFFIAN BODY. 213 with the general folding in of the body, cause it to appear to change its place and travel downwards (p. 193). While the shifting is going on, the cells lining the upper end of the pleuroperitoneal cavity (the kind of bay which, as seen in sections, is formed by the diver- gence of the somatopleure and splanchnopleure) become columnar, and constitute a distinct epithelium. This epithelium, which is clearly shewn in Fig. 64, and is also indicated in Fig. 65, is often called the germinal epitheliwm, because some of its cells subsequently take part in the formation of the ovary. Soon after the ap- pearance of the germinal epithelium, the intermediate cell-mass increases in size and begins to grow outwards into the pleuroperitoneal cavity, as a rounded projection which lies with its dorsal surface towards the somato- pleure, and its ventral surface towards the splanchno- pleure, but is in either case separated from these layers by a narrow chink. The Wolffian duct (Figs. 65, 68, Wa) travels down, and finally before the end of the third day is found in the upper part of this projection, near that face of it which is turned towards the somatopleure. The tubules of the anterior part of the Wolffian body have by the end of the fourth day almost entirely disappeared; but the tubules of that part of the Wolf- fian body which is found behind the 16th segment undergo a further development. Each increases in size especially that portion which proceeds from the Malpighian body and is known as the coiled tubulus (region No. 3, see p.193). This becomes much elongated and twisted. As a consequence of this increase in size the intermediate cell-mass comes to project more and more into the pleuroperitoneal cavity. 214 THE FOURTH DAY. [CHaP. The large size of the hinder part of the Wolffian body as compared with that of the anterior part is due to the presence of the dorsally placed secondary tubules, whose development was mentioned on p. 192. These are more numerous in the posterior than in the anterior part of the Wolffian body. At the hind end of the Wolffian body there are as many as four to each primary tubule. The tubules, which from their contorted course are in sections (Figs. 68, 71) seen cut at various angles, possess an epithelium which is thicker than that of the Wolffian duct. From this difference it is generally easy to distinguish the sections of the tubules from those of the duct. The glomeruli of the Malpighian bodies are in sections of hardened embryos usually filled with blood-corpuscles. Towards the hind end of the embryo, the projection of the intermediate cell-mass spoken of above becomes smaller and smaller, and the Wolffian duct is thus brought nearer to the splanchnopleure, and in the region of the hind-gut comes to lie close to the walls of the alimentary canal. On the fourth day, the two ducts meet and open into two horns, into which the side-walls of the recently formed cloaca are at that time produced, one on either side. As we shall afterwards see, the ducts of the perma- nent kidneys and Miiller’s duct also fall into these two horns of the cloaca. The Wolffian bodies thus constituted perform the offices of kidneys for the greater part of embryonic life. In the chick they disappear before birth; but in most of the Ichthyopsida they remain for life as the perma- nent kidneys. Miullerian duct. After the establishment of the VII.] THE MULLERIAN DUCT. 215 Wolftian body there is formed in both sexes a duct, which in the female becomes the oviduct, but which in the male is functionless and usually disappears. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Miillerian duct of the Ichthyopsida. The first rudiment of the Miillerian duct appears at the end of the fourth day, as three successive open involu- tions of the peritoneal epithelium, connected together by more or less well-defined ridge-like thickenings of the epithelium. It takes its origin from the layer of thickened peritoneal epithelium situated near the dorsal angle of the body-cavity, close to the Wolffian duct, and some considerable distance behind the front end of the Wolffian duct. In a slightly later stage the ridges connecting the grooves become partially constricted off from the peri- toneal epithelium, and develop a lumen. The condition of the structure at this stage is illustrated by Fig. 70, representing three transverse sections through two grooves, and through the ridge connecting them. The Miillerian duct may in fact now be described as a short but slightly convoluted duct, opening into the body-cavity by three groove-like apertures, and con- tinued for a short distance behind the last of these. In an embryo not very much older than the one last described the two posterior apertures vanish and the anterior opening alone remains as the permanent open- ing of the Miillerian duct. The position of these openings in relation to the Wolffian body is shewn in Fig. 71, which probably passes through a region between two of the peritoneal openings. 216 THE FOURTH DAY. [CHAP. Fie, 70. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH GIVE RISE TO THE ANTERIOR Part oF THE MiL- LERIAN Duct (PRONEPHROS). A is the 11th section of the series. B ” 15th ” ” Cc, «18th 5 3 gr2 second groove. gr3 third groove. 72 second ridge. wd. Wolffian duct. As long as the openings persist, the Miillerian duct consists merely of a very small rudiment, continuous with the hindermost of them, and its solid extremity appears to unite with the walls of the Wolffian duct. After the closure of the two hinder openings the Miillerian duct commences to grow rapidly backwards, and for the first part of its subsequent course it appears to be split off as a solid rod from the outer or ventral wall of the Wolffian duct (Fig. 72). Into this rod the lumen, present in its front part, subsequently extends. Its mode of development in front is thus pre- cisely similar to that of the Miillerian duct in Elasmo- branchii and Amphibia. This mode of development only occurs however in the anterior part of the duct. In the posterior part of VIL | THE MULLERIAN DUCT. 217 . Fie. 71. SEcTION oF THE INTERMEDIATE CELL-MASS ON THE FouURTH Day. (From Waldeyer.) Magnified 160 times. m. mesentery. J. somatopleure. a’. portion of the germinal epithelium from which the involution to form the duct of Miiller (z) takes place. a. thickened portion of the germinal epithelium in which the primitive ova C’ and o are lying. £. modified mesoblast which will form the stroma of the ovary. WA. Wolffian body. y. Wolffian duct. its course its growing point lies in a bay formed by the outer wall of the Wolffian duct, but does not become definitely attached to that duct. It seems however possible that, although not actually split off from the 218 THE FOURTH DAY. (CHAP. Fic. 72. Two SECTIONS SHEWING THE JUNCTION OF THE TERMINAL Sorry Portion oF tHE Miiierran Duct witH THE Wourrian Doct. In A the terminal portion of the duct is quite distinct; in B it has united with the walls of the Wolffian duct. md. Miillerian duct. Wd. Wolffian duct. walls of the Wolffian duct, it may grow backwards from cells derived from that duct. The Miillerian duct finally reaches the cloaca though it does not in the female for a long time open into it, and in the male never does so. The anterior part of the commencing Miillerian duct with its three openings into the body-cavity is probably homologous with the head kidney or pronephros of the Ichthyopsida. Permanent kidney or metanephros. Between the 80th and 100th hour of incubation, the permanent kid- neys begin to make their appearance, and as is the case with the Wolffian bodies, the first portion of them to appear is their duct. Near its posterior extremity the Wolffian duct becomes expanded, and from the expand- ed portion a diverticulum is constricted off which in a vit. THE PERMANENT KIDNEY. 219 transverse section lies dorsal to the original duct, and the blind end of which points forwards, that is, towards the head of the chick. This is the duct of the perma- nent kidney or ureter. At first the ureter and the Wolffian duct open by a common trunk into the cloaca, but this state of things lasts for a short time only, and by the sixth day the two ducts have independent open- ings. The ureter thus beginning as an outgrowth from the Wolffian duct grows forwards, and extends along the outer side of a mass of mesoblastic tissue which lies mainly behind, but somewhat overlaps the dorsal aspect of, the Wolffian body. This mass of mesoblastic cells may be called the metanephric blastema. It is derived from the interme- diate cell-mass of the region reaching from about the thirty-first to the thirty-fourth somite. It is at first continuous with, and indistinguishable in structure from, the portion of the intermediate cell-mass of the region immediately in front of it, which breaks up into Wolffian tubules. The metanephric blastema remains however quite passive during the formation of the Wolffian tubules in the adjoining blastema; and on the formation of the ureter breaks off from the Wolffian body in front, and, growing forwards and dorsalwards, becomes connected with the inner side of the ureter in the position just described. In the subsequent development of the kidney col- lecting tubes grow out from the ureter, and become continuous with masses of cells of the metanephric blastema, which then differentiate themselves into the kidney tubules. 220 THE FOURTH DAY. [CHAP. The formation of the kidneys takes place before the end of the seventh day, but they do not become of func- tional importance till considerably later. From their mode of development it clearly follows that the permanent kidneys are merely parts of the same system as the Wolffian bodies, and that their se- paration from these is an occurrence of a purely secund- ary wmportance. The generative ridge. Before describing the sub- sequent fate of the Wolffian and Miillerian ducts, it will be necessary to give an account of the formation of the true sexual glands, the ovaries and testes. At the first appearance of the projection from the in- termediate cell-mass, which we may now call the genital ridge, a columnar character is not only visible in the layer of cells covering the nascent ridge itself along its whole length, but may also be traced for some little dis- tance outwards on either side of the ridge in the cells lining the most median portions of both somatopleure and splanchnopleure. Passing outwards along these layers, the columnar cells gradually give place to a flat tesse- lated epithelium. As the ridge continues to increase and project, the columnar character becomes more and more restricted to cells covering the ridge itself, over which at the same time it becomes more distinct. On the outer side of the ridge, that is on the side which looks towards the somatopleure, the epithelium under- goes, as we have seen, an involution to form the com- mencement of the duct of Miiller, and for some little time retains in the immediate neighbourhood of that duct its columnar character (Fig. 71, a’), though even- tually losing it. VIt.] THE SEXUAL EMINENCE. 221 The median portion of the ridge is occupied by the projection of the Wolffian body, and here the epithelium rapidly becomes flattened. On the inside of the ridge however, that is on the side looking towards the splanchnopleure, the epithelium not only retains its columnar character, but grows several cells deep (Fig. 71, a), while at the same time the meso- blast (Z) underlying it becomes thickened. In this way, owing partly to the increasing thickness of the epithelium, and partly to the accumulation of mesoblast beneath it, a slight eminence is formed, which when viewed from below, after opening the abdominal cavity, appears in direct light as a fusiform white patch or streak, in its early stages extending along the whole length of the Wolffian body and genital ridge, but sub- sequently restricted to its anterior portion. Its appear- ance under these circumstances has been well described by Von Baer. This ‘sexual eminence’ is present in the early stages of both sexes. In both the epithelium consists of several layers of short cylindrical cells, a few of which are con- spicuous on account of their size and their possessing a highly refractive oval nucleus of considerable bulk; in both, the underlying thickened mesoblast consists—as indeed at this epoch it does generally in all parts of the body—of spindle-shaped cells. The larger conspicuous cells of the epithelium which appear to have quite a common origin with their fellow cells and to arise from them by direct differen- tiation, and which are seen at the first in male as well as female embryos, are the primordial ova or pri- mative germinal cells (Fig. 71, 0). Thus in quite early 222 THE FOURTH DAY. [cHap. stages it is impossible to detect the one sex from the other. The ovary. In the female the primordial ova en- large and become more numerous, the whole epithelium growing thicker and more prominent, and the spindle- shaped cells of the underlying mesoblast also increase rapidly and thus form the stroma of the ovary. The primordial ova after undergoing some further changes, into which it is not within the scope of this work to enter, become surrounded by a number of the ordinary epithelium cells. These form a distinct layer, the folli- cular epithelium, round the ovum. After a time there appear numerous vascular ingrowths from the stroma, which penetrate through all parts of the germinal epi- thelium and break it up into a sponge-like structure formed of trabecule of germinal epithelium interpene- trated by vascular strands of stroma. The trabecule of the germinal epithelium form the egg-tubes of Pfliiger. In this way each ovum becomes invested by a cap- sule of vascular connective tissue lined internally by a layer of epithelium; the whole constituting a Graasian follicle. The large nucleus of the primordial ovum becomes the germinal vesicle, while the ovum itself remains as the true ovum; this subsequently becomes enlarged by the addition of a quantity of yolk derived from a differ- entiation of its protoplasm. The testis, The first traces of the testes are found in the dorsal and inner side of the intermediate cell- mass, and appear about the sixth day. From the first they differ from the rudimentary ovaries, by coming into VII.] THE TESTIS, 223 somewhat close connection with the Wolffian bodies; but occupy about the same limits from before back- wards. The mesoblast in the position we have men- tioned begins to become somewhat modified, and by the eighth day the testis is divided by septa of connec- tive tissue into a number of groups of cells; which are the commencing tubuli seminiferi. By the sixteenth day the cells of the tubuli have become larger and acquired a distinctly epithelial character. The history of the primordial cells in the male has not been so thoroughly worked out as in the female. The spermatozoa appear to arise by the division of the primitive ova (present, as we have stated, in the early stages of both sexes), which probably migrate into the adjacent stroma, accompanied by some of the indifferent epithelial cells. Here the primitive germi- nal cells increase in number and give rise to the cells lining the secretory tubules of the testes. Outgrowths from the Malpighian bodies of the Wolffian body appear to be developed, which extend into the testis and come into connection with the true seminiferous stroma. It is evident from the above account that the male and female generative products are homodynamous, but the consideration of the development of the pro- ducts in the two sexes shows that a single spermatozoon is not equivalent to an ovum, but rather that the whole of the spermatozoa derived from a primordial ovum are together equivalent to one ovum. We have now described the origin of all the parts which form the urinary and sexual systems, both of the embryo and adult. It merely remains to speak briefly 224 THE FOURTH DAY. [CHAP. of the changes, which on the attainment of the adult condition take place in the parts described. The Wolffian body, according to Waldeyer, may be said to consist of a sexual and urinary part, which can, he states, be easily distinguished in the just-hatched chick. The sexual part becomes in the cock the after- testes or coni vasculosi, and consists of tubules which lose themselves in the seminiferous tubules. In the hen it forms part of the epoophoron’ of Waldeyer, and is composed of well-developed tubes without pigment. The urinary part forms in both sexes a small rudiment, consisting of blindly ending tubes with yellow pigment ; it is most conspicuous in the hen. This rudiment has been called by Waldeyer parepididymis in the male and paroophoron in the female. The Wolffian duct remains as the vas deferens in the male. In the female it becomes atrophied and nearly disappears. The duct of Miiller on the right side (that on the left side with the corresponding ovary generally dis- appearing) remains in the female as the oviduct. In the male it is almost entirely obliterated on both sides. Vascular system. We may return to the changes which are taking place in the circulation. On the fourth day, the point at which the dorsal aorta divides into two branches is carried much further back towards the tail. A short way beyond the point of bifurcation, each vessel gives off a branch to the newly-formed allantois. 1 This is also called parovarium (His), and Rosenmiiller’s organ. VIL] THE ARTERIAL ARCHES. 225 It is not, however, till the second half of the fourth day, when the allantois grows rapidly, that these allantoic, or, as they are sometimes called, umbilical, arteries acquire any importance, if indeed they are present before. The vitelline arteries are before the end of the day given off from the undivided aortic trunk as a single but quickly bifurcating vessel, the left of the two branches into which it divides being much larger than the right. During this day, the arterial arch running in the first visceral fold becomes obliterated, the obliteration being accompanied by the appearance of a new (fourth) arch running in the fourth visceral fold on either side. The second pair of arterial arches also becomes nearly (if not entirely) obliterated; but a new pair of arches is developed in the last (fifth) visceral fold, behind the last visceral cleft; so that there are still three pairs of arterial arches, which however now run in the third, fourth and fifth visceral folds, the last of these being as yet small. If we reckon in the slight remains of the second pair of arches we may consider that there are in all four pairs of arches. When the first and second arches are obliterated, it is only the central portion of each arch on either side which abso- lutely disappears. The ventral portion connected with the bulbus arteriosus, and the dorsal portion which joins the dorsal aorta, both remain, and are both carried straight forward towards the head. The ventral por- tions of both first and second arches unite on each side to form a branch, the external carotid (Fig. 73, £.C.A.), F. & B. 15 226 THE FOURTH DAY. [CHAP. which runs straight up from the bulbus arteriosus to the head. Stare oF ARTERIAL CIRCULATION ON THE FirtH or SIXTH Day. E.C.A. external carotid. J.C.A. internal carotid. .D.A. dorsal aorta. Of.A. vitelline artery. U.A. allantoic arteries. In the same way the dorsal portions form a branch, the internal carotid, which takes its origin from the dorsal or far end of the third arch. In the venous system important changes also occur. As the liver in the course of its formation wraps round the common trunk of the vitelline veins, or meatus venosus, it may be said to divide that vessel into two parts: into a part nearer the heart which is called the sinus venosus (Fig. 74, S.V.), and into a part surrounded by the liver which is called the ductus venosus. Beyond, i.e. behind the liver, the ductus veno- sus is directly continuous with the vitelline veins, or, as we may now say, vein, for the right trunk has become so small as to appear a mere branch of the left (Fig. 74, Of). VII] THE VEINS OF THE LIVER. 227 Fie. 74, DiacGRaAM OF THE VENOUS CIRCULATION AT THE COMMENCE- MENT OF THE Firta Day. Hi, heart. d.c. ductus Cuvieri. Into the ductus Cuvieri of each side fall J. the jugular vein or superior cardinal vein, W. the vein from the wing, and ¢, the inferior cardinal vein. S.V. sinus venosus. Of. vitelline vein. J. allantoic vein, which at this stage gives off branches to the body-walls. V.C.I, vena cava inferior. 0. liver. The hepatic circulation, which was commenced on the third day, becomes completely established. Those branches which come off from the ductus venosus soon after its entrance between the liver lobes carry blood into the substance of the liver and are called vene advehentes, while those which join the ductus venosus shortly before it leaves the liver (7.e. nearer the heart) carry blood away from the hepatic substance into the ductus and are called venw revehentes. As a result of this arrangement there is a choice of paths for the blood in passing from the vitelline vein to the sinus venosus; it may pass through the capillary net-work of the liver, going in by the vene advehentes and 15—2 228 THE FOURTH DAY. [CHAP.. coming back again by the vene revehentes, or it may go straight through the ductus venosus without passing at all into the substance of the liver. As the alimentary canal by its continued closing in becomes on the fourth day more and more distinct from the yolk-sac, it gradually acquires veins of its own, the mesenteric veins, which first appear as small branches of the vitelline vein, though eventually, owing to the change in the relative size and importance of the yolk- sac and intestine, the latter seems to be a branch of one of the former. Corresponding to the increase in the size of the head, the superior cardinal veins (Fig. 74, J.) become larger and more important and are joined by the wing veins (W.). As before, they form the ductus Cuvieri (d.c.) by joining with the inferior cardinal veins (c.). The latter are now largely developed, they seem to take origin from the Wolffian bodies, and their size and importance is in direct proportion to the prominence of these bodies. They might be called the veins of the Wolffian bodies. As the kidneys begin to be formed a new single median vein makes its appearance, running from them forwards, beneath the vertebral column, to fall into the sinus venosus (Fig. 74, V.C.Z). This is the vena cava inferior. As the lungs are being formed the pulmonary vcins also make their appearance and become connected with the left side of the auricular division of the heart. The blood carried to the allantois by the allantoic arteries is brought back by two veins, which very soon after their appearance unite, close to the allantois, into vil.] THE HEART. 229 a single trunk, the allantoic vein, which, running along the splanchnopleure, falls into the vitelline vein (Fig. 74, 7). Meanwhile the heart is undergoing considerable changes. Though the whole organ still exhibits a marked curvature to the right, the ventricular portion becomes directed more distinctly ventralwards, forming a blunted cone whose apex will eventually become the apex of the adult heart. The concave (or dorsal) walls of the ventricles be- come much thicker, as did the convex or ventral walls on the third day. Well-marked constrictions now separate the ven- tricles from the bulbus arteriosus on the one hand, and from the auricles on the other. The latter constriction is very distinct, and receives the name of canals auri- cularis (Fig. 75, C.A.); the former, sometimes called the fretum Halleri, is far less conspicuous. Fie. 75. Heart oF A CHICK ON THE FourtH Day or INCUBATION VIEWED FROM THE VENTRAL SURFACE: da. left auricular appendage. C.A. canalis auricularis. ¥. ven- tricle. 6. bulbus arteriosus. 230 THE FOURTH DAY. [cHaAP, The most important event is perhaps the formation of the ventricular septum. This, which commenced on the third day as a crescentric ridge or fold springing from the convex or ventral side of the rounded ven- tricular portion of the heart, now grows rapidly across the ventricular cavity towards the concave or dorsal side. It thus forms an incomplete longitudinal par- tition, extending from the canalis auricularis to the commencement of the bulbus arteriosus, and dividing the twisted ventricular tube into two somewhat curved canals, one more to the left and above, the other to the right and below. These communicate freely with each other, above the free edge of the partition, along its whole length. Externally the ventricular portion as yet shews no sign of the division into two parts. The bulbus arteriosus (Fig. 75, 6.) has increased in size, and is now very conspicuous. The venous end of the heart is placed still more dorsal, and to the left of the arterial end; its walls are beginning to become thicker. The auricles are nearly if not quite as far forward as the ventricles, and the auricular appendages (Fig. 75, L.a.), which were visible even on the third day, are exceedingly prominent, giving a strongly marked ex- ternal appearance of a division of the auricular portion of the heart into two chambers; but Von Baer was unable to detect at this date any internal auricular septum. The chief events then of the fourth day are :— (1) The increase of the cranial and body flexure. VIL] SUMMARY. 231 (2) The increase in the tail-fold. (3) The formation of the limbs as local thickenings of the Wolffian ridge. (4) The formation of the olfactory grooves. (5) The absorption of the partition between the mouth and the throat. (6) The vacuolation of the cells of the notochord. (7) The formation of the ureter. (8) The formation of the duct of Miiller. (9) The appearance of the primitive ova in the germinal epithelium. (10) The development of a fifth pair of arterial arches, and the obliteration of the first, and partial obliteration of the second pair. (11) The development of the ‘canalis auricularis,’ the growth of the septum of the ventricles and of the auricular appendages. CHAPTER VIII. THE CHANGES WHICH TAKE PLACE ON THE FIFTH DAY. ON opening an egg about the middle of the fifth day, the observer's attention is not arrested by any new features; but he notices that the progress of develop- ment, which was so rapid during the later half of the fourth day, is being continued with undiminished vigour. The allantois, which on the fourth day began to project from the pleuroperitoneal cavity, has grown very rapidly, and now stretches away from the somatic stalk far over the right side of the embryo (which it will be remembered is lying on its left side) in the cavity between the two amniotic folds (Fig. 9, &). It is very vascular, and already serves as the chief organ of respiration. The blastoderm has spread over the whole of the yolk-sac, and the yolk is thus completely enclosed in a bag whose walls however are excessively delicate and easily torn. The vascular area extends over about two- thirds of the yolk. The splanchnic stalk or vitelline duct has now reached its greatest narrowness; it has become a solid CHAP. VIII] THE LIMBS. 233 cord, and will undergo no further change till near the time of hatching. The space between it and the so- matic stalk is still considerable, though the latter is narrower than it was on the fourth day. The embryo remains excessively curved, so much so indeed that the head and the tail are nearly in contact. The limbs have increased, especially in length; in each a distinction is now apparent between the more cylindrical stalk and the flattened terminal expansion ; and the cartilaginous precursors of the several bones have already become visible. The fore and hind limbs are still exceedingly alike, and in both the stalk is already beginning to be bent about its middle to form the elbow and knee respec- tively. The angles of both knee and elbow are in the first instance alike directed outwards and somewhat back- wards. By the eighth day, however, the elbow has come to look directly backwards and the knee forwards. In consequence of this change, the digits of the fore limb point directly forwards, those of the hind limb directly backwards. This state of things is altered by a subsequent rotation of the hand and foot on the arm and leg, so that by the tenth day the toes are directed straight forwards, and the digits of the wing backwards and somewhat ventralwards, the elbow and knee almost touching each other. While these changes are taking place the differences between wing and foot become more and more distinct. The cartilages of the digits appear on the fifth day as streaks in the broad flat terminal expansions, from the 234 THE FIFTH DAY. [CHAP. even curved edge of which they do not project. On the sixth or seventh day the three digits of the wing (the median being the longest) and the four (or in some fowls five) digits of the foot may be distinguished, and on the eighth or ninth day these begin to project from the edge of the expanded foot and wing, the substance of which, thin and more or less transparent, remains for some time as a kind of web between them. By the tenth day the fore and hind extremities, save for the absence of feathers and nails, are already veritable wings and feet. Within the mesoblast of the limbs a continuous blastema becomes formed, which constitutes the first trace of the skeleton of the limb. The corresponding elements of the two limbs, viz. the humerus and femur, radius and tibia, ulna and fibula, carpal and tarsal bones, metacarpals and metatarsals, and phalanges, be- come differentiated within this, by the conversion of defi- nite regions into cartilage, which probably are at first united. These cartilaginous elements subsequently ossify. The pectoral girdle. The scapulo-coracoid elements of the shoulder girdle are formed as a pair of cartilaginous plates, one on each side of the body. The dorsal half of each plate ossifies as the scapula, the ventral as the coracoid. The clavicles are probably membrane bones. The pelvic girdle is derived from a pair of cartilaginous plates, one on each side. Each of them is developed in con- tinuity with the femur of its side. The dorsal half of each plate ossifies as the ilium; the ventral half becomes prolonged into two processes, the anterior of which ossifies as the pubis, the posterior at the ischium. Ribs and sternum. The ribs appear to arise as cartilaginous bars in the connective tissue of the body VIII] THE CRANIUM. 235 walls. They are placed opposite the intervals between the muscle-plates, and are developed independently of the vertebra, with the transverse processes of which they subsequently become closely united by fibrous tissue. The sternum appears to be formed from the fusion of the ventral extremities of a certain number of the ribs. The extremities of the ribs unite with each other from before backwards, and thus give rise to two car- tilaginous bands. These bands become segmented off from the ribs with which they are at first continuous, and subsequently fuse in the median ventral line to form the unpaired sternum. The skull. Two distinct sets of elements enter into the composition of the avian skull. These are (1) the cranium proper, (2) the skeleton of the visceral arches. The cranium. As we mentioned in the last chap- ter, the formation of the primitive cranium commenced upon the fourth day. This primitive cranium, in its earliest stage, inasmuch as it is composed of condensed but otherwise only slightly differentiated mesoblast, may be spoken of as the membranous cranium. On the sixth day true hyaline cartilage makes its appearance as a differentiation within the membranous cranium. The cartilaginous cranium is composed of the following parts. (1) A pair of cartilaginous plates placed on each side of the cephalic section of the notochord, and known as the parachordals (Fig. 76, w.). These plates, together with the notochord (nc.) enclosed between them, form a floor for the hind- and mid-brain. The continuous plate, formed by them and the notochord, is known as the basilar plate. 236 THE FIFTH DAY. [ CHAP. Fia. 76. VIEW FROM ABOVE OF THE PaRACHORDALS AND OF THE TRABE- CUL& ON THE Firra Day oF IncuBation. (From Parker.) In order to shew this the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading. c.v. 1. cerebral vesicles (sliced off). e¢. eye. mc. notochord. wv. parachordal. 9. foramen for the exit of the ninth nerve. cl. cochlea. A.s.c. horizontal semi-circular canal. g. quad- rate, 5. notch for the passage of the fifth nerve. dg. ex- panded anterior end of the parachordals. pé.s. pituitary space. ¢r.trabecula. The reference line ¢r has accidentally been made to end a little short of the cartilage. (2) 89; of third day, 167; auricles, 259—262; ventricles, 260—262 ; auricular septum, 257—262; ventricular septum, 257 ; canalis reuniens, 257—259; bulbus ar- teriosus, 257—262; foramen ovale, 262—264; Eustachian valve, 263—264; circulation in, 263—264; structure of, 287— 289, 293—297; résumé of, 299 —30 feat oe mammals, 329; struc- ture of, 331; formation of, 406; comparison of, with birds, 407 Hemiazygos vein, mammalia, 412 Hen: formation of albumen in, 16; ovarian follicle of, 12—15 ; mesovarium of, 11; ovary of, Ir; ovarian ovum of, 11, 153 oviduct of, 15; epoophoron, paroophoron and oviduct, 224 INDEX. Hen’s egg, albumen of, 3, 16; blastoderm, y—10, 26, 27: chalaze, 4; cicatricula, 4; im- pregnation of, 17; laying of, 17; polar bodies of, 17; seg- mentation of, 18—24; vitelline membrane of, 4, 13—15; yolk of, 4—7; chorion of, 47; shell of, 1, 16; irregular develop- ment of, 48, 49; segmentation, cavity of, 50 Hepatic cylinders of chick, 179; circulation of chick, 227; veins, 288—290 Hind brain: of chick, 100; of rabbit, 329 ; of mammals, and birds, 367—370; medulla of, 367; cerebellum of, 367—370 Hippo-campus major, mammalia, 80 He saceanial fissure of cerebrum of mammalia, 385 Histological difierentiation, in chick, 269—273; of epiblast, 269, 271; of hypoblast, 269; of mesoblast, 269 Histology of placenta, 359 Holoblastic segmentation, 307 Human embryo: villi of, 335; early stages of, 335; allantois of, 336—340; yolk-sac of, 336— 340; medullary plate of, 337; amnion of, 338—340; cranial flexure of, 338—340; limbs of, 339; body flexure of, 339— 340; face of, 340; relation of, with other mammals, 341 ; pla- centa of, 355 Human ovum, size of, 307 Human placenta, histology of, 363; derivation of, 364 Humerus, chick, 234 Hyaloid membrane, chick, 144, 146 Hyoid arch of chick, 243—245; of rabbit, 334; of mammalia, 403—404 : Hyoid bone of chick, 245 Hypoblast of chick: formation of, 25, 51,593 derivation of, 26; of area opaca, 65; histological INDEX. differentiation of, 269; of di- gestive canal, 272; of respira- tory ducts, 272; of allantois, 273; notochordal, 273 Hypoblast of rabbit embryo, 316, 320, 417 Hypoblastic mesoblast of chick, 59—62; of mammal, 321 Hypogastric veins: chick, 289; mammalia, 411—413 Hypohyal, mammalia, 403 Hypophysis cerebri (see Pituitary body) Hyrax, placenta of, 358 I Tleum, chick, 234 Iliac veins, mammalia, 411—413 Imbedding, methods of, 432—434 Impregnation of hen’s egg, 17; of ovum of mammal, 310—312 Incubators, makers of, and how to manage, 423 Incus, mammulia, 398, 404 Inferior cardinal veins, chick, 228 Infundibulum: chick, rrg—r213 ventricle of, 373; tuber cinereum of, 373; of mammalia, 372; of birds, 372 Inner mass of segmented ovum, 3143; of blastodermic vesicle, 314 Innominate artery of chick, 296— 8 Insectivora, placenta of, 353 Intercostal veins, mammalia, 41I—413 Interhyal ligament, 403 Intermediate cell mass of chick, 95, 189, 190 Internal carotid artery, chick, 225 Inter-nasal plate, chick, 240 Inter-orbital plate of chick, 240 Intervertebral ligaments, mam- malia, 400 Intervertebral regions, chick, 207, 20 Intestine, mammalia, 419 Inversion of the layers, 341 479 Ischium, chick, 234 Island of Reil, 385 Iter a tertio ad quartum ventricu- lum, 121, 370 J Jugal bones, chick, 246 Jugular vein, 284—290 K Kidney : of chick, 218—220; tu- bules of, 219; of mammalia, 4l4 L Labia majora, mammalia, 416 Lacrymal bones, chick, 246; ducts, chick, 155, 156; glands, chick, 155, 156; groove, chick, 248; duct, mammalia, 390 Lagena, chick, 159; birds, 397, 398 Tien dorsalis of chick, 29, 62 Lamina spiralis, mammalia, 397 Lamina terminalis, mammalia, ee intestine of chick, 174 Larynx of chick, 177 Lateral folds of blastoderm of chick, 37; of chick of second day, 9 Lateral plates of mesoblast, 68 Lateral ventricles of chick, 117 ; of mammalia, 377; cornua of, 378 Laying of eggs, 17 Lecithin, 6 Legs of chick, 200 Lens, chick, formation of, 134, 149 oe Ligamenta suspensoria, of birds, 210 Ligamentum, pectinatum, 144; vesice medium, 351 Ligamentum longitudinale an- terius and posterius,mammalia, 402 480 Limbs, of chick, 198—200, 2333 of rabbit, 334; of human em- bryo, 339 5 mammalia, 406 Liver of chick, 178—181 ; mam- malia, 41 Lumbar veins, mammalia, 412— 413 Lungs of chick, 176—178, 267 3 mammalia, 418 M Male pronucleus, 17 Malleus, 398, 404 Malpighian corpuscles, chick, 182 ; bodies of chick, 190 Mammalia, two periods of develop- ment, 308; viviparous, 308 Mammary glands, 366; a source of nutriment for the embryo, 308 Man (see Human embryo) Mandible, chick, 246 Mandibular arch, chick, 242— 244; maxillary process of, chick, 243; rabbit, 334; mam- malia, 403—404 Manubrium of malleus, 403 Marsupialia, footal membranes of, 352 Marsupium, 308 Maturation of ovum of mammal, 10 Marilla bones, chick, 246 Maxilla-palatine bones, chick, 246 Maxillary, processes of mandibu- lar arch of chick, 243 Meatus auditorius externus, of chick, 166; of mammal, 397 Meatus venosus, of chick, 169, 287 Meckelean cartilage, chick, 244; mammalia, 403 Medulla oblongata, of chick, 122; of mammalia, 367 Medullary canal, of chick, 40, 62, 6 ietiltairy folds, of chick, 40, 62, 66 77,97; of mammal, 327 INDEX. Medullary groove, of chick, 20, 62—65; of rabbit, 320, 321; of man, 338; closure of, in mammal, 327—33! Medullary plate, of chick, 62; of rabbit, 320; of man, 338 — Membrana capsulo pupillaris of mammalia, 387—389 Membrana limitans externa, 145; granulosa, 310 Membrana propria of follicles, chick, 182 Membrane: of shell of hen’s egg, 1; serous, of chick, 32—41; vitelline of hen’s egg, 13—15 Membrane bones, 242; of skull, chick, 246 Membrane of Reissner, mamma- lia, 397 Membrane of Descemet, 389 Membrane of Corti, and tectoria mammalia, 395 Membranous labyrinth, 158 Meniscus of birds, 210 Meroblastic segmentation, 18 Mesenteric veins of chick, 228, 288—290 Mesentery, of chick, 173; mam- malia, 419—20 Mesoblast: derivatives of,in chick, 25—26; of primitive streak of chick, 54, 57; derived from lower layer cells in chick, 55, 57, 59; of area opaca in chick, 65; splitting of, in chick, 68; of trunk of embryo chick, 185— 18g; histological differentiation of, in chick, 269; of primitive streak of rabbit, 320; of mam- mal, double origin of, 321— 3233 vertebral zone of, 328, lateral zone of, 3283; somites of, 328 Mesoblastic somites, formation of in chick, 70; of chick, 81, 1:85— 187, 204—208 Mesocardium of chick, 88; forma- tion of, 264 Mesogastrium, chick, 182 Mesonephros of chick, 212 chick, INDEX. Mesovarium of fowl, 11 Metacarpus, chick, 234 Metadiscoidal placenta, histology of, 362; derivation of, 364 Metamorphosis of arterial arches, bird and mammalia, 408 Metanephos (see Kidney) Metanephric blastema, of chick, ar Bigeotemnss; and makers of, 434 4353 471 Mid brain: of chick, 100, 200; of rabbit, 329; of mammalia, 370 37 3 ventricle of, 370; nates and testes of, 371; corpora geniculata, and crura cerebri of, 372 Monotremata, foetal membranes of, 352 Mouse, inversion of the layers in, I Mouth, chick, 249, 281; of rabbit, formation of, 334 Miillerian duct: chick, 214—218; mammalia, 414—415 Muscle plates of chick, 187—189, 204—208, 211; segmentation of, 212 Muscles: hyposkeletal, chick, 211 ; episkeletal, chick, 211; cuta- neous, chick, 211; extrinsic and intrinsic of limb, chick, 212 Muscular walls of heart of chick, 88 N Nails, of chick, 283 Nares : posterior, chick, 251; an- terior and posterior, of mam- malia, 399 Nasal capsule, chick, 242; car- tilages, chick, 246; bones, chick, 246; groove, chick, 246; pro- cesses of chick, inner, 248; outer, 248; labyrinth, chick, 249— 51 Nasal organ (see Olfactory organ) Nasal pits, of birds, 71; chick, 202 Nates of mammalia, 371 F. & B. 481 Nerves, of chick of second day, 101; of mammalia, 400 Nervous system of mammalia, 367—400 Neural band, chick, 123; crest, 126 Neural canal of chick, 31—39, 66; second and third day, 122; de- velopment of, 251—256 Neurenteric canal, of chick, 71— 74, 1753; mammalia, 399; of mole, 326, 328 Ninth nerve, chick, 126—129, 203 Node of Hensen, 319 Non-deciduate placenta, 252 Nose, chick, 249 Nostrils, chick, 251 Notochord: of chick, 29, 60—62, 208—210, 237—238; of second day, 101; sheath of chick, 208, of mammal, 323, 400; forma- tion of, 325 Nuclei, 16 Nucleolus, 13 Nucleus, 13 Nucleus of Pander, 7 Nucleus pulposus, of birds, 210, or Tuition of mammalian embryo : 308; by means of placenta, 350 0 Occipital: supra-, basi-, ex-, of chick, 246; foramen, chick, 237 Gsophagus of chick, 173 ; mam- malia, 418 Olfactory organ of chick, 161; nerve of chick, 162; grooves, chick, 202; lobes of mammalia, 385 Olivary bodies, 368 Omentum, mammalia, lesser, 420; greater, 420 Opisthotic of chick, 246 Optic vesicles: of chick of second day, 79, 97; chick, 133—134: formation of, 141—144; of rabbit, 329 31 482 Optic lobes, chick, 121 Optie nerves, chick, 133, 146 Optic cup, 134 Optic chiasma, chick, 147; mam- malia, 372 Optic thalami of mammalia, 373 Orbitosphenoid, 246 Orbitosphenoidal region, chick, 240 Organ of Corti, mammalia, 395 Organ of Jacobson, mammalia, Gece: placenta of, 358 Osmic acid, how to use, 427 Osseous labyrinth, chick, 158 Otic vesicle, chick, 157 Outer layer, of blastodermic vesi- cle, 314 Ova, primordial, of chick, 221 Ovarian follicle: of hen, 12—15 ; mammal, 309 Ovarian ovum: of hen, 11—15;3 of mammals, 309 Ovary: of adult hen, 11; of chick, 222; of mammals, 309; follicles of, 309; corpus luteum of, 311 Oviduct of adult hen, 15; of chick, 224 Oviparous animals, 308 Ovum: of birds and mammals compared, 307; of mammal— in follicle, 309 ; membranes of, 310; maturation and impreg- nation of, 310—312; polar bodies of, 311; segmentation of, 312—314; blastopore of (Beneden), 314 P Palate, mammalia, 420, 421 Palatine bones, chick, 246 Pancreas: of chick, 181; mam- malia, 419 Pander, nucleus of, 7. Parachordals, chick, 235—238 Paraffin, 432—434 Parepididymis of cock, 224 Parietal bones of chick, 246 INDEX. Parieto-occipital fissure of cere- brum of man and apes, 385 PargER on the fowl’s skull, 245 Paroophoron of hen, 224 Pecten, chick, 147 Pectoral girdle, chick, 234; mam- malia, 405 Pelvic girdle, chick, 234; mam- malia, 405 Penis, mammalia, 417 Pericardial cavity, chick, develop- ment of, 264—269; of rabbit, 331; mammalia, 406 Perilymph, mammalia, 396 Periotic capsules, chick, 237 Peritoneal covering of heart of chick, 88; cavity, mammalia, 406 Peritoneum, mammalia, 419—420 PruvueGer, egg tubes, 222 Phalanges, chick, 234 Pharynx, mammalia, 418 Picric acid, how to use, 425 Picro-carmine, to make and use, I Pig? placenta, histology of, 360 Pineal glands, chick, 117—119; aca and birds, 373— 7 pituitary body: chick, 119—121; rabbit, 334; of birds, 372; mammalia, 372, 420 Pituitary space, chick, 240 Placenta: 342; discoidal, deci- duate, type of, 353, 354; meta- discoidal, type of, 354—358 ; decidua of, 356; chorion leve of, 356—358; chorion frondo- sum of, 356—358; comparison of, 358; zonary type of, 358; diffuse form, 359; polycotyle- donary form, 359; histology of, 359-—303 ; evolution of, 364; of sloth, 360. Pleural cavity, chick, development of, 264—269; mammalia, 406 Pleuroperitoneal space of chick, 28—33, 84; formation of, 40, I, Bicieveasebria nerve (see Tenth nerve) INDEX. Polar bodies, 17; of ova of mam- mals, 311 Polycotyledonary placenta, 359; histology of, 360 Pons Varolii of birds, 369; of mammals, 370 Position of embryo chick of third and fourth days, 113116 Postanal gut, of chick, 175; of rabbit, relation of, to primitive streak, 329 Posterior nares, chick, 202 Potassium bichromate, 460 Premaxilla bones, chick, 246 Prenasal bones of chick, 246 Presphenoid region, chick, 240— 246 Primitive groove of chick, 56; of rabbit, 320 Primitive streak of chick, 52—62; of chick from 20 to 24 hours, yo; of rabbit, 319 Processus infundibuli, chick, 121 Proctodeum of chick, 175; of mammal, 422 Pronephros, 218 Pronucleus, female, 17; male, 17 Prootic, chick, 246 Protovertebrea (see Mesoblastic somites) Pterygo-palatine bar, chick, 243 Pterygoid bones, chick, 246 Pubis, chick, 234 Pulmonary veins of chick, 228, 289—290 Pulmonary arteries of chick, 294— 298; mammalia, 407 Pupil, chick, 142 Pyramids of cerebellum, 368 Q Quadrato-jugal bones, 246 Quadrate, chick, 243 R Rabbit embryo, growth of, 327— 3343 Placenta of, 353 Radius, chick, 234 ‘ 483 Rat, inversion of the layers in, I Bee cseus Jabyrinthi, mammalia, 390-398 Recessus vestibuli (see Aqueductus vestibuli) chick, 203 Respiration of chick, 303; of third day, 110 Rete vasculosum, mammalia, 414 Retina, chick, 142, 144—146 Ribs, chick, 234; mammalia, 405 Rodentia, placenta of, 353 Rods and cones of retina, chick, 14! Rostrum, chick, 246 Ruminants’ placenta, histology of, 360 mo) Vv Sacculus hemisphericus, mam- malia, 390—398 Salivary glands, mammalia, 420 Scala media (see Cochlear canal) Scala tympani, mammalia, 395— BOE anos e Scala vestibuli, mammalia, 395— 3 Seapale of chick, 234 Sclerotic coat of eye of chick, 141 Sclerotic capsules, mammalia, 405 Scrotum, mammalia, 416 Sebaceous glands, 366 Secondary optic vesicle (see Optic cup) Sections, method of cutting, 434 —436; mounting of, 436 Segmentation: of hen’s egg, 18 —24; meroblastic, 18; of mam- malian ovum, 312—314; of hen’s egg to observe, 458; of mammalian ovum to observe, 61 Geratetiaatay canal: of chick, 158; mammalia, 390—398 Semi-lunar valves, chick, 258 Sense capsules of chick, 211—212 Septum lucidum, mammalia, 383 Septum-nasi, chick, 246 Serous membrane of chick, 32— 41 484 Serous envelope of chick, 107; of mammals, 346 Seventh nerve of chick, 127—129, 203 Shell-membrane of chick, 1 Shell of hen’s egg, 1; formation of, 16 Shield, embryonic, of chick, 49 Sinus rhomboidalis: of embryo chick, 71, 81; of rabbit, 329 Sinus terminalis, of chick of second day, 91, 104; in rabbit, gine venosus of chick, 169, 226, 285—290 Skeleton of limb, chick, 234 Skull of chick, 235—251; cartilage and membrane bones of, 246; of mammalia, 401—405 Sloth, placenta, histology of, 360 Somatic stalk of chick, 29—42; of mammals, 351 Somatopleure of chick, 29—33; formation of, 40—41, 68 Spermatozoa of chick, 223 Spinal nerves: of chick, 123; de- velopment of, 129—132; of mammalia, 400 Spinal cord of chick: develop- ment of, 251—256 ; white mat- ter of, 252; grey matter of, 253; canal of, 252—256; epi- thelium of, 251, 252; anterior grey commissure of, 256; an- terior fissure of, 254—256; dorsal fissure of, 255—256; posterior grey commissure of, 286; sinus rhomboidalis of, 256; anterior columns of, 256; posterior columns of, 256; lateral columns of, 256; an- terior white commissure of, 2586; posterior white commis- sure of, 256 Splanchnic stalk of chick, 29— 42, 232 Splanchnopleure of chick, 29— 33 3 formation of, 4o—42, 68 Spleen of chick, 182 Splint bones of chick, 246 Squamosal bones of chick, 246 INDEX. Staining reagents, 428—4323 he- matoxylin, 429; borax carmine, 430; carmine, 431; picro-car- mine, 431; alum carmine, 431 Stapes, of chick, 245; mammalia, 398, 404. : Sternum of chick, 235; of mam- malia, 405 , Stomach of chick, 173; mam- malia, 418 Stomodeum, of chick, 119, 203; mammalia, 420 Stria vascularis, mammalia, 397 Subclavian arteries of chick, 296 —298, of mammalia, 409 Subclavian veins, mammalia, 409 —413 Sulcus of Monro, 373 Superior maxilla of chick, 165 ; maxillary processes of hick, 202; of rabbit, 334 Superior cardinal veins of chick, 228 Supra-renal bodies, mammalia, structure of, 413; relation of, with sympathetic nervous sys- tem, 414 Subzonal membrane of mammal, 346 ' Sylvian fissure, mammalia, 384, 385 Sympathetic nervous system of mammalia, 400 Sweat-glands, 366 T. Tail-fold of chick, 29—37, 196; of second day, 96; of mammal, 32 Tal awelling of chick, 74 Tarsus of chick, 234 Teeth, mammalia, 421 Tela choroidea, 375 Tenth nerve of chick, 125, 127— 129, 203 Testis of chick, 222, 371 Thalamencephalon: of chick, 117; ofmammalia, 371—376 ; ventricle of, 372; floor of, 3472, INDEX. ote tes of, 373; roof of, 374 —37 Third nerve of chick, 129 Third ventricle of mammalia, 372 Throat of rabbit, formation of, 331 Thyroid body, of chick, 181; mammailia, 418 Tibia of chick, 234 Tongue of chick, 282 Trabecule of chick,236, 239—241 Trachea of chick, 176, 177 ; mam- malia, 418 Tuber cinereum, 373 Turbinal bones of chick, 246 Tympanic cavity of chick, 166; membrane of chick, 166 ; cavity of mammalia, 397, 418; mem- brane of mammalia, 397 Uz Ulna, of chick, 234 Umbilical, arteries (see Allantoic); veins (see Allantoic Heel vesi- cle of mammals (see Yolk-sac) ; stalk of chick of third day, 113; cord, 351 Urachus, 351 Ureter of chick, 219; mammalia, 417 Urethra, mammalia, 417 Urinogenital organs of mam- malia, 414—417; sinus of mam- malia, 415—417 Uterine crypts, 350 Uterus, mammalia, 415 Uiriculus of mammalia, 393—398 Uvea of iris, chick, 144 V. Valve of Vieussens, of birds, 369; of mammals, 370 Vagina mammalia, 415 Vagus nerve (see Tenth nerve) Vasa efferentia and recta mam- malia, 414 Vascular system of chick, 224— 230; of second day, 89—94, 102 485 —106 ; of third day, 167—170; mammalia, 406—413 Vascular area: of blastoderm of chick, 27; of third day, rro— 113; of rabbit’s ovum, forma- tion of, 326 Vas deferens: of cock, 224; mam- malia, 415 Velum medulle anterius (see Valve of Vieussens) ; posterius, 370 i Vermiform appendix, mammalia, 41 You. cava, inferior, of chick, 228, 285—290; mammalia, 4og— 413 Ven cave, superior, of chick, 286—290; of mammalia, 409 —413 Vensz advehentes of chick, 227, 287—289; revehentes of chick, 227, 287—289 Vena terminalis (see Sinus termi- nalis) Venous system: of chick, 226— 229, 283—290, 301—303; mam- malia, 409—413 Ventricles of brain of chick of second day, 102; of mammals, 117, 121—1223; of chick, 229 Ventricular septum, chick, 230, 2 Verbis of chick, primary, 205 —208; permanent, 205—208; bodies of, 207—209 Vertebral arches, osseous, of chick, 207, 210; mammalia, 409 : Vertebral artery of chick, 295— 298 Vertebral column, of chick, 205— 208 ; membranous, 205—208; secondary segmentation of, 205 —208 ; explanation of do., 205 —206; of mammalia, early de- velopment, ossification of, 400, 401 Vertebrate animal, general struc- ture of, 39 Vesicle of third ventricle (see Thalamencephalon) 486 Vessels of placenta, 360—363 Vestibule, chick, 158 Villi: of human ovum, 335; of zona in dog, 347; of subzonal membrane of rabbit, 347; of chorion of mammal, 349; of placenta, 360— 363 Visceral arches, 245; of rabbit, 334 Visceral arches of chick, 162—167; of rabbit, 334; of mammalia, 402 Visceral clefts: of chick, 162— 167, 281; closure of do., 164; of rabbit, 334; of mammalia, 402, 418 Visceral folds of chick, 163 Visceral skeleton of chick, 242 —246 Viséeral vein of chick, 284—290 ; of mammalia, 409—413 Vitellin, 5 Vitelline arteries: of chick, 167, 293—298, 225; of second day, 89, 103 Vitelline duct of chick, 196, 232; of mammals, 350 Vitelline membrane, 4; of hen’s egg, 13—15; of mammal, 310 Vitelline veins of chick, 84, 226. 288—290; of second day, 92, 104; in rabbit, 343; of mam- malia, 410—413 Vitreous humour of chick, 140, 150 INDEX. Viviparous animals, 308 Vomer of chick, 246 Ww White matter: of spinal cord of chick, 252; of brain of mam- malia, 386—387 Wings of chick, 200 Wolffian body: of chick, r90— 193 3 of mammalia, 414; of chick of second day, 106 Wolffian duct of chick, 190, 213; of second day, 94—95, 106; of mammalia, 414 Wolffian ridge of chick, 198 Wolffian tubules of chick, 106, IgI—193, 213 Y Yolk of hen’s egg, 4—7; arrange- ment of, 6; structure of, 5 Yolk-sac: of chick, 28—37, 277— 280; of mammals, 327; of marsupials, 352; of rabbit, 353; of human ovum, 355—358; of. dog, 358 Z Zona radiata, 310; of chick, 15 Zonary placenta: histology of, 360; derivation of, 364 CAMBRIDGE: PRINTED BY C, J. CLAY, M.A. AND SONS, AT THE UNIVERSITY PRESS.