1897 Human Embryology 5

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History
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I. The Law of Concrescence.

Yolk Cavity. — Concerning the formation of the yolk-cavit>' we possess verj- imperfect knowledge. Undoubtedly a patient search might collate many facts from the literature of the early stages, but until such a collation shall l>e made and supplemented by further obser^■ation^, no iwsitive history of the yolk cavity can be given. We can say that, when the notochordal canal begins to form, there is already a large cavity under the germ and entirely suiTounded by entoilermal material. In elasmobranchs and Sauropsida the floor of the cavity is the j'olk itself, while the roof is formed by cellular material ; the cavity expands over a considerable area, but is flattened; it is completely separated from the segmentation cavity ; it is designated often by the name of sub-geniihial cavity, but unfortunately the same name is also applied to the morphologically tlifFerent segmentation cavity. In Amphibia the yolk cavity has been recognized by O. Schultze; it is not lai^.

In mammals the yolk-cavity, as soon as the entodermic layer is fully developed — see below — comprises the so-called cavity of the twolayered blasto<lermic vesicle ; owing to the reduction of the yolk, it is bounded wholly by a layer of cells, not partly by a mass of yolk, as in meroblastic ova, and is very large in proportion to the ovum.

Concrescence. — The passage from the stage of segmentation to the first embryonic stage is effected in vertebrates by means of certain migrations of embryonic material from lateral ix)sitions to median positions, and subsequent union in the middle line. This process of union is known as concrescence. It consists in the growing together of the two halves of the ectental line to fonn the structural axis of the future embryo. The process is somewhat complex, and needs therefore to be described in detail, the more so as it has still to be followed in mammals.

The accompan\nng diagram may assist to render clear the process of concrescence. Fig. G4. It is intended to illustrate the spreading

of the ectoderm (germinal disc, blast<xlemi, a net,) over the yolk and the simultaneous formation of the primitive axis. The whole ovum is representeil as seen in projection; the proportions are such as have been suggested by the ova of flounders and frogs. Three successive stages of the oxpaiuling blast<xlerm are n»prc5sented ; the first position of the embrvonic rim (ectental line) corresponds to the dotted line a" a"; the concrescence reaches only to the the edge of the blastoderm. At the next stage the ectoderm has grown very much and has moved it« edge to a a\ while the margins, S^ have coalesced so that the primitive axis extends to 'Z. The extension continues, bringing the ectental line to a a a S, and carrying the primitive axis back to 3 ; behind the primitive streak a small area, Yk, of the yolk is still uncovered, and is homologous with, first, the anus of Rusconi in amphibian ova, and, secoiia {in my belief), with the so-called primitive streak of the amniota. The portion of the ectental line bounding this area diffens from that which is immediately concerned in the formation of the primitive sti*eak, S; although it now lies l)ehind the primitive streak, it was previously in front of it, when the blastoderm covered only the minor portion of the ovum, see S" aa\ Ultimately the yolk is entirely covered by the blastoderm, thus fixing the length of the primitive streak. It is essential to notice that the blastodermic rim (ectental line) divides into two portions, one, 5, which forms the primitive streak, and another, a a!' ^ which overgrows the ovum and at last closes over the yolk behind the completed primitive axis.

Fig. 64.— Diagram illuatrat injr the gn^vith of the bla«toderm P^^^^f , marked 1 . 1 llO

uncovered yolk. Compare also the text.

Historical Note, — The earliest observations on concrescence to form the embryonic axis are, so far as known to me, those of Rathke on leeches.* Nine years later Kowalewski (Mem. Acad. Sci., St. Petersburg, 7™® Ser., XVI., 1871) recorded its occurrence among insects. Its recognition as a vertebrate mode of development we owe to the brilliant investigations of W. His; in his first paper,

76. 1, he describes very accurately and clearly the process of concrescence in the salmon; in his second paper, 77. 1, he describes concrescence in the sharks, and in his third and fourth papers, 77.2,

91.2, he discusses again the general bearing of his results. Semper, in his great work on the relationship of annelids and vertebrates,

76.3, 271, was 4he first to make a direct comparison of the processes of concrescence in annelids, insects, and vertebrates. Unfortunately Balfour entirely failed to grasp the new conception, and by expressing himself very decidedly against it, "Comp. Embryol., II., 30G-308, led many embryologists to discredit the discovery. Whitman, 78.2, 91-94, has ably defended the com irison made by Semper (see above); Rauber, 76.2, Kollman,

•5.1, Ryder, 85.5, 9, and others have added to our knowledge of the phenomenon. Duval's researches on the chick, 84.1, demonstrate concrescence there also, though the author appears unacquainted with the results of his predecessors. Minot in the article "Foetus," in Buck's '* Handbook," III., 172, 173, accepts concrescence as the tjn^ical mode of vertebrate development.

Concrescence in Bony Fishes. — At the close of segmentation the germinal disc forms a cap of cells on the yolk. The disc (primitive blastoderm) spreads over the yolk gradually ; when it begins to spread its edge is already thickene<l; this thickened edge corresponds to the ectental line; the thickening is known as the Randwulst; it is also called the blastodermic rim^ which term Ryder and others have used. When the blastoderm has spread, so as to cover perhaps a sixth or less of the surface, one point of the rim ceases t to move; consequently, as the expansion continues the edge of the disc bends in behind this point on each side, until two parts of the blastodermic rim meet as they come from opposite sides, and then grow together. This is illustrated by the accompanying diagram, Fig. 65; F is the outline of the yolk ; bl is the outline of the blastoderm ; a, the fixed point ; the expansion of the blastoderm has brought the parts 1 1 together and they have united; the parts 2 2 are about to meet and unite ; then 3 3 will meet ; 1 4 and so on, until the two halves of the ectental line are brought together along their entire length ; their junction marks the axis of the future embryo, and produces a longitudinal band of thicker tissue, which has long beenknown to embryologists, and may be named the primitive axis. The fixed point Of the blastodermic rim marks the head-end of the embryo; the parts of the ectental line which grow together next behind the fixed point develop into the head, those a little farther back into the neck, and those farthest back into the rmnp and tail. The parts of the circular rim most remote from the fixed point, a, of course concresce last. The destiny of each portion of the ectental line is fixed before concrescence occurs. In fact in certain cases the differentiation of the tissues advances to a considerable degree in the "Randwulst" before concrescence. This is strikingly the case in Elacate, in the ova of which the myotomes (or segmental divisions of the mesoderm) appear in the embryonic rim l)efore its concrescence (Ryder, 85.9) ; compare also Ryder's observations on Belone, 81.2. The development of the teleostean germ-layers is not yet fully worked out. For the best history of the entoderm and mesoderm, as well also for references to conflicting authorities, sec M. Kowalewski, 86,1,2, who, however, pays no heed to the law of concrescence. That concrescence occurs in teleosts essentially as here describeil, seems to me evident from the figures given by W. His, 76.1, C. Kupffer, 84.1, Coste, 47.1, and others. Nevertheless the concrescence is denied by Henneguy, 88.1, H. Y. Wilson, 91.1, 200, and others, but the arguments I have found against concrescence have not appeared to me valid.

  • Rathke aD(l Leuckart, "Beltrii^fe zur EntwickeluQf^fC(>^'hichteder Hirudineen ;'^ Leipzig, 1882. tOr perhaps mert?ly moves more slowly.

Fig. 05.— Diaicram of concrescence in a TeleoAtean egpr: Y, outline of yolk: 6i, outline of blaMtodemi, 1 1, lateral i>artR already concreoced; 2 3, lateral parts about to concresce; 8, 4, parts to concresce later.

In the primitive axis is a mass of cells below the ectoderm; this mass subsecjuently divides into mescxlerm and entoderm. The entodermal cells form at first and for a considerable i)eriod a solid cord {cf, Balfour, **Comp. Embryol.," II., 75) in which, however, a lumen appears later ; this lumen I will tentatively homologize with the cavity of the notochordal canal of amniota.

Concrescence in Elasmobranchs. — Our knowledge rests mainly on the researches of His, 77.1, and his follower, Kollmann, 86.1. Fig. 00, A, is a generalize<l diagram of an elasmobranch ovum, representing the ectodermal disc, BU as seen from above resting upon the yolk, which is not represented in the figure. The first change noticeable in the disc after the close of segmentation is a groove running completely around its margin between it and the yolk ; as the disc grows and expands the groove is no longer present along the front edge, a a, of the blastoderm, but only on the sides and behind. Abou ' the same time there usually apjiears a distinct notch, w, which marks the fixed point of the margin and the posterior end of the disc. If now a section be made across the line, X y, the relations will be found to be essentially as represented in the diagram. Fig. GO, B; the disc rests on the yolk, T7, which contains numerous nuclei ; between the yolk and the ectoderm, Ec, ia the ee^meDtation cavity, Ac; the groove is bounded above by a layer of cells, Ell, which are larger than those of the ectoderm, and have l>eeii produced by the yolk, Vi; sometimei^ there are cells Ij'ing in the segmentation cavity at this stage, the formation of the mesoderm having already begun. The essential point to note in this stage, is, as KoHmaim has shown, the division of the margin of the A ectodermal disc into two parts, one, o a, resting directly on the yolk, the other, S, directly continuous with a layer of entodermal cells, B, En, forming a little groove under the margin of the disc. The two portions of the ect*^ntal margin have entirely distinct functions, as already stated ; the anterior, a a, is destined to grow over and cover the yolk by the estra-emlirj"onic portion of theectixlerm; the posterior S, is destined to form tl e primitive axis of the embr o Fig. 67 is similar to F t, BC, but represents a more a 1 vanced stage. The ectodermal disc, is much enlarged and its anterior grooveless n ar gin, a a a, is relatively much more increase<l than the poste rior groovetl margin, S the centre of the notch. Fig. i C has remaiued nearly if not quite stationary, while the margin, s s, of either side has been growing toward its fellow in the manner indicated by the arrows, and as they meet the two side-margins gi*ow together in the median line, making a longitudinal stnicture. The manner and results of the concrescence of the margins fntm the two sides to form an axial structure become clearer in section, Fig. 07, B, The margin at the side, /», still shows the same relations as in Fig. lili, B; in the median line, however, the margins have met and intimately united, " so that what were originally two grooves have completely united to form a single canal, Enf, bounded above by entodermal cells, below by the entodermal yolk, 17. This canal is the primitive entodermal cavity. Whether it represents, when first developed, merely the nott)chordal canal of the amniota or the fused notochordal canal and yolk cavity, we are unable to determine at present. A moment's consideratnon renders it evident that the canal must be open posteriorly; this opening is the blastopore, bl. There are some furtheP details to be mentioned : where the ectental matins have united in the mediae line there appears a lateral ouf^^wth, mes, which ia the beginning of the mesoderm ; in some cases this mesodermic tissue appears before the margins concresce; when viewed from the surface the mesoderm can oe seen through the ectoderm, as was observed long ago; it is this faint appearance which early writers call in anamniota the primitive streak, it being the foreshadowing of coming oi^imization. Fig. (IT, A, also shows in front of the primitive axis the Brst trace, N, of the central nervous system, which we shall describe later. The blastodenn is seen also to be divided already into two ]>arta, the lighter area pellucida, -4. p., and the darker area opaca, ■•A. o.; the latter alsi> shows the first bloodislands. For further descriptions of these areas, see Chap. XIII. From their obsen'ations, His, Kollmann, and others havo inferred that at the anterior ectentalmai^n, a a a, there are produced (fnim the yidk^ I cells, which grow in e'liictliB- toward the embryo, ma^e"' ^"^ coustituto jjart. of aegmro- the mesoderm and are especially concerned in forming the first blood, which is produced always in the extraembryonic area. This mesoderm of peripheral origin His has named parablast — a term which, unfortunately, has been employed differently by some subsequent writers. The ectoderm, ento<lerm, and axial mesoderm are grouped by His under the coUectivo name of archiblast. This view of the double origin of the mesoderm, although it has been adopted in a modified fonn by the brothers Hertwig, I am unable to accept. The question is discussed in Chapter VI. Concrescence In MarsipobranchB, Ganoids, and Amphibians. — As not only the constitution of the ovum, but also its early development, is very similar in the three classes named, we may consider them collectively in the present connection. The condition of the OTum at the close of s^mentation has already been described, p. 99, and figured, Fig. 49. The ectental line is not sharply defined, nor does there appear aay groove around the edge of the blastodenn as in meroblastic ova. The sinall-celled ectodenn Bpread» over the yolk ; while it is doing this a short notochordal canal appears at the hind edge of the hlastoderm with a small opening to the exterior, known us the blastopore, Fig. C8, bl. The first indication of the canal in the frog is easily recognized, being the appearance of a curved area of pigmentation of semilunar outline amid the yolk-cells at the posterior pole; the convexity of the area is directed toward the segmentation cavity; the centre of the concavity corresponds to the dorsal lip of the blastopore (Robinson and Assheton, 91.1, 4li:t)The canal runs forward toward the segmentation cavity. Fig. il8, S.C.; above and in front of the blastopore the cells have multiplied and accumulated to form the beginning of the primitive axis, Pr. In the lamprey there is at this stage no such axial accumulation of cells; according to Shipley the ectoderm consists of a single layer of cells, and the notochordal canal is bounded on its dorsal side by a single layer of cells also, between which and the overlying blastoderm there are no cells; the gathering of cells corresponding to the primitive axis does not arise until later, The canal, according to O. Schultze, ultimately fuses with the yolk cavitj* to form the definite archenteron; it is sometimes designated as the blastoporic invagination. The canal in the same measure as the blastoderm spreads over the yolk-grooves at it 3 hinder end away from the segmentation cavity, Fig. Cl», s.c, just as in elasmobranchs. A stage is s<x)n reached in which nearly the entire length of the archenteron is formed and nearly the whole yolk is covered. There is still a blastopore which leads into the cavity, and which has moved gradually backward from its original position. Behind the blastopore lies the uncovered yolk, Vk, which in the frog's ovum is very conspicuous, because its whitisli color contrasts with the dark color of the heavily pigmented ectoderm around it; this area of exposed yolk is the so-called anus of Ruacoiii. When the canal has completed its full length the

Mne,Jirr. -, tcrlor marsln (SiclieU; A.o.. are& opaca: ii.r.,Denrel ridgee; ir. oeuralor mrdullur lUveatreak; bin>tBBlDpore: &. . iK-todpni Ai, entodennic cella; Fiiyolk: tatloD cavity.

Fig. (W.— OTum of Petromjmn In longitudi ,1 nertton. After Balfour on- niest ' -' -imltlivaxiH: bf, hliMoixin- I ft jr.

following dieposition of the parts is found. Fig. 70 ; The archenteron is bounded below by the lai^ mass of yolk-cells, Vi, and above by the epithelium, Ent, of the entoderm; its posterior end curves up to open at the blastopore, Bl, passing through a mass of cells, which constitute the end of the primitive streak; this portion of the arcbenteron is sometimes called the blastoporic canal. There is further a short prolongation, Al, o£ the cavity below the blastopore. This divei-ticulum has been homologized with the allantoia, (see Chapter XII.). It is also\ erj' probably homologous with the more nearly spherical diverticulum found in a similar position in teleosts, and now known as Kupffer's \esicle, from having been especially studied by C. Kuptfer, ee.l, 475, 68.1, who has interpreted it as the ^ teleoitean allantois. Compare D. Sehwarz, 89.1, 107, Taf XIII., Figs. 35, 37, etc. Around the blastopore is a mass of cells (primitive axis) continuous on the one side with the ectoderm, on the other with the epithelial entoderm lining the arcbenteron, and, thirdly, with a sheet of cells, Me'i, between the ectodenn, Ec, and entoderm, Ent

The developmental pliases just outlined seem to me lo afford sufficient evidence of concrescence. Owing to the gradual transition between the ectoderm (blastoderm) and the entoderm (yolk-cells) there is no sliarp ectental line, as in some other types. Moreover, there is no differentiation of the tissues at the blastodermic rim, but only after the cells are united in the axis ; hence we cannot distinguish parts at the peripherj' of the blastoderm and follow their union in the primitive streak as we can in certain sharks and bony fishes. Nevertheless, we find all the essential features of concrescence; the notochordal canal and the primitive axis begin at the edge of the blastoderm and grow at their posterior end away from the segmentation cavity, and at the same rate the blastoderm overspreads the yolk. ConcreBoence in Sauropsida.— The early stages in Reptilia have long been obscure. Clarke (Agassiz' "Contributions," II.), in his paper on the embrj-ology of the turtle, mist<x>k the c*immencenient of the notochordal canal "for the commencement of the amniotic fold. Weldon, 83.1, Kupffer. 82. 1,84.1, Strahl, 80.1,2,3,82.1, 83.1, Hoffmann (Bronn's -'Thierreich," VI., Abth. iii., 1892-1807), and others partly traced out the history of the canal. Will's observations, 90.1, on the develo]tment of the gecko gave the key to the history of the canal in the reptiles. In thj geck<) there is formed a notochordal canal, which is at first verj- short, but gradually l^igthens out, apparently chiefly by growth at Jtwhind or blastoporic end. Fig. 63, Tick. c. The end of the canal, when the germinal area is examined in surface-views, is characterized by a transverse figure or sicbel, which is well known in reptilian embrj-os of all orders, and which presumably represents the portions of the Randwulst which are to concresce and thereby lengthen the primitive axis and the notochordal canal inclosed by the cells of the axis. Underneath the notochordal canal is a layer of entodermal cells, Erit, whicli form the roof of the yolk cavity, the figure does not show the inferior or lateral boundaries of the yolk cavity. In a little later stage, the tissue between the canal and the yolk cavity disappears and the two lumina fuse.

In other reptiles the development is similar, though obscured by the peculiarity that the anterior part of the notochordal caual opens into the yolk cavity liefore the posterior part is forme<l. In such cases there is only a short section of the canal to be observed with complete boundaries at any one stage. In reptiles then concrescence can only be inferred from the presence of the " sichel" and the growth backward of the primitive axis. Fig. "1 illustrates the formation of the canal in Liicerta, as described and figured by W. F. R. Weldon, 83.1. Eii is the entodenn forming the roof of the yolk cavity. B shows the not(x;hordal canal, bl, just beginning to form. C is a stage considerably more advanced; the anterior part of the canal has fusetl with the yolk cavity, and the dorsal wall of the canal has produced the notocdiord, nch; only a short posterior end, bl, remains aa a cntiBd. D is a transverse action through the blastopore.

The process of concrescence in birds was partly indicated by Keller's investigations, 79.1, 82.1, and has been carefuUy elucidated by Duval, 84. 1 , The resemblance to concrescence as Imown in elasmobranchs is very striking. Around the edge of the blastoderm appears very early a small groove ; as the blastoderm expands the front portion loses the groove ; one point, the centre of the grooved margin, ceases to move, or at least moves much more slowly than the remain,derof the blastodermic rim; as the expansion continues the edges of the two halves of the groove coalesce gradually Itehind the fixed point, thus producing the entodermal canal in the same manner as in the sharks; cells accumulate at the same time and make behind the blastopore the so-called primitive streak. There is some uncertainty in Duval's account, as, unfortunately, at the time he wrote the existence of a yolk cavity contributing to the fonnation of the archenteron had not been recognized.

In birds (hen's ova) there is a further peculiarity, which is, I think, probably to be found in all amniota, namely : that portion of the edge of the ectoderm which does £c tt|^ y ^ i Kj»a ^M-" i' .i, ' bjCT W^B*'^?^' riot share in concrescence and which " [t^S^^?^Ji--/i'-^t^^A'^ corresponds to the edge of the anus iBnLn&*jl>Ll%£££S>ii^ of Rusconi closes over the yolk behind the primitive streak, so that the portion of the yolk which is left uncovered is remote from the emhrj-ouic ,Si,SSiSr4^'VnL'^SSS,i' aP6a(orprimitiv<>Btreftk) A, a rare iiwoo Uk yolk from > lonKiiudiDsi sec- anomaly, SCO Whitman, 83.1, a nne dS™l' "" "^"""'^'^^ ■" '""■ ^'^ is visible running in the ectoderm from tho hind end of the primitive streak to the edge of the uncovered yolk; this line is to be interpreted as evidence of the growing together of the ectoderm, Itehind the streak proper. The ectoderm, as it spreads over the yolk, receives no accretions from it, but accomplishes its expansion by proliferation of its own cells. Thus the uncovered yolk is bounded by the free edge of the ectoderm, Fig. 7'Z. Tho area of uncovered yolk, which may be called the yolk olastopore,* is not homologous with the anus of Rusconi, from which it differs in position, being remote from instead of close (as is the anus of Rusconi) to the blastopore, for it is situated nearly opposite the embrj-onic area. In birds, according to Duval, 84.2, the yolk blastopore (Dvfternabel) is never closed hy ectoderm, but remains covered by the vitelline membrane only, until the mesoderm spreads over it. The growing edge of the ectoderm is somewhat thickened; it finally is reflected around the edge of the yolk blastopore, forming, eis it were, a funnel, at the bottom of which IS the yolk (see Duval, I.e.).

Concrescence in Mammals. — As shown below in the detailed historj' of the mamnmlian blastodermic vesicle, there is a fixed point (Hensen's knot) at which the formation of the primitive axis and notochordal canal begins, and from which they lengthen out backward as they would do if formed by concrescence. The main cavity of two-layered vesicle is the yolk cavity, and w^ith it the notochordal canal subsequently fuses, cf. infra. The position and history of the ectental line being absolutely unknown in mammalia, it is of course impossible to form any definite notions as to the process of concrescence in them.

  • Duval applies to ic the umuv oI umbilic. umbilical.

Concrescence : Summary. — The evidence that concrescence is the typical means of forming the primitive streak in vertebrates is : 1, detailed and conclusive observations uiK)n elasmobranchs, teleosts, and birds ; 2, exact and extensive observations on marsipobranchs, ganoids, and amphibians, which concord with the theory of concrescence ; 3, a great probability of its occurrence in reptiles, owing to the similarity of their development with that of birds ; 4, a probability of its occurrence in mammals, because of the resemblance in the growth and structure of the primitive axis to that in other vertebrates. The theory seems to me inevitable that the vertebrate primitive axis is formed by the growing together in the axial line of the future embryo of the two halves of the ectental line.

The development of the primitive axis may be described in general terms as follows: At the close of segmentation the edge of the primitive blastoderm separates into two parts ; one part (the anterior) , as the blastodenn, expands, spreads over the yolk, gradually covering it with ectoderm ; the other part (the posterior) forms the primitive axis; it has in its centre one fixcil point; consequently, when the blastoderm expands the two halves of the posterior part of the ectental line are brought together and gradually unite (concresce) along a line running from the fixed point backward (radially as regards the blastoderm). Conseciuently, the segmentation cavity, which is underneath the primitive blastodenn, lies in front of the developing axis. While this goes on cells grow out from the concrescing part of the ectental line into the space between the ectoderm and entoderm (or yolk) ; vmdemeath the lino of junction a cavity is formed lined by entoderm ; this cavity is the notochordal canal ; it lengthens backward as concrescence progresses ; it has, whatever its length, a small entrance, the blastopore, at its hind end ; the blastopore is ultimately obliterated. The cells which grow out from the ectental line constitute the first anlage of the middle germinal layer or mesoderm, and shining through the ectoderm they produce the appearance of a whitish line, which has led to the name of primitive axis. The characteristics of the mesodemi are described in the next section. Along the line of junction there often appears a slight furrow in the ectoilerm, which is known as the primitive groove.

Significance of Concrescence. — It will at once be evident that if the process of concrescence went on without the actual meeting of the two portions of the ectental line the result would be to leave the archenteron open along its entire length ; the borders of the opening would be the ectental line; and this line, as we have seen, corresponds to the lips of the gastnila mouth ; consequently, we should have a gastrula with an elongated mouth. This condition is illustrated by the accompanying diagram, Fig. 73. It agi-ees in all respects with the gastrula type ; its most noteworthy peculiarities are two : first, the enormous mass of yolk accumulated in the aboral portion of the entoderm; second, the elongation of the gastrula or archenteric cavity in a direction at right angles to the gastrula axis, xy. If now the lips of gaatnila, Fig, 64, .1, meet and unite we should obtain at once the vertebrate type. According to W. His' discovery, this is precisely what takes pla<^ — only the lips are brought together first at one end, where they at once unit«, while behind they are widely separated ; but gradually they are brought together and unite throughout their entire length.

Concrescence is, then, a modified method of uniting the lips of a greatly elongated gastrula mouth. Why this modification is estabushed we caimot say with certainty, though we may surmise with confidence that it is consequent upon the great accumulation of yolk ih vertebrate ova.

The view here adopted enables us to speak positively as to the |>oint where we are to look in vertebrates for the horaologue of the invertebrate mouth. In annelids concrescence is very well marked, whenever the ova contains much yolk; now in leeches and earthworms the ectental line does not concresce along the entire axial line but, on the contrary, as shown by Kleinenbei^ and Whitman, the foremost part of the germ bands (gastrula lips) do not unite, but leave a small opening; when the permanent mouth is formed this opening is carried in and serves as the passage between the mouth cavity {Vorderdarm, stomodseum) and the archenteric cavity. The foremost part of the line of concrescence lies, according to His' observations, on fishes just where the optic outgrowths arise. Fig. 74, vi; hence we have to search between the origins of the optic nerv-es for traces of the invertebrate moutiis. Further reference to this question ie made later in connection with the development of the ner%'ous system.

The Notochordal Canal

The existence of this canal was, so fta as I am aware, first satisfactorily recognized by Lieberkuhn, 82. 1 , 84.1, who discovered that in mammals it produces the notochord, and by loning its lower walla fuses with the yolk cavity. The canal is a narrow tube which nins forward in the tissue of the primitive axis (KoUiker's heatl process) ; it ends blindly in front, but its posterior end communicates with the exterior by a funnel-shaped opening (the blastopore) through the ectodenn. Immediately behind the blastopore lies the accumulation of cells, termed the primitive streak in amniota, the anus of Rusconi in amphibians. The canal is lined by epithelium, which is thickened on the dorsal side to form theanlage of thenotochord. At the sides the epithelium merges into cells belonging to the mesoderm.

The manner in which the canal is formed by concrescence is explained in the preceding pages, and the manner in which it fuses with the yolk cavity is de8cribe<l in the following section. For additional details and references see the histor>- of the notocliord in Chapter VIII,

Fusion of the Notochordal Canal and Tolk Cavity

The fusion of these two cavities has been caref II tud ed n n ammals and reptiles. The fusion in ampl b ans is briefly mentioned by O. Scl Ize 88.1. In the gecko (L. Will, 90 1) and in mammals (Lieberkiilin, 82 1 84. l,Van Beneden, 88.3, and oti ers) the canal l>econies qnite long, and I acquires a series of irregular opei ^ Fig. 75, nch, on its ventral side the very lai^e yolk cavity, whi I h this stage underlies the whole gem 1 area. The anlnge of the notocho I a]rea<ly differentiated on the lopv 1 side of the canal. The ventral pe ings increase Iwtb in nunilwr an I z until the entire canal has fused tl the yolk Ciivity except at the hind en I where it persists for a while as tl callal blastoi)oric canal. The f pa "hnw occurs in guinea-pigs the fourteenti to uiiim. ™o.ajra1j|iiM:«^ni. ortansUufifteenth day, in rabbits the eighth day. cidoj'NrA.Qniairpof iiiV uoi.rhopaMii

In lizards (Stralil, Kupffer) and tur- ^'lh^:^ilIxlJ™l'BBldl.^"'^'i4Ui'aE^ett^ ties (Will) the fusion occurs in a similar manner, but sooner, so that the anteriiir portion of the canal has fused with the yolk cavity before the posterior portion of the canal is completed.

The union of the two cavities prexluces the ftejin if ii^e nrrhenteron, which is a spacious cavity line<l by entoderm, having the anlage of the notochonl in its median dorsal line and opening to the exterior by the blastopore, which is situated, at the caudal end of the primitive axis and the headward end of the primitive streak.


The blastopore is the small opening which leads into the notochordal canal, or. after the canal has fuseil with the yolk cavity, leads into the archenteron. It is situatetl at the hind end of the primitive axis (head-pnvcess), and marks the anterior boundary of the anus of RuHci>ni in amphibia, or of the primitive Btreak, properly so-calle<.l, of amniota, Fig. 71, B.

While the concrescence of the tx^tontal line is going on the ])lastopore changes its position, being always at the end of the notochordal canaJ. When the canal fuses with the j^olk cavity the end of the canal persists for a time iis a passage at the end of the primitive axis, and this passage is sometimes designated as the blastoporic canal, see Figs. 70 and 71. The opening is finally obliterated.

The blastopore is not homologous with the gastioila mouth, but is merely a small portion thei'eof ; in front of it the gastnila mouth is closed by concrescence ; w^hile concrescence is going on there will be a part of the gastrula mouth open behind the blastoi)ore ; when concrescence is completetl the blastopore is at the end of the elongated gastrula mouth, the lips of which are united throughout the remainder of their length. The blastoi^re is not a fixeil i)oint, being merely the opening of the notochordal canal, and as by concrescence the canal is elongated, in precisely the same measure the blastopore travels backward.

The Meroblastic Embryo

Considerations of practical convenience have led to the custom of distinguishing in the development

of meroblastic ova the embryonic from the extra-embryonic portions. The distinction is in reality entirely arbitrary, for the whole of the ovum is included, morphologically speaking, w^ithin the body oif the embr^x^. Custom has led to designating the two parts as the embryo and the yolk ; but the student should be careful not to allow himself to l)e misled by these tenns. In the laboratory it is a general practice to remove the so-calleil '* embryo " from the yolk, and in doing this the archenteric cavitv loses its inferior wall, t<^ wit : the entoilermic yolk. Let the i-elations be represented

ram showinjr tho n-iaiiona of a bv the accompanying diagram,

with an embryo in croKs-wo- x

yolk. £<-.ecui;ienn; .v.n(.urai Fig. i (>, tho embryo being drswn

Fig 70. — Diapram Tertebrate oviini lion and a lar

groove; mcj», mescKlerm; h c . sejrnientation eav- verv much too lartrp in nronortion ity; Ent, arthentt^ric cavity; « r/. wtwlmnal > 1 1> illiu u lOO img« ill pruporwuu

rim, where the ectoderm is growing over the to the yolk, for the Sake of CleftT ness. Supjx^se the layers to be

cut through on the lines ,i' .r; we could then remove the embryonic portion. This is what is actually done in practice. It is very important to imderstand clearly that the yolk is part of the embryo, and that our sections usually ivpresent only a torso.

II. The Primitive Axis and Streak

The term primitive axis is a new one, which it has seemed necessary to introduce to avoid confusion. It is nearly synonymous with the term head-process (Kolliker's Kopffnrtsatz) . It is applied in all vertebrates to the median band of c*ells which rims forward from the blastopore ; the central cells of the band are entodermal* and form the epithelial wall of the notochordal canal; the lateral cells of the band contribute to the production of the mesoderm. At the blastopore the primitive axis merges into the primitive streak, sensu strict u, and on that account has been interpreted and described by many authors as the anterior prolongation of the primitive streak. After the ventral wall of the notochordal canal has disappeared and the canal has fused with the yolk cavity, the entire tissues of the primitive axis lie on the dorsal side of the archenteron.

The term primitive streak may be conveniently and properly restricted hereafter to the accumulation of cells lying immediately beliind the blastopore. In amphibia this accumulation is known as the anus of Rusconi ; it belongs to the entoderm (and later to the mesoderm also), and is very conspicuous owing to the absence of pigment in its cells. In amniota the corresponding accumulation comprises the cells in the region around the primitive groove, as described in detail below ; in amniota the accumulation has the yolk cavity (later archenteron) extending under it, Fig. 71, A B, pr, and it is therefore not directly continuous with the yolk proper, as in amphibians.

The conceptions of the axis and streak above presented appear to me necessary consequences of our present knowledge, but until they are accepted by other embr^^ologists, the reader must view them as largely my personal opinions, and must remember that morphologists are not yet agreeil as to the nature of the primitive streak.

The Primitive Axis

As above defined, the primitive axis is the median band of cells resulting from concrescence and overlying the definitive archenteron.

It is advisable to begin with the consideration of the arrangement as we find it in eggs of marsipobranchs, ganoids, and amphibians, since these eggs are probably more primitive in their mode of development than those of other vertebrates. The points of most importance in my judgment are illustrated in Fig. 77, A and B. In A we have a section through the middle portion of a young primitive axis of an axolotl, the axis still requiring considerable additions at its hinder end before attaining its full length ; the archenteric cavity, Ae, is a, large space bounded above by an epithelium, Euy and below by the large mass of yolk cells, Yk; the two-layered ectoderm, Ec% everywhere bounds the section ; above the archenteron and below the ectodenn lies the accumulation of cells constituting the primitive axis, Pr; the lateral prolongations, Mes, of the axis represent the commencing mesodermic outgrowths; whether the mesodenn grows out from the primitive axis and subsequently expands solely by its own proliferation, or whether it receives at its periphery accretions from the yolk cells is uncertain. I am inclined to think that the mesoderm doe.s not receive additions from the yolk. In B we have a similar section, but of an older stage, and through the hind end of the nearly full-grown axis ; the general arrangement is the same as in A ; we note the following differences : the archenteric cavity is a mere slit, Ae; the primitive axis, P7\ is very thick and composed of numerous small cells, and its lateral mesodermic expansion, Mes, extends farther around the ovum. In both sections we see that the cells of the primitive axis are not marked off from those of the adjoining entoderm. In a lon^tudinat section, as is illustrated by that of a sturgeon, Fig. TO, the mesoderm of tho primitive axis is seen to extend far forward from the blastopore, Bl. The disposition of the parts and the appearance of the cells vary in the three groups we are considering, bux for our purpose it is unnecessary to describe these secondary differences. The points essential to note are that the primitive axis produce chiefly mesoderm, which is accumulated along the axial line, and is tliickest around the blastopore, where it joins the primitive streak, and which spreads laterally between the ectoderm and entoderm ; in the axial region the mesoderm is not separated from the entoderm.

  • Prenant ("Embryolojrie**) resrards them a.s ectodermal, followinff O. Hertwig's suggestion; the terminology in this case is largely a question of previous definition.

In elasmobranchs the differentiations of the axial tissues begins in the embryonic rim before concrescence takes place, so that while the type affords peculiarly conclusive evidence of concrescence, it is less convenient for the study of the primitive axis, since the hind end of the primitive axis is, as it were, divided, being continued aa the embryonic rim, right and left. Tho degree of differentiation in the rim varies extremely: i:i Pristiurus the mesoderm grows out; In Scylliiun the mesoderm grows out and the differentiation of the notochord begins; in Torpedo (Riickert, 87.1, Inl) the myotomes ap{)ear in the embryonic rim before concrescence, as in Elacate among teleosts. The relations are further complicated by the advance in development of the axial structures while concrescence is going on, so that, as for instance in Pristiurus, Rabl, 89.2, 11(3-129, the axial notochord may bo differentiated, while the ineso<lerm is still developing in the embryonic rim. The precocious changes in the embryonic rim demand especial attention when the origin of the mesoderm is discussed (c/. ChajiterVI.). the ectoderm, as ssoon as it becomcH nne-layere<l (secondary hlastwlemi, see Cliapter IV.), consists of high-cylinder cells. As development pnigresses the ectoderm thins out except on either side of the axial line. The mesoderm arises from the entoderm close to the ectental line and is there quite thick, but as it stretches away it thius out. Now if it be remembered that the ectental line becomes the axial line, when concrescence occurs, it is evident that this mesodermic thickening of the entoderm is in reality an axial tliickening, and when concrescence takae place it fuses with the corresponding thickening of the opposite side and constitutes an actual axial thickening or true primitiver streak ; but in elasmobranchs, as soon as the anterior axial structures have concresced, we find by precocious development that the notochord and medullary groove appear; now, as shown in Chapter VII., the appearance of these structures causes the division of the axial mesoderm into completely separated right and left portions. It is only by keeping the process of concrescence and the precocious development of the parts constantly in mind that we can understand the development in elasmobranchs or compare it rightly with that of other types. From what has been said it is clear that a section of the blastodermic rim from which the mesoderm was just growing out would correspond to half a section of, say, a bird's ovum, though the primitive axis, and upon comparison it will be found that all the essential relations are identical.

The primitive axis and streak of 'birds have been much investigated and discussed, and may be conveniently treated together. I follow in the main Duval, 78. 1 , 84. 1 , many of whose statements are confirmed by Zumstein, 87.1, Other important authorities to be consulted are Kolliker in both his text- books ; His, 68.1, 77.2, 82. 1, etc.; KoUer, 82. 1 ; Diase, 78. 1, 79. 1 ; Waldeyer, 69.1, 83.1; M. Braun, 82.3; Gasser, 77.1, 79.1; Rauber, 76.2; C. Eabl, 89.2, et al.

The following description applies to the hen's egg. When the egg is laid the centre of the segmented blastodisc presents a circular area of lighter color; during the first few hours of incubation this area pellucida, as it is called, becomes more dis- , tinct; as the area pellucida expands, the primitive streak appears in it eccentrically between the eighth and twelftli hour. By the sixteenth hour the primitive streak has its full length. The rate of development is extremely variable, autumn eggs developing more slowly than spring eggs; the e^s vary also individually, and are, moreover, much influenced by the temperature of their incubation. For a fuller diticussion of these variations see His, 68.1, 56-63. Seen from the surface the areajpellucida with completed Btreak presents the following features, Fig. 78. The area pellueida, a. p., is considerably elongated and somewhat |)ear-8haped, being widest at the anterior end of the primitive fjro<)ve, j>/. ; this groove is well marked as a narrow and shallow furrow, which begins some distance from the anterior edge of the area and ends just l>efure reaching the posterior edge of the area; the front end of the furrow usually bends alightly to the left, but not invariably, as Koller and Eabl have maintained, for it sometimes bends to the right or is quite straight; aline of granules is sometimes noticeable above the primitive groove; they were seen byDursy, I.e., irad are called by Duval, 78.1, 15, the fila?iieiif epiaxtal — compare Gaseer, 79. 1 . The portion of the area pellueida immediately around the primitive groove appears sliglitly darker than the rest. The anterior portion of the pellueida is further distinguished by the anterior crescent, ct, the "vordere Aussenfalte" of His, 68.1, and other German writers. The anterior crescent is a temporaiy appearance, due, according to Duval, to a series of f oUls of the entoderm, which form a curving row of shallow pockets, that, shining through, mark out the crescent. The crescent disappears a little later, and there arises nearly, if not quite, in its place a different fold, the amniotic. The similarity of position has led to the anterior crescent's being identified by some authors with the true amniotic fold.

Longitudinal and transverse sections are very instnictive. We begin with the examination of a longitudinal section of a somewliat younger stage. Later the ectoderm closes behind the primitive streak, as already stated and spreads Iwickward over the yolk. The section shows that the yolk is not divided into cells, although nuclei are scattered through it; the nuclei are represented as black dots in

Tia TO LoDKltudiaal SHTtionofthpr ■Ix houni Aftpr Du al 1) jtraeral ir with hie>wr mnen IkAtiim Er Mtod rn pan. itu, KUiuinal »all iheiaiiralli , A

nciD ut tl pMn KlTeunwk of a Hra sovuu ticubated RiBKnilled ubuut 40 dlameU-ni A. B C IKailnotD. nen nmvidinu i, entoder tt til X ival h blasto.

A, B, and C. The cavity of the archenteron, ^4(7«,* is enlarged by the formation of a deep pit in the yolk, while the posterior half of the cavity remains a narrow fissure between the cellular entoderm, En, and the yolk; the archenteron communicates, according to Duval, with the exterior by an opening, 6/, which he calls the blastopore; as this supposed opening is apparently at the posterior extremity of the primitive streak it cannot be the true blastopore. The entoderm is a loosely put together stratum of cells, which passes over anteriorly into a ridge of the yolk in which cells are being produced around the already accumulated nuclei ; this ridge, kw, is the germinal wall. Posteriorly the cell layers are much thicker, A; the ectoderm, Ec, is clearly differentiated from the underlying cells, which are all more or less alike; though they represent both the entoderm and mesoderm. From this connection and from the fact that the connection between the ectoderm and mesoderm which is so well known to exist after the primitive streak has attained its full length, Duval concludes that the mesoderm arises primitively from the entoderm. Transverse sections afford additional information.

Fig. 80. —Transverse sections of a germinative area, with half-forme<l primitive streak, of a Hen's egK. After Duval. A, throuj^h the anterior region of the area i^ellucida. B, through the primitive streak. C, part of A enlarged. Ec, ectoderm; mes, mesoderm; i?nf, entoderm; W, blastopore; /rif, germinal wall (^KeimivcUl) ; Ach^ archenteric cavity; sg. c, segmentation cavity.

The accompanying Fig. 80 represents cross-sections of a germinal area, the primitive stre^ of which had attained about one-half its full length. The first section. Fig. 80, A, passes through the anterior region of the area pellucida, and therefore in front of the primitive groove; it shows the large cavern, Ach, of the archenteron (or yolk cavity?) hollowed out in the yolk; the entoderm, C, Ent, above the cavity is a thin layer of cells, connected laterally with a projecting shelf of yolk kw (the bourrelet entodermO'Vitellin of )uval), which is rich in nuclei ; it subsequently expands and acquires a more cellular character ; this shelf is the commencement, therefore, of the Keimicall of German writers. Immediately above the entoderm, and intimately connected with it, are a few cells, which belong to the mesoderm, C, mes; the ectoderm is quite thick, C, Ec, and consists of high columnar cells ; toward its periphery the ectoderm thins out, and its edge rests upon the yolk, witn which it has no connection. In the region of the primitive streak, Fig. 80, B, there are important diflferences in th^ germ layers to note. The entodermic cavity, Ach^ is very much smaller; the mesoderm is much thicker and in the axial region fuses with both the outer layer of cells * and the entoderm, thus forming the Ach^enstrang (axial cord) of German writers; the mesoderm also spreads out over the yolk far beyond the archenteric cavity, and about one-third of the way from the axial line to the distal edge of the ectoderm : the ectoderm merges in the median line with the mesoderm, and presents externally a small notch, B, pr, corresponding to the primitive groove.

  • As previouslv Ktate<l. Duval was unacquainted with the existence of yolk cavity; it is probable that the cavity here termed archenteric is really the yolk cavity.

Whether at the stage from which Fig. 80 is taken the formation of the primitive axis (head-process) has fairly begun is uncertain. In slightly older stages the "head-process" is present (KoUiker, "Gnmdriss," 2te Aufl., 3G). During these changes the archenteron (yolk cavit\'?) expands rapidly, the entoderm becomes very thin in the area pellucida, and passes more and more abruptly, as development progresses, into the so-c^led germinal wall of the area optica ; finally the ectoderm becomes thinner periphenUly, so that the axial thicker part is gradually marked off more and more abruptly. Sections of a stage with the primitive groove at its maximinn — a stage which is usually found t<)\yard the end of the first day of incubation — show these changes clearly. A cross-section through the area opaca in front of the area pellucida shows the thin ectoderm, the thick cellular entoderm overlying the archenteric cavity and charginl with yolk granules; the entodermic nuclei are very- variable in form and irregular in distribution ; the cell lx>undaries are indistinct. There is no mesoderm. A cross-section near the front of the area pellucida likewise shows only e<!to<lerm and ento<lenn ; the foniicr is a high cylinder epithelium ov^^r the area i)ellucida and thins out toward the opaca on each side; the latter is a thin layer over the area pellucida and passes quickly but not abruptly mio the ver}" tliick yolk-bearing entoderm {or Keimwalt) of the area opaca. Sections a short distance in front of the primitive groove show that the head-process (Kopffortfiatz) is a forward ])rolongation of the primitive strejik, and consists of an axial accumuIatioN of mesodernnc cells fused nith the entodenn^ and having broiul extensions sideways to fonn the mesoderm between the outer and inner genn-layers; the lateral portions of the mescxlerm have no connection with the other germ yers, and at its distal edge the mesoderm thins out and rests upon the entoderm of the opaca, but without l)ei ng connected with it ; I cannot find any satisfactory evidence that it receives any additions from the opaca entoderm, as many authors have maintained. The ectoderm in the I'egion of the " Kopffort^siltz" resenilJes that further forward, but it very soon shows a faint median furrow, the so-called dorsjd groove (Riickenrinne) ^ which is the commencement of the medullary groove (see Chapter VII.). In the anterior half of the primitive streak the relations are different from those* in the "headprocess." The outer layer shows the primitive gnnn^?, Fig. SI, pr(j,

  • This outer layer is iimially termed ectcxlerm, but I liohl that U in not eottxU'rui, but tin* houiologfue of the outer layer of yolk cells iu the aniphibiau uuus of Huscoiii.

and is fused with the axial cord (Arksevstrang) of the mesoderm; laterally the outer layer paBses ioto the true eotodenn, Ec. In the posterior region of the primitive groove the connection of the mesoderm with the inner germ-layer is dissolved. Behind the primitive groove the mesoderm extends, but lies free between the ectoderm and entoderm. To recapitulate: there is a long axial mesodermic thickening, which has the pi-imitive groove over its posterior two-thirds; the thickening in front of the groove is united with the entoderm, and constitute!} the primitive axis; the thickening under the front half of the groove is united with the entoderm ; in the median line its external surface is freely exposed, and laterally it merges into the ectoderm; the thickening under the hind half of the'gro^ve is not united with the entodenn.

¥ia. SI. — TranavetW! wiHIor Hen's OTiiiii: Ec. wtnrtemi: Itna black dots reprmeot folk

III. The Mammaliajj Blastodermic Vesicle.

In all placental mammalia, owmg presumably to the absence of the lai^ amount of yolk present in the ova of other amniota, the early development is motlifiefl, and the germinal area instead o£ resting on a mass of yolk rests upon a vesicle. When the vesicle is fully developed its main cavity is lined by entodennal cells, and must be, in my opinion, homolt^zed with the yolk cavity of other vertebrates, for it fuses with the notochordal canal to develop the definitive archenteron.

We may conveniently distinguish four stages of the vesicle, which are describe<l below in order: 1, with one layer constituting the vesicle, except over the gemiinative area; 2, with two layers; 3, with primitive streak: 4, with "head-process." or primitive axis.

1. Vesicles with One Complete Layer,— After the close of segmentation we find that the inner mass liecomes flattened out, and in the reigon it occupies \ve can distinguish three layers of cells, as previously descril)e<l: first, counting from the outside, the thin layer of cells laiown as Rauber's " Deckschicht ;" second, a middle layer of cylindrical cells, which becomes the ectoderm; third, an inmost layer of thin flattenetl cells, which belong to the entodenn; the Deckschiclit continues round the whole vesicle as a single layer; the other layers do not so continue, compare Figs. 57 and 58. The next step in development is the formation of a second layer, which spreads out in all directions from the regiim of the inner mass; hence as far as the new layer reaches the blastodermic vesicle becomes two layered. Meanwhile the Deckuchicht disappears, leaving two layers' in the region of the innef mans ; it is to be remarked that the Deckschicht ia retained in certain rodents, undergoing special modification, as described in the section on inversion of the germ-layers.

RahbiVa Vesicle at Six Days. — The following is a summary of Ed. van Beneden's description, 80.1, 1H5-20O, The vesicle measured 3.3 mm. in diameter; it was nearly spherical; the wall of one hemisphere consisted of one layer of cells; the other hemisphere had two layers of cells, and besides in its central portion a third layer inter\-ening between the other two. The area with three layers Van Beneden designates as the tache embryonnaire; it showed no trace of the primitive streak; it was oval in outline and had one point, which the author identifies aaHensen'a hnot, where the layers adhere together closely. Transverse sections show that the outermost layer of cells is a low cylinder epithelium, which, at the edge of the area, passes into a thin epithelium, quite abruptly; it corresponds to Rauber's Deckscbicht, and has been said by him to flatten out and disappear, leaving the cells underneath as the permanent outer layer or the embryonic vesicle. The cells of the innermost layer are thin and wide; they are called the hypoblast (entoderm) by Van Beneden; the cells themselves have round nuclei, around each of which is accumulated a court of granular protoplasm; the adjacent courts are connected by a coarse meshwork of protoplasmatic threads; treatment with nitrate of silver brings out the cell boundaries and divides the reticulum into polygonal areas. The cells of the present outermost layer have distinct boundaries and contain granules and long bacilliform bodies, wliich Van Beneden sawalso in the fresh specimens and found to be constant appearances. Similar bodies are found in the germinal vesicles of sheep, and are held by Bonnet, 84.1, to be derived from the uterine milk; the rabbit is not known to have uterine milk. The liistological peculiarities of these two layers remain about the same fnim the fifth to the eighth day. the middle layer consists of rounded cells with numerous granules ; seen from the surface their diameter is greater than that of the cells outside them, but much less than that of the cells underlying them. While we know that the middle layers are ectodermal, it is uncertain whether the inner layer is entodermal or not.

Blastnnjst of the Rabbit of

Flo. R3.— BlaModermlc Tericle of nB^bltor ffpi-on Dnils The devplonment

KevraOtye: nu, »reBireniiln«tlva, orembryonio "*^t*^" tMiifK. liie lie veil tpiiitrui,

•hwii: tfp. iim, shovpwrhtcii theiifku- iKfwo- \n exceedmgly variable, so that

l«wred. From Killllker m,

exacttimescannot l)egiven. Ihe general appearance is illustrated by Fig. 82, from Kolliker, The vesicle figured was 4.4 mm. in length; the envelopes of the ovum are not shown, though they were still present ; at the upper pole is the small embryonic shield, corresponding in position to the region of the inner mass ; it is marked out by the greater thickness of the walls of the vesicle; the developing second layer extends over more than half the vesicle, reaching to the line ge,

2. Blastodenxuc Vesicle with Two Layers. — Of this stage we have several descriptions ; for the rabbit by KoUiker {" Gnindriss/' p. 91); Hensen, 76.1; C. Rabl, 89.2. 141; as well as the older accounts by Bischoff, 42.1, and Coste, 47.1, and the brief mention by Heape in Foster and Balfour's ** Embryology," 2d edition, 316320; for the mole by Heape, 83.1; for the dog by Bischoflf, 46.1: f or the cat by Schaf er, 76.1; for the sheep by Bonnet, 84.1; and for several rodents, as indicated in the section on inversion of the germlayers, p. 141.

The two-layered stage is fomid in the rabbit about seven, in the sheep about thirteen, days after coitus. The dimensions for the sheep are about 4 mm. for the greatest diameter and 2.3 mm. for the lesser diameter.

The two layers form each a closed sack ; the embryonic shield is well marked as a round spot, less translucent than the walls elsewhere. The outer layer has a distinctly epithelial character; in the region of the shield its cells are columnar with spherical nuclei ; in the rabbit the cells are low and the nuclei lie nearly at one level; in the sheep the cells are taller and the nuclei are at various levels; in the mole (for a good figure see Heape, 83. 1, PI. XXL, Fig. 49), and in various rodents there are several layers apparently, but perhaps in them also the epithelium is columnar, as it certainly is later. At the edge of the shield there is an abrupt change to a very thin layer, with widely expanded cells ; consequently, in the region of the slueld the nuclei are close set, while outside the shield they are wider apart. The change at the edge of the shield is at first less abrupt, but at the present stage is very marked. A similar difference exists in the inner ayer, although its cells are very much thinned out everywhere, yet the layer is slightly thicker in the region of the shield ; the nuclei of the inner layer are somewhat flattened, and they are larger and farther apart than the nuclei of the outer layer — a difference which is very obvious in surface views, both during this and the no. 83. —Transverse section of the embryonic chield of the Ti£ivf frkllr\-wriT^fy afarrou blastodermic vesicle of a Sheep, thirteen days pregnant. After iieAt luuuwiug »W4$t;a. g^nnet. a, outer layer of vesicle; 6, inner layer of vesicle.

The inner layer has an epithelial character in the region of the shield, but farther away the cells move apart, and being connected by processes resemble embryonic connective tissue (Bonnet, 84.1, 192; Hensen, 76.1, Figs. 15 and 11, B on Taf. VIT. ; E. van Beneden, 80.1). The relations are illustrated by the accompanying Fig. 83, representing the shield in the sheep at thirteen days and of a vesicle measuring 4 mm. by 2 mm. ; at the left of the figure the layers are accidentally folded. The next changes which occur are principally those of growth both of the vesicle as a whole and of the embryonic shield, which aJao begins to arch up ; the vesicle and shield both become oval ; usualKthe oval shield Ues lengthwise, but in the deer, as shown by Bischofi, it lies trarksversely of the vesicle. The size of the shield is quite nearly uniform among the placental mammals in which it has been studied, but the size of the vesicle varies extremely ; especiallj' noteworthy ia the excessively rapid elongation in ungulates (pig, sheep, goat, and deer) ; in the sheep, for example, it trebles or sextuples its length in less than a single day after tne shield appears. The next step 19 the appearance of a middle layer, at least in sheep (Bonnet, 84. 1, 192-llliJ, 89. 1, i2), which shows in the fresh specimen as a slight turbidity, Fig. 84, mes, of the vesicular wails just outside tlia edge of the shield, while in the region of the shield there is no middle layer whatever. Sections show that the new layer consists of loosely sca'tCT;-' "c"s — noc'^et' b" anastomosing processes; it iseverywhere absolutely distinct from the outer layer, but merges at many points with the inner layer ; from this connection Bonnet concludes that the middle layer is derived from the inner layer by what must be called a process of dehunination. So far as known to me nothing analogous to this middle layer has yet been obser\'ed in other mammals. The next iniix>rti!mt step, again according to Bonnet, 81.1, 1!I5, is the appearance of Hensen's knof, which takes place while the peripheral middle layer is developing. The knot. Fig. 84, An, is at nrst a small thickening on the under-side of the outer layer; it ia situated on the middle line of the shield a little nearer one end than the other; it is distinctly separated from the inner layer, but is connected with the cells of the middle layer, which have now developed themselves in the middle region of the shield also. Bonnet mantjiins that the knot gives off cells which contribute to the formation of the middle layer. The knot marks the front end of the future primitive streak, and is the beginning of the primitive axis.

The appearances in a sheep's ovum at this stage are illustrated by Fig. 84 of a vesicle of twelve to thirteen days from u sheep ; the vesicle measured 55 mm. in length by alxmt 1.5 in breadth, but the length of the vesicle is extremely variable at this stage ; the specimen had been staine<l to bring out the small, close-set nuclei of the outer layer and the larger, more widely set imclei of the inner layer. The upward arching embryonic shield, .S'A, shows Hensen's knot, k»: around the edge of the shield, Hh, the middle layer makes an irregular shadow, mes.

Fig.84.— Central portion ote Sheep's hluitodermic vesiclr of IhtIvp ti) tlilrieeu dayn, Sfl. 8t>leM:tn. HPiixeii's kncl: pr. (rMW o[ primitive BinM-k ; iii»ji. " Mi*oblaMhof . " At

A condition of tbo blastodermic vesicle similar to that described is figured by Coeto for the rabbit, 47. 1, by Bischoff for the rabbit, 42. 1 , Taf. IX., fig. 43, c, for the dog, 46.1, Taf. III., fig. 28, B; and the cadual extension of the second layer is recorded for the mole by eapo, 83. 1 . Since it is knowTi to occur in rodents, camivora, and insectivora, it is probably true of all placental mammals that the onelayered vesicle becomes two-layered by the outgrowth of celts from the "inner mass" found at the close of segmentation ; this is the first step of development after segmentation.

Raubeh's Deckschicht has evidently great importance. It was first described by him in the rabbit, 76.2; and waa also discovered by E. van Bcueden, 76.1, who, however, made the error of consid oring it as the permanent ectoderm, and the true ectoderm below it as tho mesoderm; this error has been amply corrected by Kt'iUiker and is now admitted by Van Beneden (seo Van Beneden and Julin, 84.1). Its disappearanc-o in the rabbit has also been studied by Lieberktilm, 79.1, Balfour ("C'omp. Embryol.," II., 219) from investigations on the rabbit by himself and Heape, conclude*! that the cells of the Dusckchicht disappear by biding incorporated in the true ectodermal layer becoming at the same time columnar; this view is verified by Liel>erkuhn, 82.1, 400. 401. As already stated the rodent modification of the I>eckschicht is discussed below, p. 141. In the rabbit the Dcisckchicht disappears lieforo the second layer of cells gnjws completely round the vesicle.

3. Blastodermic Vesicles with Primitive Streak

The knot of Hen-ien marks the front end of the pnmiti^ e streak which lengthens backward; during the same period the vesicle as a whole enlarges; m rum in in ts the enhirgement is enormous and ver\ rapid * The primitive strejik always lie^ m the lon^ axis of the shield. The formation of the prim itive axis begins with the union of Henatii 'i knot with the inner layer, so that at the knot all three layers are actually united — the condi tion originally discovered by Henstn 76 1 2(!8. The union of the knot with the inner layer spreads backward until it rear lies the etl^e of the shield, thus generating the priniiti\e streak. Next follows the elongation of the streak and shield, the latter becoming, pointed at its hinder end. We thus have a i)ear shaped shield with the primitive streak running for ward from its pointed end; the anterior end of the primitive streak is somewhat enlarged and or a i the posterior end is considerably thitkcned the ^imi. -.,,. ,..»...,.....» three layers are uniti-d along the primitive of iniii.i™1a)«. m Bi"'."Afstreak. Fig. 85 represents the embryonic shield '" ""'" ^- * """"^ of a rabbit embr3-o; the shield measured 1.34 mm. in length and 0.85 mm. in width; the primitive streak is a broad band, corresponding to the axial thickening, and extends about two-thirds of the length of the shield; the mesoderm, m\ m\ occupies a circular area around the hind end of the streak ; for a similar stage in the opossum see Selenka, 86.1, Taf. XVIIL, Fig. 6; in the mole, Heape, 83.1, PI. XXVIII., Fig. 12; in the sheep, Bonnet, 84.1, Taf. X., Fig. 39, 40. Cross-sections show the union of the three layers in the axis; the greater width of the streak in front (to this wide anterior end of the streak the term Hensen's knot continues to be applied) ; and show also the increasing thickness of the streak posteriorly. The primitive groove, which is a shallow depression of the outer layer, appears first over Hensen's knot, and thence extends gradually backward along the median line of the primitive streak.

• BoniKt Htaten ihat In th^ slip^j^ [hf lilaHtiBlt^niiio vwit-l^ nmal >^li>nif»l>- ilurlun ihu period it

Fig. 86.— Section of the primitive streak of the Mole: p. gr. primitive crroove; ECy ectoderm r vneB. mesoderm ; JSVt, entoderm ; iV. primitive streak. (In sections nearer the end of the CToove, Pr does not appear, and the inner layer is distinct, though not separated axially from the middle layer. ) After Heape

A transverse section through about the middle of the streak at this stage in the mole is represented in Fig. 86, and may be considered thoroughly typical.

4. Blastoaermic Vesicles with Primitive Streak and Head-Process

In the stage we are now considering the axial thickening becomes subdivided into two parts, an anterior known aa the head-process (Kopffortsatz) ^ and the true primitive streak. The two are distinguish^ by the fact that the axial thickening in the region of the process is separated from the outer layer but fused with the inner layer, while in the region of the streak it includes, as in birds, the outer layer. Except at its anterior end, the axial thickening is not connected with the inner layer. Hence cross-sections may give us three different appearances according to the level at which they are taken.

The head-process was first distinguished, so far as I am aware, by Kolliker {" Entw.-Ges.," 18r9,p. 271), also 83. 1. Lieberkiihn, 82. 1, first showed that in it appears a small longitudinal canal, the walls of which form the notochord. Heape, 83.1, discovered that the hinder end of this canal opens exteriorly in the mole, and Bonnet, 84.1, made the same olxservation on sheep. Strahl describes the "process" in the rabbit incidentally in his paper on the cloaca, 86.2; additional information is given by Bonnet, 89.1, concerning the sheep, and by C. Rabl, 89.2, concerning the rabbit. Especially valuable is Fr. Carius' dissertation, 88.1. In the guinea-pig, according to Carius, after the formation of the primitive streak the middle layer grows out in all directions and lies free between the inner and outer layers. In front of the primitive streak the outgrowth takes place in three divisions — one median, two lateral. The median outgrowth is the head-process proper, and it becomes later united with the inner layer, but at first lies entirely free (embryo of thirteen to fourteen days) . The first indications of the formation of a canal is an alteration of form in the cells, which elongate in directions at right angles to the axis of the head-process, so that their oval nuclei are radially placed ; the change begins posteriorly and progresses forward ; while it is going on the anterior extremity of the head-process fuses with the inner layer. The radial cells move apart so that there arises a longitudinal canal ; subsequently the canal loses its inferior wall, so that it becomes continuous as a cavity with the cavity of the vesicle formed by the inner layer; compare antey p. 127. In the rabbit the head-process is also free at first, but very early unites with the inner layer, in which condition it was found by Carius, 18-11>, at seven and a half days.* In the rabbit Hensen's knot presents at this stage a small depression (the front end of the primitive groove into which a small plug of tissue projects up from the underlying axial thickening (Carius' Fig. 7); Van Benedc^n homologizes this with the anus of Rusconi. The relations of the head-process in the sheep are verj' much as in the rabbit. Bonnet, 89.1, 65-67; the cells of the middle layer are at first free, as thev grow forward to fonn the process, but subseipieiitly are found united with the inner layer.

The head-process {rf. Lieberkuhn), 84. 1, probably always grows — as is certainly the case in the guinea-pig — at its hinder end and at the expense of the primitive streak; it is, I think, in this maimer that the often-noticed shortening and final disappearance of the streak is effected. This mode of growth concords with the concrescence theory.

Homologies of the Mammalian Blastocyst

The homologies with corresponding stages of other vertebrates are uncertain. It seems clear that the main cavity of the two-layered vesicle corresponds to the yolk cavity and that the head-process is identical with the primitive axis. But the homologies during the stages of transition from the segmented ovum to the two-layered vesicle are uncertain, and must remain so until we understand the genesis, first of the yolk cavity, second of the primitive axis. Nor can the development be clear to us until the growth of the primitive axis by concrescence is elucidated.

Inversion of the Germ-Layers in Rodents

In many but not in all rodents the outer layer, Rauber's Deckschicht, of the embryonic shield undergoes a remarkable hyj)ertrophy immediately after the close of segmentation projxir ; the Deckschicht, together with the ectoderm underlying it, becomes a plug which pushes in the other layers, thereby profoundly altering the topography of the ovum. In the mole, Heape, 83.1, the hypertrophy is not very great and the plug disappears soon, so that there is no great change; in guineapigs, mice, and Arvicola, the plug becomes very large and remains for a long time. The plug is very long and the ovum elongates with it, changing into an almost cylindrical vesicle (Selenka's Keimcylifider) . The plug becomes hollow, and the cells corrysponding to the Deckschicht become separated from those which are to form the ectoderm of the embrj-o. Three modifications of the lioUowing out of the plug and of the separation of its t^vo parts are known. The changes referred to are very clearly illustrated by Selenka, 84.1, Taf. XVI., in a series of comparative di^^mmatic figures. In the simplest case, Fig. 87, the plug acquires a single cavity, ci; the cells around the upper end, TV, correspond to the Deckschicht and serve partly to attach the omm to the uterine walls; the cells, Ec, around the lower end of the cavity become the embryonic ectoderm ; all the cells around the cavity a are homologous with the outer layer of the embryonic shield of other mammals. The cavity c of the vesicle is very much reduced; the inner side of the shield, i.e. of the plug, ia lined by an inner layer, En, which gives rise to the entoderm. The outer layer of the vesicle is very thin; it unites ^ verj- closely with the walls of the uterus, and later disappears. Hence, when the uterus is opened, only the hollow plug and its covering of entoderm can be removed ; as it makes a two-walled vesicle it was considered to represent by itself the twolayered stage of the blastodermic vesicle. Thus it came that Bischoff believed that in various rodents the ectoderm lies inside, the entoderm outside. Bischofif's observations, 62. 1, 70. 1, which were confirmed by Reichert, 63.1, are correct; hut the inversion of the layers is apparent, not real. The actual homologies were not discovered /- tn until the improvements in microscopical technique enabled Selenka, 88.1, 84.1, and Kupfifer, 82.3, to make sections of uteri with the ova iu situ, and in their sections to follow the history of the outer *esici» oThuii »ji«i^ layer. Their results have been in the main con^-^'t;J'!^^'-J'^l' firmed by Fraser, 83.1, and extended to another iienii:o*.i>utrriB)-;r;(i, species by Bichringer, 88.1, 81.1. ^erm"'^?S'!^^Q^. lo Mus decumanus the ectodermal cells early become a separate spherical mass, thus dividing the plug into two parts; a cavity appears in each jtart; these two cavities soon become confluent, and the inner layer of cells having meanwhile developed, the relations become essentially identical with those in Mus sylvaticus, Fig. S7. In Mus muscuhis the development is similar, but there is the additional peculiarity that the Deckschicht is regularly invaginated at first so as to form a small pit, into which uterine tissue grows. In Arvicola this invagination is more marked and lasts longer, but in both cases it is early obliterated.

  • C. Rabl. 89.2. 14.'i-145, stat«»s expressly that in the rabbit the axial thickenlnj? is not connected with the inner layer either under the head-proct»s8 or under the primitive streak. He differs from other inve.sti>fator.s in this so much that I think his pre|>arations were probably defective: indeH<l, his own flgiu'es sup^est at once that the inner layer has l>eeu artificially separated from the overlyiuj? one.

Arvicola represents the second mwiification mentioned above ; it has not only the invagination to distinguish it, but also the very early fonnation of the cavity of the plug iw a fissure between the Deckschicht and the true ectodeimal cells.

The guinea-pig offers the third motiification and, is characterized by the early complete sei>aration of the plug Jnt't its two parts; the Deckschicht remains at one end of the ovum and forms the Triiger; it acquires an indejiendent cavity of its own ; the ectodermal portion of the plug forms a solid spherical mass which is transported to the opposite pole of the ovum ; it subsequently becomes hollowed out, presenting a space, which, as the later development shows, is the amniotic cavity. The inner layer passes from the edge of the Trager around the sphere of ectoderm ; if the two parts of the plug were connected the relations of the inner layer would be the same as in Mus sylvaticus. Fig. 87.

The subsequent development of the rodents with inverted layers is modified in various secondary features, which it will be unnecessary for us to study. In all typical respects the embryonic development agrees with that of other mammals even as to details.

Duval, 90.2, has shown that in the rabbit the outer layer of the blastodermic vesicle degenerates and disappears, though at a much later stage than in the species just considered. Hence there is in the rabbit also potentially an inversion of the germ-layers.

Graf Spee, 89.1, 170, suggests, and I think with considerable reason, that the earliest development of the human ovum takes place by inversion of the layers. If this hypothesis is correct, it explains many of the remarkable peculiarities of the youngest human ova known at the present time,

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Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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