1897 Human Embryology 5

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

Note - this online text is only at a very early draft stage and contains many errors from the original scanning.

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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

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

uncovered yolk. Compare also the text.

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.

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,

81

•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, -B/, 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, Fig. (IT, f s

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 observ-ed 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 ta'TiaiT' \ n.

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.

Blastopore. — 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>fH ; _ _

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.

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Cite this page: Hill, M.A. (2024, April 16) Embryology 1897 Human Embryology 5. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/1897_Human_Embryology_5

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