The practice of obstetrics, designed for the use of students and practitioners of medicine . PJV \.5i=- Fig. 63.—Sagittal Section of Frog Embryo showing the Three Layers. The blasto-pore now becomes the proctodeal opening and the neurenteric canal joining enteronwith neural canal.—{After Gotte.) BF, Fore-brain; BH, hind-brain; BM, mid-brain; CH, notochord; M, mesoblast; NC,cavity of neural tube; NT, neurenteric canal; PN, pineal body; PT, point wherefuture mouth arises; TI, intestinal region of mesenteron; TP, pharyngeal region ofmesenteron; J/, proctodeal aperture; VK, liver; Y, yolk-cells. i
The practice of obstetrics, designed for the use of students and practitioners of medicine . PJV \.5i=- Fig. 63.—Sagittal Section of Frog Embryo showing the Three Layers. The blasto-pore now becomes the proctodeal opening and the neurenteric canal joining enteronwith neural canal.—{After Gotte.) BF, Fore-brain; BH, hind-brain; BM, mid-brain; CH, notochord; M, mesoblast; NC,cavity of neural tube; NT, neurenteric canal; PN, pineal body; PT, point wherefuture mouth arises; TI, intestinal region of mesenteron; TP, pharyngeal region ofmesenteron; J/, proctodeal aperture; VK, liver; Y, yolk-cells. is possible in that animal, the formation of germ-layers has been more fullyand frequently studied in the chick than in any other species. The followingchanges, however, have been taken from the embryology of the rabbit, whichis tolerably well understood, because this mammal naturally has more resem- FORMATION OF GERM-LAYERS. 53 blance to man. The concentrated mass of cells above described at the poleof the ovum flattens out into a disc called the blastoderm, which is seen to. Fig. 64.—Section THROUGH Early Human Ovum. X 24. c?.^., Embryonic disc; e(;., ecto-derm; m., mesoderm; , yolk-sac; c, chorion; ,am., amnion; , allantoic stalk;, allantoic canal.—{After Graf Spee.) consist of two kinds of elements (Fig. 58, c), with two layers next to the outer sphere of flattened cells; and more or less continuous with it, which together are regarded as the ectoderm. The cells which complete the sphere are called, from their discoverer, Raubers layer, and in the rabbit they disappear (see Membranes). There are also cells lying next to thecavity which form the entoderm,. Thistwo-layered germ, though arising in amuch modified manner, is properly com-parable with the two-layered or gastrulastage of the amphioxus (Fig. 60), andthe cavity is also called the segm-entationcavity.* The blastoderm of the rabbit,as seen from above, soon takes the formof a shield, in
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectobstetrics, bookyear1
