. The physiology of reproduction. Reproduction. 388 THE PHYSIOLOGY OF REPRODUCTION much reduced, and was "presumably enveloped by the ; It had previously been shown by Turner that the yolk-sac disap- peared at an early period. RoDENTiA.âIn Rodents the conditions are entirely different. The mesoblast never extends, in the rabbit, rat, or mouse, completely round the ovum, and the yolk-sac hypoblast remains long in contact with the trophoblast, and carries on the nutrition of the embryo till the tardily fortned allantoic placenta is de- veloped. Regarding the partial extension
. The physiology of reproduction. Reproduction. 388 THE PHYSIOLOGY OF REPRODUCTION much reduced, and was "presumably enveloped by the ; It had previously been shown by Turner that the yolk-sac disap- peared at an early period. RoDENTiA.âIn Rodents the conditions are entirely different. The mesoblast never extends, in the rabbit, rat, or mouse, completely round the ovum, and the yolk-sac hypoblast remains long in contact with the trophoblast, and carries on the nutrition of the embryo till the tardily fortned allantoic placenta is de- veloped. Regarding the partial extension of the mesoblast, Minot ^ says : " That it represents a modified condition is. â Eiit Fia. 83.âBlastodermic vesicle of the rabbit. (Minot.) cos, ocelom ; Oho, chorion (diplo-trophoblast); Yh, yolk-sac; mes, mesoderm ; vt, vena terminalis; Ent, entoderm ; Ec, ectoderm. evident, since in all non-mammahan Vertebrates both mesoderm and coelom extend completely round the yolk. Hence the com- plete separation of the yolk-sac in Man and the sheep is nearer the ancestral type than the relations of the extra-embryonic germ-layers to one another in the rabbit and ; In the rabbit, the mesoblast begins to spread out from the embryonic region about the end of the first week of gestation, and it gradually reaches half-way round the circumference of the blastocyst. It spHts into two layers over its whole extent, and it is limited below by the sinus terminalis (Fig. 83). The lower half of the yolk-sac is non-vascular, and its wall of hypoblast is closely invested by trophoblast. Later the yolk-sac begins to ^ Minot, Human Embryology, Boston, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Marshall, F. H. A. (Francis Hugh Adam), 1878-1949; Cramer, William, 1878-1945; Lochhead, James. London, New
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