. The physiology of reproduction. Reproduction. FCETAL NUTRITION: THE PLACENTA 387 only in the early stages. In the dog the yolk-sac is large and extends at first to the end of the citron-shaped ovum (Fig. 97). According to Bischoff ^ it persists till birth, but this is denied by Duval.^ The allantois grows out on the dorsal side of the embryo, and fuses with the diplo-trophoblast over a small discoidal area. Later, as the cavity of the allantois en- larges, it adheres to the whole of the blastocyst waU except the poles. Subsequently the zone of adhesion is reduced in extent (see p. 413). Prob
. The physiology of reproduction. Reproduction. FCETAL NUTRITION: THE PLACENTA 387 only in the early stages. In the dog the yolk-sac is large and extends at first to the end of the citron-shaped ovum (Fig. 97). According to Bischoff ^ it persists till birth, but this is denied by Duval.^ The allantois grows out on the dorsal side of the embryo, and fuses with the diplo-trophoblast over a small discoidal area. Later, as the cavity of the allantois en- larges, it adheres to the whole of the blastocyst waU except the poles. Subsequently the zone of adhesion is reduced in extent (see p. 413). Proboscidea and Hyrax.— The elephant and the aberrant genus Hyrax have at full-time, hke the Carnivores, a zonary placenta, but Uttle is known regarding the development of the foetal mem- branes. Assheton^ has recently ^^^ 82-Transverse section given an account of two early through the blastocyst of the embryos of Hyrax. In the younger, the yolk-sac occupied about three- quarters of the surface of the blastocyst, and the allantois the remaining quarter, the ovum pos- sibly being whoUy embedded in the uterine mucosa. The yolk-sac was covered with a network of vessels, and the head of the embryo dipped into it. It was invested externally with a mass of trophoblastic cells, honey- combed with spaces and filled with maternal blood, but no vilU were developed. In the second embryo the yolk-sac was ' Bischoft, Entwickelungsgeachichte der Saugethiere und des Menschen, 1842. ^ Duval, "Le Placenta des Carnassiers," Jour, de I'Anat. et de la Phys., 1893. ^ AsshetoD, Phil. Trans. Roy. Soc, London, loc. sheep at the twenty-fifth day. (From Assheton, " The Mor- phology of the Ungulate Placenta," Phil: Trans. Roy. Soc, London, Ser. B., vol. cxcviii., 1906.) A., allantois ; A S, splanchno- pleur of allantois; , allan- toic blood-vessel; C, coelom ; V, commencing folds from which villi spring; Y, solid Please note that these images are extracted from scann
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