. The physiology of reproduction. Reproduction. F(ETAL NUTRITION: THE PLACENTA 389 shrink, taking a raushroom shape, and its,vascular haK comes against the non-vascular half (Fig. 84). The specially large ccslomic space, thus left by the shrinking of the vesicle, is filled with fluid through which the aUantois extends to reach the part of the wall not covered by the yolk-sac. Hence at this stage the whole wall of the blastocyst is vascularised, one half by the vitelhne and the other half by the allantoic vessels.^ From- an investigation of the early stages in the mouse and. Fig. 84 —Diagram of
. The physiology of reproduction. Reproduction. F(ETAL NUTRITION: THE PLACENTA 389 shrink, taking a raushroom shape, and its,vascular haK comes against the non-vascular half (Fig. 84). The specially large ccslomic space, thus left by the shrinking of the vesicle, is filled with fluid through which the aUantois extends to reach the part of the wall not covered by the yolk-sac. Hence at this stage the whole wall of the blastocyst is vascularised, one half by the vitelhne and the other half by the allantoic vessels.^ From- an investigation of the early stages in the mouse and. Fig. 84 —Diagram of the blastodermic vesicle of the rabbit in longitudinal section. (From Hertwig's Entwicklungsgeschichte des Menschen und der Wirbelthiere.) e, embryo ; a, amnion ; al, aUantois with blood-vessels; fd, vascular layer of mushroom-shaped yolk-sac; , cavity of yolk-sac; , sinus terminalis ; r, large space filled with fluid. rat, Robinson ^ attaches much importance to the yolk-sac in providing for the nutrition of the embryo. On the seventh day the yolk-sac is large, and becomes invaginated with the inver- sion of the germinal layers (see p. 438). Outside its thin wall hes extravasated maternal blood, which is absorbed into the cavity. Over a large area, the wall of the yolk-sac becomes villous with a covering of columnar hypoblast. Over a small ' Hertwigi EntwicMungsgeschichte des Menschen und der Wirbelthiere, 1906. ^ Robinson, " The Nutritive Importance of the Yolk-Sac," Jour, of Anat. and Phya., vol. xxvi., 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 York, Longmans, Green
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