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 ; d.s, cavity of yolk-sac ; s.t, 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., 1892. 



