422 THE PHYSIOLOGY OF REPRODUCTION 



of the mesoblast, Minot 1 says: "That it represents a modified 

 condition is evident, since in all non-mammalian Vertebrates both 

 mesoderm and coBlom extend completely round the yolk. Hence 

 the complete separation of the yolk-sac in man and the sheep is 

 nearer the ancestral type than the relations of the extra-embryonic 

 genii-layers to one another in the rabbit arid opossum." 



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 blasto- 

 cyst. It splits into two layers over its whole extent, and it is 

 limited below by the sinus terminalis (Fig. 108). 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 shrink, taking a 

 mushroom shape, and its vascular 

 half comes against the non-vascular 

 half (Fig. 109). The specially large 

 ccelomic space, thus left by the 

 shrinking of the vesicle, is filled 

 -Ent with fluid through which the 



allantois extends to reach the part 

 'Ec 



of the wall not covered by the 



yolk-sac. Hence at this stage the 



FIG. lOa-Blastodermic vesicle of whole wal1 of the blastocyst is 



the rabbit. (Minot.) vascularised, one-half by the vitel- 



coe, Cojlom ; Cho, chorion (diplo- line and the other half by the 



trophoblast) ; Yk, yolk-sac ; mes, a li an f o j VPSS pl s 2 

 mesoderm ; v.t, vena terminalis ; 

 A'///, entoderm ; E<\ ectoderm. From an investigation of the 



early stages in the mouse and rat, 



Robinson 3 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 inversion of the germinal 

 layers (see p. 468). Outside its thin wall lies 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 

 !i\loblast. Over a small area the trophoblast is thickened and 

 maternal blood circulates in its spaces. But the allantois lias not 

 yet come in contact with it, and the blood "must serve only for 

 the nutriment of the trophoblast itself." At the eleventh day the 

 trophoblast is vascularised by the allantoic vessels, by which the 

 nutriment is now transmitted as well as by the vitelline vessels in 



1 Minot, Human Embryology, Boston, 1892. 



! Hertwig, Entwicklungsgeschichte des Menschen und der Wirbelthiere, 1906. 

 3 Robinson, "The Nutritive Importance of the Yolk-Sac," Jour, of Ana f. ,><1 

 I'h>/*., vol. xx vi., 1892. 



