422 THE PHYSIOLOGY OF REPRODUCTION 



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

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

 mesoderm and coelom 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 

 germ-layers to one another in the rabbit and 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 (Pig. 109). The specially large 



coelomic space, thus left by the 



shrinking of the vesicle, is filled 



'-^Ent with fluid through which the 



allantois extends to reach the part 



of the wall not covered by the 



yolk-sac. Hence at this stage the 



Fig. lOa-Blastodermic vesicle of ^^ole waU of the blastocyst • is 



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



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



trophoblast) ; Tk, yolk-sac ; m^, allantoic vessels.^ 



mesoderm ; v.t, vena terminalis ; 



Snt; entoderm ; Ec, ectoderm. From an mvestigation of the 



early stages in the mouse and 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 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 



hypoblast. Over a small area the trophoblast is thickened and 



maternal blood circulates in its spaces. But the allantois has 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 transniitted as well as by the vitelline vessels in 



I 



1 Minot, Human Embryology, Boston, 1892. 



2 Hertwig, Entwieklungsgesckichte des Menschen und der Wirhelthiere, 1906. 



^ Robinson, "The Nutritive Importance of the ydk-Sac," Jour, of Anat. and 

 Phys., vol. xxvi., 1892. 



