THE EARLY DEVELOPMENT OF THE MAMMAL 431 



cerned with placentation and will be considered in connection 

 with the development of the placenta. 



3. The Yolk-sac 



We. may summarize what has already been said concerning 

 the yolk-sac and add a few facts of importance, chiefly from 

 the comparative standpoint. We are to think of the yolk-sac 

 of the Mammals as typically occupying the chief part of the 

 early blastodermic vesicle, its cavity opening widely into the 

 embryonic gut by the broad yolk stalk, and its wall separated 



FIG. 180. Diagrammatic section through the fully formed blastodermic vesi- 

 cle of the rabbit, showing the reduced yolk-sac. From Hertwig (Lehrbuch, etc.) 

 after Bischoff. a, Amnion; al, allantois; ds, yolk-sac; e, embryo; ed, ed f , ed" 

 yolk-sac endoderm; fd, vascular layer (mesoderm) of yolk-sac; pi, villi; r, exo- 

 coelom; st, sinus terminalis; u, allantoic stalk. 



from the chorion by the exoccelom (Figs. 161, 179). The yolk- 

 sac is thus a splanchnopleuric structure, in contrast to the 

 somatopleuric amnion and chorion. This typical relation, how- 

 ever, is subject to varied and often profound modification. In 

 the rabbit we have seen that the endodermal wall of the vesicle 

 develops slowly, so that for a considerable early period the yolk- 

 sac is incomplete ventrally. Finally the endoderm does form 

 a complete sac, in contact with the chorionic ectoderm; the 



