432 OUTLINES OF CHORDATE DEVELOPMENT 



extra-embryonic mesoderm then develops very slowly pushing 

 down between the ectoderm and endoderm, but limited to the 

 upper half of the vesicle (Fig. 176). The yolk-sac in the rabbit 

 is therefore strictly splanchnopleural only in its upper half; 

 below this the chorionic ectoderm is in direct contact with the 

 yolk-sac endoderm. When the exoccelom expands so consider- 

 ably as it does in the rabbit, the yolk-sac is compressed and 



va 



FIG. 181. Area vasculosa (yolk-sac circulation) of an eleven-day rabbit. 

 After Van Beneden and Julin. Veins black, arteries white, s, Marginal sinus 

 (sinus terminalis) ; va, vitelline artery; vv, vitelline veins. 



reduced to an umbrella-shaped structure with a very narrow 

 cavity, connected with the embryonic gut by a long narrow 

 yolk stalk, the wall of which includes mesoderm as well as 

 endoderm (Fig. 180). 



As in the chick the mesodermal wall of the yolk-sac becomes 

 very vascular. Its rich network of blood vessels is supplied 

 by the vitelline or omphalomesenteric arteries arising from the 

 dorsal aorta (Fig. 181) . After spreading over the surface of the 

 sac these collect into a well-marked sinus terminalis, which is a 

 complete ring in the rabbit, and from which the blood is re- 

 turned to the embryo through the vitelline or omphalomesenteric 



