THE DEVELOPMENT OF THE VASCULAR SYSTEM. 239 



the area vasculosa converge to form another pair of vessels, the omphalomesen- 

 teric (vitelline) arteries, which extend into the embryo through the splanchno- 

 pleure until they reach a point ventro-lateral to the notochord, thence extend 

 cranially and caudally as the two primitive aorta. Later as the germ layers 

 close in ventrally, these fuse longitudinally, except in the cervical region, to 

 form the single dorsal aorta. The proximal ends of the omphalomesenteric 

 arteries also fuse into a single trunk which may then be considered as a branch 

 of the aorta. The double portion of the aorta in the cervical region sends a 



Aortic arches 



Sinus terminalis 



Ant. cardinal 

 vein 



Aorta 



Sinus 

 venosus 



Right vitelline vein 

 Right vitelline artery 



Duct of Cuvier 

 Post, cardinal vein 

 Left vitelline artery- 



Left vitelline vein 



Fig. 213. — Diagram of the vitelline (yolk) circulation of a chick embryo at the end 

 of the third day of incubation. Balfour. 



branch ventrally on each side through each branchial arch, forming the aortic 

 arches. The aortic arches on each side then unite ventrally to form the ventral 

 aortic root, and the two ventral aortic roots unite in the medial line to form the 

 single ventral aortic trunk which joins the cranial end of the heart (p. 224). 

 Thus the vitelline (or yolk) circulation is completed. And from this time on, the 

 area vasculosa gradually enlarges, as the mesoderm extends farther and farther 

 around the yolk, until finally it surrounds the entire mass of yolk. 



In Mammals, as in the chick, the vascular anlagen first appear in the extra- 

 16 



