THE DEVELOPMENT OF THE VASCULAR SYSTEM 



219 



trunk, the two vitelline veins, however, remaining separate. In each 

 branchial arch on each side a vessel develops which joins with the corre- 

 sponding dorsal aortic root. These vessels the aortic arches arise from a 

 single vessel on each side ventral to the pharynx which is known as the 

 ventral aortic root. The two ventral aortic roots arise from a single medial 



FIG. 197. Dorsal surface view of chick embryo with 18 segments, including the area vasculosa. 

 Photograph, X 15. The blood vessels were injected with India ink, the dark blotch in 

 the upper left corner indicating some ink which escaped during the injection. 



vessel, the aortic trunk, or truncus arteriostis, which in turn is a continuation 

 of the early tubular heart. 



The heart, having developed and become a contractile organ in the 

 meantime, receives the blood in its caudal end through the vitelline veins 

 and ejects it from its cephalic end through the aortic trunk. The blood 

 then passes through the aortic arches to the dorsal aorta whence it is dis- 

 tributed to the vitelline plexus by the vitelline arteries. The blood is 



