THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



187 



channels of the vascular area unite to form a vessel the sinus terminalis 

 which is continuous around the border except at the head end of the embryo 

 (Fig. 159). At the same time the vascularization of the visceral layer of 

 mesoderm gradually extends through the clear area of the blastoderm 

 (area pellucida) toward and finally into the embryonic body. Reaching 

 the region just lateral to the notocord, the vessels unite longitudinally in the 

 embryo to form a continuous channel, the primitive aorta, which thus con- 

 stitutes a natural selvage to the vascular area on each side of the blastoderm 

 (Fig. 159). Some of the channels of the vitelline plexus increase in size 

 and coalesce to form a large trunk which is a branch of the primitive aorta 



Ccelom 



Parietal mesoderm 



Ectoderm 



Visceral mesoderm 



Blood islands 



IG. 158. Section of blastoderm of chick of 42 hours' incubation. Photograph. The cells of 

 the blood islands are differentiated into primitive blood cells and the endothelium of 

 the vessels. 



on each side and leads off into the smaller vessels in the peripheral part of 

 the vascular area. This trunk is known as the vitelline, or omphalomesenteric, 

 artery and is at first located near the caudal end of the embryo. When cir- 

 culation is established through contractions of the heart it carries blood 

 from the aorta to the surface of the yolk sac (Fig. 159). Other channels of 

 the vitelline plexus nearer the head end of the embryo likewise form a large 

 trunk, the vitelline, or omphalomesenteric, vein which collects the blood from 

 the surface of the yolk sac and conveys it to the heart (Fig. 159). 



So long as the germ layers lie flat the two primitive aortae remain separate, 

 but with the ventral flexion and fusion of the germ layers to form the tubular 

 body the aortae fuse into a single medial vessel, the dorsal aorta, except in 

 the cervical region where the two original vessels persist as the dorsal aortic 

 roots. The proximal ends of the vitelline arteries also fuse into a single 



