THE PRIMITIVE BLOOD VASCULAR SYSTEM 



26l 



mon venous trunks are taken up into its wall, and thus three veins open 

 into each horn of the sinus venosus: (i) the umbilical veins from the 

 chorion; (2) the vitelline veins from the yolk sac; (3) the common cardinal 

 veins from the body of the embryo. 



The descending aorta have now fused caudal to the seventh interseg- 

 mental arteries and form the single dorsal aorta as far caudad as the origin 

 of the umbilical arteries. 



First cervical artery. 

 Pulmonary artery 

 Ant. cardinal vein 

 Post, cardinal vein 



Bulbus cordis 

 Ophthalmic artery 

 Ant. cerebral artery 



Common cardinal vein 



Vitelline artery 

 (Superior mesenteric) 



Caudal artery 

 Umbilical artery 



Subclavian artery 

 Casliac artery" 



Dorsal aorta 



Inf. mesenteric artery 



FIG. 271. Arteries and cardinal veins of the right side in a 4.9 mm. human embryo (modified 

 after Ingalls). X 20. H, Heart; I-VI, aortic arches. 



Of the numerous vitelline arteries one pair is prominent; it fuses into 

 a single vessel which courses in the mesentery and later becomes the 

 superior mesenteric artery. By the enlargement of capillaries connecting 

 the ventral and dorsal aortae, a second pair of aortic arches is formed at 

 this stage (Fig. 270). 



In embryos 4 to 5 mm. in length, five pairs of aortic arches are 

 successively developed: the first, second, third, fourth, and sixth (Fig. 

 271). An additional pair of transitory vessels, which extend from the 

 ventral aorta to the sixth arch, appear later in embryos of 7 mm., 

 but soon degenerate (Fig. 272 B}. They are interpreted as being the 



