DEVELOPMENT OF THE ARTERIES 261 



plexus of ventral vessels in series with the vitelline arteries. At this stage the 

 vitelline circulation of the yolk sac is established. 



In embryos of 15 to 23 somites (Fig. 270) the veins of the embryo proper de- 

 velop as longitudinal anastomoses of branches from the segmental arteries. The 

 paired anterior cardinal veins of the head are developed first, and, coursing back 

 on either side of the brain, they join the vitello-umbiHcal trunk. In embryos of 

 23 somites the posterior cardinals are present. They lie dorsal to the nephrotomes, 

 and, running cranially, join the anterior cardinal veins to form the common cardinal 

 veins. Owing to the later enlargement of the sinus venosus, the proximal portions 

 of the common venous trunks are taken up into its wall and thus three veins open 

 into each horn of the sinus venosus: (1) 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. 



Posterior cardinal veins Ant. cardinal veins 

 Vitelline artery . / Descending aorta 



Umbilical arteries. 



^Aortic arches i and 2 

 Body stalk/ U \ ^v==^ / \ \ ^""''^ 



Umbilical veins v^^^^/ \ Sinus venosus 



Vitelline veins 

 Fig. 270. — Diagram of the blood vessels of human embryos of 2.6 mm. 



The descending aortm have now fused caudal to the seventh intersegmental 

 arteries and form the single dorsal aorta as far caudad as the origins of the um- 

 bihcal arteries. 



Of the numerous vitelline arteries one pair is prominent; these fuse 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); 



DEVELOPMENT OF TrfE ARTERIES 

 -Transformation of the Aortic Arches. — 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 



