PRIMITIVE BLOOD VASCULAR SYSTEM 



269 



on either side of the brain they join the vitello-umbilical 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 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 



l& Cervical artery 

 Pulmonary artery 



Vertebral artery 



Vena capitis 

 media 



Com. card- 

 mat vein 



Porsal 

 aorta 



Caudal artery 

 Umbilical artery 



'Inf. mesenteric artery 



Fig. 264. — Arteries and cardinal veins of the right side in a 4.9 mm. human embryo (modified after 

 Ingalls). H, heart; /, II, III, IV, and VI, first, second, third, fourth, and sixth aortic arches. 



vitelline veins from the yolk-sac; (3) the common cardinal veins from the body of 

 the embryo. 



The descending aortas, have now fused caudal to the seventh intersegmental 

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

 bilical arteries. 



Of the numerous vitelline arteries, one pair are prominent and they fuse to 

 form the single vessel which courses in the mesentery and later forms the superior 



