26o 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



heart tubes paired vessels, the ventral aorta, extend cephalad, then bend dorsad as 

 the first aortic arches and extend caudad as the dorsal or descending aortce. These, 

 as the umbilical arteries, bend sharply ventrad into the belly stalk and branch 



Dorsal intersegmental arteries Descending aorta 

 Umbilical veins 



Umbilical arteries 



Body stalk 

 Umbilical vein 



Primitive aortic arch 

 Primitive heart 

 Vitello-umbilical trunk 

 Vitelline veins 

 Yolk sac 



Vitelline arteries 

 Fig. 268— Diagram, in lateral view, of the primitive blood vessels in human embryos of 1.5 to 2 mm. 



in the wall of the chorion. The chorionic circulation is thus the first to be es- 

 tabhshed. 



In embryos 2 to 2.5 mm. long (5 to 8 somites) the heart has become a single 

 tube (Fig. 269). From the yolk sac numerous veins converge cephalad and form 

 a pair of vitelline veins. These join the umbihcal veins, and, as the viteUo-umbili- 



Dorsal intersegmental arteries A nt. cardinal veins 



Descending aorta 

 Umbilical arteries 



Body stalk 



Umbilical vein 



Vitelline arteries 



Aortic arch i 

 Heart 

 Vitello-umbilical trunk 

 Vitelline veins 



Yolk sac 

 Fig. 269. — Diagram, in lateral view, of the primitive blood vessels in human embryos of 2 to 2.5 mm- 



cal trunk, traverse the septum transversum and open into the sinus venosus. 

 The descending aortae give off, dorsally and cranially, several pairs of dorsal 

 intersegmental arteries, and, ventrad and caudad, a series of non-segmental vitelline 

 arteries to the yolk sac. The umbihcal arteries now take their origin from a 



