268 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



a pair of vitelline veins. These join the umbilical veins and, as the vitello-umbili- 

 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 umbilical arteries now take their origin from a plexus 



Dorsal intersegmental arteries 



Umbilical arteries 



Ant.cardinal veins 



Descending aortae 



Aortic arch I 

 \rteart 



Wife/lo- umbilical frunK 

 Wife/line veins 

 \YoIK-sac 



Fig. 262. — Diagram, lateral view, of the primitive blood-vessels in embryos of 2 to 2.5 mm. (adapted 



from Felix). 



Body-stalk 



Umbilical vein 



Vitelline arteries 



Posterior cardinal veins 

 Vitelline artery , 



Ant. cardinal veins 



descending aorta 



Umbilical arteries 



Sady-stc Ik 



fi t 4& t Aortic 

 arches 



Umbilical veins 



Wear* 



Vitelline Veins * VS/nu5 Venosus 

 Fig. 263. — Diagram of the blood-vessels of embryos with 15 to 23 somites (modified from Felix). 



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. 263) 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 



