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 aortse give off dorsally and cranially several pairs of dorsal 

 intersegmental arteries and ventrad and caudad a series of non-segmental mtelline 

 arteries to the yolk-sac. The umbilical arteries now take their origin from a plexus 



Umblli 



Dorsal intersegmental arteries 

 ilical arteries 



Ant. cardinal veins 



Descending aorf&e 



Body-sfalk 



Umbilical vein 



ortic arch I 



\Heart 



\\Ji1cllo- umbilical frunK 

 \VlteJHltt veins 

 i. Yolk-sac 

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



Vi-felline arteries 



from Felix). 



Posterior cardinal veins 

 VUdline artery 



Ant. cardinal veins 



andiny aorta 



Umbilical aneries 



Bcdy-5loiK / y \ / \ \ ^eart 



Umbilical veins ^^^___^-/ \ \ 



Vitelline Veins \ VjwS 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 



