260 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



caudad, a series of vitelline arteries to the yolk sac. The umbilical arteries 

 now take their origin from a plexus of ventral vessels in series with the 

 vitelline arteries. At this stage the vitelline circulation of the yolk sac 

 is established. 



Dorsal inter segmental arteries Ant. 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 numan embryos of 2 to 



2.5 mm. 



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

 proper develop as longitudinal anastomoses of branches from the segmental 

 arteries. The paired anterior cardinal veins of the head are developed first, 



Posterior cardinal veins 

 Vitelline artery 



Ant. cardinal veins 



Descending aorta 



Umbilical arteries. 



^Aortic arches i and 2 

 Heart 



Sinus vcnosus 

 Vitelline veins 

 FIG. 270. Diagram of the blood vessels of a human embryo of 2.6 mm. 



Body stalk' 



Umbilical veins 



and, coursing back on either side of the brain, they join the vitello-um- 

 bilical trunk. In embryos of 23 somites, the posterior cardinals are present. 

 They lie dorsal to the nephrotomes, and, running cephalad, 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 com- 



