THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



191 



importance in supplying the growing embryo with nutritive materials. In 

 mammals the vitelline circulatory system develops as extensively as in the 

 lower forms but, since little yolk is present, does not assume the same impor- 

 tant role of carrying food supply; yet the portions of the vessels inside the em- 

 bryo, viz. : the heart, aortic arches, aorta, the proximal part of the vitelline 

 artery, and the vitelline veins, form parts of the permanent vascular system. 

 In reptiles and birds a second set olyessels develops in connection with 

 the allantois and serves to carry away the waste products of the body and 

 deposit them in that sac-like structure. Two arteries, one on each side, 



Yolk stalk 



Allantois 



Umbilical artery 

 Umbilical vein 



Amnion 



Chorionic villi 



FIG. 163, Diagram of the umbilical vessels in the belly stalk and chorion. Kollmann's Atlas. 



arise as branches of the dorsal aorta near its caudal end and pass out of the 

 body along with the allantoic duct to ramify upon the surface of the allantois. 

 These are the umbilical, or allantoic, arteries. The blood is collected and 

 carried back by the umbilical veins which pass along the 'allantoic duct to the 

 body and then forward, one on each side, through the somatic layer of 

 mesoderm to join the ducts of Cuvier. The duct of Cuvier, formed on each 

 side by the junction of the anterior and posterior cardinal veins, which will 

 be considered in a subsequent section, pour their blood into the sinus venosus. 

 This venous trunk is formed by the junction of the ducts of Cuvier with -the 

 vitelline veins and empties directly into the heart. 





