332 



COMPARATIVE ANATOMY 



the yolk-sac, and uniting into a single trunk before opening into 

 the heart. As the liver becomes developed, a portal circulation 

 arises, and the main trunk of the vein, where it passes through the 

 liver, disappears. In the meantime, the coeliac and mesenteric 

 veins have become developed, and all the blood from them, as well 

 as from the vitelline veins, now passes through a common trunk, 

 the hepatic portal vein, into the capillaries of the liver, whence it 



Fig. 270, c. — Reference to lettering on previous page. 



reaches the sinus venosus through the hepatic veins. The vitelline 

 veins gradually disappear as the yolk-sac becomes reduced. 



In addition to these vessels, the umhilical vein must also be 

 mentioned. This vessel is originally paired, and corresponds 

 genetically to the lateral veins of Elasmobranchs and to the 

 abdominal or epigastric vein of Ceratodus and Amphibians. It 

 is situated originally in the body-walls, and comes into rela- 

 tion with the allantois (pp. 9 and 337), opening eventually into the 



