VEINS 309 



which ramify among the liver tubules, finally connecting with both 

 omphalomesenterics on the anterior side of the liver (fig. 328, B). 

 As the liver increases in size the network of sinusoids increases in 

 complexity, supplying all of the tubules. For a time the left ompha- 

 lomesenteric retains its primitive importance on the side of the 

 liver and is known as the ductus venosus (Arantii), but soon this 

 preeminence is lost and all blood coming from behind passes through 

 the network of ca pillaries in the hver be fore it enters the heart (fig. 

 328, O.'tSuch acapillary circulation occu rring in the course of a 

 vein is known as a portal systejB, and this one occurring in the hver 

 is the hepatic portal circulation. It consists of the vessels bringing 

 the blood to the liver (portal vein) — a part of the original omphalo- 



FiG. 329. — Diagram of embryonic circulation in a large-j-olked vertebrate; compare 

 with fig. 319. aa, aortic arches; al, allantois; an, anus; ca, cv, caudal artery and vein; 

 da, dorsal aorta; dc, Cuvierian duct; /;, heart; ha, hypogastric (allantoic) artery; », 

 jugular vein; /, liver; otna, om, omphalomesenteric artery and vein; pc, postcardinal vein; 

 si, subintestinal vein; st, sinus terminalis; va, ventral aorta; y, yolk; ys, yolk stalk. 



mesenteric — the capillary vessels, and the bases of both omphalo- 

 mesenterics, now known as the hepatic veins, which convey the blood 

 from the liver to the heart. 



In eggs with a large yolk (elasmobranchs, sauropsida) the presence of this 

 large food supply exercises a modifying influence on these ventral veins (fig. 

 329). From the junction of the omphalomesenteric and the subintestinal veins a 

 pair of large vitelline veins run out into the yolk sac, over the yolk, and play 

 a large part in the transfer of material to the growing embryo. The distal 

 parts of these veins follow the margin of the yolk sac, forming a tube (sinus 

 terminalis) into which smaller veins empty. Blood is brought to the yolk by 

 the omphalomesenteric arteries, which are also distributed to the yolk sac, 

 dividing up distally into a network of capillaries connecting distally with the 

 vitelline veins. By these latter the blood is carried to the liver and through 

 the portal circulation to the heart. In the mammals a similar vitelline circula- 



