346 OUTLINES OF CHORDATE DEVELOPMENT 



omphalomesenteric vein; and now a large vein coming directly 

 from the posterior part of the yolk-sac similarly opens into the 

 same trunk. 



By the end of the fourth day the two omphalomesenteric 

 veins again anastomose still farther back, and now the inter- 

 mediate portion of the right vein disappears. The embryonic 

 course of the omphalomesenteric veins may therefore be de- 

 scribed as follows (Fig. 136) ; they enter the body symmetrically, 

 passing directly to the ventral side of the intestine just in front 

 of the anterior intestinal portal; here they fuse into a single 

 vessel which passes anteriorly around the left side of the intes- 

 tine to its dorsal surface and thence across to the right side, 

 where it enters the liver. 



This portion of the omphalomesenteric vein becomes the 

 trunk of the hepatic portal vein in the following manner. As 

 the vein passes through the liver to the ductus venosus, which 

 is now embedded in it, it branches abundantly supplying the 

 vascular spaces of the liver tissue, and soon the strands of liver 

 cells push into the large vessel so that it becomes entirely 

 broken up into small vessels and capillaries in the liver. The 

 ductus venosus then remains as the efferent vessel, or hepatic 

 vein, while the base of the omphalomesenteric vein itself be 

 comes the afferent hepatic vessel, the hepatic portal vein. This 

 arrangement is practically completed during the sixth day. 

 Before this time the veins of the digestive tract appear; these 

 collect into the mesenteric vein, which becomes the chief branch 

 of the hepatic portal. A typical subintestinal vein is indicated 

 the fourth day, coming from the tail and connecting with the 

 left omphalomesenteric vein; it soon disappears without taking 

 an essential part in the formation of any permanent venous 

 structure. 



Veins of considerable phyletic importance are the umbilical 

 veins, which represent the lateral veins of the Elasmobranchs 

 and the abdominal vein of the Amphibia. These appear early 

 in the body wall, primarily as the veins of the limb-buds, open- 

 ing into the ductus Cuvieri (Fig. 137). During the fourth day 

 they connect with the veins of the allantois, and shortly there- 



