218 FREDERICK TILNEY 



longitudinal growth of the inter-renal segment of the cava as 

 compared with the post-renal segment of the aorta. The shift- 

 ing of the sex veins from their more primitive point of inoscula- 

 tion may be due to a caudal migration of the angle of confluence 

 of these vessels with the post-renal segment of the cava or it 

 may be the result of certain mechanical changes due to the caudal 

 migration and descent of the testis. 



The intra-hepatic portion of the right omphalo-mesenteric vein 

 has gained ascendency over all the venous spaces of the liver 

 and appears as a definite channel situated in the right dorso- 

 mesial angle of that organ. It constitutes the hepatic portion 

 of the post-cava. At the cephalic pole of the liver the vessel 

 is large, receiving, in this region, the two major revehent trunks 

 which drain the hepatic sinusoids. Immediately below the inos- 

 culation of these revehent vessels the cava diminishes in size, 

 taking up as it proceeds caudad, several lesser, hepatic revehent 

 tributaries. When the caudal pole of the liver is reached the 

 vessel swings slightly mesad and dorsad, to pass over into the 

 sub-cardinal portion of the cava. The mesenteric portion of the 

 omphalomesenteric vessel has now become the portal vein and 

 drains into one of the largest advehent branches of the um- 

 bilical channels. The post-caval drainage line thus utilizes the 

 right intra-hepatic portion of the omphalomesenteric vein in pass- 

 ing through the liver, and the right sub-cardinal vein as far 

 caudad as the inter-renal segment. 



The umbilical drainage system presents itself as the typical 

 single channel of foetal life. It makes its way through the um- 

 bilical fissure of the liver and then enters that organ. In the 

 liver it breaks up into the rich plexus of the umbilical portal 

 system. The ductus venosus is given off from one of the main 

 stems of this plexus and passes obliquely upward to enter the 

 post-cava in common with the confluence of the major hepatic 

 revehent veins. These observations apply equally to the 20 mm. 

 and 23 mm. embryos. 



