Frank A. Stromsten 457 



just craniad of the origin of the oniphalo-mesenteric artery to form an 

 impaired stem. A short branch is then formed in the caval mesentery 

 which connects the unpaired stem just mentioned with the right hepatic 

 revehent vein in lizards, but with the common hepatic vein in snakes. 

 A continuous path is thus formed f]'om the caudal vein direct to the 

 heart, and the result is that most of the blood from the hinder parts of 

 the body now reaches the heart by this route. The new path thus formed 

 is the first anlage of the postcava. 



Craniad of the origin of the omphalo-mesenteric artery the postcava is 

 an unpaired vessel formed from the following parts: the fused portion 

 of the two branches of the caudal vein, the new formation in the caval 

 mesenter}', sinusoids of the liver, and the riglit hepatic revehent vein 

 (lizard), or the common hepatic vein (snakes). Caudad, the postcava 

 is paired and is formed by the two branches of the caudal vein. The 

 postcava loses its connection with the caudal vein, however, while at the 

 same time a strong anastoinosis is formed between the caudal vein and 

 the postcardinals. It thus appears that while the postcava is still con- 

 nected with the caudal vein, it may function for a short time both as a 

 revehent and as an advehent vein to the mesonephroi. 



Shortly after the postcardinals have joined the caudal vein they lose 

 their connection with the sinus venosus and the reversal of the primitive 

 portal system of the mesonephroi is thus completed. The postcardinal 

 veins which now receive the blood from the caudal vein act as advehent 

 vein^, while the postcava with its branches becomes the revehent system. 



Further changes in the postcava. — During the time the above changes 

 are taking place, the two branches of the postcava (original branches 

 of the caudal vein) have fused just caudad of the origin of the 

 omphalo-mesenteric artery so that this artery appears to pass through 

 a foramen in the unpaired postcava. The unpaired postcava craniad 

 of the omphalo-mesenteric artery receives a branch on each side from 

 the cranial regions of the mesonephroi. The left cranial mesonephric 

 branch then begins to split away from the main stem backward as 

 far as the origin of the omphalo-mesenteric artery so that finally this 

 artery no longer passes through the postcava, but to the left side of it. 

 The postcava now extends as an unpaired vessel from the sinus venosus 

 to the origin of the omphalo-mesenteric artery, immediately caudad of 

 which it divides into two branches. The right branch passes backward 

 as a direct continuation of the postcava, while the other branch bends 

 sharply to the left behind the origin of the omphalo-mesenteric artery, 

 and then proceeds backward. The left cranial mesonephric vein opens 



