456 Anatomy and Development of Veins of Chelonia 



The transformation of the paired oinpJialo-mesenteric veins into a 

 single vessel to form the anlage of the portal vein. — The two omphalo- 

 mesenteric veins fuse dorsal to the intestine just caudad of the first 

 anlage of the pancreas to form the dorsal anastomosis and again further 

 back ventral to the intestine to form the posterior ventral anastomosis. 

 A ring is thus formed around the intestine which, however, is soon broken 

 by the atrophy of that part formed by the right vein. In the meantime 

 both omphalo-mesenteric veins craniad of the dorsal anastomosis are 

 more or less broken up into smaller vessels by the liver tubules. In 

 snakes, the left omphalo-mesenteric vein loses its connection with the 

 sinus venosus before the posterior ventral anastomosis takes place, while, 

 on the other hand, in lizards this connection is retained until the for- 

 mation of the ring mentioned above is completed. In both reptiles it is 

 the right vein that persists in front of the dorsal anastomosis and the left 

 vein behind it. There is thus formed a single vessel, arising from the 

 vitelline veins at the umbilicus, which takes a spiral course around the 

 intestine and enters the right half of the liver where it breaks up to 

 form the hepatic network. The vein thus formed constitutes the anlage 

 of the portal vein. The cardiac end of the right omphalo-mesenteric 

 vein is retained as a part of the i-ight hepatic revelient vein. 



The formation of the postcava, and the reversal of the primitive renal 

 portal system. — The revehent veins of the primitive renal portal system 

 of reptiles are represented by the postcardinals. In lizards, the post- 

 cardinal veins arise in the caudal region of the body and extend forward 

 dorsal to the mesonephric ducts to join the precardinals and with the 

 latter form the ducts of Cuvier. They are broken up near the cranial 

 ends of the mesonephroi into a number of broad blood sinuses (Blut- 

 rdumen) and lose their dorsal position. In snakes, the postcardinals 

 are connected Avith the caudal vein from the first and do not lose their 

 integrity as distinct vessels at their cranial ends. 



The advehent veins are formed by the bifurcation of the caudal vein. 

 The two liranches thus formed extend forward along the ventromedial 

 border of the mesonephroi and appear to end blindly a short distance in 

 front of the origin of the omphalo-mesenteric artery. As a matter of 

 fact, however, they are connected with the postcardinals by means of the 

 mesonephric network. All of the blood from the caudal vein must, 

 therefore, in order to reach the heart, stream through the mesonephroi. 



The primitive renal portal system just described soon undergoes a 

 complete reversal. One of the most important events connected witli 

 this change is the formation of the postcava. This is brought about in 

 the following manner : first, the two branches of the caudal vein fuse 



