482 Anatomy and Development of Veins of Chelonia 



subeardinal eraniad of the origin of the omphalo-mesenteric artery, and 

 (f) the fused subcardinals caiidad of this point. 



The posterior verlehral veins are formed in exactly the same manner 

 as in lizards except that the longitudinal fusion of the dorsal interseg- 

 mental branches of the postcardinals takes place dorsal to the rib anlages 

 instead of ventral to them. These veins persist in the adult form. 



The anterior and posterior advehent veins of the permanent kidneys 

 represent the remains of the postcardinals. The connection between the 

 anterior renal advehent vein and posterior vertebral vein is formed 

 through the persistence of one of the dorsal branches of the primitive 

 postcardinal. 



RESUME. 



In Chelonia the unpaired mesenteric portal vein is formed, in the 

 abdominal region, as in other reptiles, from the originally paired 

 omphalo-mesenterics. Within the liver it is split off from the right 

 omphalo-mesenteric vein. 



The cranial termination of the left hepatic revehent vein as well as 

 the greater part of the right revehent of the liver is also split off from 

 the right omphalo-mesenteric. 



Both umbilical veins enter the liver and at first open into the common 

 hepatic vein; later they join the portals. 



During development the umbilical veins are carried mediad by the 

 ingrowth of the connective tissue folds which are to form the pericar- 

 dium; and also enter the liver further and further caudad. Eventually 

 they come to form the pre-umbilical portion of the abdominals. The 

 portion of the abdominals behind the umbilicus is developed from a 

 pair of veins which extend forward along the ventral abdominal walls 

 from the iliac veins and open into the umbilicals at the umbilicus. 



The portal vein of Bojanus is not developed from the omphalo-mesen- 

 terics, but from a venous trunk formed by the union of the left umbilical 

 with the gastric vein. 



The postcava is formed from the following parts: (a) the common 

 hepatic vein, (b) the right hepatic revehent vein, (c) sinusoids of the 

 liver, (d) a new venous formation in the caval mesentery, (e) the right 

 subeardinal eraniad of the origin of the omphalo-mesenteric artery, and 

 (f ) the fused subcardinals caudad of this point. 



The vertebral veins develop as a longitudinal fusion of the dorsal 

 intersegmental branches of the postcardinals above the costal anlages. 



The caudal portions of the postcardinals are retained, as in other rep- 

 tiles, as the anterior and posterior advehent veins of the permanent 

 kidneys. 



