346 HISTORY OF THE HUMAN BODY 



in stage e, in which the left limb of the loop (ss of stage d), 

 has for the most part disappeared, while the right has become 

 nearly median, and has thus straightened the entire vessel. 

 The left spermatic (or ovarian) vein, which in stage d enters 

 the left limb of the loop, has caused the retention of that part 

 through which its connection with the main system was origin- 

 ally established, while the right spermatic vein enters the post- 

 cava directly, since this was originally the part to which it 

 was attached. 



This last diagram (e) is taken from the human embryo, 

 since in man the relation of the iliacs differs considerably 

 from that in the rabbit, from which the other diagrams of 

 this series are taken. In other respects there is no appreciable 

 difference between the two forms. 



This embryological history explains the composite structure 

 of the postcava as seen in the adult. Anteriorly a sprout 

 from the liver capillaries, it is composed more posteriorly cf 

 a vein connected with the erftbryonic kidney and a portion of 

 the right posterior cardinal, and to this is added, still more 

 posteriorly, the caudal vein, primarily a portion of the sub- 

 intestinal. 



Concerning the abdominal vein, which seems to appear in 

 the amphibians as suddenly as does the postcava, the em- 

 bryology of urodeles shows it first in the form of paired lateral 

 vessels lying in the body wall and emptying into the duct.us 

 Cuvieri without connection with the liver. Its embryonic 

 position and relationships thus render it probable that this 

 vein is the same as the lateral vein of fishes, which likewise 

 runs in the body wall and empties into the ductus Cuvieri 

 or near it. The connections of this vein with the iliacs pos- 

 teriorly and with the liver anteriorly appear later on in em- 

 bryonic development and are thus shown to be secondary 

 modifications, and not features of the original vein. 



Above the amphibians there is nothing which at first sight 

 resembles an abdominal vein, but the two lateral elements 

 of which it is composed are probably identical with the simi- 

 larly related umbilical veins, which in the embryo supply 



