458 Anatoniv and Dcvclopincni of \'cins of Chelonia 



into this left branch of the ])Ostcava, while the right cranial nicsoncphric 

 vein opens directly into the main stem of the postcava. 



Development of the posterior vertebral veins. — The degeneration of 

 the cranial portions of the mesonephroi is accompanied by the gradual 

 atrophy of the postcardinal veins from before backward. As these veins 

 degenerate, their dorsal intersegmental branches fuse ventral to the ribs 

 to form a ])air of longitudinally running vessels, the posterior vertebral 

 veins of the adult. They arise in the caudal region of the body and 

 extend along its dorsal wall, one on each side of the vertebral column, 

 to open into the subclavian veins. A connection Ijetween the liver circu- 

 lation and the posterior vertebral veins is also made by means of two or 

 three branches. 



The posterior vertebral veins are, at first, symmetrically developed on 

 each side of the body in snakes just as they are in the lizards. In snakes, 

 however, anastomoses early develop between this pair of veins and the 

 veins of the oesophagus, stomach, and portal system, which soon result 

 in the degeneration of the vertebral veins, so that they are represented 

 in the adult system only as fine anastomoses between the intercostal 

 veins. According to Eathke. they may in some cases disappear alto- 

 gether. Hochstetter attriliutes the degeneration of these veins to the 

 peculiar locomotion of snakes. 



Advelicnt veins of the permanent kidneijs. — The caudal portion of the 

 postcardinal veins in snakes, as well as in lizards, is still retained in the 

 adult system as the advehent veins of the permanent kidneys. Since the 

 permanent kidneys arise dorsal to the mesonephroi, the persisting por- 

 tions of the postcardinal veins are found on their ventral surfaces, but 

 still retain their original position dorsal to the mesonephric ducts. 



Tlie nmhilical veins and their transformations. — The umbilical veins 

 of a very young embryo of the lizard arise in the allantois and enter the 

 body through the urribilicus. In the umbilicus they unite to form a 

 single vein, l)ut separate again on entering the body. They extend for- 

 ward in the body-wall and 0]>en into the sinus venosus together with the 

 omphalo-mesenteric vein of the same side. At the umlnlicus they receive 

 a pair of small veins which extend backward along the ventral body-wall 

 to join the postcardinals. This small pair of veins constitutes the 

 posterior anlage of the abdominal veins. In a later stage than this just 

 described, the left umbilical vein connects with the hepatic network. 

 Soon a direct channel is formed through the liver between the left um- 

 bilical vein and the hepatic revehent vein. The left umbilical vein 

 completely degenerates craniad of the point where it enters the liver so 

 that all of the blood from this vein must pass through the liver in order 



