CIRCULATORY SYSTEM OF THE GRASS-SNAKE. 627 



point where tliey open into the sinus venosns. An anastomosis 

 between the two omphalo-mesenteric veins forms on the dorsal 

 side of the gut just posterior to the pancreas rvidiment, and the 

 portion of the left vein between this point and the sinus venosus 

 disappears. A similar anastomosis between the two veins now 

 forms on the ventral side of the gut, and thus a complete ring is 

 formed. In a short time, however, the right half of this ring 

 disappears, leaving a single vein which runs in a spiral manner 

 round the gut. While these latter changes are taking place, the 

 middle part of the right omphalo-mesenteric vein between the 

 sinus venosus and the dorsal anastomosis spreads out and forms a 

 venous network in the liver. The portion of the right omphalo- 

 mesenteric vein in front of the hepatic network persists as the 

 hepatic vein, and the part behind it always remains as the anterior 

 end of the hepatic portal vein (text-fig. 90). 



Text-fiij. 90. 



Diagram of posterior veins in the embryo. It shows the change from an early to 

 a late embrj'onic condition. The shaded vessels being the first to disappear. 



A.C. Alimentary canal. D.C. Ductus Cuvieri. H.N. Hepatic network. L.O. 

 Left omphalo-mesenteric (soon disappears). L.U. Left umbilical. 0. Omphalo- 

 mesenteric. P.C. Post-caval. R.O. Right omphalo-mesenteric (disappears later 

 than L.O.). K.U. Right umbilical. S.V. Sinus venosus. 



Adapted from Hochstetter (20) . 



The posterior cardinal vein originates at the caudal end of the 

 mesonephros, and runs forward along the dorso-lateral edge of 

 that body. After leaving the kidney, however, it breaks into 

 several branches, which soon reunite and then it runs into the 

 ductus Cuvieri. When the caudal vein develops it divides into 

 two branches at its anterior end, each of which runs to the 

 extremity of the corresponding cardinal vein (not along the mesial 

 wall of the kidney as in Lacerta). The post-caval vein springs 

 from the union of the right umbilical and omphalo-mesentei-ic 



