474: Anatomy and Development of Veins of Chelonia 



wall, as it docs in birds, but is merely the result of the fusion of the two 

 branches of the right and left umbilicals, as shown by Fig. 8. Further- 

 more, in turtles, it is only a temporary condition and merely marks one 

 stage in the gradual shifting of the position of the umbilical veins. 



The right umbilical vein craniad of its connection with the median 

 vein continues forward for a short distance in the body-wall and finally 

 disappears (Fig. 8). 



The left umbilical vein continues still further forward and enters the 

 liver a little to the left of the mid-line (Fig. 9). It now forms an 

 entirely new channel through the liver and opens into the gastric vein 

 mentioned in the earlier part of this paper. The gastric vein, as we re- 

 member, opened at first into the ventral hepatic anastomosis between the 

 two omphalo-mesenteric veins, then into the right omphalo-mesenteric 

 vein. After the mesenteric portal vein is split oS. from the right hepatic 

 the gastric vein is split off with it, so that in the present 

 stage it opens into the mesenteric portal vein. That part of the gastric 

 vein which conveys the blood from the left umbilical vein to the mesen- 

 teric portal becomes very much larger than the other part and may be 

 considered as the continuation of the umbilical vein. We may, therefore, 

 now say that the left umbilical vein opens directly into the mesenteric 

 portal; and that one of its branches is the gastric vein. The small 

 common trunk formed by the union of the gastric and left umbilical 

 veins is the first anlage of the portal vein of Bojanus. 



The original path of the left umbilical vein through the liver now 

 becomes a part of the left hepatic vein, the terminal end of which, as wo 

 have already seen (Fig. 6, Z), is split off from the right omphalo-mesen- 

 teric vein. The left hepatic vein is, therefore, formed from two parts : 

 one, the terminal portion, is split off from the right omphalo-mesenteric 

 vein, while the other portion is the new path formed through the 

 sinusoids of the liver by the left umbilical vein when it first entered that 

 organ. 



The ground-plan arrangement of the veins of the liver has now been 

 laid down. The chief events in the further development of the uml)ili- 

 cal veins and their derivatives arc as follows: 



The right umbilical vein makes direct connections with the mesen- 

 teric portal, or with one of its branches. 



The portal vein of Bojanus lengthens as the liver increases in width. 



The common hepatic vein continually grows shorter, being absorbed 

 by the backward extension of the sinus venosus. The result is that the 

 left hepatic vein gradually approaches the sinus venosus and eventually 

 opens into it. A still further caudal extension of the venous sinus 



