368 THE DEVELOPMENT OF THE CHICK 



vessels. (On the right side similar connections appear, according 

 to Brouha, but as the entire right umbilical vein soon degenerates 

 thev need not be considered farther.) The blood of the left um- 

 bilical vein thus divides and part flows into the duct of Cuvier by 

 way of the original termination, and part flows through the liver 

 into the meatus venosus. The original connection is then lost 

 and all of the blood of the umbilical vein flows through the liver 

 into the meatus venosus. Although the intrahepatic part is 

 at first composed of several channels, yet the blood of the um- 

 bilical vein flows fairly directly into the meatus venosus, and 

 thus takes no part in the hepatic portal circulation. On the 

 eighth day the entrance of the umbilical vein into the cephalic 

 part of the meatus venosus is still broken into several channels 

 by liver trabeculae (Fig. 182) ; these, however, soon disappear, 

 and the vein then empties directly into the meatus venosus, which 

 has in the meantime become the terminal part of the inferior 

 vena cava. As the ventral body-wall closes, the umbilical vein 

 comes to lie in the mid-ventral line, and in its course forward it 

 passes from the body-wall in between the right and left lobes 

 of the liver. The stem of the umbilical vein persists in the adult, 

 as a vein of the ventral body-wall opening into the left hepatic 

 vein. 



The System of the Inferior Vena Cava (Post-cava). The 

 post-cava appears as a branch of the cephalic portion cf the meatus 

 venosus, and in its definitive condition the latter becomes its 

 cephalic segment; thus the hepatic and umbilical veins appear 

 secondarily as branches of the post-cava. The portion of the 

 post-cava behind the liver arises from parts of the postcardinal 

 and subcardinal veins, and receives all the blood of the posterior 

 portion of the body and viscera, that does not flow through the 

 hepatic portal system. The history of the development of this 

 vein, therefore, involves an account of (1) the origin of its proxi- 

 mal portion within the liver, and (2) of the transformation of the 

 postcardinals and subcardinals. 



The proximal portion of the post-cava arises in part from 

 certain of the hepatic sinusoids in the dorsal part of the liver 

 on the right side at about the stage of ninety hours, and in part 

 from a series of venous islands found at the same time in the 

 caval fold of the plica mesogastrica (Figs. 211 and 212. See 

 Chap. XI). As the caval fold fuses Avith the right dorsal lobe of 



