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 
they 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 trabecule (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 of 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 posteardinal 
and subeardinal 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 
posteardinals and subeardinals. 
The proximal portion of the post-cava arises in part from 
certain of the hepatie 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 with the right dorsal lobe of 
