468 David M. Davis. 
plates vessels extending still farther and draining the oesophagus 
and other tissues between the two aortic roots. This area about 
corresponds with the posterior mediastinum in the adult. In 
Plate III, the slightly greater size of that portion of the right sub- 
cardinal draining the caval mesentery is apparent. Now, return- 
ing to fig. 2, we can understand the situation. The factors are, 
first, the hepatic capillaries reaching down into the caval mesen- 
tery, draining their blood into the hepatic veins, vena hepatica 
communis, and sinus venosus; and, second, the subcardinal capil- 
laries, pushing out through the caval mesentery toward the liver. 
The consequence is obvious; growing toward each other, they 
come into contact, and inosculate. It was at this period of devel- 
opment that Zumstein made his observation—and Plate V shows 
the* condition when the new channel has become somewhat 
enlarged. The sectional appearance is shown in fig. 3, where it is 
seen that hepatic tubules still obstruct the caval lumen to some 
extent. The changes at this period, both progressive and regres- 
sive, are very rapid. The division of the posterior cardinals into 
anterior and posterior portions has already commenced. It soon 
becomes complete (Plate VI), all the blood from the posterior por- 
tion of the body pouring through the new channel. The enlarge- 
ment is rapid, the condition shown in Plate VI (16.2 mm.) being 
fully reached in a remarkably short time—about two days; a 
closer estimate of the period of change is difficult to arrive at in 
the case of pigs. In Plate V (9.5 mm.) the ultimate course of the 
complete cava is well seen—it is formed by the subcardinal for 
about half the length of the mesonephros; from there caudad it is a 
part of the right posterior cardinal. The designations of pars 
hepatica, pars subeardinalis, and pars cardinalis, are self-explana- 
tory. Of course, the relative length of the pars subcardinalis 
becomes greatly reduced as the liver grows downward and the 
thorax expands. 
The connections of the veins of the caval mesentery, which aided 
in the formation of the cava, remain as they were originally—as a 
result we have a good sized vein flowing from the oesophageal 
(posterior mediastinal) region into the cava just where it bends 
ventrally to enter the liver (Plates V and VI). The network of 
S 
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