Veins in Pig Embryos. 469 
Pre-aortic 
\ Capillaries 
Ventricle 
Coelom 
. A.Subclavia 
Dextra 
Coelom 
Glomerular 
spaces of Wolffian 
body. 
Fie. 3. Embryo 9.5 mm. No. 2309. Sagittal section. 304. Showing the 
lumen of the newly-formed vena cava inferior still partly obstructed by liver 
cylinders. Just ventral to the aortic root are to be seen the forward prolongations 
of the subeardinal communicating with the peri-oesophageal plexus. 
veins surrounding the oesophagus has been called in this paper the 
peri-oesophageal plexus. In tracing the history of this peri-oeso- 
phageal plexus, it will be noted, first, that, while the subcardinal- 
portal anastomoses mentioned above, which lie caudal to the cir- 
cum-intestinal rings of the portal system, become completely 
obliterated before the vena cava appears; second, these other 
anastomoses, by way of the peri-oesophageal plexus and gastric 
veins, remain patent during the entire period, and do not show any 
special enlargement. This would indicate some inherent advan- 
tage in the route through the caval mesentery. These connections 
through the gastric veins are also noteworthy as the forerunners of 
the vessels making up the gastro-oesophageal collateral channel 
in portal obstruction. 
It would, then, appear that the vena cava is no exception to the 
rules governing the development of other vessels—having its 
anlage in capillary sprouts at the critical point in the caval 
