Veins in Pig Embryos. A471 
trunks, such as the pulmonary, the carotid, and the limb arteries 
can be traced to their origin from a capillary plexus. From the 
same injections Miss Smith has shown clearly that the thoraco- 
epigastric vein is formed from certain early body wall capillaries. 
It is evident that the final act in the formation of the vena 
cava inferior furnishes another clear instance of a capillary plexus 
ancestry, for this portion of this great vessel. It has already been 
pointed out that Zumstein deserves the credit of first having seen 
these capillary connections, which are the precursors of the cava 
here, and more recently F. T. Lewis has described and figured 
the invasion of the caval mesentery by hepatic sinusoids. No 
one, however, has interpreted these phenomena in the cava’s 
development in accordance with the theory of the development 
of the vascular system from capillary plexuses. These injections 
show conclusively that here we are dealing with the fusion of 
two capillary plexuses, one, as already mentioned, from the he- 
patic vessels, the other from the cephalic portion of the right sub- 
cardinal vein; and the writer has chanced upon the stage in which 
the fusion has just taken place, and in which, consequently, 
for the first time a complete vascular path is afforded from the 
subeardinal to the hepatic channels. 
CONCLUSIONS 
The conclusions drawn from this study are as follows: 
1. Open connections exist between the subcardinal veins and 
portal system at an early stage, but they reach their maximum 
and are obliterated before the vena cava is formed. 
2. At the time that subecardinal capillaries form the anlage 
of the cava in the region of the caval mesentery they have pre- 
viously proliferated in a cephalic direction surrounding the ceso- 
phagus; consequently before the cava is formed this peri-cesphageal 
plexus drains into the cephalic tip of the v. subeardinalis, and 
with the formation and rapid enlargement of the cava it naturally 
comes about that this plexus is now drained into the latter vessel. 
3. At the critical period in the history of the vena cava, its 
channel is definitely formed by the fusion in the caval mesentery of 
capillary sprouts from the hepatic and subeardinal vessels, respec- 
