LATER DEVELOPMENT OF VASCULAR SYSTEM 367 
teric vein begins to form as a small vessel situated in the dorsal 
mesentery and opening into the omphalomesenteric vein behind 
the dorsal pancreas. This vein increases in importance as the 
development of the viscera proceeds, and becomes the definitive 
hepatic portal vein; it receives branches from the stomach, in- 
testine, pancreas, and spleen. The development of these branches 
proceeds pari passu with the development of the organs from 
which they arise, and does not require detailed description. It 
should be noted, however, that part of the veins from the giz- 
zard and proventriculus form an independent vena porta sinistra 
which enters the left lobe of the liver. 
A distinct subintestinal vein extends forward from the root of the 
tail at the stage of ninety-six hours to the posterior intestinal portal, 
where it opens into the branch of the left omphalomesenteric vein, 
that extends forward from the posterior end of the sinus terminalis. 
This vein appears to take up blood from the allantois at an early stage. 
However, it disappears at about the time when the umbilical vein be- 
comes the functional vein of the allantois. Originally it appears to 
open into symmetrical right and left branches of the omphalomesen- 
teric vein that encircles the splanchnic umbilicus. The right branch 
is, however, much reduced at ninety-six hours (ef. Hochstetter, 1888). 
The Umbilical Veins. The umbilical veins appear as vessels 
of the lateral body-wall opening into the ducts of Cuvier (Fig. 
210 C; ef. Fig. 117); at first they show anastomoses with the 
latter, which, however, soon disappear. They are subsequently 
prolonged backwards in the somatopleure along the lateral closing 
folds of the septum transversum (Chap. XI). Up to the end of 
the third day of incubation they have no direct connection with 
the blood-vessels of the allantois,and function only as veins of the 
body-wall. 
However, they obtain connection with the efferent vessels 
of the allantois during the fourth day, apparently by widening 
of parts of an intervening vascular network, and then the allan- 
toic blood streams through them to the heart. The right um- 
bilical vein disappears on the fourth day, and the left one alone 
persists. 
In the meantime the central ends of the umbilical veins have 
acquired new connections. (Middle of third day, Fig. 210 D.) 
This takes place through the formation of anastomoses, especially 
on the left side, between the umbilical vein and the hepatic 
