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 yari 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 (cf. 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; cf. 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 



