214 Venous System of Didelphys Marsupialis (L) 
the two veins as being formed, in places, through a fusion of the two 
umbilical veins. On reaching the ventra! surface of the liver, the large 
umbilical vein enters the latter through which it continues as a single 
channel which at first turns caudad, and then dorsad before opening into 
the postcava as in the 8 mm. embryo, in common with the left hepatic 
revehent vein (Text Fig. 25). 
In the 10.5 mm. pouch young both umbilical veins can be followed 
for only a short distance in front of the umbilicus which is now closed. 
Further forward, however, only one large umbilical vein can be clearly 
distinguished, which now returns blood solely from the body-walls, but 
0. A J ) 
11. 5-12 MM. EMBRYO UO SUSI HEM a! Vieuiine 
POSTCAVA 
POSTCAVA 
) PORTAL 
LIVER— 
OMPHALO = 
MESENTERIC 
LIVER 
* * UMBILICAL 
FIG. 44 
UMBILICAL FIG. 45 
Pie. 25. Iie, PAey 
ADULT 
POSTCAVA 
LIVER 
HEPATIC 
REVEHENT 
( UMBILICAL 2) eae 
PORTAL, 
Figs. 25, 26 and 27. Diagrams illustrating the transformations which the 
umbilical and omphalomesenteric veins undergo in the embryos and pouch 
young. Lateral views. 
which is still continued through the liver where it opens into the post- 
cava, as in the preceding stages, without anastomosing directly with the 
omphalomesenteric vein (Fig. 44, Plate III, and Fig. 45, Plate IV, and 
Text Fig. 26). 
In the older pouch young the abdominal portion of the left umbilical 
vein ceases to be of prominence, and, so far as the writer can determine, 
entirely disappears before the pouch young have attained a length of 
about 17 mm. Its presence was noted, however, in the 14 and 15 mm. 
pouch young, although its connection with the hepatic circulation could 
not be determined. 
The hepatic continuation of the left umbilical vein, after it has given 
