Charles F. W. McClure - 213 
mesenteric veins do not anastomose with each other at any point within 
(except by sinusoids) or without the liver. Near their entrance at the 
umbilicus considerable variation was met with in the 8 mm. embryos as 
regards the size of the umbilical veins. In some cases the right (Fig. 38, 
Plate III) and in others the left umbilical vein was the larger of the two, 
so it can be said that at this stage of development the umbilical vein of 
a particular side does not invariably predominate as the principal chan- 
nel between the allantois and the liver. Although a marked difference 
in size may characterize the umbilical veins in the region of the umbilicus, 
the smaller of the two veins invariably increases in size as the liver is ap- 
LEFT 
HEPATIC 
OMPHALO=— 
MESENTERIC 
POSTCAVA 
LEFT 
UMBILICAL UMBILICAL 
POSTCARDINAL 
/ASTBISIINAL ABDOMINAL 
EXT, ILIAC 
Wig. 24. 
Fic. 24. Diagram of the venous system of an 8 mm. embryo of Didelphys 
showing the umbilical and abdominal veins. Ventral view. 
proached, so it can be said that both umbilical veins are highly developed 
up to a relatively late stage of development; a circumstance which, as 
shown by Broom, 98, for Trichosurus, appears to be characteristic of the 
marsupials as thus far examined. 7 
In the 11.5-12 mm. embryos of Didelphys one large umbilical vein 
now forms the principal channel between the allantois and the liver; a 
vein which | regard as the left umbilical vein. This large vein (Fig. 53, 
Plate V) lies in the ventral body-wall shghtly to the left of the mid-ven- 
tral line, and to its right is situated a much smaller vessel which is diffi- 
cult to follow in consecutive sections, but which is probably the remains of 
the right umbilical vein. The two umbilical veins appear to anastomose 
in places with each other, so that one might almost regard the larger of 
