212 Venous System of Didelphys Marsupialis (L) 
it is evident that the cardinal collateral vein must have entered into the 
formation of the postcava rather than the postcardinal, otherwise the 
left ureter would occupy the same relative position that it does in Text 
Fig. 17, and pass caudad between the postcava and the aorta. 
Text Fig. 23 is a diagram illustrating the probable modifications 
which the venous system has undergone in establishing the abnormality 
represented by Text Fig. VII in Part I of this paper. The shaded por- 
tions indicate the veins which have atrophied and the crosses (++) 
the new formation by means of which the postcava anastomoses with the 
ilae tributary of the right side, dorsal to the aorta. 
AORTA POSTCAVA 
LEFT 
SPERMATIC 
VEIN 
POSTCARDINAL 
LEFT 
* CARDINAL 
RIGHT COLLATERAL 
CARDINAL 
COLLATERAL URETER 
VEIN 
NOUS RING 
Vase EXT. ILIAC 
VEIN 
INT. ILIAC 
VEIN 
Fic. 23. Diagram illustrating the probable modifications which the venous 
system has undergone in establishing the abnormality represented by text 
Fig. VII in Part I of this paper. Ventral view. 
THE UMBILICAL, ABDOMINAL AND OMPHALOMESENTERIC VEINS. 
The Umbilical Veins.—In the youngest embryos of Didelphys (8 mm.) 
studied by the writer, the right and left umbilical veins, usually after 
fusing at the umbilicus to form an umbilical sinus, can be traced for- 
ward as independent vessels (Figs. 38, 37, Plate III, and Figs. 36, 34, 33, 
Plate II) to the ventral surface of the liver. Here they again fuse to 
form a sinus (Fig. 32, Plate IL) from which they are continued dorsad 
through the parenchyma of the liver in a channel common to both which 
opens into the postcava in common with the left hepatic vein (Text 
Figs. 10 and 24). 
The hepatic continuation of the umbilical veins opens into the post- 
eava slightly craniad of that of the omphalomesenteric vein (Text Fig. 
10) and, so far as the writer can determine, the umbilical and omphalo- 
