194 Venous System of Didelphys Marsupialis (1L) 
divisions of the postcardinal veins and the establishment of this new sec- 
tion of the postecava (cardinal collateral) is clearly illustrated by the 
following transverse sections (Figs. 59, 60 and 61, Plate V) and recon- 
structions of the venous system (Text Figs. 14, 15, 16, 17 and 18) of 
the 14 and 15 mm. pouch young. 
In the case represented by Text Fig. 14, the mesonephric divisions of 
the postcardinals are still, as in the 11.5 mm. pouch young, the chief 
channels through which the blood reaches the pars subcardinahs from 
the hind limbs and hinder end of the body. This condition is unusual 
for this period of development and represents the only instance met with 
among the 14 and 15 mm. pouch young, in which both of the postcard- 
inal veins function in such a manner. An advance in development over 
that in the preceding stages is evident, however, since the cardinal col- 
lateral of the right side has made a connection at its caudal end with the 
postcardinal vein and the posteardinals have anastomosed with each other 
ventral to the caudal artery. In all probability the cardinal collateral of 
the left side also joins its corresponding posteardinal, although I am un- 
able to establish definitely such a connection, on account of the vessel not 
being filled with blood at its caudal end. 
Text Figs. 15, 16 and 17 represent examples of unilateral atrophy in 
which, in one case the left (Text Figs. 16 and 17), and in the other the 
right (Text Fig. 15) mesonephric division of the postcardinal has 
atrophied, and in which there is an hypertrophy of the corresponding 
cardinal collateral vein. In each case the atrophied postcardinal now 
functions as a vein which returns blood to the pars subcardinalis solely 
from the mesonephros (Urnierenvenen) and not, as hitherto, from the 
hind limb and hinder end of the body. 
Finally, Text Fig. 18 represents a case in which the postcardinals 
have atrophied on both sides, with the result that both of the cardinal 
collateral veins collect all of the blood from the tributaries of the external 
and internal iliac veins and constitute the caudal end of the postcava, 
while both of the postcardinals now function solely as veins of the 
mesonephroi (Urnierenvenen). 
It has been stated in Part I of this paper (p. 398) that in 42 of the 
101 adult opossums examined by the writer the postcava was either bi- 
fureated as far craniad as the level of the internal spermatic veins, or 
otherwise presented some indication of an incomplete fusion between the 
two vessels (cardinal collateral) which form the postcava caudal to the 
level of the internal spermatic veins; also, that in some of the adults, 
either one or both of the posterior internal spermatic arteries were found 
to pass between the two divisions of the postcava or through a foramen 
