208 Venous System of Didelphys Marsupialis (L) 
cava, although, at the present writing, it is quite impossible to establish 
a relationship between a particular type of postcava and the manner in 
which the Vv. pudendovesicales join the iliac veins. 
From what has already been stated above concerning the evanescent 
character of the ventral portion of the lateral venous rings it is clear that 
the type of postcava which at present characterizes the 15 mm. pouch 
young (Text Fig. 17) and which would most likely prevail in the adult, 
is Type III, B, and possibly of the variety represented by Fig. 19 on 
Plate IV of Part I. It is also possible that the ventral median vessel 
in Text Fig. 17 (ventral portion of medial rings) might become com- 
pletely atrophied before the adult state was reached so that a postcava of 
Type II would result, possibly of the variety represented by Fig. 7 on 
Plate II of Part I, in which the connections between the internal and 
iliac veins present a marked difference in caliber. In case of either of 
the two possibilities the ventral portions of the lateral rings could persist 
in the adult as in Fig. 10, right side (Plate IJ, Part I); or, they could 
give up their connections with the external iliacs so that each V. pudendo- 
vesicalis would open into an internal iliac vein as in Fig. 7 (Plate 
II, Part I). It is also evident that the condition represented in Fig. 9 
(Plate II, Part I) might result, in which the V. pudendovesicalis opens 
on one side into the external and on the other into the internal iliac 
vein. 
We are now in a position to further consider the character of the an- 
astomosis, ventral to the common iliac artery, which exists between the 
left external and internal iliac veins in the 14 mm. pouch young (Text 
Fig. 15). ‘It has already been stated that this anastomosis, ventral to 
the artery, has probably been brought about either as the result of the 
persistence of a vein whieh corresponds to the ventral portion of an em- 
bryonic cireumarterial venous ring, or as the result of the persistence 
of a vein which has been developed secondarily in the pouch young in 
connection with the V. pudendovesicalis. It is not improbable that the 
ventral portion of one of the lateral rings might, in some cases, become 
so enormously hypertrophied, that it would function in the adult as the 
sole channel through which the blood reached the postcava from the pel- 
vic region. It is impossible, however, to state definitely what is actually 
the case in the 14 mm. pouch young, although I am inclined to believe 
that in this particular case (Text Fig. 15, left side) it is the ventral por- 
tion of a lateral ring which has persisted. My reason for holding this 
view is based on the relations of the V. pudendovesicalis to the ventral 
anastomosis in question. 
Text Fig. 18 represents another reconstruction of the venous system 
