206 Venous System of Didelphys Marsupialis (L) 
the dorsal portion of the more laterally situated rings (lateral rings). 
The ventral portion of the more laterally situated ring presents a marked 
difference in caliber on opposite sides and receives a vein which the writer 
regards as the V. pudendovesicalis (McClure, 00, 2, p. 457). 
There can be no doubt as to the postcardinal origin of the dorsal por- 
tion of both sets of rings. There is some doubt, however, regarding the 
origin of the median ventral vein which forms the ventral portion of 
the two medial rings, as well as that of the ventral portion of the two 
lateral rings into which the V. pudendovesicalis opens. 
The median ventral vein presents the same relative position with re- 
spect to the dorsal portion of each of the two medial rings, as is the case 
with the ventral portion of the circumarteria! venous rings of the 8 mm. 
embryo (Text Fig. 13), the 14 mm. pouch young (Text Fig. 15), and 
the case of the adult (Fig. 17, right side, Plate IV, Part I), and, for this 
reason, has been most likely derived from the same class of vessels as 
those which form the ventral portions of the venous rings in the 8 mm. 
embryos. Whether, however, it corresponds to the ventral portion of one 
ring or has been formed as the result of a fusion between the ventral por- 
tion of two rings, as represented in the diagram, Fig. 20, it is impossible 
to state. Whatever its mode of origin may be, its presence in the pouch 
young undoubtedly accounts for the presence of a similarly situated ves- 
sel which is frequently met with among the adult variations (see Fig. 19, 
Plate IV, Part. 1): 
The ventral portion of each of the lateral rings, on the other hand, 
occupies an entirely different position with respect to the dorsal portion 
of the ring from what is the case with the ventral portion of the medial 
rings, since it lies lateral instead of medial to the dorsal portion of the 
ring. I am, therefore, inclined to conclude, for this reason as well as 
others given below, that the ventral portions of the lateral rings are 
secondary formations which are first met with in the pouch young and 
which are developed here in connection with the V. pudendovesicalis. 
In addition to the reason already mentioned, my reasons for so thinking 
are as follows: (1) On account of the presence of these veins in the 
15 mm. pouch young (Text Fig. 17) in addition to the median ventral 
vein; as well as the occasional persistence in the adult of the ventral por- 
tion of a right lateral ring in addition to a vein whose origin cannot be 
accounted for unless it has been derived from the ventral portion of an 
embryonic circumarterial venous ring of the same side (see Fig. 4, right 
side, Plate I, Part I, and compare with Fig. 10, right side, on Plate II, 
Part I) and, (2) because the presence of these veins in the pouch young 
explains, for the most part, the variable character of the V. pudendovesi- 
calis in the adult. 
