Charles F. W. McClure 219 
is concerned, principally affects that portion of the vein (cross anasto- 
mosis) which lies between its junction with the two postcardinals and 
that with the left anterior revehent vein. 
The left renal vein of Didelphys is developed in connection with the 
left anterior revehent vein at the point where the latter opens into the 
postcava. The right renal vein is developed, as a rule, slightly craniad 
of the left and, possibly, from one of the urogenital tributaries of the 
pars subeardinalis of the posteava. Both renal veins were met with for 
the first time in the 14 mm. pouch young. 
2. In the rabbit, the portion of the right subcardinal which, in part, 
forms the stem of the postcava terminates, approximately, at the level 
at which the renal veins will be developed; while the stem of the post- 
cava caudad of the renal veins is developed from the right postcardinal, 
together with a derivative of the latter which is formed on the medial 
side of the ureter. In Didelphys, however, the portion of the subeardinal 
which, in part, forms the stem of the postcava does not terminate at the 
renal level, but at the level at which the internal spermatic veins subse- 
quently open into the postcava; while caudad of the spermatic level the 
posteava is formed by two veins which usually fuse in the median line 
ventral to the aorta. This last feature, so far as I know, is distinctively 
a marsupial character. These two veins have been described in the pre- 
ceding pages under the name of the cardinal collateral veins. In both 
the embryos and pouch young the cardinal collateral veins occupy a po- 
sition ventrolateral or ventral to the aorta and, in this respect, differ from 
that of the posteardinal derivative which is developed on the medial side 
of the ureter, lateral to the aorta, and which forms, in part, the stem of 
the postcava in the rabbit. They are also to be distinguished from the 
posterior reyehent veins (subeardinals) with which, however, they form 
frequent anastomoses. In correlation with the degeneration of the post- 
cardinal veins the cardinal collaterals increase in size and subsequently 
function in place of the postcardinals in returning the blood to the root 
of the postcava from the hind limbs and pelvic region. 
3. In the adult rabbit the postrenal division of the postcava lies to 
the right of the aorta and is usually formed through a union of its iliac 
tributaries which takes place dorsal to the arteries. In the adult Didel- 
phys, on the other hand, the postrenal division of the posteava lies ventral 
to the aorta and its iliac tributaries normally unite in such a variable 
manner to form the postcava that it is actually impossible to assign any 
one mode of union for the iliae veins that may be regarded as typical of 
the species. In consequence of this variability, the writer has classified 
the different modes of union which characterize the external and internal 
