Charles F. W. McClure 197 
migration than in the rabbit and cat, since the renal veins in Didelphys, 
as in the embryos of the rabbit and cat, open into the postcava only 
shghtly caudad of the origin of the omphalomesenteric artery and con- 
tiguous to the junction of the postcava and the left anterior reyehent 
vein (subcardinal). The opening of the latter vein into the postcava, 
however, lies relatively much further craniad of the junction of the post- 
cava and postcardinal veins in the pouch young, than is the case in the 
8 mm. embryos of Didelphys and the embryos of the rabbit and cat. 
This circumstance I can only account for on the basis that the junction 
of the postcardinal veins with the postcava remains, more or less, as a 
fixed point, in front of which the vessels elongate more rapidly than 
those that lie behind. This growth in length, so far as the postcava 
is concerned, principally affects that portion of the vein which lies be- 
tween its junction with the two postcardinals and that with the left an- 
terior revehent vein. Strictly speaking, this portion of the posteava cor- 
responds to the original cross anastomosis between the two subcardinals ° 
(pars subcardinalis of postcava and left anterior revehent vein; see Text 
Figs. 10 and 12). 
A comparison of Text Figs. 11 and 17 gives a clear idea of the relative 
positions occupied by the permanent kidneys during their migration and 
after it is completed. The permanent kidneys are not represented in Fig. 
7, but the renal veins sufficiently indicate their position. 
In the 11.5 mm. pouch young a renal vein was met with for the first 
time. Here a left renal vein was present, which opened into the post- 
cava in common with the left anterior revehent vein (Fig. 49, Plate IV). 
Both renal veins were present in the 14 and 15 mm. pouch young. 
In the adult opossum the left suprarenal body lies in close contact 
with the left renal vein (Fig. 28, Plate I) so close, in fact, that it is im- 
possible to speak of the existence of a left suprarenal vein. On account 
of the intimate relation, therefore, which exists betwen the left renal 
vein and suprarenal body, it appears probable that the slight venous con- 
nections between the two have been derived, as in the rabbit, from the 
left anterior revehent vein. 
®Hochstetter (1893, Taf. XXIII, Fig. 25) suggests a somewhat similar 
explanation for the development of that portion of the unpaired postcava in 
Dasypus novemcinctus which lies, ventral to the aorta, between the renal 
veins and the origin of the posterior mesenteric artery. Referring to this 
portion of the postcava, he says (page 621): ‘ das der zwischen 
Miindiing der Nierenvenen und der Theilung in die beiden hinteren Hohlvenen 
gelegene Abschnitt der V. cava posterior einem starkeren Wachsthum der 
Lendenwirbelséule seine Entstehung verdanke.” 
