196 Venous System of Didelphys Marsupialis (L) 
posteava is fully established and the posteardinals now function as veins 
which return blood to,the postcava from the still functional mesonephroi 
and the anlages of the genital glands. 
THrE MIGRATION OF THE PERMANENT KIDNEYS AND THE DEVELOPMENT 
OF THE RENAL VEINS. 
After the permanent kidneys have completed their forward migration 
in the embryos of the rabbit and cat, renal veins are developed which 
open into the posteava, approximately, at the level at which the latter 
joins the two posteardinal veins (cross anastomosis, Text Fig. 7). When 
UROGENITAL LEFT 
ARTERY 
SPERMATIC Gol eae 
CARDINAL 
Pp. 
COLLATERAL OST CARDINAL 
COMMON 
ILIAC 
ARTERY 
PUDENDO- 
VESICALIS 
COMMON 
INT. ILIAC 
TGs ass 
INT. ILIAC 
Fig. 18. Reconstruction (slightly schematic) of the venous system of a 15 
mm. pouch young of Didelphys in which a postcava of Type III, B is already 
established. Ventral view. 
a double or bifurcated posteava is met with in an adult rabbit or cat the 
bifurcation reaches, approximately, as far forward as the level at which 
the renal yeins open into the postcava. 
In Didelphys, on the other hand, after the permanent kidneys have 
completed their forward migration, renal veins are developed which open 
into the postcava some distance craniad of the latter’s junction with 
the two posteardinal veins (Text Fig. 17). This circumstance now 
explains why the postcava of the adult opossum was never found by 
the writer (Part I, page 398) to be bifurcated as far forward as the level 
at which the renal veins opened into the posteava. The position of the 
renal veins in Didelphys with respect to the junction of the postcardinal 
veins and the posteava, does not appear to indicate that the permanent 
kidneys in Didelphys have undergone, relatively, a more extensive 
