400 Venous System of Didelphys Marsupialis 
entirely or for the most part by vessels which unite ventral to the common 
iliac arteries, only four cases were met with in which it was bifur- 
cated or presented indications of an incomplete fusion. 
It appears from the above statistics that we have grounds for assuming 
that the presence of a double postcava in the adult Didelphys is in some 
way related to the manner in which the internal iliac veins unite with the 
external iliacs to form the postcava, since ninety per cent (90¢) of the 
observed cases in which the postcava was bifurcated or presented indi- 
cations of a previous bifurcation, occurred under Types II, IIT, B, and 
III, C, in which the anastomosis between the iliac veins ventral to the 
iliac arteries is either wanting, or does not exceed in size that 
which is situated dorsal to the iliac arteries. 
Fie. X. Fie. XI. 
Fic. X, Posteava of Echidna embryo No. 44. Ventral view. After Hochstetter. 
Fie. XI. Postcava of Echidna embryo No. 45. Ventral view. After Hochstetter. 
A.i.e., A. iliaca externa; A.i.i., A. iliaca interna; A.m.a., A. mesenterica anterior ; 
A.m.p., A. mesenterica posterior; Ao., Aorta; A.om., A. omphalomesenterica; V.i.e., 
V. iliaca externa; V.i.i., V. iliaca interna; V.sr., V. suprarenalis. 
If this relationship can be definitely established for Didelphys, through 
further investigation, it would seem to account for the presence of the 
bifurcated postcava in the adult of Hchidna aculeata, since this condition 
in Echidna is reached through exactly the same venous transformations 
as those which have apparently produced the bifurcated condition in 
Didelphys, as can be seen from the following brief account of the devel- 
opment of the posteava in Echidna. 
Hochstetter, 96, who has studied the development of the postcaval 
vein in Echidna aculeata, finds that, at an early stage, it consists, caudal 
to the liver, of an unpaired and a paired portion. In a young embryo 
(No. 44, Text Fig. X), the unpaired portion extends from the liver, on 
the right side of the A. omphalomesenterica (A. 0. m.), to a point some- 
what caudad of this artery. From this point the two postcaval veins 
