Charles F. W. McClure 193 
internal iliac veins, as in the 8 mm. embryo, through the mesonephric 
divisions of the postcardinal veins. 
From a functional standpoint the mesonephroi may be said to be at 
the height of their development as long as the mesonephric divisions of 
the postcardinals retain the function of returning most of the blood to 
the pars subeardinalis which is collected by the tributaries of the external 
and internal iliac veins. 
In the 14 and 15 mm. pouch young there is a noticeable degeneration 
of the mesonephroi, and correlated with this also a degeneration of the 
r 3 POSTCAVA 
RIGHT 
UROGENITAL 
ARTERY 
SPERMATIC 
UROGENITAL 
ARTERY 
SPERMATIC 
RIGHT : x 
CARDINAL = Fs AORTA 
POSTCARDINAL COLLATERAL 
AORTA CARDINAL 
COLLATERAL 
an: “TUR 
COLLATERAL COMMON ARTERY a rs, POSTCARDINAL 
ILIAC a ; 
ARTERY -* x 
EXT. ILIAC EXT. ILIAC EXT. ILIAC 
PUDENDO- PUDENDO 
UREN BE: VESICALIS VESICALIS 
VESICALIS PUDENDO 
INT. ILIAG VESICALIS 
INT. ILIAC pone INT. ILIAC 
VENOUS RING 
Fic. 14. Fig. 15. 
Fic. 14. Reconstruction (slightly schematic) of the venous system of a 14 
mm. pouch young of Didelphys in which a postcava of Type II is already 
established. Ventral view. 
Fic. 15. Reconstruction (slightly schematic) of the venous system of a 
14 mm. pouch young of Didelphys in which a postcava of Type III, A is al- 
ready established. Ventral view. 
mesonephric divisions of the postcardinals. As a result of this degenera- 
tion the blood from the hind limbs and the pelvic region is directed 
toward the pars subcardinalis of the postcava through the cardinal col- 
lateral veins; while the mesonephric divisions of the postcardinals (Ur- 
nierenabschnitt) retain the function of returning the blood from the 
mesonephroi and constitute the mesonephric veins (Urnierenvenen). 
The correlation which exists between the atrophy of the mesonephric 
