Charles F. W. McClure 183 
revehent (subeardinal) veins of the 8 mm. embryo of Didelphys, we are 
now in a position to consider the circumarterial venous rings or loops 
which encirele the umbilical arteries. 
The Circumarterial Venous Rings.—It has been stated above, as well 
as in a preceding paper (McClure, 02), that in the 8 mm. embryos of 
Didelphys the umbilical arteries, instead of lying ventral to the post- 
cardinal veins, as in most mammals, or dorsal to the same, as in Echidna 
POSTCAVA LEFT ’ 
PARS SUBCARDINALIS ANT, REVEHENT 
SUBCARDINAL 
RIGHT 
POSTCARDINAL POSTCARDINAL 
ROSS ANASTOMOSIS 
POST. REVEHENT 
POST. REVEHENT SUBCARDINAL 
SUBCARDINAL 
CARDINAL 
COLLATERAL 
CARDINAL 
COLLATERAL POSTCARDINAL 
POSTCARDINAL VENOUS RING 
UMBILICAL 
UMBILICAL ARTERY 
ARTERY 
INT, ILIAC 
Fig. 13. 
Fic. 13. Partial reconstruction of the venous system of an 8 mm. embryo of 
Didelphys showing the cardinal collateral and posterior revehent veins and 
the venous rings which encircle the umbilical arteries. Ventral view. 
* 
(Hochstetter) and Dasyurus (Text Fig. 9), are encircled near their origin 
by complete circumarterial venous rings. These venous rings are situated 
slightly craniad of the point of junction of the external and internal 
iliac veins, and, so far as their general make-up is concerned, are ex- 
tremely variable in character, not only in the different embryos, but even 
upon opposite sides of the same individual. Two main types of venous 
rings may be distinguished :—One in which the portion of the ring which 
