Charles F. W. McChire 203 
POSTCAVA 
POSTCAVA 
AORTA 
RIGHT 
CARDINAL 
COLLATERAL 
AORTA 
RIGHT _ 
POSTCARDINAL 
VENOUS RING 
COMMON 
VENOUS RING nities EXT. ILIAC 
UMBILICAL aS COMMON 
ARTERY INT. ILIAC 
EXT. ILIAC) INT. ILIAC 
CAUDAL 
INT. ILIAC iereay 
Hig. 19: Fie. 20. 
Fic. 19. Diagram of the venous system of an 8 mm. embryo of Didelphys 
showing the circumarterial venous rings. Ventral view. 
Fic. 20. Diagram of the venous system of Didelphys in which the internal 
iliac veins have fused ventral to the caudal artery and in which a postcava of 
Type III has been established as the result of the persistence of the dorsal 
and ventral portions of both embryonic circumarterial venous rings. Ventral 
view. 
POSTCAVA POSTCAVA 
SPERMATIC 
SPERMATIC 
' AORTA—+- 
AORTA ’ 
° ‘ 
. LEFT ’ ae 
. CARDINAL RIGHT ¢ Sa SHC ARDINAL 
. 
A . («COLLATERAL CARDINAL i 
’ ‘ z 
POSTCARDINAL = ate COLLATERAL * ' 
. 4 
COMMON VENOUS RING 
COMMON VENOUS RING ILIAC 
A ARTERY 2 
(Eke EXT, ILIAC EX L 
ARTERY 
COMMON COMMON 
INT, ILIAC INT. ILIAC 
INT, ILIAC 
CAUDAL ' CAUDAL 
ARTERY EXT. ILIAC ARTERY 
Wie. 21. MTG. 225 
Fic. 21. Diagram of the venous system of Didelphys in which a postcava 
of Type I has been established as the result of the complete atrophy of the 
dorsal portions of the circumarterial venous rings. Ventral view. 
Fic. 22. Diagram of the venous system of Didelphys in which a postcava 
of Type II has been established as the result of the complete atrophy of the 
ventral portions of the circumarterial venous rings. Ventral view. 
