THE EMBRYOLOGY OF THE OPOSSUM 155 



postcava. The branches from it which extend through the 

 coronary appendage and dorsal mesentery to communicate 

 with similar branches from the right subcardinal, constitute 

 a new vessel, the vein of the caval mesentery, but this con- 

 tributes a relatively small part of the definitive postcava con- 

 necting the omphalomesenteric and subcardinal portions. 

 From the liver back to the level of the metanephroi the post- 

 cava is the original right subcardinal. At the level of the 

 kidneys it is the subcardinal anastomosis. Posterior to this 

 point up until birth the only important channels are the sub- 

 cardinal-postcardinal anastomoses and the postcardinals ; for 

 the subcardinals posterior to the anastomosis are not well 

 developed, and there are no supracardinals. 



The posterior portion of the postcardinals originally passed 

 the common iliac artery on its dorsal side (fig. 42, J). In 

 stage 33 it has sent sprouts around the artery which fuse 

 to form a circumarterial ring (figs. 50 and 52), which was 

 first described by McClure ('06). At this time another set 

 of vessels is developing ventral to the aorta. These cardinal 

 collateral veins, as McClure named them, develop posterior 

 to the large subcardinal-postcardinal anastomosis from the 

 mesonephric venous sinusoids which connect the postcardi- 

 nals and subcardinals. They later become organized into two 

 channels which replace first the subcardinals and later even 

 the postcardinals in this region. In the formation of the 

 postrenal division of the postcava they play the part which 

 in placental mammals is played by the supracardinals; but 

 unlike the latter the cardinal collaterals are entirely ventral 

 to the aorta. In accordance with Butler's generalization ( '27) 

 the high development of the mesonephros in the opossum 

 prevents the postcardinal from taking any part in the forma- 

 tion of the postcava; but why the vessels which replace it 

 should be unlike those of all eutherians is not explained. 



Posteriorly the cardinal collaterals are connected with the 

 venous ring around the common iliac arteries already de- 

 scribed. And about 1 week after birth when the postcardinals 

 in this region degenerate, the circumarterial rings discharge 



