156 EDWAED MCCRADY, JR. 



all of their blood into the cardinal collaterals. McClure has 

 shown that either the right or the left or both cardinal col- 

 laterals can persist, and either the dorsal or the ventral or 

 both halves of the circumarterial ring can persist. By various 

 combinations of these possible routes several types of post- 

 cava are commonly formed in the opossum. In the adult the 

 postcava posterior to the spermatic veins may be single or 

 double with about equal frequency. It may also pass ventrad, 

 dorsad, or both, around the common iliac artery. All of these 

 types must be considered 'normal' (McClure, '03). The 

 opossum is simply plastic in this respect, and has not settled 

 upon a single ' normal' method of forming the postcava. 



Aside from the plasticity, the two principal peculiarities 

 about the opossum's postcava are: 1) that the hepatic por- 

 tion is formed almost entirely from a persistent right omphalo- 

 mesenteric vein, instead of from hepatic sinusoids; and 

 2) that the postrenal portion is formed from the unique 

 cardinal collateral veins instead of from the postcardinals 

 or supracardinals. 



The septum primum. The interatrial septum has grown 

 ventrad until it has reached the endocardial cushions. In 

 stage 33 it is always cribriform, and the older the specimen the 

 finer the mesh (fig. 51). Since in the previous stage there is 

 sometimes only a single foramen (fig. 47, central reconstruc- 

 tion), and in later stages there are always many, and the later 

 the stage the smaller the individual foramina, it seems neces- 

 sary to assume that the muscular slips grow across and sub- 

 divide the original aperture. This process carried to its limit 

 is presumably the mechanism of closure. 



No septum secundum forms. At birth there are still a few 

 small perforations through the septum primum, but they be- 

 come gradually grown over and a few days after birth the two 

 atria are separated by an imperforate partition. 



In having no septum secundum, and in having many small 

 interatrial foramina instead of a single large one, the opossum 

 is similar to the chick and unlike the placental mammals. It 

 has been known at least as far back as 1868 (see reference to 



