THE EMBRYOLOGY OF THE OPOSSUM 



149 



part of the adrenal gland, form fairly compact masses just 

 dorsal to the subcardinal veins (fig. 47, C). 



The foramen ovale. In stage 32 (fig. 47, reconstruction) 

 the septum primum has descended some two-thirds of the way 

 toward the atrioventricular canal, and has become secon- 



Fig. 48 Photograph by Dr. A. A. Ziiiiinerinann showing a pori-intimal space 

 with its own emlothelium forming while that of the venule is still intact. 



darily perforated by one or more foramina. AVhen there 

 are more than one, which is always the case in later stages, 

 usually one of the more ventral ones is decidedly larger than 

 the others (fig. 51). This multiple condition is a primitive 

 feature not found in placental mammals. 



