THE EMBRYOLOGY OF THE OPOSSUM 



157 



Owen in Cunningham, 1882, p. 150) that the adult marsupial 

 heart has no fossa ovalis. I believe, however, that the ex- 

 planation of this fact, namely, that it is due to the total lack 

 of a septum secundum in the embryo and the presence of an 

 avian type of cribriform septum primum, has not hitherto 

 been recorded. 



Evidently the equalization of pressure on the two sides of 

 the septum due to the increased circulation of blood through 

 the lungs after respiration begins, has nothing to do with the 



right cerium 



right 

 prccava 



pulmonary vein 



apvr'ium 



pulmonary 

 trunk 



vent end cushion sept primum dors. end. cushion hepatic 



Fig. 51 Reconstruction of dorsal half of heart from 17125 showing the 

 cribriform septum primum and the absence of any septum secundum. 



closure of the interatrial foramina in the opossum, as the 

 process of closure is almost complete before respiration be- 

 gins. Also in prerespiratory stages there is no tendency for 

 the septum to bulge into the left atrium. Figure 51 shows the 

 foramina wide open, but the septum flat. If the pressure 

 is unequal, then the septum is rigid enough to withstand it. 

 Under these circumstances there is no way in which equaliza- 

 tion of pressure could close these foramina. They must be 

 closed by growth and fusion of the muscular bands. 



