94 MAMMARY APPARATUS OF THE MAMMALIA 



the case in the genus Peramys — the marsupial 

 pockets, even if they have already appeared 

 here, remain isolated. If, on the other hand, 

 the nipples are so close together that the 

 walls of the surrounding marsupial pockets 

 come into contact, the possibility arises for 

 the fusion of their lateral parts to form the 

 folds of the marsupium. From the lateral 

 pouch folds so arising it is quite easy to 

 derive the different types of the pouch by 

 variations in their mode of union. 



In the fusion of the lateral walls of the 

 marsupial pockets into the pouch folds, the 

 pockets themselves did not necessarily cease 

 to exist. They mostly do so by flattening 

 out, with resulting increase of the marsupial 

 area, as we have seen in Didelphys marsu- 

 pialis. But in other cases they have persisted 

 perhaps because they aided in the fixation of 

 the young. 



Such, then, is my idea of the phylogeny of 

 the mammary apparatus in the Marsupialia — 

 certainly a very different one from that put 

 forward bv Gea:enbaur and Klaatsch. I am 

 well aware that my views in respect of certain 

 details still require confirmation. It is above 



