NIPPLE DEVELOPMENT OF MARSUPIALS 85 



by their small size. It is evident that from 

 this nipple type both the actual eversion nipples 

 of the Australian Marsupials, as well as the 

 proliferation nipples, can be derived — the 

 one through progressive development, the 

 other through involution of the nipple-pouch 

 stage. 



Thus, study of nipple development, like that 

 of pouch development, leads us to the con- 

 clusion that the pouchless Didelphyidfe exhibit 

 the most primitive conditions of the mammary 

 apparatus in the Marsupialia. Now, Winge 

 and Bensley, as the result of their investigations 

 on the foot structure and teeth of the Marsu- 

 pialia, have placed just those pouchless forms 

 — namely, Marmosa and Peramys — at the very 

 base of the marsupial series. We have here, 

 then, quite a striking confirmation of the 

 conclusions reached above from the evidence 

 of the mammary apparatus alone. 



If we now proceed to consider the phylogeny 

 of the mammary organs in the Marsupialia, it 

 is clear that we must ignore the old theory of 

 Darwin and Gegenbaur, according to which 

 nipples and milk glands were superadded to a 

 previously existing pouch. For, as we have 



