TYPES OF NIPPLES IN PLACENTALS 111 



quite another conclusion. They maintained 

 that it was necessary to distinguish at least 

 four types of nipples among the Placentals, 

 each of which was regarded as having arisen 

 independently of the others out of the con- 

 ditions existing in the different stages of the 

 development of the eversion nipples of the 

 Marsupials. 



Gegenbaur and Klaatsch thought they could 

 find the nearest link to the Marsupials in the 

 Muridai amongst the Rodents, which possess 

 deep sheaths, in which the nipples are for a 

 long time enclosed, and from which they only 

 emerge when the period of lactation begins 

 (Fig. 39). These sheaths, therefore, were 

 regarded as being derived from knob-shaped 

 primordia surrounded by a cutis wall, just like 

 the nipple pouches in the kangaroo, and it was 

 held that the nipples arose in a similar way as 

 there through eversion. In a second type, 

 represented in the Prosimise and Primates, 

 including man, a nipple pouch, somewhat 

 shallower, was also said to develop in the same 

 way. But this pouch only partially became 

 raised to a nipple, whilst the rest of it, adjacent 

 to the original cutis wall, remained as the 



