114 MAMMARY APPARATUS OF THE MAMMALIA 



distinct from the true nipples of the rest of the 

 Mammals. 



But this part also of the Gegenbaur-Klaatsch 

 theory is based on premises entirely false. 

 For, quite apart from the fact of the non- 

 existence of the mammary pouches, there are 

 no such fundamental differences between the 

 nipples of the different groups of Placentals 

 as Gegenbaur and his followers assumed on 

 the strength of insufficient embryological 

 investigations. The various types of nipples 

 of the Placentals are rather to be regarded, in 

 spite of differences in detail, as further develop- 

 ments of a single type of nipple, which was 

 peculiar to the common ancestors of the Mar- 

 supials and Placentals. 



I propose now to substantiate this view with 

 the help of the accompanying semi-diagram- 

 matic figures (Fig. 40). 



We have already traced the development 

 of the nipples of the Placentalia to the stage 

 of the knob-shaped primordia, which stage is 

 equally characteristic of the nipple primordia 

 of Marsupials. But whereas in the Marsupials, 

 with the exception of the Didelphyidae, the 

 knob- shaped primordia go on growing, this is 



