122 MAMMARY APPARATUS OF THE MAMMALIA 



nipple forms have developed independently in 

 the series of the Placentalia, and from such 

 a condition as is seen in the hedgehog or the 

 Carnivora. Here, indeed, we have essentially 

 the same type of nipple as is observable in 

 Marmosa murina amongst the Marsupials. 

 This simple type of eversion nipple represents, 

 then, the original nipple form as it arose in 

 the Metatheria, and constitutes, therefore, the 

 common point of departure for the phylo- 

 genetic development of the marsupial as well 

 as of the placental nipples. 



If then the nipples of the marsupials and 

 Placentals have had a common origin, it is not 

 astonishing that the milk glands in both orders 

 should also completely harmonize with each 

 other. Onto2;eneticallv we have obtained in 

 recent years especially clear evidence of this 

 homology through the proof that in some 

 Placentals (horse, ass, cat, man) the milk-gland 

 buds arise in connection with rudimentary 

 primordia of mammary hairs and sebaceous 

 glands. The milk glands of the Placentalia 

 thus belong, like those of the Marsupials, to 

 the sweat-gland type. As regards the mam- 

 mary hairs, it may be mentioned that whilst 



