MARSUPIAL AND PLACENTAL NIPPLES 121 



Taking into consideration the relative size of 

 the adult animals, we find exactly the same 

 condition in the Insectivora and Muridae, where 

 there is also only a single milk duct. 



Thus the fundamental differences, postulated 

 by Gegenbaur and Klaatsch between their four 

 types of placental nipple, do not exist. In all 

 the Placentalia the nipple develops just as in 

 the Marsupials, through the co-operation of 

 nipple pouch and cutis wall ; only these two 

 factors participate in a more or less varying 

 degree in the different types. According as 

 the nipple pouch or the cutis wall takes the 

 greater share in nipple formation nipples result 

 belonging to either the eversion or the prolifer- 

 ation type. But these types cannot be sharply 

 contrasted, since there is a whole series of 

 connecting stages between them. 



If, on the strength of these facts, an inquiry 

 is made as to the mutual relations of the mar- 

 supial and placental nipples, the answer is 

 plain — namely, that the proliferation or the 

 eversion nipples of the latter are not to be 

 derived directly from the corresponding nipple 

 forms of the former. On the contrary, all the 

 evidence points to the fact that the different 



