84 MAMMARY APPARATUS OF THE MAMMALIA 



siderably lengthened through the growth of 

 the neck of the knob, and then, by a process of 

 cornification, are hollowed out into nipple 

 pouches, whilst at their bases the primordia of 

 the mammary hairs, of the milk and sebaceous 

 glands, appear just as in the proliferation 

 nipples. These nipple pouches then continue 

 for a shorter or longer time, until the final 

 nipples appear through the eversion of the 

 pouches. The epithelial covering of these 

 nipples is thus for the most part furnished by 

 the nipple pouches — that is, by the original 

 knob-shaped primordia. The cutis wall only 

 enters into the formation of the basal part of 

 the nipple, if at all (Fig. 31, b-d). 



The transition between these very distinct 

 types of nipples (proliferation and eversion) is 

 afforded by the conditions met with in certain 

 Didelphyidfe — namely, the pouchless species, 

 as, for example, Marmosa murina, and others. 

 Here, in the development of the nipples 

 (Fig. 31, e), the knob-shaped primordia are not 

 reduced as in Dide/phj/s marsiipialis, nor are 

 they so considerably lengthened as in the 

 Australian forms, but are directly hollowed 

 out into nipple pouches, characterized especially 



