82 MAMMARY APPARATUS OF THE MAMMALIA 



the sebaceous glands. The secondary give rise 

 to the milk glands, which in their first stages 

 are quite similar to the primordia of the sweat 

 glands in the rest of the skin (Fig. 31). At 

 first the remains of the nipple primordia are only 

 slightly raised above the level of the surround- 

 ing skin. But in the following stages the 

 so-called " cutis wall" — as Klaatsch has named 

 the part of the integument surrounding the 

 nipple primordia, coloured black in the figures — 

 grows gradually, and so brings about the 

 elevation of the nipples. These, then, are 

 produced essentially by the cutis wall, their 

 apical portions only being derived from the 

 original knob-shaped primordia. During the 

 development of the nipples the mammary hairs 

 and the appertaining sebaceous glands become 

 reduced, so that finally only the milk glands 

 open on the fully formed nipples. 



Didelphys marsupialis, so far as I have ob- 

 served hitherto, stands alone amongst the 

 Marsupials in possessing nipples of this kind, 

 which may be termed " proliferation nipples " 

 (Fig. 31,/ and ^). 



In all the other species the nipples arise from 

 the nipple pouches discovered by Morgan, and 



