50 MAMMARY APPARATUS OF THE MAMMALIA 



two nipples (Fig. 18, p) each surrounded by 

 a marsupial pocket. 



The number of nipples varies considerably 

 within the order of Marsupialia, as you will 

 gather from the table. The number two corre- 

 sponding to the two gland areas of the Mono- 

 tremes occurs but seldom. Very high numbers 

 are met with in the case of the Didelphyidfe, 

 where the arrangement of the nipples shows, 

 moreover, some peculiar features. The nipples 

 here do not simply occur in pairs, as in the 

 other families, but as their number is odd, a 

 single nipple is to be found in the middle of 

 two lateral rows. Several pouchless species 

 (Fig. 17) show lateral nipple rows stretch- 

 ing over the whole length of the trunk. Still 

 more striking is the fact that sometimes not 

 only one but two nipple rows are placed on 

 either side, as in the case of Peramys henseli 

 (Fig. 17). 



It has already been mentioned that the 

 efferent ducts of the milk glands end on the 

 apex of the everted nipples, and it only remains 

 to add that mammary hairs are not present in 

 the adult. The number of milk ducts in the 

 individual nipple is, in general, small. The 



