88 MAMMAKY APJPAEATUS Ol' THE MAMMALIA 



torgtcatus), which is quite similar, structurally 

 and developmentally, to the nipple-pouches. 



We therefore can picture to ourselves the 

 development of nipple primordia, nipple- 

 pouches, and nipples from the primary-primor- 

 dia without any direct reference to the gland 

 areas. In correspondence with the higher 

 number of young in the Marsupials, we find that 

 here the area of the primary-primordia was 

 utilized to the greatest possible extent. Thus 

 we see a four-rowed arrangement of the nipple 

 primordia in all Didelphyidse and also in several 

 species of the more primitive Dasyuridge, which 

 possess a high number of nipples. Fig. 32 

 shows the nipple arrangement in Fhascologale 

 Jlavipes, which would exactly correspond to the 

 Didelphyd condition, if an unpaired central 

 nipple were present. 



The fact that the area of the original primary- 

 primordium is divided into a number of separate 

 nipple primordia explains why there develops 

 in connection with each of these primordia only 

 a relatively small number of mammary hairs 

 and glands. In Didelpkys marsupialis there are, 

 for example, only eight, as already mentioned. 

 But if we multiply that number by the number 



