MAMMAEY HAIRS 91 



opened up, and shows clearly the remains of 

 the marsupial pockets and the nipples, which 

 have just emerged from their pouches, each 

 carrying on its tip a pencil-like tuft of 

 mammary hairs. 



Primary-primordia, nipple primordia, nipple 

 pouches with their appertaining hair and 



Fir. 33. — Nipple or Phascolarctos, showing 

 Mammary Hairs. 



gland buds, and finally the nipples — this is 

 the order of developmental succession of the 

 mammary organs in the lowest pouchless 

 Didelphyidse. In the higher forms we find, 

 in addition, the raarsupium. How, then, is the 

 appearance of this organ to be explained ? 



In order to answer this question we must 

 bear in mind that even in the pouchless 



