NIPPLES IN PLACENTALS 117 



of the area of the original nipple pouch a solid 

 circular fold, which becomes hollowed out 

 shortly after birth, and so forms the secondary 

 nipple sheath. Into this the papilla, until now 

 situated on the surface, withdraws, and does 

 not emerge until the approach of the lactation 

 period, when it forms the definitive nipple. 

 Thus these nipple sheaths are by no means 

 identical with the nipple pouches of the 

 Marsupials, which originate by the direct 

 hollowing out of the knob-shaped primordia. 



The homologues of the marsupial nipple 

 pouches are here only observable in the first 

 stages of development, and disappear very 

 quickly, just as in the case of the hedgehog. 

 If, then, after birth, the nipple sheaths arise, 

 it is clear that they are secondary formations. 

 Indeed, consideration of the mode of life of 

 the mole and the Muridfe suggests a very simple 

 explanation of the acquisition of such organs. 

 In these small burrowing animals freely pro- 

 jecting nipples would be liable to injury, and 

 the nipple sheaths therefore may be regarded 

 as of the nature of an adaptive device. 



The conditions in the Carnivora, too,are easily- 

 connected with the type of nipple develop- 



