116 MAMMARY APPARATUS OF THE MAMMALIA 



depression upon its surface. This is of con- 

 stant occurrence, and indicates the transforma- 

 tion of the primordia into nipple pouches, even 

 where these pouches do not develop further, 

 as is the case in Ruminants. 



If, now, we turn to consider the details of 

 nipple development in certain of the Placentalia, 

 we find in the hedgehog that the knob-shaped 

 primordia become hollowed out into real nipple 

 pouches. These are then everted before the 

 end of embryonic development in such a way 

 that they flatten out on the surface. At the 

 same time there arises in the centre of each 

 a little papilla, which here, as in many other 

 forms, gives origin only to one gland bud. 

 This papilla grows further, and so produces 

 the nipple, the epithelial investment of which 

 is essentially derived from the original nipple 

 pouch, even though the cutis wall may par- 

 ticipate in the formation of its basal part. 



Very similar conditions are found in the 

 Muridse and in the mole, where at first a nipple 

 pouch likewise originates. But with eversion, 

 the further development takes a different 

 course. After the elevation of the papilla 

 there arises around it and within the limits 



