NIPPLE DEVELOPMENT IN SQUIRREL 123 



these in the Marsupials attain complete develop- 

 ment, in the Placentals they never advance 

 beyond the first stages of development. The 

 process of retrograde development indicated 

 in the Marsupials by the premature shedding 

 of the mammary hairs has in this case pro- 

 ceeded much farther. 



Curiously enough, there is an exception to 

 this statement, which is interesting also for 

 other reasons. This is the case of the squirrel, 

 whose nipple development we may now con- 

 sider (Figs. 41-45). 



In the literature of the last twenty years the 

 statement has recurred that the squirrel pos- 

 sesses double nipple rows on each side, and 

 it has been supposed that these arose from 

 double milk lines. That seemed to me im- 

 probable, and so I have recently looked into 

 this question. As I had expected, the older 

 supposition proved to be incorrect. Only a 

 single milk line is developed on either side 

 of the body, which extends from the axilla 

 to the inguinal region (Fig. 41). In its pectoral 

 region a nipple primordium (p) has already been 

 differentiated, and a little behind, the first ab- 

 dominal primordium is appearing. In somewhat 



