130 MAMMART APPARATUS OF THE MAMMALIA 



squirrels, so, for instance, in the Ceylon 

 Giant Squirrel {Ratiifa macrura), where six- 

 teen vibrissai in two rows extend from 

 the navel re2:ion forwards over the whole 

 chest (Fig. 47). The striking feature here 

 is that in connection with the prolific pro- 

 duction of vibrissa the pectoral pair of 

 nipples has been lost, and that thus a part 

 of the milk-line system has failed in its 

 original purpose of producing nipples and 

 milk glands. As regards the function of 

 these hairs they are Avithout doubt of use to 

 the squirrel in connection with its arboreal 

 habits, since from their position they come 

 directly into contact with the surfaces of 

 the branch, alona: which it moves in such 

 characteristically rapid fashion. 



It would be quite unintelligible that such 

 vibrissas could spring from the nipple pri- 

 mordia if we did not know, from the phylo- 

 genesis of the mammary apparatus, of the 

 close relations between hair and milk glands, 

 traces of which we have already recognized 

 as occurring in the Placentalia. Normally 

 here the nipple primordia produce only vestiges 

 of mammary hairs, but in the accessory 



