90 MAMMARY APPAKATUS OF THE MAMMALIA 



many authors assume to-day, nor yet of so 

 hetero^^eneous a nature as to necessitate the 

 association of the one with the sweat glands 

 and the other with the sebaceous glands as 

 Gegenbaur asserted. Both represent rather 

 two forms of development from an indifferent 

 type of skin gland, from which also are derived 

 the sweat glands. 



The fact that the mammary hairs, unlike 

 those of the Monotremes, have only a tem- 

 porary existence is no doubt connected with 

 the formation of the nipples. Whilst these hairs 

 in the Monotremes probably serve as paths for 

 the conveyance of the mammary secretion, 

 there can be no question of such a function in 

 the Marsupialia. Indeed, it is obvious that 

 their existence on the adult nipple would be 

 none other than a nuisance. As a matter of 

 fact they are always shed early, usually before 

 the eversion of the nipple pouches. It is only 

 in the Phascolarctid^e and perhaps also in the 

 Phalangerid;T2, where the eversion of the nipple 

 pouches is very precocious, that it is possible 

 for the mammary hairs to occur for a short 

 time at the apex of the nipple, as may be seen 

 in Fig. 33. This figure represents the pouch 



