ORIGIN OF MAMMARY APPARATUS 41 



primitive Prototheria, is very difficult to re- 

 solve, but for my part I am inclined to give 

 an affirmative answer to the latter alternative. 



The great importance, then, of the primary- 

 primordia is that they throw light upon the 

 origin of the mammar}' apparatus, for they point 

 to a far older relation of the young to the ab- 

 dominal skin of the parents than that which 

 the appearance of the incubatorium conditioned. 

 We have no longer to suppose that the mam- 

 mary apparatus of the Mammals arose de novo 

 in this highest class of vertebrates, but we now 

 know that it resulted from primitive structures 

 developed in their oviparous ancestors for 

 brooding purposes. 



In conclusion, it may be mentioned that this 

 point of view throws a new light upon the 

 much -discussed question of the reasons for 

 the development of the mammary organs in 

 both sexes. Although the mammary organs 

 function only in the female, they exist also 

 in the male, and indeed in the male Monotreme 

 are fully developed. It has always been diffi- 

 cult to explain how this came to be without the 

 active participation of the male in lactation. 

 Various, and sometimes very curious, hypotheses 



