86 MAMMARY APPARATUS OF THE MAMMALIA 



seen, the most primitive Marsupials completely 

 lack a jjouch. Moreover, in those forms 

 where a pouch exists, it does not commence to 

 develop until long after the appearance of the 

 nipple primordia.* 



We have therefore to start with the primary- 

 primordia, which here, as in the Monotremes, 

 constitute the primordia of the mammary 

 apparatus. Here, as there, we have to trace the 

 orio-in of the mammarv organs to the same 

 source — namely, to the brooding organs with 

 which the pre-mammalian ancestors were 

 provided. 



Whilst in the Monotremes the two primary- 

 primordia give rise only to the two gland areas, 

 in the Marsupials they produce tj'pically several 

 nipple primordia — indeed, fifteen and more in 

 the lowest Didelphyidas. This circumstance 

 precludes the possibility of deriving the nipple 

 primordia directly from the gland areas, as 

 hitherto has mostly been done. Their relation 

 to each other becomes clear, when we remember 



'■• In the Dasyurus, for example, the first primordia of the 

 marsupial pockets, leading to the formation of the pouch, 

 do not appear until fourteen days after the development 

 of the nipple primordia, according to Professor Hill's exact 

 statements as to the age of the young ones examined. 



