60 MAMMARY APPARATUS OF THE MAMMALIA 



and 20), and, with the disappearance of the 

 membrana reuniens inferior, approach each 

 other in a medioventral direction. But while 

 in the Echidna they remain passive until the 

 formation of the mammary hairs and glands, 

 in the Marsupialia the differentiating processes. 



/ 



./ 



Fig. 20.— Embryo of Perameles. (After Hill.) 



already started in the Didelphys embryo, go 

 steadily forwards, so that even before birth the 

 primary-primordia have entirely disappeared 

 as such, and their place is taken by a number 

 of small separate epidermal thickenings, which 

 constitute the primordia of the individual 

 nipples. These at first appear in the form of 



