NIPPLE DEVELOPMENT 81 



coming to the conclusion that the marsupium 

 first originated within the limits of the mar- 

 supial series itself. 



We may now return to the consideration of 

 the nipple primordia, whose development we 

 traced to the knob-shaped stage, and which 

 have not materially altered during the develop- 

 ment of the marsupium. It is only after the 

 completion of the latter that they become 

 transformed into the definitive organs. 



If we again take Didelphys marsupialis as 

 our type, it is to be noted that the processes 

 of nipple development apparently do not agree 

 with those observed by Morgan in the kangaroo. 

 The knob-shaped nipple primordia, instead of 

 developing into nipple pouches, gradually 

 become reduced, so that in young ones about 

 15 centimetres long there are only insignificant 

 remains of them, corresponding to the bases 

 of the original knob-shaped buds. During this 

 reduction there grow out from them eight 

 epithelial sprouts, and each primary sprout 

 produces one secondary and a pair of tertiary 

 outgrowths. 



The primary sprouts represent the primordia 

 of the mammary hairs ; the tertiary those of 

 6 



