NIPPLE ARRANGEMENT IN DIDELPHIA 61 



biconvex, lens-shaped thickenings, as seen 

 already in the caudal part of the primary- 

 primordia (Fig. 19c). But shortly before or 

 after birth they begin to grow further, and 

 finally present the knob-shaped form described 

 by Gegenbaur. They remain in this stage for a 

 long time after birth ; in the case of the opossum 

 young, indeed, during the whole period it is 

 attached to the nipple. Meantime other interest- 

 ing developmental processes are going on, which 

 lead to the appearance of the marsupium. But 

 before considering these processes we have to 

 pay attention to the mode of arrangement of 

 the nipples peculiar to the Didelphyidfe, to 

 which I referred at the beginning of this 

 chapter. How can such an arrangement, char- 

 acterized by an unpaired median nipple, arise 

 when symmetrical, paired primary-primordia 

 form the starting-point of the nipple primordia ? 

 We are provided with an answer to this 

 question if we examine Fig. 21, representing 

 the abdominal wall of one of three new-born 

 opossum young belonging to Semon's collec- 

 tion, and measuring in dorsal contour length 

 2-15 centimetres. In all the three female 

 young the same condition was clearly obser- 



