70 MAMMARY APPARATUS OF THE MAMMALIA 



reality there are four rows of nipples, and the 

 formula to be used is 3-5-3, and not 5-1-5. 



The same four-rowed arrangement is quite 

 obvious in many other species of Didelphyidaj. 

 In others it is sometimes more or less obscured, 

 as, for example, in Didelphyx marsupialis^ as 

 the result of secondarj' displacements or of 

 reduction in number of the nipples. In young 

 specimens, however, of this species, the four- 

 rowed arrangement is perfectly distinct. 



We now return to follow the further de- 

 velopment of the mammary apparatus, and 

 more especially the mode of formation of the 

 pouch, and, as our point of departure, we may 

 take the stage in Didelphys represented in 

 Fig. 22. The individual nipple primordia have 

 become differentiated, and have reached the 

 knob- shaped stage. The marsupium is non- 

 existent as yet, but the darker stained areas 

 in the neighbourhood of the three caudal pairs 

 of nipples indicate the beginning of its de- 

 velopment. If we look now at sections of the 

 abdominal skin in the region of those nipple 

 primordia (Fig. 25), we find in the case of young 

 3 '55 centimetres long that only these primordia 

 project into the cutis as knob-shaped buds of the 



