ARRANGEMENT OF NIPPLES 67 



touch and undergo fusion. At the same time, as 

 the young ones grow in length, the individual 

 nipple primordia are pulled somewhat apart in a 

 longitudinal direction, as shown by the arrows 

 (Fig. 23c). Thus results an arrangement o£ 

 the nipple primordia, from which we can de- 

 rive all the diflFerent types met with in the 

 Didelphyidfe. 



And first you see at once that here, there 

 is a four-rowed arrangement of the nipple 

 primordia, a complex of five central nipples 

 being shut in by five on either side, corre- 

 sponding to the nipple formula 5-5-5, as usually 

 vrritten. 



In natvire this condition is exactly realized 

 in Peramys americana, as is seen in the de- 

 scription given of it by Oldfield Thomas in his 

 excellent catalogue of Marsupialia. He says : 

 " Mammae 5-5-5, the outer series placed in 

 such a curve, that the anterior pair are in line 

 with those of the inner set, and might almost 

 be reckoned as belonging to them." With 

 increase of the number of nipples, in front of 

 the primordia a single row of nipple primordia 

 differentiates on each side without affecting the 

 inner nipple complex, as in Peramys henseli. If, 



