NIPPLE ARRANGEMENT IN DIDELPHIA 65 



of the primarj^-primordia. Consideration o£ this 

 evidence leads us to a series of conclusions, 

 which further throw an unexpected light on 

 the conditions of the nipple arrangement in 

 the Didelphyidse. 



Let us make these conclusions clear with the 

 help of the diagrams (Fig. 23, a, 6, c). Our point 

 of departure is the primary-primordia, which, 

 as we saw in the Didelphys embrj^o, formed two 

 oval areas on the lateral parts of the abdomen. 

 Projected on to a plane, we can draw them as 

 two ellipses, whose long axes converge in a 

 caudal direction. Suppose now that the nipple 

 primordia differentiate within these areas, there 

 would be a waste of space were all the primordia 

 to arrange themselves in a single row behind 

 one another. The fact is that the primary- 

 primordia leave space in breadth as well as in 

 length for the differentiation of individual 

 nipple primordia. We can therefore represent 

 their arrangement as you see in Fig. 23a, 

 where eight nipple primordia, partly unpaired 

 {a and e), partly in two rows (iyS, cy, rfS), are 

 indicated in each primary-primordium. 



Through the disappearance of the mem- 

 brana reuniens, the areas of the two primary- 

 5 



