64 MAMMARY APPAKATUS OF THE MAMMALIA 



this figure it will be noticed that there is now no 

 trace of the navel-scar in the region of the linea 

 alba. On the other hand, and in spite of 

 the higher magnification, the two lateral rows 

 of nipple primordia lie much nearer to each 

 other than in the preceding preparation ; but 

 the single primordia are more separated from 

 each other in a longitudinal direction. Further, 

 at the spot occupied in Fig. 21 by the two 

 central paired primordia, there is present only 

 the tj'pical unpaired primordium. There is no 

 doubt as to the explanation of this. At the 

 closing of the navel-scar, the areas of the two 

 primary-primordia became approximated by 

 rather more, indeed, than the distance between 

 the two central nipple primordia, as I ascer- 

 tained by exact measurement. These primordia 

 in consequence became pressed together, and 

 finally fused completely — a phenomenon quite 

 similar to that observable in the group of the 

 placentalia in the case of the nipple primordia 

 of the horse and ass, as long ago known. 



Thus the central nipple of the Didelphia is 

 only apparently an unpaired formation, having, 

 in fact, sprung from paired primordia, just as 

 we should presume from the paired appearance 



