58 MAMMAKY APPARATUS OF THE MAMMALIA 



The thickening of the epidermis in the 

 sections through the primary-primordia was not 

 everywhere luiif orml}^ distributed. In a number 

 of sections taken from the caudal region the 

 thickening was lens-shaped (Fig. 19c, lower 

 figure), just as we shall see later is the case 

 in the first primordia of the single nipples. 

 Towards the cranial region the area in ques- 



.^^, ^^ *^ 



Fig. 19c. — Transverse Section of Primary-Primor- 



DIUM OF MAJIMARY APPARATUS OF SiX-DaYS-OlD 



Embryo of "Didelphys marsdpialis." 



tion was on the whole less convex, sometimes 

 thinner, sometimes thicker ; but it was not 

 possible to distinguish individual primordia 

 from each other. The whole behaviour sua;- 

 gested that the common blastema, represented 

 by the primary-primordia, had just begun to 

 differentiate into a number of small individual 

 primordia. 



