72 MAMMAEY APPARATUS OF THE MAMMALIA 



ring-shaped walls partly in contact with each 

 other. 



This structural condition, which leads to 

 the formation of the marsupial pockets, is very 

 clearly seen in preparations of the mammary 

 region made from these and the following 



Fig. 26. — Abdominal Wall of Young Didelphys. 

 (Brbsslau.) 



Micro-photograph, 18 diameters. 



stages (Figs. 26, 27, 28). In Fig. 26 is illus- 

 trated the condition of the mammary apparatus 

 in the stage of 44 centimetres, where, in cor- 

 respondence with the last sections, there was 

 as yet no external pouch. The ring-shaped 

 epidermal ingrowths surrounding the three 



