74 MAMMARY APPARATUS OF THE MAMMALIA 



— namely, 4*6 centimetres long — we find that 

 the lateral margins of the three pairs of caudal 

 rings have straightened out to form a continuous 

 horseshoe-shaped fold bounding a slightly de- 

 pressed area. This area represents the future 

 pouch, and the fold its wall. The way in which 



Fig. 28. — Abdominal Wall of Young Didelphys. 

 (Bresslau.) 



the originally solid ingrowths are changed into 

 folds is explained by diagrammatic sections 

 (Fig. 25, a-f). They show that within the in- 

 growths there sets in a process of cornification, 

 as the result of which they gradually become 

 hollowed out. Fig. 25, c, representing the stage 



