DEVELOPMENT IN DIDELPHi'S 75 



illustrated in Fig. 26, shows the beginning of 

 this process ; Fig. 25, d, a further advanced 

 stage, in which the original ingrowths are 

 already hollowed out into regular folds, which 

 are deepest laterally. Thus, in photographs 

 representing the stage 5-5 centimetres, in place 

 of the earlier solid primordia we get deep lateral 

 pouch folds, and in connection ring folds sur- 

 rounding the nipple primordia (Fig. 27). It is 

 therefore clear that the ring-shaped epidermal 

 ingrowths are nothing else than the primordia 

 of the marsupial pockets, of which we have 

 already spoken, whose value for the pouch 

 formation was so justly recognized by Klaatsch, 

 but whose phylogenetic significance he entirely 

 misunderstood. 



In the course of further development, the 

 pouch folds and marsupial pockets get com- 

 pletely hollowed out. At the same time they 

 flatten, so that the pouch area increases con- 

 siderably (Fig. 25, e, f). Meantime, marsupial 

 pockets have been formed round the cranial 

 nipple primordia (Fig. 27), but these, owing to 

 the four-rowed arrangement of the nipple 

 primordia, take no part in the formation of the 

 pouch folds. 



