76 MAMMARY APPARATUS OF THE MAMMALIA 



In the next stages, the marsupial pockets and 

 their remains graduall}' disappear through the 

 continuation of the flattening process ; thus 

 the lateral walls of these pockets alone persist 

 as the wall of the marsupium. This leads us 

 to the condition seen in the 6 "4 centimetres 

 stage, where only feeble remains of the mar- 

 supial pockets are to be observed. But these 

 also are finally lost, no traces of them being 

 found in the almost completely developed 

 marsupium of, for example, a young one 

 8-7 centimetres long (Fig. 28). The inner 

 surface of the pouch, now much increased in 

 area, is perfectly smooth with the exception 

 of the nipple primordia, and the last indication 

 of the previous existence of the marsupial 

 pockets is seen in faint wrinkles and in the 

 arrangement of the hair primordia, which have 

 in the meantime begun to develop. The great 

 increase in size resulting from the flattening 

 of the inner folds is even more clearly seen 

 if one compares the figures of sections from 

 three successive stages (Fig. 25, rf, e,/"), d and 

 e taken at the same magnification, f at one- 

 quarter less. It should be noted that the nipple 

 primordia remain unchanged at the knob-shaped 



