ORIGIN OF INCUBATOEIUM 9 



practically all authorities have agreed that a 

 pouch was already in existence before the 

 sweat glands assumed the function of mam- 

 mary glands. The incubatorium was there- 

 fore regarded as phylogenetically the older 

 organ of the mammary apparatus. The fact 

 that it does not occur in Ornithorhynchus 

 did not constitute a serious objection, for 

 one might suppose that it disappeared with 

 the adoption of the aquatic habit. 



With this Darwin-Gegenbaur hypothesis the 

 question of the origin of the mammary ap- 

 paratus was, of course, not totally answered ; 

 since there now arises the question, How did the 

 incubatorium itself originate ? Up to the 

 present there have been two conflicting theories 

 — the one promulgated by Gegenbaur and de- 

 veloped by Klaatsch, the other expressed by 

 Ruge. 



Gegenbaur and Klaatsch started in their 

 assumptions from Owen's famous description 

 of the mammary apparatus of the Echidna, in 

 which, for the first time, the fact that the 

 nipples are absent was established. Nothing 

 was known then of the incubatorium. Owen 

 said that each gland area was situated in a sort 



