40 MAMMARY APPARATUS OF THE MAMMALIA 



in another respect a difference appeared with 

 their separation into two divergent stems, 

 leading to the still extant groups of the Mono- 

 tremes — the terrestrial Echidna, and the aquatic 

 Ornithorhyuchus. In the former, perhaps 

 owing to the greater development of the cutis- 

 thickenings of the brooding organs, the skin- 

 muscle failed to extend over the region between 

 and around the gland areas. The existence of 

 the muscle-free region thus left rendered pos- 

 sible the appearance, during each brooding 

 period, of a median inpushing or depression ; 

 a change in itself unimportant, but which 

 gradually led to the formation of a storing- 

 place for the developing egg — that is, an in- 

 cubatorium. In the aquatic Ornithorhynchus, 

 on the other hand, the skin muscle spread over 

 the whole abdomen, leaving only narrow slits 

 in the places of the brooding organs. What- 

 ever the origin of this difference, there can be 

 no doubt that the absence of an incubatorium 

 in Ornithorhynchus is an adaptive feature. 

 The question whether the Ornithorhynchus 

 condition developed from the Echidna c6ndi- 

 tion, or whether both were derived indepen- 

 dently of each other from the conditions in 



