78 MAMMARY APPARATUS OF THE MAMMALIA 



marsupium can in no way be compared with 

 the incubatorium. 



The incubatorium is an unpaired single 

 formation ; the marsupium, on the other 

 hand, a complex. As we have seen, the genesis 

 of the incubatorium is conditioned by the 

 behaviour of the skin muscle, in so far as 

 primarily a muscle-free area of the abdominal 



Fig. 29. — Marsupial Pockets in Trichosurus. 

 (Bresslau.) 



wall, surrounded by the sphincter incubatorii, is 

 formed, and then becomes invaginated by a 

 tension from within. In the Marsupialia, on 

 the other hand, the sphincter marsupii takes 

 not the slightest part in the genesis of the 

 pouch. The marsupium here arises as the 

 result of formative processes in the epidermis ? 

 leading to the development of marsupial 

 pockets, wholly unrepresented in Echidna. 



