RUGE's views on INCUBATORIUM 13 



of the body wall plays a part in the formation 

 of the incubatorium, inasmuch as that area 

 remains unprovided with muscles, and thus 

 may readily become invaginated to form a 

 pouch-like depression. At all events, accord- 

 ing to Ruge, the incubatorium is certainly 

 not developed from paired primordia. What 



Fig. 4. — Ornithorhynchus : The Mammary Pouch. 

 ( After Klaatsch.) 



Klaatsch regarded as the paired primordia were 

 simply the lateral halves of the incubatorium 

 temporarily divided in the newly hatched 

 young by the navel-scar. Furthermore, there is 

 no evidence that mammary pouches are present 

 in the embryo. These could not develop until 

 later, when the mammary glands, growing out 

 laterally under the sphincter incubatorii, pro- 



