14 MAMMARY APPARATUS OF THE MAMMALIA 



diice a drag upon the gland-area and so cause 

 a depression in it. Thus the secretion of the 

 mammary glands can flow into this depression, 

 and can be absorbed more easily by the young. 

 According to Ruge, then, the successive stages 

 in the development of the mammary apparatus 

 are as follows : (1) A single muscle-free area 

 on the umbilical region of the abdominal wall ; 

 (2) the incubatorium ; (3) the mammary glands ; 

 (4) the mammary pouches. 



Such, then, was the state of our knowledge 

 when, about eleven years ago, Semon's material, 

 already so often examined, was committed to 

 me for the purpose of further research. I had 

 the advantage of being entrusted with a much 

 more complete set of specimens than any of 

 my predecessors. Only for this reason could 

 the work be brought to completion, and an 

 explanation furnished of the contradictory 

 opinions which I have mentioned. The residts 

 were in the highest degree astonishing. I was 

 able to show (1) that no such structures as 

 mammary pouches exist, thus demolishing the 

 theory of Gegenbaur and Klaatsch,and (2) that 

 the incubatorium is not the first structure to 

 make its appearance in the development of the 



