10 MAMMARY APPARATUS OF THE MAMMALIA 



of pouch-like cavity of the integument, the so- 

 called mammary pouch, destined to serve for 

 the reception of the young. At the same time 

 he illustrated these statements with excellent 

 drawina-s, the most effective of which is re- 

 produced as Fig. 3. 



On the basis of this description of the 

 mammary pouches, Gegenbaur and Klaatsch 

 built up an ingenious theory by the help of 

 which they tried to explain the phylogenetic 

 development of the mammary apparatus 

 throughout the whole series of the Mammalia. 

 I shall refer again to this theory in con- 

 nection with the Didelphia and Monodelphia 

 in the succeeding chapters. 



But I must here point out that Klaatsch, on 

 the basis of this theory, thought he could 

 provide the solution of the problem before us — 

 namely, the question of the origin of the 

 incubatorium. He chose Ornithorhynchus as 

 his point of departure, and he assumed that the 

 presence of the two slits in the panniculus 

 muscle which transmit the ducts of the 

 mammary glands was the primary condition. 

 These slits were said to have made it possible 

 for the animal to seize and carry the egg in 



