12 MAMMARY APPARATUS OF THE MAMMALIA 



producing the mammary pouches. Later these 

 united, forming the incubatorium. In proof of 

 this Klaatsch relied upon observations he had 

 made on the splendid material collected by 

 Professor Semon during his well-known ex- 

 pedition in Australia. He maintained that the 

 mammary pouches were the first parts of the 

 mammary apparatus to appear during onto- 

 genesis (Fig. 4), and, further, that occasionally 

 they even persisted in the adult. Moreover, 

 Klaatsch insisted that every possible transition 

 from the stage of separate mammarj^ pouches 

 to their fusion in a sino;le incubatorium was to 

 be observed. The progress of the phylo- 

 genesis, according to Klaatsch, was marked, 

 then, by the following stages : (1) Paired slits ; 

 (2) mammary pouches ; (3) incubatorium ; 

 (4) mammary glands. 



Ruge, on the contrary, came to an entirely 

 different conclusion through his researches, 

 which were likewise made on Semen's material. 

 Ruge began by fixing the formation of the 

 incubatorium on that spot of the abdomen 

 occupied during embryonic development by 

 the navel. This coincidence led him to the 

 inference that the position of the last closure 



