DEVELOPMENT IN MAESUPIALlA 59 



After the examination of the sections had 

 led to the discovery of these primary-primordia, 

 it was, of course, interesting to know if they 

 had also been macroscopically visible. Pro- 

 fessor Hill's custom of photographing his 

 objects before cutting them made it possible 

 to decide this. We were both surprised to see 

 in the photograph of the Didelphys embryo the 

 right primary-primordium clearly appearing as 

 a whitish spot on the abdomen (Fig. 19a, ma). 

 Once having noted the primary-primordia, I 

 could then ascertain their appearance in sections 

 of Dasyurus, Phascolarctos, and Phascolomys, 

 and in a well-preserved, entire embryo of Pera- 

 meles (Fig. 20). In Petaurus and ^pyprymnus 

 too, although the earliest stages were lacking, 

 formations were found which coidd be nothing 

 else but remains of the primary-primordia. 

 Thus it may be regarded as an established fact 

 that in the Marsupials, as well as in the Mono- 

 tremes, the development of the mammary 

 apparatus begins with the formation of the 

 primary-primordia. 



In the following stages, just as in the Mono- 

 tremes, the primary-primordia move from the 

 lateral position they occupy at first (Figs. 19a 



