DEVELOPMENT IN MAESUPIALIA 



57 



perhaps six days old. Upon examining the 

 sections of the region of the trunk, which 

 probably contained the primordia of the mam- 

 mary apparatus, I at once saw that here exactly 

 the same primary-primordia were present as 



Fig. 19b. — Transverse Section of Six-Days-Old 

 Embryo of "Didelphys maksupialis." 



ma, Primary-primordia of mammary apparatus ; sm, primordium o( 

 trunk musculature. 



exist in Echidna — viz., little circumscribed 

 areas of the abdominal wall, characterized by 

 thickened epidermis and an underlying cutis 

 thickening, penetrated by numerous blood- 

 vessels, and down to which the primordia of the 

 trunk muscles reached (Fig. 19b, ma, and 19c). 



