Text-fig. 1. 
Diagrams to sliow the formation of the hlastocyst in : A, Proto- 
theria after Caldwell, Semon, and Hill and Wilson's description ; 
B, Metatheria after Hill's description of Dasynrus ; C. Meta- 
theria, after Selenka's description of the opossum. White = 
epihlast ; black = hypoblast and yolk ; dotted area = tropho- 
blast ; shaded area = embryonic knob. 
