REVIEW OF BIOLOGY OF SEX-DETERMINATION 
21 
and thereby sex^ as in the case of Seiler’s moths and Riddle’s 
pigeons. Where^ however, the male is heterogametic, sex could 
be controlled only by an agency which would differentially aid 
or inhibit the progress of one of the two types of sperm in its 
approach to or penetration of the ovum. The way is not closed 
and the facts so far learned about sex-determination make it 
seem not at all unlikely that definite control of sex will be es- 
tablished for many organisms in years not far distant. 
