188 



HEREDITY AND SEX 



Showing the number of male- and female-producers in the progeny 

 of five sister individuals of Hydatina senta, one line being reared in 

 spring water, the others in various concentrations of the filtrate 

 from old food cultures. 



Mitchell found that a sudden change in food causes 

 male-producing individuals to appear in Asplanchna; 

 and Whitney can now produce, at will, in Hydatina a 

 high percentage of male producers. Females feeding 

 on the colorless flagellate Polytoma were fed on the 

 green flagellate Duniella, and gave birth to 80 % of 

 male producers ; while the control female fed on Poly- 

 toma gave birth to only 9 % of male producers. Since 

 the eggs of the male producer give rise to sexual fe- 

 males, or to males, according to whether they are fer- 

 tilized or whether they are not, sex itself is not here 

 determined by the environment, but by fertilization ; 

 the environment determines the kind of reproduction. 



ARTIFICIAL PARTHENOGENESIS 



We have now considered some of the most striking 

 examples of natural parthenogenesis in the animal 

 kingdom. The facts show that fertilization of the egg 

 is not in itself essential for development. The in- 



