SEX INHERITANCE 97 



female organs which are characteristic of this sex. 

 Baltzer found, however, that only 90 per cent, of 

 these free embryos became females; the remaining 

 10 per cent, developed into hermaphrodites. He 

 speaks of Bonellia as a protandric hermaphrodite in 

 which one or the other sort of reproductive organs 

 may be suppressed by the environment, but this is 

 only another way of describing the results. 



There are several groups in which a change from 

 parthenogenesis to sexual reproduction takes place 

 in response to changes in the environment. The 

 best known cases are the rotifer (Hydatina senta), 

 some of the daphnians (Moina and Simocephalus) 

 and certain insects (aphids). Hydatina gives the 

 clearest evidence (Fig. 33). It has been shown by 

 Whitney and by A. F. Shull that if this rotifer is fed 

 on a colorless flagellate and kept in water from old 

 cultures it can be kept indefinitely reproducing by 

 parthenogenesis, i.e., by eggs that are not fertilized. 

 If taken out of these solutions and put into spring 

 water, a certain percentage of the females will give 

 rise to daughters whose eggs may be fertilized. These 

 daughters behave therefore as sexual females. But 

 if they are not impregnated their eggs remain viable 

 and develop parthenogenetically into males. If in 

 addition to being transferred to spring water the 

 females are fed with a green flagellate, Dunaliella, 

 then, as Whitney has shown, almost all of their 

 daughters (80 per cent.) are changed into the sexual 

 form, i.e., a form producing eggs capable of being 

 fertilized (or if not fertilized, developing into males) . 



