PARTHENOGENESIS 187 



ness of this general conclusion would be hard to find ; 

 for, in the first place, as we have seen, the same indi- 

 vidual that produces males will produce out of the same 

 eggs females if she happens to be fertilized. In 

 the second place the older evidence which was supposed 

 to establish the view that certain specified changes in 

 the environment cause the production of males has 

 been overthrown. 



The French zoologist, Maupas, is deserving of high 

 praise for working out some of the most essential facts 

 in the life cycle of hydatina, and for opening up a 

 new field of investigation. But the evidence which 

 he brought forward to show that by a low tempera- 

 ture a high production of males is caused has not 

 been confirmed by very careful and extensive repeti- 

 tion of his experiments by Whitney and by A. F. 

 Shull. The evidence that Nussbaum obtained which 

 seemed to him to show that food conditions de- 

 termined the production of males has likewise not 

 borne the test of more recent work by Punnett, Shull, 

 and Whitney. 



It has been found, however, that the production of 

 the sexual phase of the cycle can be suppressed so that 

 the animals continue almost indefinitely propagating 

 by parthenogenesis. In several ways this may be 

 accomplished. If hydatina is kept in a concentrated 

 solution of the food culture, the sexual phase does not 

 appear. The result has nothing to do with the abun- 

 dance of food, for, if the food be filtered out from 

 the fluid medium, the filtrate gives the same result. 

 The following table given by Shull shows this very 

 clearly. 



