232 Biology in America 



yet been discovered. Furthermore in some cases the males 

 have lost the sexual instinct and the females have partly 

 developed hermaphroditism, producing sperms, which enter 

 the eggs but take no further part in their development, the 

 latter developing by parthenogenesis. 



The principal American workers on control of the sexual 

 cycle in parthenogenetic forms have been Shull and Whitney. 

 It has been claimed by some European zoologists that tem- 

 perature or food are controlling factors in the production of 

 males in daphnids and rotifers. The influence of the former 

 factor is seemingly borne out by the occurrence in nature 

 of differences in the cycle in different races of the same 

 species. Thus Chydorus sphaBricus, a European daphnid, 

 reproduces both parthenogenetically and sexually in the low- 

 lands of central Europe, while in the mountains, reproduction 

 is ' said to be exclusively parthenogenetic. The influence of 

 these factors is denied by the former workers. They have 

 demonstrated however a marked influence of the purity of 

 the culture medium on production of males, the proportion 

 of males to females in Hydatina senta being greater the 

 more the culture medium was diluted with spring water. 

 Possibly the controlling factor here was the relative acidity 

 of the medium, for Banta has shown the influence of this 

 factor in controlling the production of males in daphnids. 

 It has been argued that these experiments have nothing to 

 do with sex control, that all they prove is the possibility of 

 shifting one way or another the time of appearance of males, 

 in an alternating sex cycle, in support of which argument 

 is presented the fact that the same mother will produce males 

 or females from the same eggs, depending on whether or not 

 these are fertilized, similarly to the case of the bee mentioned 

 above. But is not this begging the question, for what is it 

 that determines the appearance of the males in the first place? 

 The evidence indicates that it is an external factor, and in 

 so far as this is true we certainly are controlling sex. 



There is some recent work by Riddle which indicates that 

 sex in birds is a question of metabolism, males arising from 

 germ cells of higher metabolism, larger water content and 

 less fat and phosphatides. Riddle claims the sex may be 

 controlled by controlling these factors in spite of the sex 

 chromosomes, which are "but a sign or index, not an efficient 

 cause of sex." His results, while seemingly conclusive, need 

 to be extended, and correlated with those of the cytologists 

 before any certain conclusions can be drawn however. 



The conditions found in some of the Crustacea and.gephy- 

 reans indicate that sex is not a predetermined, unchange- 

 able condition. Some of the parasitic isopods are first males 



