No. 524] CHROMOSOMES AND HEREDITY 



481 



The careful experiments of Cuenot and of Schultze on 

 mice have positively shown that no such relation exists 

 in these animals. 



There are two groups of animals that seemed for a 

 long time to furnish evidence in favor of the view that 

 sex is determined by the environment. I must refer to 

 these in more detail. 



The plant lice, or aphiils, produce throughout the sum- 

 mer by means of parthenogenesis a series of partheno- 

 genetic females, hi the autumn, when the food begins to 

 fail, there appear males— and vernal fan ales. If the 

 aphids and their food plants are brought into the green 

 house the males and sexual females may not appear, but 

 the animals go on reproducing by parthenogenesis. It 

 seems, therefore, that external conditions determine the 

 appearance of males and are therefore sex determining, 

 since the parthenogenetic forms are ranked as females. 



It has become apparent in recent years that these re- 

 sults have nothing to do with sex determination in the 

 sense that external conditions determine the production 

 of males or females. The results show that external 

 conditions cause the cessation of the parthenogenetic re- 

 production and the beginning of sexual reproduction. 

 i. e., the appearance of males and sexual females. 

 Whether the one, or the other, seems not to be deter- 

 mined by the environment, but to some internal mechan- 

 ism to which I shall refer later. 



In the rotifer, Hydatina scuta, Maupas claimed that 

 temperature determines sex. Later Xussbaum tried to 

 show that food conditions determine sex in this animal. 

 Still more recently both views have been disproves It 

 has been shown in the first place that here, as in the 

 aphids, the external conditions affect the life-cycle in 

 such a way that parthenogenesis ceases and sexual re- 

 production begins. Recently A. F. Shull has determined 

 that if the animals are kept in old culture water, •. e„ 

 water in which the food has been kept-parthenogenesis 

 goes on indefinitely. At least nineteen generations of 

 purely parthenogenetic individuals have been reared. 



