148 THE THEORY OF THE GENE 



able conditions of food, or when fed on the protozoon 

 Polytoma, only female rotifers occur. Each female is 

 diploid, and her eggs are at first diploid. Each egg gives 

 off only one polar body — each chromosome splitting into 

 like halves. The full number of chromosomes is retained 

 in the egg that develops by parthenogenesis into a female. 

 When fed on other food (Eugiena, for example), a new 

 type of female appears. If she is fertilized by a male at 

 the moment she emerges from the egg, she produces 

 sexual eggs only, which give off two polar bodies and 

 retain the haploid number of chromosomes. The sperm 

 nucleus, already within the egg, unites with the egg nu- 

 cleus to form a diploid female that starts once more a 

 parthenogenetic line. If, however, the special type of 

 female, just described, is not fertilized, she produces 

 smaller eggs which give off two polar bodies and prob- 

 ably retain the half number of chromosomes. They de- 

 velop by parthenogenesis into male haplonts. The male 

 is sexually mature a few hours after birth; he never 

 grows any larger and dies after a few days. 



The males of the white ''fly," Trialeurodes vaporari- 

 orum, have been shown by Schrader to be haplonts. It 

 had been discovered by A. W. Morrill that, in America, 

 virgin females of this fly give rise to male offspring only, 

 and Morrill and Back found this in another member of 

 the same family. On the other hand, in England, virgin 

 females of the same white fly give rise to females only, 

 according to Hargreaves and later to Williams. Schrader 

 (1920) studied the chromosomes in the American 

 form. There are 22 chromosomes in the female and 11 in 

 the male. The mature eggs have 11 bivalent chromosomes. 

 Two polar bodies are given off, leaving 11 single chromo- 

 somes in the egg. If the egg is fertilized 11 chromosomes 

 are added by the sperm nucleus. If the egg is not fertil- 

 ized it develops by parthenogenesis with 11 chromosomes 



