1 6 STUDIES ON THE GERM CELLS OF APHIDS. 



SEX DETERMINATION. 



Though this study was undertaken with the hope that it, together 

 with experimental work, might throw considerable light on the much 

 discussed question of sex determination, it has yielded more abundant 

 results along other lines. 



The discovery of embryos and winter eggs in the same partheno- 

 genetic individual has been confirmed, but shown to be of rare occur- 

 rence. However, the fact that this can occur is evidence that the 

 parthenogenetic and winter eggs are different in method of develop- 

 ment, rather than in their original constitution. 



Several new points as to the relation of the sexual to the partheno- 

 genetic generations have been gained (Summary of Results, No. 7). 

 It will be seen at once that these relations are such as to prove con- 

 clusively that the changes in sex usualty attributed to changes in ex- 

 ternal conditions are really a change from the parthenogenetic to the 

 sexual mode of reproduction. This point has been fully discussed by 

 Castle in his paper on the Heredity of Sex, page 190 ('03). It might 

 still be maintained that different external conditions had caused some 

 eggs to develop into male and others into female embr) T os in the same 

 individual, had not embryos been repeatedly found of different sexes 

 and of exactly the same size, and presumably of too near the same age 

 to make it probable that the developing eggs had been given their sex 

 impulse by different external conditions. Careful examination was 

 made to see whether, in cases where embryos of both sexes occurred, 

 all from one ovary were of the same sex, and also whether, if both 

 sexes come from the same ovary, there is any definite order of succes- 

 sion in the oviducts. These are not easy questions to answer from 

 sections which cut the embryos in various planes, but it was ascer- 

 tained that both sexes occur in the same oviduct, and that embryos of 

 different sexes may alternate or two or more successive embryos may 

 be of the same sex. 



From the results recently obtained in the study of the germ cells of ) 

 other insects [McClung ('99-'o5), Sutton ('02, '03), Wilson ('05)^ 

 Stevens ('05) ] , it now seems probable that the sex character is repre-/ 

 sented in the chromatin in the germ cells and behaves like other Men- 

 delian characters, as suggested by Castle ('03). Assuming that this 7 

 is true, the spermatozoa must be dimorphic with respect to the sex 

 character — one half of them containing the male character, the other 

 half the female character — since the mothers of the males may pro- 

 duce both male and female offspring, and there is no reduction of 

 chromosomes in the eggs which develop into male embryos. 



