328 N. M. Stevens. 



produced, and that all the male eggs are suppressed in the female 

 generations while all the female eggs are suppressed in the male 

 generations. There is no histological evidence of any such degen- 

 eration of the germ-cells. 



In trying to fit the Mendelian theory of dominance, as elaborated 

 by Castle, to the sex-conditions in Aphids, we meet with a peculiar 

 contradiction in the fact that the same external conditions lead to 

 the production of males and of sexual females. On the CEnothera 

 this condition is very conspicuous, for in the autumn partheno- 

 genetic reproduction is completely changed over into the sexual 

 method of reproduction. Certain apterous individuals are pro- 

 ducing male offspring, and at the same time or slightly earlier 

 other winged individuals are producing the winter egg-layers. 

 Three generations at least are involved in the winter egg produc- 

 tion — an apterous generation followed by a winged generation 

 and that by the apterous sexual females. In the case of the males 

 only two generations are necessarily involved, an apterous gen- 

 eration and the generation of winged males. The food conditions 

 which probably lead to the change in method of reproduction, 

 may therefore differ in degree, the earlier conditions starting the 

 sexual female line, and later conditions the male hne. In favor 

 of this argument is the fact that on the rose a scattered genera- 

 tion of sexual females is often met with before there are any 

 males and before the regular female sexual generation appears. 



It is perfectly evident that histological study of the germ-cells 

 combined with observation of the living insects has not settled 

 the question of sex-d: termination in the Aphid, but it has, in a 

 way, cleared the field for interpretation of the results of experiment. 

 We know, (i) that there is no "accessory chromosome," and (2) 

 that the formation of a second polar body with reduction of chro- 

 mosomes does not occur in the male generation. 



In the discussion of sex-determination by Cuenot ('99), Beard 

 ('02), O. Schultze ('02), and Lenhossek ('03) we find that the 

 evidence is overwhelmingly on the side of the view that sex is 

 determined in the egg; but to the question how sex is determined 

 in the egg, no thoroughly convincing answ^er has yet been given. 



