2IO Basis of Sex Determination 



proof for the correctness of this interpretation as far as 

 gynandromorphism in the bee is concerned.^ 



It seems to be generally true that where sexual 

 reproduction leads only to the formation of females 

 the case finds its explanation in the fact that the 

 male-producing spermatozoa perish and only the 

 female-producing spermatozoa survive. Such an ob- 

 servation was made by Morgan on a certain species 

 of phylloxerans. 



The slight preponderance in the ntimber of one sex 

 which is occasionally found — an excess of six per cent, 

 males over females in the human race — may well find 

 its explanation on the assumption of a slightly greater 

 mortality of the female-determining spermatozoa. 



In certain forms parthenogenetic and sexual reproduc- 

 tion may alternate in a cycle, e, g., in plant lice, Daphnia, 

 and rotifers. In plant lice it has been observed for a 

 long time that when the plant is normal and the weather 

 warm the aphides remain wingless, reproduce par- 

 thenogenetically, and only females exist, and this may 

 last for years and for more than fifty generations ; but 

 that when the plant is allowed to dry out both sexes 

 appear. 



Here we are dealing with a limited determination of 

 sex inasmuch as the experimenter has it in his power to 

 prevent or allow the production of males. The facts 

 do not in all probability contradict the statements 



* Boveri, Th., Arch.f. Entwcklngsmech., 1915, xlii., 264. 



