SEX DETERMINATION — A SUGGESTION. 11 



tozoa of the hermaphrodite generation, one chromosome lags 

 behind at reduction division and fails to reach the pole of the 

 spindle, remaining on or near the plane of division. In this 

 way two kinds of spermatozoa are formed, one with and one 

 without the X chromosome. This is believed to be a normal 

 and regular occurrence. In other cases however, a chromo- 

 some is lost quite irregularly, as in non-disjunction described 

 by Bridges 2 "' in Drosophila. Here, during the reduction 

 division, two chromosomes sometimes fail to separate, both 

 going to one daughter cell. This apparently happens most 

 often in the case of the X chromosomes, but may occur with 

 any pair of autosomes. If then, at any time during the 

 evolution of a species, one member of any pair of chromosomes 

 be lost during division of the germ cells, at meiosis the chromo- 

 some without a partner will pass to one daughter cell only. 

 Now, whether the movement of the chromosomes at anaphase 

 be due to activity at the poles of the spindle, or in the 

 chromosomes themselves, or in both, it seems probable that 

 the daughter cell which obtains the odd chromosome is in 

 way some " stronger " than its sister cell. If then there is an 

 odd number of chromosomes in the male germ mother cells the 

 " strongest " sperms will probably be the ones which have 

 obtained the full complement of chromosomes, i.e., the ones 

 with the X chromosome : also these " strongest " sperms 

 will be more likely than their weaker fellows to have the power 

 to determine the type of metabolism of the zygote at fertiliza- 

 tion, and will give female embryos, as is the case in the 

 Mammalia and Diptera. If, on the other hand, the female 

 has the odd number, the " stronger " eggs will be those which 

 have obtained the odd chromosome, and these will also have 

 a greater chance of determining the type of metabolism ;ii 

 fertilization than their weaker sisters, and so will give males. 

 as is the case in Aves and Lepidoptera. 



This idea, of course, involves the supposition that the 



