THE FACTORS WHICH DETERMINE SEX 641 



female characters after the destruction of the testicles, while in 

 the female, castration merely arrests development. ^ 



Ziegler's Theory. — Ziegler ^ has put forward a theory which 

 assumes that the chromosomes which are derived from a grand- 

 parental female tend to produce a female, and that those de- 

 rived from a grandparental male tend to give rise to a male. 

 Ziegler points out that the parental chromosomes as such cannot 

 determine the sex, since these are equal in niunber. He there- 

 fore assumes that the grandparental chromosomes are the 

 directing factor, and consequently that sex is a matter of chance 

 depending on the result of the reduction division during matura- 

 tion — that is to say, upon which member of a pair of homologous 

 chromosomes goes to one pole of the spindle and which to the 

 other. If the number of chromosomes derived from the male 

 grandparent is greatest, the sex will be male, and if the 

 chromosomes from the female grandpairent are most numerous 

 the offspring will be a female. Ziegler's theory has been ad- 

 versely criticised by Morgan, who writes as foUows : — " On Ziegler's 

 theory of sex it is evident that whenever the reduced number of 

 chromosomes is even, there may occur an exact balance of grand- 

 mother and grandfather chromosomes, hence the child can have 

 no sex at aU. . . . It seems improbable that the equal balance 

 of the maternal and paternal chromosomes could be counter- 

 balanced by the presence of chromosomes derived from the 

 grandparents, especially since these have also been contained in 

 one or the other parent whose sex, on the theory, should have in- 

 fluenced them to acquire the character of that parent. These and 

 other difficulties make Ziegler's hypothesis very improbable." ^ 



Heafe's Views. — Heape * has recently expressed the belief 

 " that while each ovum and spermatozoon in the generative 

 glands contains within itself sex, which is probably determined 

 by the laws of heredity, the proportion of those male and 

 female ova and spermatozoa which are developed and set free 



1 For farther discussion see Bateson, Mendel's Principles, Cambridge, 

 1909. 



' Ziegler, Der Vererbungslehre in der Biologie, Jena, 1905. 



' Morgan, loc. cit. See also "Ziegler's Theory of Sex Determination 

 and an Alternative Point of View,'' Science, vol. xxii., 1905. 



* Heape, " Note on the Proportion of the Sexes in Dogs," Proc. Gamb. 

 Phil. Soc, vol. xiv., 1907. 



2s 



