Internal Factors of Sex Determination 419 



tion that rests on the assumption that the chromosomes that 

 arise from a female individual have a greater tendency to pro- 

 duce a female ; and those that originate from a male individual 

 have a greater tendency to produce a male. Since the child 

 gets as many chromosomes from the father as from the mother, 

 the parental chromosomes as such cannot determine the sex. 

 But it is to be recalled that amongst the parental chromosomes 

 some have come from the grandfather and some from the 

 grandmother. The relative number of chromosomes from the 

 maternal and paternal lines will be variable in number on the 

 current assumption that at the reduction division it is merely a 

 question of chance which member of a pair of homologous chro- 

 mosomes goes to one pole of the spindle and which to the other. 

 If the chromosomes of the grandfather predominate in the 

 offspring, it will be a male ; if the grandmother chromosomes 

 predominate, a female develops. 



To take an example. If the somatic number of chromosomes 

 for the human species be assumed to be 24, the child gets 12 

 from the father and 1 2 from the mother. If amongst the former 

 there are 8 grandmother chromosomes and amongst the latter 

 7 grandmother chromosomes, the child will be a girl, for there 

 are at least 15 of the 24 derived from the grandmother's 

 side. 



On Ziegler's theory of sex it is evident that whenever the re- 

 duced number of chromosomes is even, there may occur an exact 

 balance of grandmother and grandfather chromosomes, hence 

 the child can have no sex at all. In the list of cases given by 

 Ziegler himself, we find that the reduced number of the chro- 

 mosomes is an even one in 29 species and odd only in 10. 



It seems improbable that the equal balance of the maternal 

 and paternal chromosomes could be counterbalanced by the 

 presence of chromosomes derived from the grandparents, espe- 

 cially since these have also been contained in one or the other 

 parent whose sex, on the theory, should have influenced them to 

 acquire the character of that parent. These and other diffi- 

 culties make Ziegler's hypothesis very improbable. 



