DETERMINATION OF SEX 499 



of gametogenesis as the result of nutritive oscillations in the 

 body. 



But accepting the idea for the moment, it would appear that, 

 as the fertilised ovum of the next generation has received, 

 let us say, (for man) twelve chromosomes from the mother (by 

 hypothesis tending to the development of female offspring) and 

 twelve chromosomes from the father (by hypothesis tending to the 

 development of male offspring), it will not be biassed towards 

 either direction as far as its complement of chromosomes is con- 

 cerned. Thus, unless the cytoplasms of the ovum and the sperm, 

 which are also mingled in fertilisation, have some determining 

 influence on the chromosomes, it does not seem as if fertilisation 

 could have anything to do with the determination of sex. (We 

 are here excluding the cases already referred to where there is an 

 accessory or extra chromosome in one of the gametes, and we are 

 supposing that the chromosomes throughout their history within 

 the parents have retained their original tendencies towards pro- 

 duction of a male or towards production of a female.) 



At this point we encounter the suggestion of Ziegler, that 

 since the parental chromosomes include contributions from the 

 grandfather and grandmother, and since the relative numbers 

 of these depend on the chances of the reduction divisions in 

 maturation, it will be, so to speak, a " toss-up" whether grand- 

 fatherly or grandmotherly chromosomes predominate. If the 

 former, the child will be a boy ; if the latter, a girl. In fact, it is 

 a matter of chance. 



Suppose the potential offspring has twelve chromosomes 

 from the father and twelve from the mother, as in the human 

 species. " If amongst the former there are 8 grandmother 

 chrom.osomes and amongst the latter 7 grandmother chromo- 

 somes, the child will be a girl, for there are at least 15 of the 

 24 derived from the grandmothers' side " (T. H. Morgan, 1907, 

 p. 419). 



Retrospect. — In the Evolution of Sex (revised edition, 



