I INTBODUCTION 15 



resulted from the fusion of an egg with a spermatozoon which 

 contained the larger idiochromosome ; the formula of its nuclei 

 therefore will be 2x + 2a, where a denotes the large idiochromosome. 

 The male on the other hand has developed from a zygote which has 

 resulted from the fusion of an egg with a spermatozoon containing 

 the small idiochromosome, and the formula of its nuclei will be 

 2x + a + b, where b denotes the small idiochromosome. Hence the 

 presence of two a chromosomes in the zygote determines the formation 

 of a female, and we can reduce the case of idiochromosomes to that 

 of the hetero-chromosome by 'supposing b diminishes until it dis- 

 appears altogether. 



Other modifications have been described by Wilson ; thus a may 

 be represented by a group of chromosomes, but this group acts as 

 a unit in the reducing division and passes to one of two daughter 

 spermatocytes. The principle therefore is the same, and Wilson, in his 

 final summary, suggests that what determines a zygote to be a female 

 is an excess of peculiar trophic chromatin in its nuclei. Whilst a zygote 

 which is characterized by a defect in this regard becomes a male. 



No such clear cases of differences between the sexes in the 

 number of chromosomes have been found outside insects. Many 

 statements as to the existence of such a dimorphism in other groups 

 have been made but have not been proved to be true. It is quite 

 obvious that such a dimorphism can only be demonstrated where the 

 number of chromosomes is few and easily counted, and that where 

 they are numerous the matter must remain in doubt. 



It is clear that the existence of special sex-determining chromo- 

 somes is irreconcilable with Weismann's conceptions of the chromo- 

 somes as bundles of equipotential "ids" closely resembling one another, 

 any one of which was able to direct the whole development. 



But the discovery of what Wilson calls sex-chromosomes has led 

 to other results of a far-reaching character. The reducing division 

 is a separation of whole chromosomes which immediately before this 

 had paired with one another. Now the idiochromosomes, when 

 present, always pair with one another, and the question arises whether 

 the pairing of the other chromosomes is not just as definite a matter 

 as that of the idiochromosomes. An examination of cases like the 

 germ cells of insects, where the number of chromosomes is small, 

 reveals the fact that in the nucleus, before "pairing" has taken place, the 

 chromosomes are of different sizes, but that there are always except 

 in the case of idiochromosomes two chromosomes of the same size, 

 and that these two pair with one another. The pairing therefore, and 

 the subsequent distribution of the members of the pair to different 

 gametes at the reducing division, is a definite and not a haphazard 

 phenomenon, and Montgomery (1904), to whom we owe this important 

 observation, has suggested that the two homologous groups of 

 chromosomes, which he asserts can be seen in all the nuclei of the 

 body, are derived from the male and female gametes respectively, to the 

 union of which the adult, which produces the germ cells, owes its origin. 



