276 RECENT CYTOLOGY 



absent in the male. As will be seen later, there are 

 other cases in which this relation is reversed, and in 

 which the male has a chromosome which is lacking in 

 the female.] 



In various other insects, some of them closely allied 

 to Protenor, the somatic cells of the male, like those of 

 the female, contain each a pair of idiochromosomes ; 

 but in the male one member of the pair is much larger 

 than the other, whilst in the female they are of equal 

 size. The behaviour of the larger member of the 

 unequal pair of chromosomes, in the various nuclear 

 processes which occur during the life-history, is pre- 

 cisely like that of the single heterotropic chromosome 

 of Protenor. It is still possible to regard this chromo- 

 some as representing a single sex-chromosome, and to 

 suppose that the process of sex determination is pre- 

 cisely similar in the two cases. On this supposition, 

 the smaller idiochromosome is regarded as being with- 

 out function so far as sex is concerned. 



In a third group of insects, some nearly related to 

 Protenor, both male and female sexes bear alike a pair 

 of idiochromosomes of equal size. Here, again, it is 

 possible to apply the same theory of sex determination 

 by simply disregarding one of the idiochromosomes of 

 the male as unimportant. We may suppose, in fact, 

 that one of these chromosomes corresponds to the 

 smaller idiochromosome of the preceding case, and that 

 it takes no essential part in these phenomena. The 

 fact that this chromosome takes no active part in these 

 processes may, indeed, have led to its reduction in the 



