258 RECENT CYTOLOGY 



In another species of insect 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 precisely like that of 

 the single heterotropic chromosome of Protenor. It is 

 still possible to regard this chromosome as representing 

 a recessive male determinant, and to suppose that the 

 process of sex determination is precisely similar in 

 the two cases. On this supposition, the smaller idio- 

 chromosome is regarded as being without function so 

 far as sex is concerned. 



In a third insect belonging to the same natural group 

 both male and female sexes bear alike a pair of idio- 

 chromosomes 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 

 second of the three species, and to its final disappear- 

 ance in the first. 



Thus, by dint of a good deal of speculation, Wilson 

 has arrived at a possible Mendelian description of the 

 phenomenon of sex in a species in which the chromo- 



