HEREDITY OF SEX 253 



indication in the behaviour of the chromosomes, any 

 more than they can really see the redistribution of the 

 supposed factors carried by the chromomeres. But 

 apart from this it must be allowed that the facts of 

 experiment and of microscopic observation fit in with 

 one another in a remarkable way, and that the Men- 

 delian theory throws considerable light on the minute 

 features of cell anatomy. 



The possibility still remains that in certain cases 

 particular characters may be associated with par- 

 ticular chromosomes as a whole, and we shall next 

 proceed to describe what actually seems to be an 

 example of this sort. 



The case we have to describe is directly concerned 

 with one of the most interesting and elusive of bio- 

 logical problems namely, the problem of the heredity 

 of sex. Prof. E. B. Wilson has recently investigated 

 the behaviour of the chromosomes in the somatic cells 

 and in the germ-cells of a particular species of insect 

 known as Protenor belfragi. The case afforded by this 

 animal is remarkable, inasmuch as the somatic cells 

 in the male, and only in the male, contain an odd 

 number of chromosomes. An irregularity is accord- 

 ingly introduced into the process of fusion of the 

 chromosomes in pairs, which, as already described, 

 always precedes the formation of the germ-cells with 

 their reduced number of chromosomes. In the case 

 of the male Protenor all the chromosomes fuse in pairs 

 except one, which is, of necessity, left over. This 

 odd chromosome is described as the heterotropic 

 chromosome. The female Protenor has one more 



