180 HEREDITY AND SEX 



A study of the chromosomes has explained how some 

 of these changes in successive generations are brought 

 about. It has explained, for instance, how males are 

 produced by parthenogenesis, and why the sexual egg 

 produces only females. Let us take up the last point 

 first. 



When the spermatocytes are produced, we find, as in 

 many other insects, that at one division a sex chromo- 

 some passes to one cell only (Fig. 93). Two classes of 

 cells are produced — one with three, one with two, 

 chromosomes. The latter degenerates, and in conse- 

 quence only the female-producing spermatozoa become 

 functional. All fertilized eggs give rise therefore to 

 females. 



The second point that has been made out concerns 

 the production of the male. When the small egg 

 produces its single polar body, all of the chromosomes 

 divide, except one, which passes out entire into the 

 polar body. In consequence the number of chromo- 

 somes left in the egg is one less than the total number. 

 In a word, there are five chromosomes in the male, 

 while there are six chromosomes in the female (Fig. 93). 

 By throwing out one chromosome, the change is effected. 

 The chromosome is the mate of the sex chromosome, 

 that appeared as a lagging chromosome in the spermato- 

 genesis. 



In the large egg no such diminution takes place, 

 consequently the diploid number of chromosomes is 

 present in the female. These unite in pairs and are 

 reduced to three when the two polar bodies of the 

 sexual egg are produced. 



We see that by means of the chromosomes we can 



