96 THE DETERMINATION OF SEX 



first of the two pictures is from the cell of a female, the second 

 from the cell of a male. In these cases we recognize at once 

 that the female cells are distinguished from the male by having 

 two accessory chromosomes. Wilson was able to demonstrate 

 that the eggs of these insects always contain one accessory 

 chromosome. When such an egg is fertilized by a spermato- 

 zoon that contains an accessory chromosome, then the egg 



FIG. 44. FIG. 45. 



FIG. 44. Protenor belfragei. Photogram of a group of young spermatozoa with 

 and without the accessory chromosome. (See text.) 



FIG. 45. Alydus pilosulus. Second division of the spermatocyte. The acces- 

 sory chromosome lies separated from the others, does not divide, and is going toward 

 one of the poles. 



develops with two accessory chromosomes in its nucleus, and 

 there arises a female, but if such an egg is fertilized by a sper- 

 matozoon that contains no accessory chromosome then a male 

 is produced. Fig. 43 is a somewhat incomplete picture, but 

 shows clearly that during the second division the accessory 

 chromosome has migrated undivided toward one pole. An 

 extremely interesting photograph, Fig. 44, shows a group of 

 spermatozoa. The so-called heads are circular. Half of 



