THE DETERMINATION OF SEX 



a. Microscopical Evidence 



207 



A, 1. The "X" Chromosome 



In 1891 Henking called attention to the presence 

 of two kinds of spermatozoa in the firefly, Pyrrho- 

 coris, and later McClung (1901), in studying the 

 spermatogenesis of the grasshopper, discovered a 

 similar phenomenon with respect to the chromosomes 

 of its spermatozoa. Soon after, Stevens and Wilson 



FIG. 62. Diagram to show how numerical equality of the sexes results 

 when one parent is homozygous (the female in this instance) and the 

 other is heterozygous for the sex character. 



working independently on various species of insects, 

 and Boveri, on sea-urchins, found that when the 

 male is characterized by two kinds of sperm-cells, 

 one of which has an "extra" chromosome (the so- 

 called "accessory" or "z" chromosome), while the 

 [other does not, the female of the same species, upon 

 [maturation of the eggs, produces mature eggs, all of 

 which possess one "z" chromosome. The result 

 jof this heterozygous condition of the male and homo- 

 Izygous condition of the female with respect to the 

 x chromosome is the theoretical equality of the 

 sexes among the individuals formed by their union, as 



