206 THE THEORY OF THE GENE 



insects belong to this group, with the exception of the 

 Lepidoptera (moths and butterflies). In the Hymenop- 

 tera, however, another mechanism determines sex (see 

 below). The roundworms (Nematodes) and sea urchins 

 belong also to the XX-XO type. 



The Avian Type {WZ-ZZ). 



The other type of sex mechanism, the avian-moth type, 

 is shown in the diagram (Fig. 118). The male has two 

 like sex-chromosomes that may be called ZZ. These sepa- 





M&le Female 



Fig. 119. 

 Male and female chromosome groups of the fowl. (After Shiwago.) 



rate at one of the two maturation divisions and each ripe 

 sperm-cell comes to contain one Z. The female has one 

 Z-chromosome and a W-chromosome. When the eggs ma- 

 ture, each egg is left with one or the other of these chro- 

 mosomes. Half the eggs contain a Z- and half contain a 

 W-chromosome. Any W-egg fertilized by a Z-sperm pro- 

 duces a female (WZ). Any Z-egg fertilized by a Z-sperm 

 produces a male (ZZ). 



Here again we find a mechanism that automatically 

 produces two kinds of individuals, females and males, 

 in equal numbers. As before, a 1 to 1 sex-ratio results 

 from the combination of chromosomes that takes place 



