212 THE PERPETUATION OF ADAPTATIONS 



Ill 



FIG. 92. The chromosomes of the squash bug, ylrazsa tristis, twenty-one in the 

 male, twenty-two in the female. At maturation these chromosomes pair two by 

 two, similar ones mating together (/), one odd one (A) remaining unpaired in the 

 male, but paired in the female (/) and (ti). After the maturation divisions of 

 each primordial germ cell, two of the resulting spermatozoa will have eleven 

 chromosomes and two will have ten, while all of the egg cells will have eleven. 

 If one of the first two fertilizes the egg the result will be a female with twenty-two 

 chromosomes; if one of the latter two unites with the egg the resulting individual 

 will be a male with twenty-one chromosomes. (From Wilson.) 



