The Role of the Chromosomes 



209 



hands, and that tlio most wliich niaii can experimentally 

 accomplisli is to intlnence tlie survival ratio between males 

 and females. 



In many animals, including insects, myriapods, arachnids, 

 nematodes, echinoderms, fowls, amj)liil)ians, rats, guinea pigs 

 and man, difiPerences occur in the number or size of the chro- 

 mosomes of the male and the female. In some cases one sex, 

 usually the female, has from one to several more chromosomes 

 than the male ; in others there is a size difference between 

 two chromosomes of a corresponding pair, which consists in 

 the female of two large chromosomes, in the male, on the 



Gynandromorph Fruit Flies 

 Courtesy of Professor T. H. Morgan. 



contrary, of a large and a small element. The ** accessory" 

 or sex chromosome may occur either free or attached to 

 another chromosome, while the relative differences in size 

 between the unequal members of a pair of sex chromosomes 

 varies all the way from equality in the two members to 

 absence of the smaller one. 



The process of sex determination in these forms is briefly 

 as follows : In the reduction division in the maturation of the 

 sex cells in the male, the members of the unequal pair of sex 

 chromosomes separate from each other, the larger passing into 

 one cell and the smaller into another cell; or one or more 

 elements may pass into one cell, while its sister cell receives 

 none or a smaller number. The details vary, but the general 



