CHAPTER XVII 

 SEX DETERMINATION 



The subject of sex determination must be included in 

 any consideration of genetics, but it is extensive enough 

 as measured by the amount of investigation to form the 

 subject of a book. In consequence in this connection it 

 can only be given brief consideration. The current the- 

 ories of sex determination fall naturally into two general 

 categories : 



1. Many biologists believe that sex is predetermined 

 by the chromosome equipment that enters into the 

 zygote. 



2. Other biologists believe that sex may be deter- 

 mined otherwise than by chromosomes; that it may be 

 determined at fertilization, or before, or even after- 

 ward; in any event, that it is not the chromosomes 

 that determine sex but some general physiological 

 condition. 



These are the two schools in regard to sex determina- 

 tion; in fact, each school includes a number of theories, 

 but the fundamental idea in each group is the same. 

 These two general views will be referred to as the 

 chromosome theories and the physiological theories. 

 Representative examples of each will be given first, and 

 since sex determination has been investigated more 

 thoroughly and for a much longer time in animals than in 

 plants the illustrations will be taken from animals. 



177 



