CHAPTER VIII 



DETERMINATION OF SEX, SECONDARY SEXUAL CHARAC- 

 TERS, AND SEXUAL INSTINCTS 



I. The Cytological Basis of Sex Determination 



I. It is a general fact that both sexes appear in 

 approximately equal numbers, provided a sufficiently 

 large number of cases are examined. This fact has 

 furnished the clue for the discovery of the mechanism 

 which determines the relative number of the two sexes. 

 The honour of having pointed the way to the solu- 

 tion of the problem belongs to McClung. 1 It has 

 been known that certain insects, e. g., Hemiptera and 

 Orthoptera, possess two kinds of spermatozoa but only 

 one kind of eggs. The two kinds of spermatozoa differ 

 in regard to a single chromosome, which is either lacking 

 or different in one-half of the spermatozoa. 



The first one to recognize the existence of two kinds 

 of spermatozoa was Henking, who stated that in Pyr- 

 rhocoris (a Hemipteran) one-half of the spermatozoa 



1 McClung, C. E., "The Accessory Chromosome — Sex Determinant? " 

 Biol. Bull., 1902, iii., 43. 



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