222 



AN INTRODUCTION TO MODERN GENETICS 



sexes and even of normal males and females is affected by duplications 

 and deficiencies of parts of the X. A controversy has developed between 

 Goldschmidt^ and Bridges^ as to whether these genes should (Bridges) 

 or should not (Goldschmidt) be considered as true sex genes; Gold- 

 schmidt wishes to argue that there is only one primary F gene in the 

 X chromosome. It is probably true, as Bridges points out, that there is 

 at present no way of classifying the genes which affect the anatomical 

 development of the sex organs into true sex genes and others. But 



=ig. 103. Time of Development of Sexual Characters in D. me/onogoster and 

 :heir stability in intersexes. — ^The table gives the age in hours of pupae in which 



their stability in intersexes. — i he table gives the age 



various stages in development are reached. Since the intersexes all start develop- 

 ment as males and later switch over to female differentiation, male characters 

 are less likely to be normal if they develop late (after the switch-over) while 

 female characters are more likely to be normal if they develop late. 



Pupal Age 

 Character 

 Male— 



Anal plates lateral 



Penis normal in shape .. 



Testes spiral 



Testes connected with vas deferens 



Penis present 



Male ducts present 



Genital arch present 

 Female — 



Anal plates dorso-ventral 



Chitinous spermathecae present 



Vaginal plates present . . 



Vagina present 



Gonads ovarian in structure 



Goldschmidt is also right in so far that there is a possible way in which 

 such a classification might be attempted. If we accept Goldschmidt's 

 hypothesis that sex differentiation is controlled by male and female 

 substances, these substances presimiably act by stimulating the develop- 

 ing organs, and the actual results of development will depend on the 

 nature of the stimulated rudiments as well as on the nature of the 

 stimulant. For instance, it may be that the sex substances act as evoca- 

 tors, when the character of the organs developed would also depend on 

 the competence of the reacting tissues. If any reaction of this kind 

 occurs, it should theoretically be possible to classify genes affecting 

 sexual differentiation into those sex genes which affect the production 

 of the evocating sex-stuffs, and those genes which affect the com- 

 petence of the organ rudiments. The distinction is essentially the same 

 1 Goldschmidt 1935^. ' Bridges 1932. 



