GONADS AS CONTEOLLERS OF CHARACTERISTICS 159 



best to be very poor criteria of maleness and femaleness. The 

 negative influences, such as failure of the penis and seminal 

 vesicles of the male rat to grow, are nothing more than we could 

 expect in any castrated form. It has been shown repeatedly 

 that many structures of this kind depend upon the presence of 

 the testis for their growth and development. And since the 

 primordium of these are absent in the female, we could not ex- 

 pect their development. 



Guinea-pigs as well as rats afford good material for these con- 

 siderations only from the ease with which they are handled and 

 with which they withstand operation, but they afford very poor 

 material from which to draw demonstrable conditions and defi- 

 nite conclusions in regard to sex modification of this kind. They 

 possess no distinct sexual differences, aside from the internal and 

 external sexual organs, that will specifically classify them as a 

 male or female, and hence are decidedly inadequate for experi- 

 mental purposes to decide the question at hand. 



The writer purposely postpones further discussion of the bear- 

 ing of these results until the observations on guinea-pigs are 

 complete. 



Hull Zoological Laboratories, 

 University of Chicago 



