HEREDITY OF SEX 259 



somes of male and female are alike ; and it is a descrip- 

 tion which has its basis in actual phenomena observed 

 in two other related animals. 



Other explanations of the sex phenomena of Pro- 

 tenor and its allies may, as Wilson points out, be pos- 

 sible ; but if Cuenot's result meets with confirmation 

 from other observers, the one given above will certainly 

 take the rank of a very probable hypothesis. In any 

 case these observations of Wilson's mark a consider- 

 able advance along the road towards a complete 

 interpretation of the problem of sex determination. 

 One thing, at any rate, seems certain, and that is that 

 the male or female character is already fully deter- 

 mined in the fertilized egg, so that no subsequent 

 action of the environment can have any influence upon 

 the sex of the offspring. This definite determination 

 of sex in the very earliest stage of the zygote follows of 

 necessity from any Mendelian view of the phenomenon, 

 and the evidence afforded by Protenor points very 

 clearly in the same direction. 



By way of further illustrating the far-reaching im- 

 portance of the information which has been rendered 

 available by the combined use of experimental and 

 cytological methods, we may here briefly criticise the 

 celebrated theory of inheritance put forward by Weis- 

 mann in 1892 under the name of the ' Germ Plasm 

 Theory.' Some notice of this theory, which might 

 otherwise have been permitted to go the way of similar 

 valuable provisional hypotheses, is rendered almost 

 necessary by the circumstance of its having been 



172 



