148 HEREDITY AND ENVIRONMENT 



male-producing gametes is not changed by 

 such cases. 



On the other hand there are many observa- 

 tions which seem to indicate that the sex ratio 

 may be changed by environmental conditions 

 acting before or after fertihzation and that 

 therefore sex is determined by extrinsic rather 

 than by intrinsic causes. Many of these ob- 

 servations, as already remarked, are now 

 known to be erroneous or misleading, since 

 they do not prove what they were once sup- 

 posed to demonstrate. But there remain a 

 few cases which can not at present be ex- 

 plained away in this manner. Perhaps the 

 best attested of these are the observations of 

 R. Hertwig and some of his pupils on the 

 effects of the time of fertilization on the de- 

 termination of sex. If frog's eggs, which are 

 always fertilized after they are laid, are kept 

 for some hours before spermatozoa are mixed 

 with them, or if the female is prevented for 

 two or three days from laying the eggs after 

 they have entered the oviducts, the proportion 

 of males to females is enormously increased. 

 A wholly similar result has been reached by 



