166 Heredity and Environment 



cate that the sex ratio may be changed by environmental condi- 

 tions acting before or after , fertilization and therefore it has 

 been concluded that sex is determined by extrinsic rather than by 

 intrinsic causes. Many of these observations, as already remarked, 

 are now known to be erroneous or misleading, since they do not 

 prove what they were once supposed to demonstrate. But there 

 remain a few cases which cannot at present be explained away in 

 this manner. Perhaps the best attested of these are the observa- 

 tions of R. Hertwig and some of his pupils on the effect of the time 

 of fertilization on the determination 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 pre- 

 vented for two or three days from laying the eggs after they have 

 entered the oviducts, the proportion of males to females is enor- 

 mously increased. A wholly similar result has been observed by 

 Pearl and Parshley in the case of cattle, where delayed fertiliza- 

 tion of the egg leads to a great preponderance of males. Hertwig 

 attempts to explain his extremely interesting and important ob- 

 servations as due to the relative size of nucleus and cytoplasm of 

 the egg ; but in general this nucleus-plasma ratio may vary greatly 

 irrespective of sex and there is no clear evidence that it is a cause 

 of sex determination. 



Miss King also, working on toad's eggs, has increased the 

 proportion of females by slightly drying the eggs or by with- 

 drawing water from them by placing them in solutions of salts, 

 acids, sugar, etc., but the manner in which drying increases the 

 proportion of females is wholly unknown. All of these experi- 

 ments on sex determination in frogs and toads are somewhat 

 complicated by the difficulty of determining the sex of tadpoles and 

 young animals and the evidence is by no means conclusive that in 

 these cases sex is determined by extrinsic causes. 



Quite recently Whitney- has shown that in several species of 

 rotifers a scanty diet produces in the second filial generation only 

 female offspring while a copious diet produces as high as 95 per 



