SEX DETERMINATION 255 



other into a female. Riddle has shown that the female-pro- 

 ducing egg is the larger of the two and contains the larger 

 amount of potential chemical energy. If the eggs are re- 

 moved from the nest as fast as laid, the female is induced to 

 lay a larger number of eggs than she would otherwise have 

 laid and the majority of these are female-producing. Toward 

 the end of the season nothing but females may come from 

 eggs the production of which is forced in this way. 



In such cases sex is subject to a certain amount of control 

 through the state of nutrition of the egg itself. But, neither 

 in this case nor in that of most other animals is the state 

 of nourishment of the single eggs directly affected by nourish- 

 ment of the mother. 



In certain cases (Daphnia) poor nutrition of the mother 

 may diminish the number of eggs which she liberates, without 

 increasing the proportion of males among the offspring pro- 

 duced, since nourishment of the individual egg is not lessened, 

 for the eggs under such circumstances resort to cannibalism, 

 devouring one another, and those which survive are fully 

 nourished. 



Attempts to influence the sex of an embryo or larva by 

 altered nutrition of the embryo or larva itself have proved 

 equally futile. Practically the only experimental evidence of 

 value in favor of this idea has been derived from the study of 

 insects, and this is capable of explanation on quite different 

 grounds from those which first suggest themselves. It has 

 sometimes been observed, as by Mary Treat for example, that 

 a lot of insects poorly fed produce an excess of males. In 

 such lots, however, the mortality is commonly high, and 

 more females die than males, because the female is usually 

 larger and requires more food to complete her development. 



A delayed fertilization of the egg has in certain cases, 

 notably frog's eggs, been shown to increase the percentage of 

 male offspring. This is not due to any change in the sper- 

 matozoa, as experiment clearly shows, but merely to the rela- 

 tive staleness of the egg. If the fertilization of the frog's egg 

 is delayed three or four days after its passage into the uterus, 



