HEREDITY AND SEX 



(c) It may be that in some cases, such as tadpoles, the sex 

 is not definitely fixed until a relatively late stage, and it may be 

 that in these cases the conditions of nurture (notably nutrition 

 and temperature) may swing the balance to one side or to the 

 other — may be the deciding moment in determining whether the 

 organism is to become a sperm-producer or an egg-producer, 

 and whether the potentialities of feminine characters or of mas- 

 culine characters are to find expression in development. 



As we have said, there are many reasons for believing that in 

 mammals the sex is determined ah ovo, that, once conception has 

 occurred, no influence whatever has any effect on the sex of 

 the offspring. But it does not follow from this that influences 

 brought to bear upon the parents before liberation of ova from 

 the ovary, or of spermatozoa from the testis are of no account. 

 Nor does it follow that because the sex of the offspring is fixed 

 in the fertilised ovum of mammals, it is similarly fixed in the 

 fertilised ova of frogs or much lower animals. 



§ 3. Alleged Determining Influence of External Factors 



Influence of Food. — Various experimenters have been led 

 to conclude that the amount and quality of the food determine 

 whether an embryo or larva becomes a male or female. Thus 

 Landois, Treat, and Gentry believed that this was true of certain 

 caterpillars, but what they seem to have shown is rather that 

 the mortality is greater among the females than among the 

 males when the food is insufficient. The fact is that the sex of 

 the caterpillar is fixed when it leaves the e^g, therefore the 

 experiments were irrelevant. Even apart from this fatal ob- 

 jection, it should be noticed that the experiments alluded to 

 have not been confirmed by other experimenters such as Riley. 

 In regard to silkworms it has been shown by Kellogg and Bell 

 that the sex of the offspring is not appreciably affected by the 

 nutrition of the parents or grandparents. Cuenot found that 



