WHAT DETERMINES SEX? 101 



pay sufficient attention to differential mor- 

 tality, and this defect in method has vitiated 

 a number of similar experiments. 



Several experimenters, for instance, have 

 noticed that when a crowd of caterpillars 

 are under-fed, there is an unusually large pro- 

 portion of males among the survivors. But 

 as the sex of the caterpillar is really deter- 

 mined before it hatches out of the egg, the 

 experiments only showed that there may be 

 great differences in the rate of juvenile 

 mortality in the two sexes. The females 

 seem to be more susceptible to starvation. 



It used to be supposed that between the 

 beginning of the individual life (the fertilisa- 

 tion of the ovum) and the time when sex- 

 characters become clearly apparent, there is 

 an intervening indifferent period. But more 

 penetrating research has shown in many 

 cases that this so-called indifferent period, 

 if it is a reality at all, is very short. In many 

 cases it is possible to distinguish micro- 

 scopically between the male and female gonads 

 in the very young embryo. 



It is important, however, to bear in mind 

 cases like that of the male crab referred to 

 in Chapter IV, where a testis destroyed by 

 a parasite was replaced by an ovary pro- 

 ducing eggs. There was no indefiniteness 

 about the crab's sex-determination; what 

 happened was the activation of the germs of 

 femaleness, and also of feminine characters, 

 of whose latency there is evidence in many 

 different kinds of males. Now and again 

 these awake a little if there is a thoroughgoing 

 change of constitution, such as the parasitised 



