428 Evolution and Adaptation 



fertilization. He has also examined critically the evidence 

 that appeared to show that external conditions, acting on the 

 embryo, may determine the sex, and has pointed out some 

 possible sources of error that had been overlooked. The 

 best-known case is that of the tadpole of the frog, but Cuenot 

 shows not only that there are chances of error in this experi- 

 ment as carried out, but also, by his own experiments and 

 observations, that the facts themselves are not above suspi- 

 cion. He points out that at the age at which some of the tad- 

 poles were when the examination was made, it was not always 

 possible to tell definitely the sex of the individual, and least 

 of all by means of the size alone of the reproductive organs, 

 as was supposed, in one case at least, to be sufficient. In his 

 own experiments he did not find an excess of one sex over 

 the other as a result of feeding. 



Cuenot points out that Brocadello found that the larger 

 eggs laid by the silkworm give rise to from 88 to 95 per cent 

 of females, and the small eggs to from 88 to 92 per cent of 

 males. Joseph has confirmed this for Ocneria dispar, and Cudnot 

 himself also reached this conclusion. Korschelt found that 

 the large eggs of Dinophilus produced females and the small 

 ones males. Cuenot experimented with three species of flies, 

 and found that when the maggots were well nourished the 

 number of the individuals of the two sexes was about equal, 

 and when poorly nourished there were a few more females in 

 two cases, and in another about the same number of males 

 and females. 



It has been claimed that the condition of nourishment of 

 the mother may determine the number of eggs of a particular 

 sex, but Cuenot found, in three species of flies which he 

 raised, that there was a slight response in the opposite di- 

 rection. He concludes that the condition of the mother is 

 not a factor in the determination of sex. 



The first egg of the two laid in each set by the pigeon is 

 said, as a rule, to produce a male, and the second a female. 



