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 Cuénot 
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. 
Cuénot 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 Cuénot 
himself also reached this conclusion. Korschelt found that 
the large eggs of Dinophilus produced females and the small 
ones males. Cuénot 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 Cuénot 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. 
