External Factors of Sex Determination 381 
Thus while poor nourishment increases the percentage of 
males, good nourishment does not increase the percentage of 
females above the normal. The results may be due, Pictet 
admits, to mortality rather than to a change in sex affected by 
the food. 
A number of experiments have been carried out with tadpoles 
of frogs. An observation made by Born seemed to him to 
show that the sex of the tadpole of the frog is determined by the 
amount or by the kind of food eaten by the young animal. He 
found that when the tadpoles of Rana temporaria (from 
eggs artificially fertilized) were fed on a mixed vegetable and 
meat diet, that 95 per cent of them were females and 5 per cent 
were males. This was a general result obtained from several 
lots of 1443 tadpoles in all. In some aquaria all the indi- 
viduals were females. 
The experiments of Yung (1883) are more discriminating. 
He fed tadpoles of Rana esculenta on different kinds of food. 
In one lot (A) the flesh of fish was given. These tadpoles were 
large and well developed. In another lot (B) the flesh of beef 
and of fish was used. These tadpoles were also large. Ina 
third lot (C) the white of eggs was used. These tadpoles were 
smaller. In a fourth lot (D) the yolk of hen’s egg was given. 
These tadpoles were even smaller than the last. The number 
of males and females that developed is shown in the following 
table: — 
ae Matzs |Femates| P°V2™| Losr see 
Frocs nue 3 FEMALES 
Lot A (fish) 24 4 7 2 I 70 % 
Lot B (fish and beef) 33 6 25 2 - 15% 
Lot C (white of egg) Io 3 7 - I 470 % 
Lot D (yolk of egg) 7 fe) 5 2 - 41% 
There are also a few other statements by other authors in 
regard to the influence of food. Thus Balbiani and Henneguy 
state that an excess of females is found in tadpoles fed on yolk 
