FOOD AS A FACTOR IN THE DETERMINATION OF SEX. 4 1 



was considered to be a male. Such a method of distinguishing 

 the sexes in young frogs has been found to be unreliable, as at 

 the time of metamorphosis the genital organs are not very well 

 developed and it is often impossible to determine the sex of an 

 individual with any degree of certainty without making a histo- 

 logical examination of the gonads. 



The experiments of Yung (13) on Rana esculenta were made, 

 primarily, to study the influence of various kinds of food on the 

 development of the tadpoles, but the results seem to furnish posi- 

 tive evidence that the sex of Rana is influenced by nutrition. 

 Yung's experiments were carried out with great care, the differ- 

 ent lots of eggs being kept under similar external conditions and 

 the food alone differing in the various cases. In considering his 

 results, Yung also failed to take into account the tadpoles that died 

 during the course of the experiments, and he ascertained the sex 

 of only those individuals that underwent metamorphosis. In 

 these experiments the number of females that developed varied 

 from 70 per cent, to 75 per cent, in different cases, the greatest 

 number being found among the lot of frogs that had received 

 only animal food. In a later series of experiments Yung (14) 

 found that in a lot of 100 young frogs that had been fed exclu- 

 sively on beef, 78 per cent, were females ; the number of females 

 was found to be increased to 81 per cent, in a second lot of 100 

 tadpoles that had been fed on fish ; while in a third lot of 100 tad- 

 poles that had received the flesh of frogs as food the number of 

 females was 92 per cent. From an investigation of the sex of 

 300 young Rana esculenta that had developed under natural con- 

 ditions, Yung concluded that normally the number of females in 

 this species of Rana is about 53 per cent. The results of Yung's 

 experiments, therefore, support Born's conclusion that nutrition 

 is a decisive factor in sex determination, an abundance of food 

 leading to the development of a large proportion of females. 



In a recent paper, Cuenot (3) gives the results of a series of 

 feeding experiments which he made on the larvae of Rana tem- 

 poraria in order to test the conclusion reached by Born and Yung. 

 Cuenot's results do not agree with those obtained by the earlier 

 investigators, as in two lots of frogs that had been well nourished 

 on animal food he found an excess of males ; while in another 



