382 Experiinejital Zoology 



of egg ^ as compared with those that obtain only a vegetable 

 diet. 



These results on tadpoles have been shown to be inconclusive 

 for the following reasons: — The method followed by Born of 

 determining the sex of the tadpoles at their time of metamor- 

 phosis was very inexact. He relied on the size of the gonads 

 (testis or ovary), but histological examination has shown that the 

 female gonad is not always larger than the male, and there are 

 always so many cases intermediate in size as to render the con- 

 clusions invalid. Even the method of teasing out the gonads, 

 a method also used by Born, cannot be relied upon. Further- 

 more, unless the number of tadpoles that die is taken into ac- 

 count (and the number may be considerable), we cannot be cer- 

 tain that the results may not be due to greater mortahty of one 

 or the other sex under certain conditions. If further evidence 

 were needed to invalidate these results, they are to be found in 

 Pfliiger's observations on the proportion of males and females 

 in certain species of frogs. As already pointed out, the eggs 

 from certain localities give a high percentage of females, and the 

 same disproportion of adult frogs is found under natural condi- 

 tions. It is true that this does not in itself show that the sex 

 may not be determined by the external conditions; but if the 

 natural disproportion of males to females is very great, error 

 may easily creep into the experimental results. 



The most important objections to the results of Born and of 

 Yung are found in the more recent experiments of Cuenot. His 

 results are as follows: The eggs of Rana temporaria were used. 

 The first lot lived on a vegetable diet. They suffered from the 

 confined space, and the tail was often malformed. Their de- 

 velopment was retarded and the genital organs (gonads) were 

 small. The 26 young that underwent metamorphosis were all 

 females. In the second lot the conditions were the same as the 

 last, and 3 females and 4 males were found. The third lot was 

 kept in a large aquarium with cool, running water. The tails 

 were cut off several times with the intention of prolonging thereby 



^ It is not clear that yolk of egg is a good food. See Yung's results. 



