382 Experimental 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 mortality 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 Cuénot. 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 isa good food. See Yung’s results. 
