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ZOOLOGY: J. LOEB 
and Bancroft, but the eggs were so few in number that it was safe to 
assume that the two organisms examined would have developed into 
normal males had they lived; and the other structure of the gonads was 
such as to support this conclusion. Still it seemed desirable to make 
sure of this conclusion by examining older specimens of parthenogenetic 
frogs if they could be obtained. This opportunity offered itself in the 
case of the ten-months old parthenogenetic frog. Its gonads were 
FIG. 3. 
hardened in Zenker's fluid^ and sectioned. It was found that the gonads 
(figs. 2 and 3) were normal testicles containing normal spermatozoa 
in large numbers, which leaves no doubt as to the sex of this partheno- 
genetic frog. 
According to our present knowledge of the determination of sex it 
would appear from this that in the frog the male is heterozygous for 
sex; i.e., that the eggs are all alike and that there are two kinds of sperma- 
