DEVELOPMENT OF THE FROG, 



585 



For a considerable time the tadpole is neither male nor 

 female, but hermaphrodite. Differences in nutrition and 

 other conditions cause one kind of sexual organ to pre- 

 dominate over the other, and the tadpole becomes unisexual. 

 In nature there is no marked disproportion in the number 

 of the sexes in a brood, but Yung made experiments from 

 which he concluded that by changing the food given to 



FIG. 286. Life history of a frog. After Brehin. 



1-3. Developing ova ; 4. newly-hatched forms hanging to water- 

 weeds ; 5-6. stages with external gills ; 7-10. tadpoles during 

 emergence of limbs ; n. tadpoles with both pairs of limbs appa- 

 rent ; 12. metamorphosis to frog. 



young tadpoles from fish-flesh to beef, and from beef to 

 frog-flesh, he could raise the percentage of females to about 

 ninety. 



In many respects the development of the tadpole is very 

 interesting, especially because it is a modified recapitulation 

 of that transition from aquatic to aerial respiration, which 

 must have marked one of the most momentous epochs in 

 the evolution of Vertebrates. 



