DEVELOPMENT OF THE FROG. 



555 



In the reduction of the tail the epidermis thickens and is partly cast, 

 partly dissolved ; the muscles break up, and their substance undergoes 

 intracellular digestion or is dissolved in the body juices ; the notochord 

 is repeatedly bent on itself and is also disrupted ; the same is true of 

 nervous system and blood vessels. It is a pathological process which 

 has become" normal. Some credit the leucocytes with playing a very 

 important part in the reduction of the tail ; but others restrict their 

 function to engulfing solid particles, such as pigment granules, and say 

 that most of the material degenerates until it becomes virtually fluid, 

 when it passes directly into the vascular fluid. 



Fig. 239. — Life history of a frog. — After Brehm. 



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 ; 11. tadpoles with both pairs of limbs appa- 

 rent ; 12. metamorphosis to frog. 



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 has shown that by- 

 changing the food given to young tadpoles from fish-flesh 

 to beef, and from beef to frog-flesh, he could raise the per- 

 centage of females to about ninety. 



In many respects the development of the tadpole is very 

 interesting, especially because it is a modified recapitulation 



