604 



AMPHIBIA. 



While these changes are in progress, and as the supply of 

 food afforded by the tail begins to be exhausted, the tadpole 

 recovers its appetite, but is now exclusively carnivorous, 

 feeding on any available animal matter, or even on its 

 fellows. The change is not, however, so great as it seems, 

 for even at a very early stage animal food is eagerly 

 devoured. 



With the change of diet, the abdomen shrinks, stomach 

 and liver enlarge, the intestine becomes relatively narrower 

 and shorter. The tail shortens more and more, and as it 

 does so the disinclination for a purely aquatic life seems to 

 increase. Eventually it is completely absorbed, the hind- 

 limbs lengthen, and the conversion into a frog is completed. 



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 phagocytes 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 almost liquid, 

 when it passes directly into the vascular fluid. 



In many respects the development of the tadpole is very 

 interesting, especially because it is a modified recapitulation 



