322 



THE VITAL FUNCTIONS. 



shows their interior structure, composed of large 

 cells, into which the trachea (t) opens. 



Few subjects in animal physiology are more 

 deserving the attention of those whose object is 

 to trace the operations of nature in the progres- 

 sive developement of the organs, than the 

 changes which occur in the evolution of the 

 tadpole from the time it leaves the egg till it has 

 attained the form of the perfect frog. We have 

 already had occasion to notice several of these 

 transformations in the organs of the mechanical 

 functions, and also in those of digestion and 

 circulation : but the most remarkable of all are 

 the changes occurring in the respiratory appa- 

 ratus, corresponding with the opposite nature of 

 the elements which the same animal is destined 

 to inhabit in the different stages of its existence. 



