390 . THE MECHANICAL FUNCTIONS. 



higher developement. Preparations are silently 

 making for a change of habitation, for the animal's 

 emerging from the waters, for the reception of 

 atmospheric air into new cavities, for the acquisi- 

 tion of limbs, suited to new modes of progression ; 

 in a word, for a terrestrial life, and for all the attri- 

 butes and powers which belong to quadrupeds. 

 The succession of forms, which these metamor- 

 phoses present, are in themselves exceedingly 

 curious, and bear a remarkable analogy to the 

 progress of the transformations of those insects, 

 which in the first stages of their existence are 

 aquatic. To the philosophic inquirer into the 

 marvellous plans of creation, the series of changes 

 which mark these singular transitions cannot fail 

 to be deeply interesting ; and occurring, as we here 

 iind them, among a tribe of animals allied to the 

 more perfect forms of organization, they afford us 

 a better opportunity of exploring the secrets of 

 their developement by tracing them from the 

 earlier stages of this complicated process, so full 

 of mystery and of wonder. 



The egg of the frog (Fig. 196) is a round mass 

 of transparent nutritive gelly, in the centre of which 

 appears a small black globule. By degrees this 

 shapeless globule exhibits the appearance of a 

 head and tail ; and in this form it emerges from 

 its prison, and moves briskly in the water. From 

 the sides of the neck there grow out feathery tufts 

 (Fig. 198, B. b), which float loosely, and without 

 protection, in the surrounding fluid. These, how- 

 ever, are mere temporary organs, for they serve the 

 purposes of respiration only until the proper gills 

 are formed, and they then shrink and become 



