304 THE MECHANICAL FUNCTIONS. 



former being a side, and the latter an upper view of that 

 animal. 



Yet, with all this apparent conformity to the structure of 

 a strictly aquatic animal, the tadpole contains within its 

 organization the germs of a higher development. Prepara- 

 tions are silently making for a change of habitation, for the 

 animal's emerging from the waters, for the reception of at- 

 mospheric air into new cavities, for the acquisition of limbs 

 suited to new modes of progression; in a word^ for a terres- 

 trial life, and for all the attributes and powers which belong 

 to quadrupeds. The succession of forms, which these meta- 

 morphoses present, are in themselves exceedingly curious, 

 and bear a remarkable analogy with the progress of the trans- 

 formations 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 in- 

 teresting; and occurring as we here find them, among a tribe 

 of animals allied to the more perfect forms of organization, 

 they afibrd us a better opportunity of exploring the secrets 

 of their development by tracing them from the earlier stages 

 of this complicated process so full of mystery and of won- 

 der. 



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

 parent nutritive jelly, 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, however, are mere tempo- 

 rar}^ organs, for they serve the purposes of respiration only 

 until the proper gills are formed, and they then shrink and 

 become obliterated. The true gills, or branchise, are con- 

 tained within the body, and are four in number on each 

 side, constructed on a plan very similar to those of fishes. 

 Retaining this aquatic constitution, the tadpole rapidly in- 



