622 REPRODUCTION. 



straight tube, running directly from the mouth to the anus. It then 

 begins to grow faster than the abdominal cavity which encloses it r 

 becoming longer and narrower, and at the same time thrown into 

 numerous curvilinear folds. 



Arrived at this period, the young tadpole ruptures the vitelline mem- 

 brane, and leaves the cavity of the egg. He at first attaches himself to 

 the remains of the albuminous envelope, and feeds upon it for a short 

 time. He soon, however, acquires sufficient strength and activity to 

 swim about in search of other food, propelling himself by his large, 

 membranous, and muscular tail. The alimentary canal increases in 

 length and becomes spirally coiled in the abdominal cavity, attaining 

 a length from seven to eight times greater than that of the entire 

 body. 



Afterward, a change takes place in the external form of the animal. 

 The posterior limbs are the first to make their appearance, by budding 

 or sprouting from the sides of the body at the base of the tail. The 

 anterior extremities are for a time concealed beneath the integument, 

 but afterward become liberated, and show themselves externally. At 

 first both the fore and hind legs are very small, incomplete in structure, 

 and useless for locomotion. They subsequently increase in size and 

 strength ; while the tail, on the contrary, ceases to grow, and becomes 

 shrivelled and atrophied. The limbs, in fact, are destined to replace 

 the tail as organs of locomotion ; and a time at last arrives when the 

 tail has altogether disappeared while the legs are fully developed, mus- 

 cular, and powerful. Then the animal, heretofore confined to an aquatic 

 mode of life, becomes capable of living on land, and the tadpole is trans- 

 formed into the frog. 



During the same time, other changes take place in the internal organs. 

 The tadpole at first breathes by gills ; but these organs subsequently 

 become atrophied, and are replaced by lungs. The structures of the 

 mouth, of the integument, and of the circulatory system, are altered 

 to correspond with the varying conditions of the growing organism ; 

 and these transformations, taking place in part successively and in part 

 simultaneously, bring the booty at last to a state of completion. 



The development of a young animal from the egg consists therefore 

 of a series of changes, in which different organs make their appearance 

 from modifications of the blastoderm. Many of these organs are tem- 

 porary, serving for the growth of the embryo during a certain period ; 

 while others are of more permanent structure, and, after passing through 

 various alterations of size and form, become component parts of the 

 adult organism. 



