DEVELOPMENT OF THE IMPREGNATED EGG. 



themselves, by budding or sprouting from the sides of the body, 

 just at the base of the tail. (Fig. 202.) The anterior extremities also 

 grow at this time, but are at first concealed underneath the integu- 

 ment. They afterward, however, become liberated, and show them- 

 selves externally. At first both the fore and h;nd legs are very 

 small, imperfect in structure, and altogether useless for purposes of 

 locomotion. They soon, however, increase in size and strength; 

 and while they keep pace with the increasing development of the 

 whole body, the tail on the contrary ceases to grow, and becomes 

 shrivelled and atrophied. The limbs, in fact, are destined finally 

 to replace the tail as organs of locomotion ; and a time at last 

 arrives (Fig. 203) when the tail has altogether disappeared, while 



Fig. 202. 



Fig. 203. 



TADPOLE, with limbs beginning to be formed. 



Perfect FROG 



the legs have become fully developed, muscular and powerful. 

 Then the animal, which was before confined to an aquatic mode 

 of life, becomes capable of living also upon land, and a trans- 

 formation is thus effected from the tadpole into the perfect frog. 



During the same time, other changes of an equally important 

 character have taken place in the internal organs. The tadpole at 

 first breathes by gills; but these organs subsequently become 

 atrophied and disappear, being finally replaced by well developed 

 lungs. The structure of the mouth, also, of the integument, and 

 of the circulatory system, is altered to correspond with the varying 

 conditions and wants of the growing animal ; and all these changes 

 taking place in part successively and in part simultaneously, 

 bring the animal at last to a state of complete formation. 



