728 DEVELOPMENT OF THE IMPREGNATED EGG. 



a time at last arrives (Fig. 249) when the tail has altogether disappeared, 

 while the legs have become fully developed, muscular, and powerful. 

 Then the animal, heretofore confined to an aquatic mode of life, becomes 

 capable of living upon land, and a transformation is effected from the 

 tadpole into the frog. 



Fig. 248. Fig. 249. 



TA DPOLE, with limbs beginning to be formed. Perfect FROG. 



During the same time, other changes of equal importance 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 

 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 organism ; and all these changes taking place, in part 

 successively and in part simultaneously, bring the animal at last to a 

 state of complete formation. 



The process of development, as thus far described, may be recapitu- 

 lated as follows : 



1. The germinal membrane or blastoderm, produced b} r the segmenta- 

 tion of the vitellus, consists of two cellular layers, namely, an external 

 and an internal blastodermic layer. 



2. The external blastodermic layer incloses by its dorsal plates the 

 cerebro-spinal canal, and by its abdominal plates the abdominal or 

 visceral cavity. 



3. The internal blastodermic layer forms the intestinal canal, which 

 becomes lengthened and convoluted, and communicates with the exterior 

 by a mouth and anus of secondary formation. 



4. Finally, the cerebro-spinal axis and its nerves, the skeleton, the 

 organs of special sense, the integument, and the voluntar}^ muscles, are 

 developed from the external blastodermic layer ; while the anterior and 

 posterior extremities are formed from the same layer by a process of 

 sprouting, or continuous growth. 



