FORMATION OF ORGANS. 503 



at the anterior extremity, while a similar perforation, at the poste- 

 rior extremity, results in the formation of the anus. 



All these parts, however, are as yet imperfect ; and, being merely 

 in the course of formation, are incapable of performing any active 

 function. 



By a continuation of the same process, the different portions of 

 the external blastodermic layer are further developed, so as to re- 

 sult in the complete formation of the various parts of the skeleton, 

 the integument, the organs of special sense, and the voluntary 

 nerves and muscles. The tail at the same time acquires sufficient 

 size and strength to be capable of acting as an organ of locomo- 

 tion. (Fig. 201.) The intestinal canal, which has been formed from 



Fig. 201. 



TADPOLE fully developed. 



the internal blastodermic layer, is at first a short, wide, and nearly 

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

 soon, however, begins to grow faster than the abdominal cavity 

 which incloses it, becoming longer and narrower, and is at the 

 same time thrown into numerous convolutions. It thus presents 

 a larger internal surface for the performance of the digestive 

 process. 



Arrived at this period, the young tadpole ruptures the vitelline 

 membrane, by which he has heretofore been inclosed, and leaves the 

 cavity of the egg. He at first fastens himself upon the remains of 

 the albuminous matter deposited round the egg, and feeds upon it for 

 a short period. He soon, however, acquires sufficient strength and 

 activity to swim about freely in search of other food, propelling 

 himself by means of his large, membranous, and muscular tail. 

 The alimentary canal increases very rapidly in length and becomes 

 spirally coiled up in the abdominal cavity, so as to attain a length 

 from seven to eight times greater than that of the entire body. 



After a time, a change takes place in the external form of the 

 young animal. The posterior extremities or limbs begin to show 

 38 



