DEVELOPMENT OF THE IMPREGNATED EGG. 727 



in by the abdominal walls, and still forms a closed sac, of slightly 

 elongated figure, without inlet or outlet. Afterward, the mouth is 

 formed by means of a perforation, which takes place through both 

 external and internal layers at the anterior extremity; while a similar 

 perforation, at the posterior extremity, results in the formation of the 

 anus. 



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

 external blastodermic layer are further developed, resulting in the com- 

 plete formation of the various parts of the skeleton, the integument, 

 the organs of special sense, and- the voluntary muscles and nerves. The 

 tail at the same time acquires sufficient size and strength to be capable 

 of acting as an organ of locomotion. (Fig. 247.) The intestinal canal, 



Fig. 247. 





TADPOLE, fully developed. 



which has been formed from 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 curvilinear folds. 



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

 brane, by which he has heretofore been inclosed, and leaves the cavity 

 of the egg. He at first fastens himself upon the remains of the albu- 

 minous 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 in length and becomes spirally coiled up in the abdominal 

 cavity, attaining 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 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, (Fig. 

 248.) The anterior extremities are for a time concealed beneath the 

 integument, but afterward become liberated, and show themselves ex- 

 ternally. At first both the fore and hind legs are very small, incom- 

 plete in structure, and useless for purposes of locomotion. They soon, 

 however, 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 finally to replace the tail as organs of locomotion ; and 



