196 



REPTILES AND BATRACHIANS 



that of the external gills. A few days after hatching the 

 tail elongates and becomes bordered above and below with 

 a membranous crest ; the external gills, two or three in 

 number on each side, become branched, and the mouth 

 acquires fleshy lips, a parrot-like beak, and horny teeth. 



Fig. 12. — Metamorphosis of the Common Frog. 



a. The newly laid egg. 



c. On hatching. 



e. Hind-limbs growing. 



g. On leaving the water. 



^. On the fifth day. 

 d. After loss of external gills. 

 f. Just before leaving the water. 

 //. Frog in the perfect condition. 



The external gills then disappear, and are replaced by 

 internal gills, inserted on the gill arches, which persist 

 until the final transformation ; the water which has passed 

 through the gills is expelled by a tube-like opening on the 

 left side, called the spiraculum. The limbs then develop ; 

 both pairs grow simultaneously, although, from the fact 

 that the fore limbs remain concealed under the skin, they 



