Chap. II.] EXTERNAL CHARACTERS. 5 



ments of legs near the root of the tail. They are at first 

 enclosed in the skin of the tail, and acquire a considerable size 

 before any sign of fore-legs appears, for the fore-limbs lie hidden 

 beneath the operculum, so as to be invisible without dissection. 

 In Fig. 2, v., the right foreleg (/. I.) has been pulled out through 

 a hole cut in the opercular membrane ; the left remains in- 

 visible. 



Thus by acquiring legs the organism passes from a fish-like to 

 a truly amphibian condition. It is still, however, a tailed 

 amphibian, like the newt or triton of our ponds. But as the 

 legs increase in size, the tail shortens and begins to atrophy ; 

 and at last, by a final ecdysis, or throwing off of skin, the oper- 

 cular membrane is got rid of, and the fore-legs are set free ; the 

 horny jaws are lost, and the mouth loses its suctorial form ; the 

 eyes, hitherto covered by skin, become freely exposed ; the gills 

 atrophy, and the gill-slits close ; and the little frog breathes 

 entirely (he has for some time breathed partially) by means of 

 lungs. The short and stumpy rudiment of a tail gradually dis- 

 appears, and the long series of changes is complete. 



The student should be careful to verify the facts for himself. 

 The frog-spawn is readily obtainable in the spring; and the 

 tadpoles are easily reared in an aquarium. 



Such a series of changes as is undergone by the frog is called 

 metamorphosis, which essentially consists in the reduction and 

 atrophy of provisional embryonic organs, and the appearance of 

 adult organs in their place, the series of changes taking place 

 during the free life of the organism. It is well to restrict the 

 word metamorphosis to this use. If the changes take place 

 before birth or before hatching, the word transformation should 

 be used for the prenatal embryonic changes. 



The time occupied in the hatching and metamorphosis of the 

 frog varies with the temperature. At about 15 C. the eggs are 

 hatched in ten days, and the metamorphosis is complete in 

 about seventy-three days; but at about 10 C. the eggs are not 

 hatched for twenty-one days, and metamorphosis is not com- 

 plete until about the 235th day. 



Even when metamorphosis is complete the frog still continues 



