INTRODUCTION 197 



make their appearance externally long after the hinder 

 ones ; they grow rapidly, the fingers and toes soon budding 

 out. In the final stages of the metamorphosis the tail 

 begins to shrink, the tissues becoming absorbed and 

 afford food to build up the various organs of the body ; 

 the gill arches disappear, and the lungs, which had co- 

 existed as accessory respiratory organs, assume alone the 

 respirating functions ; the lips are likewise absorbed, the 

 beak is shed, and the cleft of the mouth extends ; the eyes 

 acquire movable lids ; the extremely long intestine, due 

 to the fact that vegetable matter forms the principal 

 article of food during the tadpole stage, shortens. In fact, 

 the entire body changes in form, the creature, on leaving 

 the water, appearing, but for the presence of a short, 

 stumpy tail, as a perfect frog. 



In the newts the transformation is much more gradual, 

 without the intercalation of a tadpole stage. The tail and 

 the external gills are well developed when the young leaves 

 the egg, and the fore-limbs appear some time before the 

 hind pair. Horny teeth are never present, and the 

 intestine does not assume the extraordinary development 

 which is characteristic of a tadpole. The final transforma- 

 tion in the Urodeles consists essentially in the loss of 

 the gills and in the development of the eyelids. I may 

 mention that, although the larvae of Urodeles are often 

 called tadpoles, the term, as applied to them, is quite 

 misleading, the larvae of Frogs having an altogether differ- 

 ent form and organization. 



Some Batrachians have a tendency to prolong the larval 

 life, and in the case of many Urodeles are even able to 

 breed in that condition. A few members of this class are 



