90 



CHArTER III. 



THE DEVELOPMENT OF THE FROG. 



Fkogs belong to the class Amphibia, of which toads, newts, 

 and salamanders are other well-known members, while less 

 familiar examples are afforded by the axolotl of Mexico, the 

 Proteus of the caves of Carniola and Dalmatia, the crypto- 

 branch of Japan, which attains a length of three feet or more, 

 and the curious snake-like Coecilia of tropical countries. 



As a group, Amphibia are characterised more especially 

 by the double nature of their breathing organs. When adult, 

 they all have lungs ; but in the early stages of almost all genera, 

 and throughout life in a large number, true gills are present, 

 corresponding in structure and in mode of use to those of fish. 



In the frog itself these gills are only present during the 

 early, or tadpole, period of existence ; in the later stages they 

 are replaced functionally by lungs, and in the adult they have 

 disappeared completely. The frog is thus, in the course of its 

 own life history, transformed from a water-breathing to an air- 

 breathing animal ; and, in accordance with the principle of 

 Recapitulation explained in the introductory chapter, this trans- 

 formation is to be interpreted as indicating that frogs are 

 descended from fish-like ancestors, each frog in its own develop- 

 ment repeating the ancestral history. 



The frog thus holds a position midway between Fish and 

 the higher Vertebrates ; and as frog's eggs can readily be 

 obtained in large numbers, and the embryos and tadpoles develop 

 well in captivity, the frog becomes a very convenient and 

 instructive form for practical laboratory study. 



GENERAL ACCOUNT OF THE DEVELOPMENT 

 OF THE FROG. 



Frogs' eggs are laid in water, usually during March or the 

 early part of April. 



During the act of oviposition, which may last several days, 



