II.] 



THE COMMON FROG. 



17 



do not appear externally, and indeed a membrane, 

 termed the operculum (Fig. 2, op), is developed 

 from the front of each series of branchial apertures, 



Tig 3.- Tadpoles in different stages of development, from those just hatched 

 (i) till the adult form is ailamed (S). 



and which, extending backwards by degrees, ul- 

 timately covers over and conceals them. 



Little by little the limbs bud forth and grow, 

 the hind ones being the first visible because the 



