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, 



Fig. 



3.— Tadpoles in different stages of development, from those just hatchet 

 (i) till the adult form is attained (8). 



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 



