208 



THE FROG 



Ifft side the spout-like aperture remains open for some time 

 longer, and the water from both sides passes out through it 

 (D, E). All this time the tadpole is to all intents and 

 purposes a fish ; apart from the possession of gills and a 



_ Fig. 66.— Stages in the later development of the tadpole. 

 A, stage with e.vternal gills ; B, stage showing the formation of the operculum ; 

 C, later_ stage, in which the external gills are disappearing ; D, stage with in- 

 ternal gills and budding limhs ; iJl, dissection of same stage to show the heart, 

 gills and lungs ; E, later stage, in which the limbs are further differentiated. 

 (After Howes.) 



tail-fin, the structure of the circulatory organs and various 

 other parts is more like that of a fish than of a frog. 



The lungs (D^) now appear, and the tadpole is for a time 

 truly amphibious, rising periodically to the surface of the 

 water to breathe air. At a later stage, however, the single 



