558 



COLLEGE ZOOLOGY 



Older tadpole (side view) 



Older tadpole (side view) 



f 



Newly hatched 

 tadpole (side view) 



Older tadpole (ventral 

 view dissected to show 

 intestine and gills) 



Forelimb-" 

 developing 

 under 

 operculum 



Frog after 

 metamorphosis 



Stage in metamorphosis 

 Figure 238. Life cycle of the frog from egg to adult. 



Resorption of gills 

 Development of lungS 



Advanced tadpole juSt 

 before metamorphosis 



238). The tadpole breaks out of the mem- 

 branes, hves for a few days on the yolk in 

 the digestive tract, and then feeds on algae 

 and other vegetable matter. The external 

 gills grow out into long branching tufts. A 

 skin fold or operculum grows over the ex- 

 ternal gills, which then degenerate, and are 

 replaced by internal gills; water enters the 

 mouth, passes through the gill slits, and 



out of an opening on the left side of the 

 body, the spiracle. 



The hindlimbs appear first; later the fore- 

 limbs break out. The tail decreases in size 

 because it is gradually reabsorbed as the end 

 of the larval period approaches. The gills 

 are reabsorbed, and the lungs develop to 

 take their place. Finally the form resembling 

 that of the adult frog is acquired (Fig. 238). 



