THE CYCLE OF LIFE 461 



objects. The gills become branched ; the mouth opens ; 

 the food-canal lengthens till it is hke a watch-spring ; 

 four gill-clefts open from the pharynx to the exterior ; 

 the larvae feed greedily on vegetable matter, and grow 

 rapidly ; as their power of locomotion increases, the cement 

 organs dwindle. 



The true tadpole stage then begins. A skin-fold covers 

 the gills, which are absorbed, only, however, to be replaced 

 by a second very similar set. Both sets are comparable to 

 the external gills of the double-breathing lung-fishes (or 

 Dipnoi) rather than to the gills of ordinary fishes. The 

 mouth acquires horny jaws and the fleshy hps bear horny 

 papillae. A gill-chamber is formed on each side, with one 

 exhalant opening, however, to the left. The circulation 

 is hke that of a fish, and the heart is two-chambered ; 

 the tadpole is about a month old. The third period is 

 marked by the appearance of the limbs and by the 

 development of the lungs. The tadpoles come to the sur- 

 face to take gulps of air ; the circulation ceases to be 

 piscine ; the heart becomes three-chambered ; the tadpole 

 is two months old. 



The tadpole reaches its full size, and the metamorphosis 

 is close at hand. It seems to fast, but the tail, which under- 

 goes internal dissolution, furnishes, through the medium 

 of the amoeboid phagocytes, some nourishment to other 

 parts of the body. The horny jaws are lost ; the friUed 

 hps shrink ; the hitherto rounded mouth becomes frog- 

 hke ; the tongue enlarges and gains mobihty ; the eyes are 

 exposed; the fore-Umbs, which have been kept back by the 

 gill-cover, become free. The animal recovers its appetite, 

 becomes thoroughly carnivorous, gets a relatively shorter 

 intestine, has its hind legs relatively lengthened, and, having 



