4 Vertebrate Embryology 



The young tadpole has now practically the 

 structure and habits of a fish, but very soon 

 the rudiments of the hind legs appear as small 

 protuberances at the base of the tail, one on 

 each side of the cloaca ; and by the eighth week 

 the joints and toes are formed, and the legs 

 have about the same structure as in the adult. 



The fore legs are formed at about the same 

 time as are the hind legs, but they are hidden, 

 for some time, by the operculum. The left 

 fore leg, in the course of two or three weeks, 

 projects through the above-mentioned opercu- 

 lar spout, while the leg of the opposite side 

 has to force its way directly through the oper- 

 cular fold. 



The lungs, in the meantime, have become 

 functional and the tadpole frequently comes to 

 the surface to breathe, although, for a time, 

 respiration takes place by means of both lungs 

 and gills. 



Before this remarkable metamorphosis is 

 complete and the frog is ready to begin life as 

 an air-breathing animal, the tail must be com- 

 pletely absorbed, the mouth, eyes, and other 

 structures must be greatly changed, and the 

 gills, gill-clefts, and other fish-like structures 

 must diminish or entirely disappear. 



