DEVELOPMENT OF THE COMMON FROG. 121 



it are those of the limbs, with the shoulder-girdle and 

 pelvis, and the lungs. The gills continue active; water, 

 however, no longer enters through the nostrils, but through 

 the mouth, and the nostrils have become very much 

 smaller in proportion to the increased size of the body. 



Small rudiments of the hind limbs in the form of a pair 

 of papillae were visible at the close of the second period ; 

 and they continued without change through the third 

 period, except that they increased in size with the in- 

 creased size of the body ; but at the commencement of the 

 present period (that is, on the fiftieth day) the ends oi 

 the papillae enlarge and become club-shaped. Afterwards 

 they grow broader and flattened, and the divisions of the 

 toes begin to appear : these are well-marked on the fifth 

 day, when the knee and ankle are also indicated by 

 b endings in the lengthening limbs. In ten days more 

 both fore and hind limbs may be said to be fully formed, 

 though the toes of the hind pair are still without webs. 

 The fore limbs, budding out at the back of the gills within 

 the gill-chamber, and also being much smaller than the 

 hind limbs, were not seen in their earliest stages ; but when 

 examined on the tenth day they were found to agree in 

 development with the corresponding hind limbs. During 

 the remaining fourteen days both fore and hind limbs 

 grow rapidly, and the hind toes become webbed ; at the 

 close of the period the shoulder-girdle and pelvis are com- 

 pletely formed.. 



The development of the lungs appears to proceed gra- 

 dually during almost the whole of the fourth period. The 

 first indication of their presence was on the fifth day, 

 when the tadpoles were seen to come' to the surface from 

 time to time and put out their mouths, from which a 

 small bubble of air escaped. These organs, however, were 

 not actually examined until the twentieth day, when they 



SER. III. VOL. VIII. K 



