xii METAMORPHOSIS 209 



gradually disappear, the single opercular aperture closes, 

 and thenceforth respiration is purely aerial. 



In the meantime the limbs are developed. The hind- 

 limbs appear as little rounded buds, one on either side 

 of the root of the tail (D) : they gradually elongate and 

 divide into thigh, shank, and foot (E). The fore-limbs 

 appear beneath the operculum and are therefore hidden 

 at first (E) ; at a later stage each divides into upper 

 arm, fore-arm, and hand, and emerges from its conceal- 

 ment. As the limbs increase, the tail undergoes a 

 progressive shrinking ; its tissues become, as it were, 

 digested, and are carried off by the blood ; so that for a 

 time the metamorphosing tadpole is nourished largely 

 by its own tail. 



The mouth widens, the horny jaws and papillae dis- 

 appear, and teeth are formed. The suckers vanish, and 

 the intestine not only begins to grow less rapidly than 

 the body, but even becomes reduced in length and loses 

 its spiral arrangement, while vegetable is exchanged 

 for animal diet. The dark colour of the tadpole gradu- 

 ally gives place to the bright hues of the frog. The 

 little tailed frog can now leave the water and hop about 

 on land : its tail is soon absorbed, and the metamorphosis 

 is complete (Figs, i, 7, 8). 



It will be noticed that in the course of development 

 a substitution or replacement of organs occurs. Thus, for 

 instance, the notochord is replaced by a vertebral 

 column, the gills by lungs, and the horny jaws by teeth. 



Fate of the Germinal Layers. In concluding this 

 chapter we may enumerate in rather more detail the chief 

 parts and organs which are derived from each of the 

 three embryonic tissues respectively. From the ectoderm 

 are formed the epiderm and the cutaneous glands ; 

 the whole of the nervous system, central and peripheral, 

 and the essential parts of the sensory organs, as well 



PRACT. ZOOL. r> 



