XII DEVELOPMENT 209 



opercular aperture on the left side 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 concealment. 

 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 feeds largely on its own tail. 



The mouth widens, the horny jaws and papillae disappear 

 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 gradually 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 (Fig. I, 7, 8). 



It will be noticed that in the course of development of 

 the frog 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 various parts 

 and organs which are derived from each of the three 

 embryonic tissues respectively. From the ectoderm are 

 formed — the epidermis and the cutaneous glands; the 

 whole of the ner\'ous system, central and peripheral, and the 

 essential parts of the sensory organs, as well as the crystalline 



Pract. Zool. P 



