182 ANIMAL METAMORPHOSIS. 



escape from the egg, the young frog presents itself as a little fish- 

 like tadpole, with a broad head and flattened tail. From the side 

 of the head shoot out a few filamentous tufts, which float loosely 

 in the water, and imbibe from it the oxygen which it contains. 

 These branchial tufts rapidly develop till they attain their maxi- 

 mum ; they are now two on either side, each consisting of about 

 five leaves, and in them the circulation of the blood, as seen 

 through the microscope, is a most beautiful object. About the 

 seventh or eighth day, these external gills fade away, and more 

 complex branchiae are developed in chambers situated on either 

 side of the neck. After a few weeks, the hind limbs make their 

 appearance ; the fore-legs succeed these. In the meantime the 

 lungs have been developed, and the gills begin to be obliterated, 

 the circulation being gradually adapted to the altered condition of 

 the respiration. The tail now shrinks by a process of absorption, 

 and ultimately disappears, and the frog is complete. 



A similar transformation takes place in the Newts, the only 

 differences being that the branchial tufts are retained longer than 

 n frogs, while the tail is not absorbed, and the fore-legs are the 

 first to be developed. 



Among the other Amphibians are some which commence life 

 as water-breathing larvae, but in the adult condition also possess 

 air-breathing lungs, while with others the gills are transitory, wast- 

 ing and disappearing on the development of the lungs. The 

 curious Mexican Axolotl usually possesses only branchiae, and 

 passes all its life in water, but under certain circumstances there is 

 no doubt that it may lose its gills and become less aquatic in 

 its habits without thereby suffering any apparent change. Not 

 long ago, at the Paris Zoological Gardens, some of these creatures 

 crept out of their tank on to dry land, and when discovered were 

 found to have become gill-less Salamanders, breathing only 

 through their lungs ; thus rendering it probable, as suggested by 

 Cuvier, that these species are merely larval Salamanders, whose 

 development is arrested in their progress to the adult form. 



With the Amphibians the process of extra-uterine metamor- 

 phosis in the animal kingdom ends, and it now only remains, in 

 conclusion, to see what is the object of the various changes we 

 have been considering, and what deductions may be drawn from 



