674 EEPTILIA. 



the first time, begins to emit air from its mouth, showing that the 

 lungs have begun to be developed. The branchiae are still large. The 

 fingers upon the fore-legs are completely formed ; the hind-legs begin 

 to sprout beneath the skin ; and the creature presents, in a transitory 

 condition, the same external form as that which the Siren lacertina per- 

 manently exhibits. 



(1871.) By the thirty-sixth day the young Salamander (fig. 330, E) 

 has arrived at the development of the Proteus anguinus ; its hind-legs 

 are nearly completed, its lungs have become half as long as the trunk 

 of its body, and its branchiae more complicated in structure. 



(1872.) At about the forty-second day the Tadpole begins to assume 

 the form of an adult Triton (fig. 330, F) : the whole body becomes 

 shorter ; the fringes of the branchia3 are rapidly obliterated, so that in 

 five days they are reduced to simple prominences covered by the skin of 

 the head ; and the gill-openings at the sides of the neck, which, as in 

 fishes, allowed the water to escape from the mouth, and were in like 

 manner covered with an operculum formed by a fold of the integument, 

 are gradually closed ; the membranous fin of the tail contracts, the skin 

 becomes thicker and more deeply coloured, and the creature ultimately 

 assumes the form and habits of the perfect Newt, no longer possessing 

 branchiae at all, but breathing air, and in every particular completely 

 converted into a reptile. 



(1873.) But however curious the phenomena attending the deve- 

 lopment of the tadpoles of the amphibious Eeptiles may be to the 

 observer who merely watches the changes perceptible from day to day in 

 their external form, they acquire a tenfold interest to the physiologist 

 who traces the progressive evolution of their internal viscera more 

 especially when he finds that in these creatures he has an opportunity 

 afforded him of contemplating (displayed before his eyes, as it were, 

 upon an enlarged scale) those phases of development through which 

 the embryo of every air-breathing vertebrate animal must pass while 

 concealed within the egg. The division, therefore, of Eeptiles into such 

 as undergo a metamorphosis, and such as do not, is by no means philo- 

 sophical, although convenient to the zoologist : all Reptiles undergo a 

 metamorphosis, though not to the same extent. In the PERENNIBRAN- 

 CHIATA the change from the aquatic to the air-breathing animal is never 

 fully completed ; in the CADTJCIBRANCHIATA the change is accomplished 

 after the embryo has escaped from the ovum ; and in the REPTILIA 

 proper, as well as in BIRDS and MAMMALS, which are generally said to 

 undergo no metamorphosis, the changes referred to are accomplished 

 in ovo during the earliest periods of the formation of the foetus. 



(1874.) The second order of Reptiles (OPHIDIA) includes the Serpent 

 tribes, animals entirely deprived of external locomotive extremities, and 

 nevertheless endowed with attributes at once formidable and surprising. 

 Absolutely without limbs or any apparent means of progression, the 



