GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



365 



§5 

 The loss of power of regeneration has been studied in connexion 

 with the tail and limbs in Amphibia. As is well known, the Urodela 

 will regenerate tails and limbs even in the adult, but in the Anura 

 the adult has lost this power, which is present only in the young 

 tadpole. It is further to be noticed that the Anuran tadpole loses 

 the power to regenerate its limbs before it loses the power of 



Fig. 176 

 Absence of regeneration after total extirpation of the field. Triton from which 

 the entire tail-field has been removed. No regeneration at all. (From Guyenot, 

 Rev. Suisse de Zool. xxxiv, 1927.) 



regenerating its tail. This may be compared with the fact that adult 

 lizards can regenerate a tail, but not a limb. 



In analysing the problem as to why the power of regeneration in 

 the Anura is limited, it is possible straightway to discard the view 

 that the degree of histological differentiation of the tissues is the 

 deciding factor. The differentiation of the leg of the adult newt, 

 with its bony skeleton, functional muscles, and fibrous connective 

 tissue, is much greater than that of the leg of the tadpole which has 



