THE MOSAIC STAGE OF DIFFERENTIATION 



197 



found that, although they have the capacity for regeneration, the 

 regenerate still shows the defective organisation (e.g. with regard 

 to the ventral fin membrane). It appears in these cases that the 

 organism cannot regenerate a structure which has never been 

 formed in its own ontogeny : a fact of great interest in itself and 



.>■'■- \^ ' ', ' 



.^- 





^^. 





Fig. 94 

 Effect of extirpation of tail-rudiment at early stage, in Triton ; left, the operation ; 

 right, the resulting larva, with total absence of tail. (From Schaxel, Arch. 

 Entwmech. l, 1922.) 



with an important bearing on the problem of gradient-fields, to be 

 discussed in Chap. x. It is still, however, uncertain whether this 

 limitation is universal. ^ 



1 Cases are known in which an abnormal limb (which owes its abnormality to 

 the fact that its rudiment was grafted at an early embryonic stage and failed to 

 develop normally) can after amputation regenerate a normal limb (Swett, 1924). 

 Clearly, the conditions here are different from those in which a structure is ab- 

 normal, imperfect, or absent as a result of retnoval of its rudiment. The abnor- 

 mality of the grafted limb is a consequence of some local conditions due to the 

 experiment, and does not reflect any intrinsic restrictions of potency in the limb- 

 rudiment. Consequently, when a new set of conditions supervenes as a result 

 of amputation, these potencies are present and able to control the regeneration 

 of a normal limb. 



