278 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



observed that the covering of a wound in Siredon pisciformis not only with 

 transplanted living skin, which heals on rapidly, but also with dead material, 

 may prevent regeneration. In this case purely mechanical factors are probably 

 responsible for the result and we might even conclude that if the organismal 

 or organ differentials are active after transplantation of extremities, their 

 effect is only an indirect one, permitting the graft to remain in perfect apposi- 

 tion to the wound and thus to exert the needed mechanical pressure; but if 

 the differentials are not compatible with each other and the right contact 

 substances do not interact in the area of the wound, then the transplant is not 

 able to exert the required mechanical pressure on the wound surface and 

 regeneration takes place. But, there is reason for assuming that the type of 

 inhibition of the regenerative process which occurred in Schaxel's experiment 

 is different from that caused by the transplantation of an extremity bud. In 

 the former case regeneration was not actually prevented ; it began to take 

 place and then the pressure of the scar-tissue apparently did not allow the 

 regenerating extremity to break through. Therefore, in this instance the 

 regenerative processes were presumably merely inhibited and made abnormal 

 by the mechanical pressure of the overlying skin. Perhaps the inhibition of 

 the development of transplanted buds of extremities was also a pressure 

 effect of the overlying skin, although here the homoiogenous nature of the 

 transplanted skin may also have played a role. On the other hand, if two 

 well-fitting surfaces of extremities or tails are joined together, even the 

 beginning of regeneration can be obviated. In this case we have probably to 

 deal with specific contact effects rather than with non-specific mechanical 

 pressure. 



In accordance with this interpretation, and somewhat different from the 

 conclusions suggested by the experiments of Schaxel, are the results obtained 

 by Godlewski, who noted that only living tissue, especially skin with the 

 underlying cutis, was able to prevent regeneration of a tail in axolotl ; further- 

 more, only auto- and homoiotransplants, or transplants belonging to different 

 races but to the same species, were effective. Thus, according to Godlewski, 

 skin of the white axolotl grafted onto wounds in the black axolotl prevented 

 regeneration of the tail in the latter, which would otherwise have followed 

 an amputation. Godlewski assumes that this result is due to the specific effect 

 of the cutis, which remains alive after transplantation and which prevents the 

 epidermis from growing down into the underlying coagulum and initiating 

 the regenerative process. As usual, under similar conditions the inhibition of 

 regeneration is complete only if the wound has been covered in an exact man- 

 ner. If certain small areas have been left uncovered, finger-like, thin, prolifer- 

 ative buds may grow out. 



However, there is considerable difference in the conclusions of various 

 investigators as to the manner in which the regeneration of the extremity 

 takes place. We may cite the more recent experiments of Harrison, who be- 

 lieves that the extremity is produced by the mesenchyme of the extremity bud 

 and not by the ectoderm. Still, the ectoderm may exert some influence on the 

 formation of the limb and different types of ectoderm may vary in the effects 



