8 



THE AMERICAN NATURALIST [Vol. XLI1 



Conclusions 



Although the experiments undertaken in the hope of 

 inciting regeneration in parts that do not do so ordi- 

 narily, or imperfectly and only after a long time, have 

 not given any positive results as yet, still even the nega- 

 tive results are not without a certain theoretical interest. 

 The experiments with the frog showed that the lack of 

 power to regenerate, or only to regenerate imperfectly 

 after a long time, is not due to the pressure of the skin 

 on the cut end of the parts beneath. It is interesting to 

 find that when after several months the internal parts 

 begin to push out a new stump, as sometimes occurs, 

 the skin is then also incited to regeneration, and will 

 form a suitable covering for the parts beneath. It is 

 plausible to suppose that this growth of the skin is due 

 to the pressure of the new part from within. Other 

 factors may also enter into the result, but that the pres- 

 sure must be the main factor is shown by the failure of 

 the skin to regenerate when other tissues, those of the 

 tail for instance, are present, that have themselves the 

 power to regenerate, but not finding suitable conditions 

 for regeneration fail to form an embryonic knob. In its 

 absence the skin is not incited to regenerate. The fact 

 that the skin does not show anv tendency to complete 

 itself alone, although it has the power of regeneration, is 

 important as showing that the possession of this power 

 is not itself a stimulus that will lead to its development. 

 Some other condition is necessary to call it forth, and in 

 this case, that condition seems to be pressure from 

 beneath. 



Other tissues in the leg may also have the power to 

 regenerate, but fail to do so unless certain conditions are 

 realized. This state of affairs may lead us to hope that 

 with a better knowledge of the conditions we may ulti- 

 mately control them, and I trust this may be a first con- 

 That the muscles in the frog's leg have the power to 



