PHYSIOLOGY OF REGENERATION 21 



cession, an enormous amount of growth energy being thus uti- 

 lized, which would otherwise have remained dormant in the organ- 

 ism. The evidence of economy with which this inherent power 

 of growth of the organism is used to compensate for a mutila- 

 tion is a problem which we cannot go into at this moment. The 

 interesting facts brought out by most of the recent researches 

 are, that the lost portion is not fully restored, and, as I have also 

 observed in Podarke, that regeneration produces smaller individ- 

 uals but such as have normal proportions, the new growth being 

 apparently brought to a termination when a definite form has 

 been established. It should be emphasized here that the cessa- 

 tion of regenerative growth does not imply an exhaustion of the 

 regenerative energy, for, as experiments on regeneration after 

 successive injuries show, a repeated stimulation will overcome 

 the inertia of the organism and set its formative forces into activ- 

 ity once more. Whether or not a repetition of the injury causes 

 an acceleration of the regenerative process — and apparently 

 either condition may exist — it causes an additional output of 

 regenerative energy, and the quantity of tissue generated after 

 several operations greatly exceeds that produced after a single 

 operation. Likewise, a greater degree of injury — whether increas- 

 ing or leaving unaffected the regenerative power or even decreas- 

 ing it, as in the case of Podarke — results in a larger output of 

 regenerative energy. 



The old problem of the cessation of growth recurs under a new 

 aspect: Why does the regeneration of an organism cease? And 

 why does it cease before the original size relations have been 

 restored, notwithstanding the reserve potentiality to further re- 

 generation? These are questions which must still await a solution. 



