163] RATE OF REGENERATION— ZELENY 163 



erated material. This consisted in a comparison of the ratio between tail 

 length and body length in the operated individuals with that in control 

 unoperated individuals. This was done after regeneration had been 



tail 

 going on during the whole larval period. If the } r~T~ period is the same 



in operated as in unoperated individuals it is proper to suppose that 

 regeneration has been complete. If however the ratio is lower the 

 conclusion that regeneration is incomplete is very probably correct 

 though absolute certainty can not be assumed because of the possibility 

 of the changed ratio being due to regulatory changes in other parts of the 

 individual. 



The experiment consists of a comparison of the relative degree of 

 completeness of regeneration of the tail in four groups, (1) with no 

 operation, (2) with one-fourth of the tail removed, (3) with one-half 

 of the tail removed and (4) with three-fourths removed. The operations 

 were made as soon as possible after the animals left the egg envelopes 

 and the experiment proceeded until all four legs were well developed 

 and absorption of the gills had begun. This allowed practically the entire 

 larval period for regeneration. There were seventy individuals at the 

 start but a high mortality reduced the number very considerably. Lim- 

 nodrilus was used as food. 



The data are given in Tables 108 to 112. The average ratio between 

 tail and body length in control individuals at the end of the experiment 

 is 1.09, in individuals with one-fourth of the tail removed it is 1.01, in 

 those with one-half removed 0.93 and with three-fourths removed 0.86. 

 This progressive relative decrease in the tail length as compared with the 

 body length is very probably due to lack of completeness of regeneration 

 even though the whole larval period has been allowed for such completion. 



Discussion 



Apart from the starting stimulus in regeneration the most interesting 

 problem is undoubtedly that of the stopping stimulus. With the growth 

 once started what are the factors involved in checking it 1 In general it 

 has been assumed that regeneration goes on until the removed organ is 

 entirely replaced and that over- and under-regeneration occur but rarely. 

 The present data make it probable that incompleteness is more general 

 than has been supposed. The factors at work in bringing regeneration 

 to a close tend to overdo rather than underdo their function. 



A further investigation of the problem of completeness of regener- 

 ation would be of interest. 



