480 Charles Zeleny 
493, 494, 498, 499, 502). Inevery instance all the operations of an 
experiment were made at the same intervai after the molt, since the 
neglect of this precaution has been found to be an important source 
Gierror. 
3. It is a notorious fact that under prolonged subjection to 
laboratory conditions many animals gradually lose their vigor. 
This has been demonstrated for a large number of activities. It 
often merely indicates bad living conditions in the laboratory. 
In other cases, however, it is undoubtedly due to change of envi- 
ronment alone. At any rate the effect is one that cannot be neg- 
lected and seems always to be present. In the experiments on 
successive regenerations, comparisons were made only between 
individuals having the same laboratory histories.‘ 
4 A regenerating organ does not grow at a uniform rate from 
the time of the operation until the completion of the process. 
Two sets of individuals must therefore be compared only during 
corresponding periods. Miss Durbin® has studied in detail the 
change in rate following the removal of the tail of a frog tadpole 
and finds that the rate is low for a short period, then rises very 
rapidly to a maximum at from six to nine days, then sinks rapidly 
to a lower level and finally gradually approaches zero. ‘The rapid- 
ity of the change shows the absolute necessity of the use of identi- 
cal periods in a comparison. 
5 The level of the cut must be the same in all operations of 
any two compared sets of individuals, because the rate varies 
with the level of the cut. Ellis* has shown that in the tail of the 
frog tadpole the length regenerated after several days varies directly 
as the length removed. Care was taken to avoid this source of 
error in dealing with successive removals.7 | 
6 Since the rate of regeneration varies with the temperature, 
it is necessary to have the compared sets of animals as nearly 
alike in this regard as possible. By alternating the individ uals of 
3 See especially Emmel ’06 and Zeleny ’osb. 
4See Parker ’03. 
5 See Durbin ’o9. 
6 See Ellis ’og. 
7 See especially Morgan ’o06 and Stockard ’o8. 
