Effect of Successive Removal upon Regeneration 479 
Changes in rate during the regeneration period. 
Differences in the level of the cut. 
Additional injury to the individual. 
Changes in temperature 
Changes in character and quantity of the food. 
g Individual variation in rate of regeneration due to causes not 
otherwise controlled. 
Most of these factors have been made the subjects of separate 
study, but we are concerned here merely with their elimination. 
It should, however, be stated that these studies have proved con- 
clusively the necessity of disregarding data on successive regenera- 
tion from experiments that are not carefully controlled. ‘This 
statement is made with a full realization of the obvious gaps in the 
present data, especially in some of the series. ‘These gaps are 
emphasized in the discussions of the individual methods. A 
statement of the principal ways by which the chief disturbing fac- 
tors were removed follows. The more special treatment of the 
various devices is reserved for the individual sets of experiments. 
1 The age factor was eliminated by means of several devices. 
The two principal ones are given here; (a) Whenever possible 
individuals of a single brood raised under identical conditions 
were used. Comparisons of later regenerations with the first 
were not made in the case of the successive regenerations of a 
single individual, but between two sets of individuals operated upon 
at the same time, one set for the later regeneration and the other for 
the first regeneration. (b) In twoof the experiments the first regen- 
eration of one organ or part of the body was compared with the 
second regeneration of a similar organ or part of the same individ- 
ual. This device is of value only in determining the local effect 
of successive removal since it can show no difference between the 
two rates in case the effect is wholly constitutional.’ 
2 In Crustacea the periodic physiological changes due to the 
molting habit constitute the most formidable of the sources of 
error. A detailed account of the manner in which their elimina- 
tion was undertaken is given under the separate experiments (pp. 
COMI Qn — 
*Spallanzani (1769) recognized the importance of the age factor. 
