Effect of Successive Removal upon Regeneration 481 
two compared sets in their positions on the laboratory tables this 
source of error was eliminated. Preliminary experiments by 
Ellis (og) indicate that the regeneration rate 1s even more sus- 
ceptible to temperature changes than is the rate of ordinary 
growth.® 
7 Morgan (’06) has shown that in the salamander, Diemyc- 
tylus viridescens, the rate of regeneration in length of the tail is 
independent to a large degree of the food supply. A similar state- 
ment is made by Spallanzani (1769).° That there is some influ- 
ence is, however, shown by the observations of Miss Durbin on the 
effect of starving or change in food upon the rate of regeneration of 
frog tadpoles.*® In order to avoid any error due to this factor, all 
animals in an experiment were fed as nearly the same quantity and 
quality of food as possible. 
8 It has been shown that the rate of regeneration of an organ 
in many cases is influenced by injuries to other parts of the same 
individual. In the present experiments animals accidentally 
injured in any part of the body are discarded. 
g Experience with a great many sets of individuals leads me 
to believe that so-called individual variations in rate of regeneration 
are due not so much to inherent differences in the animals as to 
differences in the environment. Close attention to the minor 
details of the operation and care of the animals with special refer- 
ence to keeping the two compared sets as nearly alike as possible 
often reduces individual variations in the results to a marked 
degree. As illustrations of such details may be mentioned (1) 
the alternating of individuals of the two series on the laboratory 
tables so that differences in external conditions due to position may 
be equalized, and (2) the handling of all specimens in the same 
manner at the time of the operation, including the unoperated as 
well as the operated ones. ‘The first precaution in large measure 
removes differences due to temperature, to order of feeding, to 
8 See Spallanzani 1769 and Davenport (1899). 
9 Spallanzani speaking of the regeneration of the legs and tails says, ‘‘Yet when salamanders are 
kept fasting a longer time they begin to grow more lean and tapering than those that are fed. But the 
reproduction continues still in the same way.” 
10 This journal, vol. vili, no. 3. 
11 See this journal, vol. vi, no. 3, 
