516 | Charles Zeleny 
the level of the cut. In the frog tadpole’s tail Ellis has shown 
that the amount of regeneration in length except for the first few 
days following an operation is directly proportional to the length 
removed.* Special care must therefore be taken to have the 
levels from which regeneration is to be compared as nearly iden- 
tical as possible. 
6 Successive regenerations. In a number of cases’ it has been 
shown that the rate of regeneration changes with successive 
removal. It is therefore necessary to make comparisons for the 
effect of degree of injury between such groups only as have first 
regenerations of the organ in question. Second and succeeding 
regenerations may also be compared but the danger from disturb- 
ing factors is greater than in the first. 
7 Temperature has a very direct effect upon growth and 
regeneration and it must be carefully controlled. It is probable 
that in some cases at least the influence of temperature upon 
rate of regeneration is more marked even than upon rate of 
ordinary growth. 
8 Food. Morgan has shown for salamanders that abundance 
of food has very little if any influence upon rate of regeneration 
in length of the tails.* In a study of the frog tadpole Miss Durbin 
has however found that change in quantity or in character of 
food influences the rate.* In the present experiments compared 
sets were fed alike. A source of error that could not be con- 
trolled is due to the fact that individuals with the greater injury 
are frequently handicapped in catching or manipulating their 
food. When present this error probably favors the animals with 
the lesser degree of injury. 
g Differences in manipulation. The extra handling to which 
individuals with the greater degree of injury are subjected may 
in itself be harmful apart from the removal of the appendages. 
This is especially true of aquatic animals that must be operated 
upon in the air. It was frequently seen that individuals with the 
® See this journal, vol. vii, no. 3. 
7 See this journal, vol. vii, no. 3. 
8 Morgan, 1906. The physiology of regeneration. 
9 See this journal, vol. vii, no. 3. 
