Degree of Injury and Rate of Regeneration 515 
‘compared groups with different degrees of injury as nearly alike 
in all other respects as possible. Each of the disturbing factors 
is taken up in turn. 
I Age. It is necessary to have the individuals of two com- 
pared groups of the same age, of the same brood if possible, 
because it is known that rate of regeneration changes with age. 
According to general belief rate decreases with age but excep- 
tions to this rule are not unknown. } 
2. Periodic physiological changes. In the Arthropods in gen- 
eral thé outer covering is cast off at intervals and it has been 
shown that growth and regeneration have a very intimate relation 
to this habit. In Crustacea especially it has been determined that 
the time of the operation with respect to the molting period is 
of importance.‘ In experiments on Crustacea it is therefore neces- 
sary to take special pains in making the molting habit the center 
for adjustment of the compared groups. Notwithstanding the 
greatest care taken it is not possible to wholly eliminate the molt 
as a disturbing factor. The data dealing with animals having 
a molting habit are separated from those dealing with animals 
not having a molting habit. 
3 Character of laboratory history. It is known that animals 
transferred from their natural environment to the laboratory show 
changes in their behavior. ‘These changes are usually associated 
with a general decrease in vitality. It is therefore necessary to 
have the compared groups with identical laboratory histories. 
4. Changes in rate during the regeneration period. Following 
an operation the rate of regeneration is not the same for the 
whole time up to the completion of the organ. In the frog tad- 
pole and probably in other forms it is at first slow, then increases 
rapidly to a maximum, then decreases, rapidly at first, and then 
more and more slowly to zero at the time of completion of the 
regeneration.’ It is necessary in our present experiments to com- 
pare the rates in identical periods only. 
5 The level of the cut. The rate of regeneration varies with 
3 See this journal, vol. vii, no. 3. 
4Emmel, 1904. 
5 See this journal, vol. vii, no. 3. 
