Degree of Injury and Rate of Regeneration 521 
at their bases the degree of injury is evidently beyond the optimum. 
A point has been reached at which the injury is rapidly or grad- 
ually fatal in all cases and the rate of regeneration in those sur- 
viving for a time is slow. ‘The rate of regeneration thus increases 
up to the optimum which is reached when four arms are removed. 
Beyond the optimum the rate slows down. 
4. The Oral Arms of Cassiopea xamachana* 
The rate of regeneration of a single oral arm in each of the 
following cases was determined: 
a One arm removed at its base. 
b Two arms removed at their bases, 
c Four arms removed at their bases. 
d Six arms removed at their bases. 
e All eight arms removed at their bases. 
In the last four cases the averages of the rates of regeneration 
of the individual arms were taken. As in the case of the arms of 
the brittle-star there is a complication due to the age or size 
factor and to individual variation. The general result 1s how- 
ever clear that there is an increase in rate of regeneration up to 
an optimum degree of injury beyond which further injury causes 
a decrease in rate. In this experiment the removal of four to six 
arms seems to constitute the most favorable degree of injury. 
When four arms are removed the rate is not as great as with six 
removed but the difference is so small that it may not be signifi- 
cant. On both sides of the optimum of four or six arms however 
there is a marked decline. ‘The average specific amount of regen- 
eration, 1. e., the rate per unit of disk diameter, was 0.058 for a 
single removed arm, 0.112 for each of two removed arms, 0.144 
for each of four removed arms, 0.157 for each of six, and 0.117 
for each of eight. The data showing the considerable individual 
variation are given in Table I of the paper on regeneration in 
Cassiopea (Journal of Exp. Zodl., vol. v, no. 2, 1907, p. 267). 
13 Journal Exp. Zodl., 1907. 
