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Degree of Injury and Rate of Regeneration 561 
the cause of the decrease in size. If it is due to factors only secondarily connected 
with the removal, such as ability to get food, freedom in movement, etc.,it seems 
to me that it is more valid to use the final rather than the original diameter as a 
basis. ‘The decrease in size of the animals of necessity means a decrease in the rate 
of regeneration quite apart from the matter of degree of injury. The new arms 
are being regenerated in a proportion that is proper for the decreased disk and not 
for the original one. The smaller animal is certainly not regenerating arms of the 
original size. If the animals were fed an amount just sufficient to retain their 
original disk diameters it is probable that the specific regeneration would be greater 
in those with the greater number of removed arms. 
The greatest degree of injury, however, in Stockard’s experiments shows the 
greatest rate for the individual arms under eitherinterpretation. Inmy Cassiopea 
materials the optimum came at a lower degree. ‘The difference is evidently due 
to some factor in the conduct of the experiments, probably better living conditions 
in Stockard’s animals. “a 
In the brittle-star Ophiocoma riisei the same criticism may be offered of the use 
of the original rather than the final disk diameter though here it makes only a 
slight difference in the interpretation of the results. “The data show very little if 
any difference in rate with a different number of removed arms. 
In the brittle-star Ophiocoma echinata the data furnished by Stockard evidently 
show a decrease in rate of the individual arms with increase in the removed number. 
No criticism of the interpretation of the data occurs to the writer. 
In comparing these brittle-star experiments with mine on Ophioglypha lacertosa 
it should be borne in mind that apart from specific differences there were also 
differences in the living conditions in the two cases. ‘The level of the cut inthe arms 
was higher up in Stockard’s animals, the latter also had a considerable food supply 
and a much greater chance of active movement. 
Stockard’s three experiments thus give one case with increase, a second with no 
change and a third with a decrease in rate of regeneration as a result of increase in 
degree of injury. All three such instances are given in the data of my paper though 
the majority belong to the first group. His data therefore do not modify the general 
conclusion that on the whole the optimum degree of injury in an individual is not 
necessarily the lowest degree. In the majority of cases studied so far the optimum 
degree of injury is above the lowest degree. 
The problem now is therefore not whether a// animals under all conditions 
regenerate parts more rapidly or less rapidly with increase in number of removed 
parts. It seems to me to be the determination of the conditions affecting the loca- 
tion of the optimum and the relation of these conditions to the general problem of 
growth. 
