Effect of Successive Removal upon Regeneration 493 
individuals and the great individual variability cannot be neglected 
in an estimate of the validity of the conclusions, at least, as far as 
Table 9 is concerned. The data are thus open to the general 
criticism of insufficient control of the accessory factors. | 
Experiments on Molting Animals 
The second group of experiments includes those on Crustacea. 
A statement of the advantages and difficulties common to all of 
these may be given here. The chele were used in all four species 
for the study. The presence of a definite breaking joint enables 
the operator to be absolutely sure that he has the same plane of 
removal in all cases. This is a matter of great importance since 
it has been shown that the rate of regeneration is very closely 
related to the location of the plane of removal.* 
An advantage due to the molting habit is the convenience in mak- 
ing measurements of the various stages in the process of growth 
and regeneration. ‘The cast skin and the removed chelz, as well 
as the preserved animals may be kept indefinitely in alcohol with- 
out danger of shrinkage and a complete record of the growth of 
all parts can be kept for future measurement. The cast must, 
however, be removed from the dish containing the animal soon 
after the molt, since it is eaten as soon as the mouth parts become 
sufficiently hardened to be capable of chewing. 
The special difficulty with Crustacea is found in the same molt- 
ing habit. It is not possible to use a definite period of time for 
the comparison because the amount of regeneration during such a 
period is dependent not only on its length, but also upon its rela- 
tion to the molt.** It is therefore most important that the opera- 
tions be made at the same time with reference to the molt. 
Furthermore, in making comparisons of amounts of regeneration in 
individuals of approximately the same size or age it is necessary to 
use not a definite period of time, but a single molting period or a 
definite number of molting periods as a unit. ‘This is true because 
it has been found that in individuals of the same size or age the 
15 Ellis: 709, Morgan ’06. 
16 Emmel ’06; Zeleny ’o5. 
