514. Charles Zeleny 
degree of injury is meant the effect of additional injuries to the 
individual. Special emphasis needs to be laid on this fact to 
avoid confusion in interpretation. A comparison of the rate of 
regeneration from different levels of a particular organ 1s not of 
direct value in connection with this problem because difference in 
level involves a great many factors not connected with degree of 
injury to the individual as a whole. The local conditions at 
different levels are obviously different and these differences have 
no necessary connection with degree of injury. Furthermore 
level cannot be assumed to be a measure of degree of injury, 
especially in the case of long, slender organs. 
The lack of knowledge of the several subsidiary factors influ- 
encing the rate of regeneration makes it impossible to compare 
the changes in rate within single individuals. In every case it 
is necessary to compare the rate in different groups of individuals, 
each group being as nearly like the others as possible except in the 
degree of injury. The success of the method is dependent upon 
the similarity between the different groups in all respects except 
the factor under consideration. 
The principal sources of error arise in connection with the 
following factors: 
Age. 
Periodic physiological changes. 
Character of the laboratory history. 
Changes in rate during the regeneration period. 
Level of the cut. 
Successive regenerations. 
Temperature. 
Food. 
Differences in manipulation. 
Departure of the living conditions from the optimum. 
The relation of the degreé of injury to the optimum 
degree. 
12 Individual variation. 
qe OO ON ANN FW WN 
— 
The general methods employed in the elimination of these 
sources of error are based upon the attempt to make every two 
