510 Charles Zeleny 
normal growth because the conditions are so entirely different. 
The only thing that can be made out is that the replacement 
follows in general the same rate changes as in ordinary growth. 
In the case of successive replacements, however, it is possible to 
make a direct comparison. Here a test can be made of the general 
question of growth potential. Does the accomplishment or partial 
accomplishment of the normal growth process or of its first repetition 
imply the using up of an original supply of growth potential? If 
it does, should it not be natural to infer that a history of successive 
removals and replacements ought to be followed by less rapid and 
less perfect regeneration? It does not matter whether this growth 
potential is put in terms of chromatin units, accessory germs or any 
other kinds of physical units or units of ‘“entelechy” efficiency. 
The fact remains that regeneration occurs as readily or more 
readily after previous replacements than before. ‘The organism 
does not act as if it had used up any of its essential supplies. 
Rather it shows a tendency to a more rapid performance upon 
previous trial. 
The facts seem thus to point strongly toward external rather 
than internal initiative in the causation of growth limitation. ' 
There is an inexhaustible supply of material within the organism 
for the production and reproduction of its parts. “The undimin- 
ished force of the developmental processes during successive 
regenerations shows in a very obvious way that the cause of restric- 
tion and cessation of growth is to be sought in the presence of cer- 
tain relations between the parts of the organs or between them and 
outside forces and not in the absence of any specific materials 
necessary for further growth. When the proper relations are pro- 
duced by injury, growth starts again with undiminished force. 
VII SUMMARY 
1 The rate of successive regenerations was studied with special 
reference to the determination of the pure effect of successive 
removal after elimination of the accessory factors. 
2 In the best controlled series, it was found that successive 
removal does not cause a decrease in rate of replacement, but 
either no change or an increase in rate. 
