Effect of Successive Removal upon Regeneration 509 
VI DISCUSSION 
The data at hand show that successive removal during the first 
four removals does not diminish the rate of regeneration. It 1s 
either the same throughout or is visibly increased. For non- 
molting cases which seem to be the better adapted for an accurate 
determination the rate is increased with successive removal. In 
molting cases it is apparently not changed. 
Leaving out of consideration any general discussion of the facts 
as an instance of adaptive response on the part of the organism two 
points deserve attention. The first concerns the mechanism of 
the process and the second involves the general bearings of the 
results. 
1 The removal of an organ according to the generally accepted 
view is followed by the proliferation of partly or wholly undifferen- 
tiated cells near the cut surface. ‘This proliferation is accom- 
panied by degenerative changes in the differentiated cells and 
makes up the early slow period of regeneration. It is followed 
by a period of very rapid multiplication and resulting rapid growth. 
Then, finally, with the differentiation of the cells the rate gradually 
diminishes. If the new organ is removed for a second time before 
the completion of the old process the second organ grows more 
rapidly because the proliferating cells are already present, and the 
conditions for new growth are more favorable. ‘This, however, 
does not explain an advantage, if any, that the third or later 
regenerations may have over the second. 
2 The special interest of successive regenerations is largely 
in connection with the problem of the limitation and cessation of 
growth. In the development of most organisms, growth follows a 
definite cycle with a maximum rate near its beginning followed by 
a rapid decrease and finally a more gradual decrease ending in 
cessation. We are not in a position at present to discuss the prob- 
able causes of this limitation, but it is known that certain kinds of 
injuries such as those involving the removal of a part of the body 
bring about a renewal of the process. “The new organ inits growth 
follows the same cycle as before. It is not possible, however, in 
this case to make a comparison of the absolute rate with that of 
