Analysis of Rate of Regeneration 419 
between the rapid decline in rate and the rapid differentiation of 
the materials proliferated during the time represented by the 
first and second regions in the rate curve. It is easy to under- 
stand how differentiation by decreasing the number of dividing 
cells will decrease the rate at which the regenerating tail becomes 
longer. ‘The larger part of this reduction in rate by differentia- 
tion comes immediately after the rate has been high, when there 
are a large number of cells that are capable of differentiating in 
the regenerating bud of the new tail. It is with the completion 
of this differentiation on a large scale, that the third region of the 
curve ends and the fourth begins. After a large per cent of the 
undifferentiated cells have been differentiated both the growth 
of the new organ by cell division and the reduction of its rate of 
growth by differentiation must proceed more slowly because (1) 
there are fewer undifferentiated cells to divide; and (2) there are 
fewer to differentiate. These two processes may go on for some 
time, the proportion of the undifferentiated cells to the differen- 
tiated ones becoming constantly less and less until finally the per 
cent characteristic of these animals at their present age is reached. 
This agreement between the time of rapid decrease in rate of 
regeneration and the time of rapid differentiation of the new tissue 
is an evidence in favor of the view that the differentiation of these 
cells in the new tail is the cause of the reduction in the rate of its 
growth. However, it must be remembered that no satisfactory 
explanation has been given of the factors which cause cells to begin 
to differentiate. It is quite possible that the beginning of differ- 
entiation depends upon a reduction of the rate of cell division 
in which case some other factor, at present unknown, is respon- 
sible for the changes in rate of regeneration after the maximum 
has been passed. 
SUMMARY 
1 In tadpoles of Rana clamitans (Latreille), 35-40 mm. long, 
the removal of aboout 50 per cent of the tail is followed by the 
regeneration of 56 to 62 per cent of the amount removed. 
2 The rate at which this amount is replaced is not the same 
