Amount of Tail Regenerated to Removed in Tadpoles 447 
the same conditions of pressure and tension in the two sets of tails. 
The series with 10 mm. would probably have to regenerate 5 mm. 
to bring about a condition similar to that in the tail, that had 
regenerated two and one-half millimeters, but had lost only 5 mm. 
through injury. This is precisely the method of regeneration 
followed by all of the series observed. Each regenerated an 
amount proportional to that removed. ‘This is to be expected in 
view of the last statement, for the regeneration would cease when 
the cells dividing had reached a point at which the extra-cellular 
force was equal to that force causing them to divide; and since 
these cells had equal regenerative potential to start with, the con- 
ditions of pressure and tension would only be equalized when the 
amount regenerated was proportional to the amount removed. 
Another point in favor of this view of the cause of the propor- 
tional regeneration from various levels, is the sequence of the 
regions comprising the curves of regeneration. ‘The first region, 
that of low rate of regeneration, is due to the immediate shock of 
injury and the healing over of the exposed surface; the second, of 
rapid regeneration, is the period during which the minimum resist- 
tance is offered to cell-division; the third, that in which the rate 
of regeneration suddenly decreases, is produced by the rapid cell- 
division of the second region, which has brought up the tension of 
the tail to a point almost equal to that, at which the extra-cellular 
forces are equal to those causing cell-division; the fourth and last 
region, that in which regeneration gradually ceases, is that in which 
the extra-cellular forces finally overcome cell-division. 
GENERAL SUMMARY 
1 The tadpoles observed did not completely regenerate the 
part of tail removed. 
2 The absolute amount regenerated was proportional to the 
amount removed, whether this was large or small; the per cent 
regenerated, by series under uniform conditions, was the same, 
if they were of the same relative age, regardless of the absolute 
amount removed. 
3 The per cent regenerated, other conditions being the 
same, varied with the relative age of the tadpole. 
