ZOOLOGY: C. ZELENY 
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in the cells at the different levels. If differentiation in the tail proceeded 
from the tip toward the base, the more rapid rates from the more basal 
levels might be explained by the more embryonic character of the cells 
at these levels. As the tip is approached the material would become 
more and more highly differentiated and therefore less and less capable 
of readjustment. There is however no evidence that differentiation 
proceeds in this way in this case. 
The progressive increase in rate with depth of level of the cut is un- 
doubtedly due to reactions which involve a more central control, a coor- 
dination of the functional activity as a whole. The period of cell mi- 
gration probably is only slightly subject to such control. It is a period 
in which the response is largely local in character. The rate of cell 
division which is the important factor during the period of rapid in- 
crease in length is however undoubtedly under central control. 
FIG. 3. SPECIFIC RATES OF FIRST AND SECOND REGENERATIONS AT DIFFERENT TIMES 
AFTER THE OPERATION. RANA CLAMITANS TAIL REGENERATION. UPPER FIGURE = SEC- 
OND, LOWER FIGURE = FIRST REGENERATION. 
3. Time within the Regenerative Cycle. — The present analysis of change 
in rate of regeneration during the regenerative cycle was made in ex- 
tension of previous studies which showed that the increase in amount 
of material during regeneration follows the general rule of increase 
during an ordinary life cycle. The rate is slow at first, increases very 
rapidly to a maximum, then declines rapidly at first and then more 
and more slowly as zero is approached. 
The present study deals with tail regenerations in frog and salamander 
larvae. Large tadpoles of Rana clamitans which remained fairly con- 
stant in size during the course of the experiment were found to be the 
most satisfactory. They yielded results which were uniform enough 
for an analysis of the change in rate. The second regenerations are 
taken up here. The levels of removal averaged 1.5, 2.8, 4.9, 8.4, 
