134 ILLINOIS BIOLOGICAL MONOGRAPHS [134 



is very high between the first and second periods but decreases very 

 rapidly at first and then more slowly. 



It is evident that there is a close similarity between the change in 

 rate of growth during the regeneration cycle and the change in rate 

 during an ordinary developmental cycle and there is every reason to 

 believe that the factors controlling the one are similar to those controlling 

 the other. The problem of the factors is particularly interesting when 

 it is noted that for widely different levels the rates of regeneration differ 

 in such a way that length regenerated in a given time is proportional to 

 the length removed. The process of regeneration apparently is initiated 

 in a similar manner at each level but is kept under such control that only 

 a certain per cent of the length is regenerated in a given time. 



Knowledge of the process is at present insufficient to enable one 

 to discuss with profit the nature of the control of rate of regeneration. 

 All that can be done is to point out the relations of certain phenomena. 

 The initial slow period is coincident with the period of cell migration 

 without cell division, the period of rapidly increasing rate is coincident 

 with the period of rapid cell multiplication without pronounced cell 

 differentiation and the period of rapidly decreasing rate is associated 

 with the appearance of pronounced differentiation in the cells. There is 

 certainly some causal relation between these phenomena. 



Summary 



1. In second regenerations of the tail in Rana clamitans the average 

 specific rates are 0.019 mm. for the to 4 day period, 0.066 for the 4 to 6 

 day period, 0.051 for 6 to 8 days, 0.033 for 8 to 10 days, 0.017 for 10 

 to 12y 2 days, 0.001 for 12y 2 to 18 days and —0.001 for 18 to 56 days. 



2. The average accelerations of rate are -f-0.095 mm. per day from 

 the first to the second period, — 0.015 from the second to the third, 

 —0.030 from the third to the fourth, —0.058 from the fourth to the fifth, 

 — 0.028 from the fifth to the sixth and — 0.001 from the sixth to the 

 seventh. 



3. The average percentage increments between the same periods 

 are respectively 106, 28, 12, 5, and 0. 



4. The average accelerations of specific rate for the four deepest 

 levels between the same periods are respectively -4-0.011 mm., 0.000, 

 —0.005, —0.006, —0.004 and 0.000. 



5. In first regenerations of the tail in Rana clamitans the average 

 specific rates are 0.018 mm. for to 4 days, 0.046 for 4 to 6 days, 0.057 

 for 6 to 8 days, 0.037 for 8 to 10 days, 0.026 for 10 to 12y 2 days, 0.002 

 for 12y 2 to 18 days and —0.001 for 18 to 56 days. 



6. The average accelerations of rate are +0.078 mm. per day from 



