58 ILLINOIS BIOLOGICAL MONOGRAPHS [58 



regenerative or developmental energy or of determiners by repeated 

 regeneration but from changes in the non-regenerating part associated 

 with age. In another place there is a discussion of the possibility that 

 there may be an effect upon the rate of developmental processes in 

 the organism as a whole due to continued regeneration of a part. This 

 is studied particularly in connection with the effect of regeneration 

 upon rate of metamorphosis in Amphibia. 



Regeneration studies in general and those on successive regener- 

 ation in particular make it improbable that there is a definite number 

 of cell generations between the fertilized egg and the end product, the 

 differentiated cells. The possibility that certain cells may remain in 

 an early cell generation can not be wholly excluded as an explanation 

 of at least a part of first regeneration phenomena. Under suitable stimu- 

 lation such cells may be postulated to take up development where it 

 had left off. The definite descriptions of de-differentiations of cells 

 as well as other facts of regeneration argue against this conclusion. 

 The view that there can be no such definite number of cell generations 

 is strengthened by the facts of successive regeneration. It does not 

 seem probable that embryonic cells of an early cell generation can be 

 held in reserve through repeated regenerations. 



The explanation of regeneration by the theory of duplicate sets of 

 determiners meets difficulties in undiminished successive regenerations. 

 The greater the number of repeated regenerations the greater the diffi- 

 culties of explanation on this basis. Of course the difficulty does not 

 hold for the hypothesis that every cell or nearly every cell contains a 

 full set of determiners. 



The earlier appearance of the maximum rate in the second than 

 in the first regeneration may be due to the more rapid progress of the 

 cells in the early cell migration period alone or it may be due to the 

 acceleration of the whole developmental cycle. 



Summary 



1. The age factor was eliminated in Experiments I to IV. Ex- 

 periments I and II deal with tadpoles of Rana clamitans and Experi- 

 ments III and IV with larvae of Amblystoma punctatum. 



2. In Experiment I approximately one-half of the tail was re- 

 moved. At six days the average first regeneration length is 2.01 mm. 

 and the average second regeneration length 2.18 mm. In five cases the 

 first exceeds the second and in six the second exceeds the first. The 

 corresponding specific lengths are 0.194 and 0.205. The first regen- 

 eration exceeds the second in two sets, the second exceeds the first in 

 eight and one is tied. The second regeneration has the advantage in all 

 the comparisons. 



