24 ILLINOIS BIOLOGICAL MONOGRAPHS [24 



Discussion 



While tlie knowledge of the relative rates of regeneration for old 

 and new tissue is essential for accurate determination of other factors 

 its main interest is in its bearing on the question of the character of 

 control of the process of regeneration. Evidence from a great many 

 directions points toward the conclusion that regeneration is not wholly 

 a direct response of the injured cells at the cut surface nor of those in 

 the immediate neighborhood of the cut surface. It is more and more 

 evident that conditions in parts of the body remote from the injured 

 region are involved. If rate of regeneration were determined wholly 

 by the character of the cells at the cut surface we would expect that 

 cells in process of active proliferation, such as those that are starting 

 to build up a new tail, would respond much more promptly than those 

 which have become more highly diffei-entiated and hence more stable. 

 Regenerating cells ought to furnish a much better basis than old ones. 

 "We find however that there is no striking difference in the two cases. 

 Regeneration proceeds at approximately the same rate whether old or 

 new cells have furnished the basis for the new material. It is true that 

 the data show on the average a slight advantage in favor of the new 

 tissue, especially during the early periods, but this advantage is small 

 and it is doubtful Avhether it can be considered as significant. There is 

 some evidence that the earliest stages of regeneration, those due to cell 

 migration exclusively, are more rapid from new than from old tissue. 

 If this evidence is reliable an explanation is found for the slight advan- 

 tage in favor of the new tissue at later periods. 



Summary 



1. A comparison of the rate of regeneration in tadpoles of Raiia 

 clamitans in eases where there are newly regenerated cells at the cut 

 surface with those in which only old cells are present shows, on the 

 whole, little difference between the two. 



2. The slight difference favors the new cells but may not be 

 significant. 



3. In Experiment I the specific length of regeneration at the 

 end of 6 days was 0.196 from old tissue and 0.204 from new tissue. 



4. In the same experiment at the end of 8 days the specific 

 length from the old was 0.303 and from the new 0.310. 



5. In Experiment II the general result was similar to that in 

 Experiment I. The amounts of regeneration in the two cases are very 

 nearly equal and the slight difference is in favor of the new tissue. 



6. Experiment III shows that as regards completeness of regen- 



