112 



ILLINOIS BIOLOGICAL MONOGRAPHS 



[112 



the values are respectively +0.011, 0.000, —0.005, —0.006, —0.004 and 

 0.000, putting the greatest rate of decrease between the 9 and the 11*4 

 day periods. 



An examination of the curves of specific rate and a comparison 

 with the facts of histogenesis shows that acceleration of rate is a plus 

 quantity only during the period before active differentiation of the 

 cells, i. e. until the end of the fifth or seventh day. As soon as tissue 

 differentiation is fairly begun the retarding influence is apparent and 

 by the ninth to eleventh days when muscle fibres and other cells are in 

 full process of differentiation the negative acceleration is at its height. 



Following the percentage increment method used by Minot (1908) 

 for ordinary growth and using length instead of weight because the latter 

 could not be determined with sufficient accuracy the results given in 

 Table 59 are obtained. The values for the six periods excluding the first 

 one are 106, 28, 12, 5 and 0. The regenerated material present at the 

 end of four days is made up almost wholly of cells that have migrated 

 from the old tissues and have not as yet undergone division. After the 

 fourth day the additions to regenerated material are almost wholly 

 the result of cell division. From the end of the fourth to the end of 

 the sixth day the material is on the average more than doubled in length 

 each day. After this time the percentage increment decreases rapidly. 

 The change from period to period is represented in graphic form in 

 Figure 55. The curve is a logarithmic one quite similar to that obtained 



eS 

 •a 



9 

 M 

 eS 



4-> 



C 



9 

 O 



$~ 

 9 



a* 



100 



80 



60 



40 



20 



7 9 11*4 15% 



— ►■ Days after the operation 

 Figure 55 Percentage increment per day at different periods after the opera- 

 tion First and second regenerations of the tail of Rana clamitans Un- 

 broken line=first regeneration. Broken line=second regeneration. 



