13] RATE OF REGENERATION— ZELENY 13 



it was too near the tip of the tail and the cells at the cut surface were 

 newly regenerated ones. 



The operations were at different levels in different individuals but 

 the determination of the specific amounts of regeneration according to 

 the method given in the explanation of Experiment I eliminates these 

 differences within wide limits. It does not hold when the level of the 

 cut is very near the tip or near the base of the tail. In the present 

 experiment the specific amount is a fair constant for all removed 

 lengths of over 4 mm. The individuals with a removed length of 

 less than 4 mm. are therefore treated separately. Likewise it does 

 not hold for the first few days of regeneration during which regenera- 

 tion is confined to active migration of cells over the cut surface without 

 any new formation by cell division. Separate comparisons are made at 

 4, 6, 8, 10, 12y 2 , 18 and 56 days of regeneration. The data are given 

 in Tables 2 to 17. 



Taking first the cases with a removed length of over 4 mm. there 

 is at four days a specific amount of 0.043 for old tissue and of 

 0.045 for new tissue. At six days the amounts are respectively 0.135 

 and 0.143, at eight days 0.216 and 0.224, at ten days 0.292 and 0.293, 

 at twelve and a half days 0.331 and 0.337, at eighteen days 0.352 and 

 0.348, and at fifty-six days 0.345 and 0.346. The two are approximately 

 equal though in six out of the seven cases the new tissue is ahead. The 

 average difference in favor of the new tissue is 0.003. 



For removed amounts of less than 4 mm. the data are un- 

 satisfactory because there are only three individuals with regeneration 

 from new tissues. The data are however of value in comparison with 

 the others. The specific amounts at the different days, again putting 

 the old tissue first in each case, are 0.119 and 0.160 for four days, 0.317 

 and 0.327 for six days, 0.444 and 0.467 for eight days, 0.506 and 0.520 

 for ten days, 0.517 and 0.517 for twelve and a half days, 0.501 and 0.507 

 for eighteen days, and 0.475 and 0.325 for fifty-six days. In the last 

 the absorption of the tail had begun before the measurement was made 

 and the comparison is therefore not valid for our purposes. In the 

 first, 0.119 for old and 0.160 for new at four days, the great difference 

 between individual cases on each side makes a comparison of doubtful 

 validity. There are other data however which make it probable that the 

 initial migration of the cells takes place more rapidly from new than 

 from old tissue. For the other levels there is on the average a slight 

 difference (0.011) in favor of the regeneration from new tissue. With but 

 a single exception, which is a tie, the new tissue is ahead of the old. 

 The differences favoring the new tissue are greater than those for the 

 larger removals. This again may be due to the fact that a larger percent- 



