59] RATE OF REGENERATION— ZELENY 59 



3. At eight days in Experiment I the average first regeneration 

 length is 3.06 mm., and the second 3.42 mm. The first exceeds the sec- 

 ond in three sets and the second exceeds the first in seven. The corre- 

 sponding average specific lengths are 0.298 and 0.323. In four sets 

 the first regeneration exceeds the second and in six the second exceeds 

 the first. The second regeneration has the advantage in all the com- 

 parisons. 



4. The advantage of the second regeneration over the first in 

 Experiment I holds true of second regenerations from both old tissue 

 and new tissue levels. 



5. In Experiment II observations were made at the 1/10, 1/3, 1/2 

 and 2/3 levels in a sufficient number of individuals to yield valid data. 

 ^Regeneration measurements were made at each of these levels 4, 6, 8, 

 10, 12^, 18 and 56 days after the operations. The second regen- 

 eration at all of them tends to be ahead of the first until the tenth day, 

 after which the first regeneration catches up. The maximum rate for 

 both regenerations is reached before this time and earlier for the second 

 than for the first regeneration. 



6. In Experiment III two-thirds of the tail was removed. A 

 comparison of the first, second and third regenerations was made at 

 2, 4, 6, 8, 10 and 14 days.. At two days the first, second and third 

 regenerations average respectively 0.22, 0.25 and 0.26 mm. The cor- 

 responding values at four days are 0.66, 0.75 and 1.00; at six days 

 1.36, 1.40 and 1.46; at eight days 2.18, 2.68 and 2.68; at ten days 3.55, 

 3.82 and 4.20; at fourteen days 5.34, 6.12 and 6.08. The advantage is 

 in favor of the second and third regenerations as opposed to the first 

 and of the third as opposed to the second. Individual comparisons at 

 each of the different times as well as in the experiment as a whole 

 show the same results. 



7. The removed tails in the preliminary procedure of Experi- 

 ment III furnish the data of Experiment IV and allow a comparison 

 of the first and second regenerations at eleven days. The procedure is 

 however subject to greater error than that of Experiments I to III. 

 Twenty-five individuals for each regeneration give an average of 4.55 

 ± 0.11 mm. for the first regeneration and 4.50 4 0.10 mm. for the second 

 regeneration. The first regeneration is ahead of the second in ten 

 cases, the second ahead of the first in twelve cases and three are equal. 

 The two regenerations must be considered as essentially equal. 



8. In Experiments V and VI the age factor is not eliminated. 

 Successive regenerations in single individuals are compared. In Ex- 

 periment V one-half of the tail in Amblystoma larvae was removed. 

 In Experiment VI 60 per cent of the tail of toad tadpoles was removed. 



