105] RATE OF REGEXERATION—ZELENY 105 



Discussion 



That the level of the cut has an important influence upon the rate 

 of regeneration has been made out by a number of investigators (Spal- 

 lanzani 1768, King 1898, Morgan 1906, Stockard 1908, Ellis 1909, Morgu- 

 lis 1909a, b, and others). Their work indicates that regenerations from 

 deeper levels are on the whole more rapid that from more superficial ones. 

 The data obtained from the present experiments confirm tliis conclusion 

 and make possible a further analysis of the relation. They show that 

 in the regeneration of the tail of amphibian larvae there is a striking 

 relation between the level of the cut and the rate of regeneration. 

 Within wide limits the length regenerated is directly proportional to 

 the distance of the cut surface from tlie original tip of the tail. Within 

 these limits therefore regeneration at any particular time after the 

 operation has the same degree of completeness from all levels of injury. 



An anal.ysis of the progress of the regeneration brings out the fact 

 that two distinct periods are to be recognized in rate of regeneration 

 in its relation to level of the cut. During the first two to four days 

 after the operation regeneration is confined to cell migration from the 

 old tissues without cell di\'ision. During this period in the frog tad- 

 poles there is no essential difference in lengtli regenerated at the differ- 

 ent levels and the specific rate is therefore much greater after shorter 

 than after longer removals. In the second period with the initiation 

 of rapid cell multiplication the rate of regeneration is greater the deeper 

 the level and furthermore is directly proportional to the length removed. 

 As soon as the bulk of material produced by cell division is considerably 

 greater than that which was prodviced by cell migration there is an 

 approach to constancy in specific length regenerated. This holds for all 

 except the shortest removals. After the shortest removals the total 

 regeneration is so small in amount that a large part of it is made up 

 of the original migrated material. Therefore from these levels the spe- 

 cific regenerated lengtlis are greater than from the deeper levels even 

 at a late period of regeneration. 



A further complication is introduced by the fact that regeneration 

 is not complete. Only a certain per cent of the removed length is re- 

 placed and the end of the process is reached .sooner after the shorter 

 than after the longer removals. From the deepest levels regeneration 

 is still proceeding when it has stopped from the medium and shallowest 

 ones. When the process is completed in all cases the specific length is 

 therefore slightly greater after botli the longest and the shortest re- 

 movals tlian after medium ones. 



As to the cause of the tliffei-ence in rate at the different levels 



