106 ILLINOIS BIOLOGICAL MONOGRAPHS [106 



little more can be said than that it does not seem to be due to inherent 

 differences in the cells at the different levels. If differentiation in the 

 tail proceeded from the tip toward the base, the more rapid rate fi-om 

 the more basal levels might be explained by the more embryonic char- 

 acter of the cells at tliese levels. As the tip is approached the material 

 would become more and more inert. There is however no evidence 

 tliat differentiation proceeds in this way in this case. 



The progressive increase in rate with depth of level of tlie cut is 

 undoubtedly due to reactions which involve a more central control, a 

 co-ordination of the functional activity as a whole. The period of cell 

 migration probably is only slightly subject to such control. It is a 

 period in which the response is largely local in character and there is 

 correspondingly little if any difference at the different levels. The 

 rate of cell division which is the important factor during the period 

 of rapid increase in length is however undoubtedy under central control. 



Summary 



1. In frog and salamander larvae with removed tail lengths of 

 one-fifth to two-thirds, the general rule holds that the length regenerated 

 in a given time is proportional to the length removed, or in other words 

 the length regenerated per unit of removed lengtli is a constant. 



2. An analysis of the data shows however that this applies only 

 to the material produced by active cell division. 



3. During the first four days, in frog tadpoles, when the regener- 

 ating part is made up almost entirely of cells that have migrated from 

 the old tissues without division there is no such relation between length 

 i-emoved and length regenerated. The length of new material at this 

 time is not strikingly different for the different levels and the process 

 seems to be a local response of the cells to the injury. The length 

 regenerated per unit of removed length is greater at this time for the 

 shorter than for the longer removals. 



4. Since comparatively a large part of the regenerating material 

 after the shorter removals is made up of migrated cells even at the 

 later periods it follows that the specific regenerations from these levels 

 are greater than from the deeper ones. 



5. During the later periods the specific regenerated lengths tend 

 to be higher after both the shortest and the longest removals than after 

 medium ones. In tlie case of the shortest ones this is due to the rela- 

 tivel.v large part of the whole regenerated tail that is made up of mi- 

 grated cells. In the case of the longest removals it is due to the fact 

 that regeneration continues for a time after it has stopped in the medium 

 ones. 



