8 ILLINOIS BIOLOGICAL MONOGRAPHS [8 



Discussions of the results of other workers are included in the pre- 

 vious papers and need not be repeated here. The principal need at 

 present seems to be an extension of knowledge of these factors by 

 multiplying the number of series of carefully controlled experiments. 

 While it would be interesting to know why a particular series differs 

 from others with respect to a certain factor it is not always possible to 

 discuss the matter profitably in the absence of evidence as to all the 

 factors concerned. 



Particular emphasis must be laid on the fact that in connection with 

 some at least of the factors it has been possible to make out very definite 

 quantitative relations. These have been checked up in a number of cases 

 by agreement between separate series of experiments. The success in this 

 direction has made it very probable that with a more accurate control 

 of external conditions there will be a considerable further advance in 

 our knowledge of the factors controlling rate of regeneration. 



