Studies on Regulation. 283 



is perhaps sufficient to illustrate the point. I believe that the 

 absence or occurrence of functional substitution of other parts for 

 a part removed are important conditions determining whether 

 regeneration or "redifferentiation" shall occur. 



In certain cases of regulation, as for example one form of regula- 

 tion occurring in Clavellina (Driesch, '02) the old structure dis- 

 appears more or less completely and the piece seems to return to the 

 embryonic condition. Out of this apparently undifferentiated 

 mass a new complete individual arises by processes more or less 

 similar to those of ontogenetic differentiation. This is apparently a 

 case of true redifferentiation. Our knowledge of cases of this sort 

 is very incomplete as yet; we do not even know exactly to what 

 extent the old structure disappears, but it is evident that we have 

 here something widely different from regeneration and approach- 

 ing more closely to embryonic development. In the first place, 

 according to my point of view, the disappearance of the original 

 structure of the piece is only incidentally a part of the regulative 

 process; the old structure disappears simply because the conditions 

 which maintained it are no longer present. The previous differ- 

 entiation has not destroyed the capacity of the tissues for reacting 

 to altered conditions and the first effect of this altered reaction 

 is the disappearance of the old structure. The transformation of 

 the part into a "whole" is probably identical with the loss of the 

 specification which resulted from the particular conditions to 

 which it was subjected; it is thus a negative rather than a positive 

 change, the loss of visible differentiation rather than the acquisi- 

 tion of new potentialities. The development of the new individual 

 from the undifferentiated mass occurs in much the same manner 

 as in typical ontogeny. The different origin of the cell-mass in 

 the two cases does not constitute a fundamental difference, though 

 it may be found to determine some differences in detail. 



From what has been said it follows that the greater the degree of 

 differentiation, or in other words the greater the specialization of 

 conditions in different parts of the organism, the greater will be 

 the difference in the parts and the less the capacity for altering the 

 reactions in correspondence with altered conditions. Hence we 

 may expect "redifferentiation" to occur only in relatively simple 

 forms while regeneration, or destruction of other parts followed 

 by regeneration, or finally destruction without regeneration may 

 follow removal of a part in more complex forms. 



