282 C. M. Child. 



occur, one of the most important being what Driesch ('oi) has 

 designated as redifferentiation. I think it probable that in many 

 cases the so-called "redifferentiation" when subjected to a closer 

 study will turn out to be merely differentiation. Cases of this sort 

 in which the formation of new tissue is involved, such for example 

 as the formation of the new pharynx in the old tissue in Planaria, 

 differ from regeneration proper, /. .?., the outgrowth of new tissue 

 from the cut surface, chiefly in that growth is not localized at the 

 cut surface but is distributed through a larger or smaller portion 

 of the old tissue. But why should the new growth be localized 

 in the one case and not in the other .^ In attempting to answer 

 this question it is necessary to anticipate somewhat and state 

 certain conclusions from my experiments for which the data have 

 not yet been fully given. These will serve merely as suggestions 

 to make clear my point of view. In cases where regeneration, 

 /. e., outgrowth from the cut surface, takes place it will be found 

 that the old part remains essentially a part as regards function; 

 functional substitution for the part removed does not occur. But 

 when proliferation at the cut surface begins as the direct result of 

 the altered conditions the functional conditions to which this region 

 is subjected are moreor less similar to those characteristic of the part 

 removed and growth, i. ^., regeneration of this region and its differ- 

 entiation into a part more or less like that removed occur. If, on 

 the other hand, the old part is capable of performing more or less 

 perfectly the functions of the part removed, that is to say, if its 

 reactions are modified by the changed conditions so as to resemble 

 those of the part removed, corresponding changes will occur in the 

 structure of that portion which resembles functionally the part 

 removed and it will be "redifferentiated" into a part like that 

 removed. As an example let us compare the case of Leptoplana 

 (Child, '04a, '04b, '04c) with that of Stenostoma (Child, '02, '03). 

 Regulation after removal of the posterior end in Leptoplana is 

 wholly or almost wholly regeneration, /. ^., growth from the cut 

 surface, while in Stenostoma the posterior region of the old part 

 "redifferentiates" into the new tail. When we compare the 

 behavior of the pieces after removal of the posterior end, we find 

 that in Leptoplana functional substitution of the posterior end 

 of the piece for the original posterior end does not occur or occurs 

 only in slight degree, while in Stenostoma the posterior end of the 

 piece functions almost perfectly as a tail. This brief comparison 



