70 C. M. CHILD 



In an earlier paper (Child '12) the writer has shown that in 

 Planaria reconstitutional development may be inhibited in various"'\^ 

 degrees by low concentrations of narcotics and that the formation > V" 

 of a head may still occur when all development of other parts is 

 completely inhibited. Moreover, it is a familiar fact that very 

 short pieces of Planaria may give rise to a single head or to biaxial 

 heads without other parts, and the same is true forTubularia 

 and various other forms. These various facts demonstrate 

 that head-formation in Planaria is possible without any corre- 

 lative influence of other parts, i.e., it is a process of 'self-differ- 

 entiation' (Child '13 b, pp. 614-617). On the other hand, there 

 is no evidence favoring the opposite conclusion. Even as re- 

 gards pieces undergoing reconstitution, there are no facts indi- 

 cating that head-formation is determined by other regions of 

 the piece. We have simply been accustomed to consider that 

 the piece replaces lost parts and so gives rise again to a new whole. 

 But it has often been pointed out that 'in such forms as Planaria, 

 the piece does not replace exactly the parts lost, but a new whole 

 arises by the formation of a head at one end, a posterior end at the 

 other and the reorganization of remaining portions. What 

 actually occurs in these cases is that a new head region begins to 

 develop as the first step in a new individuation and this new head 

 region dominates other parts and determines a reorganization 

 of the old tissues of the piece. Head-formation is not deter- 

 mined by other parts, but it — or more specifically the formation 

 of the cephalic nervous system — represents the fundamental 

 morphogenetic reaction of the specific cellular material. 



The existence in Planaria of the inverse relation between 

 head-frequency and rate of metabolism in the piece as a whole, 

 constitutes further important evidence in support of this view 

 and is readily understood from this standpoint. It can mean 

 only that the cells of the region x (fig. 2) give rise to a head except 

 in case the rate of metabolism in the region y is sufficiently high 

 to retard or inhibit this process of "self -differentiation." 



The rate of metabolism in the cells at x, which we may call 

 'rate a:,' is probably determined largely by local conditions con- 

 nected with the altered correlative conditions and the presence 



