DOMINANCE IN RECONSTITUTION 



353 



anterior region activated by section or beginning regeneration is more 

 effective in determining a scale of organization longer than the piece than 

 is the original head. Actually, however, the effective length of the A'-pieces 

 for reconstitution of more posterior levels is somewhat less than the length 

 of the piece, for the cells of the anterior part of the piece are directly con- 

 cerned in head regeneration, while in head+A'-pieces this is not the case. 

 From isolated heads posterior ends never develop, perhaps because of 

 lack of potency ; but the table shows that at the levels A' and F length of 

 piece, rather than presence or absence of potency, determines presence 

 or inhibition of posterior development. 



TABLE 8 



Alteration of Scale of Organization in Relation to 

 Length of Piece and Body-Level in Short Anterior 

 Pieces of Dugesia ( = Enplanaria) dorotocephala (loo PIECES 

 in Each Lot 



(Data from Rulon and Child, 19376) 



Posterior development is also inhibited by delaying posterior section 

 of A'-pieces for 24 hours after anterior section, that is, by activation and 

 initiation of head regeneration before posterior section. In all these cases 

 of posterior inhibition the scale of polar organization determined is ap- 

 parently longer than the piece. Dominance and the high gradient-levels 

 are so effective in maintaining or determining the piece as an anterior 

 region that not only is activation and outgrowth of cells at the posterior 

 end largely or wholly inhibited but the piece includes no level corre- 

 sponding to the pharyngeal region. 



Exposure to KCN for a time after section decreases scale of organiza- 

 tion in these pieces, so that after return to water the frequency of pos- 

 terior development increases. In Table 9 posterior development is absent 

 or inhibited in 71 per cent of the A'-controls and in 41 per cent after KCN. 

 In the F-controls it is absent or inhibited in 31 per cent, and after KCN 



