DOMINANCE IN RECONSTITUTION 333 



bearing on the question of the nature of polarity and developmental pat- 

 tern in general. If polarity results from a molecular orientation or an 

 "intimate structure" of some sort, it is not easy to account for the gradual, 

 progressive origin of a polar axis and pattern, beginning at one end. If 

 we assume some factor which determines molecular orientation or inti- 

 mate structure progressively from one pole, that constitutes the primary 

 polarity; and the molecular or other structural orientation is a resultant, 

 an expression of polarity, like other features of polar pattern. But as re- 

 gards determination and progressive extension of a dynamic gradient from 

 an activated region, no assumptions are necessary, for these phenomena 

 are visible or can be made visible by various methods. If a structural 

 orientation or a localization of different substances is present along a 

 polar axis, it seems highly probable that it results from, rather than de- 

 termines, a physiological gradient involving dynamic factors. Without 

 operation of such factors it is not evident how the changes in structural 

 orientation or localization of substances required to account for changes 

 in polarity under experimental and other conditions are determined. 



CAN A DOMINANT REGION ORIGINATE INDEPENDENTLY 

 OF OTHER PARTS? 



If gradual progressive determination of a polarity, beginning at a domi- 

 nant region, does take place, the dominant region must originate inde- 

 pendently of other parts of the polar pattern. That it does originate inde- 

 pendently in various kinds of buds and in many reconstitutions was 

 briefly pointed out above. But the question whether it can develop inde- 

 pendently of other parts of the polar axis, whether it is a self-differentiat- 

 ing system, remains to be considered. Among animals the most striking 

 evidence of a high degree of independence is found in the partial uni- 

 polar, bipolar, and multipolar forms in hydroid, planarian, and annelid 

 reconstitution. When short pieces of Tiihularia or Corymorpha develop 

 into partial forms, these, except occasionally under inhibiting conditions, 

 represent the apical region and more or less of the normal polar pattern, 

 according to length of piece, scale of organization, and presence or ab- 

 sence of other partial polarities (Fig. 113, A-I)} The parts of the axis 

 formed in such cases appear normal in pattern, fully developed, even 

 though they consist of nothing but a hypostome or a hypostome with 

 distal tentacles. In partial forms developing from aggregates of dissoci- 



' For other bipolar and multipolar forms see Fig. 15, B, p. 42; Fig. 116, p. 346; Fig. 120, 

 p. 3()o; and Fig. 124, p. 366. 



