CHAPTER X 



PHYSIOLOGICAL DOMINANCE AND ORGANIZATION 

 IN RECONSTITUTION 



PROGRESSIVE DETERMINATION OF POLARITY 



DEVELOPMENT of bud and root axes in plants, of buds in ani- 

 mals, and the reconstitution of pieces shows that the new polar 

 pattern is not established all at once but begins with the domi- 

 nant region and extends progressively from it, as far as range of domi- 

 nance, size of isolated piece in relation to scale of organization, or pres- 

 ence of other dominant regions and polarities permit. The conclusion 

 seems to be justified that polarity itself develops progressively in these 

 cases. At an early stage of reconstitution, budding, or fission the apical 

 or anterior region may be the only part of the new axis present. What- 

 ever the level of section in the stem of Tuhularia or Corymorpha, the apical 

 region of the hydranth originates adjoining that level, except under in- 

 hibiting conditions which prevent attainment of a gradient-level high 

 enough for development of an apical region; then the level adjoining the 

 level of section sometimes gives rise to a more proximal region of the 

 hydranth, and the extreme apical region may be permanently absent. Al- 

 ways, however, the new axis develops from this level. Whatever the level 

 of section in the planarian body, if head regeneration is possible, it be- 

 gins at that level and determination of the new polar axis progresses 

 posteriorly from it. In embryonic development the so-called law of an- 

 teroposterior development indicates that, even though regional cytoplas- 

 mic differentiations are present in an egg, a physiological dominance and 

 gradient factors may determine the order of their developmental activity 

 though polarity is already present. 



That a new polarity may originate gradually and progressively from a 

 dominant region in nonembryonic forms of development which involve 

 determination of new pattern seems evident from the available data. 

 Moreover, the evidence concerning origin and progressive extension of a 

 new gradient from a dominant region is in accord with this view. And, 

 finally, the evidence pointing to this conclusion has a very important 



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