DYNAMICS OF MORPHOGENESIS 77 



rate of dynamic activity. This region dominates development, 

 becomes the apical or head region and determines the axial 

 gradient or gradients, which constitute the dynamic basis of 

 polarity and of individuation. The organization and develop- 

 ment of various parts of the organism rests upon a similar basis 

 of fundamental reaction system and dominance and subordi- 

 nation of parts resulting from differences in rate of reaction. 

 Following papers will be devoted primarily to the development 

 of this general conception on an experimental basis and its appli- 

 cation to particular aspects of the problem of experimental 

 reproduction, and secondarily, to the question of its significance 

 for inheritance and development in general. 



VI. SUMMARY 



1 . Whether or not a head shall arise at the anterior end of an 

 isolated piece of Planaria dorotocephala is determined so fixedly 

 during the first six or eight hours after section that head-formation 

 cannot afterward be prevented by conditions which do prevent 

 it when acting immediately after section. Head-determination 

 undoubtedly begins immediately after section. 



2. The period during which head-determination occurs is the 

 period of stimulation following section, and in general the more a 

 piece is stimulated by section the less likely it is to produce a 

 head. Head-formation is a process opposed or antagonistic 

 to the maintenance of the piece. 



3. The development of a new head on a headless piece is not 

 the restitution of a missing part but the first step in the develop- 

 ment of a new individual. Whether a head shall develop or not 

 depends primarily on whether the cells which give rise to new 

 tissue at the anterior end of the piece become physiologically 

 isolated to a sufficient degree to develop independently of other 

 parts of the piece, or whether other parts prevent this develop- 

 ment. In the former case a head arises, in the latter the piece 

 remains headless. 



