358 PATTERNS AND PROBLEMS OF DEVELOPMENT 



those parts is necessary for such reconstitution. In other words, reconsti- 

 tution of a hydranth or head occurs in spite of the rest of the piece, not 

 because other parts of the piece determine the replacement of parts re- 

 moved. Only when activation of the cells concerned is suflEicient to bring 

 about a certain degree of physiological isolation from other parts does a 

 hydranth or head develop from a stem piece or a postcephaHc piece. The 

 only demonstrated action of other parts on determination of a new domi- 

 nant region is inhibitory. Determination of the hydranth or head repre- 

 sents the first steps in determination of a new polar axis. The hydranth, 

 or its apical part, and the head, or probably the nervous tissue of the 

 head, are primary developmental expressions of the reaction system of 

 the species. Every stem level of Tuhiilaria, Corymorpha, and other hy- 

 droids, every postcephalic level of Dugesia and many other forms, will 

 develop as hydranth, or apical part of a hydranth, or as a head if its de- 

 velopment is not otherwise controlled or inhibited by a dominant region 

 or by external conditions. 



A region dominant in development is an inductor or "organizer." It 

 determines the spatial pattern and localization of parts along the axis 

 concerned on a larger or smaller scale, according to conditions. This de- 

 termination does not necessarily, probably not usually, take place all at 

 once, but progressively from the dominant region. Scale of organization 

 apparently depends primarily on range and effectiveness of dominance. 

 Differences in scale represent differences in spatial localization and deter- 

 mination of parts. It seems evident that they depend on a factor or fac- 

 tors operative and effective over a certain spatial range which varies with 

 the activity of the piece or system concerned. In view of the various other 

 hues of evidence, the simplest assumption is that of a dynamic gradient 

 resulting from the activation following section and from the physiological 

 isolation from more apical or anterior regions in isolated pieces, or deter- 

 mined by an external differential or in some cases by localization of an 

 activated region experimentally or otherwise without section. Such a gra- 

 dient is, of course, only the initiating factor in pattern. However it is 

 determined, its character, the changes it undergoes, and the kind of pat- 

 tern and organism that results depend on the specific constitution, nuclear 

 and cytoplasmic, of the living system in which it appears. 



