432 PATTERNS AND PROBLEMS OF DEVELOPMENT 



is that of the factor or factors determining development in one plane of 

 the sea fans (Gorgonacea). In fans developing on more or less vertical 

 rock faces with water movement chiefly vertical, the plane of develop- 

 ment is usually, if not always, vertical. These and many other cases of 

 spatial pattern and order of zooids or parts present problems of funda- 

 mental significance for our conception of development. 



CONCLUSION 



Some of the ways in which axiate patterns can be experimentally de- 

 termined have been considered in this chapter. So far as the examples 

 given have been analyzed, they appear to involve alteration, obliteration, 

 and determination of dominance and a gradient or gradients as the earliest 

 distinguishable feature of the change in pattern. In the light of the experi- 

 mental data it appears that a factor operative in the reorganization of 

 other parts in reconstitution is associated with the high end of a gradient 

 or gradient system. In isolated pieces the region or regions most intensely 

 activated following section or otherwise become more or less dominant 

 and alter a pre-existing gradient or determine a new one in a different 

 direction from that already present. A new gradient and dominance may 

 also be determined by environmental differentials or gradients of various 

 kinds. However a gradient is initiated, the specific constitution and phys- 

 iological condition of the protoplasm concerned are undoubtedly the chief 

 factors determining its physiological characteristics, its length, steepness, 

 and the effective range of dominance. 



The determination of polarity in Corymorpha cell aggregates by the 

 contact-free-surface gradient, probably an oxygen gradient in the absence 

 of any cut surface, the effectiveness of ganglionic planarian grafts in induc- 

 tion of reorganization in the host body, the obHteration of fission zones 

 by reconstitution of heads a short distance anterior to them, the destruc- 

 tion of headless parts of zooids and whole zooids in early stages by a more 

 advanced head region posterior to them in Stenostomum pieces, and in 

 general the very evident relation between dominance and dynamic, rather 

 than structural, conditions— all support the view that dominance and the 

 gradient or gradient system associated with it are primarily dynamic in 

 character, not structural, except in so far as activity and structure of 

 some sort cannot be dissociated in living protoplasms. Vital activity, me- 

 tabolism, appears to be the primary factor determining pattern rather 

 than a pre-existing structure determining metabolism. 



Reconstitution of a hydranth or head is not a replacement or restitu- 



