ORIGIN OF AXIATE PATTERN 57 



shows that for certain forms, at least, the normal sym- 

 metry of pattern is not inherent and unchangeable. 

 In the radial anemone Harenactis, for example, bilateral 

 tentacle groupings may arise under certain experi- 

 mental conditions (Child, 1909), and it is possible 

 through differential susceptibility to obliterate bilater- 

 ality and produce radial larval forms in the sea urchin 

 (Child, 1916^) and also in the starfish (unpublished). 

 Moreover, in pieces of Planaria, under experimental 

 conditions which practically obliterate the polar gradi- 

 ent the symmetry gradient may become the polar axis 

 of the new individual (Child, 1915^ pp. 163-65). 



A characteristic feature of radial symmetry is the 

 repetition about a center of parts, each of which is 

 usually axiate and often bilateral in pattern. Such 

 repetition must be largely a matter of the space rela- 

 tions of specialized growth centers. Each growth cen- 

 ter involves or dominates a certain area, and only at a 

 certain distance from it can another similar center arise. 

 Thus the number of such growth centers arising on a 

 given circumference depends on the area dominated by 

 each center, which varies with physiological condition, 

 and on the size of the circumference, and can be altered 

 experimentally in many cases. Moreover, as growth 

 in size of the circumference occurs and the distance 

 between the repetitive parts increases, we often find 

 additional new parts arising, e.g., mesenteries and 

 tentacles in many actinians, etc. Such processes are 

 physiologically similar to many forms of agamic repro- 

 duction, being essentially reproductions of specialized 

 parts instead of new wholes, resulting from physiological 

 isolation (Child, 1915^, chaps, iv, v). 



