174 c - M - CHILD. 



polarity apparently develops in definite relation to the vascular 

 supply, the apical pigmented pole arising on the arterial, the basal 

 unpigmented pole on the venous side. In these cases the oxygen 

 supply or the conditions determining rate of respiratory exchange 

 are apparently the chief factor in determining polarity, the 

 region of most rapid exchange becoming the apical pole. 



Symmetry in animals, like polarity is indicated by gradations 

 in physiological condition, and although a particular kind of pro- 

 toplasm may give rise normally to a radial or a bilateral animal, 

 experiment shows, that for certain forms at least, the normal 

 symmetry of pattern is not inherent and unchangeable. In the 

 radial anemone Harenactis, for example, bilateral tentacle group- 

 ings may arise under certain experimental conditions (Child, 

 '09) and it is possible through differential inhibition to obliterate 

 bilaterality and produce radial larval forms in the sea urchin 

 (Child, J i6rf) and also in the starfish (unpublished). Moreover, 

 in pieces of Planaria, under experimental conditions which prac- 

 tically obliterate the polar gradient the symmetry gradient may 

 become the polar axis of the new individual (Child, 15^, pp. 

 163-165). 



A characteristic feature of radial symmetry is the repetition of 

 parts, usually axiate and often bilateral, in pattern about a center. 

 Such repetition must be largely a matter of the space relations 

 of specialized growth centers. Each growth center 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 oiten find additional new parts arising, e.g., 

 mesenteries and tentacles in many actinians, etc. Such processes 

 are physiologically similar to many forms of agamic reproduc- 

 tion, being essentially reproductions of specialized parts instead 

 of new wholes, resulting from physiological isolation (Child, 

 '150, Chaps. IV., V.). 



The Problem of Symmetry. Bilaterality is inherited through 



