THE RANGE OF DOMINANCE 131 



dence indicates that when such a gradient is sufficiently 

 marked, that is to say, when the metabolic rate in its 

 apical region is sufficiently high, and when the inhibiting 

 or obliterating influence of a gradient in the opposite 

 direction is not too great, a hydranth develops. The 

 formation of a stolon, on the other hand, apparently 

 represents a gradient which is partially inhibited or 

 obliterated, or, in other words, partially dominated by a 

 gradient in the opposite direction, but in addition to this 

 relation a relatively high metabolic rate in the piece 

 or individual as a whole is also apparently necessary 

 for stolon-formation. The stem represents the lower 

 levels of a simple uninhibited gradient, and its formation 

 always occurs under the dominance of a hydranth or 

 other region of higher metabolic rate. 



It is also important for an understanding of the facts 

 to note that in general the metabolic rate of these animals 

 decreases when they are transferred from natural to 

 laboratory conditions, and the hydranths which develop 

 in the laboratory possess a lower metabolic rate than 

 those in nature; consequently the range of dominance 

 is less and physiological isolation occurs at shorter 

 distances from the dominant region than in animals in 

 nature. Moreover, the development of a new hydranth 

 at the cut end of a piece of stem is, I believe, a process 

 essentially similar to the development of a head on a 

 piece of Planaria (pp. 105-14). The new hydranth 

 region is independent of other parts and becomes 

 dominant over them, but during the early stages of its 

 development this dominance is less complete, because 

 the changes in the protoplasm of the stem in accordance 

 with the new metabolic conditions require some time; 



