194 c - M - CHILD. 



ing at any given time are regions of high rate of oxidation, while 

 those which are undergoing retraction or resorption are regions 

 of low rate. It seems probable then that in general the retrac- 

 tion of one part and the growth of another, particularly in starv- 

 ing stocks are associated with such differences in rate, the region 

 of high rate maintaining itself and even growing at the expense of 

 less active regions. 



The normal relations and proportions of parts in nature must 

 depend in large measure upon certain relations of rate of funda- 

 mental metabolism. When these metabolic relations are altered 

 the form or proportions must change, and in simple, highly 

 plastic forms such as hydroids, such changes may involve the 

 complete resorption or atrophy of previously existing parts and 

 the dominance and development of parts previously subordinate. 



The stolon represents a physiological axis, a gradient (Child, 

 '19, '21), but the data of susceptibility, KMnC>4 reduction and 

 vital staining for some ten hydroid species examined indicate 

 clearly that the stolon gradient is much less "steep" than the 

 hydranth-stem gradient and that whenever, and wherever the 

 gradient in a stolon becomes steep enough, transformation into 

 a hydranth-stem complex occurs. In other words, the difference 

 in rate between the hydranth bud or hydranth and the stem is 

 greater than that between the stolon tip and the stem or the 

 lower levels of the stolon. 



According to this viewpoint, the stolon usually appears in 

 nature as a basal structure, not because of the presence there of 

 any "stolon-forming substances" but first, because this is the 

 region of lowest metabolic or oxidative rate in the stock, and 

 second, because new buds arising in this region are more or less 

 inhibited by the dominance of the more active regions above. 

 Probably the bioelectric currents resulting from the differences 

 in electric potential between basal and higher levels are important 

 factors in such inhibition. But whatever the factors involved, 

 the partially inhibited axis develops in the form of a stolon. As 

 I showed for Tubularia (Child, '15, pp. 91-2, 130-37), when the 

 distance of the stolon tip from the hydranth becomes great 

 enough, the stolon tip becomes physiologically isolated from the 

 inhibiting action of the more active levels and transforms at once 

 into hydranth and stem. 



