SUSCEPTIBILITY IX HYDROIDS. IIQ 



species, the stichiclia in Dasya and related forms, make their 

 first appearance in the basal regions of the thallus and the zone 

 of their development progresses apically, exactly as in the hy- 

 droids. Moreover, in the algae as in the hydroids, the basal 

 regions are the regions of lowest susceptibility and the oldest 

 physiologically (Child, '16 a, b). That this similarity of localiza- 

 tion of specialized reproductive parts, as well as the similarity 

 of growth form in algae and hydroids is associated with the grada- 

 tions in physiological condition, of which the susceptibility re- 

 lations are an indicator, cannot be doubted. And finally, it 

 may be pointed out once more that this similarity in growth 

 form, susceptibility relations and localization of specialized 

 reproductive processes in algae and many other plants and hy- 

 droids must depend upon some physiological factor which is 

 independent of the specific differences in protoplasmic constitu- 

 tion of the different forms and which must therefore be quanti- 

 tative in character. In short, all the facts force us to recognize 

 the existence of a quantitative gradation in physiological con- 

 dition, in rate of fundamental metabolism and the associated 

 protoplasmic conditions, as the basis of the axial relations, of 

 which growth form, susceptibility and localization of parts are 

 various expressions. 



Concerning the nature of the relation of dominance and sub- 

 ordination which has long been known to exist in the plants and 

 is evidently present in the hydroids also, it need only be said 

 here that the conception of dominance by means of inhibiting, 

 formative or other specific substances accounts neither for the 

 origin nor for the maintenance of these relations. In order that 

 one region may produce substances that inhibit another region, 

 the two regions must already be different, i.e., in order that such 

 chemical relations may exist along a physiological axis, the axis 

 must already be established and certain differences be present. 

 Moreover, if one apical region produces substances which in- 

 hibit other apical regions, the one region must be immune to 

 the substances which it produces, while the other regions are 

 not, that is to say differences must already exist between the 

 different apical regions before one can inhibit another without 

 itself being inhibited, and if differences already exist, the assump- 



