THE AXIAL GRADIENTS IN HYDROZOA. 215 



Comparisons between the species are best of limited value be- 

 cause various physiological conditions play so large a part in 

 determining the susceptibility of the individual. 



DISCUSSION. 



The data show first, that a primary axial gradient in suscepti- 

 bility exists in the three species of hydra, with the region of high- 

 est susceptibility at the apical end, and second, that this gradient 

 is variously modified by the functional activity and particularly 

 by muscular contraction in the various regions. If the view ad- 

 vanced in various papers (see references, Child, Hyman) and 

 supported by various lines of evidence, that susceptibility is in 

 a general way a criterion of metabolic and physiological condi- 

 tion, is correct, the facts indicate that there is in hydra, as in 

 other axiate organisms examined, an apico-basal metabolic gradi- 

 ent, with region of highest metabolism at the apical end. It is 

 perhaps scarcely necessary to repeat once more that the differ- 

 ences in metabolic rate are only one feature of such a gradient. 

 Associated with them are of course differences in protoplasmic 

 condition of various sorts. The important point is that the apico- 

 basal axis in hydra, like the axis of other organisms, appears as 

 a gradient in relation to the action of external agents. Discus- 

 sion as to what constitutes the primary feature of such a gradient 

 is at present of little value, since metabolism and protoplasm 

 are always associated in active living organisms. Likewise, the 

 question how each particular agent acts on protoplasm is of 

 minor importance for the general conception. 



As in other axiate forms, the primary gradient corresponds to a 

 relation of physiological dominance and subordination, the re- 

 gion of highest susceptibility being to a greater or less degree a 

 region of initiation and control. In hydra this is true, both of 

 the tentacle and of the column. The stalk represents physio- 

 logically a more or less specialized contractile region secondarily 

 developed from the basal portion of the body, but the primary 

 gradient is altered by the contractile activity of the stalk, not 

 by its mere presence, except possibly in H. oligactis, though even 

 here the high susceptibility of the stalk observed in most cases 

 is apparently associated with more or less muscular activity. 



