METABOLIC GRADIENT IN ANNELIDS 3 



6. Electrical methods. With the aid of a galvanometer it 

 is found that permanent differences of potential exist along the 

 axes of animals. These electrical gradients correspond to the 

 susceptibility, staining, and reduction gradients, the regions of 

 highest susceptibility, greatest staining, and reducing powers 

 being electronegative (galvanometrically) to regions of lower 

 susceptibility, staining, and reducing powers. The galvanotactic 

 behavior of at least the simpler organisms is also in accord with 

 the metabolic gradient conception. 



The concordance of results obtained by these various methods 

 indicates clearly enough the existence of a definite gradation 

 along the axes of organisms. The nature of this gradation is 

 naturally open to discussion. Our explanation that the grada- 

 tion is primarily quantitative involving differences in the rate 

 of fundamental metabolic reactions and in conditions associated 

 with these is the only one that fits all of the facts at present 

 known to us. 



It would obviously be desirable, in order to establish the meta- 

 bolic nature of the gradients beyond all reasonable doubt, 

 to make quantitative tests of the rate of metabolism of different 

 levels of the axis. To do so would appear to be a relatively 

 simple matter, but in fact a number of complicating factors 

 are involved. In the first place we must consider whether the 

 isolation of pieces from different levels involves physiological 

 changes in the pieces. We have definitely shown for Planaria 

 that it does involve considerable metabolic increase, and that 

 this increase is related quantitatively to the length of the pieces 

 and the level of the body from which they are taken. To elimi- 

 nate this factor it is necessary either to cut pieces so long that the 

 stimulation of cutting is reduced to a minimum or to wait until 

 the stimulation has disappeared, as happens after a number of 

 hours. In the former case an organism of sufficient length would 

 have to be selected; in the latter case one encounters changes 

 due to the onset of regeneration. It is further necessary to select 

 organisms which present neither structural nor special functional 

 differences along their axes. The functional activity of some 

 specific part may not accord with the general gradient, and 



