is alkaline the particles become electro-negative and vice 
versa.'' 
It must be assumed that the ion-proteid is highly un- 
stable in the presence of an excess of ions, and that therefore 
the nature of the ion-proteid formed depends upon the pro- 
portions of the ions present. If this be granted (and we shall 
see that it is an indispensable assumption in accounting for 
the various phenomena observed in muscle and nerve) we can 
at once see that the reason for the proteid particles becoming 
electro-positive in an acid solution is the high velocity of the 
hydrion which is the characteristic ion of acids ; for far more 
kations are diffusing into the proteid particle than anions, 
and therefore the ion-proteid formed is, for the greater part, 
kation-proteid, and the particle becomes positively charged. 
Similarly, in alkalies the fastest ion is the anion, and there- 
fore the proteid particles become electro-negative when the 
solution is alkaline. 
3. — The Chemotaxis and Galvanotaxis of Unicellular 
Organisms. 
We have now to consider the application of the prin- 
ciples which we have enunciated to unicellular organisms. We 
have seen that it is a characteristic of the proteid part of the 
ion-proteid m,olecule that it readily forms compounds with 
any ions which happen to be present in excess, while Hardy's 
experiments, referred to in the last section, show that the 
electrical character of the resulting ion-proteid depends upon 
the relative velocities of the ions in the solution in which the 
proteid is suspended. In the first case, consider the effect 
upon a unicellular (amoeboid) organism of a constant current 
in the direction shown in the diagram (A = Anode, K = Ka- 
A(-f) 'k ^^- (-)K 
Figure 1. 
thode), the organism being supposed to be laden with kation- 
proteid by virtue of the metabolism and dissociation of which 
