_Urostyla grandis._ Interference of galvanotaxis and thigmotaxis. The freely swimming individuals move towards the cathode (left side). The creeping individuals move in transverse direction. ] These, then, are a few examples of the interference action of various stimuli on the single cell. They show us in part fairly simple, and in part very complex states. It now behooves us to obtain a general understanding of interference action, to learn the fundamental _laws_ in connection with these complex actions, to shell out, as it were, the general factors involved in the special conditions. In this connection the examples already referred to furnish all of the data necessary for our first orientation. In the simple instance in which the effect of galvanic stimulation was augmented by increase of temperature and again in the case where there was a diminution of excitation resulting from the alcohol, the interference of the two stimuli is consequent upon the fact that the location of attack is the same. The constant current acts upon a portion of the infusorium, which also responds to elevation of temperature. We have a _real_, or, as I may term it, “_homotopic interference_,” for it is an interference in which the general point of attack is the same for both stimuli. In contradistinction to this case, we have the examples of the interference of thigmotaxis and galvanotaxis in the hypotrichous infusoria. Here the effect of interference, the characteristic position of the axis of the cell body, is brought about by the fact that the galvanic stimulus affects different elements than the mechanical. The turning of a creeping Stylonychia or Urostyla, when the current is closed, in which the anterior portion of the body was previously directed towards the anode, results from excitation of the perioral cilia from the anodic pole. The mechanical stimulation, on the contrary, exerts its effect upon the locomotion and border cilia. Only when there is a turning of the anterior portion of the body towards the anode, would the galvanic stimulus affect also the anterior locomotion cilia and thereby counteract turning towards the anode. Therefore, we have before us in this case of the assuming of a characteristic position of the axis of the cell body the expression of an _apparent_, or, as I prefer to express it, a “_heterotopic interference_,” in which the two stimuli do not actually interfere in their action, but rather influence the final result, in that the condition for the state of the system in its totality is dependent upon its individual components. This heterotopic interference is of particular importance in the bringing about of the movements of the living system. The locomotion of the animal and especially the direction is in part a manifestation of heterotopic interference of response. At the same time, however, especially in the coördinated movements of nervous origin, the homotopic interference _also_ plays an important rôle and, not rarely,