2. These increases are temporary. In 30 to 45 minutes a minimum of speed is reached. This is followed by a period of recovery lasting about an hour. Finally an equilibrium is found. With inorganic acids (HCl or H₂SO₄) the final speed after 3 or 4 hours is that characteristic of prolonged exposure to pH 7.0; on the other hand, 3 or 4 hours after the application of either valeric or carbonic acid, speed is proportional to the [H^prime;] of the external and, probably, of the internal medium.

3. These facts become explicable if we assume that the ionization of an ampholyte superficially localized is essential for the execution of the ciliary stroke. Valeric and carbonic acid, in time, demonstrably penetrate the cell. As a working hypothesis we postulate that internal increase of the [H'] accelerates the rate at which this ampholyte is synthetized; but without actually penetrating the cell, hydrogen or hydroxyl ions in the external medium could also increase the degree to which this ampholyte dissociates.

4. Increased ionization of a fixed quantity of ampholyte and an increase in the rate of its production are in these experiments practically indistinguishable. Hence we assume that immediate and temporary increases of speed resulting from any change of pH, as well as final and permanently higher speed levels manifest only after prolonged exposures to organic acids, involve essentially the same mechanism.