Constant division rate in ciliates can be maintained by keeping the culture medium at constant optimum H-ion concentration.

The variations in division rate found in the typical protozoan life history, including gradual decline and death, can be reproduced experimentally by altering the pH of the medium.

When cultures are maintained under optimum conditions, encystment and conjugation can take place at any age; the life cycle disappears.

An explanation based on experiment is given for the apparently contradictory findings of Maupas.

Neither conjugation nor endomixis has any effect on the division rate under constant conditions.

The length of the endomictic period is affected by the H-ion concentration of the medium.

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