The experiments described in this paper have shown that the enzyme ribonuclease (RNase) is able to enter Spirostomum ambiguum. There is a definite concentration/time relationship in the action of RNase on the ribonucleic acid (RNA) system of the organism. The rate of action of the enzyme is proportional to its concentration in the medium. At high concentration the enzyme causes instant degeneration of the cell. At lower concentrations the enzyme acts specifically on RNA present in the cytoplasm and in the macronucleus; it causes complete loss of basiphilia. Loss of basiphilia occurs first in the cytoplasm and later in the macronucleus.

The organisms after treatment with the enzyme, when transferred back to enzyme-free Chalkley's medium, regenerate RNA. The RNA appears first in the macronucleus and later in the cytoplasm.

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