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Having elicited from the protozoan, Paramedwn a-urelia, behavior which seemed not unlike associative learning in higher organisms (3), we have been trying to discover what internal mechanism(s) might underlie such a surprising finding. During certain fertilization processes, the nuclei of single animals undergo disintegration and restructuring. Yet, acquisition seemed not to be adversely affected (6) by such ongoing drastic internal reorganization when tests of performance were administered immediately after completion of training. On the basis of such evidence, it has been suggested (6) that sites for behavioral plasticity may exist primarily in the cellular cytoplasm rather than in the nucleus. The name "z" factors has been suggested for these hypothetical sites which may generally direct behavioral modification, at least in part. Now, it is known (8) that after the macronucleus disintegrates during fertilization, its fragments are slowly resorbed into the cytoplasm. During this resorption, the "z" factors could be affected. If so, a response acquired by autogamous paramecia with disintegrating and resorbing nuclei, might be lost or reduced due to cumulative changes in the cytoplasmic "z" factors as these factors interacted with the resorbing nuclear material while paramecia with intact nuclei and hence relatively stable "z" factors might show retention of the response.With these notions, we asked first whether paramecia with intact or vegetative nuclei would exhibit behavior analogous to retention. Second, we wondered whether such retentionlike behavior might be affected by nuclear disintegration. We decided arbitrarily to use a testing period 3 hr. after completion of training.Though an excellent morphological study of the process of autogamy in Parameci-um a-urelia is available (2), no time line is indicated. Information was needed about the rate of

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