Experiments on the encystation ofOpalinainRana temporaria

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Abstract
1. The formation of Opalina cysts in Rana temporaria was compared, during September to December, in groups of untreated frogs exposed to normal seasonal fluctuations in temperature, and treated frogs kept at room temperature. Cyst formation was enhanced, to the same degree, in frogs injected with saline or chorionic gonadotrophin (Pregnyl), and, to a higher degree, in frogs injected with frog pituitaries.2. Observations were made, during September to December, on the state of the testes in Rana temporaria infected with Opalina. The presence of Opalina cysts was most closely correlated with spermiation.3. Possible factors affecting the formation of opalinid cysts are discussed. It is concluded that the seasonal production of cysts by Opalina is not due to cyclical changes inherent in the protozoa. The factor or factors provoking encystation appear to be correlated with the reproductive cycle of the host, and may be the gonadal hormones.