Studies on Excystment in the Hypotrichous Ciliate <i>Pleurotricha lanceolata</i>*

Abstract

SUMMARY. Pleurotricha lanceolata was cultured in an aqueous extract of Cerophyl with Tetrahymena sp. as the primary food organism. It was induced to form resting cysts by crowding in the absence of food in a modified Osterhout solution. Cysts formed in this medium remained viable for several months. The morphology of the cysts and of animals undergoing excystment was studied both in the living state and in stained preparations. The excystment process was recorded by camera lucida drawings. These studies revealed no endomictic nuclear reorganization, and no extrusion of nuclear material as had been previously reported. An analysis of certain chemical and physical factors which affect excystment showed the following: the rate of excystment is temperature controlled; increasing the pH of the excystment medium increases the time required for excystment; in low concentrations some aqueous plant extracts are effective excysting agents; dilute solutions of certain plant acid mixtures neutralized with KOH are likewise effective; a number of organic substances, e.g. , Bacto‐yeast extract and Bacto‐peptone, are also effective; and the presence of bacteria retards the rate of excystment.