ABSTRACTA reliable method for producing reproductive cysts inTetrahymena patulais described. The procedure involves the isolation of macrostomes without cytopharyngeal pouches in microdrops of distilled water under oil. The study of silver‐impregnated specimens has shown that a complex pattern of oral resorption and reformation occurs within the cyst that leads to the formation of a group of small cells with recessed oral apparatuses. These cells, called “cryptostomes,” swim very rapidly on excystment and are incapable of either feeding or reproducing. They are presumably dispersal forms. Oral morphogenesis during the transformation of excysted cryptostomes into microstomes and macrostomes is also desc