The recessive mp gene in syngen 1 (Species 1) of Tetrahymena pyriformis results in a characteristic set of abnormalities of the adoral membranelles, commonly including some extra membranellar segments (Kaczanowski, '75). The abnormal pattern is produced in about 10% of dividing cells, compared to 80% of cells undergoing oral replacement. Thus the percentage of abnormal cells increases during prolonged stationary phase. Abnormal oral replacement arises primarily for a separation of two kinetosomal fields: the field of kinety no. 1 (the K-1 field), and the anterior field originating from the displaced undulating membrane. Both sites of proliferation appear also during wild type morphogenesis, but they fuse early to form one integrated field (Frankel, '69). The prolonged separation of the subfields in mp/mp cells results in a separate assembly of adoral membranelles; however, the independently developing membranelles of the two subfields are united later to produce a single oral apparatus. A single gene-dependent factor apparently alters some property of the cortical layer, which in turn is responsible for a reduced integrity of the oral replacement primordium.