We have studied the development of fusiform (probably related to myoepithelial) cells in Rama 25 cultures [Bennett, D. C. Peachey, L. A., Durbin, H. & Rudland, P. S. (1978) Cell 15, 283--298]; we show that they are generated from special differentiated structures (projections) that contain a rapidly differentiating cell type (F-precursor cells). Clonal sublines isolated from projections develop in several directions under both environmental and genetic control. Some types of differentiation are reversible; others are irreversible. The various cell occurring in vitro may correspond to specific cell types in vivo.