lower plants and are implicated in the formation of flagella, cilia, and certain sensory structures and in the organization of the mitotic apparatus. Electron microscopie studies have shown that the structural pattern of these two types of organelles is almost identical from Protozoa to man, but the origin and development of this pattern remain largely unknown. Previous investigators, searching for early developmental stages, have observed at the site where the new structure will develop either a sphere of dense, amnorphous material or a young organelle shorter thaii the adult but nevertheless possessing most or all of the features characterizing the mature pattern. Intermediate stages have rarely been detected. The present paper, based on observations reported in a previous abstract,' provides evidence for these stages and establishes for the kinetosome (ciliary basal body) of Paramecium a mode of development that may be expected also to apply to basal bodies aind centrioles of other organisms. Methods.-Paramecium aurelia, stock 51, was grown at maximal rate in baked lettuce medium2 at 27?C. Success in detecting the earliest stages of basal body formatioll was based upon recogniition of three facts. First, the new ciliary basal body arises at right aingles and in close proximity to the alnterior portion of an existing basal body (Figs. 2,