The ciliate protozoan Stentor coeruleus performs a neat and localized act of cytoplasmic differentiation when it produces a new set of feeding organelles during division, regeneration, and reorganization. On the side of the cell appears a primordium which becomes visibly longer and more complex as it develops a band of ciliary membranelles and mouth-parts and migrates to its final anterior location. By quieting the animals in methyl cellulose the primordium itself can be operated on with glass needles in various ways and at different stages in its development. Following the classical studies in experimental embryology on progressive determinations of anlagen, one can remove parts of the oral primordium to determine if and when deletions result in corresponding defects in the structures produced. Corollary experiments in which whole primordia are translocated or by other means placed in new surroundings are also possible because of the graftability of stentors, and these studies will be reported in the sequel. Both types of experiment demonstrated that something like the gradual restriction of prospective fate found in the development of organ anlagen in multicellular organisms also occurs in the differentiation of the cytoplasm of the single cell.