One of the most interesting capacities of protozoans is their ability to replace lost parts following injury. Although they are structurally the equivalent of cells they are functional organisms, and a study of their behavior makes it possible to bring together concepts usually applied in the cellular field with those applied in the analysis of whole organisms. The same factors that operate to evoke a particular form in the whole organism must act in a small regenerating piece of a protozoan. Whether these factors are nuclear genes or protoplasmic organization, they act rapidly in the regenerating animal, regulating the form of the piece. Unlike the metazoan material, the morphogenetic activities are not a matter of differential growth rates, but are rather redistribution and reorganization of materials present in the regenerating piece. In this sense the repair of injury inprotozoans more closely resembles cellular differentiation than organogeny
