Abstract
Cell suspensions of the fresh-watersponge Ephydatia fluviatilis have been fractionated by means ofFicoll gradient centrifugation. Three fractions were isolated. The densest contains archeocyte-like cells only; the intermediate fraction is very rich in choanocytes, and the lightest is a mixture of cell types. Earch fraction shows specificaggregative properties and potentialities to reconstitute functional sponges. It appears that the sequence of reconstitution events can be selectively altered by certain disequilibria in the cell population These preliminary results constitute a first approach to the analysis ofcell type specificity in sponges.