This proposal concerns two fundamental aspects of development: the creation of cell diversity and the directed movements of differentiating cells. The experimental organism is the simple eukaryote Dictyostelium discoideum. During the developmental phase of its life cycle, two precursor cell types diverge from undifferentiated amoebae. Early in development, the differentiating cells move, or sort out, resulting in spatially distinct spore and stalk "tissues." The technique of density gradient centrifugation provides a means of separating the precursor populations. It will therefore be possible to make detailed biochemical comparisons of the proteins and messenger RNA composition of developing pre-spore and pre-stalk cells. Differences will be sought particularly with respect to molecules residing on the cell surface and those previously implicated in slime mold chemotaxis and early differentiation. The effects of possible physiological regulators on the relative proportions of the two cell types will be investigated. By mixing together radioactively and/or genetically labelled amoebae of different developmental tendencies, the sorting out of like cells within the aggregate can be observed. In this way the timing and the developmental significance of sorting out will be explored. It is assumed that knowledge gained in this simple system will be in large measure applicable to developmental questions in more complex organisms.