In the mound stage of Dictyostelium discoideum, pre-stalk cells sort and form a tip at the apex. The mechanisms by which this occurs is as yet unknown. A cellular level model allows us to simulate both differential cell adhesion and chemotaxis. Our simulations show that with differential adhesion only, pre-stalk cells move to the surface of the mound but form no tip. With chemotaxis to an outgoing circular wave only, a tip forms but contains both pre-stalk and pre-spore cells. Only for a narrow range of relative strengths between differential adhesion and chemotaxis, can both mechanisms work in concert to form a tip which contains only pre-stalk cells.