We have investigated the formation of the prestalk-prespore pattern in Dictyottelium
discoideum. Pattern formation occurs in clumps of Dictyostelium cells embedded in agar under
a 100% oxygen atmosphere. Agar embedding allows us to control spatially the environment
surrounding the cell clumps. Our results suggest that the ambient oxygen concentration plays
a role in controlling the size of the multicellular mass. Further, oxygen gradients established
across clumps embedded in agar or held in holes in a plastic barrier cause orientation of the
prestalk-prespore pattern such that the anterior prestalk region forms at the highest end of
the gradient. The results also indicate that developing cells have the ability to migrate up
a gradient of oxygen
