The branches in Polysphondylium pallidum whorls are arranged in a radial pattern. We have used a pattern-specific monoclonal antibody to study branch formation and characterize the origin of this pattern. A quantitative spatial analysis of antibody staining reveals that the branching pattern arises from a random distribution. This distribution passes through a series of intermediate stages to yield a radial prepattern. The origins and evolution of this prepattern are satisfactorily accounted for by models that produce spatial patterns by short-range autocatalytic and longer-range inhibitory forces.