We have shown that the apical tip of the developing sorogen prevents secondary tip formation during normal patterning in Polysphondylium pallidum. In one class of experiments the apical tip was excised and secondary tips soon formed over the surface of the remaining sorogen. In another, very large sorogens formed secondary basal tips spontaneously before regulating to a fixed radius to length ratio of 0.14, the ratio characteristic of control sorogens. These results are discussed in the light of a simple model. © 1982, International Society of Differentiation. All rights reserved.