The surface of a separate Dictyostelium discoideum cell exposed to fluorescent antiserum is initially uniformly stained. When the cell moves, labelled material is left on the glass over the area occupied by the cell while in the antiserum. The surface of new pseudopods is fluorescent. The bulk of the label soon accumulates on the rear of the cell and is towed along behind it. The rest of the surface may remain faintly fluorescent for long periods, but this residual label is immediately left behind if the cell advances while strongly compressed between agar and glass. This evidence does not clearly decide between the hypotheses of a permanent, advancing, lateral cell surface and a temporary, stationary one. The fact that the lateral surface of an advancing cell does not drag forward viscous medium in contact with it does not favour the former hypothesis.

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