Figure 1.

Demonstration that a diffusable pheromone is required for aggregation of starved S. aurantiaca cells. (A) Cells (3 × 10⁸) on a dialysis membrane were dialyzed for 24 hr against 6.8 mM CaCl₂ at 32°C. No aggregation of the cells was observed. (B) After an additional 24 hr at 32°C without dialysis, aggregates of cells have formed on the dialysis membrane shown in A. The calibration bar corresponds to 5 mm. The result (not shown) of a control experiment (cells placed on a dialysis membrane for 24 hr, but without dialysis) was indistinguishable from B.