Snapshot of a pattern generated by the cellular automaton described in Section 3 after 2000 time steps (A) and of excitation waves in D. discoideum (B). In (A) the excitability was set to g = 28, the other parameter values are k1 = k2 = 2 and n = 100. The array size is 200 × 200 cells. Elements in the excitable state (0) are displayed in black, refractory elements (n) are white. The gray values correspond to the excited state (1, ..., n - 1). The section in (B) corresponds to 7.4 × 7.4 mm. The black wave fronts are cells, which have just detected the cAMP molecules. In the bright area the cells are moving chemotactically towards the respective spiral center.