text: In the species Dictyostelium discoideum, cAMP acts outside the cell as a secreted signal.  The chemotactic aggregation of cells is organized by periodic waves of cAMP that propagate between cells over distances as large as several centimetres.  The waves are the result of a regulated production and secretion of extracellular cAMP and a spontaneous biological oscillator that initiates the waves at centers of territories.