A. Filament growing in medium with nitrogen source. Green represents nitrogen replete status. B-D. Filament shifted to medium lacking nitrogen source. Cells gradually starve (turn yellow). Random cells reaching critical stage of starvation commit to differentiate. E-G. Differentiating cells release inhibitor that diffuses to neighboring cells, preventing them from commiting. Only distant cells reach critical stage of starvation. In this model, inhibitor is itself a nitrogen source.