whatA uniform lawn of cells that cannot stay uniform. First it streams into filaments, then it collapses into evenly spaced mounds, the way slime mould amoebae gather before they become a slug.

whyKeller and Segel's equations are the textbook case of a pattern that a smooth state cannot resist. The piece shows spacing appearing out of nowhere, chosen by how far a chemical signal can travel before it fades.

howEvery cell secretes an attractant at a constant rate; the attractant diffuses and decays; every cell drifts up the attractant gradient and jitters at random. Wherever a few more cells happen to sit, more attractant accumulates and more cells drift in. A crowding term keeps mounds from collapsing to points. The attractant is drawn as contour lines, the cells as ticks along their drift. Click a mound to scatter it, or click lawn to seed a colony.