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A miraculous little movie dances across the screen of Madhav Mani’s laptop, looping hypnotically every 15 seconds. The silent drama plays out on a stage measured in microns — about the size of a typical bacterium — and depicts the movement of the morphogenetic furrow, or a wave of differentiation, that sweeps through a sheet of cells that will become the hexagonal units of the fruit fly’s eye.

Observing the scene, made possible by relatively recent advances in microscopy, even a...