—Each neuron that has been thrown randomly into the soup secretes a chemoattractant signal; they’re all trying to get the others to migrate to them. Two neurons happen to be closer than average to each other by chance, and they wind up being the first pair to be clumped together in their neighborhood. This doubles the power of the attractant signal emanating from there, making it more likely that they’ll attract a third neuron, then a fourth…Thus,
