Neuronal repulsion by a gradient of Slit. a, Explants were cultured alone for 24 hr before being cocultured with either Slit or control aggregates. Time-lapse microscopy was used to track neuronal migration. The Slit/control aggregates were placed on the right. Red dots illustrate stationary points in the pictures. b, Direction of neuronal migration before and after Slit/control aggregate placement. Time-lapse microscopy was used to track 20 neurons in each experimental group. Each colored line represents the angle of migration of an individual neuron in relation to the center of the explant and location of the aggregates. The same color is used to depict a neuron both before and after aggregate placement. Each pair of vector diagrams represents neurons from a single time-lapse experiment.