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The parallel extension of two growth cones, one optically guided (right) and one normally growing, illustrates the increase in the speed of growth cone extension in the presence of a 20-mW laser spot (red circle) (see Movie 3, which is published as supporting information on the PNAS web site). The time interval between successive pictures is 10 min. The reference marks on the left are spaced in 5-μm steps.
