Crossing the Line

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Science  15 May 2009:
Vol. 324, Issue 5929, pp. 893-894
DOI: 10.1126/science.1174216

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A key process in animal evolution was the development of a nerve-rich, bilaterally symmetric longitudinal structure, the central nervous system. Without such a symmetric body axis, Earth might still be populated by just sea anemones, sponges, and similar organisms (1). The switch from radial to bilateral symmetry created a distinct left- and right-hand side to the animal and its nervous system. The dividing line, or axis of symmetry, is known as the midline. One of the earliest decisions a developing neuron must make is whether to extend its long cellular process (the axon) across the midline. On page 944 of this issue, Yang et al. (2) uncover an unexpected level of complexity in how this initial decision is made.