Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.


Logo for

Science 324 (5929): 893-894

Copyright © 2009 by the American Association for the Advancement of Science


Crossing the Line

Thomas Kidd

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.

Department of Biology, University of Nevada, Reno, NV 89557, USA.

E-mail: tkidd{at}

To Advertise     Find Products

Science Signaling. ISSN 1937-9145 (online), 1945-0877 (print). Pre-2008: Science's STKE. ISSN 1525-8882