side is expressed dynamically in neurons and muscles during embryogenesis. It is a cell surface marker that appears to direct the path of motor axons. At embryonic stage 12, side is expressed in a belt-like pattern along the ventral midline. The aCC neuron develops next to side-expressing cells. By stage 13, the side pattern has turned into a triangular pattern, with tips pointing away from the midline. The motor axons of the intersegmental nerve project along the anterior edge of each triangle. Axons of the segmental nerve grow along the posterior edge. The motor axons are guided directly toward the exit junction and ganglionic branch of the trachea. At the trachea, motor axons fasciculate with side-positive afferent sensory axons exactly at the lateral bidendritic neuron. When the intersegmental nerve has reached the end of the sensory tracks at the bidendritic neurons, side expression in the sensory neurons is not still detectable but instead is observed in muscle fibers. The side-expressing muscles are thought to attract the motor neurons, thus causing them to leave the sensory tracks. In general, cells and tissues located ahead of motor neuron growth cones express side.