Fig. 1. Ventral cord development. (A-C) Trajectories of various axons in the ventral cord. Ventral aspect, anterior towards the left. (A) RIF axons pioneer the pathway from the anterior end of the ventral cord towards the nerve ring. RIF axons cross the ventral midline (a) and enter the nerve ring on the contralateral side (not shown). The AVG cell body is next to the RIF cell bodies in the retrovesicular ganglion (rvg). The AVG axon pioneers the right ventral cord axon tract. PVP cell bodies lie at the posterior end of the ventral cord. Their axons also cross the ventral midline (b) to grow anteriorly in the contralateral ventral cord tract, i.e. the PVPL axon grow in the right axon tract, and the PVPR axon in the left tract. The PVPR axon pioneers the left ventral cord axon tract. (B) Almost all interneuron axons growing out of the nerve ring cross over to the right side exactly where the RIF neurons crossed earlier (a). Motoneuron cell bodies lie on the ventral midline and send their axons into the right axon tract (b). DA and DB motoneuron commissure grow straight out of the cell body towards the dorsal cord (c), some on the left and others on the right side (not shown). DD motoneuron commissures branch off the ventral cord axon and always grow on the right side (d). (C) Overlay of A and B. (D) Ventral cord cross-section illustrating the position of axons within the ventral cord as well as the order of outgrowth (numbers in circles). Early outgrowing axons (AVG, DD) tend to be dorsally close to the basement membrane, whereas later outgrowing axons (DA/DB, interneurons) occupy central and ventral positions. Modified, with permission, from White et al. (White et al., 1986).