Summary

This study has identified the first five motor neurones axons out of the segmental nerves in the thoracic and has traced the pathways followed by these axons up divergence into the ganglionic nerve roots. These out axons in a stereotyped sequence over a short to 2% of embryonic development. Motor axons initially contact the dorsal basal lamina and then parallel array just beneath this membrane. At the edge axons diverge into either of two pathways: an anterior corresponding to nerve root 3 which is pioneered by axon to leave the CNS; and a posterior pathway, root 5, which is pioneered by the second motor axon. axon appears to grow circumferentially around the between the body wall and the base of the coxa, while closely associated with the filopodia or axons of the peripheral pioneer neurones. The later motor axons pathways pioneered by these first two axons. A small molecular markers would be sufficient to generate the of axon growth by these early motor neurones and some cues may be used to guide afferent axons into the CNS.