An additional discrete and unique pathway, represented by Egfr, is likely to function in the germband itself. The LG are used as a substrate during the extension of the first axons of the longitudinal tract. The differentiation of the LG has been examined in six mutations in which the longitudinal tracts are absent, displaced, or interrupted to determine whether the axon tract malformations may be attributable to disruptions in the LG scaffold. Embryos mutant for the gene prospero have no longitudinal tracts, and glial differentiation remains arrested at a preaxonogenic state. The longitudinal tracts in embryos mutant for slit fuse at the VNC midline and scaffold formation is normal, except that it is medially displaced.
Longitudinal tracts have intersegmental interruptions in embryos mutant for pebbled and midline .
In pebbled , there are intersegmental gaps in the glial scaffold. In midline , the glial scaffold retracts after initial extension. LG morphogenesis during axonogenesis is abnormal in midline . Commitment to glial identity and glial differentiation also occurs before scaffold formation. In all mutants examined, the early distribution of the glycoprotein Neuroglian is perturbed.