Late stage 16 through early stage 17 embryos were stained with anti fasciclin II (mAb 1D4) and dissected to visualize FasII positive axons in the CNS (A, C, E) and VLM(Ventro-Lateral Muscle) innervation by the ISNb (B, D,F). (A) Wild-type CNS pattern showing three pairs of longitudinal bundles flanking the midline. (C, E) CNS patterns of Ptp69D mutants in embryos that are wild type (C) or heterozygous mutant (E) for an Abl mutation. (C) Ptp69D7/Df(3L)8ex34 embryo showing a severe CNS midline defect. In contrast to wild-type, axons cross the midline in most segments (white arrows). (E) Ptp69D7Abl3/ Df(3L)8ex34 + embryo showing highly rescued CNS phenotype. Occasional segments contain a thin bundle of axon staining across the midline (white arrow) (B) Wild-type ISNb pattern showing synaptic branches in the clefts between individual VLM fibers (white arrowheads). Numbers on the right are muscle identities. (D, F) ISNbs of Ptp69D mutants in embryos that are wild type (D) or heterozygous mutant (F) for an Abl mutation. (D) Bypass phenotype in an Ptp69D1/ Ptp69D7 embryo; most ISNb axons extend past the VLM field (out of focus) and fail to innervate it (black arrows). (F) In this Ptp69D1 Abl1/ Ptp69D7 + embryo, most ISNb axons enter and innervate VLM field, although they often fail to target distal muscle fibers (black arrows). (G) Schematic of a cross section through panels B,D and F. Top : In wild type, ISN is superficial to muscles. ISNb enters muscle field. Center : Ptp69D bypasses the muscle field ; ISNb associates with ISN. Bottom: Abl rescues ISNb ability to enter muscle field, though sometimes fails to reach muscle 12. Numbered rectangles represent muscles.