Mosaics in which almost all of the eye is homozygous for sbb² while other tissues are heterozygous (clones generated using the "eyFLP" system) show striking defects in photoreceptor axon projections; photoreceptor axons project normally through the optic stalk to the brain, but fail to segregate into their distinct target layers. Only a vestigial lamina plexus is formed, while thick bundles of photoreceptor axons project through to the medulla. In mosaic larvae, most R2-R5 axons fail to recognise their lamina targets and continue through to the medulla. In mosaic adults many R1-R6 axons are mistargeted to the medulla and within the medulla, the mistargeted axons all appear to make R7-like projections. R7 photoreceptor cell axons are correctly targeted to the medulla. Both mistargeted R1-R6 axons and correctly targeted R7 axons appear to project in a topographically correct fashion along the anteroposterior axis. The dorsalmost and ventralmost photoreceptors in the eye imaginal disc project their axons retinotopically to the dorsal and ventral extremes of the optic lobe respectively in mosaic animals, as occurs in wild type. Mosaics in which almost all of the eye is homozygous for sbb² while other tissues are heterozygous show the normal complement and arrangement of photoreceptors in 96% of ommatidia. In 40/63 cases the R7 cell appears to be transformed into an R1-R6 cell, as judged by its size and the position of its rhabdomere. Lamina neurons differentiate normally. Lamina glia migrate into the lamina from peripheral precursor zones as they do in wild-type animals. R1-R6 photoreceptor axons overshoot their targets in sbb²/sbb⁰⁴⁴⁴⁰ or sbb²/sbb^k00702 transheterozygous larvae.