These images from the Karlstrom lab show retinal axon projections in 48 hour zebrafish embryos. In wildtype embryos (top), retinal ganglion cell axons cross the midline to form the optic chiasm. In you-too mutant embryos (bottom), these axons fail to cross the midline and no chiasm forms. An ectopic midline lens also forms in you-too mutants in the chiasm region. Images show ventral views of embryos labeled using the Zn-5 monoclonal antibody.