Larvae (4 dpf) tested for OKR were fixed in 4% formaldehyde in PBS for 24 hr, dehydrated through graded ethanol steps from 70% to 100%, and embedded via a standard protocol in glycol methacrylate. The eyes were sectioned (2 μm), stained with boron-dipyrromethene (1:10,000 in PBS) for membranes and with DAPI (1:8,000) for nuclei, and imaged with a Zeiss Axio Imager Z1 fluorescence microscope.