Ellipse alleles are mutations of the EGF-receptor homologue that reduce the number of ommatidia in the eye imaginal disc. Cobalt sulfide staining, expression of hairy and scabrous proteins, and mosaic analysis indicated that Elp mutations affect ommatidial precluster formation in the morphogenetic furrow. BrdU incorporation studies suggest that cells diverted from precluster formation instead enter S-phase after the morphogenetic furrow. Genetic studies suggest that the DER has multiple functions during eye development and that several recessive hypomorphic alleles affect another aspect of DER function that is required after precluster formation. Elp mutations show genetic interactions with the neurogenic mutations Notch and Delta. The small number of ommatidia that differentiate in Elp/Elp are separated more than in wildtype and have been studied to investigate what aspects of ommatidium development are intrinsic to the ommatidium itself. It appears that each developing ommatidium cues the determination of photoreceptors, cone cells, and primary pigment cells, but that the secondary and tertiary pigment cells, and the mechanosensory bristles, can form independently. The normal rotation of ommatidia in the dorsal-ventral axis does not require the presence of the ommatidial array. A short-range signal from a nearby ommatidium is important for mitosis. Cells not close to an ommatidium do not go through mitosis and many die.