Summary.1. The structure of the compound eye ofDrosophila melanogasteris described in detail.2. The development of the normal eye is traced from the late larval to the adult stage. The chief features of the development are as follows:–(i.) The optic disc differentiates into three layers. This process is affected byGlued.(ii.) The middle layer of the three. penetrates the outer layer regularly. This process is affected bysplitandGlued.(iii.) The middle layer cells next divide to form the cells of the pseudocone. Thisjprocess is affected bysplitandGlued.(iv.) The cone cells further divide to form the retinulse. This process is affected byGlued.(v.) The retinulse are excluded from the cone‐cup formed by the outer layer of the disc.(vi.) The cone cells and retinulse develop and secrete the pseudocone and rhabdome.Boughcauses this process to be irregular, and the amount of secretion is influenced bysplitandGlued.3. The structure and development of the facet‐mutantsrough, split,andGluedare analysed in detail, and a table is given comparing their structure and development with the wild‐type eye.4. The nature and origin of fused, flat, and erupted facets is described. The fused facets ofroughare different in origin from those ofsplitandGlued.Two varieties of flat facets are found, one of which is present inrougharid the other inGlued,whilst both are found insplit.The “cupless” flat facets ofroughandsplitthough similar in nature are different in origin.5. Two rules governing the relationship between the retinulse and cone cells, and the retinulse and the shape of the facets, are stated. The wild type andsplitadhere to both rules, whilstroughadheres to one rule up to th? end of the second pupal day andGlveddoes not adhere to either.6. The structure of the periopticon in the wild and mutant types is described, and the origin of the periopticon in the optic stalk is confirmed by