1. The structure of the compound eye of Drosophila melanogaster is 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 by Glued.

(ii.) The middle layer of the three. penetrates the outer layer regularly. This process is affected by split and Glued.

(iii.) The middle layer cells next divide to form the cells of the pseudocone. Thisjprocess is affected by split and Glued.

(iv.) The cone cells further divide to form the retinulse. This process is affected by Glued.

(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. Bough causes this process to be irregular, and the amount of secretion is influenced by split and Glued.

3. The structure and development of the facet-mutants rough, split, and Glued are 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 of rough are different in origin from those of split and Glued. Two varieties of flat facets are found, one of which is present in rough arid the other in Glued, whilst both are found in split. The “cupless” flat facets of rough and split though 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 and split adhere to both rules, whilst rough adheres to one rule up to th? end of the second pupal day and Glved does 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 experiments.