Flies of the mutation eyeless (Drosophila melanogaster) have small eyes on both sides or are sometimes totally eyeless on either or both sides. The eye and the optic tract in normal and eyeless stocks were studied in order to determinate what part of the optic tract is lacking in the eyeless flies. In the normal fly, three ganglia, outer, median and inner, connect the eye with the brain. In flies with small eyes all three ganglia are present and the ommatidia are normal although greatly reduced in number. In totally eyeless flies the outer ganglion is missing and the median and inner are contracted into a more or less shapeless mass which nevertheless discloses its double nature. Since the retina has been identified by modern workers as the rhabdome layer of the eye, these observations lend support to the idea that the optic nerve is a mass of fibers connecting the outer and median ganglia.