Patterning of the fly retina starts as the morphogenetic furrow sweeps through the eye imaginal disc. Successive waves of activation of the Ras pathway recruit the photoreceptors that form the ommatidia. However, the ommatidia are not all identical and later patterning events add at least two new features to the eye: one is to define the type of inner photoreceptors that are involved in detection of colored or polarized light. Another is to create chirality to the ommatidium that is essential for the correct projection pattern of the photoreceptors to the neural cartridges in the optic lobe. This new application offers to test the mechanisms underlying these two events and in particular the role of wnt's.