Pattern formation involves differentiation and morphogenesis of cells dictated by their positions within the body. In many developing organs such as the eye, cells within the organ signal to each other, instructing other cells about what to become and what shape to form. One such example is the compound eye of Drosophila melanogaster. It is comprised of 800 unit eyes or ommatidia, which each contain eight photoreceptor neurons and fourteen accessory cells. Ommatidium assembly occurs progressively from posterior to anterior across an undeveloped eye. Assembly occurs by the sequential recruitment of undifferentiated cells into distinct developmental programs. Once assembled, ommatidia change form by the coordinated morphogenesis of cells, each cell adopting a unique shape typical for its position and identity. The end result is a perfectly sculptured three-dimensional structure wholly suited to it purpose of enabling the animal to sense the visual world in a manner comparable to our own.