Fig. 2. Development of PCP in the wing and eye. (A) Wing cells display only apical-basal polarity until the pupal stage. Planar polarity is first evident by the production of a single, actin-rich structure at the distal edge of a cell. This develops into a hair. (B) Ommatidial preclusters emerge from a moving wave of differentiation, marked by an indentation called the morphogenetic furrow. As ommatidial preclusters begin to differentiate, they begin to rotate. Ommatidia in one half of the eye will rotate clockwise, and the ventral ommatidia will rotate counterclockwise. This rotation occurs in two steps of 45°, and results in dorsal ommatidia having opposite orientations from ventral ommatidia. A symmetry-breaking step occurs after rotation, resulting in ommatidia with different chiralities in the dorsal and ventral fields.