For many developmental processes, the list of the required elements (genes/molecules) is known but the resulting dynamics is often poorly understood. Thus, in most developmental patterning processes, the relative importance of external positional cues and self-organization remains an open question. Sensory bristles on the Drosophila epidermis are a classic model for cell fate patterning. Lateral inhibition via Notch selects sensory organ precursors (SOPs) within “proneural clusters”, groups of cells expressing genes of the achaete-scute complex. In the dorsal thorax, a number of bristles develop at invariant locations, defined by a genetic prepattern, and a series of proneural stripes gives rise to regular rows of finer bristles; whether these stripes also reflect an underlying prepattern is not known. We are addressing this issue with Francis Corson (Physics Dept., ENS, Paris). Using a combination of modeling and live imaging, we are testing whether Notch signaling not only selects the SOPs within each proneural stripe but also organizes the stripes themselves.