Many experiments suggest that the somitogenesis clock is not cell-autonomous (see below), but some experiments have been interpreted as ruling out the existence of coupling (see section 3.2). The Lunatic fringe secretion model was formulated to resolve this contradiction, within the framework of current experimental data. The model addresses the way segmentation-clock oscillations are generated and synchronised between cells, in the primitive streak as well as in the PSM. It does not address the relationship between the oscillations, physical somite segmentation, and polarity-establishment, but it is compatible with all major current models, which involve either the existence of a critical threshold maturity (original clock and wavefront model, Cooke and Zeeman, 1976), the spread of a signal (cell-cycle model, Primmett et al., 1989), or an FGF8 wavefront (Dubrulle et al., 2001).