Somitogenesis and the segmentation clock. (a) The pattern of expression of one of the oscillatory genes – deltaC – during somitogenesis in the zebrafish. Two specimens are shown, fixed and stained by in situ hybridization (ISH) at different phases of their somitogenesis cycle. (b) Diagram showing how the observed pattern of gene expression reflects the cyclic behavior of the individual cells. Each cell contains a gene-expression oscillator – a clock – which slows down as the cell moves from the posterior to the anterior part of the PSM, giving rise to a pattern of stripes of cells in different phases of their oscillation. The oscillation is halted as cells emerge from the PSM, leaving them arrested in different states (blue versus white shading), thereby demarcating the somite boundaries (black lines). The extent of the PSM is defined by an Fgf + Wnt signal gradient, with its origin at the tail end of the embryo.