Abstract The clock-and-wavefront model (CW) hypothesizes that the formation of somites in vertebrate embryos results from the interplay of molecular oscillations with a wave traveling along the body axis. This model however does not explain how molecular information is interpreted by cells to modulate their rearrangement into somites. Here we performed Scanning Electron Microscopy (SEM) on the pre-somitic mesoderm (PSM) of chicken embryos at stages 11-12 to describe in detail the cell shape changes occurring along the axis of the PSM. This reveals a wave of epithelialization of the dorsal PSM that precedes somite segmentation. Signs of spatially periodic apical constriction appear in this layer starting at least 3-4 somite lengths caudal to the most recently formed somite. The sizes of these clusters correspond to the typical diameter of chicken somites. We propose that a mechanical instability process leads to the separation of cells into these structures and positions the future inter-somite boundaries. We present a model in which a wave of apical constriction leads to increasing tension and periodic failure of adhesion junctions within the dorsal epithelial layer of the PSM, thus positioning somite boundaries. This model can produce spatially periodic segments whose size depends on the speed of the contraction wave ( W ) and the rate of increase of apical contractility (Λ). The Λ/W ratio determines whether this mechanism produces spatially and temporally regular or irregular segments, and whether segment sizes increase with the wave speed (scaling) as in the CW model. We discuss the limitations of a purely mechanical model of somite segmentation and the role of biomechanics along with CW during somitogenesis.