Vertebrates are characterized by their segmented structure, first visible in the somites that form along the embryonic body axis. Somites are the predecessors of the vertebrae, ribs, muscles and the skin and impose a segmented organization on the peripheral nervous system. The variability in somite number between species has presumably evolved by genetic rearrangements, leading to changes in the ratio of the segmentation clock rate to the developmental growth rate. However, it is known that physical cues, like temperature, salinity or light conditions, can modify the vertebrate body plan and change the vertebral number. Here we show that mechanical stretching, another physical cue, can induce the formation of additional somites in the developing chicken embryo.Stretching of live chick embryos and the resulting deformation of somites induces a slow cellular reorganization of somites to form two or more well-shaped and stable daughter somites.Mesenchymal cells from the somite core thereby undergo mesenchymal-to-epithelial transitions (MET), thus meeting the geometrical demand for more border cells. Our simulations, using a Cellular Potts Model of somite remodeling, suggest that this MET occurs through lateral induction by existing epithelial cells. Our results strengthen the idea that somitic mesoderm self-organizes, and show that it is phenotypically plastic under variations in the mechanical environment. These somite qualities could play out as a selective advantage by preventing the formation of transitional vertebrae and give rise to another possibility how the vertebrate body axis might have evolved towards different vertebral numbers, next to the previously proposed genetic rearrangements. DependentMechanical Cues Drive the Segmentation Clock.Cell.