The somites themselves are specified according to their location, as the segmental paraxial mesoderm from which they form it itself determined by position along the anterior-posterior axis before somitogenesis.
When the notochord is ablated during somitogenesis in the chick embryo, the proper number of somites forms, but the segmentation clock is delayed for the posterior two-thirds of the somites.
Two similar and redundant genes in mouse are required for embryonic cardiovascular development, and are also implicated in neurogenesis and somitogenesis.