any anomaly in the formation of mesodermal clusters that are arranged segmentally along the anterior posterior axis of an embryo; somites are transient structures - balls or blocks of paraxial mesoderm cells with an epithelial outer layer and a mesenchymal cell core - that emerge sequentially through a mesenchymal-to-epithelial transition (MET) from the morphologically unpatterned presomitic mesoderm; they give rise to the metameric anatomy of the vertebral column itself, and the associated skin, muscle and tendons