Somites are spheres of epithelial cells that form sequentially along the anterior-posterior axis of the embryo through mesenchymal to epithelial transition of the presomitic mesoderm.
Comment:
When the somite becomes segmented from the segmental plate, it is composed of an epithelial sac enclosing mesenchymal somitocoel cells. Thereafter the somite differentiates into two parts, the ventro-medial mesenchymal sclerotome and the dorso-lateral epithelial dermomyotome. This change in the epithelial somite depends on surrounding tissue [PMID:15906248]; [homology-note] "(...) cephalocordates and craniates belong to a group known as Somitichordata. Somitichordate synapomorphies include (1) somites (...) (reference 1); The idea that the last common ancestor of bilaterian animals (Urbilateria) was segmented has been raised recently on evidence coming from comparative molecular embryology (reference 2).[well established][VHOG]" xsd:string {date_retrieved="2012-09-17", external_class="VHOG:0000191", ontology="VHOG", source="ISBN:978-0030223693 Liem KF, Bemis WE, Walker WF, Grande L, Functional Anatomy of the Vertebrates: An Evolutionary Perspective (2001) p.40, DOI:10.1093/icb/43.1.137 Balavoine G, Adoutte A, The segmented urbilateria: a testable scenario. Integrative and Comparative Biology (2003)", source="http://bgee.unil.ch/"}