Somites are formed in a well-defined order along the antero-posterior axis

https://embryology.med.unsw.edu.au/embryology/index.php/Somitogenesis_Molecular_Movie
Follow links on the above site to view a movie showing somitogenesis in the mouse.

Identity of somites along the antero-posterior axis is specified by Hox gene expression

www.pbs.org/wgbh/evolution
The Public Broadcasting Stations production “Evolution” contains resources for studying the molecular mechanisms underlying the formation of the diverse body plans found in the animal kingdom, which are influenced by the large family of homeobox genes (Box 5E in Wolpert text). Click on http://www.pbs.org/wgbh/evolution/library/03/4/l_034_04.html for a video clip that discusses the role of homeobox genes in regulating body plans.