Peter Thorogood and James Hanken Pattern Formation

-hition of the vertebrate buupkzn is a zoological problem of classic pedigree; it is an issue in which extreme, and often contradictory, views have been maintained even up to the present day. One of the most striking f~ti.lres of the body plan is its metamerism the repeating ‘segmental’ nature of much of the anatomy, including components of the skeleton, musculature, vascukature and nervous system. For many years it was believed that, as in the protochordate Ampbtius, vertebrates are fundamentally segmented from anterior to posterior extremities. However, it was also assumed that segmentation at the anterior end had become obscured by the evolution of increasing head specialization (cephalization) and, within individual ontogenies, by the increasing anatomical complexity that accompanies development and maturation. This traditional view, encapsulated in the writings of Goodrich [ 11, is upheld by modemday anatomists such as Bjerring [2] who claim not only that virtually the entire head is segmented but that its metamerism is identical and continuous with that of xhe trunk.