<P> Cephalization in vertebrates, the group that includes mammals, birds, and fishes, has been studied extensively . The heads of vertebrates are complex structures with many features not found in close relatives such as the cephalochordates . The cephalochordate Branchiostoma (formerly called Amphioxus), which is the closest relative of vertebrates, is a burrowing marine creature which lacks most of the head structures that are so distinct in vertebrates, such as distinct sense organs; a large, multilobed brain; teeth; and a tongue . </P> <P> There was a persistent debate during the twentieth century as to whether the vertebrate head is "old" or "new". Scientists who champion the idea of an "old" head suggest that the vertebrate head resulted from the evolution of important modifications to a previously existing head . The idea of a "new" vertebrate head was proposed originally by American vertebrate morphologists Carl Gans and Glenn Northcutt in 1983 . They suggested that the vertebrate head is a new structure, which has no corresponding structure in close relatives such as Branchiostoma . </P> <P> Evidence to support a "new" vertebrate head comes from the observation that most important features of the head are derived from neural crest cells, embryonic cells found only in vertebrates . The neural crest cells are of ectodermal origin--rather than mesodermal or endodermal--and arise during the process of neurulation, the time at which the dorsal hollow nerve cord forms . </P> <P> Neural crest cells are exceptional in that they are highly mobile, migrating in streams throughout the head region and the rest of the body, and because they give rise to an unusual diversity of features . The neural crest cells are responsible for forming the bones of the face and jaws, the structures of the tongue and larynx, the teeth, and portions of the eye . Experiments in which the neural crest was removed from developing animals confirmed that these critical head structures failed to develop without it . </P>

An animal is said to show cephalization when it