Is the vertebrate head segmented?—evolutionary and developmental considerations

Because of its basal position on the phylogenetic tree of vertebrates, the lamprey embryo would be expected to exhibit segmental head mesoderm. Recent observations, however, show that the lamprey does not have any somite-like segments in the head. Coelomic head cavities that are most conspicuous in elasmobranch embryos, do not appear to represent universal vertebrate traits. From the perspective of generative constraint, segmental structures in the vertebrate body can be classified into primary segments, which arise as segmental embryonic primordia such as somites and pharyngeal pouches, and secondary segments whose patterns are determined by the presence of primary segments. Secondary segments include neural crest derivatives and epibranchial placodes that are not initially segmented. The head mesoderm of vertebrates is secondarily regionalized into several domains that do not impose any secondary segmental patterns on other structures. Thus, the vertebrate head is characterized by a lack of segmental generative constraint in its mesoderm. Classical segmental theories are now refuted because they attempted to equate the vertebrate head with that of the amphioxus, whose rostral somites are considered primary segments, which are absent from vertebrates.