© 1977 by The Society for Integrative and Comparative Biology
Segmentation of the Vertebrate Head
Biological Sciences, Northern Illinois University DeKalb, Illinois, 60115
Historical views of head segmentation are reviewed. The concensus is that the head is segmented essentially in terms of myomeres, and that other organs have responded in varying degrees to this. From the various lines of reasoning a model of the primitive vertebrate is generated. This model denies the tunicate origin of the vertebrates—rather it identifies amphioxus as most like the ancestral vertebrate. The vertebrate head is made up of a preoral segment plus four other segments. Because of sclerotomites, the head extends through five and a half segments. The nasal organs and eyes are preoral structures while the ear is located between segments three and four. The occipital portion of the head skeleton is formed from the posterior half of the fifth segment and the anterior half of the sixth; it is vertebra-like in structure. This "segment" is much altered as a result of the multiplication of the visceral pouches and is often viewed as the fusion product of several segments. Thus the idea of correspondence between somite and visceral segments posterior to the second branchial arch is rejected. In some fishes, additional vertebrae are added to the posterior part of the cranium and this can be observed in development. The bony cranium of the vertebrate appears to partially reflect segmentation; its components suggest a vertebra-like developmental influence in operation. Study of the shark head has contributed much to our knowledge of this area.