Segmentation of the Vertebrate Head

Historical views of head segmentation are reviewed. The concensusis 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 primitivevertebrate is generated. This model denies the tunicate originof the vertebrates—rather it identifies amphioxus as mostlike the ancestral vertebrate. The vertebrate head is made upof a preoral segment plus four other segments. Because of sclerotomites,the head extends through five and a half segments. The nasalorgans and eyes are preoral structures while the ear is locatedbetween segments three and four. The occipital portion of thehead skeleton is formed from the posterior half of the fifthsegment and the anterior half of the sixth; it is vertebra-likein structure. This "segment" is much altered as a result ofthe multiplication of the visceral pouches and is often viewedas the fusion product of several segments. Thus the idea ofcorrespondence between somite and visceral segments posteriorto the second branchial arch is rejected. In some fishes, additionalvertebrae are added to the posterior part of the cranium andthis can be observed in development. The bony cranium of thevertebrate appears to partially reflect segmentation; its componentssuggest a vertebra-like developmental influence in operation.Study of the shark head has contributed much to our knowledgeof this area.