The first detailed comparisons of the organization and development of craniate head structure occurred during the period 1870-1940. Most workers concluded that craniate heads were segmented, but there was no consensus regarding the nature or number of units forming head segmentation. Although a number of views are reflected in the literature, Goodrich's model has dominated subsequent discussions. Goodrich's model assumes that the head of the earliest gnathostomes consisted of 8 segments, and that each head segment, except for the premandibular segment, consisted of epimeric and hypomeric muscle, which was innervated by homologues of ventral and dorsal spinal nerves, respectively. Goodrich's model thus assumes that there is a simple one-to-one relationship between nerves and mesodermal derivatives. Review of current data on the roles of neural crest, neuromeric, placodal and somitomeric tissue in the development of vertebrate heads refutes Goodrich's contentions and suggests that at least three, possibly four, separate series of cranial nerves exist in vertebrates and that these nerve series are related to each of the iterative developmental tissues of the head in a different manner.