A theory of the relations of lateral lines to dermal bones.

Abstract

Summary. The current view that the pattern of the dermal bones of early fishes was determined primarily by the neuromasts of the lateral‐liue system is considered to be unsatisfactory for three reasons. It results in the establishment of homologies which are suspect on general grounds, it fails to account for the established relations between dermal bones and lateral lines of early amphibia, and it is not in accordance with experimental evidence. It is maintained that developmental relations between the lateral‐line system and dermal bones of at least the early fishes and early amphibia must be held to exist because in the majority of cases the lateral lines pass directly from one centre of ossification to the next, because lateral lines are in the majority of cases associated with bones which are apparently homologous, because bones associated with branches of the lateral‐line system both develop and evolve ahead of those not so associated, and because in the case of certain modern fishes the earliest appearance of bone cells is known to be in the immediate neighbourhood of neuromasts. It is contended that if the assumption is made that precursors of dermal bones affect the courses of developing lateral‐line primordia, the developmental association between lateral lines and dermal bones can be accounted for without introducing the difficulties engendered by the current hypotheses. Fields are hypothesized, with their centres at the points of future centres of ossification, which have the property of attracting growing lateral‐line primordia towards them, and changes in either the spatial relations or the relative intensities of these fields are believed to account for changes in the courses of lateral lines. It is held that this conception does not invalidate the belief that the presence of neuromasts may be responsible for the early appearance of osteoblasts in their neighbourhood. The courses of certain branches of the lateral line system close to the sutures between dermal bones are accounted for as a result of lateral lines developing under the direction of gradient‐fields through areas in which the bone‐precursor fields are relatively weak. It is suggested that the lateral‐line branches of certain labyrinthodonts which pass neither to centres of ossification nor along sutures indicate the presence of bone‐precursor fields the areas of which were later swamped by neighbouring ossifications. They probably indicate the temporary survival of the precursors of the intertemporal bones.