Development and structure of the teeth in the Dog Fish, Squalus acanthias L. and Scyliorhynus caniculus (L.).

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Abstract
SUMMARY.: The development of the teeth of Squalus and Scyliorhynus has been examined in an attempt to clarify various aspects of their detailed structure and to provide a basis for comparison with other vertebrate teeth. The enamel layer of the tooth is completely formed before calcification begins as a characteristic tissue, composed of a matrix penetrated by long cytoplasmic processes from the pulp cells and by reticular fibres passing inwards from the basement membrane. Calcification of the tooth commences in this enamel substance; during the process the matrix and reticulars alter in appearance and the cytoplasmic processes are replaced by fibres. The basement membrane calcifies where fused to the tooth, and the sharp‐edged ridges of the crown appear to be derived from this membrane. The dentine appears as fibres laid down on the sides of the pulp, its appearance overlapping the beginning of enamel calcification; these intermingle with other fibres forming an irregular boundary fringe in the enamel and both extend upwards to enclose the apex of the pulp. The pulp cells whose processes penetrated the enamel substance become odontoblasts, and as the fibres enclose the pulp the truncated ends of their processes become enclosed in dentinal tubules. The superficial similarity between the teeth of selacians and of higher types does not extend to the detailed structure of the teeth, and the question of homologies and relationships requires further work for its solution.

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