II.— Upon the development of the enamel in certain osseous fish

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Abstract
The development of the enamel in Placental Mammals has always proved a difficult subject for investigation, principally because the change from the unaltered formative cell or ameloblast into the perfected enamel prism is so abrupt that the intermediate stage occupies only an exceedingly narrow zone. Indeed, many observers have supposed that the ameloblast cell undergoes a direct conversion into an enamel, without practically any intermediate transformation, whilst others have supposed that it secreted and shed out from its free end the material of which an enamel prism is composed, namely, an almost crystalline mass of calcium salts with a mere minute trace of organic substance. In a paper recently published in the ‘Philosophical Transactions’ (8), I showed that in the Implacental Mammals, in which the ultimate enamel contains a tube system, the transition from the unaltered ameloblast to the finished enamel was more gradual, and that it was possible to follow the steps of the process more satisfactorily owing to the persistence, for a time, in the forming enamel of conspicuous organic structures of definite form and arrangement. But in my opinion the two processes are not essentially different, and that which is easily seen in the marsupial may be used to elucidate that which is only with difficulty traceable in the placental group. But in a subsequent investigation into the formation of enamel in the Plagiostome fishes (9) I found an essential, and, indeed, an almost radical, difference, for in them the tissue which has generally been termed enamel is deposited in a thick and complete organic matrix, which presents the striking peculiarity of being furnished by the mesoblastic dentine papilla, and the claim of the tissue to be called enamel rests partly upon the immense development of the ameloblasts over it during the period of its calcification. This matrix is laid down of the full thickness of the ultimate enamel before any calcification in it takes place, and calcification is exceedingly rapid.

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