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100 Purkinje and Raschkow regarded each fibre of the enamel-membrane as an excretory organ, a little gland which secreted the enamel-fibre corresponding to it. With our altered views of the growth of unorganized tissues, however, this explanation, previously so plausible, loses much of its probability. Various other explanations might be offered in place of it, but I have not made sufficient observations to enable me to decide upon the correct one. Firstly, one might suppose the organic basis of the enamel-prisms to be cells, which are formed, and continue to grow independently upon the dental substance, having no other connexion with the prisms of the enamel-membrane than that the latter furnishes their cytoblastema. This explanation, however, would compel us to regard the remarkable accordance which exists between the prisms of the enamel-membrane and those of the enamel as an accidental circumstance. But we should be obliged to adopt such a view, if it could be proved that another peculiar substance intervened between the enamel-membrane and the enamel, and I have several times observed such an one on the molar teeth of swine. It is very soft and full of vesicles, having the appearance of a slag. I think Purkinje mentions it also, but I cannot find the precise passage at this moment. It lay between the enamel-membrane and the tooth, but I am not certain whether it was also present at those points where the formation of the enamel had already commenced, and whether, therefore, it actually interrupted the continuity of the enamel-membrane with the formed enamel. We might suppose, as a second explanation, that the enamel-prisms are uninterrupted continuations of the prisms of the enamel-membrane, which become filled towards one end with calcareous salts. This is a very improbable explanation, and the connexion between the two structures is of too loose a nature to warrant its adoption. A third, and as I am at present disposed to think the most probable, explanation is, that the prismatic cells of the enamel-membrane separate from it, and coalesce with the enamel already formed, while at the same time their cavities either become filled with calcareous salts, or they become ossified throughout their entire thickness, their cavity being previously filled with an organic substance. This explanation makes the formation of the enamel accord with