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ENAMEL OF THE TEETH. however, much less striking in human teeth, so that the question as to which of the two views is correct must remain undecided.

What, then, is the process of formation of these enamel- prisms? According to Purkinje and Raschkow, the crown of the growing tooth is surrounded externally by a peculiar membrane, the enamel-membrane, the inner surface of which is composed of short hexagonal fibres, which stand perpendicularly upon the membrane, and are directed towards the enamel, so that each fibre of the enamel-membrane corresponds to an enamel-fibre. On examining a portion of this membrane, particularly that part which lies nearest to the root of the tooth, we readily recognize in it the characteristic nuclei, some of them being furnished with nucleoli. They lie in a minutely granulous substance. This granulous aspect, however, is seen to be produced, in many situations, by granulated cells which contain the nuclei. Each nucleus is surrounded by a circular areola of small granules, and seems to lie in a minutely granulated globule, which we know to be the rudimentary form of most elementary cells. Some of these cells are prolonged into very delicate fibres; they appear to be young cells of areolar tissue; most of them, however, are round. The fibres or prisms of the membrane, which have a direction from its inner surface towards the enamel-fibres, have assumed an hexagonal form, which Raschkow attributes to their close contact. They very closely resemble the columnar epithelium upon mucous membranes, only that they are prismatic in their entire length, that is, so far as they project out from the membrane to which they are attached. I am inclined therefore, to regard them as merely elongated cells. In the recent state they also contain a very distinct nucleus, which encloses its nucleolus. (See pl. III, fig. 4.) In the upper part of the enamel-membrane they lie quite close together; but in the portion nearest to the root of the tooth, they diminish in number and stand insulated, so that at this part the structure of the membrane beneath them may also be recognized, and I suppose the round cells before mentioned to be the earlier condition of these prismatic cells. What, then, is the relation which these prismatic cells of the enamel-membrane bear to the prisms of the enamel?