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to the general physiological law, and consider the doctrine which teaches that enamel is formed upon the dentine and grows by deposits from above as erroneous as the announcement that a membrane or line exists in the middle of the enamel-rods." Well, shades of Leeuwenhoek protect us! Such a complete misinterpretation

of anatomical structure and physiological action can only find a parallel in the ancient doctrine of the "primitive dental groove."

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After stating that there is a space between the dentine and enamel, in which space no structure of any sort appears, and which, in the developing tooth is a closed cavity surrounded on all sides by solid walls of dentine and enamel, he proceeds to tell us that enamel is formed in that cavity. But how formed, and from what? He says it is formed at the surface of the dentine and pushed upward from below. There is indisputable evidence in the structure of enamel that it is formed from prismatic cells. But there are no cells within this space or closed cavity. It can hardly be possible that the writer did not know that there can be no formation of tissue without proliferation of cells or corpuscles. In the formation of other epi

dermoid structures there is a constant cell proliferation from beneath. But no eye ever beheld the enamel-forming cells, in a section in which the tissues were in their natural relation, below the enamel and between the tissue and the dentine. They are always above it. After quite a lengthy quotation from Dean's translation of Legros and Magitot, the author of this reprint discloses the fact in a single sentence that he has never seen a well-mounted section showing a developed tooth. After referring to the analogy between the lower layer of cells of the enamel-organ and the prismatic cells of the intestines, he says: "It is one more point in favor of my position, that the enamel is not built by deposits from above, but pushed upward by a multiplication of cells from below." It is much to be regretted that he ever made this statement, for it is certainly destined to bring upon him the most intense humiliation.

You have now upon the screen (Plate II., Fig. 11) a photograph of a section of the jaw of a kitten at birth. The section, while representing an advance beyond those in which the "primitive dental groove" was so well portrayed, yet contains imperfections which make it of value in explaining some of the errors into which previous writers upon this subject have fallen. At the lower part of the view and rising upon either side is seen the inferior maxillary bone in transverse section. In the inferior parts of the bone is seen the medullary canal containing the nerves and blood-vessels. You will observe the very large proportionate size of the inferior maxillary nerves as shown in cross-section. Rising immediately above this, and occupying the larger part of the interior of the jaw, is shown the developing temporary tooth, to the side of which is seen the permanent tooth-germ. The cap of dentine is seen surmounting the pulp entire. But the thin line of enamel has been torn from its connection with the dentine and broken into fragments. Fortunately, however, while the space which it bears explains the space in Figs. 14 and 15, yet the structure of fully-formed enamel with the characteristic wavy lines, which nearly always run diagonally across each other at the tips of the cusps, is plainly seen in the lower side of these fragments, while the enamel-forming cells are clearly shown above. But a little further down on the side of the developing tooth the tissues remain in their natural relations, giving a perfect refutation of the hypothesis taught by Prof. Garretson and others. First, we see the tissue of the pulp surrounded by the layer of the odontoblasts; next, the band of formed dentine upon which a thin line of enamel has been deposited. Above this narrow line of

enamel are seen the ameloblasts, and outside of these cells the flat layer forming "Nasmyth's membrane," and further beyond the enamel-pulp. That there may be no shadow of doubt about the position of these enamel-forming cells outside of the formed enamel, I have slightly torn the tissues at this point in several specimens (Plate I., Fig. 5, and Plate II., Fig. 13), so as to show the isolated ameloblasts, the characteristic shapes of which, once seen, will never be forgotten. The next view (Plate I., Fig. 4) is a photograph under high power, taken low down on the left side of Fig. 11 (Plate II.,), where the enamel has just commenced to be formed.

It would not seem as though any further demonstration in support of my views could be asked for. You observe at the lower part of the view the dentine pulp surmounted by the odontoblast layer, above which a line of dentine has been formed, and upon the dentine a very thin layer of enamel has been deposited. Then come the ameloblasts, or enamel-forming cells, in intimate relation with. the enamel-organ. We will project several more views upon the screen in further demonstration of this point. These photo-micrographs are from sections of both temporary and permanent teeth (Plate I., Figs. 5-7). Plate I., Fig. 8, is a more highly-magnified view of the lower part of Fig. 11, Plate II. It shows the medullary canal with nerves and blood-vessels. Observe the size of the inferior maxillary nerve and its tripartite division.

I think the most skeptical of my hearers must now be thoroughly convinced that the development of enamel is from the line of its union with the dentine outwards. Concerning the exact manner in which the formation of the enamel takes place, I cannot speak authoritatively. In every phenomenon of physiological activity there will probably always remain a deeper depth which the human mind cannot penetrate.

The fact, as I have before stated, that there can never be seen but a single layer of enamel-forming cells surmounting the enamel, leads naturally to the inference that amelification occurs by a secretion and deposit from these cells, and that, as the ameloblasts recede before the advancing formation of enamel, the fibre of the enamel-cell, which can plainly be seen projecting from the lower part next to the line of formed enamel, remains as the persistent fibre of living matter which Dr. Bödecker has so ably demonstrated as forming an essential feature in the histology of enamel. It is also possible that the formation of the enamel may be by an intercellular deposit and gradual elongation of the cell upward.

It is not true, as asserted by certain writers, that this method of the formation of enamel finds no parallel in epidermoid structure. The process of the development of the placoid scales of the Elasmobranchii is almost identical with that by which teeth are formed. I quote from Hertwig and Balfour, than whom there are no better authorities in the world on this subject, who say, in speaking of these placoid scales, "They consist, when fully formed, of a plate bearing a spinous projection. They are constituted of an outer enamel layer on the projecting part, which is developed as a cuticular deposit of the epidermis (or epiblast), and an underlying basis of dentine (the lower part of which may be osseous)," corresponding, you see, with the cementum of the tooth, "with a vascular pulp in its axis."

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The view on the screen (Fig. 16), of a vertical section through the skin of an embryonic shark, showing the development of one of these scales, might easily be mistaken for a developing tooth.

This brings us to a specific consideration of that "general physiological law" under which the development of the teeth and other epidermoid structures takes place. The author of the paper under review studiously, or possibly innocently and "without malice aforethought," avoided all mention of a very important group of the derivatives of the epidermis. We shall see that there may be endless departures from the modifications of that which is somewhat ambiguously spoken of as a "law" in the evolution of tissues. Now, those epidermoid structures of which mention has been made originate as local growths of the outward skin in an outward direction. The slight infolding of the columnar layer of the epidermis always maintains its relations with the adjacent cells of this layer around it.

The glandular group of the epidermic derivatives, of which no mention is made, originate as local growths of the outer skin in inward direction. They are characterized in the formative stage by a dilatation of the end of the ingrowing tube or cord of the epithelium from which the glandular part of the organ is developed. To this group belong the sweat and sebaceous glands, the tear glands, and the mammary glands. The formation of the enamel-organ of the teeth, while not identical because of its slightly different functional purpose, very closely resembles the development of the skin glands. It was my observation of this fact which first led me to regard it as essentially glandular in its nature. Its process of growth in the early formative stages is the same. While the appendages which are outward growths begin as a shallow pouch or pocket in the outer skin, the development of the enamel-organ and the glandular organs begins as an epithelial cord which descends by a more or less tortuous course deeper into the dermal tissues, enlarging at the lower end. In the case of the enamel-organ, this epithelial cord is severed from its connection with the external skin layer shortly after the commencement of the formation of the dentine pulp. The cord is completely broken up and disappears, and the enamel-organ exists as an independent body having no connection with the external skin layers. This one fact completely demolishes the argument of my critics of similarity or "general physiological law," for the hair pouch always maintains its connection through the outer and inner root sheaths with the external skin layer. Another proof of the glandular nature of the enamel-organ is, that if it be removed in the early stages of the formation of the teeth, as can easily be done in the jaws of a calf, it will be found to be highly impregnated with lime-salts, which are being elaborated for the building up of the enamel. After the completion of the enamel at the tips of the cusps of the teeth, the enamel-organ at that point is thus exhausted, but still remains lower down on the sides of the tooth, where the deposit of enamel is not yet completed, until its perfect work is accomplished, after which it entirely disappears. The development of a tooth, then, is a modification of the methods by which both groups of the epidermal derivatives are formed.

The evolution of the enamel-organ is a modification of the process by which the skin glands are developed. Unlike the formations of either group, it is developed in a closed sac which has no connection with the external skin layer. Later on in the process of development, and after the completion of the crown, the pericemental

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