dependent upon a secretion of the so-called matrix. If by this it is meant that the substance of horn is secreted by the matrix and hardened in the air, the view is manifestly an erroneous one; what we call horny substance being either merely the cell-walls, when, for example, the cells are flat, and there are no cell-contents, or the cell-walls and cell-contents together, when the cells are polyhedral, as in hoofs. All these cells are independent structures, which grow organically. But if, by the above description, it is meant that the organized matrix only furnishes (or secretes) the cytoblastema, no important objection can be raised. The cells of the horny tissue require a nutritive fluid for their growth. This is supplied to them by the blood, as it is in all tissues. As, however, the blood-vessels themselves do not pass between the cells of the horny tissue, the nutritive fluid must be furnished by the nearest substance in which blood-vessels exist, and in this sense the nearest organized substance may be called, matrix. But whether this cytoblastema which exudes from the matrix have a specific character, and on that account horn-cells are formed in it—or whether their formation take place in it for the same reason that the muscle-cells, those of areolar tissue, and so on, originate in other parts of the body, that is to say, whether it is determined by the plan of the entire organism, —is a question which does not as yet admit of a decision. It is, however, a characteristic of all the cells of this class (with the exception of the crystalline lens, which I have not examined in reference to the point), that the new cells are not generated between those already formed, but only in the cytoblastema nearest to the organized substance, if not, indeed, always in immediate contact with it. The teeth were necessarily separated from this class, because, as we shall see hereafter they present quite a different relation of the cells. The new cells of cartilage, so long as it does not contain any vessels, are not only formed upon the surface of the tissue, but also between the most recently-formed cells.
The chorda dorsalis forms the transition from this class to the following one. The cell-walls remain separate in the highest stage of their development, and it is only in their rudimentary forms, in the osseous fishes for example, that they
