blending be so complete that it cannot in any way be dissolved, the simple fact being, that the cell-walls are no longer dis- cernible with the microscope. I shall not bring forward the splitting of the dental fibres as examples here, nor indeed make any reference to the teeth in this retrospect, their explanation being as yet too problematical. It has, however, been already mentioned as a doubtful point, whether a coalescence of the walls actually takes place in all cartilage-cells, for instance, in those of the higher animals.
Ossification appears to occur especially, perhaps exclusively, in those cartilages which have a greater quantity of intercellular substance. It consists probably in a chemical union between the calcareous salts and the firm portion of the cartilage substance. In the first commencement of the process the cartilage frequently acquires a granulous appearance, which subsequently disappears, the entire substance meanwhile becoming gradually dark. At the same time the cartilage-cells undergo a transformation into the osseous corpuscles, a process which must probably be explained as analogous to the formation of the stellated pigment-cells. There is reason to suppose that the osseous corpuscles and the canaliculi which issue from them, also become filled with earthy salts by the calcifying process.
The class of cells now under consideration has yet another point of especial interest for us, since it is the first in which organized structures, that is, structures provided with vessels, occur. ‘The accordance between the elementary cells of unorganized animal tissues and vegetable cells might be conceded, without granting a connexion between the organized tissues (which are especially characteristic of animals) and the structure of vegetables. A distinction had always been drawn between the growth of the organized and that of the unorganized structures; and much had already been said in a general way about a vegetative growth of the non-vascular structures, the crystalline lens for instance, though the analogy which existed between their elementary particles was not proved. Cartilage, then, is the first structure which teaches us that a tissue, which, at a later period at least, contains vessels, is composed of cells, perfectly according, in their development, with those of plants; and, therefore, that a similar formative
