developed. On the other hand, a formation of cells also takes place within the true cartilage-cells, but it is probable that they have a different signification from those within which they are generated. A deviation from the previous class seemed to occur with respect to the spot at which the young cells are formed, in relation to the entire tissue. In the former class, so far as we could perceive, they were formed at that part only where the tissue was in immediate contact with the organized substance. The formation of the new cells in cartilage, it is true, did not take place throughout the entire thickness of the tissue, but (so long at least as the cartilage itself is not furnished with vessels) only near the surface, and therefore, at the spot where it was in contact with the organized substance; still, however, it not only took place at that point of contact, but went on also between the cells most recently formed, as if cartilage had a greater capacity of imbibition, so that the cytoblastema penetrating from the blood-vessels into the parenchyma arrived at the deeper seated portions of the tissue more speedily; and, therefore, retained its fresh plastic force even in that situation; or, as if the cartilage itself possessed a higher vitality, and, therefore, the cytoblastema retained its productive power for a longer period, although penetrating quite as slowly as m the previous class.
Although the modifications in the form of the cells of this class vary but slightly from those of the preceding one, yet we see two striking changes in the cells and their cytoblastema, namely, the coalescence of the cell-walls and ossification. The thickening and transformation of the cell-walls were very distinct in the last class, for example, in feathers. Here a still more strongly-marked thickening of the cell-walls takes place in several cartilage-cells. The external contours of the walls, however, gradually disappear in such instances, and a coalescence takes place to such an extent as to leave merely the cell-cavities perceptible, lying in an homogeneous substance. The blending of the cell-walls takes place either between the walls of neighbouring cells, in imstances where they are in immediate contact, or, with the intercellular substance, when the cells are surrounded by it. Further investigations are re- quired in order to decide the question as to whether this
