STRUCTURE AND GROWTH
are not enclosed one within another, and are for the most part furnished with a nucleus, are readily recognized in the true cartilaginous substance of the unossified cartilages. I am not prepared to state whether this substance is formed by thicken- ing of the cell-walls, or by the intercellular substance. The earthy matter is first deposited in the true cartilaginous substance. It first appears in the form of isolated, extremely minute granules, by which an indistinct appearance of arched strize is sometimes produced. At other points, these little granules of earthy matter lie collected together into larger irregular heaps. I do not know whether these little collections are depositions of pure earthy matter which has not as yet united with the cartilage, and therefore merely provisional deposits which subsequently are distributed equally in the cartilaginous substance (which is not probable), or whether this earthy matter is already united with the cartilage, and that the regular aspect which the structure presents when ossified may be accounted for by the gradual union of the earthy matter with it after the same mode. I saw no such deposition of earthy matter in heaps in the incompletely ossified parietal bones of the same larva, but the whole cartilaginous substance contained it equably distributed without any perceptible granules. In both instances, however, when dilute hydrochloric acid is applied to the object under the microscope, the boundary denoting the solution of the earthy matter, and the consequent transparency of the cartilage, may be distinctly seen advancing in the form of a sharply-defined line from the edge of the preparation towards the interior, proving that, in the cartilages first mentioned, there was earthy matter equably united with the substance, in addition to the heaps and isolated granulous deposits. For this boundary line cannot be produced by the mere progressive imbibition of the acid with- out a solution of the earthy salts; at least neither an unossi- fied cartilage, nor one from which the earthy matter had been previously withdrawn and the acid again washed from it, exhibited the phenomenon of such a line advancing towards the interior. During the early period of ossification, when this line arrives at a cell-cavity, it becomes indented proportionally to the size of the cavity, because it does not come in contact with any earthy matter there; the cell-cavities, in the first instance, being free from earthy salts. The reverse, however, is
