28 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 cartilagmous sub- 
stance. It first appears in the form of isolated, extremely 
minute granules, by which an indistinct appearance of arched 
striee is sometimes produced. At other points, these little gra- 
nules of earthy matter lie collected together into larger irregu- 
lar 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 ac- 
counted 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previously withdrawn and the acid again washed from it, ex- 
hibited 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 
