28 STRUCTURE AND GROWTH 



arc 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- 

 in"- of the cell-walls, or by the intercellular substance. The 

 earthy matter is first deposited in the true cartilaginous sub- 

 Btance. It first appears in the form of isolated, extremely 

 minute granules, by which an indistinct appearance of arched 

 stride 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 previously withdrawn and the acid again washed from it, ex- 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 cavitv, 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 



