OF CARTILAGE.
also accord with the young cells of plants. Compare plate I, fig. 8, f f, with fig. 2. The form of the nucleus likewise corresponds with that of many vegetable cells. In these young cells of cartilage, it is presented to the observer as a small oval or perfectly spherical corpuscle, having, in many instances, a granulous and somewhat yellowish appearance, and containing one or two nucleoli. (Compare this with the description of the nucleus of vegetable cells in the Introduction.) The nucleus of the cartilage-cell appears to be hollow, a fact which has not been observed with regard to the cytoblast of vegetable cells,’ and the nucleoli lie close upon, or in the neighbourhood of the internal surface of its wall, whilst, according to Schleiden, they lie deep in the cytoblast of vegetable cells.
The cartilage-cells, when once formed, appear to be endued with the capacity to grow throughout the entire mass of the structure. The case is different with regard to the formation of new cells. This takes place in certain situations only, on the surface of the cartilage, for instance, or between the last formed cells. We have already seen that in the branchial rays of fishes, the least developed cells lay at the point, and lateral margins. The little rod, which the branchial ray represents, does not increase in size by the formation of new cells between the original ones throughout its entire length, but its extension in the longitudinal direction is produced by the development of new cells in the neighbourhood of the point, and it increases in breadth by the same process going on in the neighbourhood of the side walls. It is a familiar fact, that the cylindrical bones grow chiefly upon the surface and at the end of the shaft. The formation of new cartilage-cells usually takes place only in the neighbourhood of the surface which is in contact with the organized substance, (I refer throughout this passage to that period alone, at which the cartilage does not contain any vessels of its own) but it is not exclusively confined to that situation, it may also proceed in the intercellular substance between the last-formed cells.
At the period of ossification, the earthy matter is first de- posited in the cell-walls, or in the true cartilage-substance, the
' In a letter which I have received from Schleiden, he informs me that he has also found hollow nuclei in plants.
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