With the growth of the embryo the cartilag advances, and its developement as a perf tissue is completed by the separation of th cells from each other by the interposition an intercellular tissue. This latter is tran parent and dense, but without traces of de nite structure, unless it be a minutely granul tissue. Temporary cartilage is thus shown t be composed of cells having parietes and co tents, and intercellular, (seefig.4.57) or, as so have called it, hyali substance, throu which the cells or c puscles are equall tributed. In car thus constituted developement of proceeds. With t commencement of t developement of bons' great changes occur id! the arrangement of the cartilage corpuscles as the immediate prece dents of ossification.
They are no longer equally distributed through the hyaline substance, but are found arranged in pa rallel columns of varia ble lengths, in the line of the length of the bone. The corpuscles forming into columns neces sarily leave intervening columns of intercel lular tissue. A notion has pretty generally prevailed that the corpuscles already formed marshal themselves into this order. I think, however, that on further investigation it will be found that each corpuscle has developed others, and that they have been developed in one direction only, and that towards the line where osseous formation has commenced; for we find that the first perceptible change In temporary cartilage on the approach of ossifi cation is that the corpuscles, instead of being solitary, are arranged in groups of variable numbers according as they:are near or far off the site of immediate ossitication ; that they have a linear arrangement, and where there are two or three only this is somewhat semilunar, with the straight edges near each other ; and that the greatest diameter is lateral. (See fi,g. 457, b.) Moreover, the columns are not continued unin terruptedly through the cartilage, but are broken off, and near their terminations new ones com-' mence, not, hovvever, in a line with the former, but opposite their intercolumnar spaces. (See fig. 458.) Supposing this view of the subject to be correct, as I believe it to be, the growth of a bone in its length mits of easy solution, ly, by the developement of cartilage at the epiphyses. The formation of lurnns having commenced, developement in the lines proceeds with rapidity, and the corpuscles composing them hold a different tion to each other at each part of the same column. Thus at the end farthest from the point of ossification the corpuscles are flattened and closely arranged, ing an appearance not like a pile of pence. But as
we trace the line down wards the bone, each corpuscle becomes more distinct, is separated from those on either side, becomes itself enlarged, and of nearly equal dimension in each direction. (See figs. 458 and 460.) The intercellular tissue becomes distinctly visible between each corpuscle. The space also between the columns, though always able, is increased when the corpuscles have undergone the above change. (See fig. 459.) and relative position observable, but a further remarkable developenient takes place in each corpuscle. The parietes of the cell, which at first formed but a small part of the whole, at this latter situation has so far increased in mension that it forms by far the greater part of the mass, while the central portion, which at first appears to constitute the whole corpuscle, notwithstanding its increase of size, is now to be regarded only as a nucleus, presenting the pearance of a cavity or granular cell, of a form approaching that of a sphere. (Seefig.460.) Cartilage, when it has been subject to the above changes, in the next transition becomes bone. In order to derstand this process it will be necessary to bear in mind the lowing points : firat, that the enlarged lage corpuscles are ranged in vertical lumns; that each lumn is surrounded by the intercellular tissue, and that each corpuscle is separated from those above and below by a layer of intercellular tissue; and, lastly, that each corpuscle has tinct parietes with tral nucleus, or cavity, containing granules or having granular tes. Osseous tissue is in all instances developed in the form of minute granules, so the earliest appearance of bone in cartilage is marked by the presence of these spherical granules in the intercolumnar and intercellular tissue, which is thereby increased in density and opacity. This constitutes the first stage in the process of the developement of bone, and may be observed by making a thin section of a bone at the point of junction of the bone and cartilage, where the shaft is connected with the epiphysis. (See fig. 461.) The granular appearance will be Not only are the described changes in form increased if a little acid be added to the sec tion ; caustic alkali will produce a similar effect. The intercellular tissue having become gmnular, the parietes of the corpuscles next undergo a similar change, and the central nucleus or cavity can no longer be identified. The accession of granules to the parietes of the corpuscles constitutes the second stage of the process of ossification.