Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/49

OF CARTILAGE. cells is subsequently absorbed. After the cartilages of the branchial rays of fishes have been exposed to the action of water, it is only in the young cells that the nuclei are visible ; they are much more rarely seen in those cells of which the walls are already very much thickened. In many cells of the branchial cartilages of the tadpole, a small nucleus with a ragged outline may be observed, which is probably the cytoblast of the cell in the act of undergoing absorption. These cytoblasts (nuclei) of the true cartilage-cells always lie in the cell-cavity, even when its wall is thickened, and it 1s impossible to distinguish whether they lie free or are still connected with the cell-wall. A twofold explanation is here possible: either the cytoblast separates from the wall after the formation of the cell-membrane is perfected, and falls free into the cavity (as occurs in plants), and at such period a secondary deposition of substance upon the cell-wall first commences; or the thickening of the wall is due to an actual increase of the original cell-membrane, and in that manner the nucleus is pushed inwards, and may remain in connexion with the wall. If a secondary deposition of substance took place before the nucleus was disengaged from the cell-membrane, that body must be enclosed in the wall, and would not lie in the cell-cavity. As both these explanations are possible, it will be seen that no conclusion can be drawn from the position of the nucleus, as to whether the thickening of the cell-wall be a secondary deposition, or an actual growth of the cell-membrane. Sometimes a cartilage-cell presents more than one nucleus; when in such a case the original nucleus of the cell is absorbed, all those observed are probably the germs of new cells, which have not as yet commenced their development. The same fact is frequently observed in plants. The nuclei in the branchial cartilages of the tadpole have for the most part the same size; some, however, which are probably not as yet perfectly formed, are smaller than others. It also often occurs that a nucleus is seen expanded to three or four times the usual size; such instances might be mistaken for young cells without nuclei, but they may be readily recognized by their general aspect. They are more transparent and delicate, and exhibit one or two nucleoli, which are easily detected; when two are present they are widely separated from one another. According to