STRUCTURE AND GROWTH
formation of the cell, but in a much lower proportion. It is subsequently absorbed ; frequently, however, not before ossification. This is precisely what occurs in vegetables. The walls of the cartilage-cells become thickened (compare figs. 6 and 7 with fig. 5), which is also the case with many vegetable-cells. No distinction, however, between primary cell-membrane and secondary deposit can be observed in cartilage-cells, and such a deposition in strata as is often distinctly seen in thickened cells of plants cannot be made out here with sufficient certainty. The cell-nucleus in the meantime, when not absorbed, remains lying upon the inside of the thickened wall. An instance of actual thickening of the cell-membrane without a stratified deposit, does not, however, appear to be wanting in plants, e.g. the pollen-tube of Phormium tenax. (See the Introduction.) But it seems, that a thickening of the walls of the cartilage-cells does not take place universally, it does not for instance in the ossifying cartilages; the true cartilage substance may also be formed entirely, or at least chiefly of the intercellular substance. The condensed cell-walls subsequently coalesce with one another, or with the intercellular substance, so that at last only the cell-cavities remain in an homogeneous substance. Whether the walls of those cartilage-cells which do not undergo any thickening become blended with the intercellular substance or not, remains uncertain. An analogous instance of coalescence of the cell-walls is afforded by vegetables, for Schleiden has ob- served such a blending in the layer of bark which less immediately beneath the cuticle of the Cacti.
The cartilage-cells often contain either simple nuclei (i. e. without cells around them), or young cells with such nuclei. These young cells are formed free within the parent-cell, without vascular connexion. Their nucleus is first formed, and afterwards the cell around it, just as in the true cartilage-cell. This is one of the most important instances of accordance between animal and vegetable cells, for the latter, according to Schleiden, are developed in like manner from the nucleus, and likewise within a parent-cell. (See the Introduction.) We may therefore confidently compare the nucleus of these young cells, as also that of the true cartilage-cell, to the cytoblast of vegetable cells. Their shape and the eccentric position of their nucleus, placed as it is upon the internal surface of the cell-wall,
