16 STRUCTURE AND GROWTH
that the intermediate substance of the cell-cavities is not a simple structure, but one composed of the walls peculiar to the contiguous cells: that is to say, each cell is surrounded with a thick ring, its peculiar wall, the external outline of which is more or less distinct. In the preparation from which the delineation is taken, it was in some parts quite as distinct as the internal. Between two cells these external outlines blend into one line, but separate again when the contact of the cell-walls ceases; there is thus often left between the cell-walls a three or four-cornered intermediate space (c), filled with a kind of intercellular substance. No other structure, no deposition of strata, or distinction between primary cell-membrane and secondary deposit can be observed in the thickened cell-walls. The cell-contents also remain clear after the thickening of the walls. At the base of the branchial ray, it is scarcely possible to distinguish between the different cells-walls, and the cartilage presents the appearance of a homogeneous substance, in which separate small cavities only are seen. (Pl. I, fig. 7.) Around some few only of the cell-cavities, a trace of the peculiar cell-walls may be seen in the form of a ring. ‘This ring is usually somewhat thin, so that the entire intermediate substance of the cell-cavities cannot be formed of the cell-walls; but the intercellular substance, which was so small in quantity in the centre of the branchial ray, here contributes essentially to the formation of the cartilaginous substance, and often completely prevents the immediate contact of the cell-walls. This intercellular substance appears, however, to be homogeneous with that of the cell-walls, and in most situations coalesces with them. The cell-cavities, which are here transparent and without granulous contents, are now the cartilage-corpuscles.
The process of formation of this cartilage is as follows. It consists originally of cells, which lie in very close contact, but every one of which has its special, very thin cell-membrane. This follows, firstly, from the complete accordance in appearance, of cartilage in its earliest stage, with vegetable cellular tissue; secondly, from the presence of the nucleus in the young cells of cartilage, a structure which, as will subsequently be seen, occurs in almost all the cells proved to exist in other tissues ; thirdly, from the fact, that a separation of the cell-walls is often distinctly perceptible in instances where they are
