STRUCTURE AND GROWTH
The walls of the cells appear to contribute little or nothing to the formation of the substance of most of the ossifying cartilages,—those of the higher animals for example.
The cartilages of the branchial arches of the tadpole, like those of the branchial rays of fishes, consist of cells, which are, however, much larger than those of the fish, though smaller than the cells of the chorda dorsalis, with which they have, in every other respect, much similarity. The partition-walls of the cells are thicker than in the chorda dorsalis, but they may still be termed thin when compared with the cell-cavities. (See pl. III, fig. 1, which exhibits branchial cartilage from the young larva of Pelobates fuscus.) The cartilage intended to be used for investigation must be taken quite fresh from the living animal; for the structures become very indistinct if it be allowed to lie in water for any time after death, even though it be entire. After stripping off the mucous membrane, the cellular structure is readily recognized by the aid of the microscope. The cells vary much in size, and are more or less flattened against one another. The wall of each separate cell may be distinctly seen in the majority of instances, and its thickness might even be measured; that we cannot trace it so evidently in the smallest cells is probably referrible to the extreme thinness of their sides. ‘The walls of the cells are for the most part in contact, but intercellular substance may be seen in many situations,and especially where several cells are contiguous. The surface of the cartilage, which is represented on the left and lower margin of the figure, (pl. III, fig. 1,) is formed in the first place of intercellular substance, which, in as much as the cells originate in it, may be called Cytoblastema.
This cartilage may, therefore, be described as consisting of intercellular substance, or cytoblastema, in which great numbers of cells are seen. The cell-contents are generally clear as water ; but in the younger and smaller ones (for example, c,) the contained matter is less pellucid, and somewhat granulous. Each cell contains a spherical granulous nucleus, which lies upon the inner surface of the wall, and which again encloses a nucleolus. The size of the nucleus is not precisely alike in all the cells: it is somewhat larger in the large ones, but its size hears no proportion to the increased bulk of the cell; and again, the smaller cells are not much larger than the nucleus which they contain.
