STRUCTURE AND GROWTH
Schleiden, a similar enlargement of the nucleus also occurs in lants, thus affording a remarkable accordance in what seems a very unimportant circumstance. It appears to be a kind of abortion ; for I have never yet seen a cell formed around such a nucleus.
The cranial cartilages of the tadpole (Plate I, fig. 9) are distinguished from the branchial by the smaller size of the cell-cavities, and the increased strength of the firm intermediate substance. The walls of the separate cells cannot now be traced, they appear to have coalesced with the intercellular substance, which is present in greater quantity. The cells lie in groups of two or four together, and it is very probable, that in this cartilage, each group is formed of cells, which have been developed in a parent cell; for some may be seen, for example at c, which do not as yet quite fill the original cell. Such an instance, however, is rarely so very distinct as not to admit of a doubt. There is a very striking similarity between the group a, fig. 9, and fig. 3, which represents four young vegetable cells developed in a parent cell, and the thickened walls of which have coalesced with one another and with those of the parent cell, so that the four cavities only remain in an homogeneous substance. That portion of the cell-cavities which is still visible is filled with a granulous yellowish substance, in which lie one or more nuclei, or young cells provided with a nucleus: these remains of the cell-cavities are the cartilage-corpuscles discovered by Purkinje.
The intercellular substance is universally much more prominent in the cartilages of mammalia than it is in those hitherto described, and in them it forms the principal part of the firm mass of the cartilage. There is not, however, any essential difference either between the structure of the several kinds of cartilage of mammalia, or between these and the cartilage of lower animals, the only distinction being that it is a little more difficult to prove the existence of the special walls of the cartilage-cells in the former.
The intercellular substance in some cartilages of mammalia is at first so soft, that the cells fall apart under slight pressure, and float free in the fluid. If, for example, a thin lamella be cut off from the cartilage at the angle of the lower jaw of a foetal pig of three and a half inches in length (a period when
