principle is the basis both of the organized and unorganized tissues. We shall have further evidence of this presented to us in the following classes, which comprise the rest of the tissues,—those, indeed, which are most perfectly organized, and the most important to the animal organism. In them we shall also find that the formation of cells is the general principle of development, and that their elementary particles are derived from cells, although at the first glance one would scarcely imagine that any connexion could exist between them and cells.
CLASS IV.
Fibre-celis, or tissues, which originate from cells that become elongated into bundles of fibres.
Mere fibres are all that can be detected as the elementary components of the tissues of this class when they are examined in the mature animal. But when we investigate the mode in which they are generated, we see that the fibres are formed only as prolongations of cells, which, in most instances, are elongated in two opposite directions, sometimes terminating at once in a fasciculus of fibres, at other times in a single fibre, which afterwards splits into several finer ones. This constitutes the characteristic feature of the class. We are already acquainted with the type of the prolongation of cells into fibres in the pigment-ramifications, osseous corpuscles, &c. The cells next to be considered differ from them in the following particulars: the fibres originating from any one cell generally lie together in a fasciculus, and in these prolongations of the cells, it is principally the wall which is most strongly developed, whilst, in the former instances, the cells though extended into fibres, were chiefly rendered conspicuous by their cavities. This class comprises the Cellular (areolar), Fibrous, and Elastic tissues.
1. Cellular (areolar) Tissue. This tissue is known to. be composed of extremely minute, tough, smooth fibres, having a pale outline, and usually a serpentine course; they may be seen in their natural state in the mesentery with-
