Great varieties of cells are found in the various mature tissues of the higher animals, all of which have passed through the stage of being a simple nucleated mass of protoplasm in the earlier periods of their development. All cells may then be divided into two chief types, the indifferent and the differentiated.

Under the category of indifferent cells may be placed all such as retain the characters of the first embryonic cells, and have not acquired any special structure or property by which they can be distinguished from the simplest form. Such cells are the only ones in the early stages of the embryo. In the adult tissues they also occur, having various duties to perform. They are found in the blood and lymph, and scattered throughout the tissues. They are without a cell wall, and have no special contents to mark their function.

Transverse section of Blastoderm, showing the elements in the earlier stage of the development.

Fig. 5. Transverse section of Blastoderm, showing the elements in the earlier stage of the development. A, epiblast; B, mesoblast; C, hypoblast.

Among the differentiated cells we find many special characters, adapting them to certain special duties, for all these cells are modified from the original type and applied to the performance of some special function.

Space prevents even a short enumeration of the varieties of cells met with in the tissues of plants, where they not only carry on the active functions of the organism, but also form the supporting structures.

The differentiation of a cell is accomplished by its protoplasm, which forms new structural parts, and itself sometimes seems to diminish in quantity until an element is produced in which there may be no protoplasm recognizable.

We find, then, matured and differentiated cells which vary -

1. In shape, being spherical, flattened, fusiform, stellate, etc.

2. In size.

3. In their mode of connection.

Cells may also be classified according to their function, e.g., Glandular, Nervous, etc., and the greater portion of the following pages will be devoted to the functions of these various forms of cells.

So long as a cell remains in its indifferent stage it possesses the properties of ordinary protoplasm only; but by its further development it acquires special properties not common to all protoplasm. These properties may or may not be accompanied by structural change. Thus the protoplasm of a gland cell differs in little from that of any other cell except in the capabilities of its nutritive changes and its chemical products; while on the other hand, those epithelial cells which form the outer layer of the skin lose their protoplasmic characters and are completely modified in structure.