the cells.

(FIGURE 1.3. Three epithelial cells from the mucous lining of the
tongue.

FIGURE 1.4. Five spiny or grooved cells, with edges joined, from the
outer skin (epidermis): one of them (b) is isolated.

FIGURE 1.5. Ten liver-cells: one of them (b) has two nuclei.)

The cells also differ very much in size. The great majority of them
are invisible to the naked eye, and can be seen only through the
microscope (being as a rule between 1/2500 and 1/250 inch in
diameter). There are many of the smaller plastids--such as the famous
bacteria--which only come into view with a very high magnifying power.
On the other hand, many cells attain a considerable size, and run
occasionally to several inches in diameter, as do certain kinds of
rhizopods among the unicellular protists (such as the radiolaria and
thalamophora). Among the tissue-cells of the animal body many of the
muscular fibres and nerve fibres are more than four inches, and
sometimes more than a yard, in length. Among the largest cells are the
yelk-filled ova; as, for instance, the yellow "yolk" in the hen's egg,
which we shall describe later (Figure 1.15).

Cells also vary considerably in structure. In this connection we must
first distinguish between the active and passive components of the
cell. It is only the former, or active parts of the cell, that really
live, and effect that marvellous world of phenomena to which we give
the name of "organic life." The first of these is the inner nucleus
(caryoplasm), and the second the body of the cell (cytoplasm). The