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 (Fig. 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 _ passive_ portions come third; these are subsequently formed from the others, and I have given them the name of “plasma-products.” They are partly external (cell-membranes and intercellular matter) and partly internal (cell-sap and cell-contents). The nucleus (or caryon), which is usually of a simple roundish form, is quite structureless at first (especially in very young cells), and composed of homogeneous nuclear matter or caryoplasm (Fig. 2_k_). But, as a rule, it forms a sort of vesicle later on, in which we can distinguish a more solid _nuclear base (caryobasis)_ and a softer or fluid _nuclear sap (caryolymph)._ In a mesh of the nuclear network (or it may be on the inner side of the nuclear envelope) there is, as a rule, a dark, very opaque, solid body, called the _nucleolus._ Many of the nuclei contain several of these nucleoli (as, for instance, the germinal vesicle of the ova of fishes and amphibia). Recently a very small, but particularly important, part of the nucleus has been distinguished as the _central body_ (centrosoma)—a tiny particle that is originally found in the nucleus itself, but is usually outside it, in the cytoplasm; as a rule, fine threads stream out from it in the cytoplasm. From the position of the central body with regard to the other parts it seems probable that it has a high physiological importance as a centre of movement; but it is lacking in many cells.