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 (Figure 1.2 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.

The cell-body also consists originally, and in its simplest form, of a
homogeneous viscid plasmic matter. But, as a rule, only the smaller
part of it is formed of the living active cell-substance (protoplasm);
the greater part consists of dead, passive plasma-products
(metaplasm). It is useful to distinguish between the inner and outer
of these. External plasma-products (which are thrust out from the
protoplasm as solid "structural matter") are the cell-membranes and