evident, until the adult tissue seems sometimes to be composed mostly of
what we have called formed material.

[Illustration: FIG. 12.--Plant cells with thick walls, from a fern.]

[Illustration: FIG. 13.--Section of a potato showing different shaped
cells, the inner and larger ones being filled with grains of starch.]

[Illustration: FIG. 14.--Various shaped wood cells from plant tissue.]

[Illustration: FIG. 15.--A bit of cartilage.]

[Illustration: FIG. 16.--Frog's blood: _a_ and _b_ are the cells; _c_ is
the liquid.]

[Illustration: FIG. 17.--A bit of bone, showing the cells imbedded in
the bony matter.]

It must not be imagined, however, that a very rigid line can be drawn
between the cell itself and the material it forms. The formed material
is in many cases simply a thickened cell wall, and this we commonly
regard as part of the cell. In many cases the formed material is simply
the old dead cell walls from which the living substance has been
withdrawn (Fig. 14). In other cases the cell substance acquires peculiar
functions, so that what seems to be the formed material is really a
modified cell body and is still active and alive. Such is the case in
the muscle. In other cases the formed material appears to be
manufactured within the cell and secreted, as in the case of bone. No
sharp lines can be drawn, however, between the various types. But the
distinction between formed material and cell body is a convenient one
and may well be retained in the discussion of cells. In our discussion
of the fundamental vital properties we are only concerned in the cell
substance, the formed material having nothing to do with fundamental
activities of life, although it forms largely the secondary machinery
which we have already studied.

[Illustration: FIG. 18.--Connective tissue. The cells of the tissue are
shown at _c_, and the fibres or formed matter at _f_.]

In all higher animals and plants the life of the individual begins as a
single ovum or a single cell, and as it grows the cells increase rapidly
until the adult is formed out of hundreds of millions of cells. As these
cells become numerous they cease, after a little, to be alike. They
assume different shapes which are adapted to the different duties they
are to perform. Thus, those cells which are to form bone soon become
different from those which are to form muscle, and those which are to
form the blood are quite unlike those which are to produce the hairs. By