o showing its cells
filled with small granules of starch which the cells have produced by
their activities and deposited within their own bodies. At Fig. 14 are
several wood cells showing cell walls of different shape which, having
become dead, have lost their contents and simply remain as dead cell
walls. Each was in its earlier history filled with cell substance and
contained a nucleus. In a similar way any bit of vegetable tissue would
readily show itself to be made of similar cells.

In animal tissues the cellular structure is not so easily seen, largely
because the products made by the cells, the formed products, become
relatively more abundant and the cells themselves not so prominent. But
the cellular structure is none the less demonstrable. In Fig. 15, for
instance, will be seen a bit of cartilage where the cells themselves are
rather small, while the material deposited between them is abundant.
This material between the cells is really to be regarded as an
excessively thickened cell wall and has been secreted by the cell
substance lying within the cells, so that a bit of cartilage is really a
mass of cells with an exceptionally thick cell wall. At Fig. 16 is shown
a little blood. Here the cells are to be seen floating in a liquid. The
liquid is colourless and it is the red colour in the blood cells which
gives the blood its red colour. The liquid may here again be regarded
as material produced by cells. At Fig. 17 is a bit of bone showing small
irregular cells imbedded within a large mass of material which has been
deposited by the cell. In this case the formed material has been
hardened by calcium phosphate, which gives the rigid consistency to the
bone. In some animal tissues the formed material is still greater in
amount. At Fig. 18, for example, is a bit of connective tissue, made up
of a mass of fine fibres which have no resemblance to cells, and indeed
are not cells. These fibres have, however, been made by cells, and a
careful study of such tissue at proper places will show the cells within
it. The cells shown in Fig. 18 (_c_) have secreted the fibrous material.
Fig. 19 shows a cell composing a bit of nerve. At Fig. 20 is a bit of
muscle; the only trace of cellular structure that it shows is in the
nuclei (_n_), but if the muscle be studied in a young condition its
cellular structure is more evident. Thus it happens in adult animals
that the cells which are large and clear at first, become less and less