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 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.]