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  In many cases the formed material is simply the old dead cell walls from which the living substance has been withdrawn (Fig. 14).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 means of such a differentiation there arises a very complex mass of cells, with great variety in shape and function.

[Illustration:  FIG. 19.  A piece of nerve fibre, showing the cell with its nucleus at n.]

It should be noticed further that there are some animals and plants in which the whole animal is composed of a single cell.  These organisms are usually of extremely minute size, and they comprise most of the so-called animalculae which are found in water.  In such animals the different parts of the cell are modified to perform different functions.  The different organs appear within the cell, and the cell is more complex than the typical cell described.  Fig. 21 shows such a cell.  Such an animal possesses several organs, but, since it consists of a single mass of protoplasm and a single nucleus, it is still only a single cell.  In the multicellular organisms the organs of the body are made up of cells, and the different organs are produced by a differentiation of cells, but in the unicellular organisms the organs are the results of the differentiation of the parts of a single cell.  In the one case there is a differentiation of cells, and in the other of the parts of a cell.