[Illustration:  FIG. 20.—­A muscle fibre.  The nucleii are shown at n.]

[Illustration:  FIG. 21.—­A complex cell.  It is an entire animal, but composed of only one cell.]

Such, in brief, is the cell to whose activities it is possible to trace the fundamental properties of all living things.  Cells are endowed with the properties of irritability, contractibility, assimilation and reproduction, and it is thus plainly to the study of cells that we must look for an interpretation of life phenomena.  If we can reach an intelligible understanding of the activities of the cell our problem is solved, for the activities of the fully formed animal or plant, however complex, are simply the application of mechanical and chemical principles among the groups of such cells.  But wherein does this knowledge of cells help us?  Are we any nearer to understanding how these vital processes arise?  In answer to this question we may first ask whether it is possible to determine whether any one part of the cell is the seat of its activities.

==The Cell Wall.==—­The first suggestion which arose was that the cell wall was the important part of the cell, the others being secondary.  This was not an unnatural conclusion.  The cell wall is the most persistent part of the cell.  It was the part first discovered by the microscope and is the part which remains after the other parts are gone.  Indeed, in many of the so-called cells the cell wall is all that is seen, the cell contents having disappeared (Fig. 14).  It was not strange, then, that this should at first have been looked upon as the primary part.  The idea was that the cell wall in some way changed the chemical character of the substances in contact with its two sides, and thus gave rise to vital activities which, as we have seen, are fundamentally chemical.  Thus the cell wall was regarded as the most essential part of the cell, since it controlled its activities.  This the belief of Schwann, although he also regarded the other parts of the cell as of importance.

[Illustration:  FIG. 22.—­An amoeba.  A single cell without cell wall. n is the nucleus; f, a bit of food which the cell has absorbed.]

This conception, however, was quite temporary.  It was much as if our hypothetical supramundane observer looked upon the clothes of his newly discovered human being as forming the essential part of his nature.  It was soon evident that this position could not be maintained.  It was found that many bits of living matter were entirely destitute of cell wall.  This is especially true of animal cells.  While among plants the cell wall is almost always well developed, it is very common for animal cells to be entirely lacking in this external covering—­as, for example, the white blood-cells.  Fig. 22 shows an amoeba, a cell with very active powers of motion and assimilation, but with no cell wall.  Moreover, young cells are always more active than older ones,