and they commonly possess either no cell wall or a very slight one, this being deposited as the cell becomes older and remaining long after it is dead.  Such facts soon disproved the notion that the cell wall is a vital part of the cell, and a new conception took its place which was to have a more profound influence upon the study of living things than any discovery hitherto made.  This was the formulation of the doctrine of the nature of protoplasm.

Protoplasm.—­(a) Discovery.  As it became evident that the cell wall is a somewhat inactive part of the cell, more attention was put on the cell contents.  For twenty years after the formulation of the cell doctrine both the cell substance and the nucleus had been looked upon as essential to its activities.  This was more especially true of the nucleus, which had been thought of as an organ of reproduction.  These suggestions appeared indefinitely in the writings of one scientist and another, and were finally formulated in 1860 into a general theory which formed what has sometimes been called the starting point of modern biology.  From that time the material known as protoplasm was elevated into a prominent position in the discussion of all subjects connected with living phenomena.  The idea of protoplasm was first clearly defined by Schultze, who claimed that the real active part of the cell was the cell substance within the cell wall.  This substance he proved to be endowed with powers of motion and powers of inducing chemical changes associated with vital phenomena.  He showed it to be the most abundant in the most active cells, becoming less abundant as the cells lose their activity, and disappearing when the cells lose their vitality.  This cell substance was soon raised into a position of such importance that the smaller body within it was obscured, and for some twenty years more the nucleus was silently ignored in biological discussion.  According to Schultze, the cell substance itself constituted the cell, the other parts being entirely subordinate, and indeed frequently absent.  A cell was thus a bit of protoplasm, and nothing more.  But the more important feature of this doctrine was not the simple conclusion that the cell substance constitutes the cell, but the more sweeping conclusion that this cell substance is in all cells essentially identical. The study of all animals, high and low, showed all active cells filled with a similar material, and more important still, the study of plant cells disclosed a material strikingly similar.  Schultze experimented with this material by all means at his command, and finding that the cell substance in all animals and plants obeys the same tests, reached the conclusion that the cell substance in animals and plants is always identical.  To this material he now gave the name protoplasm, choosing a name hitherto given to the cell contents of plant cells.  From this time forth this term protoplasm was applied to the living material found in all cells, and became at once the most important factor in the discussion of biological problems.