If it were possible by a process of elimination to ascertain the substances which must be present in protoplasm, the physiologist might formulate a reasonable hypothesis as to the nature of this “basis.” But there is no part of any living thing, or, at any rate, no part which is not microscopic in its dimensions, which can be pointed out as protoplasm and nothing besides. It is impossible to isolate anything which can be described as pure protoplasm. Nor is it possible, by comparing various tissues which are acknowledged to be rich in protoplasm, to ascertain what chemical substances are common to them all.
If it were feasible, by analysing a number of specimens of protoplasm, to make sure that, although _x_ is absent from one, _y_ from another, and _z_ from a third, some one thing, _P_, is always present, then _P_ might be regarded as the physical basis, even though it were evident that _P_ alone was not protoplasm. Protoplasm would be _P_ combined with either _x_, _y_, or _z_. Globulins and albumins and other proteins are always present, but in varying proportions; but it is impossible to make certain that either of these chemical substances is more important than the rest. Nor is it possible to assert of either that it is essential.
Chemically, protoplasm is a mixture of substances, chiefly proteid, in a condition in which it is capable of manifesting the phenomena of life. But whether it be more complex and of heavier molecule than either globulin, nucleo-protein, albumin, fibrin, or any other of the nitrogenous compounds which take its place when it is dead; or whether it be as simple as either of these, but differ from them all in its instability, in the constant flux of its atoms, which causes it at one time to incline towards one of them, at another time to another, are questions which cannot at present be answered.
The uncertainty as to the chemical nature of protoplasm is responsible for an unfortunate irregularity in the use of the term. It is _ex hypothesi_ the most active, the most living part of an animal cell. If the cell has a nucleus and an envelope, the protoplasm must lie in the space between the two. This part of the cell is therefore often termed, without qualification, the “cell-protoplasm.” Frequently the abuse of the word is carried still further. Young cells, leucocytes, nerve-cells, etc., which have no envelope, consist of a nucleus embedded in soft cell-substance. The latter is termed its protoplasm. The cell is described as consisting of nucleus and protoplasm, the term assuming an anatomical signification. Not only is such a use of the term bad, because it indicates a confusion of thought, but it brings with it a train of ambiguities. What are the limits of the protoplasm? If the cell-body be firmer towards its exterior than it is within, is the denser substance protoplasm, or is it not? It has not the qualities which are attributed to protoplasm in so marked a degree as has the substance which it surrounds. Hence a distinction is made. The one is “ectoplasm,” the other “endoplasm.” Within the cell-body are many collections, often in the form of granules, of substances which have not the protoplasmic attributes. They constitute the “deuteroplasm” of certain cytologists. But these enclosed substances may be as far removed from protoplasm as starch grains. It is absurd to use the termination “plasm” for such well-defined products of cell activity as these. The subject is, unfortunately, obscured by conflicting terms. Nomenclatures which were invented with the object of giving definiteness to our ideas have served but to perplex them. The term “protoplasm” should be reserved as a synonym for the substance which is most alive, the substance in which chemical change is most active, the substance which has in the highest degree a potentiality of growth. Anatomical distinctions are better expressed in anatomical terms. We shall treat of such distinctions when considering the organization of the cell.