of it into surrounding watery fluids carrying the excess of soluble chemical bodies with it. Whilst proteids are the compounds of the highest stage of chemical complexity recognised in protoplasm, and appear to form the bulk of its substance, we must carefully avoid the error (which is not uncommon) of supposing that protoplasm is itself a definite chemical compound. It is not. Cell-protoplasm includes the nucleus, that denser central body: and is a structure consisting of “proteids” and of many granules and dust-like particles, and of more and of less liquid or watery parts which are less complex in chemical nature than are proteids. Some of the visible granules and invisible liquids present in protoplasm are being built up to the proteid stage of elaboration, whilst some are steps in degradation and decomposition. We have no reason to suppose that the molecules of any proteid known at present to the chemist really are the highest degree of chemical complexity attained to in living protoplasm. Probably there is present a further stage of elaboration, a chemical body even more complex than is “proteid,” which is continually attracting the lower chemical compounds to itself and as continually breaking down. This is the ultimate chemical substance of life. It is hidden invisibly in the protoplasm, yet all the chemical changes which go on in the protoplasm of a cell are either leading up to this supreme life-stuff or are leading downwards from it. This ultimate compound, which we suppose to exist but have not demonstrated, has been called “plasmogen.” It is this body in which resides the peculiar property of living matter, namely, that of attracting to itself substances containing the so-called “organic” elements—carbon, oxygen, hydrogen, and nitrogen—and of acting on them in such a way that they “nourish” it—that is to say, combine chemically with it to form more “plasmogen.” The intermediate steps leading up to plasmogen and the products arising from its incessant breaking down are formed under the influence of this unique chemical body, and by it alone. Chemists have not yet succeeded in making them; only the less elaborate kinds have been “artificially” constructed without the aid of the living plasmogen. To construct plasmogen itself is a task for the chemists of the distant future. In early geological ages plasmogen came into being; it has gone on ever since “nourishing” itself, maintaining itself, growing and spreading over the earth. It is improbable that the conditions which led to its formation have ever recurred. All subsequent plasmogen has been formed by the growth and increase of that first sample of it, which once in a remote period of the earth’s history was built up by chemical conditions, which came to an end as soon as they had produced it.