elementary substance other than occurs in the inorganic world. These elements are combined in protoplasm into certain most extremely complex compounds, which are always present where life is, and never elsewhere, and hence the essential chemical characteristic of living matter is the presence of these complex as yet unanalyzed, albuminous compounds.

It is obvious that this chemical half-knowledge of protoplasm makes no satisfying revelation to us explanatory of the qualities of this life stuff. How is it then with the physical structure of protoplasm? We know that many simple chemical substances put together in particular physical relationship to each other will give a capacity of performance or function quite different from and beyond that which they possess when simply brought together without definite order or arrangement. Is protoplasm a machine with a capacity for doing extraordinary things, with its powers due primarily to its physical make-up? Unfortunately we have no satisfying answer to this question. While chemists are balked in their analysis of the protoplasmic make-up by the complexity of the compounds they meet, a complexity too much for their present technic to resolve, physicists are similarly balked in their attempt to resolve and expose the ultimate physical structure of protoplasm.

This ultimate structure of protoplasm is ultramicroscopic, and its study is checked by the limitations of microscopes. When we examine protoplasm with the highest powers of the microscope we see plainly that it is not as it appears under lower powers, structureless and homogeneous. On the contrary it reveals an apparent granular or fibrillar or alveolar or reticular structure. We find that protoplasm varies in its physical make-up at different times or in different cells. We also find that the difficulties of interpreting just what one sees when using the highest microscopic powers make it impossible to be really certain of understanding what is seen. But however various our interpretations of the finer structure of protoplasm, they agree that any bit of protoplasm is a viscous colloidal mass composed of at least two substances of somewhat different physical make-up. One of these substances is evidently denser than the other and is arranged in the form of grains, rods, threads, or droplets scattered through a ground mass. Concerning this dimorphic condition of protoplasm practically all biologists are agreed. The names, hyaloplasm, paraplasm, or others of sim

ilar significance are applied to the viscous hyaline ground substance, while the denser parts are variously called microsomes, granules, fibrils, spongioplasm, etc.

The important part of all this is the fact that all the biologists are not agreed on any certain kind of intimate structure of

FIG. 15.-Different types of cells composing the body of the squirrel or other highly developed animal: A, liver cell; f, food materials; n, nucleus; B, complete cell; C, nerve cell, with small part of its fiber; D, muscle fiber; E, cells lining the body cavity; F, lining of the windpipe; G, section through the skin. (Highly magnified.)

protoplasm as revealed by the highest powers of the microscope, but they all agree that there is a fine and real structural organization of what at first glance appears to be homogeneous structureless life stuff. That is, as Delage expresses it, it is seen that protoplasm is not simply an organic chemical compound, but that it is an organized substance; that is, it possesses a structure of a higher order than the automatic structure of those chemical molecules which compose non-living so-called organic substances. But at the same time we are deceived if we expect

to be able to find in this physical organization of protoplasm any satisfactory explanation of its wonderful properties.

We have said that it should always be held clearly in mind that the full life capacity of protoplasm is realized only when it is in that differentiated and organized condition typical of the structural unit or cell. The essential thing about the cell is not that it has a definite shape or size or that it is truly cell- or saclike, but that it is a tiny mass of protoplasm with various

substances secreted by or held in it. The protoplasm itself is differentiated into at least two parts, an inner, denser, smaller part called the nucleus, and an outer surrounding, usually larger, portion called the cytoplasm. Such a differentiated or organized protoplasmic unit can perform all of the essential functions of life and persist in this performance indefinitely unless destroyed by extrinsic causes. The cell itself may not have any indefinite existence as a unit, but it will be the progenitor of an indefinitely prolonged series of cells. A single part of this cell, that is, a bit of protoplasm either of the nucleus or the cytoplasm, or the whole of either can perform for a while most of the activities of life; but such a part always lacks the capacity for reproduction, that is, for persistence as living matter. Thus it is obvious that

FIG. 17.--Amoeba eating a microscopic one-celled plant. (After Verworn.)

FIG. 18.-A maba polypodia in six successive stages of fusion. The dark white-margined spot in the interior is the nucleus. (After F. E. Schultze.)

if such protoplasmic cells, composed of nucleus and cytoplasm, exist singly they form living units. And we have actual exemplifications of this condition in the structure and life of the simplest organism.

The simplest organisms are independently living, single protoplasmic cells (Figs. 16-21). There are thousands of kinds of

FIG. 19.-Plasmodium of a slime mold on wood, Trichia faraginea: A, plasmodium X 2; B, spores; C, spore with contents escaping; D, ciliated swarm spore, showing flagellum, f, and nucleus, n; E, two amoeboid swarm spores; F, part of plasmodium under glass slide; G, a part of F, more highly magnified. (After Campbell.)

these single-celled organisms recognizably different by characteristics of shape and size, habit and habitat. We try to distinguish them as single-celled animals (Protozoa) and single-celled plants (Protophyta), on the basis of alleged differences in their habit of food-taking and general nutrition. This distinction is often most arbitrarily made, and botanists and zoologists are constantly claiming the same organisms as belonging to their respective fields of study. Many naturalists, conspicuously Haeckel, have repeatedly suggested the convenience and even the necessity of grouping most of these unicellular organisms into a phylum or kingdom to be called the Protista, the members