Part 3 (1/2)

This material between the cells is really to be regarded as an excessively thickened cell wall and has been secreted by the cell substance lying within the cells, so that a bit of cartilage is really a ma.s.s of cells with an exceptionally thick cell wall. At Fig. 16 is shown a little blood. Here the cells are to be seen floating in a liquid. The liquid is colourless and it is the red colour in the blood cells which gives the blood its red colour. The liquid may here again be regarded as material produced by cells. At Fig. 17 is a bit of bone showing small irregular cells imbedded within a large ma.s.s of material which has been deposited by the cell. In this case the formed material has been hardened by calcium phosphate, which gives the rigid consistency to the bone. In some animal tissues the formed material is still greater in amount. At Fig. 18, for example, is a bit of connective tissue, made up of a ma.s.s of fine fibres which have no resemblance to cells, and indeed are not cells. These fibres have, however, been made by cells, and a careful study of such tissue at proper places will show the cells within it. The cells shown in Fig. 18 (_c_) have secreted the fibrous material.

Fig. 19 shows a cell composing a bit of nerve. At Fig. 20 is a bit of muscle; the only trace of cellular structure that it shows is in the nuclei (_n_), but if the muscle be studied in a young condition its cellular structure is more evident. Thus it happens in adult animals that the cells which are large and clear at first, become less and less evident, until the adult tissue seems sometimes to be composed mostly of what we have called formed material.

[Ill.u.s.tration: FIG. 12.--Plant cells with thick walls, from a fern.]

[Ill.u.s.tration: FIG. 13.--Section of a potato showing different shaped cells, the inner and larger ones being filled with grains of starch.]

[Ill.u.s.tration: FIG. 14.--Various shaped wood cells from plant tissue.]

[Ill.u.s.tration: FIG. 15.--A bit of cartilage.]

[Ill.u.s.tration: FIG. 16.--Frog's blood: _a_ and _b_ are the cells; _c_ is the liquid.]

[Ill.u.s.tration: FIG. 17.--A bit of bone, showing the cells imbedded in the bony matter.]

It must not be imagined, however, that a very rigid line can be drawn between the cell itself and the material it forms. The formed material is in many cases simply a thickened cell wall, and this we commonly regard as part of the cell. In many cases the formed material is simply the old dead cell walls from which the living substance has been withdrawn (Fig. 14). In other cases the cell substance acquires peculiar functions, so that what seems to be the formed material is really a modified cell body and is still active and alive. Such is the case in the muscle. In other cases the formed material appears to be manufactured within the cell and secreted, as in the case of bone. No sharp lines can be drawn, however, between the various types. But the distinction between formed material and cell body is a convenient one and may well be retained in the discussion of cells. In our discussion of the fundamental vital properties we are only concerned in the cell substance, the formed material having nothing to do with fundamental activities of life, although it forms largely the secondary machinery which we have already studied.

[Ill.u.s.tration: FIG. 18.--Connective tissue. The cells of the tissue are shown at _c_, and the fibres or formed matter at _f_.]

In all higher animals and plants the life of the individual begins as a single ovum or a single cell, and as it grows the cells increase rapidly until the adult is formed out of hundreds of millions of cells. As these cells become numerous they cease, after a little, to be alike. They a.s.sume different shapes which are adapted to the different duties they are to perform. Thus, those cells which are to form bone soon become different from those which are to form muscle, and those which are to form the blood are quite unlike those which are to produce the hairs. By means of such a differentiation there arises a very complex ma.s.s of cells, with great variety in shape and function.

[Ill.u.s.tration: FIG. 19. A piece of nerve fibre, showing the cell with its nucleus at _n_.]

It should be noticed further that there are some animals and plants in which the whole animal is composed of a single cell. These organisms are usually of extremely minute size, and they comprise most of the so-called animalculae which are found in water. In such animals the different parts of the cell are modified to perform different functions.

The different organs appear within the cell, and the cell is more complex than the typical cell described. Fig. 21 shows such a cell. Such an animal possesses several organs, but, since it consists of a single ma.s.s of protoplasm and a single nucleus, it is still only a single cell.

In the multicellular organisms the organs of the body are made up of cells, and the different organs are produced by a differentiation of cells, but in the unicellular organisms the organs are the results of the differentiation of the parts of a single cell. In the one case there is a differentiation of cells, and in the other of the parts of a cell.

[Ill.u.s.tration: FIG. 20.--A muscle fibre. The nucleii are shown at _n_.]

[Ill.u.s.tration: 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, a.s.similation 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.

[Ill.u.s.tration: 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 dest.i.tute 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 a.s.similation, but with no cell wall. Moreover, young cells are always more active than older ones, 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 a.s.sociated 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 const.i.tuted 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 const.i.tutes 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.

The importance of this newly formulated doctrine it is difficult to appreciate. Here, in protoplasm had been apparently found the foundation of living phenomena. Here was a substance universally present in animals and plants, simple and uniform--a substance always present in living parts and disappearing with death. It was the simplest thing that had life, and indeed the only thing that had life, for there is no life outside of cells and protoplasm. But simple as it was it had all the fundamental properties of living things--irritability, contractibility, a.s.similation, and reproduction. It was a compound which seemingly deserved the name of ”_physical basis of life_”, which was soon given to it by Huxley. With this conception of protoplasm as the physical basis of life the problems connected with the study of life became more simplified. In order to study the nature of life it was no longer necessary to study the confusing ma.s.s of complex organs disclosed to us by animals and plants, or even the somewhat less confusing structures shown by individual cells. Even the simple cell has several separate parts capable of undergoing great modifications in different types of animals. This confusion now appeared to vanish, for only _one_ thing was found to be alive, and that was apparently very simple. But that substance exhibited all the properties of life. It moved, it could grow, and reproduce itself, so that it was necessary only to explain this substance and life would be explained.

(b) _Nature of Protoplasm_.--What is this material, protoplasm? As disclosed by the early microscope it appeared to be nothing more than a simple ma.s.s of jelly, usually transparent, more or less consistent, sometimes being quite fluid, and at others more solid. Structure it appeared to have none. Its chief peculiarity, so far as physical characters were concerned, was a wonderful and never-ceasing activity.

This jellylike material appeared to be endowed with wonderful powers, and yet neither physical nor microscopical study revealed at first anything more than a uniform h.o.m.ogeneous ma.s.s of jelly. Chemical study of the same substance was of no less interest than the microscopical study. Of course it was no easy matter to collect this protoplasm in sufficient quant.i.ty and pure enough to make a careful a.n.a.lysis. The difficulties were in time, however, overcome, and chemical study showed protoplasm to be a proteid, related to other proteids like alb.u.men, but one which was more complex than any other known. It was for a long time looked upon by many as a single definite chemical compound, and attempts were made to determine its chemical formula. Such an a.n.a.lysis indicated a molecule made up of several hundred atoms. Chemists did not, however, look with much confidence upon these results, and it is not surprising that there was no very close agreement among them as to the number of atoms in this supposed complex molecule. Moreover, from the very first, some biologists thought protoplasm to be not one, but more likely a mixture of several substances. But although it was more complex than any other substance studied, its general characters were so like those of alb.u.men that it was uniformly regarded as a proteid; but one which was of a higher complexity than others, forming perhaps the highest number of a series of complex chemical compounds, of which ordinary proteids, such as alb.u.men, formed lower members. Thus, within a few years following the discovery of protoplasm there had developed a theory that living phenomena are due to the activities of a definite though complex chemical compound, composed chiefly of the elements carbon, oxygen, hydrogen, and nitrogen, and closely related to ordinary proteids. This substance was the basis of living activity, and to its modification under different conditions were due the miscellaneous phenomena of life.

(c) _Significance of Protoplasm_.--The philosophical significance of this conception was very far-reaching. The problem of life was so simplified by subst.i.tuting the simple protoplasm for the complex organism that its solution seemed to be not very difficult. This idea of a chemical compound as the basis of all living phenomena gave rise in a short time to a chemical theory of life which was at least tenable, and which accounted for the fundamental properties of life. That theory, the _chemical theory of life_, may be outlined somewhat as follows:

The study of the chemical nature of substances derived from living organisms has developed into what has been called organic chemistry.

Organic chemistry has shown that it is possible to manufacture artificially many of the compounds which are called organic, and which had been hitherto regarded as produced only by living organisms. At the beginning of the century, it was supposed to be impossible to manufacture by artificial means any of the compounds which animals and plants produce as the result of their life. But chemists were not long in showing that this position is untenable. Many of the organic products were soon shown capable of production by artificial means in the chemist's laboratory. These organic compounds form a series beginning with such simple bodies as carbonic acid (CO_{2}), water (H_{2}O), and ammonia (NH_{3}), and pa.s.sing up through a large number of members of greater and greater complexity, all composed, however, chiefly of the elements carbon, oxygen, hydrogen, and nitrogen. Our chemists found that starting with simple substances they could, by proper means, combine them into molecules of greater complexity, and in so doing could make many of the compounds that had hitherto been produced only as a result of living activities. For example, urea, formic acid, indigo, and many other bodies, hitherto produced only by animals and plants, were easily produced by the chemist by purely chemical methods. Now when protoplasm had been discovered as the ”physical basis of life,” and, when it was further conceived that this substance is a proteid related to alb.u.mens, it was inevitable that a theory should arise which found the explanation of life in accordance with simple chemical laws.

If, as chemists and biologists then believe, protoplasm is a compound which stands at the head of the organic series, and if, as is the fact, chemists are each year succeeding in making higher and higher members of the series, it is an easy a.s.sumption that some day they will be able to make the highest member of the series. Further, it is a well-known fact that simple chemical compounds have simple physical properties, while the higher ones have more varied properties. Water has the property of being liquid at certain temperatures and solid at others, and of dividing into small particles (i.e., dissolving) certain bodies brought in contact with it. The higher compound alb.u.men has, however, a great number of properties and possibilities of combination far beyond those of water. Now if the properties increase in complexity with the complexity of the compound, it is again an easy a.s.sumption that when we reach a compound as complex as protoplasm, it will have properties as complex as those of the simple life substance. Nor was this such a very wild hypothesis. After all, the fundamental life activities may all be traced to the simple oxidation of food, for this results in movement, a.s.similation, and growth, and the result of growth is reproduction. It was therefore only necessary for our biological chemists to suppose that their chemical compound protoplasm possessed the power of causing certain kinds of oxidation to take place, just as water itself induces a simpler kind of oxidation, and they would have a mechanical explanation of the life activities. It was certainly not a very absurd a.s.sumption to make, that this substance protoplasm could have this power, and from this the other vital activities are easily derived.