Chapter 5

THE CELL AND PROTOPLASM.

==Vital Properties.==--We have seen that the general activities of the body are intelligible according to chemical and mechanical laws, provided we can assume as their foundation the simple vital properties of living phenomena. We must now approach closer to the centre of the problem, and ask whether we can trace these fundamental properties to their source and find an explanation of them.

In the first place, what are these properties? The vital powers are varied, and lie at the basis of every form of living activity. When we free them from complications, however, they may all be reduced to four. These are: (1) _Irritability_, or the property possessed by living matter of reacting when stimulated. (2) _Movement_, or the power of contracting when stimulated. (3) _Metabolism_, or the power of absorbing extraneous food and producing in it certain chemical changes, which either convert it into more living tissue or break it to pieces to liberate the inclosed energy. (4) _Reproduction_, or the power of producing new individuals. From these four simple vital activities all other vital actions follow; and if we can find an explanation of these, we have explained the living machine. If we grant that certain parts of the body can assimilate food and multiply, having the power of contraction when irritated, we can readily explain the other functions of the living machine by the application of these properties to the complicated machinery of the body. But these properties are fundamental, and unless we can grasp them we have failed to reach the centre of the problem.

As we pass from the more to the less complicated animals we find a gradual simplification of the machinery until the machinery apparently disappears. With this simplification of the machinery we find the animals provided with less varied powers and with less delicate adaptations to conditions. But withal we find the fundamental powers of the living organisms the same. For the performance of these fundamental activities there is apparently needed no machinery. The simple types of living bodies are simple in number of parts, but they possess essentially the same powers of assimilation and growth that characterize the higher forms. It is evident that in our attempt to trace the vital properties to their source we may proceed in two ways. We may either direct our attention to the simplest organisms where all secondary machinery is wanting, or to the smallest parts into which the tissues of higher organisms can be resolved and yet retain their life properties. In either way we may hope to find living phenomena in its simplest form independent of secondary machinery.

But the fact is, when we turn our attention in these two directions, we find the result is the same. If we look for the lowest organisms we find them among forms that are made of a single _cell_, and if we analyze the tissues of higher animals we find the ultimate parts to be _cells_. Thus, in either direction, the study of the cell is forced upon us.

Before beginning the study of the cell it will be well for us to try to get a clear notion of the exact nature of the problems we are trying to solve. We wish to explain the activities of life phenomena in such a way as to make them intelligible through the application of natural forces. That these processes are fundamentally chemical ones is evident enough. A chemical oxidation of food lies at the basis of all vital activity, and it is thus through the action of chemical forces that the vital powers are furnished with their energy. But the real problem is what it is in the living machine that controls these chemical processes. Fat and starch may be oxidized in a chemist's test tubes, and will there liberate energy; but they do not, under these conditions, manifest vital phenomena. Proteid may be brought in contact with oxygen without any oxidation occurring, and even if it is oxidized no motion or assimilation or reproduction occurs under ordinary conditions. These phenomena occur only when the oxidation takes place _in the living machine_. Our problem is then to determine, if possible, what it is in the living machine that regulates the oxidations and other changes in such a way as to produce from them vital activities. Why is it that the oxidation of starch in the living machine gives rise to motion, growth, and reproduction, while if the oxidation occurs in the chemist's laboratory, or even in a bit of dead protoplasm, it simply gives rise to heat?

One of the primary questions to demand attention in this search is whether we are to find the explanation, at the bottom, a _chemical_ or a _mechanical_ one. In the simplest form of life in which vital manifestations are found are we to attribute these properties simply to chemical forces of the living substance, or must we here too attribute them to the action of a complicated machinery? This question is more than a formal one. That it is one of most profound significance will appear from the following considerations:

Chemical affinity is a well recognized force. Under the action of this force chemical compounds are produced and different compounds formed under different conditions. The properties of the different compounds differ with their composition, and the more complex are the compounds the more varied their properties. Now it might be assumed as an hypothesis that there could be a chemical compound so complex as to possess, among other properties, that of causing the oxidation of food to occur in such a way as to produce assimilation and growth. Such a compound would, of course, be alive, and it would be just as true that its power of assimilating food would be one of its physical properties as it is that freezing is a physical property of water. If such an hypothesis should prove to be the true one, then the problem of explaining life would be a chemical one, for all vital properties would be reducible to the properties of a chemical compound. It would then only be necessary to show how such a compound came into existence and we should have explained life. Nor would this be a hopeless task. We are well acquainted with forces adequate to the formation of chemical compounds. If the force of chemical affinity is adequate under certain conditions to form some compounds, it is easy to conceive it as a possibility under other conditions to produce this chemical living substance. Our search would need then to be for a set of conditions under which our living compound could have been produced by the known forces of chemical affinity.

But suppose, on the other hand, that we find this simplest bit of living matter is not a chemical compound, but is in itself a complicated machine. Suppose that, after reducing this vital substance to its simplest type, we find that the substance with which we are dealing not only has complex chemical structure, but that it also possesses a large number of structural parts adapted to each other in such a way as to work together in the form of an intricate mechanism. The whole problem would then be changed. To explain such a machine we could no longer call upon chemical forces. Chemical affinity is adequate to the explanation of chemical compounds however complicated, but it cannot offer any explanation for the adaptation of parts which make a machine. The problem of the origin of the simplest form of life would then be no longer one of chemical but one of mechanical evolution. It is plain then that the question of whether we can attribute the properties of the simplest type of life to chemical composition or to mechanical structure is more than a formal one.

==The Discovery of Cells.==--It is difficult for us to-day to have any adequate idea of the wonderful flood of light that was thrown upon scientific and philosophical study by the discoveries which are grouped around the terms cells and protoplasm. Cells and protoplasm have become so thoroughly a part of modern biology that we can hardly picture to ourselves the vagueness of knowledge before these facts were recognized. Perhaps a somewhat crude comparison will illustrate the relation which the discovery of cells had to the study of life.

Imagine for a moment, some intelligent being located on the moon and trying to study the phenomena on the earth's surface. Suppose that he is provided with a telescope sufficiently powerful to disclose moderately large objects on the earth, but not smaller ones. He would see cities in various parts of the world with wide differences in appearance, size, and shape. He would see railroad trains on the earth rushing to and fro. He would see new cities arising and old ones increasing in size, and we may imagine him speculating as to their method of origin and the reasons why they adopt this or that shape. But in spite of his most acute observations and his most ingenious speculation, he could never understand the real significance of the cities, since he is not acquainted with the actual living unit. Imagine now, if you will, that this supramundane observer invents a telescope which enables him to perceive more minute objects and thus discovers human beings. What a complete revolution this would make in his knowledge of mundane affairs! We can imagine how rapidly discovery would follow discovery; how it would be found that it was the human beings that build the houses, construct and run the railroads, and control the growth of the cities according to their fancy; and, lastly, how it would be learned that it is the human being alone that grows and multiplies and that all else is the result of his activities. Such a supramundane observer would find himself entering into a new era, in which all his previous knowledge would sink into oblivion.

Something of this same sort of revolution was inaugurated in the study of living things by the discovery of cells and protoplasms. Animals and plants had been studied for centuries and many accurate and painstaking observations had been made upon them. Monumental masses of evidence had been collected bearing upon their shapes, sizes, distribution, and relations. Anatomy had long occupied the attention of naturalists, and the general structure of animals and plants was already well known. But the discoveries starting in the fourth decade of the century by disclosing the unity of activity changed the aspect of biological science.

==The Cell Doctrine==.--The cell doctrine is, in brief, the theory that the bodies of animals and plants are built up entirely of minute elementary units, more or less independent of each other, and all capable of growth and multiplication. This doctrine is commonly regarded as being inaugurated in 1839 by Schwann. Long before this, however, many microscopists had seen that the bodies of plants are made up of elementary units. In describing the bark of a tree in 1665, Robert Hooke had stated that it was composed of little boxes or cells, and regarded it as a sort of honeycomb structure with its cells filled with air. The term cell quite aptly describes the compartments of such a structure, as can be seen by a glance at Fig. 7, and this term has been retained even till to-day in spite of the fact that its original significance has entirely disappeared. During the last century not a few naturalists observed and described these little vesicles, always regarding them as little spaces and never looking upon them as having any significance in the activities of plants. In one or two instances similar bodies were noticed in animals, although no connection was drawn between them and the cells of plants. In the early part of the century observations upon various kinds of animals and plant tissues multiplied, and many microscopists independently announced the discovery of similar small corpuscular bodies. Finally, in 1839, these observations were combined together by Schwann into one general theory. According to the cell doctrine then formulated, the parts of all animals and plants are either composed of cells or of material derived from cells. The bark, the wood, the roots, the leaves of plants are all composed of little vesicles similar to those already described under the name of cells. In animals the cellular structure is not so easy to make out; but here too the muscle, the bone, the nerve, the gland are all made up of similar vesicles or of material made from them. The cells are of wonderfully different shapes and widely different sizes, but in general structure they are alike. These cells, thus found in animals and plants alike, formed the first connecting link between animals and plants. This discovery was like that of our supposed supramundane observer when he first found the human being that brought into connection the widely different cities in the various parts of the world.

Schwann and his immediate followers, while recognizing that the bodies of animals and plants were composed of cells, were at a loss to explain how these cells arose. The belief held at first was that there existed in the bodies of animals and plants a structureless substance which formed the basis out of which the cells develop, in somewhat the same way that crystals arise from a mother liquid. This supposed substance Schwann called the _cytoblastema_, and he thought it existed between the cells or sometimes within them. For example, the fluid part of the blood is the cytoblastema, the blood corpuscles being the cells. From this structureless fluid the cells were supposed to arise by a process akin to crystallization. To be sure, the cells grow in a manner very different from that of a crystal. A crystal always grows by layers being added upon its outside, while the cells grow by additions within its body. But this was a minor detail, the essential point being that from a structureless liquid containing proper materials the organized cell separated itself.

This idea of the cytoblastema was early thrown into suspicion, and almost at the time of the announcement of the cell doctrine certain microscopists made the claim that these cells did not come from any structureless medium, but by division from other cells like themselves. This claim, and its demonstration, was of even greater importance than the discovery of the cells. For a number of years, however, the matter was in dispute, evidence being collected which about equally attested each view. It was a Scotchman, Dr. Barry, who finally produced evidence which settled the question from the study of the developing egg.

The essence of his discovery was as follows: The ovum of an animal is a single cell, and when it begins to develop into an embryo it first simply divides into two halves, producing two cells (Fig, 8, _a_ and _b_). Each of these in turn divides, giving four, and by repeated divisions of this kind there arises a solid mass of smaller cells (Fig. 8, _b_ to _f_,) called the mulberry stage, from its resemblance to a berry. This is, of course, simply a mass of cells, each derived by division from the original. As the cells increase in number, the mass also increases in size by the absorption of nutriment, and the cells continue dividing until the mass contains thousands of cells. Meantime the body of the animal is formed out of these cells, and when it is adult it consists of millions of cells, all of which have been derived by division from the original cell. In such a history each cell comes from pre-existing cells and a cytoblastema plays no part.

It was impossible, however, for Barry or any other person to follow the successive divisions of the egg cell through all the stages to the adult. The divisions can be followed for a short time under the microscope, but the rest must be a matter of simple inference. It was argued that since cell origin begins in this way by simple division, and since the same process can be observed in the adult, it is reasonable to assume that the same process has continued uninterruptedly, and that this is the only method of cell origin. But a final demonstration of this conclusion was not forthcoming for a long time. For many years some biologists continued to believe that cells can have other origin than from pre-existing cells. Year by year has the evidence for such "free cell" origin become less, until the view has been entirely abandoned, and to-day it is everywhere admitted that new cells always arise from old ones by direct descent, and thus every cell in the body of an animal or plant is a direct descendant by division from the original egg cell.

==The Cell==.--But what is this cell which forms the unit of life, and to which all the fundamental vital properties can be traced? We will first glance at the structure of the cell as it was understood by the earlier microscopists. A typical cell is shown in Fig. 9. It will be seen that it consists of three quite distinct parts. There is first the _cell wall (cw)_ which is a limiting membrane of varying thickness and shape. This is in reality lifeless material, and is secreted by the rest of the cell. Being thus produced by the other active parts of the cell, we will speak of it as _formed_ material in distinction from the rest, which is _active_ material. Inside this vesicle is contained a somewhat transparent semifluid material which has received various names, but which for the present we will call _cell substance_ (Fig. 9, _pr_). It may be abundant or scanty, and has a widely varying consistency from a very liquid mass to a decidedly thick jellylike substance. Lying within the cell substance is a small body, usually more or less spherical in shape, which is called the _nucleus_ (Fig. 9, _n_). It appears to the microscope similar to the cell substance in character, and has frequently been described as a bit of the cell substance more dense than the remainder. Lying within the nucleus there are usually to be seen one or more smaller rounded bodies which have been called _nucleoli_. From the very earliest period that cells have been studied, these three parts, cell wall, cell substance, and nucleus have been recognized, but as to their relations to each other and to the general activities of the cell there has been the widest variety of opinion.

==Cellular Structure of Organisms==.--It will be well to notice next just what is meant by saying that all living bodies are composed of cells. This can best be understood by referring to the accompanying figures. Figs. 10-14, for instance, show the microscopic appearance of several plant tissues.

At Fig. 10 will be seen the tip of a root, plainly made of cells quite similar to the typical cell described. At Fig. 11 will be seen a bit of a leaf showing the same general structure. At Fig. 12 is a bit of plant tissue of which the cell walls are very thick, so that a very dense structure is formed. At Fig. 13 is a bit of a potato showing its cells filled with small granules of starch which the cells have produced by their activities and deposited within their own bodies. At Fig. 14 are several wood cells showing cell walls of different shape which, having become dead, have lost their contents and simply remain as dead cell walls. Each was in its earlier history filled with cell substance and contained a nucleus. In a similar way any bit of vegetable tissue would readily show itself to be made of similar cells.

In animal tissues the cellular structure is not so easily seen, largely because the products made by the cells, the formed products, become relatively more abundant and the cells themselves not so prominent. But the cellular structure is none the less demonstrable. In Fig. 15, for instance, will be seen a bit of cartilage where the cells themselves are rather small, while the material deposited between them is abundant. 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 mass 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 mass 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 mass 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.

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.

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 assume 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 mass of cells, with great variety in shape and function.

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 animalculæ 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 mass 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.

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, assimilation 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.

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 destitute 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 assimilation, 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 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.

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, assimilation, 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 mass 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 mass 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 homogeneous mass 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 quantity and pure enough to make a careful analysis. The difficulties were in time, however, overcome, and chemical study showed protoplasm to be a proteid, related to other proteids like albumen, 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 analysis 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 albumen 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 albumen, 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 substituting 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 passing 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 albumens, 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 assumption 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 albumen 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 assumption 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, assimilation, 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 assumption to make, that this substance protoplasm could have this power, and from this the other vital activities are easily derived.

In other words, the formulation of the doctrine of protoplasm made it possible to assume that _life_ is not a distinct force, but simply a name given to the properties possessed by that highly complex chemical compound protoplasm. Just as we might give the name _aquacity_ to the properties possessed by water, so we have actually given the name _vitality_ to the properties possessed by protoplasm. To be sure, vitality is more marvelous than aquacity, but so is protoplasm a more complex compound than water. This compound was a very unstable compound, just as is a mass of gunpowder, and hence it is highly irritable, also like gunpowder, and any disturbance of its condition produces motion, just as a spark will do in a mass of gunpowder. It is capable of inducing oxidation in foods, something as water induces oxidation in a bit of iron. The oxidation is, however, of a different kind, and results in the formation of different chemical combinations; but it is the basis of assimilation. Since now assimilation is the foundation of growth and reproduction, this mechanical theory of life thus succeeded in tracing to the simple properties of the chemical compound protoplasm, all the fundamental properties of life. Since further, as we have seen in our first chapter, the more complex properties of higher organisms are easily deduced from these simple ones by the application of the laws of mechanics, we have here in this mechanical theory of life the complete reduction of the body to a machine.

==The Reign of Protoplasm.==--This substance protoplasm became now naturally the centre of biological thought. The theory of protoplasm arose at about the same time that the doctrine of evolution began to be seriously discussed under the stimulus of Darwin, and naturally these two great conceptions developed side by side. Evolution was constantly teaching that natural forces are sufficient to account for many of the complex phenomena which had hitherto been regarded as insolvable; and what more natural than the same kind of thinking should be applied to the vital activities manifested by this substance protoplasm. While the study of plants and animals was showing scientists that natural forces would explain the origin of more complex types from simpler ones through the law of natural selection, here in this conception of protoplasm was a theory which promised to show how the simplest forms may have been derived from the non-living. For an explanation of the _origin_ of life by natural means appeared now to be a simple matter.

It required now no violent stretch of the imagination to explain the origin of life something as follows: We know that the chemical elements have certain affinities for each other, and will unite with each other under proper conditions. We know that the methods of union and the resulting compounds vary with the conditions under which the union takes place. We know further that the elements carbon, hydrogen, oxygen, and nitrogen have most remarkable properties, and unite to form an almost endless series of remarkable bodies when brought into combination under different conditions. We know that by varying the conditions the chemist can force these elements to unite into a most extraordinary variety of compounds with an equal variety of properties. What more natural, then, than the assumption that under certain conditions these same elements would unite in such a way as to form this compound protoplasm; and then, if the ideas concerning protoplasm were correct, this body would show the properties of protoplasm, and therefore be alive. Certainly such a supposition was not absurd, and viewed in the light of the rapid advance in the manufacture of organic compounds could hardly be called improbable. Chemists beginning with simple bodies like CO_{2} and H_{2}O were climbing the ladder, each round of which was represented by compounds of higher complexity. At the top was protoplasm, and each year saw our chemists nearer the top of the ladder, and thus approaching protoplasm as their final goal. They now began to predict that only a few more years would be required for chemists to discover the proper conditions, and thus make protoplasm. As late as 1880 the prediction was freely made that the next great discovery would be the manufacture of a bit of protoplasm by artificial means, and thus in the artificial production of life. The rapid advance in organic chemistry rendered this prediction each year more and more probable. The ability of chemists to manufacture chemical compounds appeared to be unlimited, and the only question in regard to their ability to make protoplasm thus resolved itself into the question of whether protoplasm is really a chemical compound.

We can easily understand how eager biologists became now in pursuit of the goal which seemed almost within their reach; how interested they were in any new discovery, and how eagerly they sought for lower and simpler types of protoplasm since these would be a step nearer to the earliest undifferentiated life substance. Indeed so eager was this pursuit for pure undifferentiated protoplasm, that it led to one of those unfounded discoveries which time showed to be purely imaginary. When this reign of protoplasm was at its height and biologists were seeking for even greater simplicity a most astounding discovery was announced. The British exploring ship Challenger had returned from its voyage of discovery and collection, and its various treasures were turned over to the different scientists for study. The brilliant Prof. Huxley, who had first formulated the mechanical theory of life, now startled the biological world with the statement that these collections had shown him that at the bottom of the deep sea, in certain parts of the world, there exists a diffused mass of living _undifferentiated protoplasm_. So simple and undifferentiated was it that it was not divided into cells and contained no nucleii. It was, in short, exactly the kind of primitive protoplasm which the evolutionist wanted to complete his chain of living structures, and the biologist wanted to serve as a foundation for his mechanical theory of life. If such a diffused mass of undifferentiated protoplasm existed at the bottom of the sea, one could hardly doubt that it was developed there by some purely natural forces. The discovery was a startling one, for it seemed that the actual starting point of life had been reached. Huxley named his substance _Bathybias_, and this name became in a short time familiar to every one who was thinking of the problems of life. But the discovery was suspected from the first, because it was too closely in accord with speculation, and it was soon disproved. Its discoverer soon after courageously announced to the world that he had been entirely mistaken, and that the Bathybias, so far from being undifferentiated protoplasm, was not an organic product at all, but simply a mineral deposit in the sea water made by purely artificial means. Bathybias stands therefore as an instance of a too precipitate advance in speculation, which led even such a brilliant man as Prof. Huxley into an unfortunate error of observation; for, beyond question, he would never have made such a mistake had he not been dominated by his speculative theories as to the nature of protoplasm.

But although Bathybias proved delusive, this did not materially affect the advance and development of the doctrine of protoplasm. Simple forms of protoplasm were found, although none quite so simple as the hypothetical Bathybias. The universal presence of protoplasm in the living parts of all animals and plants and its manifest activities completely demonstrated that it was the only living substance, and as the result of a few years of experiment and thought the biologist's conception of life crystallized into something like this: Living organisms are made of cells, but these cells are simply minute independent bits of protoplasm. They may contain a nucleus or they may not, but the essence of the cell is the protoplasm, this alone having the fundamental activities of life. These bits of living matter aggregate themselves together into groups to form colonies. Such colonies are animals or plants. The cells divide the work of the colony among themselves, each cell adopting a form best adapted for the special work it has to do. The animal or plant is thus simply an aggregate of cells, and its activities are the sum of the activities of its separate cells; just as the activities of a city are the sum of the activities of its individual inhabitants. The bit of protoplasm was the unit, and this was a chemical compound or a simple mixture of compounds to whose combined physical properties we have given the name vitality.

==The Decline of the Reign of Protoplasm.==--Hardly had this extreme chemical theory of life been clearly conceived before accumulating facts began to show that it is untenable and that it must at least be vastly modified before it can be received. The foundation of the chemical theory of life was the conception that protoplasm is a definite though complex chemical compound. But after a few years' study it appeared that such a conception of protoplasm was incorrect. It had long been suspected that protoplasm was more complex than was at first thought. It was not even at the outset found to be perfectly homogeneous, but was seen to contain minute granules, together with bodies of larger size. Although these bodies were seen they were regarded as accidental or secondary, and were not thought of as forming any serious objection to the conception of protoplasm as a definite chemical compound. But modern opticians improved their microscopes, and microscopists greatly improved their methods. With the new microscopes and new methods there began to appear, about twenty years ago, new revelations in regard to this protoplasm. Its lack of homogeneity became more evident, until there has finally been disclosed to us the significant fact that protoplasm is to be regarded as a substance not only of chemical but also of high mechanical complexity. The idea of this material as a simple homogeneous compound or as a mixture of such compounds is absolutely fallacious. Protoplasm is to-day known to be made up of parts harmoniously adapted to each other in such a way as to form an extraordinarily intricate machine; and the microscopist of to-day recognizes clearly that the activities of this material must be regarded as the result of the machinery which makes up protoplasm rather than as the simple result of its chemical composition. Protoplasm is a machine and not a chemical compound.

==Structure of Protoplasm==.--The structure of protoplasm is not yet thoroughly understood by scientists, but a few general facts are known beyond question. It is thought, in the first place, that it consists of two quite different substances. There is a somewhat solid material permeating it, usually, regarded as having a reticulate structure. It is variously described, sometimes as a reticulate network, sometimes as a mass of threads or fibres, and sometimes as a mass of foam (Fig. 23, _a_). It is extremely delicate and only visible under special conditions and with the best of microscopes. Only under peculiar conditions can it be seen in protoplasm while alive. There is no question, however, that all protoplasm is permeated when alive by a minute delicate mass of material, which may take the form of threads or fibres or may assume other forms. Within the meshes of this thread or reticulum there is found a liquid, perfectly clear and transparent, to whose presence the liquid character of the protoplasm is due (Fig. 23, _b_). In this liquid no structure can be determined, and, so far as we know, it is homogeneous. Still further study discloses other complexities. It appears that the fibrous material is always marked by the presence of excessively minute bodies, which have been called by various names, but which we will speak of as _microsomes_. Sometimes, indeed, the fibres themselves appear almost like strings of beads, so that they have been described as made up of rows of minute elements. It is immaterial for our purpose, however, whether the fibres are to be regarded as made up of microsomes or not. This much is sure, that these microsomes --granules of excessive minuteness--occur in protoplasm and are closely connected with the fibres (Fig. 23, _a_).

==The Nucleus.==--(a) _Presence of a Nucleus_.--If protoplasm has thus become a new substance in our minds as the result of the discoveries of the last twenty years, far more marvelous have been the discoveries made in connection with that body which has been called the nucleus. Even by the early microscopists the nucleus was recognized, and during the first few years of the cell doctrine it was frequently looked upon as the most active part of the cell and as especially connected with its reproduction. The doctrine of protoplasm, however, so captivated the minds of biologists that for quite a number of years the nucleus was ignored, at least in all discussions connected with the nature of life. It was a body in the cell whose presence was unexplained and which did not fall into accord with the general view of protoplasm as the physical basis of life. For a while, therefore, biologists gave little attention to it, and were accustomed to speak of it simply as a bit of protoplasm a little more dense than the rest. The cell was a bit of protoplasm with a small piece of more dense protoplasm in its centre appearing a little different from the rest and perhaps the most active part of the cell.

As a result of this excessive belief in the efficiency of protoplasm the question of the presence of a nucleus in the cell was for a while looked upon as one of comparatively little importance. Many cells were found to have nucleii while others did not show their presence, and microscopists therefore believed that the presence of a nucleus was not necessary to constitute a cell. A German naturalist recognized among lower animals one group whose distinctive characteristic was that they were made of cells without nucleii, giving the name _Monera_ to the group. As the method of studying cells improved microscopists learned better methods of discerning the presence of the nucleus, and as it was done little by little they began to find the presence of nucleii in cells in which they had hitherto not been seen. As microscopists now studied one after another of these animals and plants whose cells had been said to contain no nucleus, they began to find nucleii in them, until the conclusion was finally reached that a nucleus is a fundamental part of all active cells. Old cells which have lost their activity may not show nucleii, but, so far as we know, all active cells possess these structures, and apparently no cell can carry on its activity without them. Some cells have several nucleii, and others have the nuclear matter scattered through the whole cell instead of being aggregated into a mass; but nuclear matter the cell must have to carry on its life.

Later the experiment was made of depriving cells of their nucleii, and it still further emphasized the importance of the nucleus. Among unicellular animals are some which are large enough for direct manipulation, and it is found that if these cells are cut into pieces the different pieces will behave very differently in accordance with whether or not they have within them a piece of the nucleus. All the pieces are capable of carrying on their life activities for a while. The pieces of the cell which contain the nucleus of the original cell, or even a part of it, are capable of carrying on all its life activities perfectly well. In Fig. 24 is shown such a cell cut into three pieces, each of which contains a piece of the nucleus. Each carries on its life activities, feeds, grows and multiplies perfectly well, the life processes seeming to continue as if nothing had happened. Quite different is it with fragments which contain none of the nucleus (Fig. 25). These fragments (1 and 3), even though they may be comparatively large masses of protoplasm, are incapable of carrying on the functions of their life continuously. For a while they continue to move around and apparently act like the other fragments, but after a little their life ceases. They are incapable of assimilating food and incapable of reproduction, and hence their life cannot continue very long. Facts like these demonstrate conclusively the vital importance of the nucleus in cell activity, and show us that the cell, with its power of continued life, must be regarded as a combination of protoplasm with its nucleus, and cannot exist without it. It is not protoplasm, but cell substance, plus cell nucleus, which forms the simplest basis of life.

As more careful study of protoplasm was made it soon became evident that there is a very decided difference between the nucleus and the protoplasm. The old statement that the nucleus is simply a bit of dense protoplasm is not true. In its chemical and physical composition as well as in its activities the nucleus shows itself to be entirely different from the protoplasm. It contains certain definite bodies not found in the cell substance, and it goes through a series of activities which are entirely unrepresented in the surrounding protoplasm. It is something entirely distinct, and its relations to the life of the cell are unique and marvelous. These various facts led to a period in the discussion of biological topics which may not inappropriately be called the Reign of the Nucleus. Let us, therefore, see what this structure is which has demanded so much attention in the last twenty years.

(b) _Structure of the Nucleus_.--At first the nucleus appears to be very much like the cell substance. Like the latter, it is made of fibres, which form a reticulum (Fig. 23), and these fibres, like those of protoplasm, have microsomes in intimate relation with them and hold a clear liquid in their meshes. The meshes of the network are usually rather closer than in the outer cell substance, but their general character appears to be the same. But a more close study of the nucleus discloses vast differences. In the first place, the nucleus is usually separated from the cell substance by a membrane (Fig. 23, _c_). This membrane is almost always present, but it may disappear, and usually does disappear, when the nucleus begins to divide. Within the nucleus we find commonly one or two smaller bodies, the nucleoli (Fig. 23, _f_). They appear to be distinct vital parts of the nucleus, and thus different from certain other solid bodies which are simply excreted material, and hence lifeless. Further, we find that the reticulum within the nucleus is made up of two very different parts. One portion is apparently identical with the reticulum of the cell substance (Fig. 23, _d_). This forms an extremely delicate network, whose fibres have chemical relations similar to those of the cell substance. Indeed, sometimes, the fibres of the nucleus may be seen to pass directly into those of the network of the cell substance, and hence they are in all probability identical. This material is called _linin_, by which name we shall hereafter refer to it. There is, however, in the nucleus another material which forms either threads, or a network, or a mass of granules, which is very different from the linin, and has entirely different properties. This network has the power of absorbing certain kinds of stains very actively, and is consequently deeply stained when treated as the microscopist commonly prepares his specimens. For this reason it has been named _chromatin_ (Fig, 23, _e_), although in more recent times other names have been given to it. Of all parts of the cell this chromatin is the most remarkable. It appears in great variety in different cells, but it always has remarkable physiological properties, as will be noticed presently. All things considered, this chromatin is probably the most remarkable body connected with organic life.

The nucleii of different animals and plants all show essentially the characteristics just described. They all contain a liquid, a linin network, and a chromatin thread or network, but they differ most remarkably in details, so that the variety among the nucleii is almost endless (Fig. 26). They differ first in their size relative to the size of the cell; sometimes--especially in young cells--the nucleus being very large, while in other cases the nucleus is very small and the protoplasmic contents of the cell very large; finally, in cells which have lost their activity the nucleus may almost or entirely disappear. They differ, secondly, in shape. The typical form appears to be spherical or nearly so; but from this typical form they may vary, becoming irregular or elongated. They are sometimes drawn out into long masses looking like a string of beads (Fig. 24), or, again, resembling minute coiled worms (Fig. 21), while in still other cells they may be branching like the twigs of a tree. The form and shape of the chromatin thread differs widely. Sometimes this appears to be mere reticulum (Fig. 23); at others, a short thread which is somewhat twisted or coiled (Fig. 26); while in other cells the chromatin thread is an extremely long, very much twisted convolute thread so complexly woven into a tangle as to give the appearance of a minute network. The nucleii differ also in the number of nucleoli they contain as well as in other less important particulars. Fig. 26 will give a little notion of the variety to be found among different nucleii; but although they thus do vary most remarkably in shape in the essential parts of their structure they are alike.

==Centrosome.==--Before noticing the activities of the nucleus it will be necessary to mention a third part of the cell. Within the last few years there has been found to be present in most cells an organ which has been called the _centrosome._ This body is shown at Fig. 23, _g_. It is found in the cell substance just outside the nucleus, and commonly appears as an extremely minute rounded dot, so minute that no internal structure has been discerned. It may be no larger than the minute granules or microsomes in the cell, and until recently it entirely escaped the notice of microscopists. It has now, however, been clearly demonstrated as an active part of the cell and entirely distinct from the ordinary microsomes. It stains differently, and, as we shall soon see, it appears to be in most intimate connection with the center of cell life. In the activities which characterize cell life this centrosome appears to lead the way. From it radiate the forces which control cell activity, and hence this centrosome is sometimes called the dynamic center of the cell. This leads us to the study of cell activity, which discloses to us some of the most extraordinary phenomena which have come to the knowledge of science.

==Function of the Nucleus.==--To understand why it is that the nucleus has taken such a prominent position in modern biological discussion it will be only necessary to notice some of the activities of the cell. Of the four fundamental vital properties of cell life the one which has been most studied and in regard to which most is known is reproduction. This knowledge appears chiefly under two heads, viz., _cell division_ and the _fertilization of the egg_. Every animal and plant begins its life as a simple cell, and the growth of the cell into the adult is simply the division of the original cell into parts accompanied by a differentiation of the parts. The fundamental phenomena of growth and reproduction is thus cell division, and if we can comprehend this process in these simple cells we shall certainly have taken a great step toward the explanation of the mechanics of life. During the last ten years this cell division has been most thoroughly studied, and we have a pretty good knowledge of it so far as its microscopical features are concerned. The following description will outline the general facts of such cell division, and will apply with considerable accuracy to all cases of cell division, although the details may differ not a little.

==Cell Division or Karyokinesis.==--We will begin with a cell in what is called the resting stage, shown at Fig. 23. Such a cell has a nucleus, with its chromatin, its membrane, and linin, as already described. Outside the nucleus is the centrosome, or, more commonly, two of them lying close together. If there is only one it soon divides into two, and if it has already two, this is because a single centrosome which the cell originally possessed has already divided into two, as we shall presently see. This cell, in short, is precisely like the typical cell which we have described, except in the possession of two centrosomes. The first indication of the cell division is shown by the chromatin fibres. During the resting stage this chromatin material may have the form of a thread, or may form a network of fibres (see Fig. 27). But whatever be its form during the resting stage, it assumes the form of a thread as the cell prepares for division. Almost at once this thread breaks into a number of pieces known as _chromosomes_ (Fig. 28). It is an extremely important fact that the number of these chromosomes in the ordinary cells of any animal or plant is always the same. In other words, in all the cells of the body of animal or plant the chromatin material in the nucleus breaks into the same number of short threads at the time that the cell is preparing to divide. The number is the same for all animals of the same species, and is never departed from. For example, the number in the ox is always sixteen, while the number in the lily is always twenty-four. During this process of the formation of the chromosomes the nucleoli disappear, sometimes being absorbed apparently in the chromosomes, and sometimes being ejected into the cell body, where they disappear. Whether they have anything to do with further changes is not yet known.

The next step in the process of division appears in the region of the centrosomes. Each of the two centrosomes appears to send out from itself delicate radiating fibres into the surrounding cell substance (Fig. 28). Whether these actually arise from the centrosome or are simply a rearrangement of the fibres in the cell substance is not clear, but at all events the centrosome becomes surrounded by a mass of radiating fibres which give it a starlike appearance, or, more commonly, the appearance of a double star, since there are two centrosomes close together (Fig. 28). These radiating fibres, whether arising from the centrosomes or not, certainly all centre in these bodies, a fact which indicates that the centrosomes contain the forces which regulate their appearance. Between the two stars or asters a set of fibres can be seen running from one to the other (Fig. 29). These two asters and the centrosomes within them have been spoken of as the dynamic centre of the cell since they appear to control the forces which lead to cell division. In all the changes which follow these asters lead the way. The two asters, with their centrosomes, now move away from each other, always connected by the spindle fibres, and finally come to lie on opposite sides of the nucleus (Figs. 29, 30). When they reach this position they are still surrounded by the radiating fibres, and connected by the spindle fibres. Meantime the membrane around the nucleus has disappeared, and thus the spindle fibres readily penetrate into the nuclear substance (Fig. 30).

During this time the chromosomes have been changing their position. Whether this change in position is due to forces within themselves, or whether they are moved around passively by forces residing in the cell substances, or whether, which is the most probable, they are pulled or pushed around by the spindle fibres which are forcing their way into the nucleus, is not positively known; nor is it, for our purposes, of special importance. At all events, the result is that when the asters have assumed their position at opposite poles of the nucleus the chromosomes are arranged in a plane passing through the middle of the nucleus at equal distances from each aster. It seems certain that they are pulled or pushed into this position by forces radiating from the centrosomes. Fig. 30 shows this central arrangement of the chromosomes, forming what is called the _equatorial plate_.

The next step is the most significant of all. It consists in the splitting of each chromosome into two equal halves. The threads _do not divide in their middle but split lengthwise_, so that there are formed two halves identical in every respect. In this way are produced twice the original number of chromosomes, but all in pairs. The period at which this splitting of the chromosomes occurs is not the same in all cells. It may occur, as described, at about the time the asters have reached the opposite poles of the nucleus, and an equatorial plate is formed. It is not infrequent, however, for it to occur at a period considerably earlier, so that the chromosomes are already divided when they are brought into the equatorial plate.

At some period or other in the cell division this splitting of the chromosomes takes place. The significance of the splitting is especially noteworthy. We shall soon find reason for believing that the chromosomes contain all the hereditary traits which the cell hands down from generation to generation, and indeed that the chromosomes of the egg contain all the traits which the parent hands down to the child. Now, if this chromatin thread consists of a series of units, each representing certain hereditary characters, then it is plain that the division of the thread by splitting will give rise to a double series of threads, each of which has identical characters. Should the division occur _across_ the thread the two halves would be unlike, but taking place as it does by a _longitudinal splitting_ each unit in the thread simply divides in half, and thus the resulting half threads each contain the same number of similar units as the other and the same as possessed by the original undivided chromosome. This sort of splitting thus doubles the number of chromosomes, but produces no differentiation of material.

The next step in the cell division consists in the separation of the two halves of the chromosomes. Each half of each chromosome separates from its fellow, and moves to the opposite end of the nucleus toward the two centrosomes (Fig. 31). Whether they are pulled apart or pushed apart by the spindle fibres is not certain, although it is apparently sure that these fibres from the centrosomes are engaged in the matter. Certain it is that some force exerted from the two centrosomes acts upon the chromosomes, and forces the two halves of each one to opposite ends of the nucleus, where they now collect and form two _new nucleii_, with evidently exactly the same number of chromosomes as the original, and with characters identical to each other and to the original (Fig. 32).

The rest of the cell division now follows rapidly. A partition grows in through the cell body dividing it into two parts (Fig. 32), the division passing through the middle of the spindle. In this division, in some cases at least, the spindle fibres bear a part--a fact which again points to the importance of the centrosomes and the forces which radiate from them. Now the chromosomes in each daughter nucleus unite to form a single thread, or may diffuse through the nucleus to form a network, as in Fig. 32. They now become surrounded by a membrane, so that the new nucleus appears exactly like the original one. The spindle fibres disappear, and the astral fibres may either disappear or remain visible. The centrosome may apparently in some cases disappear, but more commonly remains beside the daughter nucleii, or it may move into the nucleus. Eventually it divides into two, the division commonly occurring at once (Fig. 32), but sometimes not until the next cell division is about to begin. Thus the final result shows two cells each with a nucleus and two centrosomes, and this is exactly the same sort of structure with which the process began. (_See Frontispiece_.)

Viewed as a whole, we may make the following general summary of this process. The essential object of this complicated phenomena of _karyokinesis_ is to divide the chromatin into equivalent halves, so that the cells resulting from the cell division shall contain an exactly equivalent chromatin content. For this purpose the chromatic elements collect into threads and split lengthwise. The centrosome, with its fibres, brings about the separation of these two halves. Plainly, we must conclude that the chromatin material is something of extraordinary importance to the cell, and the centrosome is a bit of machinery for controlling its division and thus regulating cell division.

==Fertilization of the Egg.==--This description of cell division will certainly give some idea of the complexity of cell life, but a more marvelous series of changes still takes place during the time when the egg is preparing for development. Inasmuch as this process still further illustrates the nature of the cell, and has further a most intimate bearing upon the fundamental problem of heredity, it will be necessary for us to consider it here briefly.

The sexual reproduction of the many-celled animals is always essentially alike. A single one of the body cells is set apart to start the next generation, and this cell, after separating from the body of the animal or plant which produced it, begins to divide, as already shown in Fig. 8, and the many cells which arise from it eventually form the new individual This reproductive cell is the egg. But before its division can begin there occurs in all cases of sexual reproduction a process called fertilization, the essential feature of which is the union of this cell with another commonly from a different individual. While the phenomenon is subject to considerable difference in details, it is essentially as follows:

The female reproductive cell is called the egg, and it is this cell which divides to form the next generation. Such a cell is shown in Fig. 33. Like other cells it has a cell wall, a cell substance with its linin and fluid portions, a nucleus surrounded by a membrane and containing a reticulum, a nucleolus and chromatic material, and lastly, a centrosome. Now such an egg is a complete cell, but it is not able to begin the process of division which shall give rise to a new individual until it has united with another cell of quite a different sort and commonly derived from a different individual called the male. Why the egg cell is unable to develop without such union with male cell does not concern us here, but its purpose will be evident as the description proceeds. The egg cell as it comes from the ovary of the female individual is, however, not yet ready for union with the male cell, but must first go through a series of somewhat remarkable changes constituting what is called _maturation_ of the egg. This phenomenon has such an intimate relation to all problems connected with the cell, that it must be described somewhat in detail. There are considerable differences in the details of the process as it occurs in various animals, but they all agree in the fundamental points. The following is a general description of the process derived from the study of a large variety of animals and plants.

In the cells of the body of the animal to which this description applies there are four chromosomes This is true of all the cells of the animal except the sexual cells. The eggs arise from the other cells of the body, but during their growth the chromatin splits in such a way that the egg contains double the number of chromosomes, i.e., eight (Fig. 34). If this egg should now unite with the other reproductive cell from the male, the resulting fertilized egg would plainly contain a number of chromosomes larger than that normal for this species of animal. As a result the next generation would have a larger number of chromosomes in each cell than the last generation, since the division of the egg in development is like that already described and always results in producing new cells with the same number of chromosomes as the starting cell. Hence, if the number of chromosomes in the next generation is to be kept equal to that in the last generation, this egg cell must get rid of a part of its chromatin material. This is done by a process shown in Fig. 35. The centrosome divides as in ordinary cell division (Fig. 35), and after rotating on its axis it approaches the surface of the egg (Figs. 36 and 37). The egg now divides (Fig. 38), but the division is of a peculiar kind. Although the chromosomes divide equally the egg itself divides into two very unequal parts, one part still appearing as the egg and the other as a minute protuberance called the polar cell (_pc'_ in Fig. 38). The chromosomes do not split as they do in the cell division already described, but each of these two cells, the egg and the polar body, receives four chromosomes (Fig. 38). The result is that the egg has now the normal number of chromosomes for the ordinary cells of the animal in question. But this is still too many, for the egg is soon to unite with the male cell; and this male cell, as we shall see, is to bring in its own quota of chromosomes. Hence the egg must get rid of still more of its chromatin material. Consequently, the first division is followed by a second (Fig. 39), in which there is again produced a large and a small cell. This division, like the first, occurs without any splitting of the chromosomes, one half of the remaining chromosomes being ejected in this new cell, the second polar cell (_pc"_) leaving the larger cell, the egg, with just one half the number of chromosomes normal for the cells of the animal in question. Meantime the first pole cell has also divided, so that we have now, as shown in Fig. 40, four cells, three small and one large, but each containing one half the normal number of chromosomes. In the example figured, four is the normal number for the cells of the animal. The egg at the beginning of the process contained eight, but has now been reduced to two. In the further history of the egg the smaller cells, called _polar cells_, take no part, since they soon disappear and have nothing to do with the animal which is to result from the further division of the egg. This process of the formation of the polar cells is thus simply a device for getting rid of some of the chromatin material in the egg cell, so that it may unite with a second cell without doubling the normal number of chromosomes.

Previously to this process the other sexual cell, the _spermatozoon_, or male reproductive cell, has been undergoing a somewhat similar process. This is also a true cell (Fig. 34, _mc_), although it is of a decidedly smaller size than the egg and of a very different shape. It contains cell substance, a nucleus with chromosomes, and a centrosome, the number of chromosomes, as shown later, being however only half that normal for the ordinary cells of the animals. The study of the development of the spermatozoon shows that it has come from cells which contained the normal number of four, but that this number has been reduced to one half by a process which is equivalent to that which we have just noticed in the egg. Thus it comes about that each of the sexual elements, the egg and the spermatozoon, now contains one half the normal number of chromosomes.

Now by some mechanical means these two reproductive cells are brought in contact with each other, shown in Fig. 34, and as soon as they are brought into each other's vicinity the male cell buries its head in the body of the egg. The tail by which it has been moving is cast off, and the head containing the chromosomes and the centrosome enters the egg, forming what is called the male pronucleus (Figs. 35-38, _mn_). This entrance of the male cell occurs either before the formation of the polar cells of the egg or afterward. If, however, it takes place before, the male pronucleus simply remains dormant in the egg while the polar cells are being protruded, and not until after that process is concluded does it begin again to show signs of activity which result in the cell union.

The further steps in this process appear to be controlled by the centrosome, although it is not quite certain whence this centrosome is derived. Originally, as we have seen, the egg contained a centrosome, and the male cell has also brought a second into the egg (Fig. 35, _ce_). In some cases, and this is true for the worm we are describing, it is certain that the egg centrosome disappears while that of the spermatozoon is retained alone to direct the further activities (Fig. 41). Possibly this may be the case in all eggs, but it is not sure. It is a matter of some little interest to have this settled, for if it should prove true, then it would evidently follow that the machinery for cell division, in the case of sexual reproduction, is derived from the father, although the bulk of the cell comes from the mother, while the chromosomes come from both parents.

In the cases where the process has been most carefully studied, the further changes are as follows: The head of the spermatozoon, after entrance into the egg, lies dormant until the egg has thrown off its polar cells, and thus gotten rid of part of its chromosomes. Close to it lies its centrosomes (Fig. 35, _ce_), and there is thus formed what is known as the _male pronucleus_ (Fig. 35-40, _mn_). The remains of the egg nucleus, after having discharged the polar cells, form the _female nucleus_ (Fig. 40, _fn_). The chromatin material, in both the male and female pronucleus, soon breaks up into a network in which it is no longer possible to see that each contains two chromosomes (Fig. 41). Now the centrosome, which is beside the male pronucleus, shows signs of activity. It becomes surrounded by prominent rays to form an aster (Fig. 41, _ce_), and then it begins to move toward the female pronucleus, apparently dragging the male pronucleus after it. In this way the centrosome approaches the female pronucleus, and thus finally the two nucleii are brought into close proximity. Meantime the chromatin material in each has once more broken up into short threads or chromosomes, and once more we find that each of the nucleii contains two of these bodies (Fig. 42). In the subsequent figures the chromosomes of the male nucleus are lightly shaded, while those of the female are black in order to distinguish them. As these two nucleii finally come together their membranes disappear, and the chromatic material comes to lie freely in the egg, the male and female chromosomes, side by side, but distinct forming the _segmentation nucleus_. The egg plainly now contains once more the number of chromosomes normal for the cells of the animal, but half of them have been derived from each parent. It is very suggestive to find further that the chromosomes in this _fertilized egg_ do not fuse with each other, but remain quite distinct, so that it can be seen that the new nucleus contains chromosomes derived from each parent (Fig. 42). Nor does there appear to be, in the future history of this egg, any actual fusion of the chromatic material, the male and female chromosomes perhaps always remaining distinct.

While this mixture of chromosomes has been taking place the centrosome has divided into two parts, each of which becomes surrounded by an aster and travels to opposite ends of the nucleus (Fig. 42). There now follows a division of the nucleus exactly similar to that which occurs in the normal cell division already described in Figs. 28-34. Each of the chromosomes splits lengthwise (Fig. 43), and one half of each then travels toward each centrosome to form a new nucleus (Fig. 44). Since each of the four chromosomes thus splits, it follows that each of the two daughter nucleii will, of course, contain four chromosomes; two of which have been derived from the male and two from the female parent. From now the divisions of the egg follow rapidly by the normal process of cell division until from this one egg cell there are eventually derived hundreds of thousands of cells which are gradually moulded into the adult. All of these cells will, of course, contain four chromosomes; and, what is more important, half of the chromosomes will have been derived directly from the male and half from the female parent. Even into adult life, therefore, the cells of the animal probably contain chromatin derived by direct descent from each of its parents.

==The Significance of Fertilization.==--From this process of fertilization a number of conclusions, highly important for our purpose, can be drawn. In the first place, it is evident that the chromosomes form the part of the cell which contain the hereditary traits handed down from parent to child. This follows from the fact that the chromosomes are the only part of the cell which, in the fertilized egg, is derived from both parents. Now the offspring can certainly inherit from each parent, and hence the hereditary traits must be associated with some part of the cell which is derived from both. But the egg substance is derived from the mother alone; the centrosome, at least in some cases and perhaps in all, is derived only from the father, while the chromosomes are derived from _both_ parents. Hence it follows that the hereditary traits must be particularly associated with the chromosomes.

With this understanding we can, at least, in part understand the purpose of fertilization. As we shall see later, it is very necessary in the building of the living machine for each individual to inherit characters from more than one individual. This is necessary to produce the numerous variations which contribute to the construction of the machine. For this purpose there has been developed the process of sexual union of reproductive cells, which introduces into the offspring chromatic material from _two_ parents. But if the two reproductive cells should unite at once the number of chromosomes would be doubled in each generation, and hence be constantly increasing. To prevent this the polar cells are cast out, which reduces the amount of chromatic material. The union of the two pronucleii is plainly to produce a nucleus which shall contain chromosomes, and hence hereditary traits from each parent and the subsequent splitting of these chromosomes and the separation of the two halves into daughter nucleii insures that all the nucleii, and hence all cells of the adult, shall possess hereditary traits derived from both parents. Thus it comes that, even in the adult, every body cell is made up of chromosomes from each parent, and may hence inherit characters from each.

The cell of an animal thus consists of three somewhat distinct but active parts--the cell substance, the chromosomes, and the centrosome. Of these the cell substance appears to be handed down from the mother; the centrosome comes, at least in some cases, from the father, and the chromosomes from both parents. It is not yet certain, however, whether the centrosome is a constant part of the cell. In some cells it cannot yet be found, and there are some reasons for believing that it may be formed out of other parts of the cell. The nucleus is always a direct descendant from the nucleus of pre-existing cells, so that there is an absolute continuity of descent between the nucleii of the cells of an individual and those of its antecedents back for numberless generations. It is not certain that there is any such continuity of descent in the case of the centrosomes; for, while in the process of fertilization the centrosome is handed down from parent to child, there are some reasons for believing that it may disappear in subsequent cells, and later be redeveloped out of other parts. The only part of the cell in which complete continuity from parent to child is demonstrated, is the nucleus and particularly the chromosomes. All of these facts simply emphasize the importance of the chromosomes, and tell us that these bodies must be regarded as containing the most important features of the cell which constitute its individuality.

==What is Protoplasm?==--Enough has now been given of disclosures of the modern microscope to show that our old friend Protoplasm has assumed an entirely new guise, if indeed it has not disappeared altogether. These simplest life processes are so marvelous and involve the action of such an intricate mass of machinery that we can no longer retain our earlier notion of protoplasm as the physical basis of life. There can be no life without the properties of assimilation, growth, and reproduction; and, so far as we know, these properties are found only in that combination of bodies which we call the cell, with its mixture of harmoniously acting parts. _Life, at least the life of a cell, is then not the property of a chemical compound protoplasm, but is the result of the activities of a machine._ Indeed, we are now at a loss to know how we can retain the term protoplasm. As originally used it meant the contents of the cell, and the significance in the term was in the conception of protoplasm as a somewhat homogeneous chemical compound uniform in all types of life. But we now see that this cell contains not a single substance, but a large number, including solids, jelly masses, and liquids, each of which has its own chemical composition. The number of chemical compounds existing in the material formerly called protoplasm no one knows, but we do know that they are many, and that the different substances are combined to form a physical structure. Which of these various bodies shall we continue to call protoplasm? Shall it be the linin, or the liquids, or the microsomes, or the chromatin threads, or the centrosomes? Which of these is the actual physical basis of life? From the description of cell life which we have given, it will be evident that no one of them is a material upon which our chemical biologists can longer found a chemical theory of life. That chemical theory of life, as we have seen, was founded upon the conception that the primitive life substance is a definite chemical compound. No such compound has been discovered, and these disclosures of the microscope of the last few years have been such as to lead us to abandon hope of ever discovering such a compound. It is apparently impossible to reduce life to any simpler basis than this combination of bodies which make up what was formerly called protoplasm. The term protoplasm is still in use with different meanings as used by different writers. Sometimes it is used to refer to the entire contents of the cell; sometimes to the cell substance only outside the nucleus. Plainly, it is not the protoplasm of earlier years.

With this conclusion one of our fundamental questions has been answered. We found in our first chapter that the general activities of animals and plants are easily reduced to the action of a machine, provided we had the fundamental vital powers residing in the parts of that machine. We then asked whether these fundamental properties were themselves those of a chemical compound or whether they were to be reduced to the action of still smaller machines. The first answer which biologists gave to this question was that assimilation, growth, and reproduction were the simple properties of a complex chemical compound. This answer was certainly incorrect. Life activities are exhibited by no chemical compound, but, so far as we know, only by the machine called the cell. Thus it is that we are again reduced to the problem of understanding the action of a machine. It may be well to pause here a moment to notice that this position very greatly increases the difficulties in the way of a solution of the life problem. If the physical basis of life had proved to be a chemical compound, the problem of its origin would have been a chemical one. Chemical forces exist in nature, and these forces are sufficient to explain the formation of any kind of chemical compound. The problem of the origin of the life substance would then have been simply to account for certain conditions which resulted in such chemical combination as would give rise to this physical basis of life. But now that the simplest substance manifesting the phenomena of life is found to be a machine, we can no longer find in chemical forces efficient causes for its formation. Chemical forces and chemical affinity can explain chemical compounds of any degree of complexity, but they cannot explain the formation of machines. Machines are the result of forces of an entirely different nature. Man can manufacture machines by taking chemical compounds and putting them together into such relations that their interaction will give certain results. Bits of iron and steel, for instance, are put together to form a locomotive, but the action of the locomotive depends, not upon the chemical forces which made the steel, but upon the relation of the bits of steel to each other in the machine. So far as we have had any experience, machines have been built under the guidance of intelligence which adapts the parts to each other. When therefore we find that the simplest life substance is a machine, we are forced to ask what forces exist in nature which can in a similar way build machines by the adjustment of parts to each other. But this topic belongs to the second part of our subject, and must be for the present postponed.

==Reaction against the Cell Doctrine.==--As the knowledge of cells which we have outlined was slowly acquired, the conception of the cell passed through various modifications. At first the cell wall was looked upon as the fundamental part, but this idea soon gave place to the belief that it was the protoplasm that was alive. Under the influence of this thought the cell doctrine developed into something like the following: The cell is simply a bit of protoplasm and is the unit of living matter. The bodies of all larger animals and plants are made up of great numbers of these units acting together, and the activities of the entire organism are simply the sum of the activities of its cells. The organism is thus simply the sum of the cells which compose it, and its activities the sum of the activities of the individual cells. As more facts were disclosed the idea changed slightly. The importance of the nucleus became more and more forcibly impressed upon microscopists, and this body came after a little into such prominence as to hide from view the more familiar protoplasm. The marvellous activities of the nucleus soon caused it to be regarded as the important part of the cell, while all the rest was secondary. The cell was now thought of as a bit of nuclear matter surrounded by secondary parts. The marvellous activities of the nucleus, and above all, the fact that the nucleus alone is handed down from one generation to the next in reproduction, all attested to its great importance and to the secondary importance of the rest of the cell.

This was the most extreme position of the cell doctrine. The cell was the unit of living action, and the higher animal or plant simply a colony of such units. An animal was simply an association together for mutual advantage of independent units, just as a city is an association of independent individuals. The organization of the animals was simply the result of the combination of many independent units. There was no activity of the organism as a whole, but only of its independent parts. Cell life was superior to organized life. Just as, in a city, the city government is a name given to the combined action of the individuals, so are the actions of organisms simply the combined action of their individual cells.

Against such an extreme position there has been in recent years a decided reaction, and to-day it is becoming more and more evident that such a position cannot be maintained. In the first place, it is becoming evident that the cell substance is not to be entirely obliterated by the importance of the nucleus. That the nucleus is a most important vital centre is clear enough, but it is equally clear that nucleus and cell substance must be together to constitute the life substance. The complicated structure of the cell substance, the decided activity shown by its fibres in the process of cell division, clearly enough indicate that it is a part of the cell which can not be neglected in the study of the life substance. Again the discovery of the centrosome as a distinct morphological element has still further added to the complexity of the life substance, and proved that neither nucleus nor cell substance can be regarded as the cell or as constituting life. It is true that we may not yet know the source of this centrosome. We do not know whether it is handed down from generation to generation like the nucleus, or whether it can be made anew out of the cell substance in the life of an ordinary cell. But this is not material to its recognition as an organ of importance in the cell activity. Thus the cell proves itself not to; be a bit of nuclear matter surrounded by secondary parts, but a community of several perhaps equally important interrelated members.

Another series of observations weakened the cell doctrine in an entirely different direction. It had been assumed that the body of the multicellular animal or plant was made of independent units. Microscopists of a few years ago began to suggest that the cells are in reality not separated from each other, but are all connected by protoplasmic fibres. In quite a number of different kinds of tissue it has been determined that fine threads of protoplasmic material lead from one cell to another in such a way that the cells are in vital connection. The claim has been made that there is thus a protoplasmic connection between all the cells of the body of the animal, and that thus the animal or plant, instead of consisting of a large number of separate independent cells, consists of one great mass of living matter which is aggregated into little centres, each commonly holding a nucleus. Such a conclusion is not yet demonstrated, nor is its significance very clear should it prove to be a fact; but it is plain that such suggestions quite decidedly modify the conception of the body as a community of independent cells.

There is yet another line of thought which is weakening this early conception of the cell doctrine. There is a growing conviction that the view of the organism, simply as the sum of the activities of the individual cells, is not a correct understanding of it. According to this extreme position, a living thing can have no organization until it appears as the result of cell multiplication. To take a concrete case, the egg of a starfish can not possess any organization corresponding to the starfish. The egg is a single cell, and the starfish a community of cells. The egg can, therefore, no more contain the organization of a starfish than a hunter in the backwoods can contain within himself the organization of a great metropolis. The descendants of individuals like the hunter may unite to form a city, and the descendants of the egg cell may, by combining, give rise to the starfish. But neither can the man contain within himself the organization of the city, nor the egg that of the starfish. It is, perhaps, true that such an extreme position of the cell doctrine has not been held by any one, but thoughts very closely approximating to this view have been held by the leading advocates of the cell doctrine, and have beyond question been the inspiration of the development of that doctrine.

But certainly no such conception of the significance of cell structure would longer be held. In spite of the fact that the egg is a single cell, it is impossible to avoid the belief that in some way it contains the starfish. We need not, of course, think of it as containing the structure of a starfish, but we are forced to conclude that in some way its structure is such that it contains the starfish potentially. The relation of its parts and the forces therein are such that, when placed under proper conditions, it develops into a starfish. Another egg placed under identical conditions will develop into a sea urchin, and another into an oyster. If these three eggs have the power of developing into three different animals under identical conditions, it is evident that they must have corresponding differences in spite of the fact that each is a single cell. Each must in some way contain its corresponding adult. In other words, the organization must be within the cells, and hence not simply produced by the associations of cells.

Over this subject there has been a deal of puzzling and not a little experimentation. The presence of some sort of organization in the egg is clear--but what is meant by this statement is not quite so clear. Is this adult organization in the whole egg or only in its nucleus, and especially in the chromosomes which, as we have seen, contain the hereditary traits? When the egg begins to divide does each of the first two cells still contain potentially the organization of the whole adult, or only one half of it? Is the development of the egg simply the unfolding of some structure already present; or is the structure constantly developing into more and more complicated conditions owing to the bringing of its parts into new relations? To answer these questions experimenters have been engaged in dividing developing eggs into pieces to determine what powers are still possessed by the fragments. The results of such experiments are as yet rather conflicting, but it is evident enough from them that we can no longer look upon the egg cell as a simple undifferentiated cell. In some way it already contains the characters of the adult, and when we remember that the characters of the adult which are to be developed from the egg are already determined, even to many minute details--such, for instance, as the inheritance of a congenital mark--it becomes evident that the egg is a body of extraordinary complexity. And yet the egg is nothing more than a single cell agreeing with other cells in all its general characters. It is clear, then, that we must look upon organization as something superior to cells and something existing within them, or at least within the egg cell, and controlling its development. We are forced to believe, further, that there may be as important differences between two cells as there are between two adult animals or plants. In some way there must be concealed within the two cells which constitute the egg of the starfish and the man differences which correspond to the differences between the starfish and the man. Organization, in other words, is superior to cell structure, and the cell itself is an organization of smaller units.

As the result of these various considerations there has been, in recent years, something of a reaction against the cell doctrine as formerly held. While the study of cells is still regarded as the key to the interpretation of life phenomena, biologists are seeing more and more clearly that they must look deeper than simple cell structure for their explanation of the life processes. While the study of cells has thrown an immense amount of light upon life, we seem hardly nearer the centre of the problem than we were before the beginning of the series of discoveries inaugurated by the formulation of the doctrine of protoplasm.

==Fundamental Vital Activities as Located in Cells.==--We are now in position to ask whether our knowledge of cells has aided us in finding an explanation of the fundamental vital actions to which, as we have seen, life processes are to be reduced. The four properties of irritability, contractibility, assimilation, and reproduction, belong to these vital units--the cells, and it is these properties which we are trying to trace to their source as a foundation of vital activity.

We may first ask whether we have any facts which indicate that any special parts of the cell are associated with any of these fundamental activities. The first fact that stands out clearly is that the nucleus is connected most intimately with the process of reproduction and especially with heredity. This has long been believed, but has now been clearly demonstrated by the experiments of cutting into fragments the cell bodies of unicellular animals. As already noticed, those pieces which possess a nucleus are able to continue their life and reproduce themselves, while those without a nucleus are incapable of reproduction. With greater force still is the fact shown by the process of fertilization of the egg. The egg is very large and the male reproductive cell is very small, and the amount of material which the offspring derives from its mother is very great compared with that which it derives from its father. But the child inherits equally from father and mother, and hence we must find the hereditary traits handed down in some element which the offspring obtains equally from father and mother. As we have seen (Figs. 34-44), the only element which answers this demand is the nucleus, and more particularly the chromosomes of the nucleus. Clearly enough, then, we must look upon the nucleus as the special agent in reproduction of cells.

Again, we have apparently conclusive evidence that the _nucleus_ controls that part of the assimilative process which we have spoken of as the constructive processes. The metabolic processes of life are both constructive and destructive. By the former, the material taken into the cell in the form of food is built up into cell tissue, such as linin, microsomes, etc., and, by the latter, these products are to a greater or less extent broken to pieces again to liberate their energy, and thus give rise to the activities of the cell. If the destructive processes were to go on alone the organism might continue to manifest its life activities for a time until it had exhausted the products stored up in its body for such purposes, but it would die from the lack of more material for destruction. Life is not complete without both processes. Now, in the life of the cell we may apparently attribute the destructive processes to the cell substance and the constructive processes to the nucleus. In a cell which has been cut into fragments those pieces without a nucleus continue to show the ordinary activities of life for a time, but they do not live very long (Fig. 25). The fragment is unable to assimilate its food sufficiently to build up more material. So long as it still retains within itself a sufficiency of already formed tissue for its destructive metabolism, it can continue to move around actively and behave like a complete cell, but eventually it dies from starvation. On the other hand, those fragments which retain a piece of the nucleus, even though they have only a small portion of the cell substance, feed, assimilate, and grow; in other words, they carry on not only the destructive but also the constructive changes. Plainly, this means that the nucleus controls the constructive processes, although it does not necessarily mean that the cell substance has no share in these constructive processes. Without the nucleus the cell is unable to perform those processes, while it is able to carry on the destructive processes readily enough. The nucleus controls, though it may not entirely carry on, the constructive metabolism.

It is equally clear that the _cell substance_ is the seat of most of the destructive processes which constitute vital action. The cell substance is irritable, and is endowed with the power of contractility. Cell fragments without nucleii are sensitive enough, and can move around as readily as normal cells. Moreover, the various fibres which surround the centrosomes in cell division and whose contractions and expansions, as we have seen, pull the chromosomes apart in cell division, are parts of the cell substance. All of these are the results of destructive metabolism, and we must, therefore, conclude that destructive processes are seated in the cell substance.

The _centrosome_ is too problematical as yet for much comment. It appears to be a piece of the machinery for bringing about cell division, but beyond this it is not safe to make any statements.

In brief, then, the cell body is a machine for carrying on destructive chemical changes, and liberating from the compounds thus broken to pieces their inclosed energy, which is at once converted into motion or heat or some other form of active energy. This chemical destruction is, however, possible only after the chemical compounds have become a part of the cell. The cell, therefore, possesses a nucleus which has the power of enabling it to assimilate its food--that is, to convert it into its own substance. The nucleus further contains a marvellous material--chromatin--which in someway exercises a controlling influence in its life and is handed down from one generation to another by continuous descent. Lastly, the cell has the centrosome, which brings about cell division in such a manner that this chromatin material is divided equally among the subsequent descendants, and thus insures that the daughter cells shall all be equivalent to each other and to the mother cell.

We must therefore look upon the organic cell as a little engine with admirably adapted parts. Within this engine chemical activity is excited. The fuel supplied to the engine is combined by chemical forces with the oxygen of the air. The vigour of the oxidation is partly dependent upon temperature, just as it is in any other oxidation process, and is of course dependent upon the presence of fuel to be oxidized, and air to furnish the oxygen. Unless the fuel is supplied and the air has free access to it, the machine stops, the cell _dies_. The energy liberated in this machine is converted into motion or some other form. We do not indeed understand the construction of the machine well enough to explain the exact mechanism by which this conversion takes place, but that there is such a mechanism can not be doubted, and the structure of the cell is certainly complex enough to give plenty of room for it. The irritability of the cell is easily understood; for, since it is made of very unstable chemical compounds, any slight disturbance or stimulation on one part will tend to upset its chemical stability and produce reaction; and this is what is meant by irritability.

Or, again, we may look upon the cell as a little chemical laboratory, where chemical changes are constantly occurring. These changes we do not indeed understand, but they are undoubtedly chemical changes. The result is that some compounds are pulled to pieces and part of the fragments liberated or excreted, while other parts are retained and built into other more complex compounds. The compounds thus manufactured are retained in the cell body, and it grows in bulk. This continues until the cell becomes too big, and then it divides.

If a machine is broken it ceases to carry on its proper duties, and if the parts are badly broken it is ruined. So with the cell. If it is broken by any means, mechanical, thermal, or otherwise, it ceases to run--we say it dies. It has within itself great power of repairing injury, and therefore it does not cease to act until the injury is so great as to be beyond repair. Thus it only stops its motion when the machinery has become so badly injured as to be beyond hope of repair, and hence the cell, after once ceasing its action, can never resume it again.

There are, of course, other functions of living things besides the few simple ones which we have considered. But these are the fundamental ones; and if we can reduce them to an intelligible explanation, we may feel that we have really grasped the essence of life. If we understand how the cell can move and grow and reproduce itself, we may rest assured that the other phenomena of life follow as a natural consequence. If, therefore, we have obtained an understanding of these fundamental vital phenomena, we have accomplished our object of comprehending the life phenomena in our chemical and mechanical laws.

But have we thus reduced these fundamental phenomena to an intelligible explanation? It must be acknowledged that we have not. We have reduced them to the action of chemical forces acting in a machine. But the machine itself is unintelligible. The organic cell is no more intelligible to us than is the body as a whole. The chemical understanding which we thought we had a few years ago in protoplasm has failed us, and nothing has taken its place We have no conception of what may be the primitive life substance. All we can say is that this most marvellous of all natural phenomena occurs only within that peculiar piece of machinery which we call the cell, and that it is the result of the action of physical forces in that machine. How the machine acts, or even the structure of the machine, we are as far from understanding as we were fifty years ago. The solution has retreated before us even faster than we have advanced toward it.

==Summary.==--We may now notice in a brief summary the position which we have reached. In our attempt to explain the living organism on the principle of the machine, we are very successful so far as secondary problems are concerned. Digestion, circulation, respiration, and motion are readily solved upon chemical and mechanical principles. Even the phenomena of the nervous system are, in a measure, capable of comprehension within a mechanical formula, leaving out of account the purely mental phenomena which certainly have not been touched by the investigation. All of these phenomena are reducible to a few simple fundamental activities, and these fundamental activities we find manifested by simple bits of living matter unincumbered by the complicated machinery of organisms. With the few fundamental properties of these bits of organic matter we can construct the complicated life of the higher organism. When we come, however, to study these simple bits of matter, they prove to be anything but simple bits of matter. They, too, are pieces of complicated mechanism whose action we do not even hope to understand. That their action is dependent upon their machinery is evident enough from the simple description of cell activity which we have noticed. That these fundamental vital properties are to be explained as the result of chemical and mechanical forces acting through this machinery, can not be doubted. But how this occurs or what constitutes the guiding force which corresponds to the engineer of the machine, we do not know.

Thus our mechanical explanation of the living machine lacks a foundation. We can understand tolerably well the building of the superstructure, but the foundation stones upon which that structure is built are unintelligible to us. The running of the living machine is thus only in part understood. The living organism is a machine or, it is better to say, it is a series of machines one within the other. As a whole it is a machine, and its parts are separate machines. Each part is further made up of still smaller machines until we reach the realm of the microscope. Here still we find the same story. Even the parts formerly called units, prove to be machines, and when we recognize the complexity of these cells and their marvellous activities, we are ready to believe that we may find still further machines within. And thus vital activity is reduced to a complex of machines, all acting in harmony with each other to produce together the one result--life.