General Physiology: An Outline of the Science of Life

Finally, it may be assumed that all the substances that the nucleus gives off are not employed by the protoplasm for transformations, but that some pass unused through the protoplasm and are transferred to the outside. In order to obtain a clear idea of how closely and firmly the nucleus is interwoven into the metabolism of the cell, and what complications are caused in the latter by the introduction of the nucleus into the chain, it will be advantageous to bring together the experimental facts in a schematic form, such as is shown in the accompanying Fig. 260.1 This represents a cell containing a nucleus, and each arrow signifies a quantity of substances upon their pathway through the metabolic circulation.

The cell receives certain substances from the outside ; of these some (a), upon meeting substances already present in the protoplasm, undergo decomposition and syntheses. Of the substances resulting from these transformations some (b) are at once excreted as useless, others (c) remain in the protoplasm and are there employed further, while a third class (d) is passed on tothenucleus. The nucleus, moreover, obtains a portion of the substances (e) received from the outside and passed on unchanged through the protoplasm. The substances (d-\-e) entering into the nucleus there undergo on their part certain transformations, from which again substances result ; these in part (/) are given off to the outside without being changed by the protoplasm, in part (h) pass to the protoplasm to find there further employment, and in part (g) remain in the nucleus itself.

If, now, we realize that every arrow represents a sum of substances, that the substances passing from the nucleus to the protoplasm undergo transformations as well as those entering from the outside, and that the substances arising from these transformations are in part conveyed again to the nucleus, we obtain an approximate idea of how close the metabolic connection of the nucleus with the protoplasm is. Further, it should be remembered that in all the above considerations, nucleus and protoplasm represent a great sum of different, in many cases even morphologically different, bodies, that in the conception " nucleus " are comprised all forms termed accessory nuclei, micronuclei, etc., and that by " protoplasm " there is understood the whole sum of the various products of differentiation, even chlorophyll-bodies. Only when we consider that all the various constituents of nuclear substance and, likewise, the 1 Cf. Verworn ('89, 1).

FIG. 260. — Scheme of cell -metabolism. The arrows indicate the direction of movement of substance. granules, chlorophyll-bodies, etc., of protoplasm share, at least temporarily, in the metabolism, do we obtain an approximate idea of the complexity of the cell-metabolism and of the endlessly multiform relations in which nucleus and protoplasm are united. A far-reaching correlation between the individual elements of the cell, especially between the nucleus and the protoplasm, follows from these close metabolic connections. The one is conditioned by the other. One is dependent upon the substances that the other produces. Thus the profound changes are explained that the life of the cell experiences when the individual links of the great metabolic chain are changed, whether spontaneously in the course of development or as a result of the action of external stimuli. Every change of one biotonic link brings about a change of many others and, if for any reason one drops out, the metabolic chain is broken, and necrobiosis, which finally ends in death, begins.

There now remains the question of the mechanics of this involved process of cell-metabolism. Since the metabolic relations between the nucleus and the protoplasm, like those between the whole cell and the medium, are based upon the ingestion and output of substances, this question may be simply comprised in the problem of the mechanics of these processes on the part of the cell. It is advantageous to consider separately the cells that receive and give off substances in solution only, and those that receive and give off solid substances also.

For a long time the processes of exchange of dissolved substances between the cell and the surrounding medium, both resorption and secretion, were regarded as conforming directly to the laws of filtration and diffusion. But recently attention has been directed to various facts which prove that in most cases filtration plays no rdle at all in these processes, and also that diffusion or osmosis alone is not sufficient to explain them. Especially from the later observations of Heidenhain ('94) it is known that the vital process in the cell itself plays the most important role in the exchange ; diffusion alone is unable to explain, e.g., the propelling power with which the secretion is extruded in many cases from glandcells, or the considerable energy with which certain food-stuffs are taken up by the intestinal epithelium-cells. Hence, in explaining the mechanics of resorption and secretion the two factors, diffusion and the chemism of the cell, must be taken into consideration.

By diffusion or osmosis is understood, as is well known, the fact that two different gases or liquids which are miscible will mix with one another spontaneously into a homogeneous mass, when they are brought into contact. As a rule, the word diffusion is employed when the gases or liquids are in direct contact, and osmosis, when they are separated from one another by an organic membrane. We have already become acquainted with this process. If we recall the experiment that illustrated osmosis (p. 104) and vary it somewhat, we can at the same time make clear the great importance which the second factor, the chemism of the cell, possesses in the presence of diffusion or osmosis. If in the larger vessel {Fig. 261) there is a diffusible salt solution, and in the cylinder the solution of a substance that does not diffuse, a certain quantity of salt will diffuse out of its solution into the liquid of the cylinder, while no substance can pass from the cylinder into the larger vessel. If, however, the substance in the cylinder has a chemical affinity for the salt, the salt diffusing into the cylinder goes into chemical combination at once. If the chemical compound thus arising be

continually removed and replaced by new solution of the same kind as before, the salt solution in the larger vessel will become constantly weaker and weaker, until finally all the salt has diffused into the cylinder, has become combined and is removed, so that in the large vessel there is nothing but water. This case is realized in the process of taking in gaseous and dissolved substances by living substance. The living substance is capable of mixing with the gaseous and dissolved food-stuffs, for it has a chemical affinity for them. The cell-membrane, if such be present, represents the membrane of the cylinder ; the cell-contents, the contents of the cylinder ; and the gaseous or dissolved substances, the salt solution of the larger vessel. These substances must be diffusible, if they are to be taken in ; nevertheless, the living substance cannot diffuse through the cell-membrane, since the proteids, etc., belong to the so-called colloid substances. Hence the food-stuffs will pass into the cell, but the living substance cannot pass out. Since the latter has a chemical affinity for the food-stuffs, it must enter into combination with them immediately after their entrance into the cell. But it is continually decomposing, giving off substances to the outside, and reforming ; in other words, the food-substances taken in are constantly being consumed, so that a continual balance between the inside and the outside can never take place, and new masses must constantly diffuse in. The output of substances must take place in an analogous manner. Let us, then, imagine a cell, surrounded by a membrane, existing in

a medium that contains food-stuffs, such as a bacterium in a nutrient liquid. By means of osmosis and the chemism of the cell the constituents of the nutrient liquid that are miscible with the cell-contents and do not possess too large molecules must pass through the cell-wall into the cell-substance and, vice versa, the constituents of the cell-substance that are miscible with the nutrient liquid and are able to pass through the cell-wall, must go out of the cell into the nutrient liquid. This exchange goes on in so far as the substances in question are not held fast by other forces upon the one or the other side. Theoretically, it will necessarily continue until a balance as regards the substances capable of transportation is struck between the cell-contents and the medium, when the metabolism will necessarily cease. But in the living cell such a condition is never reached, since compounds there exist which are continually decomposing and building themselves up anew. On the one hand, the substances received by the cell from the medium are always consumed at once and transformed into other compounds ; and, on the other hand, those that the cell gives off to the medium are constantly being formed. Hence the exchange between cell and medium must continue as long as the cell is still capable of taking up food-stuffs in sufficient quantity from the medium and of giving off excretory substances in sufficient quantity to the medium. If, therefore, the mass and character of the medium are fixed, and not changed from the outside, after some time the cell must perish mt this will occur either when the food-stuffs contained in the medium are consumed, or when the latter is so saturated with excretory substances, that the output of them by the cell has diminished or ceased. It is very easy to produce both cases experimentally in cultures of Bacteria.

The bacteria die either from lack of food or from the accumulation of the products of their own metabolism, because the osmotic exchange of substance between bacterium-cell and nutrient liquid gradually ceases through the gradual equalisation of the substances belonging to the two. In many cases the mechanism of exchange between cell and medium is more complex. If, for example, the nutrient substances in the surrounding medium are not present in a diffusible form, i.e., if they are either solid or possess so large molecules that they are unable to pass through the pores of the cell-wall, they must be made soluble and diffusible. This is performed through the action of ferments which the cell produces and in many cases gives off to the outside. In contact with these ferments, the polymeric molecules of proteid, of gelatine, of starch, etc., and solid masses of these substances become split up and brought into solution, and they are then able to diffuse into the interior of the cell. This process may be followed very easily in bacterial cells. If a bacterial cell be placed upon a glass plate covered with solid nutrient

gelatine, the cell begins gradually to liquefy the gelatine in its vicinity, i.e., to bring the solid substance into solution, and from the accumulation of liquid thus arising and surrounding the bacterium, the dissolved nutrient substances are able to diffuse into the cell. In cells possessing a naked protoplasmic surface and no cellwall extracellular digestion is naturally not required, because the food-stuffs, even when they cannot diffuse, are able to come into chemical relation with the surface of the protoplasm directly.

These considerations allow us to form a general idea of the mechanics of the process by which resorption and secretion in the living cell are able to go on continually and automatically. The great quantity of energy that is involved in the two can also be understood, if the chemism of the living cell be taken into sufficient consideration ; for, if very strong chemical affinities for foodstuffs exist in a cell, and if a very active transformation of substance takes place, it is then very easily conceivable that the great amount of chemical energy can lead to the performance of very considerable work. Special cases are still puzzling ; but the solution of these cases belongs elsewhere.

While, as may be supposed, the exchange of dissolved substances between cell and medium is based upon the same principle in all cases, that of solid substances is performed in very different ways in different cases. The only thing common to all these latter is the fact that the exchange is mediated solely by active movements of the cell in question, but in different cases these movements are influenced in very different ways by the food. The ingestion and output of solid substances is not wide-spread and occurs, indeed, only in naked protoplasmic masses, such as RJiizopoda and leucocytes, and in Infusoria in so far as they possess a special mouthopening. In many Infusoria and especially in those that lead a sessile life, such as Stentor and Vorticella, the ingestion of food appears to be left solely to chance, which occasionally leads small free-swimming food-particles, such as Alga-ce\\s, swarm-spores, Bacteria, etc., into the region of the lively whirlpool that is produced by the circlet of cilia upon the peristome. This whirlpool, while capable of being influenced in direction by changes in the action of the cilia, is so regulated at the time of food-ingestion that it leads directly into the mouth-opening of the cell-body. Free-swimming Infusoria and most naked protoplasmic masses seek solid food. Either they are attracted from a distance by chemical stimuli which go out from the masses of food by the diffusion of certain substances, or they are led to take up the food by mechanical stimulation through direct contact with the food-masses. In the former case the ingestion of food is the result of a positive chemotaxis; since the cell moves toward the source of the chemical stimulus, and. its protoplasm comes into very close connection

with the substance in question ; in the latter case a positive thigmotaxis exists, since the cell endeavours to extend as much as possible the surface of contact with the food -body and surrounds the latter with its protoplasm. These two factors are very frequently united. But the ball of food is always surrounded upon all sides by the protoplasm, if it has come into contact with the latter, whether upon the surface of a naked protoplasmic body, or in the mouth - opening of an infusorian cell. The process of surrounding it is explained very simply from the expansory effect which the stimulus of the food-ball exercises upon the protoplasm ; if the surface of the latter rises uparound it, it must finally be surrounded by the protoplasm. The ingestion of solid food, therefore, findsit's explanation in the mechanism of chemotactic and thigmotactic reactions; we have already become acquainted with these in detail elsewhere.1 How solid substances are given off is still little understood. This appears to be left more or less to chance. At least this is the impression obtained from Amoeba. As a rule the solid substances lie in vacuoles, and, if by the continual movement of the protoplasm the vacuole is brought close to the surface, the thin wall that separates it from the surrounding medium occasionally breaks, and the contents are set free. But perhaps stimuli of .some sort coming from the excreted particle are necessary for this rupture of the wall. The questions whether the removal of the excretion through the anal opening, which occurs in infusorian cells, likewise depends upon stimulation, must be left until the process has been studied more in detail.

A remarkable phenomenon, which has often been cited as affording special difficulties to a mechanical explanation, is the so-called selection of food on the part of certain cells, i.e., the fact that certain cells take up only certain substances among all those available.2 Thus, regarding the seeking of ^ro^yra-threads by Vampyrella Spirogyrae and the selection of fat-droplets from the intestinal contents by the intestinal epithelium-cells, Bunge ('94) says : " No chemical explanation of these phenomena is conceivable." But why this should be so is not easily understood. If in these apparently isolated phenomena the fact upon which they are based is clearly understood, i.e., that every cell takes up certain substances and not others, the action of the cells is self-evident. Every cell has its characteristic composition and its own peculiar metabolism. Is it not then comprehensible that only those substances are drawn from the medium into the metabolic circulation of the cell that have chemical relations with the constituents of the cell-body and are necessary to the maintenance of metabolism, while others which have no such relations with the living substance and are indifferent to the cell, are not taken up and, when free locomotion is possible, are not sought out ? The principle upon which this phenomenon 1 Cf. p. 498. 2 Cf. p. 146.

is based is evidently the same as that which controls in general atoms and molecules, namely, affinity. It is surely no less wonderful that an atom of phosphorus unites very easily with an atom of oxygen, but not with an atom of platinum, than that an intestinal epithelium-cell takes up fat-droplets but never pigment-granules. And it is no less comprehensible that a Vampyrella surrounds with its body-protoplasm and digests only Spirogyra threads and noother bodies, than that a drop of rancid oil, as Gad (78) has shown, sends out amoeboid processes to an alkaline liquid and uses the alkali for the manufacture of soap, but is inactive toward an acid liquid. But the behaviour of the Vampyrella and the intestinal epithelium-cell is by no means peculiar, every living cell behaves similarly. Every tissue-cell in the human body takes up from the common nutrient liquid, the blood, certain substances only, as is evident from the fact that gland-, muscle-, and cartilage-cells, produce wholly different and characteristic substances. In this respect, as Haeckel ('66) has already emphasised, the cell behaves exactly like a crystal, for example a crystal of alum, which out of a mother liquor containing numerous salts in solution always selects alum molecules only, in order to employ them for its growth, or^ if it has been injured, for its regeneration. Thus the mystical obscurity that some investigators have endeavoured to wrap about the so-called selection of food-stuffs on the part of the individual cell does not really exist. What has been called by the anthropomorphic term the "selection of food" by the cell is an absolutely necessary consequence of the fact that the living substance of every cell possesses its own specific composition and its own characteristic metabolism.

Thus, the phenomena of cell-metabolism may all be referred tochemical and physical principles, as they are found in inorganic nature, and although at present we are unable to trace in individual cases the finer details of the special metabolic processes, we are certain that the whole metabolism comes about in a purely mechanical manner, and that phenomena are never met with that cannot be explained mechanically. There can evidently be no exception to the conclusion that everything that consists of matter must obey the laws of matter.

Although in the present condition of our knowledge of cellprocesses, we do not know what special share in the whole metabolism is taken by the individual constituents of the cell, with what chemical processes in the history of the biogens the nucleus and the protoplasm with their specific constituents are associated, our discoveries so far regarding the general metabolic relations in the cell are sufficient to enable us to recognise that the phenomena

of change of form, i.e., the phenomena of growth, reproduction, development, and hereditary transmission, may also be derived as mechanical consequences from these metabolic relations. Groivth constitutes the fundamental phenomenon of changes of form in organic nature, for not only is growth of the cell the simplest case of change of form in general, but at the same time, as the following considerations will at once show, it contains the internal causes of the more complex phenomena of cell-reproduction and development. We have referred elsewhere l to the mode of growth of living substance. We know that there are molecules in living substance, that possess an extraordinary tendency toward polymerisation, i.e., under given conditions, by continually taking on similar groups of atoms, they endeavour to enlarge and to form chains of many similar links. We have become acquainted with such polymeric molecules in the native proteids. It is, a priori, probable that the so-called living proteids, or biogens, likewise possess this property, and the more probable because there is reason to assume with Pflliger, that the radical cyanogen, which tends strongly towards polymerisation, is contained in the biogen molecule. Moreover, the fact of growth requires the assumption of polymerisation in the biogen molecule. Growth can be conceived only as a process in which a biogen molecule attaches to itself little by little similar groups of atoms from the materials of the environment (food-stuffs) ; these groups then proceed in the same manner to attract to themselves certain atoms from the environment and place them in similar positions, and so on.

This process, which is here pictured in a uniform substratum, goes on in a much more complex manner in the cell whose living substance and whose metabolism are very widely differentiated. In the cell the constituents of both the nucleus and the protoplasm with all their special differentiations share more or less closely in the formation and the growth of the biogen molecule. But with this close relationship and dependence of the individual constituents of the cell upon one another, it is easily understood that the growth of certain biogens of the protoplasm by polymerisation is only possible when at the same time other constituents of the protoplasm or of the nucleus increase in a definite measure; in other words, whenever a single substance of the protoplasm or of the nucleus grows, other substances also will grow.

It is important to consider somewhat fully the relations that, with this close correlation of the individual parts of the cell, are developed by growth. For example, let us imagine a free-living, spherical cell which has at its disposal in the surrounding medium in sufficient quantity all substances necessary to its life, and let us assume that the cell grows. With the increasing size of the cell the relation of surface to mass will gradually change ; according to known mathematical laws the former will grow in comparison with the latter in the proportion of the square to the cube. In other words, the smaller the cell, the greater is the surface in proportion to the mass ; and the more the cell grows, the less does the surface grow in proportion to the mass.

This simple fact is of fundamental importance. This becomes at once clear, when it is realised that the individual parts of the cell-body are in close metabolic relations to one another and to the external world. As regards the food -stuffs and the oxygen received from the outside, the more the cell grows, the more a disproportion between the external and the internal layers of the cell-body will come about ; for, since the surface through which the food is taken in increases less than the mass of the cell-body, the time will come when the ingested food is no longer sufficient for the whole body, and the result of this must become evident in the fact that the internal cell-layers are too little nourished in comparison with the external. While nutrition goes on rapidly and richly in the latter, in the former it proceeds more slowly and more sparingly. This will affect not only the protoplasm, but also the nucleus. The nucleus will receive fewer substances from the outside, if the protoplasmic layer surrounding it becomes gradually thicker, than if it is thin. But, vice versa, the external layers of the cell will become provided with nuclear substances much less richly than the internal. In brief, with the close relationship of the individual parts of the cell the metabolism must undergo profound changes, which increase the more, the more the cell grows. Hence, so long as the cell continues to grow, at no time is its metabolism exactly the same as at the preceding and the following intervals.

This important consequence from the fact of growth contains within itself the principle of all development, i.e., with the close metabolic relations that exist between the individual constituents of the cell and of the medium, the fact of growth is alone completely sufficient to lead and must lead to all the changes that are termed " development. " It follows from these considerations that the cell can never surpass a certain size ; for, if the disturbance of metabolism that arises because of the increasing disproportion between the more superficial and the deeper layers, has reached a certain extent, the cell can no longer continue living in its existing form. Thus the remarkable fact is explained very simply, that no cells of constant form are known that are larger than a few millimetres in diameter ; and thus we are made to understand why the development of large organisms is only possible by the arrangement of the living substance into an aggregate of small cells, instead of into a single

cell, for example of the size of a man. At the same time it is comprehensible that under certain circumstances cells whose surface is considerably increased in proportion to their mass, such as the flat leaves of Caulerpa, or those cells whose protoplasm is in continual circulation between the surface and the interior, such as the plasmodia of Myxomycetes, are capable of reaching a very considerable size, especially when by multiplication the nuclear substance presents a considerable increase. In these cases the difference between the outer and the inner layers of the cell-body cannot develop in the same degree as in compact cells. But where the cell-body is a compact mass, where there is no active streaming of the contents toward the surface, and where only one nucleus is present in the protoplasm, the cell cannot surpass a certain size. If, therefore, the living substance of such a cell is not to perish by growth, at some period in its growth a correction of this disproportion between mass and surface and of the disturbance of metabolism conditioned by it must come in ; such a connection is realised in the reproduction of the cell by division.

The reproduction of the cell by division is, accordingly, to be considered merely as a result of growth, and the morphologists for a long time have rightly termed reproduction a continuation of growth, " a growth beyond the measure of the individual." Unfortunately our knowledge of the special mechanics of the process of cell-division is thus far limited ; but it is to be expected that a comparative physiological investigation of the well-known morphological facts, when especially directed to the mechanical conditions, as they are realised in various ways in different forms of cells, will yield gratifying results. It is especially important always to keep in mind and to select as the starting-point of the investigations the metabolic relations of the individual parts of the cell. The mechanical results of the metabolic relations between the individual parts of the cell and the medium, are fitted to throw some light upon the processes of cell -division, many of which appear wonderfully complex. The most important factor in the explanation of the mechanics of the characteristic figures of cell- and nuclear division is probably to be found in the mechanical movements caused by the chemical relations between the individual parts of the cell ; among these, diffusion-processes and changes of the cohesion and surface-tension of different cell-elements play a prominent role. Some time ago Biitschli (' 76) expressed the view that the radiating figure that is formed in the protoplasm about the centrosome in nuclear divison is an expression of diffusion-processes arising between the centrosome and the protoplasm; and later (' 92, 3) he showed that, when warm gelatine-foams poured upon a glass plate dry and coagulate, radiation-phenomena, exactly like those of the karyokinetic figure, are caused by the traction of the contracting air about the air-bubbles (Fig. 262). It may,

therefore, be supposed that the radiation that forms about the centrosome likewise has its origin in the traction existing between the centrosome and the foamy protoplasm, and that this traction is derived from the chemical relations and diffusion-processes that develop between the two cell-constituents, But only a systematic and comparative investigation of these processes will be able to make this supposition a certainty. The mechanical theories of cell- and nuclear division, which M. Heidenhain ('94/95/96), Driiner (' 94), Rhumbler (' 96, ' 97), and others have very recently put forward, are so contradictory, incomplete and full of hypotheses, that at present it is quite impossible to say anything certain, except of the most general nature, concerning the mechanics of

FIG. 262.— Photographic reproduction of radiating figures. /, Nuclear radiation from a cephalopod embryo. II, Eadiation about two air-bubbles in a gelatine foam which was coagulated by means of chromic acid. (After photographs by Butschli.) these complex events. Before all else, as R. Fick (' 97) very correctly emphasises, their molecular-physical relations must be given much more careful attention ; thus far Rhumbler alone has done this properly and to a considerable extent. Further, it will be of essential importance in such an investigation to start from the simplest forms of nuclear division, i.e., from the so-called direct nuclear division, in which there are no complicated figures. The lengthening and simple constriction of the nucleus giving rise to two nuclei is a simple mode of increase of the nuclear surface, the mass remaining the same ; and the subsequent constriction of the protoplasm has the same significance for the cell-body. It is the simplest form of a correction of the disproportion arising between

surface and mass with continued growth, and ought to offer relatively the least difficulties to a mechanical explanation. With the division of the cell into two independent cells, the relation of surface to mass in the two latter is very different from that existing in the large cell before division. The result is that the metabolic relations will again change, and each cell will assume the same condition which the mother-cell had when it arose by division and began to grow into an independent individual. Thus, from one cell-division to another the same cycle of changes, which is conditioned by the growth of the cell-body and the disturbance in the metabolism caused thereby, repeats itself. If these changes are slight, they will not be especially noticeable outwardly, except in an increase in size. Most cells are like this, since they simply grow and, when they have reached a certain size, divide. Where, however, the disturbances in metabolism are considerable, they will be expressed also in a change of the external form of the cell-body, constituting a typical development. A large number of free-living unicellular organisms show this, especially those whose cell-body in division breaks up, not into two halves, but into a larger number of parts, or spores. The difference in size between the spore and the adult infusorian is, indeed, very considerable. Hence the differences in the metabolism must also be very considerable, and a somewhat long development is required before the spore becomes an adult.

Thus the development of the cell, the periodic return of one and the same cycle of form-changes from one cell-division to another, from one spore-formation to another, is seen to be a simple expression of changes in the cell-metabolism that are caused by growth. During growth with the close correlation of all parts of the cell with one another and the constituents of the medium, innumerable other factors, both chemical and physical, must appear, and these must combine with one another to assist and promote the form-changes. But as the fundamental cause of all these changes no other factor than growth need be assumed. It alone suffices to explain their periodic character.

A question that has reference to the development of the multicellular organism from the ovum by continued division has recently become the focus of active discussion. It is this: — How does the division of a cell into two unequal parts, a circumstance that forms the fundamental condition of the development of every differentiated cell-community, come about ? This question, which is fundamentally important in an understanding of the development of all higher organisms, is answered in two very different ways. The view of one class of investigators follows the theory of His