General Physiology: An Outline of the Science of Life

embedded in it; but in no case can these two constituents be separated without the death of both. Many experiments have shown that protoplasm is incapable of self-preservation without the cell-nucleus, and the nucleus similarly incapable without the protoplasm. Hence, according to the above definition of individuality, neither of the two represents an individual. In all nature no organism is known which represents a lower stage of individuality than the cell. As Briicke ('61) says, the cell is the " elementary organism."

Apparently in contradiction with this idea is the fact, recently established by many experiments, that under certain conditions the cell can be artificially divided into pieces which continue to live and even reproduce. If, e.g., a free-living infusorian cell, such FIG. 5. — Budorina elegans, a colony of Flagellata, The single individuals lie embedded in a common ball of jelly. as the -delicate Stentor Roeselii (Fig. 6, A\ which lives in fresh water and is especially adapted for this experiment, be divided into two parts in such a manner that each possesses a piece of the long rod-like nucleus, the same phenomenon appears as in Hydra : the two pieces regrow into small complete Stentors (Fig. 6, B and C) and continue to live in all respects normally. In such an experiment the cell, an individual of the lowest order, has become divided into two individuals, and can even be divided into more, if the operation be performed so that each piece possesses some protoplasm as

FIG. 6. — Stentor Roeselii, a trumpet-shaped infusorian ; A, cut across at * ; B and C, the two pieces, which have become regenerated into complete Stentors. The clear extended mass in the interior is the nucleus. well as a piece of the nucleus. This fact is of fundamental importance, and we shall have occasion to recall it frequently. In the present case it stands only apparently in contradiction with the idea of the cell as the elementary individual ; for by the cutting operation there are obtained, not new stages of individuality, but complete Stentors, i.e., individuals of the value of a cell. In all such divisions of cells, wherever protoplasm and nucleus are present in the pieces, the latter have the value of cells ; in the process we do not go below the cell. If, however, the cut be made so that one piece contains protoplasm and nucleus, and the other only protoplasm, the former continues to live and represents a complete cell, while the latter, possessing no longer the individuality of a

cell, invariably perishes. In every case, therefore, the cell remains the elementary organism. If the above considerations be summarised, it is found that five stages of individuality can be distinguished in the organic world, and can be characterised as follows : elementary organisms that are not composed of lower units capable of life. An example is the unicellular, ciliate infusorian Stentor (Fig. 6). are associations of individuals of the first order, each one of which is like the others. An example is the flagellated spherical alga, Eudorina (Fig. 5).

are associations of various kinds of individuals of the second order. An example is Hydra (Fig. 2), the body of which consists of only two layers of tissues. are associations of various individuals of the third order. An example is man, whose body consists of various organs united. communities are associations of individuals of the fourth order. Examples are communities of ants and bees. This scheme requires one more remark. It shows that every individual of a higher order consists of an assemblage of individuals of the next lower order, but the constituents of an individual of the higher order are not always real individuals, i.e., they are capable of self-preservation when living in union with, but not when separated from, their fellows ; in other words, they are only virtual individuals. A person or individual of the fourth order, for example a man, consists of single organs, which are equal to individuals of the third order. These organs, however, are virtual, not real, individuals, for they perish when separated from their fellows. It is the same with individuals of all orders. E.g., the cell of an animal tissue, if separated from its fellows, is in itself incapable of life; in the tissue, therefore, it is only a virtual individual. In other cases, however, the constituents of an individual of a higher order, when separated from their fellows, can become real individuals of the next lower order, as is shown, e.g., by Eudorina, in which the single cells when separated are in themselves capable of life.

From these considerations the important facts follow that in the end all living individuals of whatever order either are composed of cells as the elementary structural components or are themselves free-living cells. The cell must, therefore, be the seat of those events the expression of which is life. In opposition to this conclusion, the attempt has lately been made by Altmann (390) to demonstrate a still lower stage of individuality than the cell, and thus to contradict the view that cells are the elementary organisms. It has long been known that roundish granules of different sizes are of wide occurrence within cells, lying in an apparently homogeneous ground-substance ; they have been termed elementary granules, granula, or microsomes (Fig. 7). In many cases only a few such granules are present in the cell, in other cases the whole cell is thickly filled with them, so that the groundsubstance between them almost disappears. Altmann considers these granules to be the true elementary organisms, and terms them " bioblasts." He believes that they represent in the cell the true living elements which are the seat of the vital phenomena.

I he Cell Itself, according to Altmann, IS tO granules. (After Altmann.) not as an elementary organism but as an individual of a higher order. Of course single bioblasts cannot be kept alive when separated from the other bioblasts of the cell. Nevertheless, according to Altmann, there are in nature free-living bioblasts, namely, the Bacteria. The great horde of Fungi or Bacteria, as Altmann says, represent nothing but free-living elementary organisms, which as regards individuality are equal to the granules or bioblasts that in part constitute the cell-contents.

But one searches in vain in Altmann's works for an adequate confirmation of the hypothesis that the bioblasts are the elementary organisms. On the contrary, it is not difficult to perceive the untenableness of such a view. The majority of investigators have not accepted it, and Altmann's attempt must be regarded as wholly unsuccessful. The following seem to be the two most important considerations which render the hypothesis of granules untenable. In the first place, Altmann brings together under the conception of the granule all sorts of different elements, which can by no means be homologised with each other. Lately, indeed, he has given up the idea that the chlorophyll bodies, which give the green colour to plant-cells, are granules, but the conception still contains the most heterogeneous elements. Thus, he considers as granules not only the minute grey particles that occur wide-spread in the most various free-living and tissue-cells, and differ greatly in chemical composition and significance for the cell-life, but he includes the fine granules of colouring matter in pigment-cells, which give to the tissues in which they lie their characteristic colour ; the fine

lamellar particles in the yolk of eggs; and even the small oildroplets and fat-globules that occur in various tissue-cells, especially in the liver and the cells of subcutaneous connective tissue. Moreover, particles of ingested food, transformed foodconstituents, undigested food -stuffs, and products of cellular metabolism, i.e., substances that are playing or have played the most various rdles in cell life, are put into the same category, and are considered as elementary organisms. In the second place, Altmann does not prove for one of all these forms that they show general vital phenomena, nor would success in such an attempt be expected, especially in the case of an oil-droplet or a pigmentgrain lying within the cell. Nevertheless, such proof must be furnished if the term " elementary organism " is to be allowed. Regarding Altmann's belief that bacteria must be considered as free-living granules, not only is there no evidence for this view, but lately the striking investigations of Biitschli ('90) have afforded proof that bacteria are complete cells, and henceorganisms that Altmann considers colonies of bioblasts.

These considerations suffice to overthrow Altmann's idea. And, in general, it seems entirely inadmissible to regard as elementary organisms structures that have no analogies with free-living organisms. If this be allowed, the conception of the organic individual collapses, for it is then not justifiable to consider any one portion of the living substance more than another as constituting the elementary organism. The term can be applied with equal justice to an atom of oxygen or carbon, or any other atom that takes a direct part in the life-process. There would be as. many elementary organisms as organic elements. How to define an organism or an organic individual is one question, what in general to call living is another. The latter will be discussed later. As to the former, if the conception of the organic individual is not to be given up, it must be regarded as an unconditional requirement that the organism be characterised by the presence of all those vital phenomena that have to do with self-preservation. Only the cell fulfils this condition ; it is, therefore, the individual of the lowest, order and the elementary organism.

The thought that the enormous number of phenomena constituting life are associated in all their essentials with the microscopic bit of living substance that constitutes the living cell is an irresistible stimulus to research. Hence, from the time when the significance of cells as elementary organisms became first recognised until now, a host of investigators have busied themselves with the detailed study of the cell and its constituents. Thanks to this, our knowledge of cell-morphology has.

been extended from year to year, and the conception of the cell has been made constantly more precise. The conception of the nature of the cell has not been always the same. As we have seen,1 the cell idea originated as a result of the microscopic observation of plants. The microscopists of the seventeenth and eighteenth centuries found that plant-tissue contained, besides long tube-like structures, small chamber-like elements set off from one another by walls, and containing liquid. Because of their similarity to the large cells of honeycomb these small structures received the name of " cells." Thus, at that time the cell was regarded as a simple droplet of liquid enclosed by a wall or membrane. The characteristic thing which led to the giving of the name " cell," a term very fitting for plant-cells, was the " cell-membrane," without which a chamber, vesicle, or cell was not possible. This idea continued to prevail even when Schleiden discovered, in addition to the cell-liquid or cell-sac, a slimy semiliquid mass, the " plant-slime," or, as Mohl called it, the " protoplasm," and when by Schwann the cell idea was extended to the elementary parts of animal tissues.

The fundamental work of Max Schultze ('61, '63) gave to the cell idea an entirely different meaning. The study of the Rhizopoda, those one-celled organisms whose naked protoplasmic bodies are capable of extending their viscous body-substance at any desired spot into fine threads and networks, led Schultze to the view that the essential part of the cell cannot be the cellmembrane, for the very numerous species of Rhizopoda have throughout life no cell-membrane ; but that it is the substance which earlier had been termed " sarcode " by Dujardin ('41) in naked fresh-water Rhizopcda and Infusoria. A comparison of Rhizopoda and plant-cells afforded Schultze the proof that sarcode, the substance of the Rhizopoda, is completely identical with protoplasm, the viscous contents of plant-cells ; and thus he founded the theory of protoplasm, according to which the essential constituent of the cell is the protoplasm. The idea that the cell is a simple bit of protoplasm has proved brilliant in results, in opposition to the old view of the necessity of the cell -membrane. Not only have an enormous number of cells that lack a membrane become known among the numerous unicellular Rhizopoda (to which belong the Polythalamia or Foraminifera having calcareous shells, the Radiolaria having silicious shells, and the Amoebce in which a shell is wholly wanting), but it has also been observed that in the development of many plants and animals one-celled stages occur as eggs, which are entirely devoid of a membrane. Hence, since Max Schultze's establishment of the protoplasm theory, the idea that the cell-membrane is a general cell-constituent has completely disappeared.

Schultze's definition, however, does not include all the essential or general cell-constituents. In 1833 Brown had discovered in protoplasm a further specific structure, the cell-nucleus, which, by its refractive power, could be distinguished clearly as a spherical granule from the protoplasm enclosing it. Schleiden ('38) took up this discovery of Brown and demonstrated the cell-nucleus to be a wide-spread constituent of the cell in many plants ; but he was misled in his theory of phytogenesis into considering the nucleus as the element from which the cell first arises in the course of the individual development of the plant. Since that time constantly more attention has been given to the nucleus. It was found not only in plant-cells, but after Schwann's labours ('39) also in the most diverse animal cells. But, especially when by means of certain colouring-matters, such as carmine, hsematoxylin, etc., it was stained, and thus was made clearly visible in the protoplasm in which it was embedded, the view was gradually adopted that it represents a very characteristic constituent of the cell ; and soon the question arose whether cells ever exist without nuclei, or whether the nucleus is a general arid, like the protoplasm, essential constituent of the cell.

Among the unicellular free-living Bhizopoda, to which Max Schultze had called attention, Haeckel (70) found a considerable number in which no trace of a nucleus was to be demonstrated, and which, since they appeared to consist of a simple bit of protoplasm and thus were the lowest and simplest conceivable organisms, he termed Monera. Another group in which no nucleus could be demonstrated was that of the micro-organisms, the Bacteria, which likewise have excited the greatest interest in recent times. They are the smallest of all existing living beings, and, although they possess a fixed unchangeable form, they reveal no trace of differentiation in their apparently wholly uniform protoplasmic bodies. If we except the red blood-corpuscles of warm-blooded animals, which likewise show no differentiation of their bodysubstance into two separate parts, protoplasm and nucleus, but which, as has been demonstrated, develop from actual nucleated cells, the two groups of the Monera and the Bacteria remain as the sole apparently non-nucleated cells.

But with the recent wonderful development of the technique of microscopic staining the conception of the Monera as nonnucleated cells has gradually changed. By the employment of the newer, complicated staining-methods constantly more of the organisms which Haeckel described as Monera are being recognised as nucleated cells: in many of them even a large number of small nuclei have been demonstrated; and Gruber ('88) has found forms in which the nuclear substance is distributed through the whole protoplasm in innumerable, extremely minute granules (Fig. 8). Thus the number of the

original Monera is constantly diminishing, and the few that cannot yet be obtained for fresh investigation are now also regarded by most investigators as nucleated cells in which the earlier incomplete technique was not able to demonstrate nuclei, just as was the case with the others that are now recognised as nucleated. The Bacteria have defied much longer than the Monera attempts to find in them a differentiation corresponding to the nucleus and protoplasm of other cells. All imaginable methods of staining and the strongest microscopic powers were not able to demonstrate the two different kinds of living substance within their minute and apparently completely homogeneous bodies. This state of our knowledge continued until a very few years ago, in spite of the great advance that bacteriology made. Recently, however, Biitschli ('90) succeeded in discovering a fine structure in the bodies of Bacteria. He found that by the use of very strong magnifying powers and not too strong illumination certain specific staining-reagents, which, as, e.g., hsematoxylin, colour only the nuclear substance and not the protoplasm, make visible two different substances in the bodies of Bacteria ; one of

these is stained intensely, the other not at all. The quantitative relations of the two substances are characteristic : the volume of the stained substance is usually greater than that of the unstained, but the relative arrangement of the two is different in different species. In one species, as, e.g.. Bacterium lineola (Fig. 9, a), the stained substance lies in the middle, and the unstained substance forms a delicate peripheral layer about it; in others, especially the corkscrew-like forms of Spirillum, such as Spirillum undula (Fig. 9, b), which is common in stagnant water, the unstained substance is accumulated at one end or both ends of the elongated body, and the latter consists otherwise wholly of stained substance. This differentiation jof the body-substance into two portions, one of which is stained and the other unstained by specific staining-reagents, appears to correspond entirely to the division of the living substance into nucleus and protoplasm that

FIG. 8.—Pelomyxa pallida. A rhizopod containing very finely-divided nuclear substance Gruber.) characterises all other cells. There are also among animals varieties of cells that show quite the same quantitative relation of the two substances to each other ; an example of such is afforded by the sperm-cells, or spermatozoa, which consist of a large quantity of nuclear substance and a very small quantity of protoplasm. Thus, from the present state of our knowledge, it appears that among the organisms now living upon the earth there are no cells in which a separation of two different substances is not present, but that every cell possesses a nucleus in addition to the protoplasm. It is, of course, another question whether during the evolution of living substance upon the earth organisms may not have existed at some earlier time, in which the whole body consisted of a single homogeneous substance, and no separation into different substances had yet taken place. If such organisms ever

FIG. 9. — Structure of various Bacteria. (After BUtschli.) a, Bacterium lineola, normal and undergoing division, b, Spirillum undula. c, Bacterium from stagnant water. existed, they could be ranked in comparison with real cells as cytodes, as Haeckel terms non-nucleated elementary organisms. Notwithstanding them, it must be granted that there belongs to the conception of the cell at present, not only a single homogeneous mass, the protoplasm, but also a substance differing from it, the nuclear substance. Accordingly, Max Schultze's morphological definition would be widened as follows : The cell is a bit of protoplasm containing a distinct nucleus.

If the protoplasm be examined with strong magnifying powers, in many cells other distinct constituents besides the nucleus are found embedded in the protoplasmic ground-substance. In many cells oil-droplets occur, in others pigment-granules, in plant-cells starch-grains, etc. ; but all these bodies do not occur in every kind of cell : they are special, not general, cell-constituents. It appeared recently, however, as if, in addition to the two previously known general constituents, the protoplasm and the nucleus, a third exists, the polar corpuscle, central corpuscle, or centrosome.

The centrosome (Fig. 10) has become known in detail only very recently. It had, indeed, been noticed when the peculiar phenomena of nuclear division in cell-multiplication were investigated twenty years ago ; but not until later was it recognised by van Beneden ('83, '87) and Boveri ('87, '88, '90) as an important element in the cell, which reproduces like the nucleus in the increase of cells by division, van Beneden came to believe that the centrosome, like the nucleus and the protoplasm, is a general cell-constituent. This idea was supported by

FIG. 10.— a, Pigment-cell from the pike ; the centrosome with its protoplasmic radiation lies between the two nuclei. (After Solger.) b, Leucocyte from the larva of a salamander; the centrosome with the aster lies at the right of the dumb-bell-shaped nucleus. (After Flemming.) c, Egg-cell in the act of dividing; there is a distinct protoplasmic radiation about each of the two centrosomes. (After Boveri.) the observations of Flemming, Solger, Heidenhain, and others, who found one or more centrosomes in other kinds of cells, such as leucocytes, pigment-cells, epithelium-cells, etc., and even when they were not undergoing division. Nevertheless, in a great number of cells it has not been possible up to the present time to demonstrate such a body. Perhaps this is due to its nature. It is a granule that is very difficult to find in protoplasm on account of its minuteness, and no structure whatever has been proved in it by the help of the microscope. Moreover, as a rule it is not stained by the usual staining-reagents. The endeavours of M. Heidenhain to find for it specific staining-media, such as exist for the nucleus, have not yet led to wholly satisfactory results. Its presence is clearly revealed by the protoplasmic radiations by which in certain conditions of the cell it is surrounded. In the division of the cell the protoplasm arranges itself around the

centrosome in the form of a radiating aster ; the centrosome forms the middle point of the star-shaped figure (Fig. 10), and is easily discovered by this peculiar investment. While many investigators, led by van Beneden, are inclined to regard the centrosome as a specific constituent of the cell, since it is always to be found in the protoplasm apart from the nucleus, O. Hertwig ('92) upholds the view that it belongs to the nucleus as a part of the nuclear substance, and passes from it into the protoplasm only during the activity of the former in fertilisation

PIG. 11.— Division and withdrawal of the centrosome in the nucleus of the spermatozoa of Ascaris megalocepkala ; the two-upper rows show successive stages of the nucleus (n, nucleolus ; c, centrosome) ; below, two spermatozoa after the withdrawal of the centrosome from the nucleus. (After Brauer.) and division, retreating afterwards again to the nucleus as a part of the substance of the latter during the resting-condition of the cell. That this view of Hertwig is applicable in certain cases has been shown recently by the striking investigations of Brauer ('93, 2) upon the development of the spermatozoa of the threadworm, Ascaris megalocephala. Brauer was able to determine that in these cells the centrosome is contained within the resting nucleus, and in certain cases even undergoes division there ; later it wanders out into the protoplasm and there produces the protoplasmic radiation which surrounds it during cell -division (Fig. 11).

On the other hand, in the large majority of cases it remains continually, even during the resting-stage of the cell, outside the nucleus. Hence, as Heidenhain ('94) and Boveri ('95) remark, there is ground for considering the centrosome a constituent neither of the nucleus nor of the protoplasm ; it must rather be regarded as an independent constituent of the cell. Since, however, many forms of cells, especially among unicellular organisms, are known in which thus far it has not been possible to discover a centrosome, it is not justifiable at present to regard it as a general cell-constituent. Among the vital phenomena of the cell, it is known thus far to share only in reproduction and fertilisation.

In accordance with these considerations the protoplasm in its entirety and the nucleus with its differentiations can be contrasted as the sole general cell-constituent, in distinction from all special constituents, such as the cell-membrane, starch-grains, pigmentgrains, oil-droplets, chlorophyll-bodies, centrosomes, etc. Five stages of individuality have been distinguished sharply from one another in organic nature; it must be remembered, however, that no sharp limits are to be found in the living world. Cells were distinguished as elementary organisms from the next higher stage of individuality, tissues ; and it might seem as if no sharper boundary exists than that between the single cell and the tissue, which consists of a number of similar cells, and as if it would be very easy to distinguish the two stages of individuality from one another. In reality this is not so. There are individual organisms in which a distinction, as to whether they are elementary organisms or tissues, does not readily appear; and such cases, like many others in which boundaries are to be established in nature, show that the fixing of sharp limits and definitions must contain finally a more or less arbitrary element, that, indeed, all limits and definitions are only psychological helps toward knowledge.

The transition-forms between typical cells and genuine tissues are numerous. They consist of a unitary protoplasmic mass containing more than the one nucleus that is characteristic of the cell-type. Double-nucleated cells are found in many tissues, such as cartilage (Fig. 12). Many epithelial cells (Fig. 13, a) contain more than two nuclei ; and the large ciliate infusorian, Opalina (Fig. 13, b), which lives parasitically in the intestine of the frog, contains a considerably larger number. Forms with innumerable nuclei are to be found among the marine algae : e.g., in the thin lamellar protoplasmic layer of Caulerpa (Fig. 14), a giant cell of the shape and size of a leaf, there lies an immense number of nuclei, all of which together with the protoplasm are moving in a constant slow stream between the cell-walls, i.e., the two surfaces of the leaf.

All these organisms containing several nuclei can be separated as multinucleate cells from multicellular tissues by the fact that in the former the protoplasmic territory immediately surrounding the individual nuclei is not sharply defined from the neighbouring protoplasm, but together with all the rest of the protoplasm constitutes a unitary mass which appears as a whole shut off from the FIG. 13. — a, Epithelium-cell, containing several nuclei from the urinary bladder of man. (After Virchow.) b, Opalina ranarum, a unicellular ciliate infusorian, containing many nuclei, from the intestine of a frog. (After Zeller.)

outside by a definite surface, while in the tissue every individual protoplasmic territory which belongs to a nucleus is sharply separated from all the rest. The multinucleate cell, therefore, represents one cell, which is characterised as a whole by a definite form of surface ; the tissue, however, consists of a sum of single cells, each one of which has its own sharply defined form. The distinction between multinucleate cells and genuine tissues becomes more difficult in the case of certain low organisms, the Myxomycetes, which have frequently been claimed by the botanists as plants and by the zoologists as animals, and which in many respects are of great interest. They are sometimes seen in leafy forests, upon mouldy leaves or decaying tree-trunks, as white, yellow, or brownish-red networks ; they often spread themselves out for several decimetres upon objects by means of their delicate arborescent strands (Fig. 15, /). Detailed examination shows that these networks, which sometimes form thicker, lumpy masses of the same appearance, are of a soft slimy consistency. If such a