General Physiology: An Outline of the Science of Life

But this theory early underwent a modification. Beginning in 1867, Frommann especially endeavoured to show by a long series of researches that the finer structure of the protoplasm of all cells is not properly fibrillar, but reticular; this view was adopted almost at the same time by Heitzmann, and soon obtained wide acceptance. According to this idea, protoplasm forms a network, or, better, a mesh work, the nodal points of which appear as individual granules. The whole mesh work of the cell is open to the

Fia. 28.— An amoeba-cell containing completely hyaline and homogeneous pseudopodial protoplasm. In the endoplasm by the side of the nucleus lies a pale contractile vacuole (droplet of liquid). outside, and between its threads exists a liquid, which, however, is different from the liquid of the medium in which the cell lives, i.e., the water, the body-juices, etc. It is difficult to understand why, as the adherents of the theory of the reticular structure of protoplasm hold to be possible, the internal cell-liquid in cells that possess no membrane, such as the leucocytes of the blood and Amoeba, the reticular structure of which has been described by Heitzmann in great detail, does not continually mix with the surrounding medium in spite of its great proportion of water. But attempts to stain such living protoplasmic masses by certain staining-solutions show clearly that the staining-fluid does not penetrate into the living protoplasm. This and similar difficulties which arise in connection with the idea of protoplasm as a meshwork open upon all sides have led many investigators to take a very sceptical attitude toward the theory of a reticular structure, although in various ways the reticular appearance of the protoplasm of many cells has been confirmed.

The striking researches with which in recent years Biitschli ('92, 1) has been surprising the scientific world, have completely clarified our ideas upon the real nature of the protoplasmic structures so much observed. The protoplasm of a cell that contains so many vacuoles or droplets of liquid that its contents have a foamy appearance, presents with high powers of the microscope a picture, not of many vacuoles or bubbles pressed tightly together, but of a network, the threads of which form the cross-sections of the thin walls of the vacuoles. This is due to the fact that with strong powers surfaces only, and never bodies, are seen. The microscope shows only optical cross-sections of bodies. But the optical cross-section of a foam is a network. This fact led Biitschli to the conviction that the finer reticular appearance of protoplasm which appears homogeneous by feeble magnification, as has been observed in so many cells, is merely the optical expression of an extremely finely vacuolated foam-structure. In order to confirm this idea, Btitschli endeavoured artificially to produce microscopic foams of a fineness equal to the hypothetical protoplasmic structures, and he succeeded in this in a most gratifying manner. He employed for his experiments oil which was very finely rubbed up with potash or cane-sugar. Small droplets of this oil-mixture, when placed upon a slide with a drop of water, covered with a cover glass, and observed under a microscope, immediately took on an extremely fine foam-structure. This was due to the fact that the particles of potash or sugar, which were finely divided in the oil-droplet, attracted particles of water ; the latter passed from the outside through the oil by diffusion, accumulated as extremely fine droplets closely about the former, and transformed the oil into a very fine foam.

The oil-foams obtained in this way show such a remarkable similarity to the structure of protoplasm that they can scarcely be distinguished from the latter. From the accompanying figures (Fig. 29, a and b), which are taken from Biitschli, the identity in structure of the two objects may be recognised at a glance. After the very careful and comprehensive investigations, the results of which Btitschli has published in his book, doubt can no longer exist that the problematic fine structure of protoplasm is in reality foam-structure, which depends upon the presence within a uniform groundmass of a large number of extremely fine

FIG. 29. — a, Foam-structure in the intracapsular protoplasm of Thalassicolla nucleata. b, Foam from olive oil and cane-sugar, c, Protoplasmic structure in a pseudopodial extension of a foraminifer-cell (Miliola). d. Protoplasmic structure of an epidermis-cell of an earthworm. (After Biitschli.) vacuoles, lying almost at the limit of microscopic visibility, and so close together that their walls consist of relatively thin lamellae. Further, Biitschli has demonstrated this foam-structure in so many wholly different forms of cells (Fig. 29, a, c, d) that its wide distribution can be disputed no longer.

As the result of these recent investigations the following picture can be formed of the finer morphological structure of protoplasm. Protoplasm consists of a ground-mass, in many cases completely homogeneous, in most cases very finely foam-like or honey-comblike, in which lies embedded a greater or less quantity of very various solid elements, or granules. In the foam-like protoplasm thegranules always lie at the corners and angles where the foamvacuoles come together, never in the liquid of the bubbles themselves.

We have already spoken 1 of the idea of Altmann, who regards the granules as the sole elementary parts of protoplasm, and the intermediate substance between the granules as non-living. In the light of Blitschli's investigations this view appears all the more untenable. In recent years the cell-nucleus has become a favourite object of morphological investigation. And here a psychological phenomenon is to be noted, which has constantly repeated itself in the history of the human mind, since mankind began to reflect upon things — this is the tendency toward exaggeration. The earlier investigators of protoplasm, especially Max Schultze, had convinced themselves that protoplasm shows important vital phenomena, and at once by excessive generalisation the view was promulgated that protoplasm is the sole bearer of vital phenomena, while the nucleus possesses an accessory significance. Since then it has been recognised that the nucleus participates prominently in certain vital phenomena ; several investigators have shown that it plays a very important role in reproduction, fertilization, secretion, etc. Immediately the original view of the all-importance of the protoplasm has by an extreme reaction become exchanged for its opposite, that of the all-importance of the nucleus. As will be seen in a later section, here, as so often, the truth lies between the two. But every reaction is exaggerated. Opinions, like a pendulum> go first to the two extremes, and only after some time does the proper mean come to be maintained. Biology is indebted to these investigations upon the nucleus for the fact that our knowledge of it has been greatly extended.2

The form of the nucleus is very different in different cells. The first conception of the nucleus was formed from cells in which within a circumscribed protoplasmic mass a single, more or less spherical nucleus exists, which as regards its refractive power and its consistency differs essentially from the surrounding protoplasm. It was found later that the nucleus stands in sharp con- * A. Zimmermann has recently made a comprehensive survey of the results of research upon the nucleus, especially in plant-cells, in his book, Die Morphologic und Physiologic des pflanzlichen Zellkernes : Eine kritische Litteraturstudie. Jena,

trast to the protoplasm by its characteristic behaviour toward certain reagents, especially colouring-matters. Nuclear masses possessing these characteristics are the most wide-spread in the organic world. The nuclei of a majority of free-living and most tissue-forming cells among animals and plants are of this type. In it the relation of the volume of the nucleus to that of the protoplasm varies greatly. There are cells in which a relatively small nucleus is surrounded by a large mass of protoplasm, as, e.g., many Foraminifera, while in other cells, the mass of the protoplasm, in comparison with the nucleus, is extremely small, as in most spermatozoa. From the type of the single, more or less spherical nucleus deviations in very different directions occur. First, as regards the number of nuclei: As has already been seen, there are organisms that consist of a unitary protoplasmic mass in which lie embedded a large number of nuclei, such as multinucleate cells and syncytia. In such cases the number of the nuclei can be so great and their size so excessively small that, as Gruber ('88) has observed in certain Rhizopoda from the harbour of Genoa, especially Pelomyxa pallida, the nuclei lie distributed through the whole protoplasm as a fine powder (Fig. 30). With such a division of the nuclear mass as is present in multinucleate forms, the nuclear surface naturally is considerably larger than with the same quantity contained in a

single large nucleus — a fact that is particularly important from the physiological point of view. The same principle of surface-enlargement is seen also in the differentiation of the form of the single nucleus. The most manifold and extreme deviations from the typical spherical form occur. Rod-shaped, band-shaped (Fig. 31, a,) and moniliform (Fig. 31, &) nuclei are very common among ciliate Infusoria.. Going still further, the same principle leads to star-shaped and branched nuclei, which are found in certain cells in the bodies of insects, and reach their highest development in antler-like branched forms in the cells of the spinning-glands of many caterpillars (Fig. 31, c). It seems noteworthy that it is the nuclei of secreting cells, i.e., cells

FIG. BO.— Pelomyxa pallida, a rhizopod-cell from the harbour of Genoa, containing finelydivided nuclear substance in the protoplasm. (After Gruber.) characterised by lively activity, in which the principle of surfaceenlargement by branching is especially expressed. As regards the nature of the substance of the nucleus, exactly the same is true as in the case of the protoplasm. The nucleus is no more a unitary substance than is the protoplasm. It is a morphological structure, an organoid of the cell, which consists of several

FIG. 31.— Cells containing different forms of nuclei, o, Vorticella, a ciliate infusorian, possessing a rod-shaped nucleus, b, Stentor, a ciliate 'infusorian, possessing a moniliform nucleus, c, c, Cells of the spinning-glands of the caterpillar possessing antler-like branched nuclei. (After Korschelt.) different constituents that may be distinguished from one another microscopically more or less clearly, and all of which are not present in all cells at all times. Because of the exceeding minuteness of the objects, it is often difficult sharply to characterise the individual constituents. Therefore, their identity in two separate species is not always beyond doubt, and extended investigations are still needed before it will be known clearly what constituents.

of one nucleus correspond exactly to those of another. Nevertheless, a number of constituents, which apparently are wide-spread, are even now fairly well characterised. The following substances occur most constantly : — 1. The nuclear sap constitutes the liquid ground-substance, in which the solid nuclear constituents are contained (Fig. 32). M. Heidenhain, Reinke, and Korschelt have lately demonstrated that in many cells, even during life, it presents an extremely finely granular appearance.

2. The achromatic nuclear substance forms in the ground-substance a supporting-structure of fine threads, which are characterised, like the nuclear sap in which they are suspended, by not staining with the typical nuclear stains, such as the carmine stains, haemotoxylin, etc. 3. The chromatic nuclear substance is distinguished from the achromatic by its property of staining with these reagents. It is contained in the strands of the achromatic substance, as a rule in the form of small granules and irregular particles, and upon its

FIG. 32. — Various nuclei from the mother-cells of the spermatozoa of the threadworm of the staining-power chiefly rests our knowledge of the finer structure of the nucleus. 4. The nucleolus is a homogeneous granule which is present comparatively rarely in nuclei ; it consists of a strongly refractive substance which appears to be closely related to the chromatic substance. Since, as a rule, the substance of the nucleoli may be stained by the nuclear stains like the chromatic substance, the nucleolus has been considered by many investigators as a special accumulation of chromatic substance — a view which, however, because of the different relations of the two substances toward certain chemical reagents, cannot strictly be maintained.

All of these substances, to which with advancing knowledge of the nucleus others will perhaps be added, are present in very different quantities in different cells. A substance that is abundant in one nucleus may be insignificant in another, and it even appears as if certain substances can be wholly wanting in certain nuclei. In many cases the nuclear substances are surrounded and marked off from the protoplasm by a special nuclear membrane, which, however, like the cell-membrane in relation to the cell, is not a general constituent of the nucleus.

Recently, Zacharias ('81-'87) and Frank Schwarz ('87) have endeavoured to replace the customary names of the individual substances by other names. Thus, the chromatic substance has been termed nuclein, the achromatic substance linin, the nucleolar substance paranuclein or pyrenin, the nuclear sap paralinin, and the substance of the nuclear membrane amphipyrenin. The adoption of these names is not recommended, for they may so easily be confounded with chemical notions as to lead to the error of seeming to deal with chemical entities, while the nuclear substances in question are purely morphological. If the term nuclein were to be employed in a chemical sense, the chromatic nuclear substance would be placed in a chemical contrast with the other nuclear substances that does not really exist, for the majority of other nuclear substances likewise belong chemically to the socalled nucleins, representing different kinds of the latter. Therefore it is more fitting to employ the original names above mentioned for the morphological nuclear constituents, and not to confuse the latter with chemical substances.

One more phenomenon relative to the differentiations of the individual substances is of interest. This is the fact that, of the substances that occur together within the nucleus in most cells, some have become differentiated in many cells into separate masses within the protoplasm, so that two entirely different forms of nucleus occur side by side within the same cell. This condition is almost universally realised in ciliate Infusoria, which possess, in addition to a larger nucleus, the macronucleus, which in some species and at certain periods seems to consist chiefly of chromatic substance, one, several, or often a great number of the so-called accessory nuclei, or micronuclei, which likewise in some species and at certain periods seem to consist mostly of achromatic substance. The claim of the two elements in the infusorian cell to be regarded as two different nuclear substances is based upon the phenomena which, according to the striking investigations of R. Hertwig ('88-'89), appear in the conjugation of two individuals. Here the chief nucleus goes to pieces completely in the protoplasm, and after conjugation a new rudiment of it is differentiated from the substance of the accessory nuclei. While in the ciliate Infusoria the two forms of nucleus remain throughout life, in the Difflugice of the PJiizopoda a localised differentiation of two nuclei appears only during the period of conjugation and gives place afterwards to the uninucleated condition.1

It has been seen that the achromatic substance forms in the ground-mass of the granular nuclear sap a supporting-structure, in the strands and nodal points of which the chromatic substance and the nucleoli lie embedded in precisely the same manner as the solid elements, the granules, etc., lie in the alveolar walls of the protoplasm. Indeed, as Biitschli has shown, the similarity of the relation even goes so far in individual cases that the achromatic substance in the nucleus shows precisely the same alveolar structure that the ground-mass of the protoplasm as a rule possesses (Fig. 33).

All these structures are characteristic Only Of the SO-Called FlG. 33._Alveolar structure of the nucleus resting-stage OI the Cell. As SOOn of a ganglion-cell. (After BUtschli.) by division, very peculiar and very complex changes in the structure of the nuclear substance appear ; these will be considered in detail in another chapter. Although the earlier investigators of the cell, such as Schleiden, Mohl and others, as the result of direct observation, considered the contents of the cell to be liquid, and compared its consistency with that of slime, later the idea found wide acceptance that protoplasm is at bottom a solid substance. This idea arose from purely theoretical considerations. Briicke ('61 ), especially, thought that the cell-contents cannot be liquid, for the reason that vital phenomena cannot possibly be associated with a liquid substratum, but presuppose a definite organisation, and the latter is not compatible with the nature of a liquid. Briicke's view soon obtained many adherents, and appeared to be supported particularly by the theory of the reticular structure of protoplasm, as maintained by Frommann and Heitzmann. It was believed that the solid supporting-structure, with the organisation of which vital phenomena are associated, was represented by the network. It has turned o«t, however, that the supposed reticular structure is an optical delusion, and thus this basis for the view of the solid consistency of protoplasm has been taken away. In reality, with the present methods of microscopic investigation, only a strong prejudice in favour of other and untenable theories can overlook the fact that, with the exception of individual differentiations in certain cells, protoplasm behaves physically like a liquid.

The idea that vital phenomena can be associated with a solid substratum only is not only unjustified, but even untenable. Not only is it unsupported on any acceptable ground, but it even contradicts facts that may easily be observed. E.g., it is quite impossible to understand how protoplasm in the more or less stiff condition of a framework or network can be capable of streaming and flowing, as can be observed so easily in certain plant-cells and in Amoeba. It is impossible for a solid network to flow in such a manner that the individual particles of its mass mix continually with one another, as may be seen so clearly in Amoeba. If at first sight the theory of the solid consistency may not be incompatible with the behaviour of cells that possess a constant form, it is absolutely so with the phenomena exhibited by naked protoplasmic masses.

Hence various investigators, especially Berthold ('86) and Btitschli ('92, 1), have recently defended strongly the idea of the FIG. 34.— a, Vaucheria tube cut open at the upper end ; the protoplasm is flowing out and taking the form of globules. (After Pfeffer.) b, Amceba-cell containing a pale vacuole and various small fat-droplets. liquid nature of the cell-contents, and no investigator who is familiar with the phenomena need hesitate to accept this view. Observation of a few facts is convincing of its truth.

The phenomena of movement, already mentioned, are the strongest proof of the liquid nature of protoplasm. In the protoplasmic strands of plant-cells and in the pseudopodia of Rhizopoda the living substance may be seen flowing like the water of a quiet stream, now slower, now faster, and in different places at unequal rates, so that, as can be observed easily in the constituents enclosed within the ground-mass, the granules, fat-droplets, etc., the particles continually mingle with one another. How would it be possible for a stiff ground-mass to flow like water in a stream ?

Another thing that throws light upon the liquid consistency of protoplasm is the fact that protoplasmic masses, when oozing out of the cell after its walls have been crushed or cut, form drops and globules. The formation of such drops and globules can be observed very beautifully in the protoplasm of the alga Vaucheria (Fig. 34, a). They can be observed also in the streaming protoplasmic strands of the uninjured plant-cell, when the electric current is sent through them. The protoplasm then collects at once into globules and small spindle-shaped masses, which, if the current be interrupted, become again extended and united, their substance flowing on (Fig. 35). The same can be seen in the pseudopodial filaments of many marine Rhizopoda upon shaking them strongly or continually (Fig. 36), and likewise in many other objects.

A third phenomenon that points to the liquid consistency of protoplasm, and one that can be observed in very different forms of cells, is the assumption of the globule- or drop-shape ~by accumulations of liquid enclosed within the protoplasm, such as the so-called vacuoles, and the fat- and oil-droplets, which appear here and there, increase in size, and under certain circumstances disappear (Fig. 34, 6). Were the ground-mass of protoplasm stiff, it would be incomprehensible that these droplets of liquid of very different sizes always assume the spherical form and preserve it during their growth, as oil-droplets do. In such cases a spherical form is mechanically possible only when the surrounding medium exercises upon all sides equal pressures and yields equally, i.e., when it is itself a liquid.

Innumerable phenomena of this kind may be cited, which are compatible only with the liquid nature of protoplasm. But those mentioned suffice completely to show that vital phenomena can very well be associated with a liquid substratum. Of course the liquid and the solid conditions of a body cannot be separated from one another by a sharp limit, but are united by imperceptible transitions. According to our present physical ideas the difference between the gaseous, liquid and solid conditions of a body depends solely upon the fact that in the first the molecules are in rapid

FIG. 35. — Tradescantia. Cell of a stamen-hair. A, Containing quietly streaming protoplasm. B, The same cell stimulated by an induced current. The protoplasm in the strands has become rounded into single globules (c, d). (After Kiihne.) motion, in the second they are moving somewhat more slowly, and in the last still more slowly. Since, therefore, only a gradual difference exists, it is impossible to establish a sharp limit. In living substance also there are different grades of mobility among the particles,, i.e., in one case the substance is like thin, in another case like thick liquid. In general, it possesses the consistency and mobility of raw white of egg, but it may be firmer, and certain constant differentiations of protoplasm may possess even the consistency of a soft jelly approximating a solid condition, without losing, however, the power of shifting its particles. Such a condition exists in muscle-fibres, flagella, cilia, the nucleus, and upon the surface of many protoplasmic masses that do not possess a.

FIG. 36. — Orbitolites. Piece of the many-chambered calcareous disc, bearing outstretched pseudopodial filaments. A , Undisturbed. B, By strong shaking the protoplasm of the pseudopodia has been stimulated to form globules and spindles. membrane, such as infusorian cells. The term . solid is applicable to such cases only, if at all. But these cases of a more viscous consistency are always locally restricted within the cell ; the rest of thfe cell-contents is always a thinner liquid.

Finally, it should not be forgotten that within the liquid there may be deposited all sorts of solid elements of very various consistencies, and that, therefore, the whole constitutes, not a homo- fBneous liquid, but a mixture, or, as Berthold terms it, an emulsion, or this reason it appears inadmissible to speak of an " aggregate condition " of protoplasm, as many observers do. Strictly speaking, the term " aggregate condition " can apply only to a homogeneous substance, not to a mixture containing substances that possess in themselves very various aggregate conditions.

The liquid nature of living substance is its most important physical characteristic. It requires that in its physical relations living substance must obey the laws of liquids. Accordingly, and in opposition to the idea that vital phenomena are associated only with a solid organisation, it will be seen that such phenomena may be understood only upon the supposition that their substratum is in a condition in which the particles are more or less capable of shifting. The structures that have a rigid consistency, like tendons, connective-tissue fibres, cell-membranes, and the groundsubstance of bone and of cartilage, show no active vital phenomena, and the old dictum, " Corpora non agunt nisi soluta," although its universality may be attacked here and there, applies perfectly to living substance.