General Physiology: An Outline of the Science of Life

network be observed some hours after it is first seen, or upon the next day, it is found that not only its place but its shape has been completely changed ; and if a small piece be cut off from it, laid upon a glass plate and kept in a moist place, it can be seen how the whole mass begins to flow slowly and to send out delicate processes in this and that direction, which branch in an arborescent manner and flow together into a network. In short, it is seen that the whole network is living. Myxomycetes consists of com-

FIG. 14.— Caulerpa, a leaf-shaped marine alga ; the single leaves are thin, protoplasmic lamella; enclosed between two flat cellulose walls and containing numberless small nuclei ; natural size. (After Reinke.) pletely naked protoplasm. Microscopic examination and staining reveal in the fine strands of its plasmodia a large number of nuclei, which are continually being dragged along by the slowly flowing protoplasm and which roll over and under one another, plainly showing that they possess no fixed position but change their places constantly and irregularly in the unitary protoplasmic mass. Here individual cell -territories are not marked off within the protoplasmic body. According to the above criterion, therefore, we would be obliged to regard the plasmodia as multinucleate

cells. But the origin of the Myxomycete plasmodia renders such a view uncertain. Myxomycetes reproduces by spores, i.e., by small microscopic capsules, the shells of which burst and give exit in each case to a small naked cell, which is capable of changing its shape and is provided with one nucleus (Fig. 15, a, b, c). A very large number of the spores always coexist and many separate cells creep out at the same time. These cells soon creep together, coalesce, and thus form a larger, unitary, protoplasmic mass, which contains a number of nuclei (Fig. 15 e,f). The mass grows by its own nutritive efforts, the nuclei multiply by division, and thus.

FIG. 15. — /, Aethalium septicum ; a piece of a reticulate Myxomycete plasmodium, natural size. II, Chondrioderma difforme ; /, piece of a plasmodium ; a, a spore, 6, the same, swelling, c, the contents of the spore is creeping out ; d, the spore has changed into a flagellated cell ; e, the flagellated cells have transformed themselves into amoebae, which are creeping together again to form a plasmodium. (II After Strasburger.) arises finally the large, reticulate plasmodium. This plasmodium, therefore, although representing a unitary protoplasmic mass containing many nuclei and without cell-boundaries, has arisen from many single cells. Hence, strictly, it is not proper to consider the plasmodium of Myxomycetes as a multinucleate cell ; at the same time it is not justifiable to speak of it as a genuine tissue, for no cell-boundaries are marked out for the single nuclei. A special name, therefore, has been created for these intermediate stages between the single cell and the tissue, and they have been called syncytia.

The chief fact that has stood in the way of a consistent extension of the cell-theory and one that still presents at first the greatest difficulties to all who study the finer structure of organisms, is the astonishing variety of forms in which the elementary constituent of organisms appears. The forms of the different cells are so manifold that it is often difficult for the inexperienced observer to realise that they are all different modifications of one and the same type. In contradistinction to this endless variety there exists a wide-spread constancy in the form of one and the same kind of cell, so that the cells of any particular tissue of the human body, e.g., the liver, the skin, the bone, or the blood, are always to be recognised at once as such, i.e., as liver-, skin-, bone-, or blood-cells. A few examples will best illustrate the great differences in the forms of cells.

There are many cells that possess no constant form, but change their shape continually, and hence are termed amceboid cells. All amoeboid cells have a naked protoplasmic body, upon the surface FIG. 16.— Amoeba, showing successively different shapes in creeping ; the hyaline exoplasm flows constantly forward ; in the middle and behind lies the granular endoplasm, containing the (darker) nucleus and the (lighter) vacuole. of which projections of the body-substance constantly appear and disappear, and thus a new shape is constantly being assumed. In different kinds of cells these projections or pseudopodia have different forms. Most fresh-water Amoebce (Fig. 16) and the eggcells (Fig. 17, a) of many animals are characterised by broad, lobate or finger-shaped pseudopodia; leucocytes (Fig. 17, &), or colourless blood-cells, by pointed and divided pseudopodia ; and many Rhizopoda (Fig. 17, c) and pigment-cells (Fig. 17, d) by threadlike and reticulate pseudopodia flowing into one another.

But by far the majority of cells possess a constant form, whether the protoplasm is enclosed in a membrane or not. The simplest form of cell that can be regarded as the type of the elementary organism is the spherical form, as it appears, for example, in many egg-cells (Fig. 18, a). From this type deviations in all sorts of directions occur. When the cells are united with other similar ones, as is the case in every tissue, their form is modified by the pressure which they receive from the surrounding cells. A cell which in itself is spherical must, therefore, in a tissue take on a polyhedral form according to simple mechanical laws, just as

FIG. 17.— a, Egg-cell of a calcareous sponge. (After Haeckel.) b, Blood-cell of a crab. (After Haeckel.) c, Biomyxa vagans, a f resh -water rhizopod. d, Pigment-cell from the tail of a tadpole. peas lose their spherical shape, and become polyhedral when they are crowded thickly in a bottle and are made to swell. In fact the polyhedral shape of cells occurs very frequently in tissues, especially in epithelium-cells of the skin (Fig. 18, b) and glandcells. Further, one essential factor in causing a deviation from the spherical type is the formation of permanent processes upon the surface. In this way permanent forms of cells often occur of the shape that amoeboid cells show temporarily. The green

alga Eiiastrum (Fig. 19, a) represents such a cell with lobate processes, and the ganglion-cells in the central nervous system FIG. 18.— a Egg-cell from the ovary of a sea-urchin. (After Hertwig.) 6, Epidermis-cells from of man, the brain and spinal cord, which give origin to the nerve - fibres, possess constant processes which appear exactly like the FIG. 19. — «, Euastrum, a unicellular alga from the group of the Desmidiacece. (After Haeckel.) b, Ganglion-cell from the human spinal cord. (After Gegenbaur.) z, Cell-body ; n, nerve-process (axis-cylinder process).

pseudopodia of many rhizopod cells (Fig. 19, &). Other cells, the ciliated cells, have upon their surface motile but permanent processes, of the shape of eye-lashes. These ciliated cells are very wide-spread ; they occur not only in tissues as ciliated epithelium-cells (Fig. 20, a), but also free-living and constituting the great host of the Ciliata, or ciliate Infusoria, Fia. 20.— a, Ciliated epithelium-cells. (After Schiefferdecker.) b, Stylonychiamytilus, a ciliate-infusorian cell possessing variously differentiated cilia ; WZ, mouth-region; C, contractile vacuole ; N, macronucleus ; N, micronucleus ; A, anal opening. (After Stein.) c, Euglena viridis, a flagellate-infusoriaii cell possessing a single flagellum. n, Nucleus ; o, eye-spot ; c, vacuole. (After Stein.)

and the Flagellata, or flagellate Infusoria, according as the onecelled body possesses many similar or variously differentiated cilia (Fig. 20, 6), or only one flagellum or several (Fig. 20, c). Finally, there are cells that deviate from the type by being enormously extended in one direction, so that they appear as slender, band- or thread-like forms. Extremes in this direction are smooth and cross-striated muscles-cells (Fig. 21, a) and many spermatozoa (Fig. 21, 6).

In contrast to the astonishing variety of form, it is surprising that the size of cells varies only within relatively narrow limits. It is a very noteworthy fact that by far the majority of all cells are microscopic. The size of organisms varies within very wide limits, from the extreme minuteness of a bacterium, measuring only a few thousandths of a millimetre, up to the enormous mass of an elephant or the huge spread of an American mammoth - tree. But large organisms are never found consisting of a single

cell. Only a very few varieties of cells having a compact protoplasmic body reach a diameter of a few millimetres, and these are amoeboid, their surface changing continually and their substance being in constant streaming motion. The fact that compact cells whose radii are approximately equal in all dimensions and whose protoplasm is not constantly streaming never surpass the size of a few millimetres, has only apparent exceptions. The bird's egg might be regarded as an exception. It is well known that the egg of a fowl before it has left the body represents a single cell; an ostrich egg would, therefore, be a single, compact giant cell, which apparently would contradict the above rule. This exception, however, as has been said, is only apparent, for the really active or living protoplasm of the eggcell has a very small bulk and in the form of an extremely thin and delicate lamella is laid over the rest of the mass, which latter consists of inactive ogg-yolk, the food-material for the further developing and reproducing cell. Hence there is here, not a solid compact mass of living substance, but merely a thin lamella. Such an extension in one or two dimensions exists also in all other cells that exceed the usual size — e.g., the cross-striated muscle-cells of the leg-muscles, which are often more than a decimetre in length ; ganglion-cells, which are extended into nerve-fibres more than a metre long, and the leaf-shaped cells of Caulerpa. In all these cases it appears that the ratio of the mass to the surface of the cell never exceeds a certain value. As will be seen later, this phenomenon is deeply grounded in the nature of

living substance, and the formation of a large and massive organism is possible only by the employment of very small autonomous elements, such as the cells. FIG. 21.— a, A smooth muscle-cell. (AfterSchiefferdecker.) 6. Spermatozoon of Salamandra macvlata. (After Hertwig.) k, Head; The mistake has frequently been made of considering protoplasm a chemically unitary substance. This idea involves a double error, for, first, the conception of protoplasm, as created by the earlier cellinvestigators, was not a chemical but a morphological conception, and, secondly, it applied to the whole contents of the cell, with the exception of the nucleus. The cell-contents is, however, in neither the chemical nor the morphological sense a unitary substance, but is a mixture of many morphological constituents; and it must constantly be borne in mind that, since it is impossible to separate one or another constituent as accessory, the limitation of the term

PIG. 22.— a, Epidermis-cells from the frog ; the living substance appears completely hyaline. /_>,. Clepsidrina blattarum, a unicellular gregarine from the intestine of a cockroach ; the protoplasm is entirely filled with granules. protoplasm to certain constituents of the cell is wholly inadmissible and leads to evil consequences. The conception of protoplasm, therefore, should be maintained under all circumstances strictly in its original sense as a comprehensive morphological conception ; protoplasm is a sum, a mixture, of very different morphological elements. Even if by degrees its individual constituents become known morphologically and chemically, the comprehensiveness of the term will not thereby be set aside. Whatever significations the various substances may have in the vital process of the cell is a wholly different question, and does not affect the conception of protoplasm.

When the contents of protoplasm are investigated, upon superficial examination two groups of constituents may be distinguished, namely, various well-defined bodies, such as grains, droplets, etc., and a uniform, semi-liquid, apparently homogeneous groundsubstance, in which the former, like the nucleus, lie embedded. But, while in many cells the ground-substance contains only a few solid bodies, as, e.g., in many epithelium-cells (Fig. 22, a), in others it can scarcely be seen because of the abundant granular constituents, as is frequently the case in many plant-cells, and especially in certain parasitic unicellular organisms, the Gregarince (Fig.

The solid constituents of protoplasm are material elements of very various natures ; they are special constituents, and do not occur in all cells. Among them occur bodies that are of the highest significance for the life of the cell in which they are contained, that impress upon the cell a characteristic feature; and also elements that play no role whatever in the vital process, such as the indigestible residue of food. There are found, further, foodconstituents which are not yet changed, other substances which have been regularly transformed from the food by the vital process or have been formed anew, and, finally, in many cells independent organisms which live continually in them as symbionts or parasites and under certain circumstances play a definite role in the life-process of the cells.

Among the solid protoplasmic constituents which are especially significant in the life of the cell, and which, therefore, can be considered as organs of the cell, or, better, since we understand by organ a structure composed of many cells, as cellorganoids, the chlorophyll-bodies of plantcells are especially important. These small, usually roundish, sometimes bandshaped bodies, which lie embedded in the ground-substance of the protoplasm (Fig.

23, a), give to the plant-cell and thus to the whole plant its magnificent green colour, for their delicate albuminoid bodies are saturated with an intensely green colouring-matter. The chlorophyll-bodies are of the greatest importance for the plant-cell, for in them occurs a considerable part of its characteristic vital process. Other organoids, which in many cases are likewise of great importance for the cell-life, are the drops of liquid, or vacuoles, as they are

Fia. 23.— a, A plant-cell containing chlorophyll-bodies. 6, A chlorophyll-body undergoing division. (After Sachs.) commonly but inappropriately termed. Of the vacuoles two kinds may be distinguished. Some collect only occasionally in the protoplasm in a place where a substance lies that attracts water. Others are permanent structures, and are present frequently in such great numbers that the mass of the protoplasm is small in proportion to them and merely forms thin walls for them ; the protoplasm then presents a frothy appearance, as, e.g., in many plant-cells (Fig. 24, a) and Eadiolaria (Fig. 24, &). Among the constant vacuoles that serve as cell-organoids there are the socalled contractile or pulsating vacuoles, drops of liquid that dis-

FIG. 24.— a, Plant-cell from a stamen-hair of Tradescantia. (After Strasburger.) b,Thalassicolla nucleata, a radiolarian cell, c, Paramaicium aurelia, a ciliate-infusorian cell, which cc within the protoplasm at each end a pulsating vacuole. appear and appear again at the same spot, usually rhythmically, while the liquid rhythmically mixes with the protoplasm and again accumulates. Many of these pulsating vacuoles have special efferent canals and a constant wall, as is the case in many unicellular free-living organisms, especially the ciliate Infusoria (Fig. 24 c\

In addition to such constant elements, in many cells solid constituents are met with that are present as such only temporarily. Here belong especially the food-bodies that are found in cells that nourish themselves by taking in solid food-constituents. Unicellular naked organisms, such as Amceba, white blood-cells, infusorian cells, and others, not rarely show in their bodycontents small Algce, Bacteria, and Infusoria, which they have taken up from the outside (Fig. 25, 7), and which sometimes are scarcely to be distinguished from other solid constituents of the protoplasm. These food-organisms become gradually digested and disappear.

There appear also frequently in the cell-body as products of digestion, both in the cells that ingest solid, and those that ingest only liquid food, definite granules, usually roundish, and varying greatly in nature (Figs. 7 and 22, &), which Altmann has grouped in part under the name gramda, and which, as has already been Fio.25.-7, Prog's leucocytes, or white blood-cells, each containing a bacterium. (After Metschnikoff .) //, A plant-cell containing starch-grains. HI, Starch-grains isolated — a, from the potato ; b, from corn ; c, from the pea.

seen, he regards as elementary organisms, the ultimate living elements of the cell. The composition and significance of most of these metabolic products of living substance which in the form of granules help to constitute the protoplasmic body, is not yet known. But some are characterised very exactly and are easily recognised, such as the concentrically stratified starch-granules in plant-cells (Fig. 25, // and III), the fat-droplets in the cells of the lacteal glands, the glycogen-granules in liver-cells, the pigmentgranules in the pigment-cells of the skin of many coloured animals (Fig. 17, d), the aleur onegrains, consisting of proteid, in the cells of sprouting plant-seeds, the crystals of calcium oxalate in plantcells, of calcium guanin in pigment-cells, and many others, special mention of which would lead us too far.

In the contents of many cells there occurs a fourth group of solid elements, which either do not take part at all in the life- Gcess of the cell, or have ceased to do so. These are indigestible ies which are taken in occasionally, such as sand-grains (Fig. 26), which are met with in many Amoebce, the indigestible residue of food-stuffs, such as shells, skeletons, and the capsules of food-organisms, and excretory substances, which remain for some time in the cell -body as useless by-products or as end-products of metabolism, to be excreted later.

Finally, among the solid elements of the protoplasm in certain cells, especially in aquatic animals, there occur not rarely symbiotic or parasitic unicellular organisms which strictly do not belong to the protoplasm of the cell in question, but in individual cases play an important role in the life of their host. Among such symbiotic organisms are especially many algae, the Zooxanthellce and the FIG. 26. — Amceba-eell containing in its protoplasm one diatom shell and two sand-grains.

FIG. 27. — Paramcecium bursaria, a ciliateinfusorian cell, the exoplasm of which is filled with small parasitic alga-cells (Zoochlorellce). Zoochlorellce, the nature of which as independent organisms has been for a long time in dispute. They occur abundantly in the cells of lower animals, particularly in many Infusoria and Eadiolaria, to which by the activity of their chlorophyll-bodies they furnish oxygen, so that as regards respiration their hosts are largely independent of the oxygen of the medium in which they live (Fig. 27).

We shall not enumerate exhaustively the solid components that are to be met with in cells. Such a list would fill many pages. It is only important here to understand how different in nature are the various solid constituents of protoplasm that may occur in individual cells, and how unjustified is the idea of the unitary character of protoplasm. We will now leave the solid elements, and turn to the consideration of the homogeneous ground-substance.

As already noted, the ground-substance of protoplasm, in which the granules, etc., are embedded, appears upon superficial examination completely homogeneous. This can be seen best in cells that contain only a few solid constituents stored in their ground - substance ; it is especially evident in many Amcebce, which are free-living cells possessing naked protoplasmic bodies that creep about at the bottom of stagnant water, constantly changing their form, and represent the lowest and simplest organisms inhabiting the surface of the earth. These interesting elementary organisms usually form upon their surface pseudopodia which are wholly free from granules, broad, finger-shaped, or lobate, and appear completely hyaline and structureless (Fig. 16, p. 75, and Fig. 28). In fact, in the Amoeboe the hyaline protoplasm not rarely is completely structureless. All investigations up to the present time which have been undertaken with the best microscopic methods agree in this.

But this actual homogeneity of the groundsubstance of protoplasm is not the rule ; on the contrary, the employment of high magnifying powers shows that by far the majority of cells possess in reality in their apparently homogeneous ground-mass an extremely fine and characteristic structure. Remak ('44) observed that not only nervefibres but also the ganglion-cells of the central nervous system possess a very fine fibrous or fibril lar structure— an observation that was confirmed and extended by a large number of investigators, especially by Max Schultze (71). A striated structure was later found in the protoplasm of various other cells, gland-cells, epithelium-cells, muscle-cells, etc., and thus the idea was formed by various investigators that a fibrillar structure is wide-spread in protoplasm ; this view is still defended today, especially by Flemming, Ballowitz, and Camillo Schneider.