Popular Science Monthly/Volume 8/November 1875/The Properties of Protoplasm
|THE PROPERTIES OF PROTOPLASM.|
By ERNST HAECKEL,
PROFESSOR OF ZOÖLOGY IN THE UNIVERSITY OF JENA.
THE tem protoplasm, from Gr. πρῶτος, first, and πλύσμα, form, is applied to the supposed original substance from which all living beings are developed, and which is the universal concomitant of every phenomenon of life. All that is comprehended for brevity under the terra life, whether the growth of plants, the flight of birds, or a train of human thought, is thus supposed to be caused by corporeal organs which either themselves consist of protoplasm, or have been developed out of it. Wherever nutrition and propagation, motion and sensation exist, there is as their material basis this substance designated in a general sense as protoplasm. The proof of it is held to be furnished by the protozoans called moners, the whole completely developed body of which consists solely of protoplasm. They are not only the simplest organisms with which we are acquainted, but also the simplest living beings we can conceive of as capable of existing; and though their entire body is but a single, formless, small lump of protoplasm, and (each molecule of it being like the other) without any combination of parts, yet they perform all the functions which in their entirety constitute in the most highly-organized animals and plants what is comprehended in the idea of life, namely, sensation and motion, nutrition and propagation. By examining these moners we shall gain a clear conception of the nature of protoplasm, and understand the important biological questions connected with the theory.
Some moners live in fresh water, and others in the sea. They are as a rule invisible to the naked eye, but some are as large as the head of a pin, and may be distinguished without the aid of a microscope. When completely at rest a moner commonly assumes the shape of a simple sphere. Either the surface of the body is quite smooth, or numerous exceedingly delicate threads radiate from it in all directions. These threads are not permanent and constant organs of the slime-like body, but perishable continuations of it, which alternately appear and disappear, and may vary every moment in number, size, and form. For this reason they are called false feet or pseudopodia. Nevertheless, by means of these pseudopodia the moners perform all the functions of the higher animals, moving them like real feet either to creep, climb, or swim. By means of these sticky threads they adhere to foreign bodies as with arms, and by shortening or elongating them they drag their own bodies after them. Each thread, like the whole body, is capable of being contracted, and every portion of it is as sensitive and excitable as the entire form. When any point on the surface of the body is touched with the point of a pin, or with another body producing a chemical alteration, as for example a small drop of acid, or when a current of electricity is passed through it, the threads are drawn in, and the entire body contracts into the form of a spherical lump. The same threads perform also the function of providing alimentation.
When a small infusorium or any other nutritive particle comes accidentally in contact with the extended pseudopodia, these run quickly over it like a fluid, wind around it with their numerous little branches, fuse into one, and press it into the interior of the body, where all the nutritive portions are rapidly absorbed and immediately assimilated, while all that is useless is quickly ejected.
The variations among the different moners, of which so far sixteen kinds have been described (Haeckel's "Monographie der Moneren)," consist partly in the various forms of the pseudopodia, but especially in the different kinds of propagation. Some of them merely divide into halves on reaching a certain size; others put forth little buds which gradually separate from them; and others experience a sudden division of the mass into numerous small spherical bodies, each of which instantly begins a separate existence and gradually reaches the size of the ancestral organism.
The chemical examination of the homogeneous protoplasmic body shows that it consists throughout of an albuminous or slime-like mass, hence of that azotic carbonate of the character of the highly-compounded connective group called proteine, albuminoids, or plasson bodies. Like other chemical compounds of this group, protoplasm exhibits several reactions which distinguish it from all others. It is easy to detect it under the microscope, on account of the facility with which it combines with certain coloring matters, as carmine and aniline; it is colored dark yellow or yellowish brown by iodine and nitric acid; and it is coagulated by alcohol and mineral acids, as well as by heat. The quantitative composition of protoplasm, though in some cases greatly varying, resembles as a whole that of other albuminoids, and hence consists of from fifty to fifty-five per cent, of carbon, probably six to eight of hydrogen, fifteen to seventeen of nitrogen, twenty to twenty-two of oxygen, and one to two of sulphur. Protoplasm possesses the quality of absorbing water in various quantities, which renders it sometimes extremely soft and nearly liquid, and sometimes hard and firm like leather; but it is usually of a medium degree of density. Its more prominent physical qualities are excitability and contractility, which Kühne and others have made a special subject of investigation.
On examining with the microscope the numerous substances constituting the various organs of the higher animals, it appears that they all consist of a large number of minute elements, known since Schleiden and Schwann (1838) by the name of cells; and in these cells protoplasm is the oldest, most primordial, and most important constituent. In every real cell there is, besides protoplasm, and while still alive and independent, a second important constituent, the cellular germ, so called (nucleus or cytoblast); but even this germ consists of an albuminous chemical compound which is closely related to protoplasm, and was orginally produced from it by an exceedingly slight chemical alteration. The germ is usually a smaller and firmer formation within the protoplasm of the cell.
Inasmuch as the idea of an organic cell, as now adopted by histologists, rests on the presence of two different essential parts in this elementary organism, the internal cell and the external protoplasm, we must distinguish also two different kinds of elementary organisms: germless cytods, as moners for example, and the real germ-inclosing cells, which originate from the former by secreting in the interior of the small mass of protoplasm a true germ or nucleus. Cells of the simplest kind consist only of protoplasm with a nucleus, while in general the cells of animal or vegetable bodies have also other constituents, particularly and frequently an inclosing skin or capsule (the cellular membrane), also crystals, grains of fat, pigments, and the like, within the protoplasm. But all of these parts came into being only secondarily through the chemical action of protoplasm; they are but the internal and external products of protoplasm. (Haeckel's "Generelle Morphologie," vol. i., p. 279). The single cell of the simplest kind is able to exist as an independent organism. Many of the lowest plants and animals, and also many neutral protista (which are neither animals nor plants), retain for life the character of a simple cell. Such unicellular organisms of the simplest kinds are the amœbæ, found in large numbers as well in fresh as in salt water. Amœbæ are simple naked cells of various and varying forms. The whole difference between them, especially protamœbæ, and certain moners, is that they have a germ. It is probable that this germ of the amœbæ (as may be supposed to be the case with many and perhaps all other cells) is only an organ of propagation, and hence of heredity; while all the other functions, alimentation, motion, and sensation, are performed by the protoplasm. This seems to indicate that at the reproduction of the cells, which is usually effected by segmentation, it is the germ which first divides in two, and that the protoplasm afterward gathers around each of the two sister germs till it also falls in two. It is impossible to distinguish from the common amoebae the cellular ovules of many of the inferior animals, as for example the sponges, medusæ, and other plant-like animals. With these the eggs are simple naked cells, which, with the sponges especially, sometimes crawl about independently in the body of the animal, giving rise to the idea that they were a class of parasitic amoebae. But with other animals also, and with most plants, the eggs of which generally obtain subsequently special and often very complicated encasements and other additions, every egg is originally a simple cell. The seminal elements of the male are also only simple cells, and the entire mysterious process of fructification is after all nothing but the fusion or concrescence of two different cells, the one a female egg-cell, and the other a male semen-cell. In consequence of this fusion the germs of the two combined cells dissolve, and therewith the young, newly-generated individual begins his existence as a simple cytod, or a small germless ball of protoplasm. But inside of this cytod soon arises a new germ, which turns it again into a cell, and this simple cell forms by oft-repeated segmentation an accumulation of cells. Out of this heap are produced by secretion certain germinal layers or "germ-leaves," and out of these proceed all the other organs of the complete being. Each of these organs again originally consists only of cells, and in all of these cells the essential constituent parts are only the germ and protoplasm: the germ as the elementary organ of propagation and heredity, protoplasm as the elementary organ of all the other functions, sensation, motion, alimentation, and adaptation. Cells and cytods, therefore, are true elementary organisms, independent minute forms of life, which either in the lowest existences continue to live independently, or in the higher organisms combine in numbers to form a community. Cells and cytods are the veritable "formers" of life, or plastids. The most ancient and primordial forms of plastids are cytods, the whole body of which consists of protoplasm, in which the germs are internally produced, and from which therefore the cells proceed.
As a matter of course, to the infinite varieties presented by the organic forms and vital phenomena in the vegetable and animal kingdom, corresponds an equally infinite variety of chemical composition in the protoplasm. The most minute homogeneous constituents of this "life-substance," the protoplasm molecules, or plastidules, as they are called by Elsberg, must in their chemical composition present an infinite number of extremely delicate gradations and variations. The atoms of carbon, hydrogen, nitrogen, oxygen, and sulphur, which compose each of the plastidules, must enter into an infinite number of diverse stratifications and combinations. The chemistry of to-day, with its imperfect methods of investigation, is totally powerless before these intricate organic compounds, and it is possible only to surmise, from the infinitely varied physiological qualities of the numberless kinds of plastids, the infinite variety of plastidules out of which they are composed.
According to the plastid theory recently advanced, the great variety of vital phenomena is the consequence of the infinitely delicate chemical difference in the composition of protoplasm, and it considers protoplasm to be the sole active life-substance. This theory puts force and matter in living organisms into the same causal connection which has long been accepted for force and matter in inorganic bodies. This conception has been rapidly matured, especially in the past twenty years, through the more exact information obtained in regard to the lowest kinds of organisms. Yet the idea had been grasped more than half a century ago; for the "primordial slime" which Lorenz Oken proclaimed in 1809 to be the original source of life, and the material basis of all living bodies, possessed in all essentials the same qualities and the same importance now ascribed to protoplasm; and the sarcode so called, which in 1835 was pointed out by the French zoölogist Félix Dujardin as the only living substance in the body of rhizopods and other inferior primitive animals, is identical with protoplasm. But when Schleiden and Schwann, in 1838, developed their cell theory, they were not acquainted with the fundamental significance of protoplasm. Even Hugo Mohl, who in 1846 was the first to apply the name protoplasm to the peculiar serous and mobile substance in the interior of vegetable cells, and who perceived its high importance, was very far from understanding its significance in relation to all organisms. Not until Ferdinand Cohn (1850), and more fully Franz Unger (1855), had established the identity of the animate and contractile protoplasm in vegetable cells and the sarcode of the lower animals, could Max Schultze in 1858-'61 elaborate this protoplasm theory of the sarcode, so as to proclaim protoplasm to be the most essential and important constituent of all organic cells, and to show that the bag or husk of the cell, the cellular membrane, and the intercellular substances, are but secondary parts of the cell, and are frequently wanting. In a similar manner Lionel Beale (1862) distinguished such primary forming and secondary formed substances in all organic tissues, and gave to protoplasm, including the cellular germ, the name of "germinal matter," and to all the other substances entering into the composition of tissues, being secondary and produced, the name of "formed matter."
The protoplasm theory received a wide and thorough illustration from the study of rhizopods which Ernst Haeckel published in 1862 in his "Monographie der Radiolarien," and its complete application in the "Generelle Morphologie der Organismen" by the same naturalist. Haeckel distinguishes in these works, for the first time, between germless protoplasm, consisting only of plastids called cytods by him, and the germ-containing real cells, the elementary organism of which consists already of two different essential parts, germ and protoplasm. He conceived the cytods and cells as two different gradations of plastids, of organic elementary individuals, or as "individuals of the first order," and adopted entirely, in regard to the individual independence of the plastids, the ideas which had been set forth by Rudolf Virchow and Ernst Brücke.
Virchow, whose "Cellular-Pathologie" contains the most complete application of the cell theory to pathology, called the cells and the "cell territories" belonging to them the individual hearth or source of life; Brücke designated them as "elementary organisms." The plastids or individuals of the first order, identical with them, were determined by Haeckel phylogenetically, to the effect that cytods and cells must be distinguished as two essentially different orders of formation; i, e., that cells were phylogenetically produced in a secondary manner from homogeneous cytods by means of the secretion of a germ by the protoplasm. This distinction is important for the reason that many of the lowest orders of organisms have no germ in the protoplasm; such is the case especially with the moners. These simplest of organisms were first discovered by Haeckel in 1864, and described by him in 1868 in his "Monographie der Moneren." Cienkowski and Huxley also made valuable investigations of various moners. The latter discovered in 1868 the famous bathybius, a very remarkable kind of moner, which at immense depths covers the bottom of the sea in immeasurable numbers, and which consists of formless and variable protoplasm tissues of different sizes.
Among the moners investigated by Cienkowski, the most interesting are the vampire-cells, which are formless little bodies of protoplasm that bore into vegetable cells by means of their pointed pseudopodia, kill them, and absorb the protoplasm they find in them. On the basis of these discoveries Haeckel elaborated his plastid theory and carbon theory, which give the extremest philosophical consequences of the protoplasm theory.
In England the monistic philosophy of protoplasm has received the most weighty support from Huxley, whose "Protoplasm, or the Physical Basis of Life" (1868), put it in its true light, and called forth numerous writings for and against it. One of the most recent treatises in favor of it is that of James Ross "On Protoplasm" (1874). Probably the name of plasson will be given to the primordial, perfectly structureless, and homogeneous protoplasm of the moners and other cytods, in contradistinction to the protoplasm of germ-containing cells, which are produced only subsequently, by the differentiation of an internal nucleus and external protoplasm by the plasson bodies of moners. Édouard van Beneden especially calls for this distinction in his "Recherches sur l'évolution des grégarines;" and Haeckel has adduced new facts in favor of it in his "Monograpphie der Kalkschwämme." For the theory of "primordial generation," the spontaneous generation of the first vitality on earth, the distinction is of special importance, as the first organisms thus produced could have been only structureless specks of plasson, like the bathybius and other moners. The great theoretical difficulties formerly in the way of the theory of primordial or spontaneous generation have been removed by the discovery of the moners and the establishment of the plastid theory. As the protoplasm of the bathybius is not yet as much as individualized, while in the case of other moners there are individual lumps of constant sizes, it follows that the moners are to be regarded as the natural bodies which effect the transition from inorganic to organic Nature.
- From the forthcoming volume of Appletons' "American Cyclopædia."