PROTOPLASM. In most of the biological articles already before the reader, whether concerned with general questions, as BIOLOGY, ANATOMY, BOTANY, EMBRYOLOGY, EVOLUTION, HISTOLOGY, MORPHOLOGY, PHYSIOLOGY, &c, or even with special groups of living beings, as ANIMAL KINGDOM, FORAMINIFERA, FUNGUS, PROTOZOA, &c, special reference has been made to protoplasm as the living matter from which all kinds of living beings are formed and developed, and to the properties of which all their functions are ultimately referred. Fundamentally important then as this substance is, whether we occupy the standpoint of morphology, physiology, or aetiology, an attempt must be made to outline the way in which our knowledge of it has been reached, to bring together by the aid of a short summary the statements of such preceding articles, and to supply means of extending the general idea thus obtained by reference to the original literature of the subject.
§ 1. History.Among the varied and fruitful observar tions of the early microscopists, Bôsel v. Bosenhof's ex-cellent account (1755) of his "Proteus animalcule" (the familiar Amoeba) is especially noteworthy as the earliest description of the form and movements of what we now know as a mass of living protoplasm. Such discoveries as those of rotation in the cell of Chara (Corti, 1772), and of similar movements in other plant cells (Vallisneria, Meyen, 1827; Tradescantia, B. Brown, 1831), are also memorable,more so indeed in this relation than is the great contemporaneous movement in general histology, since this, though aided by the rapid improvement of the microscope, eagerly carried on by the united labours of zoologists and botanists, headed by Johannes Miiller and Bobert Brown, and culminating in the hands of Schleiden and Schwann (1838-39) in the fundamental morphologi-cal generalization of the cell theory (see MORPHOLOGY), included views of the structure, origin, and function of the cell-substance alike erroneous and misleading. Know-ledge had in fact to start afresh from the level of the unappreciated discovery of Bosenhof ; and it is accordingly from the observations of Dujardin on Foraminifera (1835) that our modern knowledge of protoplasm dates. His main account is still worth reading in his own words. In proposing the term "sarcode," he says, "je propose de nommer ainsi ce que d'autres observateurs ont appelé une gelée vivante, cette substance glutineuse, diaphane, in-soluble dans l'eau, se contractant en masses globuleuses, s'attachant aux aiguilles de dissection, et se laissant étirer comme du mucus, enfin se trouvant dans tous les animaux inférieurs interposée aux autres éléments de structure." Though thus dissipating many errors, and placing the study of the lowest forms of life on its true basis, Dujardin unfortunately did not see the full bearing of his discovery. He recognized his sarcode, however, in the polyps, and noted that the ova of the slug exhibited similar move-ments. The next important step was not taken until 1846, when the botanist Hugo von Mohl, working on quite independent lines, reached a clearly defined conception of the vegetable cell, not only, as usual hitherto, distinguish-ing the cell wall and the nucleus from the cell contents (Zellsaft), but also the "tough, slimy, granular, semi-fluid" constituent from the watery cell-sap hitherto generally confused with it under the common name. For this substance (which Schleiden had already vaguely men-tioned as "Schleim") he proposed the term "protoplasma" (7rpójToc, first, 7rA.ao-/ia, formed substance). The discovery of the amoeboid movements of colourless blood corpuscles dates from the same year, and the basis was thus prepared for Ecker's acute comparison (1849) of the "formed contractile substance " of muscle with the " unformed contractile substance " of the lowest types of animal life. This speculation, so profoundly anticipatory of our present stand-point (see PHYSIOLOGY), was greatly strengthened when Donders shortly afterwards succeeded in referring con-tractility from the cell-membrane to the cell-substance. Cohn's researches among microscopic plants and animals, and particularly his study of the transition, which at that time seemed so marvellous and so perplexing, from plant-like quiescence to animal-like activity, exhibited by the protoplasm of such an alga as Protococcus on escaping from its cell-wall, led him to suggest that vegetable protoplasm and animal sarcode, " if not identical, must be at any rate in the highest degree analogous substances." This speculation again ran too far in advance of current con-ceptions, dominated as these were by the errors which accompanied the cell-theory, and another decade of research was needed for its establishment. This was effected on several simultaneous and convergent lines. The botanical evidence culminated in De Bary's classical monograph of the Myxomycetes (1859); the study of the segmentation of the ovum, and the rapid advance of animal histology, both largely due to Kolliker, were of marked importance; while the clear identification of the vegetable " protoplasm " with the animal " sarcode," requiring, as it did, a mastery of all these lines and results of inquiry, was finally effected by M. SCHULTZE (q.v.), whose researches on Foraminifera (1854), and subsequent admirable studies in animal histology, prepared him to accomplish the definite reform of the cell-theory. This he did by fully and finally replacing (1861-63) the early conception of the cell as an all-import-ant membrane enclosing a nucleus surrounded by fluid by that of a unit-mass of living matter or protoplasm (the nucleus alone being viewed as essential, the wall or mem-brane no longer so). Our present usage of the term proto-plasm for the living substance of the animal as well as of the plant dates from Schultze's paper ("Ueber Muskelkorperchen und das was man eine Zelle zu nennen habe," Arch, f. Anat. u. Physiol., 1861); and the term sarcode, notwith-standing Dujardin's priority, has since lapsed into disuse, save to some extent among French authors.
This rejuvenescence of the cell theory, in a form point-ing to a far deeper unity of the forms and processes of organic nature than its founders had ventured to dream, marks the commencement of a new epoch of detailed inves-tigation of all the forms, aspects, conditions, and products of protoplasmic life; but in this movement the workers are too numerous for mention save in so far as may be inciden-tal in the following scanty reference to some of their main results (l).
§ 2. Appearance and Properties of Protoplasm.To ob-tain a notion of the appearance and physical properties of protoplasm, it is expedient as it were to repeat the process of discovery, and acquire concrete ideas by actual observa-tion as far as possible, or at least from good figures. The Amoeba (see PROTOZOA) and the FORAMINIFERA (q.v.) thus afford convenient and classical examples of the protoplasm of the lowest animal forms ; the colourless corpuscles of blood should also be examined, and the structure of the higher tissues (see ANATOMY and HISTOLOGY) inquired into, and the segmentation of an ovum (see REPRODUCTION) observed,most conveniently perhaps in frog spawn. Vegetable examples are readily obtained from the cells of a growing shoot (see BOTANY, vol. iv. pp. 83 sq., figs. 1 and 6); while the living cells of Chara (BOTANY, fig. 7) and other examples of protoplasmic movement should be observed. Thus, with the aid of the descriptive passages to be found in the articles referred to at the outset, a tolerably clear idea of a mass of protoplasm, with its con-tained granules of various kinds and its sap-vacuoles, will be obtained; and its frequent differentiation into an outer layer or ectoplasm, clearer and denser, passing into an inner layer or endoplasm, usually more fluid and granular, will be noted. A finely reticulated structure of the protoplasm may also be made out in many cases; the nucleus (incon-spicuous since equally refracting with the protoplasm dur-ing life, but brought out clearly after death by the process of internal digestion of the surrounding protoplasm, or by the application of dyes and other reagents), and its con-tained nucleolus, as well as the cell-wall when present, will be observed. Wide variations of consistency will thus be noted from the comparatively solid, almost brittle, state of the quiescent protoplasm of some seeds to its thin, syrupy, and largely vacuolated state in a growing vegetable tissue (cf. BOTANY, fig. 6). Such structural inquiries are now in active progress, especially in connection with the process of cell division (see HISTOLOGY, REPRODUCTION), and many questions of detail are more or less under active dispute, e.g., the relation of the nucleus to the protoplasm the existence or constancy of an internal network (the "stroma") in both, the conditions of occurrence of that continuity of protoplasm lately shown to exist through the cell walls of many vegetable tissues, and so on (2).
§ 3. Manifestations of Life (Functions).The vital pro-perties or "functions" exhibited by undifferentiated living protoplasm (e.g., Amoeba) are usually enumerated as con-tractility, irritability and automatism, reception and assimi-lation of food, metabolism with secretion and excretion, respiration, and reproduction. Thus we have represented all those functions which in higher animals seem to be confined to special tissueswhich we accordingly recognize as muscular or nervous, secretory or excretory, respiratory, reproductive, or the like. Yet in these organs, however apparently specialized to one function only, a residue of all or nearly all the other fundamental properties of pro-toplasm remains and may be redeveloped ; and thus those functional changes (necessarily accompanying morpholo-gical evolution or change of environment) which we call "adaptation" and those pathological disturbances which we term " disease " are alike provided for. See BIOLOGY, PATHOLOGY, SELECTION AND VARIATION, also (3).
§ 4. External Conditions of Life.See BIOLOGY.
§ 5. Experimental Modification of the Conditions of Life. _The behaviour of protoplasm under various modifications ©f physical conditions has been investigated by Schultze, Kiihne, Strasburger, Engelmann, and others, while not a few researches are also extant as to the behaviour of living cells under various chemical stimuli, among which those of Darwin (see INSECTIVOROUS PLANTS) and Frommann may be mentioned as especially suggestive. See also SCHIZO-MYCETES, NUTRITION, and (4).
§ 6. Chemical Composition and Processes.This aspect of protoplasm is of constantly increasing importance, since for the chemist all functions alike can only be viewed in terms of those specific anabolic or katabolic changes which to the physiologist, on the other hand, seem mere accom-paniments of them (see PHYSIOLOGY, NUTRITION, REPRODUCTION). The determination of the chemical nature of protoplasm is thus the supreme problem of physiological chemistry; and, while, thanks to the labours of Beinke, E. Schultze, and others, there has been a rapidly increasing knowledge of its anastates, but more especially of its katastates, and of many cases of the unity of metabolic processes throughout nature, several daring general hypo-theses are already in the field. Of these that of Schiitzen-berger, who views proteid bodies as complex ureides, and that of Loew and Bokorny, who regard them as a complex mixture of aldehyde groups, are*»examples. See (5).
§ 7. Molecular Constitution.Many hypotheses as to the minute structure of protoplasm have been proposed ; thus Spencer resolves protoplasm into "physiological units," Haeckel into " plastidules," while Darwin accounts for heredity by reference to the properties of supposed " gem-mules," Engelmann suggests the existence of " contractile units " (isotagmen), &c. ; but those various hypotheses, framed mostly for special purposes, still await more general criticism. See (6).
§ 8. Origin of Protoplasm.See ABIOGENESIS, BIOLOGY, REPRODUCTION, and (7).
Bibliography.In addition to the articles above referred to, the reader may with advantage consult the following works, from which complete bibliographical details can be obtained. (1) For general history see Sachs, Geschichte d. Botanik ; Carus, Ge-schichte d. Zoologie; Engelmann, "Physiol, d. Protoplasma u. Flimmerbevvegung," in Hermann's Handb. d. Physiologie, i., Leipsic, 1879 ; and for special history, Dujardin, " Recherches s. 1. .organismes inférieurs," especially § iii., "S. 1. prétendus estomacs d. animales Infusoires et s. u. subst. appelée Sarcode," Ann. d. Sci. Nat., 1835, p. 367; H. v. Mohl, " Ueb. d. Saftbewegung im Innern d. Zelle," Bot. Zeitung, 1846, p. 73; Max Schultze, Ueb. d. Organismus d. Polythalamien, Leipsic, 1854; Id., "Ueb. Muskelkbrperchen," &c, Arch. f. Anat. u. Physiol., 1861; Id., D. Protoplasma d. llhizopoden u. d. Pjîanzenzellen, Leipsic, 1863 ; De Bary, "Die Mycetozoen," Zeitsch. f. wissensch. Zool., x., 1859; Haeckel, Generelle Morphologie, i., Berlin, 1866. (2) For general structure and properties see Hofmeister, D. Lehre d. Pfl.-Zelle, Leipsic, 1867 ; Sachs, Lehrb. d. Botanik, or Engl, translation, Oxford, 1882 ; Sachs, Vorlesungen üb. Pflanzen-Physiol., Leipsic, 1882 ; and other text-books of vegetable physiology, especially those of Beinke and Pfeffer ; Strasburger, D. Botan. Practician, Leipsic, 1884 ; and Engelmann, op. cit. Among special works see Kühne, Untersuch, üb. d. Protop. u. d. Contractilität, Leipsic, 1864 ; Strasburger, "Studien üb. d. Protoplasma," Jena. Zeitsch., x., 1876, also Ueb. Zellbildung u. Zelltheilung, 2d ed., Jena, 1876. (3) Dohm, Das Principd. Functionswechsels, Leipsic, 1875; Foster, Handbook of Physiology, 4th ed., London, 1884. (4) For experimental researches see Engelmann, Kühne, and other authors cited under (2) passim ; also Darwin, Insectivorous Plants, London, 1876 ; Frommann, "Ueb. Structur, Lebenserschein, &c, d. Thier, u. Pfl.-Zellen," Jena. Zeitsch., xviii., 1884. (5) Gamgee, Physiol. Chem. of Anim. Body, London, 1880 ; Foster, Handbook of Physiology ; Gautier, Chimie appliquée à la Physiologie ; Schützenberger, " Recherches s. l'Albumine, &c," Bull. d. I. Soc. Chimique, 1875 ; Leow and Bokorny, Biol. Central-Blatt, i., No. 7 ; and Reinke, " Ueber d. Protoplasma," in Untarsuch. a. d. bot. Lab. .Univ. Göttingen, 1881. (6) Spencer, Principles of Biology, vol. i. ; Haeckel, Gen. Morph., i. ; Darwin, Variation under Domestication; Engelmann, op. cit. (7) Engelmann, op. cit. (P. GE.)
These numbers refer to the bibliography at p. 830.