2 The Intermediate Nuclear Substance.-Besides the nuclear elements of definite form, whatever that form may precisely be, all investigators describe an intermediate substance of variable consistence, usually semi-liquid, amorphous and structureless, but with fine granules. It is a clear unstainable plasma' filling up the chinks, but nothing definite is known as to its composition. 13 The nucleolus which lies within the nucleus varies greatly in size and position, and more than one are very generally present. Flemming has defined them as portions of the nuclear substance, distinct in structure from network and plasma, definitely limited and smoothed, always rounded in outline, usually suspended in the network, but often independent of it. But when the minute structure and the relation of nucleoli to nuclear framework are inquired into, or the question of physiological role raised, very great diversity of opinion is found to obtain. (4) Bodies different in appearance from nucleoli may occur inside the nucleus, but of these httle is known. (5) The wall which bounds the nucleus seems to be a true integral part of the latter, but disappears at the beginning of division.

The Cell-wall.-In the older conception of the cell, which was practically that of a closed bag, the wall of the cell figured very prominently. Bat Nageli showed (1845) that some vegetable cells were destitute of walls, Leydig (1857) defined the cell in respect to its substance, Schultze and others described naked Protozoa, and the progress of the protoplasmic movement' led to the abandonment of the position that the wall was a necessary or important part of the cell. In many cells, indeed, a limiting layer is very clearly present, and a sheath or cyst is especially characteristic of passive cells. Plant-cells are almost always distinguished by the possession of a limittag wall, of definite chemical composition, consistmg of what is known as cellulose. An analogous wall occasionally occurs round animal cells. In the latter, however, the membrane is usually a ¦ comparatively slight thing, and may arise (1)

49

from an aggregation of the threads and knots of the framework; (2) as a cuticle or capsule formed from the matrix or ground substance; (3) from a combination of both these elements. Leydig has shown that in a very wide series of animal cells the membrane, such as it is, is penetrated by small but definite pores. It is very important further to remember that both in plants and animals the cells are in a great number of cases connected with one another by intercellular bridges of protoplasm, and are in nowise to be thought of as closed bags. The cell-wall of plants, which, be it again noted, is a definite chemical substance, grows in extent and thickness by an intricate organic process, in the course of which new infinitesimal elements form apparently as intercalations between the old. The growth is in very many cases far from uniform; pits, ridges, and manifold kinds of sculpturing thus appear, and give rise to numerous detailed variations. The formation of new boundaries when a cell divides is a question of much difficulty; but in plant, and apparently in some animal cells, the formation of a cellular plate' is one of the last events in the dividing process.

III. Physiology of the Cell.-When the entire organism is simply a cell, as in most of the Protozoa and Protophyta, all the vital processes which in higher forms have their seat in special sets of cells, known as tissues and organs, are of course discharged by the unit-mass. Thus a unicellular organism like the Amoeba takes in energy as food in nutrition, works it up into living matter in digestion and assimilation, and expends it again in contraction and locomotion. As in any higher organism the oxygen required for the chemical breaking up of the protoplasmic molecules, the air for the vital flame, is taken in by the absorption known as respiration, and the waste carbonic acid gas is in an essentially similar way got rid of. Further, more solid ashes' of the vital combustion are formed in the Amorba and in other actively living cells, and may pass out in excretion along with the refuse of unusable foodmaterial. The absence of a circulating fluid, of digestive glands, nerves, sense-organs, lungs, kidneys, and the like, does not in any way restrict the vital functions of a unicellular organism. All goes on as usual, only with greater chemical complexity, since all the different processes have but a unit-mass of protoplasm in which they occur. The physiology of independent cells, instead of being very simple, must be very complex, just because structure or differentiation is all but absent. It is, however, possible to express the manifold processes in a comparatively simple way by remembering what Claude Bernard was one of the first clearly to emphasise, that vital processes must be really only twofoldbuilding up and breaking down of living matter. On the one hand the protoplasm or real living matter is being by a series of chemical processes built up or constructed; on the other hand, in activity it is breaking down or being destroyed. The income of food or energy is, at the expense of the cellular organism, gradually raised into more and more complex and unstable compounds, until the genuine most complex and more unstable living matter itself is reached. On the opposite side, with liberation of energy in the form of work, this living matter breaks down into simpler and simpler compounds, until only the work, the waste products, and heat remain as the equivalent of the income of energy or food on the other side of the life-equation." On the one hand there are constructive processes, on the other, destructive; chemical synthesis and chemical dis. solution is another expression of the contrast; while the two sets of processes are in more modern

language respectively termed anabolism and katabolism (see PROTOPLASM).

But only a few cells, comparatively speaking, live a free and independent life. The majority are component elements in higher unities. In these the original many-sidedness of function is more or less lost, or at anyrate in abeyance, and that exactly in proportion to their degree of subordination. Even in individual cells there is a tendency, obviously within narrow limits, towards differentiation-that is, to the restriction and specialisation of certain parts for certain functions. But when the cells form elements of a larger whole, the division of labour finds full effect. From position and other conditions the cells cease to be uniform or metaphorically many-sided. Certain sets predominate in contractility, others in irritability, others in secretion, others again in storage, and so on. In such cases one function predominates over the others, which are subordinate or only dormant possibilities. Thus arise muscle-cells, nerve-cells, glandular cells, fat-cells, and the like. Compared with Amoebae, those cells must have a simpler physiology; they may have gained in complexity of structure, but have lost in manifoldness of function. The aggregation of similar cells, usually with one predominant habit or function, results in the formation of tissues, and the discussion of the different sets may well be referred to the article HISTOLOGY.

One general physiological fact may, however, be referred to which will greatly assist in understanding the life both of independent cells and of those which form the elements of tissues. A survey of the unit-organisms, both among plants and animals, reveals the existence of three wellmarked phases. Some cells are emphatically active, equipped with motile lashes (cilia or flagella), and obviously liberal in their expenditure of energy. Others are just the reverse of this, emphatically passive, wrapped up in themselves and without motile processes, obviously economical in their expenditure, conservative of their income. A third set form a mean between these two extremes, are neither encysted like the latter nor lashed like the former, but furnished with the relatively slow-moving processes characteristic of Amoebae, and living in a via media between activity and passivity. These three types may be termed respectively ciliated, encysted, and amoeboid, or active, passive, and moderate. That these types generally correspond to the three great divisions of the Protozoa shows that they represent the three main possibilities of cellular life. Now in the very simplest forms all the three phases occur in one life-history; no step has, as it were, been taken in any one of the three directions; the primitive cells are in a state of physiological indifference. What has happened in the

accentuated to the more or less marked subordination of the others. Not that the emphatic adoption of one line of cell-life excludes the others; they may in fact occur as temporary stages, or as pathological deviations.

But while simple observation is sufficient to establish the existence of a cycle of phases in the life of primitive cellular organisms, such as Protomyxa, and the existence of three main lines of specialisation among the Protozoa, the importance of this conception of a 'cell-cycle' becomes increased and justified when the facts are considered physiologically. If we start from a simple cell, such as an Amoeba, it is evident enough, from what has been already said as to the twofold nature of all vital processes, that the principal physiological possibilities are the three phases above indicated. On the one hand, with preponderance of income over expenditure, of constructive over destructive changes, of anabolism over katabolism, the cell must tend to become larger in size, more weighted with stored material, more sluggish or passive in habit, and more rounded in form. But if the reverse take place, the cell will tend to become less bulky, more active or locomotor in habit, and more elongated

Fig. 5.-Protomyxa : 1, encysted: 2, dividing; 3, spores escaping as ciliated bodies,

passing into 4, amoeboid state; 5, plasmodium' forming from fusion of amoeboid cells.

in form. The sweated-off cyst of the former, the motile processes of the latter, are expres sions of exactly opposite constitutions and conditions. A third physiological possibility remains, that namely of continuing in a position of average equilibrium between income and expenditure, between anabolism and katabolism, in a middle way between the fitful fever of extreme ciliated activity and the sluggish sleep of encysted passivity.

Now if we take these two facts-the existence of a primitive cycle through which cells tend to

a

Fig. 6. The Cycle of Cell-life:

a, encysted; b, ciliated; c, amoeboid; d, plasmodial. pass, and the existence of three main physiological possibilities which lie behind the cycle-we are in a better position to understand both the changes exhibited in normal and pathological conditions by individual cells, and the various forms of cells as they occur in the tissues of the higher organisms. Thus lashed cells such as those of the windpipe of mammals, the skin of many lower worms, the inside of a Hydra, the male elements of most animals and many lower plants, emphasise one phase in the cycle, and it is not surprising to find that in certain conditions they may sink down into the amoeboid type. Or again, the amoeboid character of young ova, preceding the more passive and encysted condition of the mature cells, is in view of the cell-cycle' a most natural procedure. In many cases artificial stimulus of various kinds has been shown to make cells pass from one phase to another of the primitive life-cycle theoretically possible to all. In the same way the preponderance of cellulose in cells

a natural consequence of common descent and similar conditions, is not without its marvel when the complexity of the process (see below) receives due consideration. Even in detail there is in structural as well as in physiological changes a deep-seated unity of process. But while the essential similarity of all cases of simple indirect' division must be allowed, it is only fair to recognise that in minor details very manifold variations occur. Even in those Protozoa where the nuclear changes of division have been followed, considerable diversity of detail obtains; nor, within a single genus do the ova of two different species of threadworm (Ascaris) divide in exactly the same fashion. But neglecting at present the detailed divergences, whether these occur normally and constantly, or as they often do atypically and arbitrarily, it is necessary now to notice the general steps usually observed in cell-division.

We have already described the nucleus as consisting of a readily stainable (chromatin) network or ribbon, and of another substance (so-called achromatin) which does not stain so deeply. As a preliminary to division, the nucleus loses its definite boundary, and the chromatin threads no longer exhibit the regular disposition they have when at rest. The threads form an irregular wreath, and as the loops break, their arrangement is comparable to a star in which the open ends of the loops are directed outwards, and the closed ends lie in the centre. By subsequent movement this position is reversed, the loops gather into two groups, which lie with their open ends towards one another in the middle. Meanwhile, the achromatin elements also exhibit regular arrangement, forming fine streaks stretching from the centre towards the poles, and exhibiting an appearance which is often compared to a striated spindle. At the two poles of the cell the granules of the general protoplasm

be in any way confused with unsubstantial polar stars previously mentioned. Soon after this stage is reached, the real cell-division occurs. The protoplasm constricts across the middle of the cell, and the division is accomplished. In plant-cells, and apparently in some animal cells also, the division of the protoplasm is accompanied by the formation of a cellular plate, which bounds the open surfaces of the two daughter-cells. With or without cellular plate, the result is the formation of two daughter-cells, each with half of the original nucleus. But this is not all, the half nucleus formed after the above fashion has to be reconstructed into the original resting form. A series of retrogressive stages occur, in the course of which the nucleus passes from star to wreath, and from wreath into the typical network or twisted coil. In some cases the steps of reconstruction seem to correspond very closely to the various steps of the antecedent upbreaking.

Death. It seems tolerably certain, as Weismann and others have suggested, that the unicellular Protozoa are in the great majority of cases practically immortal. These simple organisms have nobody to keep up, in their functions they appear to be continually self-recuperative, and except from entirely abnormal conditions such cells probably never die. The pool in which they live may dry up for ever, or other animals may swallow and digest them, but such casualties are very different from natural death. They may indeed lose their individuality by doubling in division, or the whole cell may break up into spores, but where there is nothing to be buried we can hardly speak of death. It seems in fact justifiable to say that death began with the formation of a many-celled body. Even there, a certain amount of immortality may be claimed for the reproductive cells, which, becoming separate from the parent organism, proceed to divide into a body which will of course eventually die, but also into reproductive cells, which, as some of them at least will form again fresh organisms and reproductive cells, may be said to be links in a continuous and immortal cellular chain. But leaving aside the really immortal Protozoa, and the logically immortal successful reproductive elements, it must be allowed that cells, like organisms, die. And that not only with the body as a whole, but by themselves. Certain superficial cells are constantly being brushed off and replaced by others; the red blood-corpuscles break up in the fluid; others become hardened in death into the 'mummified cells of supporting and epidermic structures; others surrender themselves into mucus or in the ejection of lassoes as in the Colenterates; others practically die away into fat and reserve products, or may in manifold ways degenerate. Many surfaces, especially in secreting regions of the body, exhibit continual death of cells, and regeneration by the division of the survivors.

IV. Modern Aspect of the Study of the Cell.With the improvement of appliances and the perfecting of staining methods, the study of the cell has within late years become at once more accurate and more complex. On the one hand, the labours of the early histologists are being amplified and corroborated with ceaseless industry. The forms of cells in different animals and tissues, the minutiae of their structure, the processes observed in their multiplication, are being each year more and more perfectly investigated. On the other hand, the emphasis which has been laid on the protoplasm is finding expression in numerous attempts to explain the forms and phases of cell-life in terms of the underlying plasmic changes, and such investigations as hich seek to disclose the mechanics of cell- and ovum-segmentation, the conditions of e nilibrium and change,

1

CELLE

or the chemistry of the various parts, mark the linit and high-water mark of cellular biology.

Practical Study.-To gain a preliminary acquaintance with the cell, the student should examine with ¦ a good microscope—(1) Free cells as seen in unicel lular plants, such as yeast, green mould, simple ale, or in pollen grains, &c.; in unicellular animals like Amaba, Paramæcium, Vorticella; in the elements of the blood; in the ova of animals, as found in spawn of frog, &c. (2) Simple vegetable tissues as seen in root-hairs, transparent leaves, epidermis of plants, and common fresh-water algae like Spirogura; simple animal tissues readily obtained from frog, earthworm, Hydra, and the like. For research in details of structures, staining and sertion-cutting must be resorted to.

Literature.-(1) For history, see BIOLOGY, HISTOLOGY, MAPHOLOST, PROTOPLASM; M'Kendrick, On the Modern C) theory (1888); Drysdale, Protoplasmic Theory of

Life (1874)

2 For structure of cell and process of division, conFat nrst modern text-books of histology, such as those of Erana, Fol, Frey, Klein (English), Leydig, Ranvier, ani Stohr. For recent researches, see Journal of Royal Mpical Society. As one research is rapidly superseling another, detailed references need not be given. Frgeneral bibliography, see Professor M'Kendrick's paper over; for nucleus, Van Bambeke, Etat actuel de nos fas suamances sur la Structure du Noyau (Gand, 1885); for a-division, Waldeyer, Uber Karyokinese,' Archiv. f. Amat, u. Physiol. (1887); for the vegetable cell in par| tia.ar, Zimmerinann, Die Morphologie und Physiologie der Pflanzenzelle,' Schenck's Handbuch d. Botanik (1887). See also Professor Carnoy's cell journal, La Cellule. The Memoirs, which will always be classic in the history of lore, both in themselves and on account of the ** -alus which they supplied, will be found in the followg and those to which they chiefly refer: Van Beneden, Becherches sur la Maturation de l'Euf, dec. (1883); Fermin ng, Zell-substanz, Kern und Zell-theilung (1882); and later papers in Archiv. f. mikr. Anatomie; From mann, Unters. über Struktur, Lebenserscheinungen und Braatiomen thierischer und pflanzlicher Zellen (1884);

and R. Hertwig, Beiträge zur Morphologie der Zellen 178; Leydig. Zelle und Gewebe (1885), and prevas works; Strasburger, Zellbildung und Zell-theilung કે તું તું, 1880),

5 For general physiology, consult first Foster's Physi *** chap. 1, then general works on physiology of pants and animals-e.g. Sachs' Text-book of Botany and Lortures on the Physiology of Plants, also Vines' similar

117, Hermann's Handbuch der Physiologie, &c. Further, Herbert Spencer's Principles of Biology; P. (mārs, Restatement of the Cell-theory,' Proc. Roy. Soc. 2.19 13): M. Foster's article Physiology, Encyclojerist Britannica; Berthold, Studien uber ProtoplasmaLeip 1886); Schwarz, Die Morphologische wad them she Zanimmensetzung des Protoplasmas (Bresaan, 1987); and the article PROTOPLASM.

Celle. See ZELL.

Cellini, BENVENUTO, a celebrated Italian goldsmita, sculptor, and engraver, and the author of se of the most interesting autobiographies ever written, was born in 1500 in Florence, a city which be was forced to quit in early life through having taken part in an affray.' He then travelled to Eme, where his skill as an artist in metal-work red him the favour of the highest nobles and priates. So anxious were his patrons to secure

services that they allowed him the utmost arense of conduct. By his own account he was as exert with sword and dagger as with his gold sn, th's tools, and he had apparently no scruple in mariering or maiming any who endeavoured to thwart him. He states that at the siege of Rome 1927 it was he who killed the Constable Bourbon, and that he afterwards shot down the Prince of Prange before the castle of St Angelo. He stood fe a time high in favour with Pope Clement VII., it was eventually flung into prison for the murder of a rival goldsmith. In 1534 he was pardoned

and set free by Paul III., who wished him to engrave dies in the mint; soon afterwards, having spoken contemptuously of the pope's artistic tastes, he was cast into an oubliette of the castle of St Angelo. He escaped through his knowledge of the castle's vaults, but was immediately recaptured, and was only saved from the pope's vengeance by the intercession of Cardinal d'Este. For some years he lived alternately in Rome and Florence, Mantua and Naples. In 1537 he went to the court of Francis I. of France, by whom he was honourably received, and for whom he executed a golden spice-box, the design of which, he tells us, was so exquisite that the king uttered a loud outcry of astonishment on seeing it,' and 'could not satiate his eyes with gazing on it.' In Paris he became involved in a lawsuit. Having lost his case, he had recourse, as usual, to his dagger. I attacked,' he says, 'the plaintiff who had sued me, and one evening I wounded him in the legs and arms so severely (taking care, however, not to kill him) that I deprived him of the use of both his legs. This act went unpunished. Having given offence, however, to the reigning favourite at the French court, Cellini returned to Florence, where he worked under the patronage of Cosmo de' Medici, and where he executed his most successful piece of sculpture, the famous bronze Perseus with the head of Medusa' of the Loggia de' Lanzi. began to write his autobiography in 1558, and died at Florence in 1571.

He

Cellini was a man of versatile fancy, passionately devoted to his art, and his technical skill was supreme. But his designs were often feeble and tasteless, and he seems to have had no sound knowledge of human anatomy. He has, on the whole, been somewhat overrated as an artist, and has been credited with the production of many beautiful cups and vases (such as the Cellini vase' in the British Museum) which were really the work of German silversmiths in the 16th century. But he has not been, and could not easily be, overrated as an author. His autobiography is a work of extraordinary interest. From the pages of this book,' incarnate in a single personality, leans forth and says Mr Symonds, the Genius of the Renaissance, speaks to us.' Though he had not the faculty of self-criticism, Cellini was a shrewd judge of others, and had a remarkable talent for portraying character. His book gives a faithful and a wonder fully vivid picture of Italian society in the 16th century. The animation of the narrative and the racy vigour of the style could hardly be surpassed. The keen insight and unblushing frankness of the writer make his work as fascinating to the student of human character as it is invaluable to the historian of the Renaissance. Cellini reveals all the evil and all the strength of his nature, his vindictiveness, braggartism, and self-worship, no less than his fiery energy and powerful intellect, his splendid self-reliance and passionate love of art. He is the most candid of autobiographers, and he is as ignorant of shame as he is candid. An admirable translation of his work by Mr J. A. Symonds has been published (1887). See also the magnificent monograph by Eugene Plon (1882).

Cellular Plants. It was formerly attempted by De Candolle and others to unite all the lowest plants destitute of vascular tissue (see CELL, and VEGETABLE HISTOLOGY) under the general title Cellulares, as opposed to the Vasculares, including all the higher plants. Although this classification is long disused, the term cellular plants is often familiarly employed to distinguish the Fungi, Algæ, Lichens, Characew, Liverworts, and Mosses (q.v.) from the higher or vascular cryptogams— Ferns, Horsetails, Lycopodiacea and Selagineller, and Isoeteæ. See THALLOPHYTES, VEGETABLE