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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 Amabæ, 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

Fig. 4-Phases of Cell-life. (After Geddes.) Development of passive or resting, intermediate (amoeboid), and active (motile) states.

higher classes of Protozoa Infusorians, Gregarinids, Rhizopods-is that one phase has been

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

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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 expressions 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

α

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 worns, 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

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encysted is a natural character of the passive reached, and portions of the substance are cleft
plants, and the insheathed cells of many animal apart from the main mass. From such a case
tissues may be similarly expressed as an exhibition to the separation of multiple buds, which are
of the same passive phase. But it is enough here little more than overflowings of too large a
to point out the possibility of classifying and in- cell, or to the commoner occurrence of simple
terpreting the various cells composing the tissues budding, is no great step. The difficulty begins,
of higher organisms in terms of an original life- however, when we consider the ordinary cell-
cycle, or deeper still in terms of those twofold pro- division, which appears in most cases as a deliber-
toplasmic possibilities which lie behind all forms ate and orderly process, including a well-defined
and phases whether of cells, tissues, or organisms series of nuclear changes. As to the mechanics
themselves. This conception of a cell-cycle is due of this process only a few suggestions of moment
to Geddes (see Bibliography at end of article).
have been made. Thus Platner points out that
Cell-division.-When the vital processes are so the explanation must be in terms either (1) of
related that income and upbuilding exceed ex- chemical processes influencing the cellular sub-
penditure and dissolution, the cell must obviously stance, or (2) of protoplasmic movement due to
accumulate capital and increase in size. In some the above or to external influences, or (3) of
cases the cell may expand into relatively gigantic unknown molecular and attractive forces. He
proportions, as in the alga Botrydium and in himself finds the condition of nuclear division to be
many eggs. Growth, however, brings a nemesis in part a streaming movement of the protoplasm,
with it, this namely, that the mass to be kept alive such as is familiar in many Protozoa, and would
increases more rapidly than the surface through regard the division of the protoplasm as a purely
which the vital processes are accomplished. In mechanical process. In his studies on protoplasmic
spherical cells the former increases as the cube, mechanics, Berthold has also attacked this in-
the latter as the square of the radius. The bigger tricate problem, but more in relation to the nature
the cell gets, the more difficult do its conditions of the dividing partitions than with reference to
of life become. The supplies of food and oxygen, the forces at work. Professor Van Beneden, who
has done so much in working out the details of
cell-division in the ovum, has in a recent paper
(1887) expressed himself as follows in regard to
the deeper problem: All the internal movements
which are associated with the cellular division
have their immediate cause in the contractility of
the fibres of the reticular protoplasm which form
two antagonistic groups.' All that one can at
present conclude is that the process represents, as
above noticed, a physiological necessity, and that
it takes place in connection with very intricate
physical and chemical changes within the cell.

B

1

2

3

Fig. 7.

5

6

A, Life-history of unicellular plant (Protococcus): 1, encysted; 2,
quitting its cell; 3, ciliated; 4, quiescent; 5 and 6, dividing.
B, Life-history of Amoeba: 1, encysted; 2, escaping; 3, free;
4, dividing; 5, free half with vacuole v, nucleus n, and food-
particles f; 6, encysting anew. 4 and 5 may also represent
the union of two Amoeba (conjugation).

the means of accomplishing purification and the
like, cannot keep pace with the growth of the
living mass if the surface increase only at a
much less rapid rate. A limit of growth is thus
reached. The cell must stop growing, or go on
growing at an increasing risk, or in some way
restore the balance between mass and surface.
This last course is the one most frequently
exhibited-the cell divides. However this may
be effected, the result is in all cases the same
-namely, the reduction of
mass, and corre-
sponding increase of surface. Like other organ-
isms, the cell-organism reproduces at its limit
of growth. This rationale of cell-division, due
especially to Herbert Spencer, is obviously clearest
in reference to free-living cells like Protozoa,
Protophyta, blood-corpuscles, reproductive cells,
and the like, but the general principle holds good
throughout.

It is evident, however, that such considerations as the above go to justify rather than to explain cell-division. They show why the cell ought to divide, not how it does. The real mechanism of the process is still a riddle. In its very simplest expressions, indeed, the riddle may be partly read. In a simple and primitive Protozoon like Schizogenes, the protoplasm seems literally to break. Irregular fissures appear, as well they might if a condition of unstable vital equilibrium has been

Modes of Cell-division.-After abstracting the rare occurrence of almost mechanical ruptures and of overflow buds, various modes of orderly division remain to be noticed. (a) The cell may give off a bud, usually smaller than itself. With this a portion of the nucleus is usually associated, as in many Protozoa; or the processes may occur apart from demonstrable nucleus, as in the common yeast-plant. (b) Division into two is by far the most frequent mode of multiplication, and occurs all but universally. In a small minority of cases the division is accomplished without any intricate nuclear change, the cell being in an apparently simple way divided into two, with half of the nucleus in each daughter-cell. Such divisions are said to be 'direct.' In most cases the nucleus, apparently taking the initiative, undergoes a striking series of orderly changes before the division is perfect. This is the commonly observed condition, and such divisions are termed indirect.' (c) But in many cases the division occurs in a very different way, being not single but multiple. From one cell more than two daughter-cells arise simultaneously, and that not by external cleavage, but by internal multiplication. Such a mode of multiplication is termed endogenous division or 'free' cellformation, and is well seen in many Fungi and Algae. It may be compared with the ordinary process by defining it as division taking place in limited space and time, since the daughter-cells arise within the mother-cell, and simultaneously, not successively. It is, in many cases at least, preceded by the rapid division of the nuclei, to form centres round each of which protoplasmic material then becomes aggregated. In a few cases, Arnold has described a peculiar breaking up of the nucleus which he called fragmentation.

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Karyokinesis.-One of the most beautiful results of recent histology is the demonstration of the general unity of process which obtains in the

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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

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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 unicel lular Protozoa are in the great majority of cases practically immortal. These simple organisms have no 'body' 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 Coelenterates; 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 protoplasmic changes, and such investigations as those which seek to disclose the mechanics of cell-division and ovum-segmentation, the conditions of cellular equilibrium and change,

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I

CELLE

or the chemistry of the various parts, mark the limit 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 unicellular plants, such as yeast, green mould, simple algae, or in pollen grains, &c.; in unicellular animals like Amaba, Paramecium, 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 Spirogyra; simple animal tissues readily obtained from frog, earthworm, Hydra, and the like. For research in details of structures, staining and section-cutting must be resorted to.

Literature.-(1) For history, see BIOLOGY, HISTOLOGY, MORPHOLOGY, PROTOPLASM; M'Kendrick, On the Modern Cell-theory (1888); Drysdale, Protoplasmic Theory of Life (1874).

(2) For structure of cell and process of division, consult first modern text-books of histology, such as those of Brass, Fol, Frey, Klein (English), Leydig, Ranvier, and Stöhr. For recent researches, see Journal of Royal Microscopical Society. As one research is rapidly superseding another, detailed references need not be given. For general bibliography, see Professor M'Kendrick's paper (above); for nucleus, Van Bambeke, Etat actuel de nos Connaissances sur la Structure du Noyau (Gand, 1885); for cell-division, Waldeyer, 'Uber Karyokinese,' Archiv. f. Anat. u. Physiol. (1887); for the vegetable cell in particular, Zimmermann, '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 cell-lore, both in themselves and on account of the stimulus which they supplied, will be found in the following and those to which they chiefly refer: Van Beneden, Recherches sur la Maturation de l'Euf, &c. (1883); Flemming, Zell-substanz, Kern und Zell-theilung (1882); and later papers in Archiv. f. mikr. Anatomie; Frommann, Unters. über Struktur, Lebenserscheinungen und Reaktionen thierischer und pflanzlicher Zellen (1884); O. and R. Hertwig, Beiträge zur Morphologie der Zellen (1875-88); Leydig, Zelle und Gewebe (1885), and previous works; Strasburger, Zellbildung und Zell-theilung (Jena, 3d. ed. 1880).

(3) For general physiology, consult first Foster's Physi ology, chap. i., then general works on physiology of plants and animals-e.g. Sachs' Text-book of Botany and Lectures on the Physiology of Plants, also Vines' similar work (1887), Hermann's Handbuch der Physiologie, &c. Further, Herbert Spencer's Principles of Biology; P. Geddes, Restatement of the Cell-theory,' Proc. Roy. Soc. Edin. (1883); M. Foster's article Physiology,' Encyclopædia Britannica; Berthold, Studien über Protoplasmamechanik (Leip. 1886); Schwarz, Die Morphologische und chemische Zusammensetzung des Protoplasmas (Breslau, 1887); and the article PROTOPLASM.

·

Celle. See ZELL.

Cellini, BENVENUTO, a celebrated Italian goldsmith, sculptor, and engraver, and the author of one of the most interesting autobiographies ever written, was born in 1500 in Florence, a city which he was forced to quit in early life through having taken part in an affray.' He then travelled to Rome, where his skill as an artist in metal-work gained him the favour of the highest nobles and prelates. So anxious were his patrons to secure his services that they allowed him the utmost license of conduct. By his own account he was as expert with sword and dagger as with his goldsmith's tools, and he had apparently no scruple in murdering or maiming any who endeavoured to thwart him. He states that at the siege of Rome in 1527 it was he who killed the Constable Bourbon, and that he afterwards shot down the Prince of Orange before the castle of St Angelo. He stood for a time high in favour with Pope Clement VII., but was eventually flung into prison for the murder of a rival goldsmith. In 1534 he was pardoned

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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, under the patronage of Cosmo de' Medici, and Cellini returned to Florence, where he worked 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 know

says

ledge 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,' Mr Symonds, 'the Genius of the Renaissance, incarnate in a single personality, leans forth and 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 wonderfully 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, Algae, Lichens, Characea, Liverworts, and Mosses (q.v.) from the higher or vascular cryptogamsFerns, Horsetails, Lycopodiacea and Selaginelleæ, and Isoeteæ. See THALLOPHYTES, VEGETABLE

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KINGDOM. For cellular tissue, see also CELL, HISTOLOGY, and VEGETABLE HISTOLOGY.

Celluloid, or PARKESINE. This substance was first made by Mr A. Parkes of Birmingham in 1855 or 1856. It chiefly consists of a dried solution of gun-cotton (pyroxylin), or of what is nearly the same thing, and oil. A variety of it can be made with pyroxylin and camphor. It resembles ivory, horn, tortoiseshell, and hardened india-rubber, as regards certain properties.

The pyroxylin is prepared by treating Cellulose (q.v.) from such vegetable materials as cotton or flax waste, rags, paper-makers' half-stuff, or paper itself, with a mixture of one part of strong nitric acid and four parts of strong sulphuric acid. It is convenient to call the product so obtained pyroxylin, although the two things are not quite identical. The distillate obtained by distilling wood naphtha with chloride of lime is used as a solvent for the pyroxylin, but other solvents, such as nitrobenzol or aniline, and some camphor are added with advantage. When the excess of solvent is removed from the pyroxylin, it is mixed with a considerable quantity of castor-oil or cotton-seed oil, and made into a dough or paste between heated rollers. For a hard compound the quantity of oil should be less than the pyroxylin, for a soft one it should be greater. Chloride of sulphur is sometimes added to the oil. When articles made of celluloid are in a partially manufactured state, they are soaked in bisulphide of carbon or chloride of lime to remove any trace of solvent, which would render them apt to shrink if allowed to remain. Celluloid is of a somewhat combustible nature unless the substances used to colour it are such as will neutralise this, or unless some non-combustible chemical, tungstate of soda for example, is added to it.

Properties and Uses.-Celluloid has many valuable properties. It is buff or pale brown in colour, but it can be made as white as ivory, which it much resembles, or manufactured in a transparent state. It can be moulded or pressed into any form, and turned, planed, or carved. Neither the atmosphere nor water affects it. It is elastic and can be united

by its own cement. In a plastic condition celluloid can be spread on textile fabrics, or it may be made as hard as ivory, for which it is largely used as a substitute. Billiard-balls, piano-keys, and combs are made of it, the latter two articles extensively. It can be coloured to represent amber, tortoiseshell, or malachite. In imitation of red coral it has been a good deal used for jewelry. Like vulcanite, which it excels in durability but exceeds in price, it has very numerous applications. We need only mention brush-backs, knife-handles, buttons, napkin-rings, card-cases, thimbles, and dolls. It is useful for optical instruments and for some surgical instruments. One of the most recent applications of it is for shirt fronts and collars. The manufacture of celluloid, although an English invention, has been most largely developed in the United States, where it is mostly, if not entirely, made by one firm, the Celluloid Manufacturing Co., Newark, New Jersey,

who use this word as a trade-mark.

Cellulose is the substance secreted by the living protoplasm of a vegetable cell to form its investing membrane or cell-wall. (See CELL, and HISTOLOGY, VEGETABLE, for account of its mode of formation, its ligneous, corky and colloid change, its mode of arrangement and union in cell-walls, &c.). It is obtained in a pure'state by treating any unaltered cellular tissue with alkalies and acids to remove mineral matter and protoplasm, and successive washings with water, alcohol, and ether to remove soluble substances. Cotton-pith or vegetable-ivory, although much contrasted in histological properties, are alike remarkably pure cellulose;

CELSIUS

in bast the proportion of associated mineral matter becomes much more considerable. Cellulose has the chemical composition C6H1005, and spec. grav. 1.52. Among its familiar natural modifications gum is an isomer, and starch-dextrin and grape-sugar are all of similar ultimate composition, while its woody and corky modifications (lignin and suberin) possess an increasing proportion of carbon. Iodine alone stains cellulose yellow or brown, but blue when strong sulphuric acid has been previously added. Strong hot sulphuric acid chars it, while brief im mersion in the cold converts it into a tough and dense modification, well known in parchment paper, and prolonged treatment dissolves it altogether. Dextrin may thus be prepared and next transmuted, by boiling the watery solution, into grape-sugar (see DEXTRINE, GLUCOSE). By immersion in a mixture of strong nitric and sulphuric acid we obtain Gun-cotton (q.v.), while dilute nitric acid or potash oxidises it into oxalic acid. Ammoniacal shown by its reprecipitation on dilution. By heatoxide of copper dissolves it without change, as is ing in closed vessels under pressure a dense coallike mass is formed, while in ordinary dry distillation, gas, tar, and acetic acid are given off, processes which throw light on the formation of coal in nature and on the chemistry of gas-making. In natural decomposition cellulose turns yellow and brown with gradual formation of humus. SOILS.

See

Although so constant and characteristic a proits formation are still very obscure. From that cellduct of vegetable life, the conditions and mode of cycle or rhythm of change between the passive and cellulose-walled state and an active and wall-less one, which is so characteristic of the lowest forms of life, and of which we find surviving traces (e.g. the rejuvenescence of the pollen-grain) in the reproductive processes of even the highest plants (see CELL), it would appear that there is some relation between this increased passivity and the formation of cellulose. And in this way arises the speculation that cellulose may be viewed essentially as a (mechanically coherent and thus useful) excretion, an incompletely utilised waste product corresponding to the carbonic acid and water given off by the completer respiratory oxidation and larger evolution of energy of the active phase. Once formed by the plant, it may be again absorbed, as is well seen in the union of a row of cells into a continuous vessel, or in the consumption of endosperm of a seed during germination. Many seeds, such as vegetable-ivory or date, have a great proportion of their reserve material in this form; and this must be digested into glucose by the growing embryo, and again worked up into new protoplasm, which deposits cellulose as before. Like the plant itself, the similar digestive ferments of the animal might thus be naturally expected to digest cellu lose; and this is actually, to some extent, the case with the delicate young cell-walls of many green vegetables, as can be experimentally verified, even in man; while in herbivorous animals this power is much developed, and the nutritive utilisation of their fodder is thus increased to an important extent.

The cysts of amoeba and other protozoa appear to be at least largely composed of cellulose, and the external tunic of ascidians (see TUNICATA) is of identical, or at least isomeric, composition. Cellulose has been described as a pathological product, even in brain-tissue; and Chitin (q.v.), a very characteristic and in many respects comparable animal product, has been sometimes viewed as cellulose in association with a proteid substance.

Celsius, ANDERS, the constructor of the centigrade thermometer, was born at Upsala in Sweden, 27th November 1701. He was the grandson of

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