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article PROTOPLASM, we may note some of the (6) General Substance of the Cell.-The cell is important steps. Dujardin (1835) described the much more than a mass of highly complex chemical "sarcode' of Protozoa and other cells ; Purkinje substance : it has an organised structure. (1) The (1839) emphasised the analogy between the protoplasm or living matter in the strictest sense "protoplasm of the animal embryo and the is generally supposed to be an intimate mixture of cambium' of plant-cells ; Von Mohl (1846) em. complex and highly unstable chemical compounds. phasised in the clearest way the importance of the Inspection under à microscope of such cells as protoplasm in the vegetable cell; Ecker (1849) amuebæ, white blood-corpuscles, ova, simple algae, compared the contractile substance of muscles with or such as are readily seen in thin slices of growing that of the ameba; Donders also referred the con. plant-shoots, in root-hairs, and transparent parts, tractility from the cell-wall to the contained will at once furnish an impression of the general material ; Cohn suspected that the sarcode' of | aspect of the substance of the cell. Not all that animal and the protoplasm' of plant-cells must be one sees can of course deserve the name of proto‘in the highest degree analogous substance ;' and plasm, for apart from definite inclosures like starch. 80 throughout another decade did botanists and grains and fat-globules, much of the remaining zoologists unite in laying stress rather on the slightly clouded substance is hardly to be strictly living matter than on the wall of the cell, and in called protoplasm, but rather represents steps in binting at the existence of one living substance as the ceaseless making and unmaking which form the physical basis alike of plants and animals. the fundamental rhythm of life. Keeping the This view found at length definite expression in definite inclosures and products for the moment 1861, when Max Schultze defined the modern con. | aside, we may briefly notice in general outline ception of the cell as a unit-mass of nucleated what has been with most conclusiveness observed protoplasm. Since then the protoplasmic move. | as to the structure of the general cell-substance or ment' has dominated research, and we think not cytoplasın' as it is now frequently termed. All 80 much of the cell-containing protoplasm as of the observers agree that the structure is far removed protoplasm which constitutes and gives form to the from the homogeneous, though there is much dif. cell.
ference of opinion as to the nature of the heteroII. Structure of the Cell.-While it is impossible geneity. In a large number of cases at least the to isolate the static from the dynamic aspects of substance of the cell has been resolved into two the cell, it will be convenient to discuss the two distinct portions—the one an intricate network, separately, and to consider the cell at rest and knotted and interlaced in a manner baffling descripdead, apart from the cell active and alive. In tion; the other a clear substance, filling up the other words, the form, structure, or morphology interstices or meshes of the living net. Leydig, may be studied for literary clearness apart from the Frommann, and Heitzmann have been peculiarly functions, life, and physiology.
successful in unravelling this knotted structure in la) General Form. —The typical and primitive animal cells, and much the same has been recorded form of the cell is spherical. This is illustrated by by Strasburger and Schmitz as observable in some toany of the simplest plants and animals which live plants. The reticulate structure is certainly more freely, and by young cells such as ova. But the doubtful in regard to vegetable cells, and even in typical form is in many, indeed in most cases, lost; some animal cells what some have described as a and the forms assumed are as diverse as the network others have deemed only a minutely internal and external conditions of life. The cell | bubbled emulsion. may be irregular and protean, as in Amcebæwhite But besides the real substance of the cell there blood-corpuscles, and many young eggs; or are to be seen products of various kinds formed squeezed into rectangular shape, as in much of the from the living matter. The cell may be packed substance of a lear; or flattened into thinness, as in with starch, or laden with fat, or expanded with the outer lining of the lips ; or oval and pointed, as mucus; it may contain colouring matter in various in swiftly moving Infnisorians and Bacteria ; or forms, as in the familiar chlorophyll bodies of many much branched, as in multipolar ganglion cells of plant-cells ; its structure may include, as in some animals or the latex-containing cells of some plants. Protozoa, definitely forned fibrils or yet firmer forThe typical spherical and self-contained form is mations of chitin and the like; and again there are that which would naturally he assumed by a com: concretions of retained waste and reserve products, plex coherent substance situated in a medium sometimes in the form of crystals. Not to be over. different from itself. The other forms are responses looked either is the fine * dust-cloud' of minute to internal and external conditions. Under the granules which are seen suspended in the clearer heading Cell-cycle below it will be shown how the matrix, and which apparently represent aggregarelative activity and passivity of the cell naturally | tions of diverse chemical substances formed in the expresses itself in such extremes as a long-drawn building up and breaking down of the protoplasm. out Infusorian and a rounded-off Gregarine, or in a | As the outside of any mass is bound to be in differhighly nourished ovum and a mobile spermatozoon. ent conditions from the inside, it is natural to find urther, cells, like entire animals, often show a | the appearance of distinct physical and chemical Adency to become two-ended, to have poles very | zones in the cell-substance. Thus in many Protozoa Illerent from one another. Just as an animal may the outer portion, needlessly termed 'ectoplasm,' is have a highly nourished head and a scantily often denser and more refractive than the more nourished tail, so a cell may become distinctly fluid and internal stratum of the endoplasm.' Or mlar in form. In other cases the cell is altogether this may go further, and we may have a sweated. tic, expressing every impulse of internal change off limiting cuticle, or a definitely organised wall
every impact of external influence in some of cellulose in vegetable cells. The cuticle may ification of form. Or the state of nutrition of be further substantiated with secretions of horny,
!ving, matter may cause alteration in the flinty, limy, and other material. Even within esion of the substance all over, or in particular the cell a stratified structure may be frequently
ess, and thus condition an outflowing, regular observed, and Berthold and others have recently irregular, in viven directions. Furthermore, ex. emphasised the existence of such concentric layers, nal pressure and linitation of growth may each characterised by its own special set of deare off the cell into a parallelogram, or restrict posits.
'w like a bast fibre in length alone and not Worthy of notice, too, are the various kinds of fold, and the resultant forms likewise. h. In fact the conditions are most mani. | bubbles or vacuoles which occur in the cell-sub.
Istance. These may be simply indefinite spaces,
containing some liquid not protoplasm, and includ. and active nature. In accordance with the growth ing salts and other substances in solution. In many of the cell it may occupy a position distinctly Protozoa they are ‘food-vacuoles,' formed by the nearer one of the poles. Accumulations of fat or
mucus may push it passively to the side. Or it may actively change, in response to hidden forces of attraction between it and the surrounding protoplasm, in the case of some ova exhibiting a peculiar rotation, or else distinctly shifting its ground from the centre towards the periphery.
Structure.-In many cases, as Leydig especially has shown, the nucleus seems to lie in a nest of its
own, in a clear space within the surrounding cellFig. 2.
substance. Nor is it in many cases at least deA, Embryonic cells from growing point of a root; B, older cells finitely insulated from the surrounding protoplasm, becoining vacuolated. (After Sachs.)
but is moored to the latter by strands which have
intimate relations with both. As of the entire cell, bubbles of water engulfed along with the food. so of the nucleus it must be said that in the great particles, round which the protoplasm, shrinking majority of cases it is very far from being homofrom contact, often forms a definite contour. In geneous. According to Hertwig, Schleicher, Schmitz, other cases they are more permanent, and represent Brass, and others, homogeneous nuclei may indeed minute reservoirs of secreted substance, cisterns of occur, but if they do they are rare, and it must by-products in the vital manufacture of the cell. always be remembered that the nucleus has its Finally they may be seats of special activity, where, history, and may be less complex at one time than perhaps, under the stimulus of irritant waste-pro- it is at another. To Flemming (1882) above all is ducts, the protoplasm exhibits spasmodic contrac due the credit of having elucidated the complexity tions and expansions, and forms the so-called 'con- | of the nucleus, and the labyrinthine structure to tractile vacuoles,' which in alternate dilatation and which he showed the clue, and to which Frommann bursting often seem to serve to remove fluid from (1867) had many years previously directed special the living matter to the exterior.
attention, has been studied and restudied by scores (c) Nucleus.—In the great majority of cells a of expert histologists during the last six years central body of definite composition and structure (1888). While their results disagree abundantly is present which appears to be essential to the life | on minor points, two conclusions stand out clearly and reproduction of the unit-mass. In many cases -(1) that the nucleus has a structure like that of the nucleus is well concealed, but as more skilful the general cell, consisting of firmer framework staining has revealed its presence in many cells and of more fluid intermediate substance, and (2) which used to be described as non-nucleated, it is that apart from detailed difference there is through: rash to conclude too certainly as to its absence in out the world of cells a marvellous unity of strucany particular case. Thus some of the Monera, ture and process, in the nucleus in repose and in which were formerly defined as the simplest of | the nucleus in action. simple animal organisms without even a nucleus, In the nucleus the following parts have to be dishave been shown to possess them, and the line of tinguished: (1) The readily stained firmer thread. division separating Protozoa into Monera and work, (2) an intermediate clear substance filling Endoplastica has therefore been removed. Fur. | up the interstices, (3) definite and usually globular thermore, the researches of Gruber have shown that formations known as nucleoli, (4) various granules, in some of the higher Protozoa (ciliated Infusorians) and (5) a limiting membrane or nuclear wall. where the nucleus seems entirely absent, dex. These may be briefly touched upon in order. terous staining prove its diffused presence in (1) The Nuclear Framework (reticulum, trabecn. the form of numerous granules which take on the lar framework, &c.).- A mere statement of the characteristic nuclear dye. Yet in some cases, different descriptions given of this important part such as the young spores of some Protozoa, of the nucleus would carry us far beyond the limits the greatest care has not yet been successful in of this article. The most marked difference of proving the presence of the nuclev 3. In contrast opinion is this, that some describe the framework with these cases, many cells exist in which the as distinctly of the nature of a network, while nucleus is represented not by one, but by many others are as emphatic in calling it a much-coiled bodies—the so-called polynuclear state. A further band. A third party unite both views, and rereserve requires to be made, that it is to a large garding the nucleus as variable, describe a reti. extent an hypothesis that all such definite central culum at one time and a coiled filament at another. inclosures should be slumped together under the Thus, according to Flemming, Pfitzner, Retzius, one title of nucleus. It is rather probable that in | Leydig, Van Beneden, &c., the nuclear framework this, as in other organic structures, we have to do is typically a reticulum ; according to Strasburger, with various degrees of development and definite. | Balbiani, and Korschelt, a twisted ribbon is the ness.
only or most frequent form ; according to Brass In the form also of the nucleus numerous modi and Rabl, both types may equally occur. A further fications occur. In the majority of cases, indeed, complication has been emphasised by Zacharias, it is more or less spherical, but it may be elongated, Plitzner, Carnoy, and others—this, namely, that curved, horseshoe-shaped, necklace-like, and even besides the readily stained threadwork noted above branched. In the young stages of some ova it is (the so-called chromatin), whether this be in the like the entire cell, somewhat plastic, and is pulled form of a reticulum (Pfitzner) or of a coiled ribbon in and out in amạboid movements. In special con- (Carnoy), there exists another-not readily stained ditions, furthermore, the nucleus may exhibit – framework of achromatin. This had indeed been peculiar deformations. It is in fact a peculiarly recognised though not insisted on by the first series sensitive and all-important part of the cell, suffering of investigators. To sum up, it is now generally with it in degeneration, changing with it in growth allowed that the framework or threadwork of the and division.
nucleus may exist as a network or as a coil, and In position the nucleus is typically central, where that it is in a sense double, consisting of readily as the presiding genius of the cell it shares and stainable chromatin on the one hand, and unstainperhaps controls the general protoplasmic life. But able achromatin on the other. It need hardly be it frequently suffers displacement both of a passive added that as there is considerable diversity of
opinion as to whether given nuclei have a netted or from an aggregation of the threads and knots of coiled framework, there is yet greater variety in the framework; (2) as a cuticle or capsule formed the minuter description and figuring. According | from the matrix or ground substance; (3) from a to Flemming the network is quite disorderly, but combination of both these elements. Leydig has Rauber, Leydiy, and others have described distinct 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 uni. form ; 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 Fig. 3. --( After Carnoy): A Cell and typical nuclens: a slight membrane: b. radiating
in plant, and apparently in some animal cells, the prstoplastic network; c. wall of nucleus; d, plasma of
formation of a cellular plate' is one of the last nucleus : &, nuclear coil.
events in the dividing process. B. Nuclns at rest, showing network.
III. Physiology of the Cell.- When the entire C, Nucleus before division, showing cojled filament.
organism is simply a cell, as in most of the Proto
zoa and Protophyta, all the vital processes which radial structure; according to some the nuclear coil in higher forms have their seat in special sets of is endless, while others describe it as divided into cells, known as tissues and organs, are of course portions ; and when we descend to such subtleties discharged by the unit-mass. Thus a unicellular of observation as the intimate structure of the | organism like the Ameba takes in energy as food threadwork or the relations between chromatin in nutrition, works it up into living matter in and achromatin, the diversity is so great that it digestion and assimilation, and expends it again seems desirable here to leave such minutiæ un in contraction and locomotion. As in any higher touched.
organism the oxygen required for the chemical (2) The Intermediate Nuclear Substance.-Be. breaking up of the protoplasmic molecules, the sides the nuclear elements of definite form, what. air for the vital flame, is taken in by the absorpever that form may precisely be, all investigators tion known as respiration, and the waste carbonic describe an intermediate substance of variable acid gas is in an essentially similar way got rid consistence, usually semi-liquid, amorphous and of. Further, more solid “ashes of the vital com. structureless, but with fine granules. It is a clear bustion are formed in the Amaba and in other unatainable plasma' filling up the chinks, but actively living cells, and may pass out in excrenothing definite is known as to its composition. tion along with the refuse of unusable food(3) The nucleolus which lies within the nucleus material. The absence of a circulating fluid, of varies greatly in size and position, and more than one digestive glands, nerves, sense-organs, lungs, are very generally present. Flemming has defined | kidneys, and the like, does not in any way them as 'portions of the nuclear substance, distinct restrict the vital functions of a unicellular organin structure from network and plasma, definitely ism. All goes on as usual, only with greater limited and smoothed, always rounded in outline, chemical complexity, since all the different prousually suspended in the network, but often inde cesses have but a unit-mass of protoplasm in pendent of it.' But when the minute structure which they occur. The physiology of independent and the relation of nucleoli to nuclear framework cells, instead of being very simple, must be very are inquired into, or the question of physiological | complex, just because structure or differentiation Tule raised, very great diversity of opinion is found is all but' absent. It is, however, possible to exto obtain. (4) Bodies different in appearance from press the manifold processes in a comparatively nucleoli may occur inside the nucleus, but of these simple way by remembering what Claude Bernard little is known. (5) The wall which bounds the was one of the first clearly to emphasise, that nucleus seems to be a true integral part of the vital processes must be really only twofoldlatter, but disappears at the beginning of division. building up and breaking down of living matter.
(d) The Cell-wall.-In the older conception of On the one hand the protoplasm or real living the cell, which was practically that of a closed matter is being by a series of chemical processes hag, the wall of the cell figured very prominently. | built up or constructed ; on the other hand, in But Sageli showed (1845) that some vegetalle activity it is breaking down or being destroyed. crolls were destitute of walls, Leydig (1857) de. The income of food or energy is, at the expense fined the cell in respect to its substance, Schultze of the cellular organism, gradually raised into and others described naked Protozoa, and the more and more complex and unstable compounds, progress of the protoplasmic movement' led to until the genuine most complex and more unthe abandonment of the position that the wall was stable living matter itself is reached. On the a necessary or important part of the cell. In opposite side, with liberation of energy in the Inany cells, indeed, 'a limiting layer is very clearly form of work, this living matter breaks down present, and a sheath or cyst is especially charac. into simpler and simpler compounds, until only teristic of passive cells. Plant-cells are almost the work, the waste products, and heat remain as always distinguished by the possession of a limit. the equivalent of the income of energy or food on ing wall, of definite chemical composition, consist. the other side of the life-equation. On the one ing of what is known as cellulose. An analogous hand there are constructive processes, on the other, wall occasionally occurs round animal cells. In destructive; chemical synthesis and chemical dis. the latter, however, the membrane is usually a solution is another expression of the contrast ; comparatively slight thing, and may arise (1) | while the two sets of processes are in more niodern
language respectively termed anabolism and kata accentuated to the more or less marked subbolism (see PROTOPLASM).
ordination of the others. Not that the emphatic But only a few cells, comparatively speaking, adoption of one line of cell-life excludes the others; live a free and independent life. The majority are they may in fact occur as temporary stages, or as component elements in higher unities. In these | pathological deviations. the original many-sidedness of function is more or But while simple observation is sufficient to less lost, or at anyrate in abeyance, and that establish the existence of a cycle of phases in exactly in proportion to their degree of subordina the life of primitive cellular organisms, such as tion. Even in individual cells there is a tendency, Protomyxa, and the existence of three main lines obviously within narrow limits, towards differen- of specialisation among the Protozoa, the importtiationthat is, to the restriction and specialisa- | ance of this conception of a 'cell-cycle' becomes tion of certain parts for certain functions. But increased and justified when the facts are conwhen the cells form elements of a larger whole, sidered physiologically. If we start from a simple the division of labour finds full effect. From cell, such as an Ameba, it is evident enough, from position and other conditions the cells cease to be what has been already said as to the twofold uniform or metaphorically many-sided. Certain nature of all vital processes, that the principal sets predominate in contractility, others in irrita- physiological possibilities are the three phases bility, others in secretion, others again in storage, | above indicated. On the one hand, with preand so on. In such cases one function predominates ponderance of income over expenditure, of conover the others, which are subordinate or only structive over destructive changes, of anabolism dormant possibilities. Thus arise muscle-cells, over katabolism, the cell must tend to become nerve-cells, glandular cells, fat-cells, and the like. larger in size, more weighted with stored material, Compared with Amæbæ, those cells must have a more sluggish or passive in habit, and more simpler physiology; they may have gained in com. | rounded in form. But if the reverse take place, plexity of structure, but have lost in manifoldness the cell will tend to become less bulky, more of function. The aggregation of similar cells, active or locomotor in habit, and more elongated usually with one predominant habit or function, results in the formation of tissues (see BIOLOGY, EMBRYOLOGY, FUNCTION, PHYSIOLOGY, REPRODUCTION, and cognate articles in this work).
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
Fig. 5.- Protomyxa: survey of the unit-organisnis, both among plants
| 1, encysted ; 2, dividing; 3, spores escaping as ciliated bodies,
encost and animals, reveals the existence of three well passing into 4, ainaboid state ; 5, 'plasmodium' forming from marked phases. Some cells are emphatically lusion of amoeboid cells. active, equipped with motile lashes (cilia or flagella), and obviously liberal in their expendi. | in form. The sweated-off cyst of the former, ture of energy. Others are just the reverse of the motile processes of the latter, are expres. this, emphatically passive, wrapped up in them- sions of exactly opposite constitutions and condi. selves and without motile processes, obviously tions. A third physiological possibility remains, economical in their expenditure, conservative of that namely of continuing in a position of average their income. A third set form a mean between equilibrium between income and expenditure, bethese two extremes, are neither encysted like the tween anabolism and katabolism, in a middle way latter nor lashed like the former, but furnished between the fitful fever of extreme ciliated activity with the relatively slow-moving processes charac. and the sluggish sleep of encysted passivity. teristic of Amoebæ, and living in a via media be. Now if we take these two facts-the existence tween activity and passivity. These three types of a primitive cycle through which cells tend may be termed respectively ciliated, encysted, and amcboid, or active, passive, and moderate. That
b 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
Fig. 6.- The Cycle of Cell-life: phases occur in one life-history; no step has, as it a, encysted; b, ciliated ; c, amæboid ; d, plasmodial. were, been taken in any one of the three directions; the primitive cells are in a state of physio. pass, and the existence of three main physiological logical indítference. What has happened in the 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 wind pipe of mammals, the skin of many lower wornis, 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 amaboid type. Or again, the aniqboid 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 Fig. 4.-Phases of Cell-life. (After Geddes.)
procedure. In many cases artificial stimulus of Development of passive or resting, intermediate (ameboid), and various kinds has been shown to make cells active (motile) states.
pass from one phase to another of the primitive higher classes of Protozoa - Infusorians, Gre. life-cycle theoretically possible to all. In the garinids, Rhizopods—is that one phase has been same way the preponderance of cellulose in cells CELL
encysted is a natural character of the passive reached, and portions of the substance are cleft planis, and the insheathed cells of many animal apart from the main mass. From such a case tissnes 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 delibertoplasınic 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 subpenditure 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 protoplasın, 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 inthe 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
did so much in working out the details of cell. division in the ovum, expressed himself as follows in regard to the deeper problem in a paper pub. lished in 1887: '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.
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 Fig. 7.
give off a bud, usually smaller than itself. With A Life-history of unicellular plant (Protococcus ): 1, encysted : 2,
this a portion of the nucleus is usually associated, guiitting its cell ; 3, ciliated ; 4, quiescent; 5 and 6, dividing. as in many Protozoa ; or the processes may ocem B, Lithistory of Ainoba:1, encysted; 2, escaping; 3, free;
apart from demonstrable nucleus, as in the common 4, dividing ; 5, free half with vacuole v, nucleus n, and food. articles : 6, encysting anew. 4 and 5 may also represent
yeast-plant. (6) Division into two is by far the the union of two Amabæ (conjugation).
most frequent mode of multiplication, and oecurs
all but universally. In a small minority of cases the means of accomplishing purification and the the division is accomplished without any intricate like, cannot keep pace with the growth of the nuclear change, the cell being in an apparently living mass if the surface increase only at a simple way divided into two, with half of the much less rapid rate. A limit of growth is thus nucleus in each daughter-cell. Such divisions are reached. The cell must stop growing, or go on said to be direct.' In most cases the nuelens, growing at an increasing risk, or in some way apparently taking the initiative, undergoes a strikrestore the balance between mass and surface. ing series of orderly changes before the division is This last course is the one most frequently perfect. This is the commonly observed condition, exhibited-the cell divides. However this may and such divisions are termed indirect.' (c) But in be effected, the result is in all cases the same many cases the division occurs in a very different --namely, the reduction of mass, and corre. way, being not single but multiple. From one sponding increase of surface. Like other organ: cell more than two daughter-cells arise simultane. ismax, the cell-organism reproduces at its limit ously, and that not by external cleavage, but by of growth. This rationale of cell-division, due internal multiplication. Such a mode of multipli. e-pecially to Herbert Spencer, is obviously clearest cation is termed endogenous division or 'free' cellin reference to free living cells like Protozoa, formation, and is well seen in many Fungi and Protophyta, blood-corpuscles, reproductive cells, Algæ. It may be compared with the ordinary and the like, but the general principle holds good process by defining it as division taking place in throughout.
limited space and time, since the daughter-cells It is evident, however, that such considerations arise within the mother-cell, and simultaneously, as the above go to justify rather than to explain not successively. It is, in many cases at least, precell-division. They show why the cell ought to ceded by the rapid division of the nuclei, to form divide, not how it does. The real mechanism of centres round each of which protoplasmic material the process is still a riddle. In its very simplest then becomes aggregated. In a few cases, Arnold expressions, indeed, the riddle may be partly read. has described a peculiar breaking up of the nucleus in a simple and primitive Protozoon like Schizo. | which he called fragmentation. genes, the protoplasm seems literally to break. Karyokinesis.-One of the most beautiful results Irregular fissures appear, as well they might if a of recent histology is the demonstration of the condition of unstable vital equilibrium has been | general unity of process which obtains in the