article PROTOPLASM, we may note some of the important steps. Dujardin (1835) described the 'sarcode' of Protozoa and other cells; Purkinje (1839) emphasised the analogy between the 'protoplasm of the animal embryo and the 'cambium' of plant-cells; Von Mohl (1846) emphasised in the clearest way the importance of the protoplasm in the vegetable cell; Ecker (1849) compared the contractile substance of muscles with that of the amoeba; Donders also referred the contractility from the cell-wall to the contained material; Cohn suspected that the sarcode' of animal and the protoplasm' of plant-cells must be in the highest degree analogous substance; and so throughout another decade did botanists and zoologists unite in laying stress rather on the living matter than on the wall of the cell, and in hinting at the existence of one living substance as the physical basis alike of plants and animals. This view found at length definite expression in 1861, when Max Schultze defined the modern conception of the cell as a unit-mass of nucleated protoplasm. Since then the protoplasmic movement has dominated research, and we think not so much of the cell-containing protoplasm as of the protoplasm which constitutes and gives form to the

cell.

II. Structure of the Cell.-While it is impossible to isolate the static from the dynamic aspects of the cell, it will be convenient to discuss the two separately, and to consider the cell at rest and dead, apart from the cell active and alive. In other words, the form, structure, or morphology may be studied for literary clearness apart from the functions, life, and physiology.

or

(a) General Form.-The typical and primitive form of the cell is spherical. This is illustrated by many of the simplest plants and animals which live freely, and by young cells such as ova. But the typical form is in many, indeed in most cases, lost; and the forms assumed are as diverse as the internal and external conditions of life. The cell may be irregular and protean, as in Amœbæ, white blood-corpuscles, and many young eggs; squeezed into rectangular shape, as in much of the substance of a leaf; or flattened into thinness, as in the outer lining of the lips; or oval and pointed, as in swiftly moving Infusorians and Bacteria; or much branched, as in multipolar ganglion cells of animals or the latex-containing cells of some plants. The typical spherical and self-contained form is that which would naturally be assumed by a complex coherent substance situated in a medium different from itself. The other forms are responses to internal and external conditions. Under the heading Cell-cycle below it will be shown how the relative activity and passivity of the cell naturally expresses itself in such extremes as a long-drawn ont Infusorian and a rounded-off Gregarine, or in a highly nourished ovum and a mobile spermatozoon. Further, cells, like entire animals, often show a tendency to become two-ended, to have poles very different from one another. Just as an animal may have a highly nourished head and a scantily nourished tail, so a cell may become distinctly bipolar in form. In other cases the cell is altogether plastic, expressing every impulse of internal change and every impact of external influence in some modification of form. Or the state of nutrition of the living matter may cause alteration in the adhesion of the substance all over, or in particular places, and thus condition an outflowing, regular or irregular, in given directions. Furthermore, external pressure and limitation of growth may square off the cell into a parallelogram, or restrict it to grow like a bast fibre in length alone and not in breadth. In fact the conditions are most manifold, and the resultant forms likewise.

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(b) General Substance of the Cell.-The cell is much more than a mass of highly complex chemical substance: it has an organised structure. (1) The protoplasm or living matter in the strictest sense is generally supposed to be an intimate mixture of complex and highly unstable chemical compounds. Inspection under a microscope of such cells as amoebe, white blood-corpuscles, ova, simple algæ, or such as are readily seen in thin slices of growing plant-shoots, in root-hairs, and transparent parts, will at once furnish an impression of the general aspect of the substance of the cell. Not all that one sees can of course deserve the name of protoplasm, for apart from definite inclosures like starchgrains and fat-globules, much of the remaining slightly clouded substance is hardly to be strictly called protoplasm, but rather represents steps in the ceaseless making and unmaking which form the fundamental rhythm of life. Keeping the definite inclosures and products for the moment aside, we may briefly notice in general outline what has been with most conclusiveness observed as to the structure of the general cell-substance or 'cytoplasm' as it is now frequently termed. All observers agree that the structure is far removed from the homogeneous, though there is much difference of opinion as to the nature of the heterogeneity. In a large number of cases at least the substance of the cell has been resolved into two distinct portions-the one an intricate network, knotted and interlaced in a manner baffling description; the other a clear substance, filling up the interstices or meshes of the living net. Leydig, Frommann, and Heitzmann have been peculiarly successful in unravelling this knotted structure in animal cells, and much the same has been recorded by Strasburger and Schmitz as observable in some plants. The reticulate structure is certainly more doubtful in regard to vegetable cells, and even in some animal cells what some have described as a network others have deemed only a minutely bubbled emulsion.

But besides the real substance of the cell there are to be seen products of various kinds formed from the living matter. The cell may be packed with starch, or laden with fat, or expanded with mucus; it may contain colouring matter in various forms, as in the familiar chlorophyll bodies of many plant-cells; its structure may include, as in some Protozoa, definitely formed fibrils or yet firmer formations of chitin and the like; and again there are concretions of retained waste and reserve products, sometimes in the form of crystals. Not to be overlooked either is the fine dust-cloud' of minute granules which are seen suspended in the clearer matrix, and which apparently represent aggrega tions of diverse chemical substances formed in the building up and breaking down of the protoplasm. As the outside of any mass is bound to be in different conditions from the inside, it is natural to find the appearance of distinct physical and chemical zones in the cell-substance. Thus in many Protozoa the outer portion, needlessly termed 'ectoplasm,' is often denser and more refractive than the more fluid and internal stratum of the endoplasm.' Or this may go further, and we may have a sweatedoff limiting cuticle, or a definitely organised wall of cellulose in vegetable cells. The cuticle may be further substantiated with secretions of horny, flinty, limy, and other material. Even within the cell a stratified structure may be frequently observed, and Berthold and others have recently emphasised the existence of such concentric layers, each characterised by its own special set of deposits.

Worthy of notice, too, are the various kinds of bubbles or vacuoles which occur in the cell-substance. These may be simply indefinite spaces,

and active nature. In accordance with the growth of the cell it may occupy a position distinctly 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 cellsubstance. Nor is it in many cases at least de

bubbles of water engulfed along with the foodparticles, round which the protoplasm, shrinking from contact, often forms a definite contour. In other cases they are more permanent, and represent minute reservoirs of secreted substance, cisterns of by-products in the vital manufacture of the cell. Finally they may be seats of special activity, where, perhaps, under the stimulus of irritant waste-products, the protoplasm exhibits spasmodic contractions and expansions, and forms the so-called 'contractile vacuoles,' which in alternate dilatation and bursting often seem to serve to remove fluid from the living matter to the exterior.

(c) Nucleus.-In the great majority of cells a central body of definite composition and structure is present which appears to be essential to the life and reproduction of the unit-mass. In many cases the nucleus is well concealed, but as more skilful staining has revealed its presence in many cells which used to be described as non-nucleated, it is rash to conclude too certainly as to its absence in any particular case. Thus some of the Monera, which were formerly defined as the simplest of simple animal organisms without even a nucleus, have been shown to possess them, and the line of division separating Protozoa into Monera and Endoplastica has therefore been removed. Furthermore, the researches of Gruber have shown that in some of the higher Protozoa (ciliated Infusorians) where the nucleus seems entirely absent, dexterous staining prove its diffused presence in the form of numerous granules which take on the characteristic nuclear dye. Yet in some cases, such as the young spores of some Protozoa, the greatest care has not yet been successful in proving the presence of the nucleus. In contrast with these cases, many cells exist in which the nucleus is represented not by one, but by many bodies-the so-called polynuclear state. A further reserve requires to be made, that it is to a large extent an hypothesis that all such definite central inclosures should be slumped together under the one title of nucleus. It is rather probable that in this, as in other organic structures, we have to do with various degrees of development and definite

ness.

fications occur.

In the form also of the nucleus numerous modiIn the majority of cases, indeed, it is more or less spherical, but it may be elongated, curved, horseshoe-shaped, necklace-like, and even branched. In the young stages of some ova it is like the entire cell, somewhat plastic, and is pulled in and out in amoeboid movements. In special conditions, furthermore, the nucleus may exhibit peculiar deformations. It is in fact a peculiarly sensitive and all-important part of the cell, suffering with it in degeneration, changing with it in growth and division.

In position the nucleus is typically central, where as the presiding genius of the cell it shares and perhaps controls the general protoplasmic life. But it frequently suffers displacement both of a passive

but is moored to the latter by strands which have intimate relations with both. As of the entire cell, so of the nucleus it must be said that in the great majority of cases it is very far from being homogeneous. According to Hertwig, Schleicher, Schmitz, Brass, and others, homogeneous nuclei may indeed occur, but if they do they are rare, and it must always be remembered that the nucleus has its history, and may be less complex at one time than it is at another. To Flemming (1882) above all is due the credit of having elucidated the complexity of the nucleus, and the labyrinthine structure to which he showed the clue, and to which Frommann (1867) had many years previously directed special attention, has been studied and restudied by scores of expert histologists during the last six years (1888). While their results disagree abundantly on minor points, two conclusions stand out clearly -(1) that the nucleus has a structure like that of the general cell, consisting of firmer framework and of more fluid intermediate substance, and (2) that apart from detailed difference there is throughout the world of cells a marvellous unity of structure and process, in the nucleus in repose and in the nucleus in action.

In the nucleus the following parts have to be distinguished: (1) The readily stained firmer threadwork, (2) an intermediate clear substance filling up the interstices, (3) definite and usually globular formations known as nucleoli, (4) various granules, and (5) a limiting membrane or nuclear wall. These may be briefly touched upon in order.

(1) The Nuclear Framework (reticulum, trabecular framework, &c.).-A mere statement of the different descriptions given of this important part of the nucleus would carry us far beyond the limits of this article. The most marked difference of opinion is this, that some describe the framework as distinctly of the nature of a network, while others are as emphatic in calling it a much-coiled band. A third party unite both views, and regarding the nucleus as variable, describe a reticulum at one time and a coiled filament at another. Thus, according to Flemming, Pfitzner, Retzius, Leydig, Van Beneden, &c., the nuclear framework is typically a reticulum; according to Strasburger, Balbiani, and Korschelt, a twisted ribbon is the only or most frequent form; according to Brass and Rabl, both types may equally occur. A further complication has been emphasised by Zacharias, Pfitzner, Carnoy, and others-this, namely, that besides the readily stained threadwork noted above (the so-called chromatin), whether this be in the form of a reticulum (Pfitzner) or of a coiled ribbon (Carnoy), there exists another-not readily stained

framework of achromatin. This had indeed been recognised though not insisted on by the first series of investigators. To sum up, it is now generally allowed that the framework or threadwork of the nucleus may exist as a network or as a coil, and that it is in a sense double, consisting of readily stainable chromatin on the one hand, and unstainable achromatin on the other. It need hardly be added that as there is considerable diversity of

(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. (3) 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 rôle 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 little 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.

(d) The Cell-wall.-In the older conception of the cell, which was practically that of a closed hag, the wall of the cell figured very prominently. But 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 charac teristic of passive cells. Plant-cells are almost always distinguished by the possession of a limiting wall, of definite chemical composition, consisting 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 formation of a cellular plate' is one of the last in plant, and apparently in some animal cells, the 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 Amoeba 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 dissolution is another expression of the contrast; while the two sets of processes are in more modern

language respectively termed anabolism and kata bolism (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 (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 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 Amabæ, 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

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

passing into 4, amoeboid state; 5, 'plasmodium' forming from lusion 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

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 amboid 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

encysted is a natural character of the passive plants, and the insheathed cells of many animal tissues may be similarly expressed as an exhibition of the same passive phase. But it is enough here to point out the possibility of classifying and interpreting the various cells composing the tissues of higher organisms in terms of an original lifecycle, or deeper still in terms of those twofold protoplasmic possibilities which lie behind all forms and phases whether of cells, tissues, or organisms themselves. This conception of a cell-cycle is due to Geddes (see Bibliography at end of article).

Cell-division.-When the vital processes are so related that income and upbuilding exceed expenditure and dissolution, the cell must obviously accumulate capital and increase in size. In some cases the cell may expand into relatively gigantic proportions, as in the alga Botrydium and in many eggs. Growth, however, brings a nemesis with it, this namely, that the mass to be kept alive increases more rapidly than the surface through which the vital processes are accomplished. In spherical cells the former increases as the cube, the latter as the square of the radius. The bigger the cell gets, the more difficult do its conditions of life become. The supplies of food and oxygen,

B

8

Fig. 7.

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 Aineba: 1, encysted; 2, escaping; 3, free; 4, dividing; 5, free half with vacuole v, nucleus n, and food. Tarticles 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 corresponding increase of surface. Like other organ isms, the cell-organism reproduces at its limit of growth. This rationale of cell-division, due e-pecially 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

51

reached, and portions of the substance are cleft apart from the main mass. From such a case to the separation of multiple buds, which are little more than overflowings of too large a cell, or to the commoner occurrence of simple budding, is no great step. The difficulty begins, however, when we consider the ordinary celldivision, which appears in most cases as a deliberate and orderly process, including a well-defined series of nuclear changes. As to the mechanics of this process only a few suggestions of moment have been made. Thus Platner points out that the explanation must be in terms either (1) of chemical processes influencing the cellular substance, or (2) of protoplasmic movement due to the above or to external influences, or (3) of unknown molecular and attractive forces. He himself finds the condition of nuclear division to be in part a streaming movement of the protoplasın, such as is familiar in many Protozoa, and would regard the division of the protoplasm as a purely mechanical process. In his studies on protoplasmic mechanics, Berthold has also attacked this intricate problem, but more in relation to the nature of the dividing partitions than with reference to the forces at work. Professor Van Beneden, who did so much in working out the details of celldivision in the ovum, expressed himself as follows in regard to the deeper problem in a paper published 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 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 multipli cation is termed endogenous division or 'free' cellformation, and is well seen in many Fungi and Algæ. 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.

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