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CYTOLOGY (from huros, a hollow vessel...

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Originally appearing in Volume V07, Page 720 of the 1911 Encyclopedia Britannica.
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CYTOLOGY (from huros, a hollow See also:vessel, and X6yos, See also:science)  , the scientific study of the " cells " or living See also:units of See also:protoplasm (q..v.), of which See also:plants and animals are composed . All the higher,' and the See also:great See also:majority of the See also:lower, plants and animals are' composed of a vast number of these vital units or "cells." In the See also:case of many microscopic forms, however, the entire organism,, plant or See also:animal, consists throughout See also:life of a single See also:cell . See also:Familiar examples of these " unicellular " forms are Bacteria and Diatoms among the plants, and See also:Foraminifera and See also:Infusoria among the animals . In all cases, however, whether the cell-unit lives freely as a unicellular organism or forms an integral See also:part of a multi-, cellular individual, it exhibits in itself all the phenomena characteristic of living things . Each cell assimilates See also:food material, whether this is obtained by its own activity, as in the majority of the See also:protozoa, or is brought, as it were, to its own See also:door by the See also:blood stream, as in the higher Metazoa, and builds this food material into its own substance, a See also:process accompanied by respiration and See also:excretion and resulting in growth . Each cell exhibits in greater or less degree "irritability," or the See also:power of responding. to stimuli; and finally each cell, at some See also:time in its life, is capable of See also:reproduction . It is evident therefore that in the multicellular forms all the complex manifestations of life• are but the outcome of the co-ordinated activities of the constituent cells . The latter are indeed, as See also:Virchow has termed them, " vital units." It is therefore in these vital units that the explanation of vital phenomena must be sought (see See also:PHYSIOLOGY): As Verwornl said, " It is to the cell that the study of every bodily See also:function sooner or later drives us . In the muscle cell lies the problem of the See also:heart See also:beat and that of See also:muscular contraction; in. the gland cell reside the causes of secretion; in the See also:epithelial cell, in the See also:white blood corpuscle, lies the problem of the absorption of food, and the secrets of the mind are hidden in the ganglion cell." So also the problems of development and See also:inheritance have shown themselves to be cell problems, while the study of disease has produced a " cellular See also:pathology." The most important problems awaiting See also:solution in See also:biology are cell problems . See also:Historical . ,The cell-theory ranks with the See also:evolution theory, in the far-reaching See also:influence it has exerted on. the growth of See also:modern biology; and although almost entirely a product of the 19th See also:century, the See also:history of its development gives See also:place, in point of See also:interest, to that of no other See also:general conception . The cell-theory—in a See also:form, however, very different from that. in which we now know it—was originally suggested by the study of plant structure; and the first steps to the formulation, many years later, of a definite cell-theory, were made as See also:early as the later part of. the 17th century by See also:Robert See also:Hooke, See also:Marcello See also:Malpighi and See also:Nehemiah See also:Grew .

Hooke (1665) noted and described the vesicular nature of See also:

cork and similar See also:vegetable substances, and designated. the cavities by the See also:term " cells." A few years later Malpighi (1674) and Grew (1682), still of course working with the See also:low power lenses done available at that time, gave a more detailed description of the finer structure of plant See also:tissue . They showed that it consisted in part of little cell-like cavities, provided with See also:firm cell-walls and filled with fluid,, and in part of See also:long See also:tube-like vessels . A long time passed before the next important step forward was made by C . L . Treviranus,2 who, working on the growing parts of See also:young plants, showed that the tubes and vessels of Malpighi and Grew arose from cells by the 1 Allgenzeine Physiologic, p . 53 (1895) . 2 Vom inwendigen Bau der Gewachse (1806) . latter becoming elongated and attached end to end, the intervening walls breaking down; a conclusion afterwards confirmed by See also:Hugo von ' See also:Mohl (183o) . It was not, however, until the See also:appearance of See also:Matthias See also:Jakob See also:Schleiden's See also:paper Beilrage zur Phytogenesis (1838) that we have a really comprehensive treatment of the cell, and the formulation of a definite cell-theory for • plants . It is to the See also:wealth of correlated observations and to the philosophic breadth of the conclusions in this paper that the subsequent rapid progress in See also:cytology is undoubtedly to be attributed . Schleiden in this paper attempted to solve the problem of the mode of origin of cells . The See also:nucleus (vide infra) of the cell had already been discovered by Robert See also:Brown (1831), who, however, failed to realize its importance .

Schleiden utilized Brown's See also:

discovery, and although his theory of phytogenesis is based on erroneous observations, yet the great importance which he rightly attached to the nucleus as a cell-structure made' it possible to extend the cell-theory to animal tissues also . We may indeed date the See also:birth of animal cytology from Schleiden's See also:short but See also:epoch-making paper . Comparisons between plant and animal tissues had already been made by several workers, among others by Johannes See also:Muller (1835), and by F . G . J.'See also:Henle and J . E . Purkinje (1837) . But the first real step to a comprehensive cell-theory to include animal tissues was made by Theodor See also:Schwann . This author, stimulated by Schleiden's See also:work, published in 1839 a See also:series of Mikroskopische Untersuchungen uber See also:die Ubereinstimmung in der Structur and dem •Wachstum der Tiere und Pflanzen . This epoch-making work ranks with that of Schleiden in its stimulating influence on biological See also:research, and in spite of the greater technical difficulties in the way, raised animal cytology at one See also:blow to the position already, and so laboriously, acquired by plant cytology . In the animal cell it is the nucleus and not the cell-See also:wall that is most conspicuous, and it is largely to the importance which Schwann, following the example of Schleiden, attached to this structure as a cell constituent, that the success and far-reaching influence of his work is due.' Another feature determining the success of Schwann's °work was his selection of embryonic tissue as material for investigation . He showed that in the embryo the cells all closely resemble one another; only becoming later converted into the tissue elements—See also:nerve cells, muscle cells and so forth—as development proceeded; just as a similar mode of investigation had enabled Treviranus to trace the origin from typical cells of the vascular tissue in plants more than 30 years previously .

And just as Treviranus showed that there was a See also:

union of cells to form the vessels in plants, so Schwann now showed that a union of cells frequently occurred in the formation of animal tissues . So great was the stimulus given to cytological research by the work of Schleiden and Schwann that these authors are often referred to as the founders of the cell-theory . Their theory, however, differed very greatly from that of the See also:present time . Not only did they suppose new cells to arise by a sort of " See also:crystallization " from a formative " See also:mother liquor " or " cytoblastema " (vide infra), but they both defined the cell as a vesicle " provided with a firm cell-wall and with fluid contents . The cell-wall was regarded as the essential cell-structure, which by its own See also:peculiar properties controlled the cell-processes . The work of Schleiden and Schwann marks the See also:close of the first See also:period in the history of the cell-theory—the period dominated by the cell-wall . The subsequent history is marked by the See also:gradual recognition of the importance of the cell-contents . Schleiden had noticed in the plant cell a finely granular substance which he termed " plant slime " (Pflanzenschleim) . In 1846 Hugo von Mohl applied to this substance the term " protoplasm "; a term already used by Purkinje six years previously for the formative substance of young animal embryos . Mohl showed that the young plant cell was at first completely filled by the protoplasm, and that only later, by the gradual See also:accumulation of vacuoles in the interior, did this substance come to form a thin layer on the inner See also:surface of the cell-wall . Mohl also described the spontaneous See also:movement of the protoplasm, a phenomenon already noted by Schleiden for his plant slime, and originally discovered by See also:Bonaventura See also:Corti in 1772 for the cells of Chara, and rediscovered in 1807 by Treviranus . Not only was See also:attention thus gradually directed to the importance of the cell-contents, but observations were not lacking, even in the plant See also:kingdom, tending to weaken the importance hitherto attached to the cell-wall .

Among these may be mentioned See also:

Cohn's observation that in the reproduction of Algal forms the protoplasm contracts away from the cell-wall and escapes as a naked " swarm spore." Similarly in the animal kingdom instances began to be noted in which no membrane appeared to. be present (See also:Kolliker, 1845; Bischoff, 1842), and for some time it was hotly debated whether these structures could be regarded as true cells . As a result of the resemblance between the streaming movements in these apparently naked cells (e.g. lymphocytes) and those seen in plant cells, R . Remak was led (1852–1853) to apply Moh1's term "protoplasm " to the sub-stance of these animal cells also . Similarly Max See also:Schultze(1863) and H . A. de Bary (1859), as a result of the study of unicellular animals, came to the conclusion that the substance of these organisms, originally termed " Sarcode " by F . Dujardin, was identical with that of the plant and animal cell . Numerous workers now began to realize the subordinate position of the cell-wall (e.g Nageli, See also:Alexander Braun, Leydig, Kolliker, Cohn, de Bary, &c:), but it is to Max Schultze above all that the See also:credit is due for having laid the See also:foundation of the modern conception of the cell-A conception often referred to as the proto-plasmictheory in opposition to the cell-theory of Schleiden and Schwann . Max Schultze showed that one and the same substance, protoplasm, occurred in unicellular forms and in the higher plants and animals; that in plants this substance, though usually enclosed within a cell membrane, was sometimes naked (e.g. swarm spores), while in many animal tissues and in many of the unicellular forms the cell-membrane was always absent . He therefore concluded that in all cases the cell-membrane was unessential, and he redefined the " cell " of Schleiden and Schwann as " a small See also:mass of protoplasm endowed with the attributes of life " (1861) . In the same See also:year the physiologist Brucke maintained that the complexity of vital phenomena necessitated the See also:assumption for the cell-protoplasm itself of a complex structure, only invisible because of the limitations of our methods of observation . The cell in fact was to be regarded as being itself an" elementary organism." By this time too it was realized that the formation of cells de novo, postulated by Schleiden's theory of " phytogenesis," did not occur . Cells only arose by the See also:division of pre-existing cells,—as Virchow neatly expressed it in his since famous See also:aphorism, omnis cellula e cellula .

It was, however, many years before the details of this " cell-division " were laid See also:

bare (see Cell-Division below) . General See also:Morphology of the Cell.—In its simplest form the cell is a more or less spherical mass of viscid,translucent and granular protoplasm . In addition to the living protoplasm there is present in the cell food-material in various stages of assimilation, which usually presents the appearance of See also:fine granules or spherules suspended in the more or less alveolar or reticular mesh-work of the living protoplasm . In addition there may be more or less obvious accumulations of See also:waste material, pigment, oil drops, &c.—products of the cell's metabolic activity . All these relatively passive inclusions' are distinguished from the living protoplasm by the term " metaplasm " (Hanstein), or "paraplasm" (Kupffer), although in practice no very See also:sharp distinction can be See also:drawn between them . The cell is frequently, but by no means always, bounded by a cell-wall of greater or less thickness . In plants this cell-wall consists of See also:cellulose, a sub-stance closely allied to See also:starch; in animals only very rarely is this the case . Usually the cell-wall, when this is present, is a product of the cell's secretive activity; sometimes, however, it appears to be formed by an actual See also:conversion of the surface layer of the protoplasm, and retains the power of growth by " intussusception " like the See also:rest of the protoplasm . Even when a limiting membrane is present, however, See also:evidence is steadily accumulating to show that the cell is not an isolated physiological unit, but that, in the vast majority of cases, there is a proto- 1 The Chromoplastids of the vegetable cell come under a different See also:category of cell-inclusions; see PLANTS: Cytology:plasmic continuity between the cells of the organism . This continuity, which is effected by fine protoplasmic threads (" cell-See also:bridges ") piercing the cell-wall and bridging the inter-cellular spaces when these are present, is to be regarded as the morphological expression of the physiological interdependence of the various—often widely separated—tissues of the See also:body .2 It is probable that it is the specialization of this See also:primitive See also:condition which has produced the cell-elements of the See also:nervous See also:system . In many cases the cell-connexions are so extensive as to obliterate cell-boundaries . A See also:good example of such a " syncytial " tissue is provided by the heart muscle of Vertebrates and the intestinal musculature of See also:Insects (Webber).3 In all multicellular, and in the great majority of unicellular; organisms the protoplasm of the cell-unit is differentiated into two very distinct regions,—a more or less central region, the nucleus, and a peripheral region (usually much more extensive), the cell-body or cytoplasm .

This universal morphological differentiation of the cell-protoplasm is accompanied by corresponding chemical See also:

differences, and is the expression of a physiological division of labour of fundamental importance . In some of the simpler unicellular organisms, e.g . Tetramitus, the differentiated protoplasm is not segregated . Such forms are said to have a " distributed " nucleus, and among the Protozoa correspond to See also:Haeckel's " See also:Protista." It is probable that among plants the Bacteria and Cyanophyceae have a similar distributed nucleus . In all the higher forms, however, the segregation is well marked, and a " nuclear membrane " separates the substance of the nucleus, or " karyoplasm " 4 from the surrounding "cytoplasm." Within the nuclear membrane the karyoplasm is differentiated into two very distinct portions, a clear fluid portion, the " karyolymph," and a firmer portion in the form of a coarser or finer " nuclear reticulum." This latter is again composed of two parts, the linin reticulum," s and, embedded in the latter and often irregularly aggregated at its nodal points, a granular substance, the chromatin," 8 the latter being the essential constituent of the nucleus . In addition to the chromatin there may be present in the nucleus one or more, usually spherical, and as yet somewhat enigmatical bodies, the " nucleoli." In addition to the nucleus and cytoplasm, a third body, the " centrosome," has often been considered as a See also:constant cell-structure . It is a See also:minute granule, usually lying in the cytoplasm not far from the nucleus, and plays an important part in cell-division and fertilization (see below) . Cell-differentiation.—Both among unicellular and multi-cellular individuals the cell assumes the most varied forms and performs the most diverse functions . In all cases, however; whether we examine the See also:free-living shapeless and slowly creeping See also:Amoeba, or the striped muscle cell or spermatozoon of the Metazoa (fig . 1, b and c), the constant recurrence of cytoplasm and nucleus show that we have to See also:deal in each case with a cell . The variation in the form and structure of the cell is an expression of that universal economic See also:law of nature, " division of labour," with its almost invariable accompanying " morphological differentiation "; the earliest and most fundamental example being in the differentiation of the cell-protoplasm into cytoplasm and nucleus . In multicellular individuals the division of labour to which the structural complexity of the organism is due is between the individual cell-units, some cells developing one 2 Cf .

Pfeffer's classical experiments on the physiological significance of cell-continuity in plant tissues (Uber den Einfluss See also:

des Zellkerns aufdie Bildung der Zellhaut, 1896) . The See also:recent work in physiology on the influence substances secreted by certain tissues and circulating in the blood-stream exert upon other and widely different tissues, should not be lost sight of in this connexion . The influence this protoplasmic continuity may have upon our conception of the cell as a unit of organization is referred to below (Present Position of the Cell-theory) . 4 A term (from s&pvov, See also:kernel) suggested by Flemming to replace Strasburger's hybrid term nucleoplasm (1882) . The earlier workers, e.g . Leydig, Schultze, Brucke, de Bary, &c., restricted the term protoplasm to the cell-body—the " Cytoplasm " of Strasburger, an example still followed by O . Hertwig . 4 From linum, a See also:thread, See also:Schwarz, 1887 . 6 From x,Y a, See also:colour, Flemming, 1879 . See also:uniform coating to the free surface of the cell, as in ciliated epithelium (fig . 2, a) and many infusoria, or the See also:cilia may be variously modified and restricted to See also:special regions of the body, e.g. the " undulating membrane " of the peristomial region in many infusoria, the See also:swimming combs of the See also:Ctenophora (q.v.), 712 aspect, some another, of their vital attributes . Thus one cell specializes in, say, secretion, another in contractility, another in receiving and carrying stimuli, and so forth, so that we have the gland cell, the muscle cell, and the nerve cell, each appropriately grouped with its See also:fellows to constitute the particular tissue or See also:organ—gland, muscle or See also:brain—which has for its function that of its constituent cells .

In unicellular animals we also find division of labour and its accompanying morphological differentiation, but here there is no subdivision of the protoplasm of the organism into the semi-autonomous units which so greatly facilitate division of labour in the Metazoa; instead, division of labour must be between different regions of protoplasm in the single cell . The sharply defined See also:

character of this regional differentiation in the Protozoa, and the surprising structural complexity it may produce, sufficiently clearly show that although multicellular structure has greatly facilitated regional differentiation in the Metazoa, it is by no means essential to this process (see below, Present Position of the Cell-theory) . It is not within the See also:scope of this See also:article to See also:attempt a comprehensive See also:review of the variety in structural complexity to which this division of labour among the cells of the Metazoan and the regional differentiation of the cell-bodies of the Protozoa has given rise . Some indication of the wealth of variety may be best given by taking a general survey of cell-modifications, grouped according to the cell-attributes the expression of which they facilitate . (a) Structural Complexity facilitating Movement.—One of the most striking, and hence earliest described, of the fundamental attributes of protoplasm is its power of spontaneous movement . This is seen in the walled cell of plant tissue and in b a and b from Schafer's Essentials of See also:Histology, by permission of See also:Longmans, See also:Green & Co . b, Part of a Mammalian " striated " muscle-cell (diagrammatic) . c, Spermatozoa of See also:mouse and See also:bird . the naked cell-body of Amoeba . In the latter case the streaming movements of the naked protoplasm are accompanied by the formation of " pseudopodia," and result in the highly characteristic " amoeboid " creeping movement of this and similar organisms (e.g. See also:lymph corpuscles of the blood).' In. these examples the whole protoplasm participates in the movement,—there has been no division of labour, and there is, therefore, no visible morphological differentiation . In many cells, movement (either of the entire body or of the surrounding See also:medium) is by means of slender See also:whip-like processes of the protoplasm flagella or cilia . These represent modified pseudopodia, and in the formation of the motile gametes of some of the lower forms, e.g .

Myxomycetes (de Bary,1859), Rhizopods (R . Hertwig, 1874), &c., the actual conversion of a pseudopodium into a flagellum can be witnessed . These vibratile processes may be either one or few in number, and are then large in See also:

size and move independently of one another; or they may be very numerous, covering the free surface of the cell (fig . 2, a); they are then very small and move strictly in unison . In the former case they are termed " flagella," in the latter " cilia." In some cases the flagellum is accompanied by an undulating membrane (e.g . Trypanosoma among the protozoa and in many spermatozoa), and it may be situated either at the front end (Euglena) or See also:hind end (spermatozoa) of the body during See also:motion . The cilia may form a t The formation of pseudopodia and accompanying changes in form of Amoeba were observed as early as 1755 by Raesel von Rosenhof, who named it on this See also:account the " little See also:Proteus." From A . Gurwitsch, Moephologie and Biologic der Zelle, by permission of Gustav See also:Fischer . and the See also:flame cells of the See also:Platyelmia (q.v.) . In one See also:group of infusoria (Hypotricha), the cilia, " cirri," have attained a high degree of differentiation, and reach a considerable size . Both cilia and flagella See also:spring directly from the cell-protoplasm, piercing the cell-membrane, when this is present . At the point where they become continuous with the cell-body there is usually a deeply staining " basal granule." In some cases the flagella are in See also:direct connexion with the centrosome (see below, Cell-division) , e.g .

Trypanosoma and spermatozoa, in some cases even while the centrosome is functioning in mitosis (e.g. See also:

insect spermatogenesis, Henneguy2 and Meves2 (fig . 3) . In the ability of Amoeba to See also:contract into a spherical mass, and in the presence in its protoplasm of the contractile vacuole, we see another type. of spontaneous movement—contractilityof the protoplasm . In the " musculo-epithelial " cells of See also:Hydra, From O . Hertwig, All gemeine Biologic, by permission of Gustav Fischer . the elongated basal portion of the cell alone possesses this contractility . In the higher Metazoa the whole cell—muscle cell—is specialized for contractility, and shows, as a result of its specialization, a distinct fibrillation . This fibrillation is foreshadowed in the contractile regions of many Protozoa, e.g .. 2 " Sur See also:les rapports des cils vibratiles avec les centrosomes," Archives d'anatomie microscopique (1898) . 3" Ober Zentralkorper in mannlichen Geschlechtszellen von Schmetterlingen " (Anat . Anz . Bd. xiv., 1897) .

Cf. also the papers of Lenhossek (Uber Flimmerzellen, 1898), Karl See also:

Peter (Da: Zentrum See also:fur die Flimm-und Giesselbewegung, 1899) and Verworn (Studien z r Physiologie der Flimmerbewegung, 1899) . a in the cirri of hypotrichous Infusoria, the tentacle of Noctiluca, and the myophane layer of See also:Gregarines . In the quickly contracting muscle cell of Vertebrates and insects, further specialization has produced a structure of considerable complexity (fig. r, b) . Here also the cell is fibrillated, but the fibrillae (sarco-styles) are much more distinct, and are segmented in a manner which gives to the entire cell a " See also:cross striated " appearance . Since See also:quick movement is usually (but not always) associated with voluntary See also:control, these striated muscle cells are often termed " voluntary " muscle See also:fibres . The great increase in length of these cells is accompanied by the fragmentation of the origin-ally single nucleus . (b) Cell-modification in Relation to Secretion.—Just as the complex movements considered above were the result of a great development of the power of spontaneous movement possessed by all protoplasm, so cell-secretion is the result of a development of the metabolic processes underlying all vital phenomena . But whereas specialization of the protoplasm for movement resulted in a very obvious morphological complexity, specialization for secretion results in molecular complexity, and only rarely and indirectly results in morphological differentiation . Usually indeed the specialization is only rendered evident by the appearance of the formed secretion, e.g. mucus-secreting epithelial cells (fig . 2, b), the ovarian ovum and the See also:fat cell (fig . 1, a) . In some cases a distinct fibrillation of the cytoplasm accompanies or precedes the appearance of the cell-secretion (See also:Mathews, See also:pancreas cell of See also:Amphibia) .

In many cases the See also:

internal secretion is no See also:mere accumulation, e.g. the internal See also:skeleton of the See also:Radiolaria, and the nematocysts of the See also:Coelentera . Frequently in animal tissues the cell-secretions are accumulated in the intercellular spaces, and result in the formation of the various " connective tissues," all of which are characterized by the immense amount of intercellular substance, e.g. fibrous tissue, See also:cartilage and See also:bone . Cell-modifications facilitating the general See also:metabolism, but not necessarily indicating specialized secretion, also occur, e.g. the " gullet " of many Protozoa, the suctorial tubules of the Acinetaria, and the " nutritive processes" of the ovarian ova in many See also:Lepidoptera . Mention may be made here of the network or See also:canal system of the cytoplasm, described for many cells by Golgi, Holgren and others . An enigmatical structure, the " yolk-nucleus " of many e•ya, has been frequently regarded as a structure of considerable metabolic importance, e.g . Bambeke (1888) for Pholcus.' Striking modifications resulting from specialization in secretion are frequently presented by the nucleus . In many secreting From Prof . E . B . See also:Wilson's The Cell in Development and Inheritance, by permission of the author and of the See also:Macmillan Co., New See also:York . a, Permanent spireme-nuclei in cells from the intestinal epithelium of a dipterous larva, Ptychoptera . (After See also:van Gehuchten.) From Korschelt and Heider, Lehrbuch der verg .

Entwicklungsgeschichts der wvbeUosen Tore, by permission of Gustav Fischer . b, Branched nucleus of the " nutritive " cell, from a portion of an ovarial tube of Forficula auricularia . cells this structure is extensively branched, e.g. many gland cells and ovarian nutritive cells of insects (fig . 4, b) . In some cases the nucleus of the gland cell contains a persistent spireme thread (fig . 4, a); while almost all actively secreting cells I Cf., however, the present writer's See also:

interpretation of this structure in the oocyte of Antedon . Phil . Trans . Royal .See also:Soc . (1906), B . 249.are characterized by the See also:possession of large or numerous nucleoli . (c) Specialization for the Reception and See also:Conduction of Stimuli.—One of the most striking of the fundamental attributes of living protoplasm is its " irritability," that is to say, its power of responding to See also:external impressions, " stimuli," by movement, which, both in See also:kind and intensity, is wholly See also:independent of the amount of See also:energy expended by the stimulus .

The stimulus conveyed by the nerve fibre to the muscle is out of all proportion C From Schafer's Essentials of Histology, by permission of Longmans, Green & Co . A and B, Ganglion cells from the cerebral cortex; in A the only slightly branched axon may extend the whole length of the See also:

spinal See also:cord . (After Schafer.) C, Body of a ganglion-cell showing " Nissl's granules." D, Sensory cells from olfactory epithelium . (After Schultze.) E, Diagrammatic See also:representation of the sensory epithelium of retina (See also:rod and See also:cone layer) . (After Schwalbe.) to the amount of work it may cause the muscle to do . Although protoplasmic irritability is thus incapable of a See also:simple See also:mechanical explanation, See also:science has rejected the assumption of a special " vital force," and interprets protoplasmic response as being a long series of chemico-See also:physical changes,2 initiated, but only initiated, by the See also:original stimulus; the latter thus See also:standing in the same relation to the response it produces as the pull on the trigger to the propulsion of the See also:rifle See also:bullet . The function of receiving stimuli from the See also:outer See also:world, originally possessed to a greater or less extent by all cells, has, in the Metazoa, been relegated to one class of cells, the sensory cells' (fig . 5, D and E) . Another class of cells—the " ganglion cells " or " neurones " (fig . 5, A and B), are concerned with the conduction of the stimuli so received . The contractile elements in the Metazoa are thus dependent for their stimuli on the nervous elements—the sensory cells and neurones . Origin of Cells.—In the preceding sections we have considered the structure of the cell in relation to the fundamental attributes of cell-metabolism, irritability, and movement .

We have how 2 See also:

Claude See also:Bernard expressed the same conclusion in 1885 . Rejecting both the view that vital phenomena were identical with chemicophysical phenomena, and that which regarded them as totally distinct, he suggested a third point of view: ` l'See also:element ultime du phenomene est physique; l'arrangement est vital." 2 Many forms of response to stimulus involve no visible specialization, e.g. See also:positive and negative heliotropism, chemiotropism, geotropism, &c., seen more especially in plants, but occurring also in the animal kingdom . to consider the cell in relation to yet another vital attribute, that of reproduction . Just as we now know chat the phenomena of assimilation, respiration, excretion, response, movement and so forth, characteristic of living things, are but the co-ordinated expressions of the corresponding activities of the constituent cells, so we now know that the reproduction of the organism is, in its ultimate See also:analysis; a cell-process . Our knowledge of the essential fact that cells only arise by the division of pre-existing cells, now a fundamental See also:axiom of biology, and of the details of this process, have been acquired during recent years by the strenuous efforts of numerous workers.' Matthias Jakob Schleiden (1838) supposed that in plants the new cell, arose from the See also:parent cell by a sort of " crystallizing " process from the cell fluid or "cytoblastema "; the nucleolus appearing first, then the nucleus, and finally the cell-body . Theodor Schwann (1839) extended Schleiden's theory to animal tissues, with this yet greater See also:error, that new cells might arise, not only within the mother cell as Schleiden had supposed, but also in the inter-cellular substance so See also:common in animal tissues (to which he also gave the term " cytoblastema ") . By 1846, however, the botanists, thanks mainly to the efforts of Hugo: von Mohl and Nageli, recognized as a general law that cells only arise by the division of a pre-existing cell . But it was long before the universal application of this law was recognized by zoologists; the delay being largely due to pathological phenomena . The work of Kolliker (1844-1845), Karl Bogislaus Reichert (1841-1847), and Remak (1852-1855), however, finally enabled Virchow in 180 to maintain the law of the genetic continuity of cells in the since `famops aphorism omnis cellula e cellula . At this time, however, nothing was known of the details of cell-division,—one school: (Reichert, L . See also:Auerbach, and the majority of the botanists) maintaining that the nucleus disappeared See also:prior to cell-division, the' other . ,scOhool (von See also:Baer, Remak, Leydig, Haeckel, &c.) maintaining that it took a leading part in the process .

It is not until the appearance of Anton See also:

Schneider's work in 1843, followed by those of Fol, Auerbach, Strasburger See also:awl. many others, that we begin to gain an insight into the process . In 1882 W . Flemming was able to extend Virchow's aphorism to the nucleus also: omnis nucleus e nucleo . Outline of Cell-division.—There are two very distinct methods of cell-division . The more general and also more complicated method is accompanied by the formation of a complex fibrillar mechanism, and was on this account termed " mitosis" (giros, a, thread) by W . Flemming (1882), and " karyokinesis " (Kapvov, See also:nut, nucleus, and uivgats, See also:change, movement) by W . See also:Schleicher (1878) . The other method, "amitosis," or direct division, is unaccompanied by any visible mechanism and is of relatively exceptional occurrence . In the more usual method of cell-division, or " mitosis," we can distinguish two distinct but parallel processes, the one undergone by the chromatin and resulting in the " See also:chromatic figure," the other usually only concerning the cytoplasm and resulting in the " achromaticspecies . Thus in the parasitic See also:worm Ascaris megalocephaia, See also:var. univalens, there are only two . In the crustacean Artemia See also:Bauer found 168, while in the amphibian Salamandra maculata, as also in the See also:lily, the number is 24 . While these changes have been proceeding in the nucleus, changes in the cytoplasm have resulted in the formation of the achromatic figure .

These cytoplasmic changes are initiated by the division into two of a minute body, the " centrosome," originally discovered by P . J. van Beneden in 1883,' and usually lying not far from the nucleus (fig . 6, a) . The daughter centrosomes See also:

separate from one another, travelling to opposite poles of the nucleus . At the same time radiations extend out into the cytoplasm from the centrosomes, and, as the nuclear membrane disappears, invade the nuclear See also:area (fig . 7, a) . Some of the fibrillae in the latter region become attached to the chromosomes and are b a; b and c from Prof . E . B . Wilson's The Cell in Development and Inheritance, by permission of the author and the Macmillan Co., New York; d from A . Gurwitsch, Mor phologie u . Biologic der Zelle, by permission of Gustav Fischer .

termed "See also:

mantle fibres"; others become continuous from one centrosome to the other and constitute the " spindle fibres." The remaining radiations at the two poles of the spindle are the " astral rays." (The details of the formation of the achromatic figure vary considerably, some indication of this is given in the next See also:section in connexion with the question of the origin of the mitotic mechanism.) The chromosomes now arrange themselves in the " See also:equatorial See also:plate " of the spindle and each splits longitudinally into twos (fig . 6, b and c) . The See also:sister chromosomes now pass to opposite poles of the spindle (fig . 6, d), and there, returning to the " resting '.' condition, constitute the daughter nuclei . Division of the cell follows, usually, in animals, by simple constriction . Both Theodor Boveri and van Beneden, in their papers of 1887, regarded the centrosome as initiating, not only the division of the cell-body but that of the chromatin also; Beneden even suggested that the pull of the mantle fibres caused the division of the chromatin in the equatorial plate . W . Pfitzner in 1882 was the first to show that the splitting of the chromosomes in the equatorial plate was only the reappearance of a split in the spireme thread and was due to a corresponding ' " Recherches sur la maturation de l'ceuf, la fecondation et la division cellulaire " (Archives de biologie, vol. iv.) . e First discovered by Flemming in 1879 and confirmed by Retzius in 1881 figure." 2 We will consider the chromatin changes first . The chromatin granules lose their scattered arrangement on the nuclear reticulum, and become instead arranged in a linear series to form a coiled and deeply staining "spireme thread" 3 (fig . 6, a) . As the thread contracts, its granular origin becomes less evident, and at the same time the coils become fewer in number; the " close " spireme of earlier stages becomes the " loose " spireme of later stages .

• As the spireme thread contracts, it segments into a number of short, and usually U-shaped, segments-the " chromosomes " (Waldeyer, 1888) . The number of these chromosomes is always constant for the cells of any given See also:

species of plant or animal, but varies greatly in number in different Prominent among these are: Schleiden (1873), Fol (1843-1877), Auerbach (1874), Biitschli (1876), Strasburger (1875-1888), O . Hertwig (1875-1890), R . Hertwig (1875-1877), Flemming (1879-1891), van Beneden (1883-1887), Rabl (1889), Boveri (1887-19o3) . 2 This distinction between the chromatic and achromatic portions of the mitotic figure is due to Flemming . ' The See also:genesis of the spireme thread was first described by E . G . Balbiani in 1876 . division into two of each of the chromatin granules . In the spermatogenic cells of Ascaris, A . Brauer has. shown that the chromatin granules See also:divide while still scattered over the nuclear reticulum and before either the formation of a spireme thread or the division of the centrosome . In many other cases the See also:reverse of this condition occurs, the centrosome dividing long before there is any indication of division in the nucleus (e.g. See also:salamander spermatogenic cells, Meves, &c.) .

We must there-fore, with Boveri and Brauer, regard the division of the chromatin in mitosis as a distinct reproductive See also:

act on the part of the chromatin granules, the chromosomes being merely aggregates (temporary or permanent, vide infra) of these self-propagating units . For convenience of description it is usual to recognize four periods in mitosis: (i.) Prophase, (ii.) Metaphase, (iii.) Anaphase, and (iv.) Telophase (Strasburger, 1884) . The 'prophase covers all changes up to the completion of the mitotic figure . The metaphase is the parting of the sister chromosomes in the equatorial plate; their passage to opposite poles of the spindle constitutes the anaphase; and their reconstruction to form the resting daughter nuclei, the telophase . The Achromatic Figure.—The mode of origin of the achromatic figure varies greatly . In some cases a distinct and continuous spindle, the " central spindle " of F . See also:Hermann, is visible from the very first separation of the daughter centrosomes (e.g. salamander . spermatogenic cell)' (fig . 7, b) . In other From Prof . E . B . Wilson's The Cell in Development and Inheritance, by permission of the author and of The Macmillan Co., New York .

a, Leucocyte from a Salamander, showing permanent See also:

aster and centrosome . From A . Gurwitsch, Morphologie u . Biologic der Zelle, .by permission of Gustav Fischer . b, Sperm-mother cell of Salamandra maculate, showing Hermann's " central spindle." cases the rays only invade the nuclear area and become continuous in the equatorial See also:plane after the centrosomes have assumed their definitive positions at the two poles of the nucleus, and may even appear to indent the disappearing nuclear mein-. brane as they invade the nuclear area .2 . In the salamander testis cell (fig . 7, b), and in many other cases, the whole of the achromatic figure is obviously of cytoplasmic origin . In many cases, however, it equally obviously arises within the nucleus,3 while in yet other cases" the spindle fibres are of mixed origin . The question, therefore, of the cytoplasmic or nuclear