The cell in development and inheritance (1900) 1
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Wilson EB. The Cell in Development and Inheritance. Second edition (1900) New York, 1900.
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Chapter I General Sketch of the Cell
" Wir haben gesehen, dass alle Organismen aus wesentlich gleichen Thcilen, namlich aus Zellen zusammengesetzt sind, dass diese Zellen nach wesentlich denselben Gesetzen sich bilden und wachsen, dass also diese Prozesse iiberall auch durch dieselben Kraft e hervorgebracht werden miissen." Schwann. 1
In the passage quoted above Schwann expressed a truth which subsequent research has established on an ever widening basis ; and we have now more than ever reason to believe that despite unending diversity of form and function all cells may be brought into definite relation to a common morphological and physiological type. We are, it is true, still unable to specify all its essential features, and hence can give no adequate brief definition of the cell. For practical purposes, however, no such definition is needed, and we may be content with the simple type that has been familiar to histologists since the time of Leydig and Max Schultze.
It should from the outset be clearly recognized that the term " cell " is a biological misnomer ; for cells only rarely assume the form implied by the word of hollow chambers surrounded by solid walls. The term is merely an historical survival of a word casually employed by the botanists of the seventeenth century to designate the cells of certain plant-tissues which, when viewed in section, give somewhat the appearance of a honeycomb. 2 The cells of these tissues are, in fact, separated by conspicuous solid walls which were mistaken by Schleiden, followed by Schwann, for their essential part. The living substance contained within the walls, to which Hugo von Mohl gave the name protoplasm 8 ( 1 846), was at first overlooked or was regarded as a waste-product, a view based upon the fact that in many important plant-tissues such as cork or wood it may wholly disappear, leaving only the lifeless walls. The researches of Bergmann, Kolliker, Bischoff, Cohn, Max Schultze, and many others showed, however, that most living cells are not hollow but solid bodies, and that in many cases — for example, the colourless corpuscles of blood and lymph — they are naked masses of protoplasm not surrounded by definite walls. Thus it was proved that neither the vesicular form nor the presence of surrounding walls is an essential character, and that the cell-contents, i.e. the protoplasm, must be the seat of vital activity.
1 Untersuchungen, p. 227, 1839.
2 The word seems to have been first employed by Robert Hooke, in 1665, to designate the minute cavities observed in cork, a tissue which he described as made up of " little boxes or cells distinct from one another " and separated by solid walls.
8 The same word had been used by Purkinje some years before (1840) to designate the formative material of young animal embryos.
Within the protoplasm (Figs. 6-8) lies a body, usually of definite rounded form, known as the nucleus, 1 and this in turn often contains
Plaslidi lying in the ground -subsume.
one or more smaller bodies or nucleoli. By some of the earlier workers the nucleus was supposed to be, like the cell-wall, of secondary importance, and many forms of cells were described as being devoid of a nucleus ("cytodes" of Haeckel). Nearly all later researches have indicated, however, that the characteristic nuclear material, whether forming a single body or scattered in smaller masses, is always present, and that it plays an essential part in the life of the ceil.
Besides the presence of protoplasm and nucleus, no other structural features of the cell are yet known to be of universal occurrence.
Robert Itruwn ir
u»
•781..,
ecogniied as a normal element of the c
We may therefore still accept as valid the definition given more than thirty years ago by Leydig and Max Schultze, that a cell is a mass of protoplasm containing a nucleus} to which we may add Schultze's statement that both nucleus and protoplasm arise through the division of the corresponding elements of a preexisting cell. Nothing could be less appropriate than to call such a body a " cell " ; yet the word has become so firmly established that every effort to replace it by a better has failed, and it probably must be accepted as part of the established nomenclature of science. 2
A. General Morphology of the Cell
The cell is a rounded mass of protoplasm which in its simplest form is approximately spherical. The form is, however, seldom realized save in isolated cells such as the unicellular plants and animals or the egg-cells of the higher forms. In vastly the greater number of cases the typical spherical form is modified by unequal growth and differentiation, by active movements of the cell-substance, or by the mechanical pressure of surrounding structures, but true angular forms are rarely if ever assumed save by cells surrounded by hard walls. The protoplasm which forms its active basis is a viscid, translucent substance, sometimes apparently homogeneous, more frequently finely granular, and as a rule giving the appearance of a meshwork, which is often described as a spongelike or netlike " reticulum." 8 Besides the active substance or protoplasm proper the cell almost invariably contains various lifeless bodies suspended in the meshes of the network ; examples of these are food-granules, pigment-bodies, drops of oil or water, and excretory matters. These bodies play a relatively passive part in the activities of the cell, being either reserve food-matters destined to be absorbed and built up into the living substance, or by-products formed from the protoplasm as waste-matters or in order to play some rdle subsidiary to the actions of the protoplasm itself. The passive inclusions in the protoplasm may be collectively designated as mctaplasm (Hanstein) or paraplasm (Kupffer), in contradistinction to the active protoplasm.
1 Leydig, Lehrbuch der Histologic, p. 9, 1857; Schultze, Arch. Anat.u. PAys.,p. 11, 1861.
2 Sachs has proposed the convenient word energid {Flora, '92, p. 57) to designate the essential living part of the cell, i.e. the nucleus with that portion of the active cytoplasm that falls within its sphere of influence, the two forming an organic unit both in a morphological and in a physiological sense. It is to be regretted that this convenient and appropriate term has not come into general use. (See also Flora, '95, p. 405, and cf. Kupffer C96), Meyer ('96), and Kolliker ('97).)
8 Such meshworks are sometimes plainly visible in the living protoplasm (p. 44). It is always more or less an open question how far the appearances seen in fixed (coagulated) material correspond with the conditions existing in life. See pp. 42-46.
It is often difficult to distinguish between such metaplastic bodies and the granules commonly supposed to be elements of the active protoplasm ; indeed, as will appear beyond (p. 29), there is reason to believe that "protoplasmic" and " metaplasmic " granules cannot be separated by any definite limit, but are connected by various gradations. Among the lifeless products of the protoplasm must be reckoned also the cell-wall or membrane by which the cell-body may be surrounded ; but it must be remembered that the cell-wall in some cases arises by a direct transformation of the protoplasmic substance, and that it often retains the power of growth by intussusception like living matter.
Fig. 7— Spei
Above, two cells showing laige
each cell an attraction-sphere will
showing chromatin-reticulum, cent
threads and scattered chroma tin -granules ; in Below, three contiguous spermatogonia. , and sphere-bridges.
It is unfortunate that some confusion has arisen in the use of the word protoplasm. When Leydig, Schultze, Briicke, De Bary, and other earlier writers spoke of "protoplasm," they had in mind only the substance of the cell-body, not that of the nucleus. Strasburger, however, in 1882, extended the term so as to denote the entire active cell-substance, including the nuclear material, suggesting that the latter be called nucleoplasm, and that of the cell-body cytoplasm.
Fig. 8 — Various cells showing the typical pans.
A. From peritoneal epithelium of the salamander-larva. Two cenrro: Nucleus showing net-knols. [F LEMMING.)
B. Spermatogonium of frog. Attraction-sphere (aster) containing a Nucleus with a single plasmosome. [Hermann.]
C. Spinal ganglion-cell of frog. Attraction-sphere near the centre, contaii some with several £enlrioles. [L.ENHOS5KK.]
D. Spermatocyte of Prateut. Nucleus in the spireme-stage. Cenlrosomi sphere containing rod-shaped bodies. [Hermann.]
These terms have been adopted by many, but not all, later writers, the hybrid word nucleoplasm having, however, at Flemming's suggestion, been changed to karyoplasm. At the present time, therefore, the word protoplasm is used by some authors (Biitschli, Hertwig, Kolliker, etc.) in its original narrower sense (equivalent to Strasburger's cytoplasm), while perhaps the majority of writers have accepted the terminology of Strasburger and Flemming. On the whole, the terms cytoplasm and karyoplasm seem too useful to be rejected, and, without attaching too much importance to them, they will be employed throughout the present work. It must not, however, be supposed that either of the words denotes a single homogeneous substance ; for, as will soon appear, both cytoplasm and karyoplasm consist of several distinct elements.
The nucleus is usually bounded by a definite membrane, and often appears to be a perfectly distinct vesicular body suspended in the cytoplasm — a conclusion sustained by the fact that it may move actively through the latter, as often occurs in both vegetable and animal cells. Careful study of the nucleus during all its phases gives, however, reason to believe that its structural basis is similar to that of .the cell-body ; and that during the course of cell-division, when the nuclear membrane usually disappears, cytoplasm and karyoplasm come into direct continuity. Even in the resting cell there is good evidence that both the intranuclear and the extranuclear material may be structurally continuous with the nuclear membrane l and among the Protozoa there are forms (some of the flagellates) in which no nuclear membrane can at any period be seen. For these and other reasons the terms "nucleus" and "cell-body" should probably be regarded as only topographical expressions denoting two differentiated areas in a common structural basis . The terms karyoplasm and cytoplasm possess, however, a specific significance owing to the fact that there is on the whole a definite chemical contrast between the nuclear substance and that of the cell-body, the former being characterized by the abundance of a substance rich in phosphorus known as nuctein t while the latter contains no true nuclein and is especially rich in albuminous substances such as nucleo-albumins, albumins, globulins, and the like, which contain little or no phosphorus.
Both morphologically and physiologically the differentiation of the active cell-substance into nucleus and cell-body must be regarded as a fundamental character of the cell because of its universal, or all but universal, occurrence, and because there is reason to believe that it is in some manner an expression of the dual aspect of the fundamental process of metabolism, constructive and destructive, that lies at the basis of cell life. The view has been widely held that a third essential element is the centrosome t discovered by Flemming and Van Beneden in 1875-76, and since shown to exist in a large number of other cells (Figs. 7, 8). This is an extremely minute body which is concerned in the process of cell-division and in the fertilization of the egg, and has been characterized as the " dynamic centre " of the cell. Whether it has such a significance, and whether it is in point of morphological persistence comparable with the nucleus, are questions still subjudicc y which will be discussed elsewhere. 1
1 Conklin ('97, 1), Obst ('99), and some others have described a direct continuity in the resting cell between the intranuclear and extranuclear meshworks.
B. Structural Basis of Protoplasm
As ordinarily seen under moderate powers of the microscope, protoplasm appears as a more or less vague granular substance which shows as a rule no definite structure organization. More precise examination under high powers, especially after treatment by suitable fixing and staining reagents, often reveals a highly complex structure in both nucleus and cytoplasm. Since the fundamental activities of protoplasm are everywhere of the same nature, investigators have naturally sought to discover a corresponding fundamental morphological organization common to all forms of protoplasm and underlying all of its special modifications. Up to the present time, however, these attempts have not resulted in any consensus of opinion as to whether such a common organization exists. In many forms of protoplasm, both in life and after fixation by reagents, the basis of the structure is a more or less regular framework or meshwork, consisting of at least two substances. One of these forms the substance of the meshwork proper ; the other, often called the ground- substaticc (also cell-sap, enchylema, hyaloplasma, paramitome, interfilar substance, etc.), 2 occupies the intervening spaces. To these two elements must be added minute, deeply staining granules or " microsomes " scattered along the branches of the meshwork, sometimes quite irregularly, sometimes with such regularity that the meshwork seems to be built of them. Besides the foregoing three elements, which we may provisionally regard as constituting the active substance, the protoplasm almost invariably contains various passive or metaplasmic substances in the form of larger granules, drops of liquid, crystalloid bodies, and the like. These bodies, which usually lie in the spaces of the meshwork, are often difficult to distinguish from the microsomes lying in the meshwork proper — indeed, it is by no means certain that any adequate ground of distinction exists. 3
From the time of Frommann and Arnold (^S-^) onwards, most of the earlier observers regarded the meshwork as a fibrillar structure, either forming a continuous network or reticulum somewhat like the fibrous network of a sponge ("reticular theory " of Klein, Van Beneden, Carnoy, Heitzmann), or consisting of disconnected threads,
Fig. 9. — Living cells of salamander-larva. [FLEMMINCJ.] A, Group of epidermal cells at different foci, showing protoplasmic bridges, nuclei, and cytoplasmic fibril] ie; the central cell with nucleus in the spire me-5tage. B. Connective tissue cell.
C. Epidermal cell in early mitosis (segmented spireme) surrounded by protoplasmic bridges.
D. Dividing cell. J-.t-'. Cartilage-cells with cytoplasmic fibrillse (the latter somewhat exaggerated in the plait).
whether simple or branching ("filar theory of Flemming), and the same view is widely held at the present time. The meshwork has received various names in accordance with this conception, among which may be mentioned reticulum, thread-work, spongioplasm, mitome, filar substance} all f of which are still in use. Under this view the "granules" described by Schultze, Virchow and still earlier observers have been variously regarded as nodes of the network, optical sections of the threads, or as actual granules (" microsomes ") suspended in the network as described above.
Widely opposed to these views is the " alveolar theory " of Biitschli, which has won an increasing number of adherents. Biitschli regards protoplasm as having a foam-like alveolar structure ("Wabenstruktur"), nearly similar to that of an emulsion (Fig. 10), and he has shown in a series of beautiful experiments that artificial emulsions, variously prepared, may show under the microscope a marvellously close resemblance to living protoplasm, and further that drops of oil-emulsion suspended in water may even exhibit amoeboid changes of form. To restate Biitschli's view, protoplasm consists of separate, closely crowded minute drops 2 of a liquid alveolar substance suspended in a continuous interalveolar substance, likewise liquid, but of different physical nature. The latter thus forms the walls of closed chambers or alveoli in which the alveolar drops lie, just as in a fine emulsion the emulsifying liquid surrounds the emulsified drops. The appearance of a network in protoplasm is illusory, being due to optical section of the interalveolar walls or partitions as viewed at any given focus of the microscope. As thus seen, the walls themselves appear as fibres, while the "spaces of the network" are in like manner optical sections of the alveoli, the alveolar substance that fills them corresponding to the "ground substance." As explained beyond, 3 Biitschli interprets in like manner the radiating systems or asters formed during cell-divison, the astral rays (usually considered as fibres) being regarded as merely the septa between radially arranged alveoli (Fig. 10).
The two (three) general views above outlined may be designated respectively as the fibrillar (reticular or filar) and alveolar theories of protoplasmic structure ; and each of them has been believed by some of its adherents to be universally applicable to all forms of protoplasm. Beside them may be placed, as a third general view, the granular theory especially associated with the name of Altmann, by whom it has been most fully developed, though a number of earlier writers have held similar views. According to Altmann's view, which apart from its theoretical development approaches in some respects that of Biitschli, protoplasm is compounded of innumerable minute granules which alone form its essential active basis; and while fibrillar or alveolar structures may occur, these are of only secondary importance.
1 See Glossary.
a Measuring on an average about .00 1 mm. in diameter. * Cf. p. no.
A. Epidermal cell of ihe e urchin egg. C. lnlracapsuli D. Peripheral cytoplasm of sea-urchin egg. E. Artificial emulsion of olive-oil. sodium chloride,
It is impossible here adequately to review the many combinations and modifications of these views which different investigators have made. 1 On the whole, the present drift of opinion is toward the conclusion that none of the above interpretations has succeeded in the attempt to give a universal formula for protoplasmic structure ; and many recent observers have reached the conclusion, earlier advocated by Kolliker {'89), that the various types described above are connected by intermediate gradations and may be transformed one into another, in different phases of cell-activity. Unna C95), for example, endeavours to show how an alveolar structure may pass into a sponge-like or reticular one by the breaking down of the interalveolar walls. Flemming, for many years the foremost and most consistent advocate of the fibrillar theory, now admits that protoplasm may be fibrillar, alveolar, granular, or sensibly homogeneous, 3 and that we cannot, therefore, regard any one of these types of structure as absolutely diagnostic of the living substance. In plant-cells Strasburger 8 and a number of his pupils maintain that the "kinoplasm" (p. 322) or filar plasm, from which the spindle-fibres and astral rays are formed, is fibrillar, while the " trophoplasm " or alveolar plasm forming the main body of the cell is alveolar, the former, however, assuming the fibrillar structure, as a rule, only during the mitotic activity of the cell. My own long-continued studies on various forms of protoplasm have likewise led to the conclusion that no universal formula for protoplasmic structure can be given. 1 In that classical object, the echinoderm-egg, for example, it is easy to satisfy oneself, both in the living cell and in sections, that the protoplasm has a beautiful alveolar structure, exactly as described by Butschli in the same object (Fig. n). This structure is here, however, entirely of secondary origin ; for its genesis can be traced step by step during the growth of the ovarian eggs through the deposit of minute drops in a homogeneous basis, which ultimately gives rise to the interalveolar walls. In these same eggs the astral systems formed during their subsequent division (Fig. 12) are, I
Fig. 11, — (a) Protoplasm of the egg of (he sea-urchin (Taxopnmtia) in section sho meshwork of microsomes; (*) protoplasm from a living siar-fish egg (Aslerias) showing alvi
• form larger sphor
1 a young o
1 For full discussion, with litcratur
1. showing alveoli and
sperm-nucleus, middle piei
believe, no less certainly fibrillar; and thus we see the protoplasm of the same cell passing successively through homogeneous, alveolar, and fibrillar phases, at different periods of growth and in different conditions of physiological activity. There is good reason to regard this as typical of protoplasm in general. Butschli's conclusions, based on researches so thorough, prolonged, and ingenious, are entitled to great weight; yet it is impossible to resist the evidence that fibrillar and granular as well as alveolar structures are of wide occurrence ; and while each may be characteristic of certain kinds of cells, or of certain physiological conditions, 1 none is common to all forms of protoplasm. If this position be well grounded, we must admit that the attempt to find in visible protoplasmic structure any adequate insight into its fundamental modes of physiological activity has thus far proved fruitless. We must rather seek the source of these activities in the ultramicroscopical organization, accepting the probability that apparently homogeneous protoplasm is a complex mixture of substances which may assume various forms of visible structure according to its modes of activity.
1 Wilson, '99.
Some of the theoretical speculations regarding the essential nature of that organization are discussed in Chapter VI., but one quasi-Xhzoretical point must be here considered. Much discussion has been given to the question as to which of the visible elements of the protoplasm should be regarded as the "living" substance proper; and the diversity of opinion on this subject may be judged by the fact that although many of the earlier observers identified the "reticulum" as the living element, and the ground-substance as lifeless, others, such as Leydig and Schafer, held exactly the reverse view, while Altmann insisted that only the " granules " were alive. Later discussions have shown the futility of this discussion, which is indeed largely a verbal one, turning as it does on the sense of the word "living." In practice we continually use the word " living " to denote various degrees of vital activity. Protoplasm deprived of nuclear matter has lost, wholly or in part, one of the most characteristic vital properties, namely, the power of synthetic metabolism ; yet we still speak of it as " living," since it still retains for a longer or shorter period such properties as irritability and the power of coordinated movement ; and, in like manner, various special elements of protoplasm may be termed " living " in a still more restricted sense. In its fullest meaning, however, the word " living " implies the existence of a group of cooperating activities more complex than those manifested by any one substance or structural element. I am therefore entirely in accord with the view urged by Sachs, Kolliker, Verworn, and other recent writers, that life can only be properly regarded as a property of the cellsystem as a whole ; and the separate elements of the system would, with Sachs, better be designated as "active" or "passive," rather than as "living" or "lifeless." Thus regarded, the distinction
1 Thus the alveolar structure seems to be characteristic of Protozoa in general, and of the protoplasm of plant-cells when in the vegetative state, the fibrillar of nerve-cells and muscle-cells. The granular type is characteristic of some forms of leucocytes and glandcells; but many of the granules in these cells are no doubt metaplasmic, and it is further very doubtful whether such a granular or " pseudo-alveolar " structure can be logically distinguished from an alveolar (cf. Wilson, '99). In the pancreas-cell granular and fibrillar structures alternate with the varying phases of secretory activity (cf. Mathews, '99).
between " protoplasmic " and " metaplasmic " substances, while a real and necessary one, becomes after all one of degree. I believe that we are probably justified in regarding the continuous substance as the most constant and active element, and that which forms the fundamental basis of the system, transforming itself into granules, drops, fibrillae, or networks in accordance with varying physiological needs. 1
Thus stated, the question as to the relative activity of the various elements becomes a real and important one. It now seems probable that the substance of the meshwork (fibrillar or interalveolar structure) is most active in the processes of cell-division, in contractile organs such as cilia and muscle-fibres, and in nerve-cells; but the groundsubstance, while apparently the most frequent seat of metaplasmic deposits, is certainly also the seat of active chemical changes. This subject has, however, not yet been sufficiently investigated.
C. The Nucleus
A fragment of a cell deprived of its nucleus may live for a considerable time and manifest the power of coordinated movement without perceptible impairment. Such a mass of protoplasm is, however, devoid of the powers of assimilation, growth, and repair, and sooner or later dies. In other words, those functions that involve destructive metabolism may continue for a time in the absence of the nucleus ; those that involve constructive metabolism cease with its removal. There is, therefore, strong reason to believe that the nucleus plays an essential part in the constructive metabolism of the cell, and through this is especially concerned with the formative processes involved in growth and development. For these and many other reasons, to be discussed hereafter, the nucleus is generally regarded as a controlling
i Wilson, '99. Cf. Sachs ('92, '95), Kollikcr ('97), Meyer ('96), and Kupffer ('96) on energies. Sachs sharply distinguishes between the energid (nucleus and protoplasm), which forms a living unit, and the passive energid-//W«*7j, placing in the former the nucleus, nucleolus, general cytoplasm, centrosome and plastids (chloroplasts and leucoplasts), and in the latter the starch -grains, aleurone-crystals, and membrane. Meyer carries the analysis further, classifying the active energid-elements into protoplasmatic and alloplasmatic organs, the former (nucleus cytoplasm, chromatophores, and perhaps the centrosomes) arising only by division, the latter (cilia, and according to Kollikcr, also the muscle- and nerve- hbrilbe) formed by differentiation from the protoplasmatic elements. The passive energid-products (ergastic structures or " formed material " of Beale) are formed as enclosures (starch-grains, etc.), or excretions (membranes). These general views are accepted by Kolliker; but none of these writers has undertaken to show how " alloplasmatic " structures are to be distinguished from metaplasmic or ergastic. I believe Sachs' view to be in principle not only true but of high utility. Practically, however, it involves us in considerable difficulty, unless the terminology adopted above — itself directly suggested by and nearly agreeing with the usage of Sachs and Kolliker — be employed.
THE NUCLEUS 3 1
centre of cell-activity, and hence a primary factor in growth, development, and the transmission of specific qualities from cell to cell, and so from one generation to another.
1. General Structure
The cell-nucleus passes through two widely different phases, one of which is characteristic of cells in their ordinary or vegetative condition, while the other only occurs during the complicated changes involved in cell-division. In the first phase, falsely characterized as the "resting state," the nucleus usually appears as a rounded sac-like body surrounded by a distinct membrane and containing a conspicuous irregular network (Figs. 6, 7, 13), which is in some cases plainly visible in the living cell (Fig. 9). The form of the nucleus, though subject to variation, is on the whole singularly constant, and as a rule shows no very definite relation to that of the cell-body, though in elongated cells such as muscle-cells, in certain forms of parenchyma, and in epithelial cells (Fig. 49), the nucleus is itself often elongated. Typically spherical, it may, in certain cases, assume an irregular or amoeboid form, may break up into a group of more or less completely separated lobes (polymorphic nuclei, Fig. 49), sometimes forming an irregular ring (" ring-nuclei " of leucocytes, giant-cells, etc., Fig. 14, D). It is usually very large in gland-cells and others that show a very active metabolism, and in such cases its surface is sometimes increased by the formation of complex branches ramifying through the cell (Fig. 14, E).
These forms seem in general to be fairly constant in a given species of cell, but in a large number of cases the nucleus has been seen in the living cell (cartilage-cells, leucocytes, ova) to undergo more or less active changes of form, sometimes so marked as to merit the name of amoeboid (Fig. 77). Perhaps the most remarkable deviations from the usual type of nucleus occur among the unicellular forms. In the ciliate Infusoria the nuclei are massive bodies of two kinds, viz. a large macronucieus and one or more smaller micronuc/ci, both of which are present in the same cell, the former kind being generally regarded as the active nucleus, the latter as a reserve nucleus from which at certain periods new macronuclei arise (p. 224). The macronuclei show a remarkable diversity of form and structure in different species. Still more interesting are the so-called scattered or distributed nuclei, described by Biitschli in flagellates and Bacteria, by Gruber in certain rhizopods and Infusoria, and by several authors in the Cyanophyceae (Figs. 15, 16). The nuclear material is here apparently scattered through the cell in the form of numerous minute, deeply stained granules, which, if this identification is correct, represent the most primi
32 GENERAL SKETCH OF THE CELL
tive known types of nucleus ; but this subject is still sub judice (p. 39). A transition from this condition to nuclei of the ordinary type appears to be given in the nuclei of certain flagellates (e.g. Chilomonas and Trachelmonas), where the chromatin-granules are aggregated about a nucleolus- like body, but are not enclosed by a membrane. 1 In considering the structure of the nucleus, as seen in sections, we must, as in the case of the cytoplasm, bear in mind the possibility, or rather probability, that some of the elements described may be coagulation - products ; for the nucleus is in life composed of liquid or semi-liquid substance, and Albrecht ('99) has recently shown that nuclei isolated in the fresh condition will flow together to form a single body. Most of the main features of the nucleus, both in the resting and in the dividing phases, have, however, been seen in life (Fig. 9), and the principal danger of mistaking artifacts for normal structures relates to the finer elements, considered beyond.
In the ordinary forms of nuclei n their resting state the following structural elements may as a rule be distinguished (Figs. 6, 7, 10):Fig. 13. -Two n«c!« from ihe crypis of a The nuclear membrane, a
. {Heidenhain.]
well-defined delicate wall which
(tasifhromatm) ,s accurately shown: Theupptr gives the nucleus a sharp contour nucleus coniains three piasmosomes or irue an( j differentiates it clearly from
nucleoli; ihe lower, one. A few fine lmin-lhreads ,, ,. , r~ .
[oxychromatw) arc seen in the upper nucleus tne Surrounding Cytoplasm. ThlS running off from the chro matin-masses. The wall sometimes Stains but Very
s,ance! PflC ™ *" ° eCUpied * *" ground - S1 "* slightly, and can scarcely be differentiated from the outlying cytoplasm. In other and perhaps more frequent cases, it approaches in staining capacity the chromatin.
b. The nuclear reticulum. This, the most essential part of the nucleus, forms an irregular branching network or reticulum which consists of two very different constituents. The first of these, forming the general protoplasmic basis of the nucleus, is a substance known as linin ■QUkini, '98, 1.
THE NUCLEUS 33
(Schwarz), invisible until after treatment by reagents, which in sections shows a finely granular structure and stains like the cytoplasmic substance, to which it is nearly related chemically (Figs. 7, 49). The second constituent, a deeply staining substance known as chromatin (Flemming), is the nuclear substance par excellence, for in many cases it appears to be the only element of the nucleus that is directly handed on by division from cell to cell, and it seems to have the power to produce all the other elements. The chromatin often appears in the form of scattered granules and masses of differing size and form, which are embedded in and supported by the linin-substance (Figs. 7, 19). In some cases the entire chromatin-content of the nucleus appears to be condensed into a single mass which simulates a nucleolus ; for example, in Spirogyra and in various flagellates and rhizopods (e.g. Actinosph(Brinm> Arcel/a) ; or there may be several such chromatin-masses, as in some of the Foraminifera and in Noctiluca. More commonly the chromatin forms a more or less regular network intermingled with and more or less embedded in the linin, from which it is often hardly distinguishable until the approach of mitosis, when a condensation of the chromatin-substance occurs.
In contradistinction to the other nuclear elements, chromatin is not acted upon, or is but slowly affected, by peptic digestion. It may thus be easily isolated for chemical analysis, which shows it to consist mainly of nuclcin> i.e. a compound in varying proportions of a complex phosphorus-containing acid known as mtcleinic acid, with albuminous bodies such as histon, protamin, or in some cases albumin itself. 1 Upon this, as will be shown in Chapter VI., probably depends the pronounced staining capacity when treated with the so-called " nuclear stains " {e.g. hematoxylin, methyl-green, and the basic tar-colours generally) from which chromatin takes its name. This capacity always increases as the nucleus prepares for division, reaching a climax in the spireme- and chromosome-stages, and it is also very marked in condensed nuclei such as those of spermatozoa. These variations are almost certainly due to varying proportions in the constituents of the nuclein, the staining capacity standing in direct ratio to the amount of nucleinic acid. •
c. The nucleoli, one or more larger rounded or irregular bodies, suspended in the network, and staining intensely with many dyes. In some nuclei they are entirely absent. When present the nucleoli vary in number from one to five or more ; and the number is often inconstant in the same species of cell, and even varies in the same cell with varying physiological conditions. In the eggs of some animals, at certain periods of growth {e.g. lower vertebrates), the nucleus may contain hundreds of nucleoli. An interesting case is
1 See p. 334.
34
GENERAL SKETCH OF THE CELL
that of the subcutaneous gland-cells of Piscio/a, the nuclei of which contain in early phases of secretion but a single nucleolus. During growth of the cell the nucleolus fragments, finally giving rise to several hundred nucleoli which then appear to migrate out into the cytoplasm, leaving but a single nucleolus to repeat the cycle. 1
The bodies known as nucleoli are of at least two different kinds. The first of these, the so-called true nucleoli or plasmoso?nes (Figs. 6, 8, B y 13), are of spherical form, and are shown by the staining reactions to differ widely from chromatin, being in general sharply stained by dyes which, like eosin, orange or acid fuchsin, stain the linin and the general cytoplasm. The plasmosomes sometimes seem to have no envelope, but in many cases {e.g. in leucocytes) are surrounded by a thin layer that stains like chromatin. Nucleoli of a quite different type are the "net-knots" (Netzknoten), chromatinnucleoli, or katyosomes, which agree in staining reaction with chromatin and are doubtless to be regarded as merely a portion of the chroma tin-network (Figs. 8, 49). These are sometimes spherical, more often irregular (Fig. 8), and often are hardly to be distinguished, except in size, from nodes of the chromatin-reticulum. 2 The relations between these two forms of nucleoli are far from certain, and the variations in staining reaction shown by true nucleoli render it not improbable that intermediate forms exist which may represent an actual transition from one to the other. 3 In many of the Protozoa, as described beyond, the " nucleolus " is shown by its behaviour during mitosis to be comparable with an attraction-sphere or centrosome ("nucleolo-centrosome," Keuten); and even in higher forms there are some cells in which the centrosome is intranuclear
(Fig. 148).
There is good reason to believe that the chromatin-nucleoli are merely more condensed portions of the chromatin-network, since during cell-division they have the same history as the remaining portion of the chromatin-substance. 4 The nature of the true nucleoli is still imperfectly known. By some observers, including Flemming, O. and R. Hertwig, and Carnoy, they have been regarded as storehouses of material (para-nuclein, plastin) which contributes to the
1 Montgomery, '98, 2.
2 Flemming first called attention to the chemical difference between the true nucleoli and the chromatic reticulum ('82, pp. 138, 163) in animal-cells, and Zacharias soon afterward studied more closely the difference of staining reaction in plant-cells, showing that the former are especially coloured by alkaline carmine solutions, the latter by acid solutions. Other studies by Carnoy, Zacharias, Ogata, Rosen, Schwarz, Heidenhain, and many others, show that the medullary substance (pyrenin) of true nuclei is coloured by acid tar-colours and other plasma stains, while the chromatin has a special affinity for basic dyes. Cf. p. 337.
8 For very full review of the literature of the nucleoli sec Montgomery ( '98, 2).
Cf. p. 67.
THE NUCLEUS
35
formation of chromosomes during division, and hence may play an active' role in the nuclear activity. Strasburger ( '95) likewise believes them to contain a store of active material which, however, has no direct relation to the chromosomes but consists of " kinoplasm "
Fig. 14. — Special forms of nuclei
A. Permanent spireme-nucleus. salivary gland of Ck&onomm larva. Chromatin In a single thread, composed o( chromatin-discs (chromomeres). terminating ai cacti end in a Irue nucleolus or plasmosoine. [BaLBIaNI.]
B. Permanent spireme-nuclei, intestinal epithelium of dipterous larva Ptychopltra. [VAN Gehuchtek] C. The same, side view.
D. Polymorphic ring-nucleus, giant-cell of bone-marrow of the rabbit; c. a group of centrosomes or centrioles. [HeiDeNkaiN.]
E. Branching nucleus, spinning gland of butterfly-larva (Pierii). [KorScHelt.]
(p. 322), from which arises the achromatic part of the divisionfigure (p. 82). On the other hand, Hacker ( '95, '99) and other observers regard the nucleolar material as a passive by-product of the chromatin-activity destined to be absorbed by the active substances. This is supported by the fact that in some forms of mitosis the nucleolus is at the time of division actually cast out of the nucleus into the cytoplasm, where it degenerates without further apparent function. This seems to constitute decisive evidence in support of Hackers view as applied to certain cases; but without further evidence it must remain doubtful whether it applies to all. 1
d. The ground-substance \ nuclear sap, or karyolymph> a clear substance occupying the interspaces of the network and left unstained by most of the dyes that colour the chromatin, the linin, or the plasmosomes. By most observers the ground-substance is regarded as a liquid filling a more or less completely continuous space traversed by the nuclear network. By Biitschli, however, and some of his followers the nucleus is regarded as an alveolar structure, the walls of which represent the "network," while the ground-substance corresponds to the alveolar material. Nearly related with this is the view of Reinkc ( '94) that the ground-substance consists of large pale granules of " lanthanin " or " oedematin."
The configuration of the chromatic network varies greatly in different cases. It is sometimes of a very loose and open character, as in many epithelial cells (Fig. 1); sometimes extremely coarse and irregular, as in leucocytes (Fig. 49); sometimes so compact as to appear nearly or quite homogeneous, as in the nuclei of spermatozoa and in many Protozoa. In some cases the chromatin does not form a network, but appears in the form of a thread closely similar to the spireme-stage of dividing nuclei (cf. p. 65). The most striking case of this kind occurs in the salivary glands of dipterous Xaxvaz^Chironomus\ where, as described by Balbiani, the chromatin has the form of a single convoluted thread, composed of transverse discs and terminating at each end in a large nucleolus ( Fig. 14, A). Somewhat similar nuclei (Fig. 14, B) occur in various epithelial cells of other insects (Van Gehuchten, Gilson), and also in the young ovarian eggs of certain animals (cf. p. 273). In certain gland-cells of the marine isopod Anilocra it is arranged in regular rosettes (Vom Rath). Rabl, followed by Van Gehuchten, Heidenhain, and others, has endeavoured to show that the nuclear network shows a distinct polarity, the nucleus having a " pole " toward which the principal chromatinthreads converge, and near which the centrosome lies. 2 In many nuclei, however, no trace of such polarity can be discerned.
The network may undergo great changes both in physical configuration and in staining capacity at different periods in the life of the same cell, and the actual amount of chromatin fluctuates, sometimes to an enormous extent. Embryonic cells are in general
1 ty PP« 126-130. 2 Cf. the polarity of the cell, p. 55.
THE NUCLEUS
37
characterized by the large size of the nucleus; and Zacharias has shown in the case of plants that the nuclei of meristem and other embryonic tissues are not only relatively large, but contain a larger percentage of chromatin than in later stages. The relation of these changes to the physiological activity of the nucleus is still imperfectly understood. 1
2. Finer Structure of the Nucleus
A considerable number of observers have raised the question whether the nuclear structures may not be regarded as aggregates of more elementary morphological bodies, though there is still no general agreement regarding their nature and relationships. The most definite evidence in this direction relates to the chromatic network. In the stages preparatory to division this network resolves itself into a definite number of rod-shaped bodies known as chromosomes (Fig. 21), which split lengthwise as the cell divides. These bodies arise as aggregations of minute rounded bodies or microsomes to which various names have been glvcn(chromomeres, Fol ; ids, Weismann). They are as a rule most clearly visible and most regularly arranged during celldivision, when the chromatin is arranged in a thread {spireme), or in separate chromosomes (Figs. 8, D, 53, B); but in many cases they are distinctly visible in the reticulum of the "resting" nucleus (Fig. 54). It is, however, an open question whether the chromatin-granules of the reticulum are individually identical with those forming the chromosomes or the spireme-thread. The larger masses of the reticu
1 Both chromatin -granules cells (Fig. 9}. Favourable oh glands of caterpillars, where
nd nucleoli have been seen in a considerable number of living
cts for this purpose are according to Korachelt ('96) the silkie whole nucleus may be seen to be filled with fine granules i are scattered many larger granules (" macrosomes "). The later studies of Meves ('97, 1) make it probable that the latter are true nucleoli and the former chromatin-granules. Korschelt, however, regards the "macrosomes" as composed of chromatin and the *' microsomes " as representing the so-called " achromatic substance."
38 GENERAL SKETCH OF THE CELL
lum undoubtedly represent aggregations of such granules, but whether the latter completely fuse or remain always distinct is unknown. Even the chromosomes at certain stages appear perfectly homogeneous, and the same is sometimes true of the entire nucleus, as in the spermatozoon. It is nevertheless possible that the chromatin-granules have a persistent identity and are to be regarded as morphological units of which the chromatin is built up. 1
Heidenhain ('93, '94), whose views have been accepted by Reinke, Waldeyer, and others, has shown that the " achromatic " nuclear network is likewise composed of granules, which he distinguishes as lanthanin- or oxyc/iromatin-granules from the dasic/iromatin-gr&nules of the chromatic network. Like the latter, the oxychromatin-granules are suspended in a non-staining clear substance, for which he reserves the term linin. Both forms of granules occur in the chromatic network, while the achromatic network contains only oxychromatin. They are sharply differentiated by dyes, the basichromatin being coloured by the basic tar-colours (methyl-green, saffranin, etc.) and other true " nuclear stains " ; while the oxychromatin-granules, like many cytoplasmic structures, and like the substance of true nucleoli (pyrenin), are coloured by acid tar-colours (rubin, eosin, etc.) and other "plasma stains." This distinction, as will appear in Chapter VI I., is possibly one of great physiological significance.
Still other forms of granules have been distinguished in the nucleus by Reinke ('94) and Schloter ('94). Of these the most important are the " oedematin-granules," which according to the first of these authors form the principal mass of the ground-substance or " nuclear sap " of Hertwig and other authors. These granules are identified by both observers with the " cyanophilous granules," which Altmann regarded as the essential elements of the nucleus. It is at present impossible to give a consistent interpretation of the morphological value and physiological relations of these various forms of granules. The most that can be said is that the basichromatin-granules are probably normal structures; that they play a principal rdle in the life of the nucleus ; that the oxychromatin-granules are nearly related to them ; and that not improbably the one form may be transformed into the other in the manner suggested in Chapter VII.
The nuclear membrane is not yet thoroughly understood, and much discussion has been devoted to the question of its origin and structure. The most probable view is that long since advocated by Klein ('78) and Van Beneden ('83) that the membrane arises as a condensation of the general protoplasmic substance, and is part of the same structure as the linin-network and the cytoplasmic meshwork. Like these, it is in some cases "achromatic," but in other cases
1 Cf. Chantrr VI.
THE NUCLEUS
39
tt shows the same staining reactions as chromatin, or may be double, consisting of an outer achromatic and an inner chromatic layer. According to Reinke, it consists of oxychromatin granules like those of the linin-network.
Interesting questions are raised by a comparison of these facts with the conditions observed in some of the lowest organisms, such as the flagellates and lower rhizopods among animals and the
Fig. IS. — Forms of Cyanophycese, Bacteria, and Flagellates showing the so-called scattered or distributed nuclei. [A-C. BUtschli ; D-F. Schkwiakoff; G-J. Calkins.]
A. Oicillaria. B. Orematium, C, Bacttrium lintola. D. Achromatium. E. The same in division. F. Fission of the granules. G. Tttramitm, with central sphere and scattered granules. H. Aggregation of the granules. I. Division of the sphere. J. Fission of the cell.
Cyanophyceae and Bacteria among plants. In many of these forms (Fig. 16) no distinct nucleus can be demonstrated, the cell consisting of a mass of protoplasm in which are scattered numerous deeply staining granules. Many of these granules stain intensely with hematoxylin and other "nuclear" dyes; like chromatin, they resist the action of peptic digestion, and in at least one case (the bacteriumlike Aehromathim, according to Schewiakoff, '93) they have the power of division like the chromatin-granules of higher forms. For these
40 GENERAL SKETCH OF THE CELL
reasons most observers (Biitschli, Gruber, Schewiakoff, Nadson, etc.) regard them as true chromatin-granules which represent a scattered or distributed nucleus not differentiated as a definite morphological body. If this identification is correct, such forms probably give us the most primitive condition of the nuclear substance, which only in higher forms is collected into a distinct mass enclosed by a membrane; and the scattered granules are comparable to those forming the chromatin-reticulum and chromosomes in the higher types. The identification is, however, difficult, owing to the impossibility of actual chemical analysis; and Fischer ('97) has shown in the case of the Bacteria and Cyanophyceae that we cannot safely trust either the staining reactions or the digestion test, since the former are variable, while the latter does not differentiate the granules from some other cytoplasmic constituents. 1 It is, however, certain that the staining power of chromatin in the higher forms varies with different conditions, and furthermore there is reason to believe that these granules may divide by fission. Besides these observations of Schewiakoff on Achromatinm (see above), we have those of several authors on Infusoria, and more recently those of Calkins on flagellates, both pointing to the same conclusion. Balbiani, Gruber, Maupas, and others have described various Infusoria (Urostyla, Trac/telocerca, Holosticha, Uroleptus\ as well as some rhizopods (Pelomyxa), in which the body contains very numerous minute chromatin-granules of " nuclei " (Fig. 15), which Gruber ('87) showed to multiply by division. Balbiani ('6i) long since showed that in Urostyla these bodies become concentrated toward the centre of the cell at the time of division, and Bergh ('89) demonstrated that they then fuse to form a macronucleus of the usual type, that elongates, assumes a fibrillar structure, and divides by fission. After division of the cell-body the macronucleus again fragments into minute scattered granules, which in this case certainly represent a distributed nucleus. In the flagellate Tctramitus Calkins ('98, 1) likewise finds numerous scattered chromatin-granules, which at the time of division become aggregated into a single dividing mass (p. 92); while in other forms the mass (nucleus) persists as such without (Trachelomonas y Lagenella, Clrilomonas) or with (Euglena, Synura) a surrounding membrane.
Taken together, the foregoing facts, while certainly not conclusive, give good ground for the provisional acceptance of Biitschli's conception of the distributed nucleus, and indicate that nucleus and cytoplasm have arisen through the differentiation of a common protoplasmic mass. The nucleus, as Carnoy has well said, 2 is like a
1 It should be remembered that we have no unerring " chromatin-stain." Cf. p. 335.
2 '84, P- 251.
THE CYTOPLASM 4 1
house built to contain the chromatic elements, and its achromatic elements (linin, etc.) were originally a part of the general cell-substance. Moreover, as Carnoy points out, the house periodically goes to pieces in the process of mitotic division, the chromatin afterward " building for itself a new dwelling."
3. Chemistry of the Nucleus
The chemical nature of the various nuclear elements will be considered in Chapter VII., and a brief statement will here suffice. The following classification of the nuclear substances, proposed by Schwarz in 1887, has been widely accepted, though open to criticism on various grounds.
1. Chromatin. The chromatic substance (basichromatin) of the network and of
those nucleoli known as net-knots or karyosomes.
2. Linin. The achromatic network and the spindle fibres arising from it.
3. Paralinin. The ground-substance.
4. Pyrenin or Parachromatin. The inner mass of true nucleoli.
5. Amphipyrenin. The substance of the nuclear membrane.
Chromatin is probably identical with nuclein (p. 332), which is a compound of nucleinic acid (a complex organic acid, rich in phosphorus) and albuminous substances. In certain cases (nuclei of spermatozoa, and probably also the chromosomes at the time of mitosis) the percentage of nucleinic acid is very large (p. 333). The linin is supposed to be composed of "plastin" — a substance identified by Reinke and Rodewald ('8i) and probably a nucleo-albumin or a related substance. " Pyrenin " is related to plastin ; and Carnoy and Zacharias apply the latter word to. the nucleolar substance, while O. Hertwig calls it paranuclein. "Amphipyrenin" has no very definite meaning ; for the nuclear membrane sometimes appears to be of the same nature as the linin, while in other cases it stains like chromatin. For critique of the staining reactions see page 334.
D. The Cytoplasm
It has long been recognized that in the unicellular forms the cytoplasmic substance is often differentiated into two well-marked zones : viz. an inner medullary substance or endoplasm in which the nucleus lies, and an outer cortical substance or exoplastn (ectoplasm) from which the more differentiated products of the cytoplasm, such as cilia, trichocysts, and membrane, take their origin. Indications of a similar differentiation are often shown in the tissue-cells of higher plants and animals, 1 though it may take the form of a polar differentiation of the cell-substance, or may be wholly wanting. Whether the distinction is of fundamental importance remains to be seen ; but it appears to be a general rule that the nucleus is surrounded by protoplasm of relatively slight differentiation, while the more highly differentiated products of cell-activity are laid down in the more peripheral region of the cell, either in the cortical zone or at one end of the cell. 1 This fact is full of meaning, not only because it is an expression of the adaptation of the cell to its external environment, but also because of its bearing on the problems of nutrition. 2 For if, as we shall see reason to conclude in Chapter VI I., the nucleus be immediately concerned with synthetic metabolism, we should expect to find the immediate and less differentiated products of its action in its neighbourhood, and on the whole the facts bear out this view.
1 This fact was first pointed out in the tissue-cells of animals by Kupffer ('75), and its importance has since been urged by Waldeyer, Reinke, and others. The cortical layer is by Kupffer termed paraplasm, by Pfeffer hyaloplasm, by Pringsheim the Hautschicht. The medullary zone is termed by Kupffer protoplasm, sensu strictu; by Strasburger, Kdrnerplasma; by WigtXi, polioplasm.
The most pressing of all questions regarding the cytoplasmic structure is whether the sponge-like, fibrillar, or alveolar appearance is a normal condition existing during life. There are many cases, especially among plant-cells, in which the most careful examination has thus far failed to reveal the presence of a reticulum, the cytoplasm appearing, even under the highest powers and after the most careful treatment, merely as a finely granular substance. This and the additional fact that the cytoplasm may show active streaming and flowing movements, has led some authors, especially among botanists, to regard the reticulum as non-essential and as being, when present, either a secondary differentiation of the cytoplasmic sub• stance specially developed for the performance of particular functions or a mere coagulation-product due to the action of fixatives. It has been shown that structureless proteids, such as egg-albumin and other substances, when coagulated by various reagents, often show a structure closely similar to that of protoplasm as observed in microscopical sections. Flemming ('82) long since called attention to the danger of mistaking such coagulation-products for normal structures as seen in fixed and stained material, and his warning has been emphasized by the later experiments of Berthold ('86% Schwarz ( J Sy), and especially of Biitschli ('92, '98), Fischer ('94, '95, '99), and Hardy (*99). Butschli's extensive studies of such coagulation-phenomena show that coagulated or dried albumin, starch-solutions, gelatin, gum arabic, and other substances show a fine alveolar structure scarcely to be distinguished from that which he believes to be the normal and typical structure of protoplasm. Fischer and Hardy have likewise made extensive tests of solutions of albumin, peptone, and related substances, in various degrees of concentration, fixed and stained by a great variety of the reagents ordinarily used for the demonstration of cell-structures. The result was to produce a marvellously close simulacrum of the appearances observed in the cell, alveolar, reticulated, and fibrillar structures being produced that often contain granules closely similar in every respect to those described as
1 c f- P- 55* 2 Sce Kupffer O90), pp. 473-476.
" microsomes " in sections of actual protoplasm. After impregnating pith with peptone-solution and then hardening, sectioning, and staining, the cells may even contain a central nucleus-like mass suspended in a network of anastomosing threads that extend in every direction outward to the walls, and give a remarkable likeness of a normal cell. These facts show how cautious we must be in judging the appearances seen in preserved cells, and justify in some measure the hesita
Fig. 14. - Ciliated cells, showing cytoplasmic microsomes to which the cilia are attached. [Enc
A. From intestinal epithelium of Anodonta. B. From gill of Aiwdonta, CD. I (helium or Cycltu.
tion with which many existing accounts of cell-structure are received. The evidence is nevertheless overwhelmingly strong, as I believe, that not only the fibrillar and alveolar formations, but also the microsomes observed in cell-structures, are in part normal structures. This evidence is derived partly from a study of the living cell, partly from the regular and characteristic arrangement of the thread-work and microsomes in certain cases. In many Protozoa, for example, a fine alveolar structure may be seen in the living protoplasm ; and Flemming as well as many later observers has clearly seen fibrillar structures in the living cells of cartilage, epithelium connective-tissue, and some other animal cells (Fig. g). Mikosch, also, has recently described granular threads in living plant-cells.
Almost equally conclusive is the beautifully regular arrangement of the nbrillse in ciliated cells (Fig. 17, Engelmann), in muscle-fibres and nerve-fibres, and especially in the mitotic figure of dividing cells
Fig. iS. — Cells of the pancreas in Amphibia. [Mathkws.]
A-C. fitclums ; D. /faaii. A and B represent two siages of the "loaded" cell, showing ymogen-granul<:5 in the peripheral and fibrillar structures in the basal part of the cell. C shows ells after discharge of the granule- material and invasion of the eniirc cell by fibrillce. In D por
(Figs. 21, 31), where they are likewise more or less clearly visible in life. A very convincing case is afforded by the pancreas-cells of Necturns, which Mathews has carefully studied in my laboratory. Here the thread-work consists of long, conspicuous, definite fibrillar, some of which may under certain conditions be wound up more or less closely in a spiral mass to form the so-called Nebenktm. In all these cases it is impossible to regard the thread-work as an accidental coagulation-product. In the case of echinoderm eggs, I have made ('99) a critical comparison of the living structure, as seen under powers of a thousand diameters and upwards, with the same object stained in thin sections after fixation by picro-acetic, sublimate-acetic, and
The centre of the cell is occupied by a large vacuole plasm forms a very regular and distinct reticulum with sc large in the peripheral lone. The larger pale bodies, lyi granules (ij. metaplasm). The nucleus, at the right, is brane, is traversed by a very disiincl linin-network, i granules, and a single targe nucleolus within which is a showing nucleoli and chro matin-granules suspended in tl
ium by Arnold Graf from
ed with a watery liquid. The cytored microsomes which become very i the ground-substance, are excretory rounded by a thick chromatic mem
Other reagents. The comparison leaves no doubt that the normal structures are in this case very perfectly preserved, though the sections give at first sight an appearance somewhat different from that of the living object, owing to differences of staining capacity. In these eggs the microsomes, thickly scattered through the alveolar walls, stain deeply (Figs, u, 12), while the alveolar spheres hardly stain at all. When, therefore, the stained sections are cleared in balsam, the contours of the alveolar spheres almost disappear, and the eye is caught by the walls, which give at first sight quite the appearance of a granular reticulum, as it has been in fact described by many observers. Careful study of the sections shows, however, that the form and arrangement of all the elements is almost identically the same as in life.
This result shows that careful treatment by reagents in some cases at least gives a very faithful picture of the normal structure ; and while it should never be forgotten that in sections we are viewing coagulated material, much of which is liquid or semi-liquid in life, we should not adopt too pessimistic a view of the results based on fixed material, as I think some of the experimenters referred to above have done. Wherever possible, the structures observed in sections should be compared with those in the living material. When this is impracticable we must rely on indirect evidence ; but this is in many cases hardly less convincing than the direct.
It is a very interesting and important question whether living protoplasm that appears to the eye to be homogeneous does not really possess a structure that is invisible, owing to the extreme tenuity of the fibrillse or alveolar walls (as was long since suggested by Heitzmann and Butschli), 1 or to uniformity of refractive index in the structural elements. It is highly probable that such is often the case ; indeed, Butschli has shown that such " homogeneous " protoplasm in Protozoa may show a typical alveolar structure after fixation and staining. This explanation will not, however, apply to the young echinoderm eggs (already referred to at p. 28), where the genesis of the alveolar structure may be followed step by step in the living cell. The protoplasm here appears at first almost like glass, showing at most a sparse and fine granulation ; but after fixing and staining it appears as a mass of fine, closely crowded granules. This may indicate the existence of an extremely fine alveolar structure in life ; but on the whole I believe that these granules are for the most part coagulation-products, since they cannot be demonstrated by staining intra vitam, and they very closely resemble the coagulation-granules found in structureless proteids like egg-albumin after treatment by the same reagents. In common with many other investigators, therefore, I believe that protoplasm may in fact be homogeneous dozen to the present limits of microscopical vision.
1 Cf. Butschli, '92, 2, p. 169.
Flj. ao. — Spinal ganglion-cell of the frog. [LENHOSSgK.] The nucleus contains a single intensely chromatic nucleolus, and a paler linin-network with rounded cliro matin -granules. The cytoplasmic fibrillse are faintly shown passing out into the nerve-process below. (They are figured as far more distinct by Flemming.) The dark cytoplasmic masses are the deeply staining "chromophilic granules" (Nissl) of unknown function. (The centrosome, which lies near the centre of the cell, is shown in Fig. 8, C.) AI the left, two
One of the must beautiful forms of cyto-reticulum with which I am acquainted has been described by Bolsius and Graf in the nephridial cells of leeches as shown in Fig. 19 (from a preparation by Dr. Arnold Graf). The meshwork is here of great distinctness and regularity, and scattered microsomes are found along its threads. It appears with equal clearness, though in a somewhat different form, in many eggs, where the meshes are rounded and often contain foodmatters or deutoplasm in the inter-spaces (Figs. 59,60). In cartilagecells and connective tissue-cells, where the threads can be plainly seen in life, the network is loose and open, and appears to consist of more or less completely separate threads (Fig. 9). In the cells of columnar epithelium, the threads in the peripheral part of the cell often assume a more or less parallel course, passing outwards from the central region, and giving the outer zone of the cell a striated appearance. This is very conspicuously shown in ciliated epithelium, the fibrillae corresponding in number with the cilia as if continuous with their bases (Fig. 17). 1 In nerve-fibres the threads form closely set parallel fibrillae which may be traced into the body of the nerve-cell ; here, according to most authors, they break up into a network in which are suspended numerous deeply staining masses, the " chromophilic granules'* of Nissl (Fig. 20). 2 In the contractile tissues the threads are in most cases very conspicuous and have a parallel course. This is clearly shown in smooth muscle-fibres and also, as Ballowitz has shown, in the tails of spermatozoa. This arrangement is most striking in striped muscle-fibres where the fibrillae are extremely well marked. According to Retzius, Carnoy, Van Gehuchten, and others, the meshes have here a rectangular form, the principal fibrillae having a longitudinal course and being connected at regular intervals by transverse threads ; but the structure of the muscle-fibre is probably far more complicated than this account would lead one to suppose, and opinion is still divided as to whether the contractile substance is represented by the reticulum proper or by the ground-substance.
Nowhere, perhaps, is a fibrillar structure shown with such beauty as in dividing cells, where (Figs. 21, 31) the fibrillae group themselves in two radiating systems or asters, which are in some manner the immediate agents of cell-division. Similar radiating systems of fibres occur in amoeboid cells, such as leucocytes (Fig. 49) and pigmentcells (Fig. 50), where they probably form a contractile system by means of which the movements of the cell are performed.
The views of Btitschli and his followers, which have been touched on at p. 25, differ considerably from the foregoing, the fibrillae being regarded as the optical sections of thin plates or lamellae which form the walls of closed chambers filled by a more liquid substance. Butschli, followed by Reinke, Eismond, Erlanger, and others, interprets in the same sense the astral systems of dividing cells which are regarded as a radial configuration of the lamellae about a central point (Fig. 10, £). Strong evidence against this view is, I believe,
1 The structure of the ciliated cell, as described by Engelmann, may be beautifully demonstrated in the funnel-cells of the nephridia and sperm-ducts of the earthworm.
2 The remarkable researches of Apathy ('97) on the nerve-cells of leeches have revealed the existence within the nerve-cell of networks far more complex and definite than was formerly supposed, and showing definite relations to incoming and outgoing fibrillae within the substance of the nerve-fibres.
afforded by the appearance of the spindle and asters in cross-section. In the early stages of the egg of Nereis, for example, the astral rays are coarse anastomosing fibres that stain intensely and are therefore very favourable for observation (Fig. 60). That they are actual fibres is, I think, proved by sagittal sections of the asters in which the rays are cut at various angles. The cut ends of the branching rays appear in the clearest manner, not as plates but as distinct dots, from which in oblique sections the ray may be traced inwards toward the centrosphere. Driiner, too, figures the spindle in cross-section as consisting of rounded dots, like the end of a bundle of wires, though these are connected by cross-branches (Fig. 28, F). Again, the crossing of
the rays proceeding from the asters (Fig. 128), and their behaviour in certain phases of cell-division, is difficult to explain under any other than the fibrillar theory.
We must admit, however, that the meshwork varies greatly in different cells and even in different physiological phases of the same cell ; and that it is impossible at present to bring it under any rule of universal application. It is possible, nay probable, that in one and the same cell a portion of the meshwork may form a true alveolar structure such as is described by Biitschli, while other portions may, at the same time, be differentiated into actual fibres. If this be true the fibrillar or alveolar structure is a matter of secondary moment, and the essential features of protoplasmic organization must be sought in a more subtle underlying structure. 1
' See Chapter VI.
Space would not suffice for a comparative account of the endless modifications shown by the cytoplasmic substance in different forms of cells. Many of these arise through special differentiations of the active substance, the character of the structure thus being sometimes so highly modified, as in the striated muscle-fibre, that it is difficult to trace its exact relation to the more usual forms. More commonly the cytoplasm is modified through the formation of passive or metaplasmic substances which often completely transform the original appearance of the cell. The most frequent of such modifications arise through the deposit of liquid drops and " granules " (many of the latter, however, being no doubt liquid in life). When the liquid drops are of watery nature the cavities in which they lie are known as vacuoles, which are especially characteristic of the protoplasm of plant-cells and of Protozoa. These may enlarge or run together to form extensive cavities in the cell, the protoplasm becoming reduced to a peripheral layer, or to strands and networks traversing the spaces ; while in some forms of unicellular glands the spaces may form branching canals traversing the protoplasm.
The vacuolization or meshlike appearance arising through the formation of larger vacuoles or the deposit of other metaplasmic material is not to be confounded with the primary protoplasmic structure. When, however, smaller vacuoles or metaplasmic granules are evenly distributed through the protoplasm, a " pseudo-alveolar " structure (Reinke) arises that can often hardly be distinguished from the "true" alveolar structure of Butschli. 1 Comparative study shows that all gradations exist between the " false " and the " true " alveolar structures and that no logical ground of distinction between the two exists. 2 We thus reach ground for the conclusion that the coarser secondary alveolar or reticular formations are to be regarded as only an exaggeration of the primary structure, and that the alveolar material of Butschli's structure belongs in the same general category with the passive or metaplasmic substance. 8
E. The Centrosome
The centrosome 4 is usually an extremely minute body, or more commonly a pair of bodies, staining intensely with haematoxylin and
1 In the latter the alveolar spheres are, according to Butschli, not more than one or two microns in diameter.
This has been demonstrated in the cells of plants by Craio ('96), and more recently by the writer ('99)1 in the case of echinoderm and other eggs.
• Cf. p. 29.
4 The centrosome was apparently first seen and described by Flemming in 1875, * n the egg of the fresh-water mussel Anodonta, and independently discovered by Van Beneden, in some other reagents, and surrounded by a cytoplasmic radiating aster or by a rounded mass known as the attraction-sphere (Figs. 8, 49, etc.). As a rule it lies in the cytoplasm, not far from the nucleus, and usually opposite an indentation or bay in the latter ; but in a few cases it lies inside the nucleus (Fig. 148). In epithelia the centrosomes (usually double) lie as a rule near the free end of the cell
(Fig. 23). 1
There is still much confusion regarding the relation of the centrosome to the surrounding structures, and this has involved a corresponding ambiguity in the terminology. We will therefore only consider it briefly at this point, deferring a more critical account to Chapter VI. In its simplest form it is a single minute granule, which may, however, become double or triple (leucocytes, connective tissuecells, some epithelial cells) or even multiple, as in certain giant-cells (Fig. 14, D\ and as also occurs in some forms of cell-division (Fig. 52). In some cases (Figs. 8, C f 120, 148) the " centrosome " is a larger body containing one or more central granules or " centrioles " (Boveri); but it is probable that in some of these cases the central granule is itself the true centrosome, and the surrounding body is part of the attraction-sphere. During the formation of the spermatozoon the centrosome undergoes some remarkable morphological changes (p. 171), and is closely involved in the formation of the contractile structures of the tail.
The nature and functions of the centrosome have formed the subject of some of the most persistent and searching investigations of recent cytology. Van Beneden, followed by Boveri and many later workers, regarded the centrosome as a distinct and persistent cellorgan, which like the nucleus was handed on by division from one cell-generation to another. Physiologically it was regarded as being the especial organ of cell-division, and in this sense as the "dynamic centre " of the cell. In Boveri' s beautiful development of this the following year, in dycyemids. The name is due to Boveri ('88, 2, p. 68). Van Beneden's and Boveri's independent identification of centrosome in A scan's as a permanent cell-organ ('87) was quickly supported by numerous observations on other animals and on plants. In rapid succession the centrosome and attraction-sphere were found to be present in pigment-cells of fishes (Solger, '89, '90), in the spermatocytes of Amphibia (Hermann, '90), in the leucocytes, endothelial cells, connective tissue-cells, and lung-epithelium of salamanders (Hemming, '91), in various plant-cells (Guignard, '91), in the one-celled diatoms (Butschli, 9i), in the giant-cells and other cells of bone-marrow (Heidenhain, Van Bambeke, Van der Stricht, '91), in the flagellate Noctiluca (Ishikawa, '91), in the cells of marine algae (Strasburger, '92), in cartilage-cells (Van der Stricht, '92), in cells of cancerous growths (epithelioma, Lustig and Galeotti, '92), in the young germ-cells as already described, in gland-cells (Vom Rath, '95), in nerve-cells (Lenhossek, '95), in smooth muscle-fibres (Lenhossek, '99), and in embryonic cells of many kinds (Heidenhain, '97). Many others have confirmed and extended this list. Guignard's identification of the centrosomes in higher plants is open to grave doubt {cf p. 82). l Cf. p. 57.
view it was regarded further as the especial fertilizing element in the spermatozoon, which, when introduced into the egg, endowed the latter with the power of division and development. Van Beneden's and Bovcri's hypothesis, highly attractive on account of its simplicity and lucidity, is supported by many facts, and undoubtedly contains an element of truth ; yet recent researches have cast grave doubt upon its generality, and necessitate a suspension of judgment upon the entire matter. Many of the most competent recent workers on the cytology of higher plants have been unable to find centrosomes, whether in the rcsting-cclls, in the apparatus of cell-division, or during the process of fertilization, notwithstanding the fact that undoubted centrosomes occur in some of the lower plants. Among zoologists, too, an increasing number of recent investigators, armed with the best technique, have maintained the total disappearance of the centrosome at the close of cell-division or during the process of fertilization, agreeing that in such cases the centrosome is subsequently formed de novo. Experimental researches, also, have given strong ground for the conclusion that cells placed under abnormal chemical conditions may form new centrosomes (p. 306). If these strojigly supported results be well founded, Van Beneden's hypothesis must be abandoned in favour of the view that the centrosome is but a subordinate part of the general apparatus of mitosis, and one which may be entirely dispensed with. Thus regarded, the centrosome would lose somewhat of the significance first attributed to it, though still remaining a highly interesting object for further research. 1
F. Other Organs
The cell-substance is often differentiated into other more or less definite structures, sometimes of a transitory character, sometimes showing a constancy and morphological persistency comparable with that of the nucleus and centrosome. From a general point of view the most interesting of these arc the bodies known zsplastids or pro toplasts{¥\g. 6), which, like the nucleus and centrosome, are capable of growth and division, and may thus be handed on from cell to cell. The most important of these arc the chromatophores or chromoplastids, which are especially characteristic of plants, though they occur in some animals as well. These are definite bodies, varying greatly in form and size, which possess the power of growth and division, and have in some cases been traced back to minute colourless plastids or leucoplastids in the embryonic cells. By enlargement and differentiation these give rise to the starch-builders (amyloplastids), to the chlorophyll-bodies (chloroplastids), and to other pigment-bodies (chromoplastids), all of which may retain the power of division. The embryonic leucoplastids are also believed to multiply by division and to arise by the division of plastids in the parental organism ; but it remains an open question whether this is their only mode of origin, and the same is true of the more highly differentiated forms of plastids to which they may give rise.
1 fy PP- m » 3°4* Ei»cn ('97) asserts that in the blood of a salamander, Batrachoseps* the attraction-sphere ("archosome ") containing the centrosomes may separate from the remainder of the cell (nucleated red corpuscles) to form an independent form of bloodcorpuscle or " plasmocyte," which leads an active life in the blood.
The contractile or pulsating vacuoles that occur in most Protozoa and in the swarm-spores of many Algae are also known in some cases to multiply by division ; and the same is true, according to the researches of De Vries, Went, and others, of the non-pulsating vacuoles of plant-cells. These vacuoles have been shown to have, in many cases, distinct walls, and they are regarded by De Vries as a special form of plastid (" tonoplasts ") analogous to the chromatophores and other plastids. It is, however, probable that this view is only applicable to certain forms of vacuoles.
The plastids possess in some cases a high degree of morphological independence, and may even live for a time after removal from the remaining cell-substance, as in the case of the " yellow cells " of Radiolaria. This has led to the view, advocated by Brandt and others, that the chlorophyll-bodies found in the cells of many Protozoa and a few Metazoa {Hydra, Spongilla, some planarians) are in reality distinct Algae living symbiotically in the cell. This view is probably correct in some cases, e.g. in the Radiolaria ; but it may be doubted whether it is of general application. In the plants the plastids are almost certainly to be regarded as differentiations of the protoplasmic substance.
The existence of cell-organs which have the power of independent assimilation, growth, and division is a fact of great theoretical interest in its bearing on the general problem of cell-organization ; for it is one of the main reasons that have led De Vries, Wiesner, and many others to regard the entire cell as made up of elementary self-propagating units.
G. The Cell-membrane
The structure and origin of the cell-wall or membrane form a subject somewhat apart from our general purpose, since the wall belongs to the passive or metaplasmic products of protoplasm rather than to the living cell itself. We shall therefore treat it very briefly. Broadly speaking, animal cells are in general characterized by the slight development and relative unimportance of the cell-walls, while the reverse is the case in plants, where the cell-walls play a very important rdle. In the latter the wall sometimes attains a great thickness, usually displays a distinct stratification, and often has a complex sculpture. Such massive walls very rarely occur in the case of animal tissues, though the intercellular matrix of cartilage and bone is to a certain extent analogous to them, and the thick and often highly sculptured envelopes of some kinds of eggs and of various Protozoa may be placed in the same category.
It is open to question whether any cells are entirely devoid of an enclosing envelope; for even in such "naked" cells as leucocytes, rhizopods, or membraneless eggs, the boundary of the cell is usually formed by a more resistant layer of protoplasm or " pellicle " (Butschli) which may be so marked as to simulate a true membrane, as is the case, for example, in the red blood-corpuscles (Ranvier, Waldeyer, etc.). Such pellicles probably differ from true membranes only in degree ; but it is still an open question both in animals and in plants, how far true membranes arise by direct transformation of the peripheral protoplasmic layer (the " Hautschicht" of botanists), and how far as a secretion-product of the protoplasm. In the case of animal cells, Leydig long since proposed l to distinguish between " cuticular " membranes, formed as secretions and usually occurring only on the free surfaces (as in epithelia), from " true membranes " arising by direct transformation of the peripheral protoplasm. Later researches, including those of Leydig himself, have thrown so much doubt on this distinction that most later writers have used the term cuticular in a purely topographical sense to denote membranes formed only on one (the free) side of the cell, 2 leaving open the question of origin. The formation and growth of the cell-wall have been far more thoroughly studied in plants than in animals, yet even here opinion is still divided. Most recent researches tend to sustain the early view of Nageli that the cell-wall is in general a secretion-product, though there are some cases in which a direct transformation of protoplasm into membrane-stuff seems to occur. 8 In the division of plant-cells the daughter-cells are in almost all cases cut apart by a cell-plate which arises in the protoplasm of the mother-cell as a transverse series of thickenings of the spindle-fibres in the equatorial region (Fig. 34). This fact, long regarded by Strasburger and others as a proof of the direct origin of the membrane from the protoplasmic substance, is shown by Strasburger's latest work ('98) to be open to a quite different interpretation, the actual wall being formed by a splitting of the cell-plate into two layers between which the wall appears as a secretion-product. Almost all observers further are agreed that the formation of new membranes on naked masses of protoplasm produced by plasmolysis are likewise secretion-products, and that the secondary thickening of plant-membranes is produced in the same way. These facts, together with the scanty available zoological data, indicate that the formation of membranes by secretion is the more usual and typical process. 1
1 Cf. '85, p. 12. 2 Cf. O. Hertwig, '93. 8 Cf. Strasburger, '98.
The chemical composition of the membrane or intercellular substance varies extremely. In plants the membrane consists of a basis of cellulose, a carbohydrate having the formula C 6 H 10 O 6 ; but this substance is very frequently impregnated with other substances, such as silica, lignin, and a great variety of others. In animals the intercellular substances show a still greater diversity. Many of them are nitrogenous bodies, such as keratin, chitin, elastin, gelatin, and the like ; but inorganic deposits, such as silica and carbonate of lime, are common.
H. Polarity of the Cell
In a large number of cases the cell exhibits a definite polarity, its parts being symmetrically grouped with reference to an ideal organic axis passing from pole to pole. No definite criterion for the identification of the cell-axis has, however, yet been determined ; for the general conception of cell-polarity has been developed in two different directions, one of which starts from purely morphological considerations, the other from physiological, and a parallelism between them has not thus far been fully made out.
On the one hand, Van Beneden ('83) conceived cell-polarity as a primary morphological attribute of the cell, the organic axis being identified as a line drawn through the centre of the nucleus and the centrosome (Fig. 22, A). With this view Rabl's theory ('85) of nuclear polarity harmonizes, for the chromosome-loops converge toward the centrosome, and the nuclear axis coincides with the cellaxis. Moreover, it identifies the polarity of the egg, which is so important a factor in development, with that of the tissue-cells ; for the egg-centrosome almost invariably appears at or near one pole of the ovum.
Heidenhain ('94, '95) has recently developed this conception of polarity in a very elaborate manner, maintaining that all the structures of the cell have a definite relation to the primary axis, and that this relation is determined by conditions of tension in the astral rays focussed at the centrosome. On this basis he endeavours to explain the position and movements of the nucleus, the succession of divisionplanes, and many related phenomena. 1
1 Strasburger ('97, 3, '98) believes membrane-formation in general to be especially connected with the activity of the " kinoplasm," or filar plasm of which he considers the " Hautschicht," as well as the spindle-fibres, to be largely composed. In support of this may be mentioned, besides the mode of formation of the partition-walls in the division of plantcells, Harper's ('97) very interesting observations on the formation of the ascospores in Erysiphe (Fig. 33), where the spore-membrane appears to arise directly from the astral rays.
Hatschek ('88) and Rabl ('89, '92), on the other hand, have advanced a quite different hypothesis based on physiological considerations. By "cell-polarity" these authors mean, not a predetermined morphological arrangement of parts in the cell, but a polar differentiation of the cell-substance arising secondarily through adaptation of the cell to its environment in the tissues, and having no necessary relation to the polarity of Van Beneden (Fig. 22, B, C). This is
Fig. 32. — Diagram A. Morphologic! Chromatin -threads Halscliek, ffinagl.
l)-polanly, lassing through nucleus and a B.C. Physiological polarity of Rabl a
typically shown in epithelium, which, as Kolliker and Haeckel long since pointed out, is to be regarded, both ontogenetically and phylogenetically, as the most primitive form of tissue. The frcd and basal ends of the cells here differ widely in relation to the foodsupply, and show a corresponding structural differentiation. In such cells the nucleus usually lies nearer the basal end, toward the source of food, while the differentiated products of cell-activity are formed cither at the free end (cuticular structures, cilia, pigment, zymogeyigranules), or at the basal end (muscle- fibres, nerve-fibres). In tr(e non-epithelial tissues the polarity may be lost, though traces of \\ arc often shown as a survival of the epithelial arrangement of the, embryonic stages.
' Cf. p. 105.
But, although this conception of polarity has an entirely different point of departure from Van Beneden's, it leads, in some cases at least, to the same result; for the cell-axis, as thus determined, may coincide with the morphological axis as determined by the position of the centrosome. This is the case, for example, with both the spermatozoon and the ovum ; for the morphological axis in both is also the physiological axis about which the cytoplasmic differentiations are grouped. Recent researches have further shown that the same is the case in many forms of epithelia, where the centrosomes He in the outer end of the cell, often very near the surface. 1 (Fig. 23)
D
F
in epithelial and oiher cells. [A, D, ZIMme and CoHN; fr\ HEIDENHAIK.]
n; E, Heidenhain
3 of men ; dead cell al [he led. II. Uterine epiih t-cell, with cenlrosomc in Ihe middle. D, Cornea new blasl-so mi les, embryo duck. F. Red blood-co es are double in nearly all cases.
epithelium or monkey
and the recent observations of Henneguy ('98) and Lenhossek ('98,1) give reason to believe that the "basal bodies" to which the cilia of ciliated epithelium are attached may be the centrosomes. 1 These facts are of very high significance; for the position of the centrosome, and hence the direction of the axis, is here obviously related to the cell-environment, and it is difficult td. avoid the conclusion that the latter must be the determining condition to which the intracellular relations conform. When applied to the germ-cells, this conclusion becomes of high interest; for the polarity of the egg is one of the
1 Zimmermann, '98; Henlenhain and Cohn, '97.
" Cf. p. 356.
primary conditions of development, and we have here, as I believe, a clue to its determination. 1
r C I. The Cell in Relation to the Multicellular Body
In analyzing the structure and functions of the individual cell we are accustomed, as a matter of convenience, to regard it as an independent elementary organism or organic unit. Actually, however, it is such an organism only in the case of the unicellular plants and animals and the germ-cells of the multicellular forms. When we consider the tissue-cells of the latter, we must take a somewhat different view. As far as structure and origin are concerned the tissuecell is unquestionably of the same morphological value as the one-celled plant or animal ; and in this sense the multicellular body is equivalent to a colony or aggregate of one-celled forms. Physiologically, however, the tissue-cell can only in a limited sense be regarded as an independent unit ; for its autonomy is merged in a greater or less degree into the general life of the organism. From this point of view the tissue-cell must in fact be treated as merely a localized area of activity, provided it is true with the complete apparatus of cell-life, and even capable of independent action within certain limits, yet nevertheless a part and not a whole.
There is at present no biological question of greater moment than the means by which the individual cell-activities are coordinated, and the organic unity of the body maintained; for upon this question hangs not only the problem of the transmission of acquired characters, and the nature of development, but our conception of life itself. Schwann, the father of the cell-theory, very clearly perceived this ; and after an admirably lucid discussion of the facts known to him ('39), drew the conclusion that the life of the organism is essentially a composite ; that each cell has its independent life ; and that " the whole organism subsists only by means of the reciprocal action of the single elementary parts. 2 This conclusion, afterward elaborated by Virchow and Haeckel to the theory of the "cell-state," took a very strong hold on the minds of biological investigators, and is even now widely accepted. It is, however, becoming more and more clearjy apparent that this conception expresses only a part of the truth, and that Schwann went too far in denying the influence of the totality of the organism upon the local activities of the cells. It would of course be absurd to maintain that the whole can consist of more than the sum of its parts. Yet, as far as growth and development are concerned, it has now been clearly demonstrated that only in a limited sense can the cells be regarded as cooperating units. They are rather local centres of a formative power pervading the growing mass as a whole, 1 and the physiological autonomy of the individual cell falls into the background. It is true that the cells may acquire a high degree of physiological independence in the later stages of embryological development. The facts to be discussed in the eighth and ninth chapters will, however, show strong reason for the conclusion that this is a secondary result of development, through which the cells become, as it were, emancipated in a greater or less degree from the general control. Broadly viewed, therefore, the life of the multicellular organism is to be conceived as a whole ; and the apparently composite character which it may exhibit is owing to a secondary distribution of its energies among local centres of action. 2
1 fy PP- 384» 424. We should remember that the germ-cells are themselves epithelial products.
2 Untcrsuchungen, Trans., p. 181.
In this light the structural relations of tissue-cells become a question of great interest ; for we have here to seek the means by which the individual cell comes into relation with the totality of the organism, and by which the general equilibrium of the body is maintained. It must be confessed that the results of microscopical research have not thus far given a very certain answer to this question. Though the tissue-cells are often apparently separated from one another by a non-living intercellular substance, which may appear in the form of solid walls, it is by no means certain that their organic continuity is thus actually severed. Many cases are known in which division of the nucleus is not followed by division of the cell-body, so that multinuclear cells or syncytia are thus formed, consisting of a continuous mass of protoplasm through which the nuclei are scattered. Heitzmann long since contended ( '73), though on insufficient evidence, that division is incomplete in nearly all forms of tissue, and that even when cell-walls are formed they are traversed by strands of protoplasm by means of which the cell-bodies remain in organic continuity. The whole body was thus conceived by him as a syncytium, the cells being no more than nodal points in a general reticulum, and the body forming a continuous protoplasmic mass.
This interesting view, long received with scepticism, has been to a considerable extent sustained by later researches, and though it still remains subjudice, has been definitely accepted in its entirety by some recent workers. The existence of protoplasmic cell-bridges between the sieve-tubes of plants has long been known ; and Tangl's discovery, in 1879, °f similar connections between the endosperm-cells was followed by the demonstration by Gardiner, Kienitz-Gerloff, A. Meyer, and many others, that in nearly all plant-tissues the cell-walls
Cf. Chapters VIII., IX. 8 For a fuller discussion see pp. 388 and 413.
are traversed by delicate intercellular bridges. Similar bridges have been conclusively demonstrated by Ranvier, Bizzozero, Retzius, Flemming, Pfitzner, and many later observers in nearly all forms of epithelium (Fig. i ) ; and they are asserted to occur in the smooth muscle-fibres, in cartilage-cells and connective tissue-cells, and in some nervecells. Dendy ( '88), Paladino ( '90), and Retzius ( '89) have endeavoured to show, further, that the follicle-cells of the ovary are connected by protoplasmic bridges not only with one another, but also with the ovum ; and similar protoplasmic bridges between germ-cells and somatic cells have been also* demonstrated in a number of plants, e.g. by Goroschankin ( '83) and Ikeno ( '98) in the cycads and by A. Meyer ( '96) in Volvox. On the strength of these observations some recent writers have not hesitated to accept the probability of Heitzmann's original conception, A. Meyer, for example, expressing the opinion that both the plant and the animal individual are continuous masses of protoplasm, in which the cytoplasmic substance forms a morphological unit, whether in the form of a single cell, a multinucleated cell, or a system of cells. 1 Captivating as this hypothesis is, its full acceptance at present would certainly be premature ; and as far as adult animal tissues are concerned, it still remains undetermined how far the cells are in direct protoplasmic continuity. It is obvious that no such continuity exists in the case of the corpuscles of blood and lymph»and the wandering leucocytes and pigment-cells. In case of the nervous system, which from an a priori point of view would seem to be above all others that in which protoplasmic continuity is to be expected, its occurrence and significance are still a subject of debate. When, however, we turn to the embryonic stages we find strong reason for the belief that a material continuity between cells here exists. This is certainly the case in the early stages of many arthropods, where the whole embryo is at first an unmistakable syncytium ; and Adam Sedgwick has endeavoured to show that in Peripatus and even in the vertebrates the entire embryonic body, up to a late stage, is a continuous syncytium. I have pointed out ( '93) that even in a total cleavage, such as that of Amphioxus or the echinoderms, the results of experiment on the early stages of cleavage are difficult to explain, save under the assumption that there must be a structural continuity from cell to cell that is broken by mechanical displacement of the blastomeres. This conclusion is supported by the recent work of Hammar ( '96, '97), whose observations on sea-urchin eggs I can in the main confirm.
1 '96, p. 212. Cf. also the views of Hanstein, Strasburger, Russow, and others there cited. 2 Cf also E. A. Andrews, '98, I, '98, 2.
Among the most interesting observations in this direction are those of Mrs. Andrews C97), 2 who asserts that during the cleavage
of the echinoderm-egg the blastomeres " spin " delicate protoplasmic filaments, by which direct protoplasmic continuity is established between them subsequent to each division. These observations, if correct, are of high importance ; for if protoplasmic connections may be broken and re-formed at will, as it were, the adverse evidence of the blood-corpuscles and wandering cells loses much of its weight. Meyer ('96) adduces evidence that in Volvox the cell-bridges are formed anew after division ; and Flemming has also shown that when leucocytes creep about among epithelial cells they rupture the protoplasmic bridges, which are then- formed anew behind them. 1
We are still almost wholly ignorant of the precise physiological meaning of the cell-bridges ; but the facts indicate that they are not merely channels of nutrition, as some authors have maintained, but paths of subtler physiological impulse. Beside the facts determined by the isolation of blastomeres, referred to above, may be placed Townsend's recent remarkable experiments on plants, described at P a g e 346. If correct, these experiments give clear evidence of the transference of physiological influences from cell to cell by means of protoplasmic bridges, showing that the nucleus of one cell may thus control the membrane-forming activity in an enucleated fragment of another cell. The field of research opened up by these and related researches seems one of the most promising in view; but until it has been more fully explored, judgment should be reserved regarding the whole question of the occurrence, origin, and physiological meaning of the protoplasmic cell-bridges.
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Alex. Bruce. London, 1896. Van Beneden, E. — (See Lists II., IV.) De Vries, H. — Intracellular Pangenesis. Jena, 1889. Waldeyer, W. — Die neueren Ansichten liber den Bau und das Wesen der Zelle :
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Tlibingen, 1893. Id. — Die Morphologie und Physiologie des Pflanzlichen Zellkernes. Jena, 1896.
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