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Russell ES. The interpretation of development and heredity. (1930) Oxford. Univ. Press.
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XII The Cell and the Organism
Those organisms which, consisting of one or of many energids, show no differentiation into separate cells are as a rule comparatively small in size and simple in structure; large and complex organisms are invariably multicellular. It appears then that a cellular structure is a necessary condition for any great degree of growth and differentiation. The reasons for this are not far to seek.
In plants, the formation of resistent cell-walls is enormously important in imparting rigidity, and so enabling a great elaboration of external form and internal differentiation to take place. The supporting structures of the cellular plant, its exo- and endo-skeleton, are essentially formed by its cell-walls and their derivatives. The separation of protoplasmic cells by means of definite walls, though the cells may be connected by inter-cellular bridges, also permits of a certain segregation and diversity of metabolic processes, which are carried out, as it were, in separate little laboratories. Compared with the Siphoneate algae, which are the largest and most highly developed of non-cellular, polyenergid organisms, cellular plants differ mainly and primarily in this formation of definite cell- walls; the other differences are derivative from this primary one. The point is well put by Sachs , 1 who writes of the Siphoneae (which he calls Coeloblasteae) as follows:
‘. . . in the protoplasm of these plants a certain quantity of nuclear substance (especially the nuclein characteristic of it) is distributed in formed portions and at small intervals, and is especially aggregated in the growing points. From this fact, we obtain once more, as we have already obtained from other sides, a certain insight into the true significance of the cellular structure of plants. We need only imagine in a not too complex cellular plant (a higher Alga, a Moss, or even a vascular plant) that in the substance enveloped by the outer walls of the epidermis, the cell- walls are simply wanting; whereas the protoplasm, with the cell-nuclei distributed in it, behaves essentially as if these cell-walls were present. Thus we have, on the whole, the structure of a Coeloblast. On the contrary, we need only imagine the inner cavity of such a Coeloblast to be divided up by numerous transverse and longitudinal partition-walls into very numerous small chambers, each of which encloses one or several of the cell-nuclei present, and we should thus have an ordinary cellular plant. It is however very easily intelligible that not only the solidity, but also the shutting off of various products of metabolism, the conduction of the sap from place to place, and so forth, must attain greater perfection if the whole substance of a plant is divided up by numerous transverse and longitudinal walls into cell-chambers sharply separated off from one another’ (p. 109).
1 J. von Sachs, Lectures on the Physiology of Plants (Eng. Trans.), Oxford, 1887 .
Even for plants, however, this does not give the whole story; there are other advantages in a cellular structure besides those mentioned by Sachs. In particular, the matter of surface-area has to be considered. For the life of the energid a supply of oxygen and nutritive substances is essential, and these must be taken up byits surface. It would be impossible, for instance, for energids to exist in a mass of any thickness, for those in the middle would be asphyxiated and starved. For this reason, the energids of a Siphoneate alga form a thin layer, bounded externally by the common wall and internally by the common vacuolar system. In cellular plants, access to nutrient salts is arranged for by a system of water-transport, and to oxygen by a system of intercellular air-spaces, and most cells have their own private milieu interne in the form of an intracellular vacuole.
In animals, the general circulating body-fluid, blood or lymph, plays a greater role than in plants, as the source of the means of life, and all animal cells are bathed in greater or lesser degree by this internal medium. In animals, rigid cell-walls are of little importance, and the rationale of cellformation is to be sought rather in the necessity for exposing at least part of the surface of the energid to the internal medium, and still more in the advantages offered by separate cells for histological differentiation and the building up of elaborate organs.
The advantages of cellular structure in animals were briefly discussed by Rauber in his paper of 1883. 1 Following Leuckart, he pointed out that by the formation of separate cells the surface area was increased, thus facilitating the metabolic processes of the energids. Chemical and histological differentiation was also furthered, and cells could be arranged and built up into structures of great complexity — they had thus an architectonic value (pp. 333-4). Of more modern writers, O. Hertwig may be mentioned, who discusses the question on much the same lines as Sachs and Rauber. 2 It merits fuller treatment, especially from a biochemical standpoint, than can be accorded to it here. We may however safely draw the conclusion that a cellular structure is physiologically an advance upon a plasmodial or syncytial state, and a necessary pre-condition of any great elaboration of internal organization.
2. While adhering to the view that an energid organization is the most general and fundamental of all, and has its raison d'etre in metabolic necessities, we must recognize that the majority of organic types are composed of more or less discrete, more or less independent cellular units.
We may now profitably consider, purely on the basis of observed fact, just how far the tissue cells of multicellular organisms are separate and independent morphological units, just how far, that is to say, the cell theory is an accurate description of the intimate structure of these organisms. Are the cells of Metazoa and Metaphyta separate and distinct, so that the organism may be properly regarded as a cell-state or cell-colony, or are they so intimately connected with one another that the multicellular organism must be regarded as a syncytial whole, secondarily differentiated into energids and cells ?
Both views are held at the present day, and there is not a little conflict of evidence as to which is observationally correct. Though, as we shall see, the question is not of crucial or decisive importance in relation to the theoretical aspect of the celltheory, it will not be without interest, in connexion with the general problem of the relation of the cell to the organism, briefly to survey the more important evidence bearing upon thequestion. We shall find that neither view is wholly correct, but that each contains essential elements of the truth.
Morph . Jahrb . viii, 1883, pp. 233-338. Allgemeine Biologie , 6th and 7th edit., Jena, 1923, pp. 501-2,
3. In general, those who have looked for and found intercellular connexions have been impressed with the continuity of substance throughout the organism and have accepted the view of the organism as syncytial. Thus Heitzmann in 1873 held that even when distinct cell-walls were present they were traversed by strands of protoplasm, by means of which the protoplasts remained in protoplasmic continuity. He conceived the whole body as being a more or less continuous mass, with cells as nodal points in a general network of protoplasm. 1 Intercellular connexions were demonstrated to be widespread in plants by Tangl, Gardiner, KienitzGerloff, A. Meyer and others, and Sachs’s view 2 that the cellular plant is essentially a continuum interrupted by perforated cell- walls received much support.
In animals also, intercellular bridges were discovered in many different kinds of tissue, and Adam Sedgwick in particular laid stress upon cellular continuity in the early stages of development. In embryos of Peripatus capensis he found that for some time all the cells were connected up by protoplasmic bridges, and in developing Elasmobranchs he described the mesenchyme tissue as being a reticulum of pale nonstaining substance containing nuclei at its nodes. Even the endoderm and ectoderm he regarded as being simply specialized parts of the reticulum, in which the meshes are closer and the nuclei more numerous and arranged in layers. In general the nerve-crest and other epithelial tissues were to be thought of as centres of growth and of the multiplication of nuclei. 3
1 Wilson, 1925, pp. 103-4.
2 'Fundamentally, every plant, however highly organized, is a protoplasmic body coherent in itself, which, clothed without by a cell-wall and traversed internally by innumerable transverse and longitudinal walls, grows; and it appears that the more vigorously this formation of chambers and walls proceeds with the nutrition of the protoplasm, the higher also is the development attained by the total organization' (Lectures, p. 84).
3 A. Sedgwick, ‘On the Inadequacy of the Cellular Theory of Development,' Q.JM.S., xxxvii, 1895, pp. 87-101.
The existence of cell-bridges and connexions in several kinds of tissue — epithelial, connective, and muscular — is generally recognized, and it is also an agreed fact that in the development of many forms, particularly of Arthropods, the earliest stages of ‘segmentation’ may be syncytial.
Some writers have attempted to establish the general thesis that all the cells of the body are, or have been at some stage, connected with one another, and that accordingly the organism is essentially a plasmodial whole, secondarily differentiated into cells. In particular, Emil Rohde has championed this view in a long series of publications. He has summarized his observations and deductions in a recent comprehensive paper, 1 which we may consider in some detail, as typifying this particular interpretation of the organism and its cells.
He deals at great length, adducing a wealth of illustrative detail, with the ontogenetic and phylogenetic origin of cells, and with the facts of histology and histogenesis, both in animals and in plants.
With regard to the origin of tissues he concludes that all tissues originate and develop from multinucleate plasmodia, and for the most part retain permanently their plasmodial character. The tissue cells are not the direct descendants of the cells of the embryo, but secondary or tertiary formations which originate de novo in the primary multinucleate plasmodium. Histological differentiation, that is to say, the formation of fibrils and ground-substances, is not bound up with cells, but takes place in the primary multinucleate plasmodium, usually before the appearance of tissue cells. Tissue cells in many cases make their appearance as a consequence, not a cause of histological differentiation (p. 469). While accepting the proved fact that all nuclei arise from pre-existing nuclei, Rohde rejects the further generalization (which we owe to Virchow) that cells arise only by division of pre-existing cells. Omnis cellula e cellula he regards as a definitely incorrect statement of the facts. He considers that there is no necessary connexion between nuclear division and cell-division, and he gives as an illustration, among others, the process of cell-formation in the Siphonocladaceae , which are a group of Algae standing just above the Sipboneae and with them classified as Siphonales. These forms begin their development as multinucleate plasmodia, just like the Sipboneae ; as development proceeds, cellulose partitions are formed cutting off multinucleate sections of the plasmodium. These partitions increase in number, and the areas separated become reduced in size, till finally cells containing only two nuclei are formed. In the development of the higher fungi a similar progressive septation takes place, which results in the formation of typical uninucleate cells. Some of the higher forms of the Siphonocladaceae have a multinucleate apical ‘cell’, which cuts off from itself basal ‘cells’, which are likewise multinucleate.
1 Emil Rohde, 'Der plasmodiale Aufbau des Tier- und Pflanzenkorpers’, Zts.f. mss. Zotl. cxx, 19*3, pp. 3*5—535.
In the development of many animals and plants there is a stage in which nuclear division goes on without formation of cells; these arise later; in some cases, e.g. in the early development of Myriapods, the cells (yolk pyramids) are formed independently of the nuclei, which later move into them (see Rohde’s Fig. 16). In general, he regards cells as having arisen by differentiation from a plasmodium, and the same applies to all manner of fibrils and other specialized products of protoplasmic activity, which develop with no particular relation to cell-boundaries, in the ground-substance of the plasmodium. A neurone for example arises by the differentiation of a part of the plasmodium; of the nuclei contained in this portion one increases in size and becomes the large nucleus of the ganglion cell, the others remain small and form the nuclei of the neuroglia cells and the sheath cells, while at the same time the protoplasm round the enlarged nucleus becomes differentiated into neural substance. The ganglion cell is therefore in Rohde’s view a new formation, arising by differentiation from a plasmodial beginning and only gradually separating itself off from the adjacent cells which are simultaneously formed as subsidiary to it. In the same way the neurofibrillae, which later form the axon fibre, are differentiated in situ from a syncytial or plasmodial continuum.
It is unnecessary to refer to the great mass of evidence that Rohde has adduced in favour of his view that tissue cells originate, not primarily by cell-division, but by differentiation from a multinucleate continuum; enough has perhaps been said to indicate the kind of evidence brought forward. We note particularly that he regards the energid conception of the organism as the fundamental one, and cells as something secondary. Assimilation, growth, and differentiation can go on perfectly well in the absence of cells, as the Siphoneae clearly demonstrate to us, and cells are formed for physiological reasons, for the purpose of giving support and rigidity to the organism, and ensuring the better nutrition of its parts (pp. 523 and 525).
His general conclusions may be summed up as follows:
‘Every animal and plant individual represents a unitary protoplasmic mass, either uninucleate or multinucleate, from which there can separate off uninucleate or multinucleate portions as independent protoplasmic masses (the so-called free cells of the text-books : red and white blood corpuscles, mesenchyme cells, sex-cells, &c.). The multinucleate plasmodium, of which the animal body consists, contains two kinds of living substance: protoplasm and metaplasm. Cells in the ordinary sense are not morphologically equivalent structures, standing at the same level of individuality or organization, but quite heterogeneous and diverse formations, fundamentally different not only in development but in structure and potencies. Plasmodia and cells alternate with one another in the development of animals and plants.
‘Metazoa and Metaphyta originate phylogenetically not from a colony of cells, but from multinucleate plasmodia; in the ontogenetic and histogenetic development of cells in Metazoa and Metaphyta there is a repetition of the processes which took place in the phylogenetic origin of cells.
‘Cells are not of the fundamental importance in the animal and the plant body which the ruling cell-theory attributes to them, but are secondary phenomena of merely secondary importance* (p. 32 6).
There is from Rohde’s point of view no fundamental difference between ‘unicellular’ organisms and multicellular. The Protozoon, for example, is not to be regarded as homologous with the tissue cell of a Metazoon, but as the equivalent of the whole body of the latter. In both, ‘histological’ differentiation takes place essentially in the same way — in one case in a small, usually monoenergid unit, in the other in a larger, polyenergid plasmodium. The cortical layer of the Protozoon is from this point of view strictly comparable with the ectoderm of Metazoa, and arises in essentially the same way, by differentiation out of a protoplasmic continuum (PP; 489-9 1 ) The contrast between Rohde’s view of the cell and the organism and the classical cell-theory is sufficiently striking. Instead of the composite, colonial cell-state, made up of semi-independent, collaborating units — essentially an analytical view — we have the organism conceived as a unitary and unified whole, with nuclei scattered throughout its substance, and cells appearing, as physiological needs dictate, in the form of cytoplasmic differentiations, more or less independently of the nuclei, which show a uniform structure and little or no differentiation. Undoubtedly it is easier to think of the organism as a unitary whole on the plasmodial conception than on the cellular.
4. There are however many facts which are not easily reconcilable with the plasmodial conception. In the first place, separate free-living and free-moving cells are found in many animal organisms, and in the second place, in many animal tissues the cells show a considerable measure of functional independence, and the new tissue cells do arise by division of the existing cells. Modern work on tissue culture has also strengthened the conviction that the cells of Metazoa possess to a considerable degree functional and structural independence.
Many writers therefore, while admitting the evidence for protoplasmic connexions between cells, prefer to keep to the cell-theory of the organism, finding in it a powerful weapon of analysis. Thus Wilson writes apropos of the cell-theory: ‘Its value as a means of biological analysis needs no other demonstration than the immense advances that it made possible. Inevitably in practice we treat cells as distinct, though closely co-ordinated, elementary organisms or organic units; and although some writers have questioned the validity of this procedure it nevertheless remains an indispensable means of analysis . 1 (That word ‘inevitably’ should give us pause — it is generally an indication of an unconscious or unacknowledged -parti pris.) Wilson accordingly throughout his book proceeds ‘upon the assumption, if only as a practical method, that the multicellular organism in general is comparable to an assemblage of Protista which have undergone a high degree of integration and differentiation so as to constitute essentially a cell-state ’. 2 It will be of interest to consider in brief outline some of the more striking evidence in favour of the functional and structural independence of the tissue cell.
The development of micro-dissection methods by Barbour, Kite, and Chambers has furnished a new means of studying directly the living cell and its connexions with its neighbours. Chambers 3 draws the general conclusion from his extensive researches by these methods that intercellular bridges, while of common occurrence in plants, are comparatively rare in animals, what has been taken as such being really cement substance.
‘In the majority of the cell groups in the metazoan body’, he writes, ‘there is no evidence whatever for the existence of actual protoplasmic bridges between the individual cells. It is highly probable that protoplasm exists as a morphological and physiological unit in each cell of the body. Some of its functions may be more highly specialized in one group of cells than in another, and the secretions of one group of cells may profoundly affect another; but as regards the fundamental vital phenomena, each cell lives out its own existence’ (p. 243).
He points out that all cells, both ectoderm and endoderm, that are in contact with the environment external or internal, are covered on their exposed surfaces by a continuous, structureless membrane; the ciliated epithelium in the mouth of the frog, for example, can be torn off in strips which, if dissociated by means of the micro-dissection needle, are found to be held together mainly by a structureless cuticular border continuous over their originally free surface (p. 254). Tissue culture work has also thrown doubt upon the reality, or the physiological significance, of intercellular connexions. Thus the Lewises 1 conclude that the mesenchyme cells of the vertebrate embryo form not a true syncytium, but an adherent reticulum. ‘Even in places where mesenchyme cells are closely connected, no interchange of granules or mitochondria can be observed. These cells thus behave as individual units that are adherent to one another because their surfaces are sticky* (p. 393). In tissue cultures nearly every type of embryonic cell has the power of independent feeding, like a phagocyte, ingesting any particulate matter found in the culture (pp. 424-5).
1 1925, p. io*. 1 1925, p. 103.
R. Chambers, ‘The Physical Structure of Protoplasm as determined by Microdissection and Injection*, in Cowdry, General Cytology, Chicago, 1924.
To take one more example, G. Levi 2 found that :
‘In the tissues of the embryo chick syncytia characterized by a complete loss of the biological individuality of the composing cells do not occur. Even the tissues in which the connections between cell and cell are most intimate (myocardium, mesenchyme) show in cultures in vitro a protoplasmic activity leading to the migration of single cells into the clot. . . . Syncytia should therefore be considered as consisting of plasmatic masses or areas which are biologically delimited by the sphere of influence of their nuclei, and though intimately united they still possess, potentially, the capacity of separating from one another .’ 3
Migration of cells as independent units is of quite common occurrence in tissue cultures, and the origin of new cells by division is frequently observed. The single cells can also in many cases be seen to grow and differentiate as individuals. All these points were established by the earliest workers on tissue cultures (Harrison and Burrows) 4 and have been fully confirmed since.
1 W. H. Lewis and M. R. Lewis, ‘Behavior of Cells in Tissue Cultures’, in Cowdiy, General Cytology, Chicago, 1924.
Arch . exper. Zellforscb. i, 1925, pp. 1-57. 3 Abstract in Jourit. Roy. Micr. Soc March 1926.
4 R. G. Harrison, ‘The Outgrowth of the Nerve Fiber as a mode of Protoplasmic Movement’, Journ. Exper . Zool. ix, 1910, pp. 787-846; M. T. Burrows, ‘The Growth of Tissues of the Chick Embryo outside the Animal Body with special reference to the Nervous System’, ibid., x, 191 1, pp. 63-83.
Harrison’s pioneer experiments were carried out on little pieces of tissue isolated from frog embryos of 3-4 mm. in length. He noted that the cells of the medullary cord at this early stage of development, though appearing syncytial in sections, fall apart into quite distinct cells when dissected out fresh. The following quotations are taken from his own summary of results :
‘Pieces of undifferentiated embryonic tissue, when isolated under aseptic precautions in clotted lymph, will live for weeks and undergo at least the initial stages of normal histological differentiation; cells from the axial mesoderm give rise to striated muscle fibers; epidermal cells form a cuticular border; typical chromatophores and a mesenchyme-like tissue are formed from pieces containing portions of the neural tube and axial mesoderm ; the walls of the neural tube and the primordia of the cranial ganglia give rise to long hyaline filaments closely resembling embryonic nerve fibers’ (p. 841).
His most striking observations were those relating to the mode of formation of nerve fibres, which he found to originate as protoplasmic extensions of the medullary nerve cells which grew to a considerable length. Harrison regarded this outgrowth as a manifestation of protoplasmic movement, that is, as an active function of the individual nerve cell.
‘One characteristic that the embryonic cells have in common is the power of movement. They change their form or move from place to place in the clot by virtue of the amoeboid activity of their hyaline ectoplasm. ... In the case of cells from the medullary tube and the primordia of the cranial ganglia the activity is so localized and the ductility of the ectoplasm is such, that the movement results in the formation of long fibers, the primitive axones. The free end of each fiber is enlarged and provided with fine processes or pseudopodia. This part continues its progression and the fiber is gradually drawn out. . . . The longest fiber observed, and this was followed throughout its whole period of growth (53 hours), was 1*15 mm. long’ (p. 842).
‘The first nerves which form are composed of few fibers and have relatively short distances to grow before establishing connection with their end organs. The long paths found in the adult are largely the result of subsequent stretching or interstitial expansion, which takes place as the various parts grow or shift apart. The fibers which develop later follow, in the main, the paths laid down by the pioneers’ (p. 843).
Burrows confirmed Harrison’s results on other material, the chick embryo, and clearly established the fact that the growth of the mesenchyme tissue consisted in the wandering out of the pre-existing tissue cells and their multiplication by mitotic division.
It will be apparent that Harrison gives a quite different account of the origin and differentiation of ganglion cells and nerve fibres from that suggested by Rohde. The problem is a very difficult one, and the interpretation to be given to the observed facts about the development of nerve-fibres is still in dispute. Sedgwick, for example, in the paper of 1895 already referred to, took the line that both nerves and muscles were special developments of the primitive reticulum or plasmodium. With regard to nerves he wrote :
‘. . . the development of nerves is not an outgrowth of cell-processes from certain central cells, but is a differentiation of a substance which was already in position; and this differentiation seems to take place from the medullary walls outwards to the periphery, both in the anterior and posterior roots, and to precede, or to proceed pari passu with, the development of other tissues. The nerve crest is, then, to be regarded as a centre for the growth of nuclei, which spread into the body of the embryo and become concerned in the formation of many tissues, nervous tissues among the rest’ (ibid., p. 94).
There is always the possibility that what Harrison observed was not the original, normal formation of the axon fibre, but the regeneration of an intercellular connexion, which had been ruptured when the piece of medullary cord was removed from the embryo.
The results obtained by Graham Kerr 1 in his very careful study of the development of nerves in Lepidosiren, a peculiarly favourable subject, on account of the large size of its cells, are difficult to harmonize with Harrison’s view.
Graham Kerr concluded from his investigation of the early development of motor nerves in this form that ‘(i) The nerve-trunk is already present as a protoplasmic bridge at a period so early in development that spinal cord and myotome are still in contact with one another. (2) As the embryo grows and the myotome recedes from the spinal cord this protoplasmic bridge increases in length and becomes fibrillated. (3) As the nerve-trunk lengthens amoeboid masses of mesenchymatous protoplasm collect round it and gradually spread out over its length to form the protoplasmic sheath’ (P- ”0).
1 J. Graham Kerr, Text Book of Embryology , vol. ii, London, 1919, pp. 106-21,
Graham Kerr subjects to severe criticism the view originally suggested by His, and apparently confirmed by Harrison’s experiments, that nerve fibres are formed by active outgrowths from the central nerve cells. He points out that in Harrison’s preparations there may well have been present the original protoplasmic bridges, broken off from their connexion with the myotomes, and that it may have bedn these nerve-rudiments, appearing as processes of the nerve cells, that exhibited the observed active growth and prolongation.
In general, Graham Kerr agrees with Sedgwick as to the great importance of intercellular connexions, though he points out that these do not seem to exist between the segmentation spheres, and only appear as a secondary formation at a somewhat later stage. He stresses the importance of regarding the individual not as an aggregate of cells and organs, but as a mass of living substance imperfectly subdivided up into subordinate units: imperfectly because each cell and each organ is closely bound up with the activity of the whole organism (p. 487).
The results of tissue-culture work on the whole favour the ‘cell-state’ theory of the organism, in that they show the individual cell to possess a certain independence of movement and power of differentiation. There may be adduced also in favour of this view the very remarkable evidence brought forward by H. V. Wilson (1907, 1910), Morgan and Drew, and others, indicating that in certain lowly organized animals, such as Sponges and Hydroids, tissue cells artificially dissociated can come together and form a new organism. For an account of this phenomenon in Sponges we may follow the recent full description provided by Galtsoff. 1 For his experiments he used the sponges Microciona -prolifer a and Cliona cellata, and these he broke up into single cells and small aggregates by squeezing them through fine-meshed bolting silk. He found that larger aggregates were formed through the amoeboid activity of the archaeocytes (unspecialized, granular mesenchyme cells) which crept about irregularly and collected other cells by adhesion. The archaeocytes are very sticky and collect all the foreign bodies they meet: they will not however pick up cells of another species. The archaeocytes do not fuse into a plasmodium, but adhere in a common hyaloplasm, which may form a large pseudopodium by means of which the aggregate moves. The aggregates are fully formed after 24 hours, and form spheres about 120-50 n in diameter. Each of the aggregates if large enough develops into a sponge possessing a single osculum.
1 P. S. Galtsoff, ‘Regeneration after Dissociation (An experimental study on Sponges), I and II. Jourrt. Exper. Zool. xlii, 1925, pp. 183-222 and 223-56.
‘Large aggregates develop faster and live longer than the small ones. A complete regeneration occurs in aggregates consisting of about 2,000 cells or more. The aggregates composed of a small number of cells, from about 40 to 500, fail to develop into perfect sponges : their canals do not fuse into one system and the osculum is frequently wanting (p. 250). The re-formation of a new sponge is primarily brought about by the coming together of cells, followed by their further differentiation. Increase in the number of cells takes place after the sponge body is formed. The rebuilding of the new sponge is then due ‘to the activities and properties of individual cells forming an aggregate. The different types of cells forming a common mass find each other and then develop flagellated chambers, canals, skeleton, mesenchyme, and other tissues’.
A similar collaboration of dissociated cells, leading towards the formation of a new organism, was observed in Antennularia by Morgan and Drew, 1 but here the process of reformation was not so complete. The experimental procedure was the same as in the Sponge work, and as a result of dissociation isolated cells and small groups were obtained, which aggregated together to form compact masses. A perisarc was formed by these masses, and a definite layer of ectoderm surrounding a plasmodial mass of endoderm, in which coenosarcal tubules developed, similar to those in the normal hydroid. Hydranths were however not formed during the course of the experiment, which was continued for 60 days. There was no sign of any multiplication of cells by division.
1 W. de Morgan and G. H. Drew, ‘A Study of the Restitution Masses formed by Dissociated Cells of the Hydroids Antennularia ramosa and A. antennina\Journ. Mar, Biol, Assoc . x, 1 91 3-1 5, pp. 440-63.
These results, especially in Sponges, are certainly of great value and significance, and show that in such forms the tissue cells, even when differentiated, still retain a considerable measure of independence, enabling them to co-operate in a most remarkable and orderly way in building up, either completely or partially, a new organism; but it must be remembered that the Sponges, in which alone a complete regeneration of typical form was achieved, are a group of peculiar organization, resembling much more closely a colonial aggregation of cells than do the higher Metazoa, and that they do normally form restitution bodies in the shape of Gemmules.
5. We have now seen in broad outline what kind of evidence is adduced in favour of their view by the upholders of the plasmodial and the cellular theory of the organism respectively. We may properly draw the conclusions — (1) that the evidence is to some extent conflicting, (2) that neither view expresses the whole truth, and (3) that both contain part of the truth. Plasmodia and independent cells do certainly both exist, and it is probable that in the early stages of histogenesis a plasmodial or syncytial type of structure is more widely spread than the cell-theory would have us suppose. Certain it is that the dictum Omnis cellula e cellula is of less wide application than the aphorism Omnis nucleus e nucleo, which appears, unlike the other, to be strictly true and to admit no exception.
But the question as to whether or not cells are distinct morphological entities is in no way crucial or fundamental for our interpretation of the relation between the organism and its cells. We have considered the question too exclusively from the morphological point of view and we must now look at it from the standpoint of physiology. As D’Arcy Thompson writes :
‘Discussed almost wholly from the concrete, or morphological point of view, the question has for the most part been made to turn on whether actual protoplasmic continuity can be demonstrated between one cell and another, whether the organism be an actual reticulum or syncytium. But from the dynamical point of view the question is much simpler. We then deal not with material continuity, not with little bridges of connecting protoplasm, but with a continuity of forces, a comprehensive field of force, which runs through and through the entire organism and is by no means restricted in its passage to a protoplasmic continuum. And such a continuous field of force, somehow shaping the whole organism, independently of the number, magnitude and form of the individual cells, which enter, like a froth, into its fabric, seems to me certainly and obviously to exist.’ 1
Physiologically, the organism is a unity and acts as such. Whether it is composed of more or less discrete cells, or is essentially continuous in substance, makes no difference to this unity of action. Intercellular bridges are not the only means at disposal to ensure co-ordination of action. The nervous system is obviously the main channel and organ through which the general integration of activity is achieved and maintained; the milieu interne, which bathes all the living parts of the body and serves as the vehicle for hormones and other dissolved substances, is also of great importance as a means of integration.
Integrative or ‘whole’ action means that the activities of the parts are subordinated to the activity of the whole. Cell-activities are carried out essentially ‘for the good oP the whole organism, and cells and energids are therefore subordinate parts, or organs in the most general sense. Their activities are not to be fully understood save in relation to the general activity of the organism. They are as a rule specialized for particular functions, in accordance with the principle of division of labour, and they are incapable of long-continued existence apart from the particular conditions which they find in the organism of which they form part. It is true that cells in tissue-cultures may live and multiply for years, but this is only because the conditions in which they normally live are artificially reproduced and steadily maintained over long periods of time. Certain freemoving cells of the Metazoan body, for instance the leucocytes of oysters, can also exist for considerable periods apart from the body, but not indefinitely.
1 On Growth and Form y p. 200.
The tissue cell, then, is always physiologically a -part, and never in normal circumstances a whole.
6. We have seen reason to conclude that the main object of the wellnigh universal nucleate organization of living things is to maintain a spatially close association between nuclear and cytoplasmic substance, and we have seen that this intimate relation is necessary for constructive metabolism and growth. This interpretation of the facts we have called the energid conception of the organism. The formation of discrete and differentiated cells is something superadded to the primary energid organization, and it appears to be a necessary pre-condition for any great elaboration of structure and any great increase in size. But the fact that histological differentiation affects primarily the cytoplasm, while the nucleus remains as a rule uniform in structure, suggests that the nuclei are there, scattered throughout the substance of the organism, primarily for their original metabolic purpose, that of rendering possible organic synthesis and growth. They may have nothing to do directly with cellular differentiation beyond supplying the necessary conditions for the continued existence and development of the cells or areas of cytoplasm with which they are associated. There is of course no fundamental difference physiologically between the energid and the cell. In the polyenergid non-cellular organism each nucleus has its own zone of influence, of restricted size; it is easy to imagine how from this physiological separation there might develop a more or less complete morphological separation, and cells be formed, either multinucleate or uninucleate. As Rohde points out, we can see this evolution taking place before our eyes in the Siphonocladaceae.
The energid conception of the organism is therefore applicable not only to plasmodial organisms but also to such as are fully cellular.
7. In multinucleate organisms, each nucleus with its associated zone of influence, whether it is separated off as a cell or not, must be regarded as a subordinate part or organ of the whole — it is not an independent whole, and cannot be regarded as a self-existent unit, save by abstracting from its relations with the whole.
This summary statement of the conclusions we have reached in this and the preceding chapter brings us back to the two laws of biological method which we formulated in Chapter IX. Since in multinucleate organisms both the energid and the cell are to be regarded as parts and not as wholes, their relations to the whole must be treated in accordance with these two laws.
The first law, which states that the activity of the whole cannot be fully explained in terms of the activities of the parts, runs definitely counter to the classical cell-theory. This theory asserts that all multicellular organisms are built up of fundamental units, the cells, as a house is built of bricks or stones. It asserts further that the activity of the organism as a whole is a summation or resultant of the activities of its component cells : the cell — not the organism — is regarded as the real unit, both morphologically and physiologically.
The theory is in fact a typical product of the method of analytical abstraction, the limitations and dangers of which we have studied in Chapter IX. Against the cell-theory in its classical form the following considerations must be adduced. There is a unity of the whole organism — it develops as a whole, and acts as a whole — and this unity is not a secondary or composite thing, but primary and original. To distinguish cells as independent unities, having their own modes of action independent of the action of the whole, is to regard them abstractly, and to introduce an artificial simplification. From such abstract elements it is impossible fully and completely to recompose the original unity from which they have been separated out by analytical abstraction. We shall see in more detail in the next chapter that in actual demonstrable fact the ovum and the embryo are from the very beginning unitary organisms — that the unity of the organism is not something which comes to be during the course of development, but is there ab initio. We shall see that the importance of cell-formation in development has been greatly overemphasized, to such an extent indeed that the primary unity of the developing organism from the egg onwards has been almost lost from sight.
Our second law, that the activities of the parts cannot be fully understood save in relation to the activities of the whole, gives us the essential rule for the interpretation of cellactivities. These must always be interpreted in relation to the general activities of the organism, as playing their part in one or other of the master-functions of the organism — maintenance, development, and reproduction.
From the organismal point of view, which is implicit in these two laws, the cells taken separately no more constitute the organism than words and letters by themselves make a sentence — the unity and the meaning are in the respective cases prior to the components.
It may be well to make quite clear once more that the organismal conception merely gives rules of method for the study of living things and their parts, that it implies no mysterious ‘action of the whole’ on the parts. There is no internal controlling agency which acts in a mysterious way to bind up the parts into a whole, no separate entelechy, either material or immaterial, which guides and controls organic processes. We accept the simple facts of observation that the organism acts as a whole, and that the activities of its parts are subordinated to, and co-operate in, whatever the organism as a whole is doing at the moment of observation. It is from this simple and objective standpoint that we must regard the relation between the organism and its cells and energids.
8. In conclusion, we may briefly consider the relation of unicellular organisms to the cell-theory. In our discussion of the energid theory of the organism we came to the provisional conclusion that the primary difference between monoenergid and polyenergid organisms must be one of size. We shall see in what follows that this view is essentially correct, that the unicellular animal or plant is primarily an organism, comparable with the whole of a Metazoan or Metaphytan body, and only secondarily a cell, and that by reason of its small size. Unlike the tissue cell the unicellular organism is a whole and not a part. It can be compared with a tissue cell only in respect of an incidental likeness — the fact that it is, as a rule, monoenergid.
This thesis as to the nature of unicellular organisms is so obvious as hardly to require elaboration. The fact that these forms live and develop and reproduce as individuals is quite enough to show that they must be regarded primarily as organisms, and only incidentally as cells. But the influence of the cell-theory has been powerful enough to obscure this obvious conclusion; it has led to the unicellular organism being regarded as the equivalent of the tissue cell of the higher forms, and to the Metazoon being looked upon as a colony of differentiated cells, each homologous with a Protozoon.
So long ago as 1883 Rauber protested against this abstract and morphological view, and maintained that on the contrary the Protozoon was to be homologized with the whole body of the Metazoon. The essential difference between them was that the Protozoon had only one surface, the external surfaceof the body, while the Metazoon possessed, in addition to an external surface, many internal cell-surfaces. In the unicellular organism the cell is the whole (1883, p. 332). He denied that the tissue cell was an ‘elementary organism’, or the Metazoon a composite assemblage of independent units. Both the Protozoon and the Metazoon were essentially wholes.
In more recent days, as a result of the modern development of ‘Protistology’, several workers in this field have emphasized the direct homology of unicellular and multicellular organisms, and pointed out the inadequacy of regarding the former as simple cells. In particular Dobell 1 has made a vigorous onslaught upon the cell-theory in its application to the Protista. By Protista he means Protozoa, Protophyta, and such other ‘unicellular’ forms as cannot be definitely allotted to either group.
1 C. Clifford Dobell, ‘The Principles of Protistology’, Arcb.f. Protistk xxiii, 1911, pp. 269-310.
He maintains that the Protista are organized upon quite a different principle from other living things. They are not really ‘cellular’, for according to Dobell a ‘cell’ is always a fart of an organism, and Protista are whole organisms. ‘An absolutely fundamental point’, he writes, ‘is this : one whole protist individual is a complete individual in exactly the same sense that one whole metazoan individual is a complete individual. Amoeba is an entire organism in just the same sense that man is an entire organism ... it is clear that a protist is no more homologous with one cell in a metazoon than it is homologous with one organ (e.g. the brain or liver) of the latter’ (p. 272). Again, ‘I would emphasize the fact . . . that a protist behaves as a whole organism, and not as a part of one, and a metazoon behaves as a whole organism in just the same way, and not as a “colony” or “state” of separate individuals’ (p. 273).
In sum, Protists are not primitive and simple organisms. ‘The truth ... is that the Protista are very small — but they are not simple. In them, we do not see vital processes in a more elementary form than in other organisms : we see them rather in a more complex form — due to what may be called the “multum in parvo” principle on which all Protista are organized’ (p. 307). That the Protista are all small is, as we have seen, easily explained by the fact that they are (with some exceptions) monoenergid.
One thing has emerged very clearly from the modern intensive investigation of Protista, and that is the amazing complexity of organization which can be developed within the confines of a single cell. This is noticeable particularly in Ciliates, and suggests that Ehrenberg was nearer the truth when he ascribed organs to the Infusoria than were his successors who saw in the Protozoa merely nucleated masses of unformed protoplasm. As an example of a high degree of intracellular organization, let us take Diflodinium ecaudatum, whose structure is thus described by Calkins: 1
‘Bars of denser chitinous substance form an internal skeleton; special retractile fibers draw in a protrusible proboscis; similar fibers closing a dorsal and a ventral operculum; other fibrils, functioning as do nerves of Metazoa, form a complicated coordinating system; cell mouth, cell anus, and a fixed contractile vesicle or excreting organ, are also present. All of these are differentiated parts of one cell for the performance of specific functions, and all perform their functions for the good of the one^cclled organism which measures less than 1/250 inch in length. Analogous, if not so complete intracellular differentiations are present in the majority of Infusoria, while many of the flagellates, notably the Trichonymphidae, have an almost equally elaborate make-up’ (p. 19).
Functionally these organellae correspond to the organs of the Metazoon, and the fact that they are intracellular, instead of being composed of differentiated cells, should not hide from us their essential similarity with organs: the difference is primarily one of scale or dimensions. Calkins, like Dobell, holds that the Protozoa are more satisfactorily regarded as organisms than as cells : ‘as organisms the Protozoa are more significant than as cells. In the same way that organisms of the metazoan grade are more and more highly specialized as we ascend the scale of animal forms, so in the Protozoa we find intracellular specializations which lead to structural complexities difficult to harmonize with the ordinary conception of cells’ (p. 19).
The distinguished Russian protozoologist Awerinzew, 2 in a paper on the systematic position of the Protozoa, has also called attention to the close analogy between the intracellular differentiations of the higher Protozoa and the specialized tissues and organs of the Metazoa. We may conveniently conclude this chapter by quoting his considered opinion that between the Protozoa and the Metazoa there exists no qualitative, but only a quantitative difference (p. 466).
1 G. N. Calkins, The Biology of the Protozoa , London, 1926.
S. Awerinzew, 4 Ueber die Stellung im System und die Klassifizierung der Protozoen’, Biol. Centrbl. xxx, 1910, pp. 465-75.
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