Talk:Russell1930 11
XI THE PHYSIOLOGICAL INTERPRETATION OF THE CELL-THEORY[edit] T HE cell-theory in its earliest form, as shaped by Schwann and Schleiden, was essentially physiological. Cells were regarded as centres of metabolic activity, and the organism as a colony or state built up of these semi-independent units. The morphology of the cell was at that time not well understood : Schwann thought that nuclei arose by a sort of crystallization or condensation; and the fundamental fact that nuclei are formed only by division from a pre-existing nucleus was not fully established until some years later. Gradually, however, as interest in the new science of cytology spread, the morphological aspect became more prominent, and the standard definition of the cell as a mass of protoplasm containing a nucleus, proposed by Max Schultze in 1861, was couched in terms of structure only. As a general definition this holds the field even at the present day.
Clearly it is a wide and formal definition: it covers for instance such diverse things as a tissue cell, a Protozoon, and a fertilized ovum. These have all a common structural characteristic — the presence of a nucleus surrounded by cytoplasm — and may therefore properly be classified as cells. It does not follow, however, that each is sufficiently defined by being called a cell ; we shall find reason later to maintain that a tissue cell cannot be adequately characterized without reference to the organism of which it forms an integral part, and that both the Protozoon and the ovum are better denned as organisms than as cells. There is a logical trap for the unwary in any simple and abstract classification which relates to only one characteristic common to a series of objects. Because certain objects can be classified as cells in respect of a common structural quality, it does not follow that they are in other respects homologous, nor that they are fully and adequately characterized by the description ‘cell’. Tins is a
THE CELL-THEORY 199
fallacy into which it is easy to slip unconsciously, and it has played its part in the elaboration of the cell-theory in its more extreme forms.
The simple definition of the cell to which we have alluded calls attention to a fundamental peculiarity of all organisms, namely the intimate association of formed nuclear and cytoplasmic substance. No living thing is at present certainly known in which this dualism of substance is not found. As a rule, one or more definite nuclei occur, but bacteria, bluegreen algae, and some other forms have, instead of a nucleus, scattered granules of nuclear matter (chromidia). In this chapter we shall consider the physiological significance of this dualism of substance, and particularly the size-relations which characterize it, attempting thus to get back to a physiological interpretation of the cell-theory. Having done this, wc shall in subsequent chapters consider in more detail the status of the tissue-cell, the Protozoon, and the ovum in relation to the cell-theory, and finally we shall attempt to correct the over-emphasis laid on cellular activity in development, and restate the early processes of development in terms of the activity of the organism as a whole.
2. Apart from the forms with chromidial nuclei, which have solved the problem of achieving an intimate association of nuclear and cytoplasmic matter by the simple device of dispersing the nuclear substance throughout the cytoplasm, all organisms display a nucleate structure. Not all however show a cellular structure, that is, a division into more or less separate units each consisting of ‘a mass of cytoplasm containing a nucleus’. Some Protozoa are bi- or multi-nucleate, as are also some tissue cells, and there is the very striking and significant case of the Siphoneae among the Algae. Forms such as Caulerpa and Acetabularia reach a very considerable size (several inches in length), and show much external differentiation of structure — ‘leaves’, axis and rhizoids in Caulerpa — while showing no internal differentiation into separate cells. There is but one cell-wall and one cell-cavity, traversed it may be by trabecular supports, but there are numerous small nuclei scattered through the cytoplasm.
200 THE PHYSIOLOGICAL INTERPRETATION being specially abundant at the growing points. Are these forms to be regarded as multicellular plants or as multinucleate unicellular plants ? If they are to be ranked as unicellular, then the simple definition of the cell must be considerably enlarged to include them. If on the other hand we keep to the definition of the cell as a nucleus with its associated zone of cytoplasm, then the Siphoneae are multicellular plants without internal cell- walls.
The confusion arises chiefly from the associations of the word ‘cell’. This in its general, non-technical meaning signifies a volume enclosed by a definite wall, and it was the rightword to adopt as a technical term in the early days, when the cell-wall was thought to be an essential part of the definition of the cell. The great majority of plants, particularly, show a cellular structure in this proper sense of the word. But the word is clearly ill-adapted, on account of its general implications, to describe the state of affairs in Metazoa generally, and particularly in multinucleate ‘cells’, syncytia and ‘non-cellular’ algae and fungi.
Toget rid of this source of confusion, J. von Sachs proposed 1 to limit the word cell to such units as are clearly demarcated by a cell-wall or cell-membrane, and to introduce the word energid for the more general concept of a nucleus with its associated cytoplasmic field of influence. A multi-nucleate Protozoon would, from this point of view, be regarded as a cell containing a number of energids; so also a siphoneate alga would be a cell inhabited by numerous energids. It is worth while to give Sachs’s own definition of the term:
, ‘By an energid I mean a single nucleus together with the protoplasm controlled by it, and I conceive it in such a way that a nucleus with its surrounding protoplasm is to be thought of as a whole, and this whole as an organic unity both in a morphological and in a physiological sense’ (p. 1150). In Sachs’s view energids are the real constitutive units both of plants and of animals — cellular structure is something superadded. The facts show, he says, that to a certain minimum amount of protoplasm there belongs a nucleus, and
1 'Energiden und Zellen’, Gesam. Abbandl . ii, Leipzig, 1893, pp. 1150-5.
OF THE CELL-THEORY 201
that when this quantity of protoplasm increases, further nuclei become necessary to maintain its energy (p. 1151). The word was chosen to emphasize the energy aspect of the nucleus-cytoplasm relation, and it is clear that Sachs considered the energid to be essentially a physiological or metabolic unit. It may of course also be a structural unit, but morphological separateness is no part of its definition. The cell on the other hand is a morphological concept, according to Sachs’s definition of it as a unit enclosed by a cell-wall or cell-membrane.
We shall consider Sachs’s general views on the cell-theory in more detail later (pp. 212-3, 217-8), but for our present purpose his conception of the energid as a physiological unit is the important thing. Our thesis in this chapter is precisely that the cell, or better the energid, is fundamentally a metabolic unit, and it is as such that it underlies all organic structure, and all the special activities of the organism, including development.
Using Sachs’s nomenclature, we can say that, with the exception of the forms possessing a chromidial nucleus, all organisms are composed of one or more energids, though not all organisms are built up of cells. Or we can use our earlier expression that all organisms show a nucleate , but not all a cellular structure. We lay stress accordingly on the nucleus with its associated plasma rather than on the ‘cell’. The formation of separate cellular units, important and widespread though it is (see below, pp. 217-9), remains a secondary phenomenon.
3. What is the nature of the physiological relationship between nucleus and cytoplasm, which necessitates their intimate spatial association ? The general answer to this question was given by Schwann himself, who considered that cells were the essential agents in nutrition and growth: ‘The 5 ground of nutrition and growth’, he wrote, ‘lies not in the organism as a whole, but in the separate elementary parts, the cells.’ 1 The anatomist Goodsir, in 1845, likewise spoke
1 Mtkroskopucbe U titer tucbungen , 1839, P* The point is fully discussed in my
Form and Function , 1916, pp. 182-5.
3727 d d
202 THE PHYSIOLOGICAL INTERPRETATION of cells as centres of growth and nutrition. 1 Coming to more modern days, we note that Rauber, in a paper of very great importance for the cell-theory, 2 took the similar view that the function of the nucleus was essentially a trophic one. He remarked upon the great uniformity of the nucleus throughout the most diverse modifications of the cytoplasm :
‘The structure of the nucleus is diversified ( vielgestaltig ) only during the periods of division. In the resting stage its structure, even with respect to the most diverse tissues of the adult body, is a monotonous one. The same thing holds good with its chemical composition. In this respect the nucleus stands in complete contrast to the multiform protoplasm. The function of the nucleus can only be one which is independent of the metamorphoses of the protoplasm, a function which the most diverse protoplasmic structures all equally demand. It can only be a trophic function’ (p. 251).
Sachs considered his energid to be ‘essentially a primary energy-element of the organism, and for that reason the primary element in the processes of form-production (Gestaltungsvorgangey . 3 He discusses the relation of nucleus and cytoplasm more fully in a paper of 1895, 4 and concludes that the nucleus is the seat not so much of chemical as of morphogenetic energy ( Gestaltungsenergie ). He considers that the function of the cytoplasm is primarily ‘kinetic’, having to do with the reception of stimuli and with response by movement or by change of form. The fact that the nuclei are crowded together at the growing points, both in cellular and in non-cellular plants, and in cellular plants are at such points very large in proportion to the size of the cells, points also to the conclusion that the nucleus has to do mainly with constructive metabolism and growth.
The same conclusion emerges from Haberlandt’s* observations on the position of the nucleus in plant cells in relation
1 Sec D’Arcy W. Thompson, On Growth and Form , 1917, p. 157.
A. Rauber, ‘Neue Grundlegungen zur Kenntnis der Zelle’, Morph . Jabrh viii, 1883, pp. 233-338. Mbid., p. 1155.
4 ‘Physiologische Notizen, ix, Weitere Betrachtungen uber Energiden und Zellen’, Flora , lxxxi, 1895, PP* 4 ° 5 - 34 • G. Haberlandt. Ueber die Beziebungen ztoiscben Funktion und Lage des Zellkerns , Jena, 1887.
OF THE CELL-THEORY 203
to the processes of growth, and particularly to the formation of the cell-wall. The nucleus lies near the point of most active growth, and is closely applied to such parts of the cell-wall as are undergoing thickening or extension. A similar study by Korschelt , 1 with reference particularly to ovarian eggs and secreting cells in Insects, also demonstrates the great part played by the nucleus in the metabolic processes of the cell. He found that in the ovarian egg the nucleus is concerned with taking up nutritive substances from the surrounding cells, stretching out processes towards the source of nourishment, increasing thus its active absorbent surface. In the same way the nucleus is concerned in secretion; and in actively secreting cells, e.g. in the silkglands of Insects, it may greatly increase its surface by branching. In general, the nucleus is to be found in that part of the cell where active absorption or secretion is taking place.
The most direct evidence as to the metabolic function of the nucleus has of course been derived from the numerous experiments which have been carried out with enucleated Protozoa and plant cells . 2 From these the fact emerges clearly that, while enucleate fragments may live for several days, exhibiting characteristic movements, carrying out the physiological functions of respiration and excretion (and even in some cases digestion), assimilation and growth are completely absent, and regeneration is limited to a mere healing of the wound. We may conclude then, with Wilson, ‘that destructive processes and the liberation of energy, as manifested by co-ordinated forms of protoplasmic movement, may go on for some time undisturbed in a mass of cytoplasm deprived of a nucleus. On the other hand, the building up of new chemical or morphological products by the cytoplasm is only initiated in the presence of a nucleus, and soon ceases in its absence. The nucleus must, therefore, play an essential part both in the operations of synthetic metabolism
1 E. Korschelt, ‘Beitrage zur Morphologie und Physiologic des Zellkerns’, ZooL Jabrb. (Anat.\ iv, 1891, pp. 1-154.
a For a general account of these experiments see M. Verworn, General Physiology 9 Eng. Trans., 1899, pp. 508-23; Wilson, 1925, pp. 657-62, and B. Sokoloff, ‘Das Regenerationsproblem bei Protozoen’, Arch. Protistenkunde , xlvii, 1924, pp. 143-252.
204 THE PHYSIOLOGICAL INTERPRETATION or chemical synthesis and in the morphological determination of these operations’ (1925, p. 660).
One must of course bear in mind that there is something abstract and artificial in thus separating and distinguishing the functions of the nucleus from those of the cytoplasm. The nucleus cannot carry out its functions save in association with the cytoplasm, and the cytoplasm deprived of its nucleus soon ‘runs down’ and dies. As Verworn points out, ‘between the protoplasm and the nucleus a mutual exchange of substance takes place, without which neither of the two parts of the cell can continue to exist. In other words, both nucleus and protoplasm take part in the metabolism of the whole cell and are indispensable to its continuance’ (1899,
f >. 518). The elementary unit is the cell or energid as a whole.
t is we who separate nucleus from cytoplasm by analytical abstraction — in actual fact their characteristic activities depend upon the maintenance of their intimate relations and interactions with each other. We must therefore correct the abstractness of our analysis by keeping in mind constantly the synthetic unity of the energid as a whole.
Even from the chemical or dynamical standpoint, the energid must be regarded as a unitary system, and its structural differentiation into nucleus and cytoplasm as conditioned by and conditioning the nature of its activity as a whole. This point is put very well by D’Arcy Thompson, 1 in the following passage, which is worth quoting in full:
‘From the moment that we enter on a dynamical conception of the cell, we perceive that the old debates were in vain as to what visible portions of the cell were active or passive, living or not living. For the manifestations of force can only be due to the interaction of the various parts, to the transference of energy from one to another. Certain properties may be manifested, certain functions may be carried on, by the protoplasm apart from the nucleus; but the interaction of the two is necessary, that other and more important properties or functions may be manifested. We know, for instance, that portions of an Infusorian are incapable of regenerating lost parts in the absence of a nucleus, while nucleated pieces soon regain the specific
« 1917, pp. 197-8.
OF THE CELL-THEORY 205
form of the organism; and we are told that reproduction by fission cannot be initiated , though apparently all its later steps can be carried on, independently of nuclear action. Nor, as Verwom pointed out, can the nucleus possibly be regarded as the “sole vehicle of inheritance”, since only in the conjunction of cell and nucleus do we find the essentials of cell-life. ‘Kern und Protoplasma sind nur vereint lebensfahig”, as Nussbaum said/
According to Hopkins, 1
‘the cell, in the modern phraseology of physical chemistry, is a system of coexisting phases of different constitutions. . . . On ultimate analysis we can scarcely speak at all of living matter in the cell; at any rate, we cannot, without gross misuse of terms, speak of the cell life as being associated with any one particular type of molecule. Its life is the expression of a particular dynamic equilibrium which obtains in a polyphasic system. Certain of the phases may be separated, mechanically or otherwise, as when we squeeze out the cell juices, and find that chemical processes still go on in them ; but “life”, as we instinctively define it, is a property of the cell as a whole, because it depends upon the organization of processes, upon the equilibrium displayed by the totality of the coexisting phases’ (p. 220).
Having established the fact that the energid is the primary unit in constructive metabolism, we must now go on to consider the significance of the characteristic size-relations of nuclei and their associated cytoplasm.
4. D’Arcy Thompson in that admirable book On Growth and Form has called attention to the importance of size or scale in organic structures, with reference particularly to the changing relation between volume and surface in structures of similar shape as their dimensions alter. Surface varies as the square, volume as the cube, of corresponding linear dimensions ; hence a small sphere, for instance, has a relatively greater surface in proportion to its volume than a larger sphere.
That nuclei are all of very restricted dimensions is therefore a fact of much significance, and suggests at once that a high ratio of surface to volume is important for the physiological activity of the nucleus.
1 F. G. Hopkins, 'The Dynamic Side of Biochemistry*, Pres. Address to Physiology Section of the British Association, 1913. Nature , vol. xcii, pp. 213-23.
206 the physiological interpretation
The nucleus, in the absurdly named resting stage, when it is in fact most fully active, is bounded by a definite membrane or cortical layer, and all its interactions with the cytoplasm are mediated by this surface of separation. It tends as a rule, unless deformed by the shape of the cell, to assume a spherical or oval form. This shape is by no means ideal from the point of view of exposing a maximum surface, for the sphere is of all bodies that which has the minimum surface per unit volume, but it is probably imposed on the nucleus by physical conditions — surface tension and the like. 1 And of course many exceptions occur — nuclei, as we have seen, may be branched or lobed in special cases, where extra surface is required.
‘Nuclei of irregular or amoeboid form are frequent in cells characterized by very active metabolism, in which case the nuclei are often not only of large size but show a marked further increase of surface by the formation of lobes, sacculations, or even, in extreme cases, of complex branches ramifying through the cell. An extreme example of this is offered by the spinning glands of certain insect larvae (Lepidoptera, Trichoptera) in which the nucleus, originally spheroidal, finally assumes a labyrinthine appearance with convolutions occupying a large area in the cell. In other cases the nucleus shows deep infoldings or incisions and sometimes even tubular ingrowths of membrane forming intra-nuclear canaliculi; and it has been shown that such infoldings may unfold or evaginate, thus increasing the nuclear size. In certain types of cells the surface of the nucleus may also be increased by its breaking up into more or less separate vesicles or karyomerites, thus forming “polymorphic” nuclei or nuclear nests.’ 1
It is of course impossible to give a mean size for all kinds of nuclei; the available data are restricted in quantity, and even if they were sufficient to be representative, the mean calculated from them would be merely a mathematical abstraction; further, the size of the nucleus may vary in the individual cell according to its functional activity, 3 and again, there is in general a regular increase in nuclear size from one
1 D’Arcy Thompson, On Growth and Form, p. 165.
Wilton, 192$, pp. 78-9. See for instance C. F. Hodge, 'A microscopical Study of Changes due to functional activity in Nerve Cells', Journ. Morph, vii, 1892, pp. 95-168.
OF THE CELL-THEORY 207
mitosis to the next, when nuclear size is again reduced — a nuclear diastole and systole, as the process was called by Ryder. 1 All that we can do is to quote a few examples of nuclear sizes in diverse groups of organisms and in diverse kinds of cell.
The nucleus of the animal ovum is as a rule relatively large, and there is a steady diminution in the size of the individual nuclei as segmentation proceeds. This may be exemplified by the sea-urchin Strongylocentrotus lividus, where the size-relations of nuclei and cytoplasm during early development have been carefully studied by Rh. Erdmann. 2 The eggs were reared under different conditions of temperature, namely at io° C., I5°-i6°C., and 20°C. It is sufficient for our purpose to consider only the first set. The following data are extracted from the table on p. 88 :
Stage of
Radius of
Volume of
Volume of
development.
Nucleus.
Nucleus.
Cell.
V fi 3
n 3
2-cells
13-88
10,037
106,250
4 „
7-264
1,605-8
5 ! > o6 3
8 „
6-362
1,08 1 -o
26,290
16 „
5-844
837*8
9,973
32 „
5-76
803-6
6,023
64-1 32 „
5 ’ 4 i
529-7
2,685-5
Blastula I
479 1
460-5
i ,343
Blastula II
4-297
332*4
549*7
Gastrula I
2-97
1 1 7*39
292-5
Gastrula II
2-466
62-87
180-7
Pluteus
i*9
28-73
1 18
The size of the nucleus decreases in steady progression as segmentation and development proceed. So also does the size of the segmentation spheres or cells. For the greater part of the time, the cell-volume decreases at a greater rate than the nuclear volume, the ratio, cell-volume: nuclear volume, falling from 31*8 at the 4-celled stage to 1-6 at the second blastula stage, increasing thereafter to 4*2 in the pluteus.
1 See E. G. Conklin, ‘Cellular Differentiation*, in Cowdry, General Cytology , Chicago, 1924, p. 550.
2 ‘Experimentelle Untersuchung der Massenverhaltnisse von Plasma, Kem und
Chromosomen in dem sich entwickelnden Seeigelei’, Arcb.f. Zellforscbung , ii, 1909,
pp . 76-136.
208 THE PHYSIOLOGICAL INTERPRETATION
If we consider the developing egg as a whole, we find that its total volume hardly increases at all until the second blastula stage is reached. This is shown by the following calculation:
Total volume at 2-cell stage — 106,250x2 =212,500 ft 3
„ „ 32-cell „ 6,023x32 =192,73^1*
„ „ Bl. I „ 1,343x170*= 228,310ft 3
„ „ Bill „ 5497x680*= 373, 79V 3
The total nuclear volume, however, increases at these stages as follows: 20,074, 25,715, 78,285, and 226,032ft 3 . Thus up to the first blastula stage, while the total volume of the cells of the developing egg remains nearly constant, the nuclear substance has increased nearly 4 times. If now we calculate the total surface area of the nuclei (on the assumption that they are spherical) we find the increase to be from 4,339/* 2 at the 2-cell stage to 49,020/t 2 at the first blastula stage, an increase of about 1 1 times. The result of segmentation and cell-division is then a considerable increase in the amount of nuclear substance relative to cytoplasmic substance, and a much greater increase in total nuclear surface. Even if there were no increase in total nuclear volume during the process, it may easily be calculated that the total nuclear surface would increase considerably — from 4,339ft 2 at the 2-cell stage to 19,788 /* 2 at the first blastula stage ( r = 3-0434). The course of events then in the early development of Strongylocentrotus affords a good object-lesson in the importance of absolute size in relation to surface area; the 170 small nuclei of the blastula expose collectively 1 1 times as much surface area as the 2 nuclei of the 2-cell stage, and, presumably, are physiologically more efficient in somewhat the same ratio.
The same phenomenon of increase in total nuclear surface during segmentation is shown clearly in the mollusc Crepidula. Conklin, 2 who has studied this form minutely, writes :
‘During cleavage the increase in nuclear surfaces is much greater than the increase in nuclear volumes. While the increase in maximum 3
Erdmann, p. 97. a £. G. Conklin, ‘Cell Size and Nuclear Size’, Journ, Exper . Zool, xii, 1912, pp. 1-98. 3 Nucleus measured at its maximum volume, i.e. just before division*
OF THE CELL-THEORY 209
nuclear volumes up to the 32-cell stage of Crepidula is about 5 per cent, for each division, the growth in the maximum nuclear surfaces during this period is about n per cent, for each division. From the 2-cell to the 70-cell stage the nuclear volume increases only 2*24 times, while the nuclear surfaces increase 5*30 times. In Styela the nuclear volume increases from the 2 -cell stage to the 256-cell stage only 4*52 times, the nuclear surfaces increase 1375 times. Unquestionably this greater growth of nuclear surfaces as compared with nuclear volumes facilitates the interchange between nucleus and protoplasm’ (p. 61).
The nucleus or germinal vesicle of the immature egg is in many animals relatively large and relatively inactive; after fertilization and during segmentation the metabolic activity of the egg increases: this is no doubt facilitated by the division of the nucleus into smaller units, which, distributed through the developing egg, expose a larger action-surface than the undivided nucleus.
Possibly also, as Conklin suggests, there is a specially intimate exchange, particularly of substance, between nucleus and cytoplasm at the time of mitosis, when large quantities of material absorbed by the nucleus during inter-mitotic growth are released again into the cytoplasm.
It is interesting to note that Sachs, in his paper of 1895 already referred to, suggests that surface action plays a great role in the physiological activities of the component parts of the energid, and that it is for this reason that these parts are of such restricted dimensions. ‘The minuteness of the parts’, he writes, ‘which react upon one another, inside the energid, both mechanically and kinetically, shows that so-called surface forces are here primarily concerned — forces that can act only at very short distances, but whose effect increases with increase of surface for a given volume’ (p. 425). In a previous paper (1893) he had pointed out that in segmentation the splitting up of the zygote nucleus into numerous smaller ones was accompanied by an increase in the activity of the nucleus, the undivided nucleus being physiologically inert, and the developmental processes progressing the more energetically the further nuclear division proceeds. He
3727 lc
2io THE PHYSIOLOGICAL INTERPRETATION
drew the tentative conclusion, as we have done, that the rationale of nuclear division in segmentation is to be found in the increase of metabolic activity brought about by increase of total surface.
Reverting to Conklin’s paper, we may note some typical dimensions of nuclei in Cre-pidula, for comparison with the data for Strongylocentrotus. Crepidula differs from the seaurchin particularly in the fact that the total volume of the cytoplasm increases considerably at the expense of the yolk, which is here abundant, and for a time at a greater rate than the total nuclear volume (p. 33). The mean diameter of the nucleus at the 2-cell stage is 18/4, at the 4-cell stage 1 6/4, at the 8-cell stage 15 /i for the macromeres and 12/1 for the micromeres. At the 70-cell stage the nuclear diameter varies from 5 n to 16/4 according to the size and type of the cells. In sexually mature individuals the nuclei are smaller — about 4-8/x in epithelial and secreting cells, up to 12 /* in large ganglion cells. In Fulgur, which has large eggs, the diameter of the nucleus in early cleavage stages is much greater — up to 96/1 in cells 4A-4D, before the seventh cleavage.
In the ascidian Styela {Cynthia), the average diameters of the nuclei are as follows (p. 43) :
Before first maturation „ „ cleavage
2-cell stage
4
8
16
32
64
128
256
54f*
12 n (<J and ? pronuclei)
i6fi
3M 1 1/4 10 /4 8/4 65/4 S-2SM
With regard to nuclear size in Vertebrates, many data are given by Hartmann 1 relating to the common toad and other Amphibia, and these are summarized in convenient form in
1 O. Hartmann, 'Ueber den Einfluss der Temperatur auf Grosse und Beschaffenheit von Zelle und Kern, Arch.f, Ent.-Mecb xliv, 1918, pp. 1 14-95.
211
OF THE CELL-THEORY Tabulae biologic ae. 1 From these tables I have calculated the following means, relating to 66 determinations of nuclear size in many different kinds of tissue at diverse stages of development in Bufo vulgaris :
Mean of length 11-3 /x „ breadth 8-55 /1
„ surface 93-48/1 2 .
These averages have only a mathematical significance, but they give an approximate idea of the general size of nuclei in this species. The nuclei of the erythrocytes in old tadpoles of Bufo measure 8 6x5-5 /*» in adult Bombinator igneus the erythrocyte nuclei measure 12-9x7-0/*, and in Triton alpestris 18-0x9-6/1.
For the sake of comparison, a few data relating to plants may be quoted. Strasburger 2 made measurements of cellsize and nuclear size in the growing points of 40 species of vascular cryptograms and phanerogams. He found considerable variations, mean nuclear diameter ranging from 3/t to 16/1, while cell-diameter varied from 5/1 to 24/1. He concluded that in these young cells, before growth and vacuolization took place, the ratio of nuclear size to cell-size was about 2 : 3 (p. 117).
From the few examples given above as to size of nuclei in diverse groups of animals and plants, it is evident that, while there is great variation according to species, kind of cell, and stage of development, the nucleus is always a structure of very restricted dimensions, with a diameter measurable in hundredths or thousandths of a millimetre. It appears reasonable to suppose that this limitation of size is connected with the need for keeping surface large in relation to volume, in order that the nuclear contents may all enter into close and intimate metabolic relations with the cytoplasm. Celldivision, in animals at least, is always initiated by nuclear division, and it may be surmised that the rationale of nuclear division is to keep the nucleus small, in order that its volume
1 Tabulae biologicae , ed. by W. Junk, vol. iv, Berlin, 1927, pp. 328-30.
E. Strasburger, ‘Ueber die Wirkungssphtire der Kern und die Zellgrosse’, Histologiscbe Beitrdge , vol. v, Jena, 1893, pp. 97-124.
212 THE PHYSIOLOGICAL INTERPRETATION
may not increase unduly in proportion to its surface, and so hinder and slow down its metabolic interchange with the cytoplasm.
We come now to the second point connected with sizerelations, namely the size of the cell or energid itself. This is a point which has been very thoroughly discussed in the literature, 1 and for this reason we need not go into it in any great detail. Certain general facts seem to be established. There is a rough correlation between the size of the cell, especially the active cytoplasmic part of it, and the size of the nucleus (karyoplasmic ratio of R. Hertwig), but there are numerous exceptions to the rule. The size of cells appears to be a specific constant, so that large individuals of a species differ from small individuals in respect of the total number of cells, but not in respect of the size of the cells — provided that their chromosome numbers are the same, for a doubling of the number of chromosomes may bring about an increase both in cell-size and in body-size. There is enormous variation in the size of cells — from a bacillus which is barely visible under the highest powers of the microscope to the relatively huge eggs of some birds and sharks. The length of the axon fibre of a neurone may be several feet. It cannot be said then that there is anything like the same restriction on cell-size that there is on nuclear size ; we note only that there must be physical continuity between all parts of the cell and its nucleus — cytoplasmic substance cannot exist for any length of time in physiological isolation from a nucleus.
Nevertheless, apart from special cases such as we have mentioned, cells are as a rule quite small objects, measurable in tenths or in hundredths of a millimetre.
For plant cells Sachs gives the mean linear dimension as about 20 /*, and points out that whatever the size of the plant may be the size of its cells is to be reckoned in small fractions of a millimetre, and its volume in cubic n (1895, p. 423). The reason why energids are small is that the sphere of influence of a nucleus is a limited one. Physiologically regarded, the cell, or rather the energid, is the zone of influence of a
1 See for initance Wilson, 1925, pp. 97-101.
OF THE CELL-THEORY 213
nucleus, which is itself minute; the size of the energid is therefore limited by the distance to which the influence of the nucleus can be transmitted, or by the volume of cytoplasmic substance with which the nucleus can effectively keep in metabolic relations.
Loeb adopts this same physiological interpretation of the cell-theory in a short paper dealing with the function of the nucleus in regeneration. 1 He points out that organic synthesis cannot go on without oxygen and adduces evidence that the nucleus is an oxidation centre. The reason why cells are small is that the cell-substance must remain within effective reach of the oxidative nucleus, otherwise it will perish through lack of oxygen. The cellular or energid structure of living things is then primarily an expression of the fact that cytoplasm must keep within a certain small distance of a nucleus.
Similar views are expressed by Watase, 2 on general grounds :
‘The division of an organism into distinct cell-entities in a multicellular organism is a phenomenon widely distributed, it is true, but still of secondary significance, due to physiological causes, I believe, emanating from the fundamental difference existing between the chromosome and the cytoplasm, the difference between the two being of such a character that makes their mutual association necessary for the existence of each. The chromosome cannot grow beyond a certain bulk, nor is the cytoplasm capable of unlimited growth, without each meeting with a restraining influence from the other, if one may express it in a metaphorical way. The formation of a nucleated cell is, in other words, a secondary adaptation to keep the nuclear and cytoplasmic material within the reach of reciprocal physiological influence of each’ (pp. 102-3).
The same is of course true of the energid — or rather it is primarily true of the energid.
5. We may now sum up our discussion as follows. An intimate association of nuclear and cytoplasmic matter underlies all organization and all vital activities. Without
1 J. Loeb, ‘Warum 1 st die Regeneration kernloser Protoplasmastucke unmoglich oder erschwert?’ Arch . Ent.-Mecb., viii, 1899, pp. 689-93.
a S. Watas£, ‘On the Nature of Cell-Organization*, Wood's Holl Biol . Lectures for 1S93, Boston, 1894, pp. 83-103.
214 THE PHYSIOLOGICAL INTERPRETATION
this dualism of substance, constructive metabolism, growth, and development are impossible. It is therefore a primary condition of all continuing life, and of all special manifestations of life — genetic continuity, fertilization, development, growth, regeneration, and so on. Apart from forms in which nuclear matter is dispersed throughout the cell, this dualism of nuclear and cytoplasmic substance appears in the shape of a small formed nucleus with its associated cytoplasm — the energid. An energid type of organization characterizes therefore the vast majority of organic forms. The energids, being primarily concerned with organic synthesis, may be regarded as the metabolic centres of the organism.
From this essentially physiological point of view, the organism must be regarded as a protoplasmic mass containing, for the purposes of metabolism, one or more nuclei, one or more energids. From this point of view, the energids cannot be properly considered as independent or semi-independent units composing the organism, but rather as structures specialized for the metabolic functions of growth and repair, and as such subservient to the needs and activities of the organism as a whole. The true biological unit is therefore not the energid, but the organism, whether this is made up of one energid or of many.
This physiological formulation of the cell-theory is one which is in fact gradually replacing the older ideas about cells.
Doncaster for example 1 states boldly that ‘to regard the organism as built up of discrete cells which co-operate physiologically but are fundamentally independent is a false conception’ (p. 2), and he contrasts the old with the new view as follows:
‘The cell theory in its original and crude form regarded an organism as composed of a horde of discrete units which co-operate for a common purpose and are modified in various ways to make that co-operation effective, much as a human community consists of many separate individuals, having different occupations and cooperating for their common good. According to this idea the individuality of any organism arose from an integration of the in
L. Doncaster, An Introduction to the Study of Cytology , Cambridge, 1920.
OF THE CELL-THEORY 215
dividualities of its separate cells, and is thus a corporate individuality such as may exist in a school or a regiment. Nowadays, however, opinion tends in the opposite direction — to regard the organism as the individual, and the cells not as units of which it is built up but rather as parts into which it is divided in order to provide for the necessary division of labour involved in so complex a process as life’ (p. 3).
There can be no doubt as to which of these two views of the organism the energid theory leads us to adopt. Physiologically regarded, the organism is one, and its cells or energies are merely organs of the whole.
The energid conception of the organism requires of course a little correction or enlargement. In most multinucleate organisms there is more implied in cell-formation than a mere multiplication of undifferentiated energids, such as takes place in ‘non-cellular’ plants.
The formation of more or less discrete cells, and their differentiation into different types, clearly plays a great part in the development of any complex multicellular organism. This is a point which we shall consider in more detail in the next chapter. But the energid conception represents the most general point of view that we can take, for it is applicable to all forms without exception that have a formed nucleus, whereas the cellular conception is difficult to apply satisfactorily to Protozoa for instance and ‘non-cellular’ plants. We are justified therefore in regarding the energid conception as the more fundamental.
We saw in our discussion of the facts that nuclei are all of very restricted dimensions, being measurable in hundredths or thousandths of a millimetre, and that energids also are limited in size, though not so strictly as are nuclei. We suggested also, as a possible or probable physiological explanation, the hypothesis that the surface of the nucleus must be kept large in proportion to its volume, in order that all its constituents may enter into intimate relations with the cytoplasm of the energid. However that may be, there is no doubt about the empirical fact of the minuteness of nuclei, nor of the conclusion reached by Sachs (see above, p. 200) that to a certain minimum amount of protoplasm there belongs a
216 THE CELL-THEORY
nucleus, and that when this quantity of protoplasm increases, further nuclei become necessary to maintain its energy. It follows then that the number of nuclei is, in the main, a function of the size of the organism. An organism composed of one energid is necessarily a small organism: a large organism must necessarily contain many energids. Accordingly the main difference between Protozoa and Metazoa, and between unicellular and multicellular plants, is simply one of size. We shall see later that the unicellular organism is strictly comparable not with the single cells of the multicellular organism, but with that organism as a whole.
Another simple deduction from the facts about the energid is that, when for any reason vital phenomena must take place in minimal dimensions, the type of the unit concerned must be mono-energid. Hence, for instance, the mono-energid character of ova and spermatozoa.
