( Originally Published 1909 )
IN the two preceding chapters living substance has been spoken of as existing in separate organic individuals, plants and animals. It is not known to exist in a mass not organized as an individual plant or animal. Many early philosophers did conceive of living matter as existing without individualization. Thus Oken (1805), in his Ur-Schleim theory, when he says that every organic thing came from primitive slime which originated in the sea from organic matter in the course of planetary evolution, simply repeated an idea passed down by the Greek philosopher Anaximenes and others.
Haeckel's theory of the Monera as the simplest of living things allies him to this belief. He says : "In the Monera, the simplest conceivable organisms, the whole body consists merely of plasm, corresponding to the 'primitive slime' of the earlier natural philosophers." And again, in 'The Natural History of Creation,' he describes the Monera as "simple, soft, albuminous lumps . . . without component parts, whose whole albuminous body is as homogeneous in itself as an inorganic crystal."
Huxley also for a time supported this view. In 1869 he described a peculiar sticky mud from the bottom of the Atlantic. The stickiness was apparently due to the presence of innumerable lumps of a transparent gelatinous substance without discoverable nuclei or membranous. envelopes. Huxley interpreted this matter to be masses of protoplasm. He thought it a new form of the simple animate things (Monera) which had been described by Haeckel, and therefore named it Bathybius Haeckelii. Haeckel himself examined the mud and agreed with Huxley's interpretation. Later studies, however, convinced Huxley that the slime was in reality some sort of inorganic precipitate, and at the British Association meeting in 1879 he made a public renunciation of Bathybius.
Leaving this interesting historical conception of living matter in extensive undifferentiated masses, the idea of the individualization of living substance prevails in modern biological science. An organic individual is a unitary mass of living substance. Microscopic study of plants and animals shows them to be made up of these unitary masses the cells. An illustration will make this point clear. Dissection shows that an organ of an animal's body is composed of several building materials, or tissues, such as muscular tissue, nervous tissue, bony tissue, etc. The microscopic study of these tissues reveals the fact that they are composed of small masses of protoplasm the cells. Hence a plant or animal with several tissues is a multicellular organism. There are many simple plants and animals whose body consists of a single cell and these are called unicellular animals. Every such simple organism is an individual. The cell then is the lowest stage of individuality.
It is true, as will be explained in detail later, that the microscope reveals different structures in cells. Especially characteristic are the main body of the cell, the 'cytoplasm,' and the 'nucleus' imbedded in it. This suggests the possibility that the cell is composed of still lower individuals. However, many studies have shown that neither nucleus nor cytoplasm can exist independently, and hence they can-not be regarded as unit masses or individuals. The cell is the lowest stage of individuality known to modern biology.
Looking at the organic world synthetically, the cell stands prominent in a scheme of the five stages of individuality found there.
Individuals of the first order are cells. They represent elementary organisms that are not composed of lower units capable of life. An example is any unicellular animal or plant.
Individuals of the second order are tissues. The tissues are associations of individuals of the first order, each one of which is like the others. Examples are the muscular and nervous tissues of an animal, the conducting and sup-porting tissues of a plant.
Individuals of the third order are organs. The organs are associations of various kinds of individuals of the second order. Examples are the stomach and heart of an animal, the leaf and root of a plant.
Individuals of the fourth order are complex organisms. These are associations of various individuals of the third order. Examples are common animals and plants, whose bodies consist of organs united.
Individuals of the fifth order are communities. The communities are associations of individuals of the fourth order.
It should be noted that an individual of a higher order consists of an assemblage of those of the next lower order. Thus communities consist of persons, persons of organs, organs of tissues, and tissues of cells. In the end all living individual plants and animals are composed of cells and the cell is the seat of those events the expression of which is life. The cell is the life unit, the elementary organism, or as Virchow has said, the vital elementary to the great promblems of biology are to be found is quite modern. Nevertheless this fact has already profoundly. modified many phases of the 'science of life. Professor Wilson of Columbia University, justly regarded as one of the foremost biologists who have worked at the problems of cytology, or the biology of cells, has well outlined the influence of the cell theory on modern biology. He says: "It was the cell-theory that first brought the structure of plants and animals under one point of view, by revealing their common plan of organization. It was through the cell theory that Kölliker, Remak, Nägeli and Hoffmeister opened the way to an understanding of the nature of embryological development, and the law of genetic continuity lying at the basis of inheritance. It was the cell theory again which, in the hands of Goodsir, Virchow and Max Schultze, inaugurated a new era in the history of physiology and pathology, by showing that all the various functions of the body, in health and in disease, are but the outward expression of cell-activities. And at a still later day it was through the cell theory that Hertwig, Fol, Van Beneden and Strasburger solved the long standing riddle of the fertilization of the egg and the mechanism of hereditary transmission. No other biological generalization, save only the theory of organic evolution, has brought so many apparently diverse phenomena under a common point of view or has accomplished more for the unification of knowledge. The cell-theory must, therefore, be placed beside the evolution-theory as one of the foundation stones of modern biology."
The history of the development of the idea that cells are the elementary units of living matter is essential to an understanding of present-day problems, and therefore a rapid survey of the important historical points may be made. These have been well traced by Oscar Hertwig, Professor at Berlin, in his famous 'Die Zelle and die Gewebe.' "The conception or idea connected with the word 'cell,' used scientifically," he says, "has been considerably altered during the last fifty years. The history of the various changes in this conception, or the history of the cell-theory, is of great interest, and nothing could be more suitable than to give a short account of this history in order to introduce the beginner to the series of conceptions connected with the word 'cells' ; this, indeed, may prove useful in other directions. For while, on the one hand, we see how the conception of the cell which is at present accepted, has developed gradually out of older and less complete conceptions, we realize, on the other hand, that we cannot regard it as final or perfect; but on the contrary, we have every ground to hope that better and more delicate methods of investigation, due partly to improved optical instruments, may greatly add to our present knowledge, and may perhaps enrich it with a quite new series of conceptions.
"The theory that organisms are composed of cells was first suggested by the study of plant-structure. At the end of the seventeenth century the Italian, Marcellus Malpighi, and the Englishman, Grew, gained the first in-sight into the more delicate structure of plants ; by means of low magnifying powers they discovered, in the first place, small room-like spaces, provided with firm walls, and filled with fluid, the cells ; and in the second, various kinds of long tubes, which, in most parts, are embedded in the ground tissue, and which, from their appearance, are now called spiral ducts or vessels. Much greater importance, however, was attached to these facts after the investigations which were carried on in a more philosophical spirit by Bahn toward the end of the eighteenth century were published.
"Caspar Friedrich Wolff, Oken, and others, raised the question of the development of plants, and endeavored to show that the ducts and vessels originated in cells. Above all, Treviranus rendered important service by proving in his treatise, entitled Vom inwendigen Bau der Gewächse,' published in 1808, that vessels develop from cells.
"The study of the lowest plants has also proved of the greatest importance in establishing the cell-theory. Small algae were observed, which during their whole lifetime remain either single cells, or consist of simple rows of cells, easily to be separated from one another. Finally, the study of the metabolism of plants led investigators to believe that, in the economy of the plant, it is the cell which absorbs the nutrient substances, elaborates them, and gives them up in an altered form. Thus, at the beginning of the last century, the cell was recognised by many investigators as the morphological and physiological elementary unit of the plant."
These views, however, only obtained general acceptance after the year 1838, when M. Schleiden, who is so frequently cited as the founder of the cell theory, published in Müller's 'Archives' his famous paper, 'Beiträge zur Phytogenesis.' In this paper Schleiden endeavored to explain the mystery of cell formation. He thought he had found the key to the difficulty in the discovery of the English botanist, R. Brown, who, in the year 1833, while making investigations upon orchids, discovered nuclei. Schleiden made further discoveries in this direction ; he showed that nuclei are present in many plants, and as they are invariably found in young cells, the idea occurred to him that the nucleus must have a near connection with the mysterious beginning of the cell, and in consequence must be of great importance in its life-history.
The way in which Schleiden made use of this idea, which was based upon erroneous observations, to build up a theory of phytogenesis, must now be regarded as a mistake ; on the other hand, it must not be forgotten that his perception of the general importance of the nucleus was correct up to a certain point, and that this one idea has in itself exerted an influence far beyond the narrow limits of the science of botany, for it is owing to this that the cell-theory was first applied to animal tissues. For it is just in animal cells that the nuclei stand out most distinctly from among all the other cell-contents, thus showing most evidently the similarity between the histological elements of plants and animals. Thus this little treatise of Schleiden's in 1838 marks an important historical turning point, and since this time the most important work in the building up of the cell theory has been done upon animal tissues.
Attempts to represent the animal body as consisting of a large number of extremely minute elements had been made before Schleiden's time, as is shown by the hypotheses of Oken, Heusinger, Raspail, and many other writers. However, it was impossible to develop these theories further, since they were based upon so many incorrect observations and false deductions that the good in them was outweighed by their errors.
Schwann, however, was the first to attempt to frame a really comprehensive cell-theory which should refer to all kinds of animal tissues. During the year 1838 Schwann, in the course of a conversation with Schleiden, was in-formed of the new theory of cell-formation, and of the importance which was attached to the nucleus in plant-cells. It immediately struck him, as he himself relates, that there are a great many points of resemblance between animal and vegetable cells. He therefore, with most praiseworthy energy, set on foot a comprehensive series of experiments, the results of which he published in 1839.
Thus Schwann originated a theory which, altho imperfect in many respects, yet is applicable both to plants and animals, and which, further, is easily understood, and in the main correct. According to this theory, every part of the animal body is either built up of elements, corresponding to the plant cells, massed together, or is derived from such elements which have undergone certain meta morphoses. This theory has formed a satisfactory foundation upon which many further investigations have been based.
However, as has been mentioned already, the conception which Schleiden and Schwann formed of the plant and animal element was incorrect in many respects. They both defined the cell as a small vesicle, with a firm membrane enclosing fluid contents, that is to say as a small chamber, or 'cellula,' in the true sense of the word. They considered the membrane to be the most important and essential part of the vesicle, for they thought that in consequence of its chemico-physical properties it regulated the metabolism of the cell.
The series of conceptions which now associate with the word 'cell' are, thanks to the great progress made during the last fifty years, essentially different from the above. Schleiden and Schwann's cell theory has undergone a radical reform, having been superseded by the Protoplasmic theory, which is especially associated with the name of Max Schultze.
The History of the Protoplasmic theory is also of supreme interest. Even Schleiden observed in the plant cell, in addition to the cell sap, a delicate transparent substance containing small granules ; this substance he called plant slime. In the year 1846 Mohl called it Protoplasm, a name which has since become so significant, and which before had been used by Purkinje for the substance of which the youngest animal embryos are formed. Further, he presented a new picture of the living appearance of plant protoplasm ; he discovered that it completely filled up the interior of young plant cells, and that in larger and older cells, it absorbed fluid, which collected into droplets or vacuoles. Finally, Mohl established the fact that protoplasm, as has been already stated by Schleiden about the plant slime, shows strikingly peculiar movements; these were first discovered in the year 1772 by Bonaventura Corti, and later in 1807 by C. L. Treviranus, and were described as "the circulatory movements of the cell sap."
By degrees further discoveries were made, which added to the importance attached to these protoplasmic contents of the cell. In the lowest algae, as was observed by Cohn and others, the protoplasm draws itself away from the cell membrane at the time of reproduction, and forms a naked oval body, the swarm-spore, which lies freely in the cell cavity; this swarm-spore soon breaks down the membrane at one spot, after which it creeps out through the opening, and swims about in the water by means of its cilia, like an independent organism ; but it has no cell membrane.
Similar facts were discovered through the study of the animal cell, which could not be reconciled with the old conception of the cell. A few years after the enunciation of Schwann's theory, various investigators, Kölliker, Bischoff and others, observed many animal cells in which no distinct membrane could be discovered, and in consequence a lengthy dispute arose as to whether these bodies were really without membranes, and hence not cells, or whether they were true cells. Further, movements similar to those seen in plant protoplasm were discovered in the granular ground substance of certain animal cells, such as the lymph corpuscles. In consequence Remak applied the term protoplasm, which Mohl had already made use of for plant cells, to the ground substance of animal cells.
Important insight into the nature of protoplasm was afforded by the study of the lowest organism, Rhizopoda (Amoebae), Myxomycetes, etc. Dujardin had called the slimy, granular, contractile substance of which they are composed 'Sarcode.' Subsequently, Max Schultze and de Bary proved, after most careful investigation, that the protoplasm of plants and animals and the sarcode of the lowest organisms are identical.
In consequence of these discoveries, investigators, such as Nägeli, Alexander Braun, Leydig, Kdlliker, Cohn, de Bary, etc., considered the cell membrane to be of but minor importance in comparison to its contents ; however, the credit is due to Max Schultze, above all others, of having made use of these later discoveries in subjecting the cell theory of Schleiden and Schwann to a searching critical examination, and of founding a protoplasmic theory. He attacked the former articles of belief; which it was necessary to renounce, in four excellent tho short papers, the first of which was published in the year 1860.
He based his theory that the cell-membrane is not an essential part of the elementary organisms of plants and animals on the following three facts: First, that a certain substance, the protoplasm of plants and animals, and the sarcode of the simplest forms, which may be recognised by its peculiar phenomena of movement, is found in all organisms; secondly, that altho as a rule the protoplasm of plants is surrounded by a special firm membrane, yet under certain conditions it is able to become divested of this membrane, and to swim about in water as in the case of naked swarm-spores ; and finally, that animal cells and the lowest unicellular organisms very frequently possess no cell membrane, but appear as naked protoplasm and naked sarcode. It is true that he retains the term 'cell,' which was introduced into anatomical language by Schleiden and Schwann ; but he defines it as a small mass of protoplasm endowed with the attributes of life.
Hence it is evident that the term 'cell' is incorrect. That it, nevertheless, has been retained, may be partly ascribed to a kind of loyalty to the vigorous combatants. who, as Brücke expresses it, conquered the whole field of histology under the banner of the cell-theory, and partly to the circumstance, that the discoveries which brought about the new reform were only made by degrees, and were only generally accepted at a time when, in consequence of its having been used for several decades of years, the word cell had taken firm root in the literature of the subject.
Since the time of Brücke and Max Schultze knowledge of the true nature of the cell has increased considerably. Great insight has been gained into the structure and the vital properties of the protoplasm, and in especial, knowledge of the nucleus, and of the part it plays in cell multiplication, and in sexual reproduction, has recently made great advances. The earlier definition, "the cell is a little mass of protoplasm," must now be replaced by the following: "The cell is a little mass of protoplasm, which contains in its interior a specially formed portion, the nucleus."
It is evident from the preceding history of the cell theory 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 a 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. 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 was at first over-looked 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. Researches showed, however, that most living cells are not hollow but solid bodies, and that in many cases—for example, the colorless 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.e. the protoplasm-must be the seat of vital activity.
Within the cell contents lies a body, usually of definite rounded form, known as the nucleus, and this in turn often contains 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 cell. Besides the presence of protoplasm and nucleus, no other structural features of the cell are yet known to be of universal occurrence.
"We may," says Wilson, "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."
The form of cells is highly variable. In isolated cells, especially those floating freely in a fluid and not subjected to unequal pressure, the spherical form is common, but even such free cells may be modified in form by internal movements and differentiations of the cell-substance. For example, some egg-cells are spherical while others are ovoidal ; muscle cells are elongated ; nerve cells much branched ; the white cells of the blood are irregular in shape because of their movements. But no matter how diverse the form of the cells, their structure is essentially the same.
As already suggested, a cell wall or membrane is usually, tho not always present. Sometimes the cell-substance has no more of a limiting membrane than has a drop of oil floating in water, that is, there is simply an undifferentiated film separating it from its surroundings. The cell-mass or cell-substance consists of protoplasm, the active living substance, in which may be embedded granules of lifeless substances. If the term protoplasm is accepted as synonymous with living matter, then there is protoplasm both in the main body of the cell and also in the specially differentiated mass, most commonly central in position, known as the nucleus. It is convenient to call the protoplasm outside the nucleus 'cytoplasm' and that within the nucleus the 'karyoplasm' or 'nucleoplasm.'
In the cytoplasm there are various lifeless substances (metaplasm). Some of these, like fat and starch, are reserve food absorbed but not yet used by the cell, others, like pigment and the cell-wall, are the lifeless products of life activity. The amount of metaplasm is frequently vastly greater than the amount of protoplasm in a cell. For example, a hen's egg just before leaving the ovary is a cell about one inch in diameter (the yolk of the egg). A small white disk on the upper surface consists of concentrated protoplasm, but the greater part of this enormous egg-cell is made up of stored yolk (metaplasm) to be used as food by the chick in its development during incubation.
The nucleus is usually surrounded by a definite membrane, the nuclear membrane. Within this membrane and embedded in the general protoplasmic basis, there are granules or masses of a substance which has a strong affinity for certain chemical dyes, hence called 'chromatin.' When this chromatin is massed into rod shaped bodies, as happens in the process of cell-division, the term 'chromosomes' is applied to these masses of chromatin. Then, too, other bodies, nucleoli, are often present. Their nature is not clearly understood, probably because they are extremely variable.
Careful study of the nucleus during all its phases gives 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. 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 'nuclein,' while the latter contains no true nuclein and is especially rich in albuminous substances such as nucleoalbumins, albumins, globulins, and the like, which contain little or no phosphorus.
"Both morphologically and physiologically," says Wilson, "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."
In addition to the cytoplasm or cell-body and the nucleus recent biologists believe that an extremely minute body lying just outside the nuclear membrane, and known as the centrosome,' is an essential element of the cell. The centrosome has been seen in a large number of cells, is known to play an important part in cell division and in the fertilization of egg cells and has been regarded by some cytologists as the 'dynamic center' of the cell. It is still under investigation, and the interested reader should refer to such technical books as Wilson's 'The Cell in Development and Inheritance' for the latest studies of its structure. Some animal and plant cells appear to differ so much from one another as to their form and contents, that, at first sight, they seem to have nothing in common, and hence it seems impossible to compare them. For instance, if a cell at the growing-point of a plant be taken and compared with one filled with starch granules from the tuber of a potato, or if the contents of an embryo cell from a germinal disk be compared with those of a fat cell, or of one from the egg of an Amphibian filled with yolk granules, the inexperienced observer sees nothing but contrasts. Nevertheless, all these exceedingly different cells are seen on closer examination to be similar in one respect i. e., in the possession of a very important, peculiar mix ture of substances, which is sometimes present in large quantities, and sometimes only in traces, but which is never wholly absent in any elementary organism. In this mixture of substances the wonderful vital phenomena may very frequently be observed (contractility, irritability, etc.) and, moreover, since in young cells, in lower organisms, and in the cells of growing-points and germinal areas, it is in the cell-substance alone (the nucleus of course being 'excepted) that these properties have been observed, this substance has been recognised as the chief supporter of the vital 'functions. It is the protoplasm or 'forming matter.'
In order to know what protoplasm is, it is advisable to examine it in those cells in which it is present in large quantities, and in which it is as free as possible from admixture with other bodies; and among such the most suitable are those organisms from the study of which the founders of the protoplasmic theory formed their conception of the nature of this substance. Such organisms are young plant-cells, Amoebae, and the lymph corpuscles of vertebrates.
The protoplasm of unicellular organisms, and of plant and animal cells appears as a viscid substance, which is almost always colorless, which will not mix with water, and which, in consequence of a certain resemblance to slimy substances, was called by Schleiden the 'slime of the cell.' Its refractive power is greater than that of water, so that the most delicate threads of protoplasm, altho colorless, may be distinguished in this medium. Minute granules, the 'microsomes,' which look only like dots, are always present in greater or less numbers in all protoplasm, and may be seen with a low power of the microscope to be embedded in a homogeneous ground substance.
As to the very minute structure of protoplasm, "most of the earlier observers," says Wilson, "regarded the meshwork as a fibrillar structure, either forming a continuous network or reticulum somewhat like the fibrous network of a sponge or consisting of disconnected threads, whether simple or branching ("filar theory" of Flem ming), 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,' `threadwork,' 'spongioplasm,' 'mitome,' 'filar substance,' all of which are still in use. Under this view the 'granules' described by Schultz, 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 net work as described above."
Widely opposed to these views is the 'alveolar theory' of Bütschli, which has won an increasing number of adherents. Bütschli regards protoplasm as having a foam like alveolar structure ("Wabenstruktur"), nearly similar to that of an emulsion and he has shown in a series of beautiful experiments that artificial emulsions variously prepared, may show the microscope a marvelously close resemblance to living protoplasm, and further that drops of oil emulsion suspended in water may even exhibit amoeboid changes of form.
The two (three) general views hereinbefore outlined may justly 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, tho 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 Bütschli, 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.
Which of these views is correct? The present tendency of cytologists is toward the conclusion that none of them is of universal application and that all exist under certain conditions. In support of this may be cited the studies of Professor Wilson, who has been led to the conclusion that "no universal formula for protoplasmic structure can be given. 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 Bütschli in the same object. 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 are, I 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. Bütschli's conclusions, based on researches so thoro, 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, 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 sub-stances which may assume various forms of visible structure according to its modes of activity.
"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, 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 existance 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, Kölliker, Verworn, and other recent writers, that life can only be properly regarded as a property of the cell-system 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 between 'protoplasm' and 'metaplasmic' substances, while a real and necessary one, becomes after all one of degree."