Plasm is the universal living substance—Definition of protoplasm, chemically and morphologically—Physical character—Viscous condition—Chemical analysis—Colloid character of albumin—Albuminoid molecules—Elementary structure of plasm—Work of plasm—Protoplasm and metaplasm—Structures of metaplasm—Frothy structure—Skeletal structure—Fibrous structure—Granular structure—Molecular structure—Plasma molecules—Plastidules and biogens—Micella and biophora—Caryoplasm and cytoplasm—Nuclear matter—Chromatin and achromin—Nucleolus and centrosoma—Caryotheka and caryolymph—Cellular matter—Plasma products—Internal plasma products—External plasma products—Cell membranes—Intercellular matter—Cuticular matter.

By plasm, in the widest sense of the word, we mean the living matter, or all bodies that are found to constitute the material foundations of the phenomena of life. It is usual to give this matter the name of protoplasm; but this older and historically important designation has suffered so many changes of meaning through the variety of its applications that it is better now to use it only in the narrower sense. Moreover, recent research on protoplasm has been greatly developed, and several new names have been invented, which are formed from the word "plasm" with a qualifying prefix. These are special varieties of the general idea of plasm, or special modifications of the general matter, such as metaplasm, archiplasm, and so on.

The botanist, Hugo Mohl, who first introduced the name "protoplasm" in 1846, used it to designate a part of the contents of the ordinary plant-cell—namely, the viscous matter that Schleiden called "cell-mucus," which is found on the inner surface of the cell-wall, and often forms a varying net-work or skeleton in the watery fluid in the cell, and exhibits characteristic movements. Mohl gave the name of "primordial skin" to this important wall-layer (the chief element of the plant-cell), and called the material of it, as being chemically different from the other parts of the cell, protoplasm—that is to say, the first (proton) or earliest formation of the organism. It is important to notice that Mohl, the author of the name, conceived it in a purely chemical, not a morphological, sense, like Oscar Hertwig and other recent cytologists. I intend to retain this early chemical idea of protoplasm—or, briefly, plasm. It was also taken in this sense by Max Schultze, who pointed out (in 1860) its extreme significance and wide distribution in all living cells, and introduced an important reform of the cell-theory which we will see later.

The mixing of the chemical and the morphological ideas of protoplasm has been very mischievous in recent biology, and has led to great confusion. It generally comes from a failure to formulate clearly the difference between the two essential elements of the modern notion of the cell—the anatomic distinction between the nucleus and the body of the cell. The internal nucleus (or caryon) had the appearance of a solid, definite, morphologically distinct constituent of the cell; the outer and softer mass which we now call the cell-body (celleus or cytosoma) seemed to be a formless and only chemically definable protoplasm. It was only discovered at a later date that the chemical composition of the nucleus is closely akin to that of the cell-body, and that we may properly associate the caryoplasm of the one with the cytoplasm of the other under the general heading of plasm. All the other materials that we find in the living organism are products or derivatives of the active plasm.

In view of the extraordinary significance which we must assign to the plasm—as the universal vehicle of all the vital phenomena (or "the physical basis of life," as Huxley said)—it is very important to understand clearly all its properties, especially the chemical ones. This is rendered somewhat difficult from the circumstance that the plasm is, in most of the organic cells, closely bound up with other substances—the various plasma products; it can rarely be isolated in its purity, and can never be had pure in any quantity. Hence we are for the most part dependent on the imperfect, and often ambiguous, results of microscopic and microchemical research.

In every case where we have with great difficulty succeeded in examining the plasm as far as possible and separating it from the plasma-products, it has the appearance of a colorless, viscous substance, the chief physical property of which is its peculiar thickness and consistency. The physicist distinguishes three conditions of inorganic matter—solid, fluid, and gaseous. Active living protoplasm cannot strictly be described as either fluid or solid in the physical sense. It presents an intermediate stage between the two which is best described as viscous; it is best compared to a cold jelly or solution of glue. Just as we find the latter substance in all stages between the solid and the fluid, so we find in the case of protoplasm. The cause of this softness is the quantity of water contained in the living matter, which generally amounts to a half of its volume and weight. The water is distributed between the plasma molecules, or the ultimate particles of living matter, in much the same way as it is in the crystals of salts, but with the important difference that it is very variable in quantity in the plasm. On this depends the capacity for absorption or imbibition in the plasm, and the mobility of its molecules, which is very important for the performance of the vital actions. However, this capacity of absorption has definite limits in each variety of plasm; living plasm is not soluble in water, but absolutely resists the penetration of any water beyond this limit.

The chemistry of living matter is the most important and interesting, but at the same time the most difficult and obscure, part of the whole of biological chemistry. In spite of the innumerable and careful investigations which have been made of it by the ablest physiologists and chemists in the second half of the nineteenth century, we are still far from a satisfactory solution of this fundamental problem of biology. This is due partly to the extraordinary difficulty of isolating pure living plasm and subjecting it to chemical analysis, and partly to the many errors and misunderstandings that have arisen through one-sided treatment of the subject, and especially through confusion of the chemical and morphological features of plasm. We can thus understand the contradictory views that are still put forward by distinguished chemists and physiologists, zoologists and botanists. As I cannot deal here with the very extensive, elaborate, and contradictory literature of the subject, I must be content to give a brief summary of the conclusions I have reached by my reading and my own studies of plasm (begun in 1859).

To begin with, we must clearly understand that protoplasm—in the most general sense in which we here take it—is a chemical substance, not a "mixture of different substances," or a "mixture of a small quantity of solid matter with a good deal of fluid." As Richard Neumeister very well observes: "We seek the nature of protoplasm in the peculiar processes which take place in its constituent matter. Protoplasm is for us a chemical matter, so pronounced, in fact, that the highest chemical actions that we know of are embodied in it." I must, from my point of view, entirely reject Oscar Hertwig's conception of living matter as a "mixture" of a number of chemical elements; because chemistry applies this phrase to various gases and powdery substances which are completely indifferent to each other—a property which we certainly do not find in the constituents of protoplasm. When we speak of the living matter or protoplasm, the general phrase does not imply that the substance may not have a distinctive composition in each particular case. And when we find many biologists still conceiving protoplasm as a mixture of various substances, the error is generally due to a confusion of the chemical idea with the morphological, and to a belief that certain structural features of the plasm are primary, whereas they are only secondary, products of the vital process itself in the cell-body.

The older biologists who first introduced the name protoplasm and studied it carefully recognized that this living matter belonged to the albuminous (or proteid) group. The many characteristics which distinguish these nitrogenous carbon-compounds from all other chemical compounds—their behavior towards acids and bases, their peculiar color-reaction towards certain salts, their decomposition-products, etc.—are found in all the plasma-substances, and in all the other albuminoids. This is quite in agreement with the results of quantitative analysis. However differently the various plasma-substances behave in detail, they always exhibit the same general composition as the other albuminoids out of the five "organogenetic elements"—namely, in point of weight, fifty-one to fifty-four per cent. carbon, twenty-one to twenty-three per cent. oxygen, fifteen to seventeen per cent. nitrogen, six to seven per cent. hydrogen, and one to two per cent. sulphur. However, there is a good deal of variety and complication in the way in which the atoms of these five elements are combined in albumin and their molecules are grouped. Hence the question of the chemical nature of the plasma-substances compels us now to look for a moment at the larger group of albuminoids to which they belong.

The carbon-compounds which we comprise under the chemical title of the albumins or proteids are the most remarkable, but also, unfortunately, the least known, of all bodies. The attempt to examine them closely encounters extraordinary difficulties, greater than in any other group of chemical compounds. Everybody is familiar with the appearance of ordinary albumin, from the transparent viscous albumin that surrounds the yolk in the hen's egg, and which becomes a white, opaque, and solid mass when it is cooked. However, this special form of albumin, which we can get so easily in any quantity from the eggs of birds and reptiles, is only one of the innumerable kinds of albumin, or species of protein, that are to be found in the bodies of the various animals and plants. Chemists have hitherto tried in vain to master the chemical structure of these obscure protein-compounds. They are only rarely to be found in chemically pure form as crystals. As a rule, they are in the colloid form, or uncrystallized jelly-like masses, which offer a much greater resistance than crystals to the passage through a porous medium by diosmosis (see p. 39). However, although we have not yet succeeded in penetrating the molecular constitution of the albumins, the laborious research of chemists has yielded some general results which are of great importance for our purpose. We have, in the first place, a general idea of their molecular constitution.

Molecules are the smallest homogeneous parts into which a body can be divided without altering its chemical character. Hence the molecules of every chemical compound are made up of two or more atoms of different kinds. The greater the number of atoms in each compound, the higher is its molecular weight. The space between the molecules and their component atoms is filled with imponderable and highly elastic ether. As even the largest molecules occupy only a very tiny space, and remain far below the range of the most powerful microscope, all our ideas of their composition depend on general physical theories and special chemical hypotheses. Nevertheless, stereochemistry, the modern science of the molecular structure of chemical compounds, is not only a perfectly legitimate section of natural philosophy, but it yields the most important conclusions as to the mutual attractions of the elements and the invisible movements of the atoms in combining. It further enables us to calculate approximately the relative size of the molecules and the number of atoms that are grouped together in them. However, the albuminoids present the greatest difficulty of all in this calculation, and their structural features are still very obscure. Nevertheless, science has reached certain general conclusions, which we may formulate in the following propositions:

1. The molecule of albumin is unusually large, and therefore its molecular weight is very high (higher than in most or all other compounds).

2. The number of atoms composing it is very large (probably much more than a thousand).

3. The disposition of the atoms and groups of atoms in the albuminous molecule is very complicated, and at the same time very unstable—that is to say, very changeable and easily altered.

These characters, which are ascribed to all albuminous bodies by modern chemistry, hold good of all plasma-substances; and, in fact, are true in a higher degree of these, as the metabolism of the living matter causes a constant displacement of the atoms. This is caused, according to the view of Franz Hofmeister and others, by the formation of ferments or enzyma—in other words, by catalysators of a colloidal structure. Verworn has, on physiological grounds, given the name of biogens to these plasma-molecules.

The profound insight which comparative anatomy has given us into the significance and nature of organs, and comparative histology into those of the cells, has naturally excited a desire to penetrate in the same way the mystery of the elementary structure of the plasm, the chief active constituent of the cell. The improved methods of modern cytology, and the great progress which this science of the cell owes to the microtome and to microchemistry with its delicate coloring processes, etc., have prompted many observers of the last three decades to study the finest structural features of the elementary organism, and on this foundation build hypotheses as to the elementary structure of protoplasm. In my opinion, all these theoretical ideas, in so far as they would explain the finer structure of pure plasm, have a very serious defect; they relate to microscopic structures which do not belong to the plasm as such (as a chemical body), but to the cell-body (or cytosoma), the chief active constituent of which is certainly the plasm. These microscopic structures are not the efficient causes of the life-process, but products of it. They are phylogenetic outcomes of the manifold differentiations which the originally homogeneous and structureless plasm has undergone in the course of many millions of years. Hence I regard all these "plasma-structures" (the comb, threads, granules, etc.), not as original and primary, but as acquired and secondary. In so far as these structures affect the plasm as such, it must take the name of metaplasm, or a differentiated plasm, modified by the life-process itself. The true protoplasm, or viscous and at first chemically homogeneous substance, cannot, in my opinion, have any anatomic structure. We shall see, when we come to consider the monera, that very simple specimens of such organisms without organs still actually exist.

By far the greater part of the plasm that comes under investigation as active living matter in organisms is metaplasm, or secondary plasm, the originally homogeneous substance of which has acquired definite structures by phyletic differentiations in the course of millions of years. To this modified plasm we must oppose the original simple primary plasm, from the modification of which it has arisen. The name "protoplasm," in the narrower sense, could very properly be retained for this originally homogeneous form of structureless plasm; but, as the term has now almost lost definite meaning and is used in many different senses, it is, perhaps, better to call this pure homogeneous primary plasm archiplasm. It is still found—firstly, in the body of many (but not all) of the monera, part of the chromacea and bacteria, and the protamoeba and protogenes; and, secondly, in the body of many very young protists and tissue-cells. In the latter case, however, there is already a chemical differentiation of the inner caryoplasm and outer cytoplasm. When we examine these young cells under a high power of the microscope, with the aid of the modern coloring methods, their protoplasm seems to be perfectly homogeneous and structureless, or, at the most, there are merely very fine granules regularly distributed in it which are believed to be products of metabolism. This is best seen in many of the rhizopods, especially the amoebÆ, thalamophora, and mycetozoa. There are large amoebÆ, which thrust out strongly mobile feet from their unicellular body, broad, flaplike processes of the naked cell body which constantly change their form, size, and place. If they are killed and examined with the aid of the best methods of coloring, it is quite impossible to detect any structure in them; and this is also true of the pseudopodia of the mycetozoa and many other rhizopods. Moreover, the slow flowing movement of the fluid protoplasm shows clearly that there cannot be any composition out of fine fixed elements in the body. This is particularly clear in those amoebÆ and mycetozoa in which a hyaline, firm, and non-granulated skin-layer (hyaloplasm) is more or less separated from a dark, softer, and granulated marrow-layer (polioplasm); as both of them are viscous and pass into each other without sharp limits, there cannot be any constant and fixed structural features in them.

Organic life—in its lowest and simplest form—is nothing but a form of metabolism, and therefore a purely chemical process. The whole vital activity of the chromacea, the simplest and oldest organisms that we know, is confined to that process of metabolism which we call plasmodomism or carbon-assimilation. The homogeneous and structureless globules of protoplasm, which represent the whole frame of these primitive protophyta (chroococcus, aphanocapsa, etc.) in the simplest conceivable way, expend their whole vital power in the process of self-maintenance. They maintain their individuality by a simple metabolism; they grow by the addition of fresh plasm obtained by it, and they split up into two equal globules of plasm when the growth passes a certain limit—reproduction by clevage, maintenance of the species. Thus these chromacea have neither special organs, or organella, that we can distinguish in their simple plasma-bodies, nor different functions in their life-process; it is wholly taken up with the primitive work of their vegetal metabolism. We shall see later on that this is a purely chemical process, something like catalysis in inorganic combinations; and for this neither special organs nor fine elementary structures in the plasm are needed. The "end" of their existence, self-maintenance, is attained just as simply as in the catalysis of any inorganic compound, or the formation of a crystal in its mother-water.

If we compare this very rudimentary life-process of the monera with that of the highly differentiated protists (diatomes, desmidiacea, radiolaria, and infusoria), the biological distance between them seems to be immense; and it is, naturally, far greater when we extend the comparison to the histona, the highly organized metaphyta and metazoa, in the bodies of which millions of cells co-operate in the work of the various tissues and organs.

In the great majority of cells—either the autonomous cells of the protists or the tissue-cells of the histona—we can detect more or less definite and constant fine structures in the plasm. We must regard these always as phyletic, secondary products of the life-process, and so call the differentiated plasm by the name of metaplasm. The very different interpretations of the microscopic pictures which this metaplasm affords have led to a good deal of controversy. In this the desire to discover in these secondary plasma-structures the first causes of vital action, or the real elementary organella of the cell, has played a great part. The most important of the theories that have been formulated are those of the frothy structure, the skeletal structure, the fibrous structure, and the granulated structure of the plasm. All these theories of structure apply to plasm in general, but particularly to its two chief forms, the caryoplasm of the nucleus and the cytoplasm of the cell-body.

Among the many different attempts to discover a definite structure in living matter, the theory of the frothy structure (also called the honeycomb structure) has lately found the most favor. Otto BÜtschli, of Heidelberg, especially, has endeavored, on the basis of many years of careful study and experiment, to make it the foundation of his view of the plasm. It is undeniable that the living matter of many cells shows a delicate structure which may best be compared with fine soap-suds; innumerable globules are crowded close together in a fluid, and flatten each other by their pressure into polyhedrical shapes. In 1892 BÜtschli artificially produced fine oil-suds by beating up cane sugar or potash in olive oil, and then put a small drop of the stuff in a drop of water under the microscope. The small particles of sugar then exercised an attractive action by diffusion on the particles of water; the latter penetrated into the oily matter, released the sugar, and formed tiny vesicles with it. As the vesicles of sugar do not mix with oil, they look like cavities isolated on all sides, and polyhedrically flattened by mutual pressure. The striking resemblance of this artificially produced "oil soap-suds" to the natural and microscopically visible structures of many kinds of plasm is strengthened from the fact that BÜtschli, Georg Quincke, and others, have also observed similar flowing movements in both; and as these apparently spontaneous movements can be explained physically and reduced to adhesion, imbibition, and other mechanical causes, there seemed a prospect of reducing the "vital" movements of the living and flowing plasm to purely physical forces. Quite recently Ludwig Rhumbler, of GÖttingen, an authority on the rhizopods, has endeavored to give in this sense a Physical analysis of the vital phenomena in the cell. To-day the froth theory is much the most popular of the many attempts to detect a fine plasm-structure as the essential anatomic foundation of an explanation of the physiological functions. It must be noted, however, that frequently very different phenomena are confused under this name, especially the coarser froth-formation by taking up water in the living matter and the invisible hypothetical molecular structure. Both these must be distinguished from the finer plasma-structure which is visible under a powerful microscope; but the limit between them is difficult to determine.

A second view of the finer structure of the plasm, which had been greatly esteemed before the acceptance of the froth theory, was formulated in 1875 by Carl Frommann and Carl Heitzmann, and supported by Leydig, Schwitz, and others. It puts another interpretation on the net-like appearance of the microscopic plasma-structure. It assumes that the plasma consists of a skeleton of fine threads or fibrils combined in the form of a net, and that these spread and cross in the body of the cell which is filled with fluid. It is also compared to a sponge, and is said to have a spongy structure. We can artificially produce such a skeletal structure by, for instance, causing coagulation in a thick solution of glue or albumin by adding alcohol or chromic acid. It is unquestionable that there are these "plasma-skeletons" both in the nucleus and the body of the cell; but they are generally (if not always) secondary products of organization in the elementary organism (or cell-organs), not primitive structures of its plasm. Moreover, an optical transverse action of a froth-structure or honeycomb, examined as a flat surface in the microscope, shows the same configuration as a fine skeleton. We can hardly see any difference between the two. We cannot accept the skeletal formation as a fundamental structure of the plasm.

As we notice very fine threads in the plasm of many cells, both in the caryoplasm of the nucleus and the cytoplasm of the cell body, the cytologist Flemming, of Kiel (1882), believed it was possible to discover them in the plasm of all cells, and based on this his filar theory of plasm. He says that we must distinguish two chemically different kinds of plasm in living matter—the filar (threadlike) and the inter-filar matter. The fine threads of the former are of different lengths, and sometimes run separately, at other times are bound in a sort of net-work (mitoma and paramitoma). In certain conditions of cell-life, especially in indirect cell-division, these filar formations play a great part; and also in the functions of highly differentiated cells, such as the ganglionic cells. But in many cases these plasma threads may be merely parts of a skeletal or frothy structure (honeycomb walls in section). In any case, we cannot regard the thread formation as a general elementary structure of plasm; in my opinion, it is always a secondary phyletic product of living matter, and never a primary feature of it.

Totally different from the three preceding theories of the finer structure of the plasm is the granular theory of Altmann (1890). He supposes that all living matter is originally made up of tiny round granules, and that these independently living bioblasts are the real "elementary organisms," the microscopic ultimate individuals; hence the cells which are formed by the combination of these granules must be looked on as individuals of the second order. Between the granules of the granulated substance (the real active living matter) there is always an inter-granular substance; the granules are regularly distributed and arranged in these. The granules themselves, or the bioblasts, are homogeneous, sometimes globular, and sometimes oval, or of other shapes. However, the distinction between these substances is quite arbitrary, and neither chemically nor morphologically well defined. Under the head of granules Altmann throws together the most different contents of the cell—fat granules, pigment granules, secretory granules, and other products of metabolism. Hence his granular theory is now generally rejected. However, there was a sound idea at the bottom of it—namely, the idea of explaining the vital properties and functions of living matter by small separate constituents which make up the plasm, and move in a viscous medium. But these real elementary parts are not microscopically visible; they belong to the molecular world, which lies far below the limit of microscopic power. In my opinion, Altmann's visible granules, like Flemming's threads and Frommann's skeleton and BÜtschli's honeycomb, are not primary structures, but secondary products of plasma differentiation.

As the special properties and activities of any natural body depend on its chemical constitution, and this is, in the long-run, determined by the composition of its molecules, it is a matter of the greatest interest in biology to form as clear and distinct an idea as possible of the nature and properties of the molecules of plasm. Unfortunately, it is only possible to do this approximately, and to a slight extent. As the hypotheses of modern structural chemistry on the molecular formation of complicated organic compounds are often very unsafe, this is bound to be the case in the highest degree as regards the albuminoids and, the most important of all, the living matter or plasm. We have as yet no knowledge of the fundamental features of its very variable chemical structure. The one thing that bio-chemists have told us about it is that the molecule of plasm is very large, and made up of a great number of atoms (over a thousand); and that these are combined in smaller or larger groups, and are in a state of very unstable equilibrium, so that the life process itself causes constant changes in them.

Since the great problem of heredity was forced by Darwin in 1859 into the foreground of general biology, many different hypotheses and theories of it have been framed. All these have in the end to trace it to molecular features in the plasm of the germ-cells; because it is this germ-plasm of the maternal ovum and the paternal sperm-cell that conveys the characteristics of the parents to the child. Hence the great progress that has been made recently in the study of conception and heredity, by means of a number of remarkable observations and experiments, has been of service to our ideas on the molecular structure of the plasm. I have dealt with the chief of these theories in the ninth chapter of my History of Creation, and must refer the reader thereto. In chronological order we have: (1) the pangenesis theory of Darwin (1868), (2) the perigenesis theory of Haeckel (1875), (3) the idioplasm theory of NÄgeli (1884), (4) the germ-plasm theory of Weismann (1885), and (5) the mutation-theory of De Bries (1889). None of these attempts, and none of the later theories of heredity, has given us a satisfactory and generally admitted idea of the plasma-structure. We are not even clear as to whether in the last resort life is to be traced to the several molecules, or to groups of molecules, in the plasm. With an eye to this latter difference, we may distinguish the plastidule and micellar theories as two different groups of relevant hypotheses.

In my essay on "The Perigenesis of the Plastidules" (1875) I formulated the hypothesis that in the last instance the plastidules are the vehicles of heredity—that is to say, plasma-molecules which have the property of memory. In this I found support in the ingenious theory of the distinguished physiologist, Ewald Hering, who had declared in 1870 that "memory is a general property of organic matter." I do not see still how heredity can be explained without this assumption! The very word "reproduction," which is common to both processes, expresses the common character of psychic memory (as a function of the brain). By plastidules I understand simple molecules; the homogeneous nature of the plasm in the monera (both chromacea and bacteria and rhizomonera) and the primitive simplicity of their life-functions do not dispose us to think that special groups of molecules are to be distinguished in these cases. Max Verworn has recently (1903) formulated his biogen-hypothesis in the same sense, as a "critical-experimental study of the processes in the living matter." He also takes the active plasma-molecules, which he calls biogens, as the ultimate individual factors of the life-process, and is convinced that in the simplest cases the plasm consists of homogeneous biogen-molecules.

The hypothesis of NÄgeli (1884) and Weismann (1885) is totally different from the hypothesis of the plastidules and biogens as simple molecules of the plasm. According to this, the ultimate "vital unities" or individual vehicles of the life-process are not homogeneous plasma-molecules, but groups of molecules, made up of a number of different molecules. NÄgeli calls them micella, and assigns them a crystalline structure. He supposes that these micella are combined chainwise into micellar ropes, and that the variety of the many forms and functions of plasm is due to the different configuration and arrangement of these. Weismann says: "Life can only arise by a definite combination of different kinds of molecules, and all living matter must be made up of these groups of molecules. A single molecule cannot live, can neither assimilate nor grow nor reproduce." I do not see the justice of this observation. All the chemical and physiological properties which Weismann afterwards attributes to his hypothetical biophora may be ascribed to a single molecule just as well as to a group of molecules. In the simplest forms of the monera (both the chromacea and the bacteria) the nature of their rudimentary life can be explained on the one supposition just as well as the other. Naturally, this does not exclude a very complicated chemical structure in the large plastidule or biogen as a single molecule. Verworn's biogen-hypothesis seems to me quite satisfactory when it represents the primitive molecule of living matter as really the ultimate factor of life.

The chief process in the evolutionary history of the plasm is its separation into the inner nuclear matter (caryoplasm) and the outer cellular matter (cytoplasm). When both kinds of plasm arose by differentiation from the originally simple plasm of the monera, there also took place the morphological separation of the nucleus (caryon) and cell-body (cytosoma or celleus). As these two chief forms of living matter are chemically different but nearly related, and as they may in certain circumstances (for instance, during indirect cell-division and the partial caryolysis connected therewith) enter into the closest mutual relations, we must suppose that the original severance of the two substances took place gradually and during a long period of time. It was not by a sudden bound or transformation, but by a gradual and progressive formation of the chemical antithesis of caryoplasm and cytoplasm, that the real nucleated cell (cytos) arose from the unnucleated cytode (or primitive cell). Both may correctly be comprised under the general head of plastids (or formative principles), as "ultimate individualities."

I regard as the chief cause of this important differentiation of the plasm the accumulation of hereditary matter—that is to say, of the internal characteristics of the plastids acquired by ancestors and transmitted to their descendants—within the plastids while their outer portion continued to maintain the intercourse with the outer world. In this way the inner nucleus became the organ of heredity and reproduction, and the outer cell-body the organ of adaptation and nutrition. I put forward this hypothesis in 1866 in my General Morphology: "The two functions of heredity and adaptation seem to be not yet distributed between differentiated substances in the unnucleated cytodes, but to inhere in the whole of the homogeneous mass of the plasm; while in the nucleated cell they are divided between the two active constituents of the cell, the inner nucleus taking over the transmission of hereditary characters and the outer plasm undertaking adaptation, or the accommodation to the features of the environment." This hypothesis was afterwards (1873) confirmed by the discoveries of Strasburger, the brothers Hertwig, and others, with regard to cell-cleavage and fertilization; it is particularly supported by the phenomena of caryokinesis(the movement of the nucleus) in sexual generation. Hence we can understand how it is that in the monera (chromacea and bacteria), which propagate by simple cleavage, there is no sexual generation and no nucleus.

The great significance of the nucleus in the life of the cell, as central organ of heredity, and also probably as "the soul of the cell," depends chiefly on the chemical properties of its albuminous matter, the caryoplasm. This one indispensable nuclear element is chemically akin to the cytoplasm of the cell-body, but differs from it in certain respects. The caryoplasm has a greater affinity for many coloring matters (carmine, hÆmatoxylin, etc.) than the cytoplasm; and the former coagulates more quickly and firmly than the latter through acids (such as acetic and chromic acid). Hence we need only add a drop of diluted (two per cent.) acetic acid to cells that seem homogeneous to make perfectly clear the separation between the inner nucleus and outer body. As a rule, the firmer nucleus then stands out sharply as a globular or oval particle of plasm; occasionally it has other forms (cylindrical, conical, spiral, or branched). The caryoplasm seems to be originally quite homogeneous and structureless, as we find in many of the protists and many young cells of histona (especially young embryos). But in the great majority of cells the caryoplasm is divided into two or more different substances, the chief of them being chromatin and achromin.

The most common division of the caryoplasm in the cells of the animal and plant body, and the one of chief significance for their vital activity, is that into two chemically different substances, which are usually called chromatin (or nuclein) and achromin (or linin). Chromatin has a greater affinity for coloring (chromos) matter (carmine, hÆmatoxylin, etc.), and so this "colorable nuclear matter" is particularly regarded as the vehicle of heredity. The achromin (or achromatin, or linin) is either not at all or less easily colorable, and is akin to the cytoplasm; in direct cell-division it enters into close relations with the latter. Achromin is usually found in the form of slender threads, and hence called "nuclear thread-matter" (linin). Chromatin is generally found in roundish or rod-shaped granules (chromosomata), which exhibit very characteristic changes of form (loop formation, etc.) in indirect cell-division. The chemical, physiological, and morphological difference between chromatin and achromin must not be regarded as an original property of cell nuclei (as is wrongly stated sometimes), but is the outcome of a very early phylogenetic differentiation in the originally homogeneous caryoplasm; and this holds also of two other parts of the nucleus—the nucleolus and centrosoma.

In a good many cells, but by no means universally, we find two other constituents of the nucleus, which owe their rise to a further differentiation of the caryoplasm. The nucleolus is a small globular or oval particle, which may be found singly or in numbers in the nucleus, and behaves somewhat differently towards coloring matter than the closely related chromatin. It has a special affinity for acid aniline colors, gosin, etc. Its substance has, therefore, been distinguished as plastin or paranuclein. The nucleolus is especially found in the tissue-cells of the higher animals and plants as an independent constituent; it is wanting in many of the unicellular protists. The same may be said of the centrosoma, or "central body" of the cell. This is an extremely small granule, on the very limit of visibility, the chemical composition of which is not known very well. We should have paid no attention to this constituent of the cell (distinguished in 1876) if it did not play an important, and perhaps leading, part in indirect cell-division. As the "polar body in the division of the nucleus," the centrosoma exercises a peculiar attraction on the granules distributed in the cytoplasm, which arrange themselves radially about this centre. The centrosomata grow independently and increase by cleavage, like the chromoplasts (chlorophyll particles, etc.). When they have split up, each of the daughter-microsomata acts in turn as a centre of attraction on its half of the cell. However, the great importance which modern cytologists have ascribed to it on this account is discounted by two circumstances. In the first place, we have not succeeded, in spite of all efforts, in discovering a centrosoma in the cells of the higher plants and many of the protists; and, in the second place, a number of recent chemical experiments have succeeded in producing centrosomata artificially (for instance, by the addition of magnesium chloride) in the cytoplasm. Hence many cytologists regard the centrosoma as a secondary product of differentiation in the cell-body, not the nucleus.

Two other parts of the nucleus that we find very often, but by no means universally, in the cells of the animal and plant body are the nuclear membrane (caryotheca) and the nuclear sap (caryolymph). A large number of cells—but not all—have the appearance of vesicles, having a thin skin enclosing a liquid content, the nuclear sap. The achromin then usually forms a frame-work of threads, with chromatin granules in its meshes or knots, within this round vesicle. This very thin nuclear membrane (often only visible as its contour) or caryotheca may be regarded as the result of surface-strain (at the planes of contact of caryoplasm and cytoplasm). The watery and usually clear and transparent nuclear sap (caryolymph) is formed by imbibition of watery fluid (like the frothy structure of the plasm in general). The separation of the nuclear membrane and nuclear sap is not a primary property of the nucleus, but is due to a secondary differentiation in the originally homogeneous caryoplasm.

Like the caryoplasm of the nucleus, the cytoplasm of the cell-body is originally a chemical modification of the simple and once homogeneous plasm (the archiplasm). This is clearly shown by the comparative biology of the protists, their unicellular organism presenting a much greater variety of stages of cell-organization than the subordinate tissue-cells in the bodies of the multicellular histona. However, in the great majority of cells the cytoplasm is separated into several, and frequently very numerous, parts, which have received diverse forms and functions in the division of labor. We then see very conspicuously the regularity of cell-organization, which is altogether wanting in the simple homogeneous plasma granules of the monera. As this great differentiation of the advanced elementary organism is incorrectly generalized by some recent cytologists and described as a universal feature of cells, it is necessary to insist explicitly that it is a secondary phylogenetic development, and is altogether wanting in the primitive organisms. The complexity of the physiological division of labor and the accompanying morphological separation of parts is extremely great in the cytoplasm. When we wish to arrange them in a few large groups from a general point of view, we may distinguish the active plasma-formations from the passive plasma-products; the former are due to a chemical metamorphosis of the living plasm, the latter lifeless excretions from it.

Under the head of plasm-formations, or products of differentiation in the cytoplasm, we comprise all formations that are due to partial metamorphosis of the living cell-body—not lifeless excretions from it, but living parts of its substance, undertaking special functions, and therefore chemically and morphologically differentiated from the primary cytoplasm. One of the commonest differentiations of this kind is the separation of the firm hyaline skin-layer (hyaloplasm) from the softer granular marrow-layer (polioplasm); though the two often pass into each other without clear limits. In most plant-cells special granules of plasm, mostly globular or roundish, are developed, called trophoplasts, and these undertake the work of metabolism. To this class belong the amyloplasts, which produce starch (amylum), the chloroplasts or chlorophyll-granules which form the green matter (chlorophyll) in the leaf, and the chromoplasts which form color-crystals of various sorts. In the cells of the higher animals the myoplasts form the special contractile tissue of the muscles, and the neuroplasts the psychic tissue of the nerve-matter. On the other hand, the distinction between the body-plasm (somoplasma) and the germ-plasm (germoplasma), which serves as the base of Weismann's untenable theory of the germ-plasm (cf. chapter xvi.), is purely hypothetical and without direct observation to support it.

The infinite variety of parts of the cell which arise as excretions of the living active cytoplasm, and so must be regarded as lifeless plasma-products, may be divided into two chief groups—internal and external. The former are stored within the living cytoplasm, the latter thrust out from it.

Internal plasma-products of common occurrence are the microsomata, very small and opaque particles which are generally regarded as products of metabolism. They consist sometimes of fat, sometimes of derivatives of albumin, sometimes of other substances of which we do not know the chemical composition. The same may be said of the large and variously-colored pigment-granules, which are very common and determine the color of tissues. Also very common in the cytoplasm are large accumulations of fat in the shape of oil-globules, fat-crystals, etc., besides other crystals of a very different sort, partly organic crystals (for instance, albuminous crystals in the aleuron-granules of plants), partly inorganic crystals (for instance, of oxalic-acid salts in many plant-cells, of calcareous salts in many animal-cells). The watery cell-sap (cytolymph) plays an important part in many of the larger cells. It is formed by the accumulation of fluid in the cytoplasm, and is found in its frothy structure. The large empty spaces which it forms are called vacuoles, with very regularly disposed alveoles. When the cell-sap gathers in great abundance within the cell, we get the large vesicular cells which are found in the tissues of the higher plants, the cartilages, etc.

As external excretions of the living cytoplasm that have acquired some importance, especially as protective organs, in the majority of cells, we have first of all the cell-membranes, the firm capsules or protective skins which enclose the soft cell-body, like a snail in its house. In the first period of the cell-theory (1838-1859) such an integument was ascribed to all cells, and often regarded as their chief constituent; but it was discovered afterwards that this protective skin is altogether wanting in many (especially animal) cells, and that it is not found in many when they are young, but grows subsequently. We now distinguish between naked cells (gymnocytes) and covered cells (thecocytes). As examples of naked cells we have the amoebÆ, and many of the infusoria, the spores of algÆ, the spermatozoa, and many animal tissue-cells.

The cell-covering (cytotheca) varies very much in size, shape, composition, and chemical character, especially in the rhizopods among the unicellular protists. The flint shells of the radiolaria and diatomes, the chalky cells of the thalamophora and calcocytea, the cellulose shells of the desmidiacea and syphonea, show the extraordinary plasticity of the constructive cytoplasm (cf. chapter viii.). Among the histona the tissue-plants are remarkable for the infinite variety of shape and differentiation of their cellulose capsules. The familiar properties of wood, cork, bast, the hard shells of fruit, etc., are due to the manifold chemical modification and morphological differentiation which the cellulose membrane undergoes in the tissues of plants. This is less frequently seen in the tissues of animals; but, on the other hand, the intercellular and the cuticular matter play a greater part in these.

The intercellular matter, an important external plasma-product, is formed by the social cells in the tissues of the histona thrusting out in common firm protective membranes. These protective structures are very common among communities of protists, in the form of masses of jelly, in which a number of cells of the same kind are united; such are the zoogloea of many of the bacteria and chromacea, the common jelly-like envelope of the volvocina and many diatomes, and the globular cell-communities of the polycyttaria (or social radiolaria). The chief part is played by intercellular matter in the body of the higher animals, in the form of mesenchyma-tissue; the connecting tissue, cartilages, and bones owe their peculiar property to the amount and quality of the intercellular matter that is deposited between the social cells.

When the socially joined epidermic cells at the surface of the tissue-body thrust forth in common a protective covering, we get the cuticles, which are often thick and solid armor-plates. In many of the metaphyta wax and flinty matter are deposited in the cellulose cuticles. The strongest formation is found in the invertebrate animals, where the cuticle often determines the whole shape and articulation, as in the calcareous shells of mollusks (mussel-shells, snail-shells, cockle-shells, etc.); and especially the coats of the articulata (the crab's coat of mail, and the skins of spiders and insects).

VII