Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

Schwann found it easy to demonstrate the cellular nature of the tissues of his first three classes. With the other two classes he had more difficulty. Fibres of all kinds, he considered, arose by an elongation of cells, which afterwards split longitudinally into long strips, forming as the case might be white or elastic fibrous tissue. Muscle-fibres and nerve-fibres were formed in a totally different way, by coalescence of cells; each separate muscle-fibre and nerve-fibre was thus a compound cell. Capillaries, Schwann held, were formed by cells hollowed out like drain-pipes, and set end to end—a mistaken view soon corrected by Vogt (Embryologie des Salmones, p. 206, 1842).

In this detail part of his book Schwann accumulates material for a general theory of the cell which he develops in the third and last section. Taking up the physiological or dynamical standpoint, he points out that one process is common to all growth and development of tissues both in animals and plants, namely, the formation of cells, a process which he conceives to take place in the following manner. There is, first of all, a structureless substance, the cytoblastem, the matrix in which all cells originate. The cytoblastem may be either inside the cells, or, more usually, in the spaces between them. It is not a substance of definite chemical and physical properties, for the matrix of cartilage and the plasma of the blood alike come within the definition. It has largely the significance of food material for the developing cells. In plants, according to Schleiden, cells are never formed in the intercellular substance—the cytoblastem is within the cells; but extracellular cell formation seems to be the general rule in animals. An intracellular formation of cells occurs only in the ovum, in cartilage cells and chorda cells and in a few others, and even there it is not the exclusive method of formation; a formation of cells within cells never occurs in muscles and nerves, nor in fibrous tissue (p. 204). In the cytoblastem granules appear, which gradually increase in size and take on the characteristic shape of nuclei; round each of these a young cell is formed. Sometimes the young cells appear to have no nuclei, as in the intracellular brood of chorda cells, but, as a rule, a nucleus is clearly visible. The nucleus is indeed the most characteristic constituent of the cell. "The most important and most constant criterion of the existence of a cell is the presence or absence of the nucleus," writes Schwann near the beginning of his book (p. 43).

As a general rule the nucleolus is formed first, and round it by a sort of condensation or concretion the nucleus, which is frequently hollow, and round this again, by a somewhat similar process, the cell. "The whole process of the formation of a cell consists in the precipitation round a small previously formed corpuscle (the nucleolus) of first one layer (the nucleus) and then later round this a second layer (the cell substance)" (p. 213). The outermost layer of the cell usually thickens to form the membrane, but this membrane formation does not always occur, and the membrane is not present in all cells. The nucleus is formed in exactly the same manner as the cell, and it might with much truth itself be called a cell—a cell of the first order, while ordinary nucleated cells might be designated cells of the second order (p. 212). In anucleate cells there is probably only a single process of layer formation round an infinitely small nucleolus. In almost all nucleate cells the nucleus is resorbed when the cell reaches its full development, and it is larger and more important the younger the cell is.

The cell was for Schwann not a morphological concept at all, but a physiological; the cell was a dynamical, not a statical unit. Cell-formation was the process at the back of all production of life, and cells were the centres of all vital activity. Each cell was itself an organism, and its life and activities were to some extent independent of the lives and activities of all the other cells. The multicellular organism was a colony of unicellular organisms, and its life was a sum of the lives of its constituent elements. This "theory of the organism," which holds so important a place in biology even at the present day, is developed by Schwann in the concluding pages of his book.

He begins by contrasting the teleological with the materialistic conception of living things. In the teleological view, a special force works in the living organism, guiding and directing its activities towards a purposeful end. According to the materialistic view there are no other forces at work in the living organism than those which act in the inorganic realm, or at least there are none but forces at one with these in their blindness and necessity. True, the purposiveness of living processes cannot be denied; but its ground lies, according to this view, not in a vital force which guides and rules the individual life, but in the original creation and collocation of matter according to a rational plan. The purposiveness of life is part of the purposiveness of the universe; just as the stars circle for ever in harmoniously adjusted paths, so do the processes of life work together towards a common end. Both are the inevitable result of the original distribution of matter in the primitive chaos, a distribution fixed by a rational and foreknowing Being (p. 222).

Which of the two conceptions is to be adopted in biology? Teleological explanations have long been banished from the physical sciences, and in biology they are only a last resort when physical explanations have proved incomplete (p. 223). And if the ground of the purposiveness of living Nature is the same as the ground of the purposiveness of the universe, is it not reasonable to suppose that explanations which have proved satisfactory for inorganic things will in time with sufficient knowledge prove adequate also for organic things?

The teleological conception, again, leads to difficulties particularly when it is applied to the facts of reproduction. If we suppose that a vital force unifies and coordinates the organism and is its very essence, we must also suppose that this force is divisible and that a part of it—separated in reproduction—can bring about the same results as the whole. If on the contrary the forces having play in the organism are the mere result of the particular combination of the matter composing it, the reconstruction of a particular combination of molecules in the ovum is all that is necessary to set development a-going along exactly the course taken by the ovum of the parent. Another argument against the teleological view is derived from the facts of the cell-theory. The cell-theory tells us that the molecules of the living body are not immediately built up in manifold combinations to form the organism, but are formed first into unit-constructions or cells, and that these units of composition are invariably formed in all development, of plants and animals alike, however diverse the goal of development may be. If there were a vital principle would we not expect to find that, scorning this roundabout way of reaching its goal, it went straight to the mark, taking a different and distinctive course for each individual development, building up the organism direct without the intermediary of cells? But since there is a universal principle of development, namely, the formation of cells, does it not seem that the cells must be the true organisms, that the whole "individual" organism must be an aggregate of cells, and that the concept of individuality applied to the organism is accordingly a logical fiction? And it is just upon this notion of the individuality of the organism that the teleological concept is based. The teleological view can perhaps not be completely refuted until the adequacy of materialistic explanations has been finally shown; but it is certain that the most promising method for research is the materialistic (p. 226).

"We start out then from the assumption that the basis of the organism is not a force acting according to a definite plan; on the contrary, the organism arises through the action of blind and necessary laws, of forces which are as much implicit in matter as those of the inorganic world. Since the chemical elements in organic Nature differ in no way from those of inorganic Nature, the ground or cause of organic phenomena can consist only in a different mode of combination of matter, either in a peculiar mode of combination of the elementary atoms to form atoms of the second order, or in the particular arrangement of these compound molecules to form the separate morphological units of the organism or the whole organism itself" (p. 226). Accepting then the materialistic conception of the organism, we have to consider this further problem. Does the ground of organic processes lie in the whole organism or in its elementary parts? Translated into terms of metabolism—note the physiological point of view—the question runs, are metabolic processes the result of the molecular construction of the organism as a whole, or does the centre of metabolic activity lie in the cell? Is it the cell rather than the organism that is the immediate agent of assimilatory processes? In the first alternative the cause of the growth of the constituent parts lies in the totality of the organism; in the other alternative:—"Growth is not the result of a force having its ground in the organism as a whole, but each of the elementary parts possesses a force of its own, a life of its own, if you will; that is to say, in each elementary part the molecules are so combined as to set free a force whereby the cell is enabled to attract new molecules and so to grow, and the whole organism exists only through the reciprocal action of the single elementary parts.... In this eventuality it is the elementary parts that form the active element in nutrition, and the totality of the organism can be indeed a condition, but on this view it cannot be a cause" (p. 227).

To help in the decision of this question, appeal must be made to the facts established as to the cellular nature of the organism and of its reproductive elements. We know that every organism is composed of cells, which are formed and grow according to the same laws wherever they are found, whose formation therefore is everywhere due to the same forces. If we find that certain of these cells—all of which we know to be essentially identical one with another—have the power when separated from the others of growing and developing into new organisms, we can infer that not only such cells but also all other cells have this assimilatory power. The ova of animals, the spores of plants, the isolated cells of lower organisms in general, all show the power of separate assimilation and development. "We must therefore, in general, ascribe to the cell an individual life, that is to say, the combination of the molecules in the single cell does suffice to produce the force whereby the cell is enabled to draw to itself new molecules. The ground of nutrition and growth lies not in the organism as a whole, but in the separate elementary parts, the cells. The fact that it is not every cell that can continue to grow when separated from the organism is not in itself an objection to this theory, any more than it is an objection to the individual life of a bee that it cannot continue to exist apart from the swarm. The activation of the forces existing within the cell depends on conditions which the cell encounters only in connection with the whole" (pp. 228-9).

Schwann's next step is to discover what are the essential forces active in the cell, and here he enters the realm of hypothesis. He finds they can be reduced to two—an attractive force and a metabolic force. The attractive force is seen in the process of cell-formation, where first of all the nucleolus is formed by a concentration and precipitation of substances found free in the cytoblastem, and in the same way the nucleus and later the cell are laid down as concentric precipitates from the cytoblastem. Cell-formation also involves the second or metabolic force, by means of which the cell alters the chemical composition of the medium surrounding it so as to prepare it for assimilation. Schwann's attractive force brings about the actual taking up of the prepared substance; his metabolic force is the cause of the digestion of food substances, and is nearly identical with enzyme action. With what inorganic process, he now asks (p. 239), can the process of cell-formation be most nearly compared, and the answer obviously is, with the process of crystallisation. Cells are, it is true, quite different in shape and consistency from crystals, and they grow by intussusception, not by apposition—their plastic or attractive forces seem therefore to be different. A still more important difference is that the metabolic force is peculiar to the cell. Yet there are important analogies between crystals and cells. They agree in the important respect that they both grow in solutions at the cost of the dissolved substance, according to definite laws, and develop a definite and characteristic shape. It might even be maintained, Schwann thinks, that the attractive force of crystals is really identical with that of cells, and that the difference in result is due merely to the difference between the substance of the cell and the substance of the crystal. He points out how organic bodies are remarkable for their powers of imbibition, and he seeks to show that the cell is the form under which a body capable of imbibition must necessarily crystallise, and that the organism is an aggregate of such imbibition-crystals. The analogy between crystallisation and cell-formation he works out in the following manner:—"The substance of which cells are composed possesses the power of chemically transforming the substance with which it is in immediate contact, in somewhat the same way as the well-known preparation of platinum changes alcohol into acetic acid. Each part of the cell possesses this property. If now the cytoblastem is altered by an already formed cell in such a way that a substance is formed that cannot become part of the cell, it crystallises out first as the nucleolus of a new cell. This in its turn alters the composition of the cytoblastem. A part of the transfomed substance may remain in solution in the cytoblastem or may crystallise out as the beginning of a new cell; another part, the cell-substance, crystallises round the nucleolus. The cell-substance is either soluble in the cytoblastem and crystallises out only when the latter is saturated with it, or it is insoluble and crystallises as soon as it is formed, according to the aforementioned laws of the crystallisation of imbibition-bodies; it forms thus one or more layers round the nucleolus, etc. If one imagines cell-formation to take place in this way, one is led to think of the plastic force of the cell as identical with the force by means of which a crystal grows" (pp. 249-50).