From these virtual bionta (parts of the body that may grow into whole organisms) we must distinguish the partial bionta which have not this property. These are separated parts of the body that live for a time after being cut off from the whole organism, but then die off. Thus, for instance, the heart of a tortoise beats for a long time after being cut out. A flower that has been plucked may, if put in water, keep fresh and alive for many days. In some highly organized cephalopods one of the eight arms of the male develops into an independent body, swims about, and accomplishes the fertilization of the female (hectocotylus among the argonauta, philonexis, etc.). It was at first thought to be an independent animal parasite. The same thing happens with the remarkable foldlike dorsal appendages of a large naked snail (thetys), which get detached and creep about. The body of many of the lower animals may be cut in pieces and yet may live for weeks. The life-properties of these partial bionta are important in view of the general question of the nature of life and its apparent unity in most of the higher organisms. As a fact, even here the cells and organs lead their separate individual life, though they are subordinate to and dependent on the whole.
It has been attempted to answer this question of organic individuality in the sense of counting all organisms individuals which develop from a single fertilized ovum. Thus, the Italian botanist Gallesio, in 1816, regarded all plants that arise by asexual generation (budding or segmentation)—sprouts, branches, slips, bulbs, etc.—as merely portions of a single individual that came from an egg (the seed). So also Huxley, in 1855, considered the sum of all the animals that have been produced by asexual propagation, but from a single sexually generated animal, to be parts of one individual. In practice, however, this principle is useless. We should have to say that the millions of plant-lice which arise parthenogenetically from unfertilized germ-cells, but are originally descended from one impregnated ovum, are one single individual; so also all the weeping-willows in Europe, because they all came from shoots of one single sexually-produced tree.
Many attempts have been made in the course of the nineteenth century to give a generally satisfactory answer to this difficult question of the content and connotation of the idea of the organic individual. None of these has found general favor. I have compared and criticised them in the third book of my General Morphology. I there paid special attention to the views of Goethe, Alexander Braun, and Nägeli among the botanists, and Johannes Müller, Leuckart, and Victor Carus among the zoologists. When we consider the striking divergence of the views of such distinguished scientists and thinkers on so important a biological question, we can understand that opinions are still very divided to-day. Hence we must not be too hard on the metaphysical philosophers when—in complete ignorance of the real facts—they rear the most extraordinary theories in their airy speculations on "the principle of individuation". Compare, for instance, the opinions of the school-men and those of recent thinkers such as Arthur Schopenhauer and Edward Hartmann. As a rule, the psychological side of the problem—the question of the individual soul—is very prominent, without much attention being paid to its material substratum—the anatomic basis of the organism. Many metaphysicians, who, in their one-sided anthropism, make man here also the measure of all things, would assign personal consciousness as the basis of the idea of individuality. It is obvious that this is not a practicable test even for the higher animals, to say nothing of the lower animals and plants. In these we have a far greater variety of individuality on the one hand, and a far greater simplicity of construction on the other. I have tried to show, in my essay on "The Individuality of the Animal Body" (1878), the easiest way to answer these complicated tectological questions, and to support it by the science of structure. It suffices to distinguish the three chief stages I have mentioned, and to explain clearly their physiological significance on the one hand and morphological on the other. We will therefore consider the cell first, then the person (or sprout), and, finally, the stock (or cormus).
Ever since the middle of the nineteenth century the cell theory has been generally and rightly considered one of the most important theories in biology. Every anatomical, histological, physiological, and ontogenetic work must build on the idea of the cell as the elementary organism. Nevertheless, we are still very far from having a general and clear agreement as to this universal and fundamental idea. On the contrary, the ablest biologists still differ considerably as to the nature of the cell or the elementary individual, its relation to the whole of the multicellular organism, and so on. This divergence of views is partly due to the intricacy of the phenomena we find in the life of the cell, and partly to the many and extensive changes that have been made in the meaning of the term in the course of its employment. Let us first cast a glance at the various stages of its history.
When in the last third of the seventeenth century a number of scientists, especially Malpighi in Italy and Crew in England, used the microscope for the first time in the anatomic study of plant structure, they noticed a certain build of the tissue that closely resembled the honeycomb. The closely packed wax cells, filled with honey, of the hive, which show a hexagonal appearance in section, are like the wood cells that contain the sap in the plant. It was the great merit of Schleiden, the real founder of the cell theory, to prove that all the different tissues of plants are originally composed of such cells (1838). Theodor Schwann soon afterwards proved the same for the animal tissues; in 1839 he extended the theory to the whole organic world. Both these scientists regarded the cell as essentially a vesicle, the firm membrane of which enclosed a fluid content, and a solid smaller body inside this, which R. Brown had recognized as the nucleus in 1833. They compared the cell, as a microscopic individual, to an organic crystal, and thought it arose by a sort of crystallization in an organic medium (cytoblastema); in this the central nucleus would serve as starting-point like the nucleus of the crystal.
In the first twenty years (1839-59) of the cell theory it was a fixed principle that there were three essential parts of the cell. Firstly, there was the strong outer membrane, which was not only regarded as a protective covering, but also credited with a great deal of importance as an element in the building of the organism. In the second place, there was the fluid or semi-fluid content (the sap); and, thirdly, the firm nucleus enclosed in the sap. In order to give a clearer idea of the relative thickness and disposition of these parts, the cell was compared to a cherry or a plum. The soft flesh of this fruit (corresponding to the cell sap) can, with difficulty, be separated from the external firm skin or from the hard stone within. A great step in advance was made in 1860, when Max Schultze showed that the external membrane was an unessential and secondarily formed part of the cell. It is, as a fact, altogether wanting in many, especially young, cells of the animal body. They are naked cells without any membrane. The distinguished anatomist also proved that the so-called "cell sap"—the real body of the cell—is not a simple fluid, but a viscous, albuminous substance, the independent movements of which had long been known in the rhizopods, and which the first to study it carefully, Felix Dujardin, had described as sarcode in 1835. Max Schultze further showed that this "sarcode" was identical with the "cell mucus" of the plant cells which Hugo Mohl had designated "protoplasm" in 1846, and that this living matter must be regarded as the real vehicle of the phenomena of life. As the membrane was now recognized to be non-essential, of secondary growth, and completely wanting in some cases, there remained only two essential parts of the cell—the outer soft cell body, consisting of protoplasm, and the inner firm nucleus, consisting of a similar substance called nuclein. The original naked cell was now like a cherry or plum without the skin. This new idea of the cell, formulated forty years ago, which I endeavored to confirm in my monograph on the radiolaria (1862), is now generally accepted, and the cell is defined as a granule or particle of protoplasm (= cytoplasm) enclosing a firm and definite nucleus (or caryon, consisting of caryoplasm).
This would be a good occasion to glance at the errors to which microscopic investigation and the conclusions based on it are liable. Although Kölliker in 1845, and Remak in 1851, had drawn attention to the existence of naked cells, and had compared their movements (for instance, in lymph-cells) to those of the protoplasm in plant-cells, the majority of the leading microscopists clung for twenty years to the dogma that every cell must have a membrane; the definite outline which even a naked cell must show in a different refracting medium was taken to be the sign of a special and anatomically separable membrane. It would be just as correct to talk of a protective membrane on a homogeneous glass ball; its outline is sharply defined. In the long controversy that "exact" observers sustained as to the presence or absence of a membrane, this optical error—the false interpretation of a sharp contour—counted for a good deal. It is much the same with other conflicts of "exact" observers who give their "certain observations" as facts, whereas they are really inferences from imperfect observations on which different interpretations may be put.
Forty years ago (1864) I tried in vain to detect a nucleus in the naked, living, mobile protoplasm of a few small rhizopod-like protists (protamœba and protogenes). Other observers, who afterwards studied similar unnucleated cells (Gruber, Cienkowski, and others), were no more successful. On the ground of these observations, which were often repeated afterwards, I formed the class of the monera—the simplest unnucleated organisms—in my General Morphology in 1866, and pointed out their great importance in solving some of the chief problems of biology. This importance has been much enhanced of late, since the chromacea and bacteria have also been recognized as unnucleated cells. Bütschli has, it is true, raised the objection that their homogeneous plasma-body behaves, not as cytoplasm, but as caryoplasm (or nuclein), and so that these simplest plastids correspond, not to the cell-body, but to the nucleus of other cells. On this view the bacteria and chromacea are not cells without nuclei, but nuclei without cell-bodies. This idea agrees with my own in conceiving the plasma-body of the monera (apart from its molecular structure) as homogeneous and not yet advanced as far as the characteristic differentiation of inner nucleus and outer cell-body. Bearing in mind that these essential parts of the cell (in the view of most cytologists) are chemically related yet different from each other, we have three possible cases of the original formation of the nucleated cell from the unnucleated cytode: (i) The nucleus and cell-body have arisen by differentiation of a homogeneous plasm (monera); (2) the cell-body is a secondary growth from the primary nucleus; (3) the nucleus is a secondary development from the cell-body.
On the first view, which I hold, the plasm, or living matter, of the earliest organisms on the earth (which can only be conceived as archigonous monera) was a homogeneous plasson or archiplasm—that is to say, a plasma-compound that was not yet differentiated into outer cytoplasm and inner caryoplasm. The rise of this chemical distinction—and the accompanying morphological division of cell-body and nucleus—was due to a phyletic differentiation; it was the outcome of a very early and most important division of labor. The hereditary matter gathered in the nucleus, the outer cell-matter controlling the intercourse with the external world. Thus, by this first ergonomy, the nucleus became the vehicle of heredity and the cell-body the organ of adaptation. Opposed to this view is the second, the hypothesis which the founder of the cell-theory, Schleiden, had put forward—that the nucleus is the original base of the cell, and the cell-body a secondary development from it. This opinion (which, in the main, corresponds to that of Bütschli) raises a number of difficulties; as does also the third hypothesis, that the unnucleated "protoplasm-body" (the outer cytoplasm-body) is the original formation, and that the nucleus arose secondarily by condensation and chemical modification of it. At the bottom, however, the difference between the three hypotheses on the primary cytogenesis is not as great as it seems at first sight. However, I am more inclined to adhere to the first; it supposes that the physiological and chemical differences between nucleus and cell-body, which afterwards became so important, were not originally present. The phenomena of caryolysis in indirect cell-division show us still how close are the relations of the two substances.