In the first place, that vital activity does not result from the fusion of two vital activities neither of which is rational. It results after the nuclei come together, by particular creation, and replaces their activity—the generation of the last vital force is the corruption of the first that existed in the separate nuclei, not a derivative of that first force. Again, when the embryo is in the two, four, eight cell stage, and so on, there are not two, four, eight vital principles present, but one. Substantial unity is essential to life of any kind, no matter how low its grade; and if each cell had an independent vital principle, any form of resultant life in the mass would be impossible. An aggregation has no unity of substance; there would be as many substances or natures as there are individual beings in the aggregate, no matter whether ordered or in a mob, consequently no life at all as a life.
The embryo in the two-cell stage is not made up of two independent organisms, any more than the right and left halves of an adult man are two independent organisms. The cells in the two-cell stage of the embryo are the right and left halves of the body, not two individuals, as has been proved repeatedly by biologists. Roux[42] punctured with a hot needle one of the cells in the two-cell stage of a frog embryo without killing the embryo, and it grew into a half-frog larva. Analogous results were obtained by operating in the four-cell stage. Later, Pflüger, Schultze, Enders, and Morgan corroborated the work of Roux. Newport[43] discovered this fact sixty years ago.
In analyzing the structure and functions of the individual cell we regard it as an independent elementary organic unit, but this view is solely a matter of convenience, almost a convention. All the billions of cell's in an adult man are inseparable parts of the single living person. No cell exists as an independent organism in multicellular animals, except the germ-cells, and these only after separation from the gland of origin. Indeed, the biological theory of heredity, already mentioned here, wherein the germ-cell is supposed to carry forward the entire heredity, is now changing toward the view which makes all the somatic cells influence the germ-cells; that is, the body-mass of cells sends on heredity through the germ-cell as the instrument. Adult organisms do not make cells de novo. New cells are formed by division from preëxisting cells, but some biologists think the body-cells so affect the new germ-cells as to influence heredity.
The cells are organs, nodal points, of a single formative power which pervades the mass of cells as a whole. The protoplasm of each cell is not only in direct apposition with its neighbors, but nearly all biologists are now inclining to the opinion, which Heitzmann proposed in 1873, that division of cell from cell is incomplete in nearly all forms of tissue; and that even where cell-walls are present (an exceptional condition in mammals) they are traversed by strands of protoplasm, by means of which the cells are in organic continuity. The whole body, he contended, is thus a syncytium (a mass of continuous protoplasm stippled with nuclei), with the cells as mere nodal points in an almost homogeneous protoplasmic mass. There are cell-bridges between the sieve-tubes of plants. In 1879 Tangl discovered such connection between the endosperm cells of plants, and later Gardiner, Kienitz-Gerloff, A. Meyer, and many others demonstrated that in nearly all plant tissues the cell-walls are connected by intracellular bridges. Ranvier, Bizzozero, Retzius, Fleming, Pfitzner, and many other observers have found these protoplasmic bridges in animal epithelium. In the skin of a larval salamander they are quite conspicuous. They are known to occur also in smooth muscle-fibre, in cartilage cells, in connective-tissue cells, and in some nerve-cells. Harrison found, in 1908, that in frogs the nerve-fibres develop out of these intracellular bridges. Dendy in 1888, Retzius in 1889, and Palladino in 1890 have shown that the follicle cells of the ovary are connected by protoplasmic bridges, not only with one another, but also with the ovum; and similar connection between somatic cells and germ-cells has been found in a number of plants. Thus even the germ-cell is not independent until it has actually broken away from the gland. A. Meyer holds that both the plant and animal individual are continuous masses of protoplasm, in which the cytoplasmic substance forms a morphological unit, no matter what the cell is. That opinion is not finally settled as regards the animal after the fetal stage, but it is much stronger as regards embryos. In the early stages of many arthropods it is certain that the whole embryo is at first an unmistakable syncytium. This is almost established also for Amphioxus, the Echinoderm Volvox, and other animals. Adam Sedgwick holds that it is true for vertebrates up to a late embryonic stage. Mitosis, then, is a form of growth of a mass, not a generation of new individuals.
Whether chromatin or any other element in the germ-cell be the idioplasm in which heredity inheres, differentiation is a progressive transformation, through physical and chemical changes, of the substance of the ovum, and this transformation occurs in a definite order and a definite distribution in the ovum. The changes result in a cleavage of the egg into cells, the boundaries of which sharply mark the areas of differentiation. These cells take on specific characters. In the four-celled stage of an annelid egg these four cells contribute equally to the formation of the alimentary canal and the cephalic nervous system, but only one of them, the left-hand posterior cell, gives rise to the nervous system of the trunk and to the muscles, connective tissues, and germ-cells. The relation between the four original cells, or blastomeres, and the adult parts arising from them, is not fixed, because in some eggs these relations may be artificially changed. A portion of the egg which normally would develop into a fragment of the body will, if split off from the others, give rise to an entire body of a diminished size.
Conklin says[44] that in the ascidian Styela "there are four or five substances in the egg which differ in color, so that their distribution to different regions of the egg and to different cleavage cells may be easily followed, and even photographed, while in the living condition. The peripheral layer of protoplasm is yellow and it gathers at the lower pole of the egg, where the sperm enters, forming a yellow cap. This yellow substance then moves, following the sperm nucleus, up to the equator of the egg on the posterior side, and there forms a yellow crescent extending around the posterior side of the egg. On the anterior side of the egg a gray crescent is formed in a somewhat similar manner, and at the lower pole between these two crescents is a slate-blue substance, while at the upper pole is an area of colorless protoplasm. The yellow crescent goes into cleavage cells which become muscle and mesoderm, the gray crescent into cells which become nervous system and notochord, the slate-blue substance into endoderm cells, and the colorless substance into ectoderm cells. Thus within a few minutes after the fertilization of the egg, and before or immediately after the first cleavage, the anterior and posterior, dorsal and ventral, right and left poles are clearly distinguishable, and the substances which will give rise to ectoderm, endoderm, mesoderm, muscles, notochord, and nervous system are plainly visible in their characteristic positions." Conklin followed these cells in every division until the embryo was developed, making a complete genealogy up to the ovum proper.
De Vries[45] assumed that the character of each cell is determined by "Pangens" that migrate from the nucleus into the protoplasm. Driesch and Oscar Hertwig held that the peculiar development of a given blastomere is a result of its relation to the remainder of the cell-mass, an outcome of the action upon it by the whole system of cells of which it is a part. Hertwig said:[46] "Each of the first two blastomeres contains the formative and differentiating forces not simply for the production of a half-body, but for the entire organism; the left blastomere develops into the left half of the body only because it is placed in relation to a right blastomere." Wilson[47] and Driesch[48] came to the same conclusion about the time Hertwig wrote. Driesch said:[49] "The relative position of a blastomere in the whole determines in general what develops from it; if its position be changed it gives rise to something different; in other words, its prospective value is a function of its position."
A discussion of this matter will be found in Wilson,[50] but the many experiments made in the study of this subject show conclusively that the cells, singly, grouped, and in mass, are a morphological unit, not an aggregation of distinct individuals. They are not, of course, absolutely homogeneous, because such a body could not have organs. The substantial form, therefore, is not confined to the first cell.
The cell-mass, then, has a unity sufficient to be the receptacle of a human vital principle; again, the basic vital operation of the human body at any age is metabolism, and this is actually carried on in the first somatic cell of the embryo as in the cells of the adult man. In the development of the human body in the embryonal stage the energy of cell-division is most intense in the early cleavage stage, and this diminishes as the limit of growth approaches because further division is not needed. When that limit is attained a more or less definite equilibrium is established. Some of the cells in the fully formed body cease to divide, the nerve-cells, for example; others divide under special conditions, as the blood-cells, the connective-tissue cells, gland-cells, epithelial and muscle cells; others continue to divide throughout life and thus replace worn-out cells of the same tissue, as the Malpighian layer of the skin. Cells grow, divide, function, reproduce themselves, and so on, all through their vital activity, sustained by the material brought to them by the blood. Weismann[51] and other biologists think that the vital processes of the higher animals are accompanied by a renewal of the morphological elements in most tissues. The material is carried to the fetus in the womb by various agents, but mostly by the maternal blood after the embryo uses up the yolk; and when the fetal circulation has been established the nutritive material is taken from the maternal blood into the fetal circulation through the placenta, and then carried to the cells by the fetal circulation itself. After the child has been born the stomach and intestines take in the food. The stomach does very little with it except in a preparatory manner; the intestines further prepare it, pass it into the body, where it is again modified by other organs, and finally it is carried by the blood to the cells. The cells really use it; the other organs are the farmers, grocers, railways, and the like; the cells are the consumers. So far as the essential processes are concerned, the embryological cells act as do the adult cells.
The first cell has contractility, protoplasmic motion; it can absorb perfectly all food-stuffs necessary for it from the deutoplasm of the ovum, and the water that passes in from without to the ovum. In a few days the embryonic cells have used up the deutoplasm and are taking up food from the maternal blood as perfectly as any adult cell does, and are exercising their function of building up and sustaining whatever part of the body they are destined for; and this with all the complicated metabolism of the adult cell. Cell metabolism is the fundamental, chief, organic act of any human body at any age. That the embryo does this impelled by the virtus formativa transmitted from the parents is a mere gratuitous assumption to fit the theory that the embryonic cell lacks organic power. The fundamental organ that conserves the body in its very existence under the government of the soul is the apparatus which effects metabolism. Incessant chemico-vital change is a characteristic of all living substances, from the single cell up to the adult man; and in all cases this activity has to do with a transformation of the complex molecules which build up the protoplasm or are associated with its operations. The totality of the chemical changes, or exchanges, in living cells, the transformation of unorganized food materials so that these may be assimilated, and the chemical processes in the tissues themselves, all are metabolism. Growth and repair (anabolism) occur side by side with the destruction of elementary tissue substance (katabolism), and the duration of life rests on these processes; and all are mere cell activities. Food-stuffs (water, inorganic salts, proteids, albuminoids, carbohydrates, and fats) undergo more or less combustion or oxidation. Oxygen unites with carbon to form carbon dioxide, and with hydrogen to form water; the nitrogen of the highly complex proteid substances reappears in combination with carbon, hydrogen, and oxygen as urea, uric acid, and other compounds; and other ingesta are thus transformed through oxidation. All maintain the temperature of the body, replace outworn parts, and accomplish the body's work. Oxidation occurs to a slight extent in the blood, but the specific reactions are intracellular. Even when nothing exists but the cells and the blood, as in the beginning embryo, the cells really do the work, and they do the work as they do in the adult.