( Originally Published 1909 )
AMONG the vital phenomena exhibited by cells and visible through the microscope, none is so strikingly distinctive of living matter as is the process of cell-division. Closely connected with it are some of the greatest problems of biology. By the continued division of an original germ cell or egg cell all the tissue cells of a multicellular animal arise and the germ-cell itself arises in the parent body from other cells by cell-division. Thus the problem is one of the central facts of development and inheritance.
The rapid advance of biological research is continually adding weight to the conclusions reached years ago that every cell originates by division of some preexisting cell (Omnis cellula e cellula). This is now regarded as one of the fundamental laws of biology and obviously is a corollary of the biogenetic law spoken of which states that all living matter (known to exist only in cells) originates from preexisting living matter.
"How do cells originate ?" was the problem which troubled the biologists of the early part of the nineteenth century. Schleiden and Schwann tried to answer the question, but their answer was entirely wrong. They held that cells, which they were fond of comparing to crystals, formed themselves like crystals in a mother-liquor. Schwann even went further, teaching that young cells developed, not only within the mother-cell (as propounded by Schleiden), but also outside of it, in an organic substance, which is frequently present in animal tissues as intercellular substance, and which he called also 'Cytoblastern.' Thus Schwann taught that cells were formed spontaneously both inside and outside of the mother cell, which would be a genuine case of spontaneous generation from formless germ substance."
These were indeed grave fundamental errors, from which; however, the botanists were the first to extricate themselves. In the year 1846 a general law was formulated in consequence of the observations of Mohl, Unger, and above all, Nageli. This law states that new plant cells only spring from those already present, and further that this occurs in such a manner, that the mother-cell becomes broken up by dividing into two or more daughter-cells. This was first observed by Mohl.
It was much more difficult to disprove the theory, that the cells of animal tissues arise from cytoblasts, and this was especially the case in the domain of pathological anatomy, for it was thought that the formation of tumors and pus could be traced back to cytoblasts. At last, after many mistakes, more light was thrown upon the subject of the genesis of cells in the animal kingdom also, until finally the cytoblastic theory was absolutely disproved by Virchow, who originated the formula, "Omnis cellula e cellula." No spontaneous generation of cells occurs either in plants or animals. The many millions of cells of which, for instance, the body of a vertebrate animal is composed, have been produced by the repeated division of one cell, the ovum, in which the life of every animal commences.
The older histologists were unable to discover what part the nucleus played in cell-division. For many decades two opposing theories were held, of which now one and now the other obtained temporarily the greater number of supporters. According to the one theory, which was held by most botanists, the nucleus at each division was sup-posed to break up and become diffused throughout the protoplasm, in order to be formed anew in each daughter-cell. According to the other, the nucleus was supposed to take an active part in the process of cell-division, and, at the commencement of it, to become elongated and constricted at a point, corresponding with the plane of division which is seen later, and to divide into halves, which separate from one another and move apart.
Later discoveries (1873-188o) revealed the extremely interesting formations and metamorphoses, which are seen in the nucleus during cell-division. These investigations have all pointed to the same conclusion, that the nucleus is a permanent and most important organ of the cell, and that it evidently plays a distinct rôle in the cell life during division. Just as the cell is never spontaneously generated, but is produced directly by the division of another cell, so the nucleus is never freshly created, but is derived from the constituent particles of another nucleus. The formula 'omnis cellula e cellula' might be extended by adding 'omnis nuclei e nucleo.'
Since about 1876 it has been recognised that there are two widely different types of cell-division. In one type there appears to be a simple constriction of the nucleus and cell body. This is known as 'direct,' 'akinetic' or 'amitotic' division. The second type is vastly more complicated, as will be described. To this are now applied the terms 'direct division,' 'karyokinesis' or 'mitosis: The terms mitosis and amitosis (referring to the presence or absence of chromosomes in the dividing cells) are preferred by most biologists.
As to the occurrence of these two types, modern re-search has demonstrated the fact that amitosis or direct division, regarded by Remak and his immediate followers as of universal occurrence, is in reality a rare and exceptional process ; and there is, reason to believe, furthermore, that it is especially characteristic of highly specialized cells incapable of long-continued multiplication or such as are in the early stages of degeneration, for instance, in glandular epithelia and in the cells of transitory embryonic erivelopes, where it is of frequent occurrence. Whether this view be well founded or not, it is certain that in all the higher and in many of the lower forms of life, indirect division or mitosis is the typical mode of cell-division. It is by mitotic division that the germ-cells arise and are prepared for their union during the process of maturation, and by the same process the oosperm segments and gives rise to the tissue-cells. It occurs not only in the highest forms of plants and animals, but also in such simple forms as the rhizopods, flagellates and diatoms. It may, therefore, be justly regarded as the most general expression of the. 'eternal law of continuous development' on which. Virchow insisted.
The phenomena which occur during the process of mitosis are very varied and very complicated; nevertheless they conform to certain laws which are wonderfully constant in both plants and animals. The main feature of the process consists in this, that the various substances which are present in the resting nucleus, undergo a definite change of position, and the nuclear membrane being dissolved, eater into closer union with the cytoplasmic substance.
During this process the whole mass of chromatin in the nucleus becomes transformed into fine thread-like segments, the chromosomes, the number of which remains constant for each species of plant or animal. These chromosomes are arranged in a characteristic manner on a spindle-like structure of achromatic material extending between the two centrosomes. Each chromosome then divides longitudinally into two daughter chromosomes which for a time lie parallel with each other and are closely connected. Next, these daughter chromosomes separate into two groups, dividing themselves equally between the two groups to form the foundation of the daughter nuclei. The cell itself meanwhile becomes divided in such a way that each of the two cells formed by the division possesses one of the daughter nuclei.
The manner in which these changes are brought about is so interesting that it seems wise to give a more detailed outline of the process. Professor Wilson, in "The Cell in Development and Inheritance" gives a description in essentials as follows:
"In the present state of knowledge it is somewhat difficult to give a connected general account of mitosis, owing to the uncertainty that hangs over the nature and functions of the centrosome. For the purpose of the following preliminary outline, we shall take as a type mitosis in which a distinct and persistent centrosome is present, as has been mostly clearly determined in the maturation and cleavage of various animal eggs, and in the division of the testis-cells.
"In such cases the process involves three parallel series. of change, which affect the nucleus, the centrosome and the cytoplasm of the cell-body respectively. For descriptive purposes it may conveniently be divided into a series of successive stages or phases, which, however, graduate into one another and are separated by no well defined limits. These are : (s) The 'Prophases,' or preparatory changes; (2) the 'Metaphase,' which involves the most essential step in the division of the nucleus; (3) the 'Anaphases,' in which the nuclear material is distributed;' (4) the 'Telophases,' in which the entire cell divides and the daughter-cells are formed.
1. Prophases. As the cell prepares for division, the most conspicuous fact is a transformation of the nuclear substance, involving both physical and chemical changes. The chromatin-substance rapidly increases in staining-power, loses its net-like arrangement and finally gives rise to a definite number of separate intensely staining bodies, usually rod-shaped, known as chromosomes. As a rule this process takes place as follows: The chromatin re-solves itself little by little into a more or less convoluted thread, known as the skein or spireme, and its substance stains far more intensely than that of the reticulum. The spireme thread is at first fine and closely convoluted, forming the 'close spireme.' Later the thread thickens and shortens and the convolution becomes more open.
"In some cases there is but a single continuous thread, in others the thread is from its first appearance divided into a number of separate pieces or segments, forming a segmented spireme. In either case it ultimately breaks trans ersely to form the chromosomes, which in most cases have the form of rods, straight or curved, tho they are sometimes spherical or ovoidal and in certain cases may be joined together in the form of rings. The staining-power of the chromatin is now at a maximum. As a rule the nuclear membrane meanwhile fades away and finally disappears, tho there are some cases in which it persists more or less completely through all the phases of division. The chromosomes now lie naked in the cell and the ground-substance of the nucleus becomes continuous with the surrounding cytoplasm.
"The remarkable fact has now been established with high probability that every species of plant or animal has a fixed and characteristic number of chromosomes, which regularly occurs in the division of all of its cells and in all forms arising by sexual reproduction the number is even. Thus in some of the sharks the number is thirty-six; in the mouse, the salamander, the trout, the lily, twenty-four; in the ox, guinea-pig and in man the number is said to be sixteen and the same number is characteristic of the onion. In the grasshopper it is twelve. Under certain conditions the number of chromosomes may be less than the normal in a given species, but these variations are only apparent exceptions.
"The nucleoli differ in their behavior in different cases. True nucleoli or plasmosomes sooner or later disappear, and the greater number of observers agree that they do not take part in the chromosome-formation.
"Meanwhile more or less nearly parallel with these changes in the chromatin a complicated structure known as the amphiaster makes its appearance in the position formerly occupied by the nucleus. This structure consists of a fibrous spindle-shaped body, the spindle, at either pole of which is a star or aster formed of rays or astral fibers radiating into the surrounding cytoplasm, the whole strongly suggesting the arrangement of iron filings in the field of a horseshoe magnet. The center of each aster is occupied by a minute body known as the centr some (Boveri, '88), which may be surrounded by a spherical mass known as the centrosphere (Strasburger, '93). As the amphiaster forms the chromosomes group themselves in a plane passing through the equator of the spindle, and thus form what is known as the equatorial plate.
"The entire structure, resulting from the foregoing changes, is known as the karyokinetic or mitotic figure. It may be described as consisting of two distinct parts, namely, the chromatic figure, formed by the deeply staining chromosomes; and the achromatic figure, consisting of the spindle and asters which in general stain but slightly.
"2. Metaphase. The prophases of mitosis are, on the whole, preparatory in character. The metaphase, which follows, forms the initial phase of actual division. Each chromosome splits lengthwise into two exactly similar halves, which afterward diverge to opposite poles of the splindle, and here each group of daughter-chromosomes finally gives rise to a daughter-nucleus. In some cases the splitting of the chromosomes cannot be seen until they have grouped themselves in the equatorial plane of the spindle, and it is only in this case that the term 'metaphase' can be applied to the mitotic figure as a whole.
"In a large number of cases, however, the splitting may take place at an earlier period in the spireme stage or even in a few cases in the reticulum of the mother nucleus. Such variations do not, however, affect the essential fact that the chromatic network is converted into a thread which, whether continuous or discontinuous, splits throughout its entire length into two exactly equivalent halves. The splitting of the chromosomes, discovered by Flemming in 1880, is the most significant and fundamental operation of cell-division, for by it, as Roux first pointed out ('83), the entire substance of the chromatic network is precisely halved and the daughter-nuclei receive precisely equivalent portions of chromatin from the mother-nucleus. It is very important to observe that the nuclear division always shows this exact quality, whether division of the cell-body be equal or unequal.
"3. Anaphases. After splitting of the chromosomes, the daughter chromosomes, arranged in two corresponding groups, diverge to opposite poles of the spindle, where they become closely crowded in a mass near the center of the aster. As they diverge the two groups of daughter-chromosomes are connected by a bundle of achromatic fibers, stretching across the interval between them and known as the interzonal fibers or connecting fibers. In the division of plant cells and often in that of animal cells these fibers show during this period a series of deeply staining thickenings in the equatorial plane forming the cell-plate or mid-body.
"4. Telophases. In the final phases of mitosis the entire cell is divided in two in a plane passing through the equator of the spindle, each of the daughter cells receiving a group of chromosomes, half of the spindle and one of the asters with its centrosome. Meanwhile a daughter-nucleus is reconstructed in each cell from the group of chromosomes it contains. When first formed the daughter-nuclei are of equal size. If, however, division of the cell-body has been unequal, the nuclei become, in the end, correspondingly unequal, a fact which, as Conklin and others have pointed out, proves that the size of the nucleus is controlled by that of the cytoplasmic mass in which it lies.
"The fate of the achromatic structures varies considerably and has been accurately determined in only a few cases. As a rule the spindle fibers disappear more or less completely, but a portion of their substance sometimes persists in a modified form. In dividing plant cells the cell plate finally extends across the entire cell and splits into two layers, between which appears the membrane by which the daughter-cells are cut apart. A nearly similar process occurs in a few animal cells, but as a rule the cell-plate is here greatly reduced and forms no membrane, the cell dividing by constriction through the equatorial plane."
Such is a general description of mitosis. The variations from the type, the origin and fate of the various structures taking part in the division, their relation to each other and the mechanism by means of which the chromosomes divide and separate to form the new nuclei are interesting problems which are to-day being subjected to keen investigation in the laboratories of the cytologists. But most interesting of all perhaps is the problem of the chromosomes in relation to inheritance. This problem will be more fully discussed in the chapter on Heredity.
Some of the most important studies on the physiological relations of nucleus and cytoplasm have been made with one celled animals. Brandt in 1877 and Nussbaum in 1884 cut certain protozoons into pieces and observed that pieces containing nuclear matter quickly regenerate and produce perfect animals, while the enucleated fragments soon die. One of the most remarkable animals with this power to regenerate is the trumpet animalcule Stentor. Gruber in 1885 found that when this animalcule was fragmented, pieces possessing a large fragment of the nucleus completely regenerated within twenty-four hours. If the nuclear fragment were smaller, the regeneration proceeded more slowly. If no nuclear substance were present, no regeneration took place, tho the wound closed and the fragment lived for a considerable time.
The only exception but it is a very significant one was the case of individuals in which the process of normal fission had begun. In these a non-nucleated fragment in which the formation of a new peristome had already been initiated healed the wound and completed the formation of the peristone. Lillie (1896) has recently found that Stentor may by shaking be broken into fragments of all sizes and that nucleated fragments as small as one twenty seventh the volume of the entire animal are still capable of complete regeneration, non-nucleated fragments perish.
Many other such experiments on one-celled animals show that life for a considerable period, perfectly normal movements, susceptibility to stimulus and the power of taking food may continue in enucleated parts of unicellular animals. They lack, however, the power of digestion and secretion and hence cannot continue to live as do the nucleated parts. These facts demonstrate that the nucleus plays an important part in metabolism. Experiments on plants have supported this conclusion. It will be noted that if a unicellular organism be divided into two parts, the part with the nucleus is a complete cell (a mass of protoplasm with a nucleus), while the other part possesses no longer the individuality of a cell and perishes.
It is well known that all animals and plants have a definite limit of growth. From the cytological point of view the limit of body size appears to be correlated with the total number of cells formed rather than with their individual size. This relation has been carefully studied by Conk-line ('96) in the case of the gasteropod Crepidula, an animal which varies greatly in size in the mature condition, the dwarfs having in some cases not more than one twentyfifth the volume of the giants. The eggs are, however, of the same size in all and their number is proportional to the size of the adult. The same is true of the tissue-cells. Measurements of cells from the epidermis, the kidney, the liver, the alimentary epithelium and other tissues show that they are on the whole as large in the dwarfs as in the giants. The body-size therefore depends on the total number of cells rather than on their size, individually considered, and the same appears to be the case in plants.
It is in the cells of both plant and animal organisms that the vital functions are carried on. All the vital proc- esses of a complex animal appear to be nothing but the highly developed result of the individual vital processes of its innumerable variously functioning cells. The study of the processes of digestion, of the changes in muscle and nerve cells leads finally to the examination of the functions of gland, muscle, ganglion and brain.