Cell nucleus

“ They are incapable of assimilating food and incapable of reproduction, and hence their life cannot continue very long. Facts like these demonstrate conclusively the vital importance of the nucleus in cell activity, and show us that the cell, with its power of continued life, must be regarded as a combination of protoplasm with its nucleus, and cannot exist without it. It is not protoplasm, but cell substance, plus cell nucleus, which forms the simplest basis of life. ”

“ Indeed, it constantly happens that the student of biology, while gazing through his microscope, hits upon discoveries that have the most far-removed implications. Thus a few years ago it was discovered that when a cell is about to bisect itself and become two cells, its nucleus undergoes a curious transformation. Within the nuclear substance little bodies are developed, usually threadlike in form, which take on a deep stain, and which the biologist calls chromosomes. ”

“ By the time the chick is ready to be hatched there are millions.
Usually, however, a cell is small—just as much protoplasm as its still more minute nucleus can keep going; though here, again, one must be guarded, for there may be several nuclei instead of only one. The protoplasm on the external surface and around the nucleus is specialized into a more or less definite membrane. To this outer envelope are attached fine fibrils, which join up to a small body within the cell, called the centrosome, and by the lengthening and shortening of these its shape can be altered. ”

“ It is possible to suggest a mechanico-physical explanation of multicellular structure which will deprive the cell of much of its assumed significance as a unit of organization. The fact that surface area becomes relatively less extensive as bulk increases would alone set a limit to the size of “unicellular” organisms; for not only is there a constant reaction between nucleus and cytoplasm through the nuclear membrane, but the surface of the cell serves both for the intake of food and the elimination of waste material. ”

“ 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. ”

“ New cells are gradually formed from the nourishing yelk around the germ, each being at first roundish in shape, and having a spot near the centre, called the nucleus. The reason why cells increase must remain a mystery, until we can penetrate the secrets of vital force—probably forever. But the mode in which they multiply is as follows: The first change noticed in a cell, when warmed into vital activity, is the appearance of a second nucleus within it, while the cell gradually becomes oval in form, and then is drawn inward at the middle, like an hour-glass, till the two sides meet. ”

“ As microscopists now studied one after another of these animals and plants whose cells had been said to contain no nucleus, they began to find nucleii in them, until the conclusion was finally reached that a nucleus is a fundamental part of all active cells. Old cells which have lost their activity may not show nucleii, but, so far as we know, all active cells possess these structures, and apparently no cell can carry on its activity without them. Some cells have several nucleii, and others have the nuclear matter scattered through the whole cell instead of being aggregated into a mass ”

“ The second and higher stage is the cell, which is already divided or differentiated into nuclear matter and cellular matter. We comprise both kinds—the cytodes and the cells—under the name of plastids (“formative particles”) , because they are the real builders of the organism. However, these cytodes are not found, as a rule, in the higher animals and plants; here we have only real cells with a nucleus. Hence, in these tissue-forming organisms (both plant and animal) the organic unit always consists of two chemically and anatomically different parts—the outer cell-body and the inner nucleus. ”

“ If it be admitted, and the evidence we have is certainly in favour of it, that in some cases, both in animal and vegetable cells, the nucleus itself divides during cell division, and in others the nucleus completely vanishes during the cell-division, it is more reasonable to suspect the existence of some connection between the two processes, than to suppose that they are entirely different in kind. ”

“ The cell is a composite organite, the primary element being a microscopic mass of protoplasm, or what may more conveniently be termed neuroplasm. It appears as finely granulated and striated or fibrillated substance on a hyaline ground, with water, fat, and diffused pigment in varying quantities. The cell contains a nucleus, and nucleolus—sometimes two. Like other animal cells, it sometimes has a distinct cell-wall, sometimes not. ”

“ In this way a nucleus was formed about which new matter gradually gathered, and in due time produced a little membrane. This theory of the formation of the cell is designated the theory of free cell formation—a theory which has been now almost entirely abandoned.
It is highly probable that the nucleus plays an extremely important part within the cell—a part less connected with the function and specific office of the cell, than with its maintenance and multiplication as a living part. ”

“ Some authors have explained the production of cells on the hypothesis of a true spontaneous generation. According to them, cells are organized in a saline solution, the first step being the deposit of a nucleolus, around which there forms an envelope called the nucleus, and finally, at a greater distance, a second envelope, or cell-wall. ”

“ It may further be added, as a general truth applicable to nucleated cells, that the nucleus rarely undergoes any considerable modification, the structures characteristic of the tissues being formed at the expense of the more super ficial protoplasm of the cells ; and that, when nucleated cells divide, the division of the nucleus, as a rule, precedes that of the whole cell. ”

“ Thus the nucleus was restored to the important position which Schwann and Schleiden had given it, but with greatly altered significance. Instead of being a structure generated de novo from non-cellular substance, and disappearing as soon as its function of cell-formation was accomplished, the nucleus was now known as the central and permanent feature of every cell, indestructible while the cell lives, itself the division-product of a pre-existing nucleus, and the parent, by division of its substance, of other generations of nuclei. ”

“ The Nucleus.—Every cell capable of living, growing, and multiplying, possesses a nucleus of constitution very analogous to the cellular mass which surrounds it. The anatomical elements in which no nucleus is found, such as the red globules of blood in adult mammals, are bodies which are [Pg 164] certain, sooner or later, to disappear. There is therefore no real cell without a nucleus, any more than there is a nucleus without a cell. ”

“ The Nucleus.—The nucleus has been demonstrated in all plants with the exception of the Cyanophyceae and Bacteria, and even here structures have been observed which resemble nuclei in some of their characteristics. The nucleus is regarded as a controlling centre of cell-activity, upon which the growth and development of the cell in large measure depends, and as the agent by which the transmission of specific qualities from one generation to another is brought about. ”

“ The most powerful microscopes disclose in this protoplasm a certain definite structure, a very fine, thread-like network spreading from the nucleus throughout the semi-fluid albuminous protoplasm. It is certainly in line with the broad analogies of life, to suppose that in each cell the nucleus with its network is the brain and nervous system of that individual cell.
All living organisms consist, then simply of cells. Those consisting of but one cell are termed unicellular; those comprising more than one cell are called pluricellular.
The unicellular organism is the unit of life on this earth. ”

“ When mitosis is about to take place, they separate from one another and pass to the poles of the nucleus, forming the achromatic spindle. After the division and cleavage of the chromosomes of the original nucleus have taken place they pass from the equator to the poles of the spindle, rearranging themselves close to the separated centrosomes to form daughter nuclei.
The cytoplasm of the cell now undergoes division in a line between the two daughter nuclei. When complete separation has taken place, we have two daughter cells formed from the original, each being a perfect cell-unit. ”

“ The word cell received a final definition as "a small mass of protoplasm supplied with a nucleus."
In this widened and culminating general view of the cell theory it became clear that every animate organism, animal or vegetable, is but a cluster of nucleated cells, all of which, in each individual case, are the direct descendants of a single primordial cell of the ovum. In the developed individuals of higher organisms the successive generations of cells become marvellously diversified in form and in specific functions ”

“ In addition to the chromatin there may be present in the nucleus one or more, usually spherical, and as yet somewhat enigmatical bodies, the “nucleoli.” In addition to the nucleus and cytoplasm, a third body, the “centrosome,” has often been considered as a constant cell-structure. It is a minute granule, usually lying in the cytoplasm not far from the nucleus, and plays an important part in cell-division and fertilization (see below) .
Cell-differentiation.—Both among unicellular and multicellular individuals the cell assumes the most varied forms and performs the most diverse functions. ”

“ The nucleii of different animals and plants all show essentially the characteristics just described. They all contain a liquid, a linin network, and a chromatin thread or network, but they differ most remarkably in details, so that the variety among the nucleii is almost endless (Fig. 26) . They differ first in their size relative to the size of the cell; sometimes—especially in young cells—the nucleus being very large, while in other cases the nucleus is very small and the protoplasmic contents of the cell very large ”

“ In quite a number of different kinds of tissue it has been determined that fine threads of protoplasmic material lead from one cell to another in such a way that the cells are in vital connection. The claim has been made that there is thus a protoplasmic connection between all the cells of the body of the animal, and that thus the animal or plant, instead of consisting of a large number of separate independent cells, consists of one great mass of living matter which is aggregated into little centres, each commonly holding a nucleus. ”

“ In epithelial and gland cells, the size of nuclei is limited not only by the presence of metabolic products in the cells, but also by the occurrence of cell division and the consequent limitation of the growing period of the nucleus. ”

“ All organisms consist either—as do the simplest, mostly microscopic, plants and animals—of a single minute mass of protoplasm, or of a few, or of many, or of an enormous aggregation of such before-mentioned particles, each of which is one of those bodies named a "cell" (Fig. 3) . Cells may, or may not, be enclosed in an investing coat or "cell-wall." Every cell generally contains within it a denser, normally spheroidal, body known as the nucleus. ”

“ In cell-division the first stage is the aggregation, on opposite sides of the nucleus, in a line with the long axis of the cell, of a quantity of pale, slightly granular protoplasm. This is perhaps derived from the peripheral layer, and travels inward along the fibers, since minute thickenings, of similar appearance, show themselves upon these fibers. The nucleolus swells out into two protuberances, in the same axial line, joined by a bridge of denser matter. ”

“ Other as simple but larger organisms may consist of a minute mass of semi-fluid protoplasm, containing granules, as we find to be the case in the plant Vaucheria, [85] and many other Algæ, and in the animal Amœba primitiva. [86]
An organism of this simplest kind or a fragment of a higher organism which presents this simplest condition is called a cell. [87] Very generally [Pg 326] such cell has within it a more or less distinctly marked generally denser and spheroidal body called a nucleus, within which, again, other minute spots may appear called nucleoli. ”

“ The nucleus represents the most vital part of the cell protoplasm, and has a more granular appearance than the latter. The nucleolus is a minute solid spot in a nucleus, appearing to be more highly refractive. ”

“ The nucleus is looked upon by the majority of cytologists as the formative center of the cell in a chemical, and also, consequently, in a morphological, sense. Active exchanges of material take place between the nucleus and the protoplasm during the nutritive processes of the cell. ”