Cell (biology)

“ During the time protoplasm was being observed the cell had likewise come under close scrutiny, and naturalists had now an extensive collection of facts upon which to found a theory. It has been shown that many animal cells have no cell-wall, and the final conclusion was inevitable that the essential part of a cell is the semifluid living substance that resides within the cavity when a cell-wall is present. Moreover, when the cell-wall is absent, the protoplasm is the "cell." ”

“ In the amœba no definite parts of the cell appear to be set off to perform certain functions; but any part of the cell can take in food, can absorb oxygen, can change the food into protoplasm, and excrete the waste material. The single cell is, in fact, an organism able to carry on the business of living almost as effectually as a very complex animal.
Complex One-celled Animals.—In the paramœcium we find a single cell, but we find certain parts of the cell having certain definite functions: the cilia are used for locomotion ”

“ This moving substance is living matter, the protoplasm of the cell. The green bodies (the chlorophyll bodies) we shall learn more about later; they are found only in plant cells. All plant and animal cells appear to be alike in the fact that every living cell possesses a structure known as the nucleus (pl. nuclei) , which is found within the body of the cell. This nucleus is not easy to find in the cells of Elodea. Within the nucleus of all cells are found certain bodies called chromosomes. These chromosomes in a given plant or animal are always constant in number. ”

“ Cellular structure begins, as has long been known, by the division of a cell into two, each of the parts then proceeding to grow by the assimilative power of protoplasm and in due time to divide in its turn. A mass of these cells is called ‘cellular tissue.’ The so-called ‘budding’ of a small cell from the side of the parent is, of course, simply a form of division. ”

“ What may be called the ‘female’ cells (germ cells) are large and quiescent; the ‘male’ (sperm cells) are smaller and active. The primary meaning of this is that the larger cells have stored up a supply [Pg 50] of nutriment for the young organism, and are therefore bulkier and less active, while the others contain only the bare elements of cell-structure and are therefore able, as they are obliged, to be active in order to search out their quiescent mates. A strictly vegetable organism, in this stage, may therefore possess organs of locomotion, and be as free-moving as a fish. ”

“ there can surely be no difficulty in understanding how, under appropriate conditions, a cell may, in the course of untold millions of years, give origin to the human race. ”

“ Even the latest theory, developed especially by Ludwig in Germany—that protoplasm contains a separate enzyme for each separate function appointed to a particular task—amounts to nothing more than an analysis of the living organism.
The Formation of Multicellular Organisms
We cannot say that the cell is the element of life, because, in an absolute sense, it is not alive; it lives only when it constitutes an individual. Even the brain cells, the muscular fibres, the leucocytes, etc., are cells; but they do not live independently; their life depends upon the living individual that contains them. ”

“ 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. ”

“ We know that the progress of anatomy has resulted in the cellular doctrine—i.e., in the two-fold affirmation that the most complicated organism is composed of microscopic elements, the cells, all similar, true stones of the living building, and that it derives its origin from a single cell, egg, or spore, the sexual cell, or cell of germination. ”

“ 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 ”

“ 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. ”

“ Cells were then recognized as the ultimate units of organic structure, which were variously modified to adapt them to diverse special purposes. This cell theory of organized structure was generally adopted, and cells were defined as closed membranes or sacs, containing a more or less fluid substance which served to nourish them. The cells were looked upon as independent units, which multiplied by a process of budding or by self-division, and a new factor was introduced in the discussion of the mooted question as to what constitutes the individual in plants. ”

“ 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. ”

“ The somatic or tissue cells, which are associated in the metists to form one organic whole, are of the same essential type as germ cells and unicellular organisms, although the parallelism is more close between the unicellular organism and the germ cell. The germ cell, like the protist, is equipped with all the potentialities of life, whereas tissue cells are specialized for one function rather than another. ”

“ Nevertheless a large and important class of cells, forming a kind of borderland between the vegetable and animal kingdoms, still remains to be dealt with, in which the cell contents are only imperfectly differentiated, while numerous other unicellular organisms, owing to their extreme minuteness, 360 tenuity, and want of all colour, are apparently devoid of any nucleus, and when present can only be differentiated by a resort to a specially conducted method of preparation and staining. ”

“ Inasmuch as the idea of an organic cell, as now adopted by histologists, rests on the presence of two different essential parts in this elementary organism, the internal cell and the external protoplasm, we must distinguish also two different kinds of elementary organisms: germless cytods, as moners for example, and the real germ-inclosing cells, which originate from the former by secreting in the interior of the small mass of protoplasm a true germ or nucleus. ”

“ 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. ”

“ Cells are those simple organic individuals, a large number of which constitute the bodies of most organisms, the human body not excepted. With the exception of the organisms of the lowest order, which have not even the perfect form of a cell (Monera) , or during life only represent a single cell (many Protista and single-celled plants) , the body of every organic individual is composed of a great number of cells. ”

“ In the second, life reaches its greatest development. The body of man, which represents the highest type of life, is recognized as a group of cells. In this group each cell is usually separate and distinct from the others, but is attached to them, and is held in place by the intercellular material.
Protoplasm, the Cell Substance.—The cell is properly regarded as an organized bit of a peculiar material, called protoplasm. This is a semi-liquid and somewhat granular substance which resembles in appearance the white of a raw egg. ”

“ With regard to the former, we have seen that in the single cells, constituting unicellular organisms, there is always a bounding membrane of denser texture than the rest of the protoplasm. As the cell develops its capabilities, we have a shell or case of non-living matter secreted around it, with apertures for communication with the outside world, and increasingly effective protection is provided as protoplasm, whether in the single cell or the body, leaves the water, and has to face the inclemencies of terrestrial life. ”

“ 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. ”

“ Structure of the Cell.—Though all portions of the cell are formed from the protoplasm, this essential substance differs both in structure and in function at different places in the cell. For this reason the cell is looked upon as a complex body having several distinct parts. At or near the center is a clear, rounded body, called the nucleus. This plays some part in the nourishment of the cell and also in the formation of new cells. If it be absent, as is sometimes the case, the cell is short-lived and unable to reproduce itself. ”

“ Each separate cell of an organism would be in exactly the same state as a unicellular organism. Just as these have to make a selection from amongst the disharmonious substances by which they are continually bathed, so, too, would the cells of the body have to pick out the substances they need, according to the conditions presented. Not only would the work of the single cells be enormously increased, but also, without doubt, the mutual influence of different kinds of cells, by means of certain secretions, would be much hindered. ”

“ Where the cell containing the germ-plasm happens to be a free cell, it is called an “organism”; but whether it be a germ-cell or a protozoan, it alike ceases to be a cell when it has given origin to a multitude of other cells, whether these happen to be other germ-cells (plus somatic-cells) or other protozoan cells. In short, quâ cell, all cells are mortal: it is only the substance of heredity which some cells contain that can be said, in any sense of the term, to be immortal. ”

“ These free-moving living particles are known as “swarm cells”. Each is an individual with a film-like skin separating it from all other individuals. That is, the protoplasm of each cell is enclosed within a boundary and in the center of each is a nucleus. These one-celled “animals” wander about freely for a few days. During this time they may mate, as individuals. More commonly each loses its flagellae and splits into several fragments. ”

“ In many cells the protoplasm has formed on its periphery a layer of metaplasm which is frequently called a cell-wall. This cell-wall prevents amœboid movements of the protoplasm, and a cell possessing it is said to be encysted. ”

“ The cell, which consists of protoplasm and a nucleus, is an elementary organism, that, by itself, or in combination with other cells, forms the basis of all animal and vegetable organisation. In minute structure it is so extraordinarily complicated that its essential constitution (its micellar or molecular structure) eludes our observation. It is a medley, composed of numerous, chemically distinct particles that may be divided into two groups, organised and unorganised. ”

“ As the cells increase in number, the mass also increases in size by the absorption of nutriment, and the cells continue dividing until the mass contains thousands of cells. Meantime the body of the animal is formed out of these cells, and when it is adult it consists of millions of cells, all of which have been derived by division from the original cell. In such a history each cell comes from pre-existing cells and a cytoblastema plays no part. ”