Cellular theory

“ 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 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. ”

“ It is also important to remember that a cell that has synthesized DNA is a cell that, with a few exceptions, will divide in the very near future. Thus, for an understanding of the mechanisms that control cellular proliferation, it is important to investigate the factors that control DNA synthesis. ”

“ Mitosis is a continuous process; the following stages of the process are designated only for convenience. During interphase the cell is busy metabolizing, synthesizing new cellular materials, and preparing for self-duplication by synthesizing new chromosomes. In prophase the chromosomes, each now composed of two identical strands called chromatids, shorten by coiling, and the nucleolus and nuclear membrane disappear. During metaphase the chromosomes line up in one plane near the cell equator. ”

“ So long as purely chemical research is unable to answer questions concerning the finer structure of the cell elements, we are bound to resort to indirect methods. The latter have, in a relatively short time, opened up a vast field, and discovered views of the widest interest in regard to every kind of cellular process. It is the duty of the future to follow up with exact methods all the observations that have been made, and to replace with known quantities the many unknowns with which our present-day methods have still to reckon. ”

“ There are also workers who melt down outworn letters and still other workers who make new letters for further use; these are the enzymes, special kinds of proteins. If we wish to continue the analogy, we may say that each kind of cell in the organism has a different subeditor, who writes that cell’s own editorial. Actually we might say that all cells have the same board of editors in common, but only one editor functions in any given type of cell. In biological terms this means that only a portion of all the cellular DNA is active in each cell. ”

“ The young cells increase in size and fill up the cavity of the old cell, which is in time resorbed. Cell-development always takes place within existing cells, and either one or many new cells may be formed within the mother-cell. Schleiden's views on cell-formation were drawn from some rather imperfect observations on the embryo-sac and pollen-tube, but he extended his theory to cell-formation in general. Though wrong in almost all respects the theory had at least the merit of fixing attention upon the really important constituents of the cell, the nucleus and the cell-plasma. ”

“ The nucleus is the essence of the cell. And the doctrine of cellular continuity emphasizes the fact that the nuclei of all the cells of the body are derived by a process of divisional growth from the first segmentation-nucleus which results from the union of the nuclei of the ovum and the sperm. In this sense, then, however late the germinal cells appear as such, they are in direct continuity with the germinal cell from which they, in common with all the cells of the organism, derive their origin. In this sense there is a true continuity of germ-cells. ”

“ If that is really so, the cell cannot possibly be the primitive organism; if it were, we should have a miracle at the beginning of organic life on the earth. The theory of natural evolution clearly and distinctly demands that the cell (in this sense) is a secondary development from a simpler, primary, elementary organism, a homogeneous cytode. There are still living to-day very simple protists which do not tally with this definition, and which I designated monera in 1866. As they must necessarily have preceded the real cells, they may also be called "precellular organisms." ”

“ But when an organism undergoes great changes of structure during development, the cells, which at each stage are supposed to be directly derived from previously existing cells, must likewise be greatly changed in nature; this change is attributed by the supporters of the cellular doctrine to some inherent power which the cells possess, and not to any external agency. Others maintain that cells and tissues of all kinds may be formed, independently of pre-existing cells, from plastic lymph or blastema. ”

“ The systematic elaboration of the theory of cells, founded on careful observation and mature reflection, began with the year 1844. Almost at the same time in this year appeared Nägeli's detailed enquiries into the occurrence of the cell-nucleus and into cell-division, von Mohl's observations on the primordial utricle and its behaviour in the process of cell-division in young tissue, and lastly those of Unger on merismatic cell-formation (cell-division) as a general mode of proceeding in the growth of organs. ”

“ 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. ”

“ Omnis cellula ex cellula, enunciated by Virchow in 1855, and Flemming’s principle: Omnis nucleus ex nucleo, proclaimed in 1882. In this way, Cytology supplemented Redi’s formula that every living being is from a preëxistent living being, by adding two more articles, namely, that every living cell is from a preëxistent cell, and every new cellular nucleus is derived by division from a preëxistent cellular nucleus. Now neither the nucleus nor the cell-body (the cytoplasm or extranuclear area of the cell) is capable of an independent existence. ”

“ 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. ”

“ Our recent knowledge of the cell cycle has therefore led to a shift in the focus of investigation from mitosis to DNA synthesis.
Another point to remember is that not all cells keep going through the cell cycle indefinitely. As shown in Figure 25, when a cell divides, the daughter cells have two alternatives, either to go through another cycle or to leave it altogether. Cells that leave the cycle are called differentiated cells and will eventually die without any further division. Many cells in an adult organism also have lost the capacity to make DNA and therefore the capacity to divide. ”

“ 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. ”

“ A critic who thinks the above statements are “rather misleading” admits that the lowest types of organisms yield them support, saying that “there are certainly masses of protoplasm containing many nuclei, but no trace of cellular structure, in both animals and plants. Such non-cellular masses may exist during development and later become separated up into cells, but there are certain low organisms in which such masses exist in the adult state. They are called by some botanists non-cellular, by others multi-nucleate cells. ”

“ 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. ”

“ These artificial cells have heterogeneous cell-contents, and produce in their interior granulations which are disposed in a regular order. Thus they are both in constitution and in form absolutely similar to the cellular elements which constitute living organisms.
"8. It is probable that the inorganic elements which are present in the natural protoplasm may play an important part {118}in determining the form which is assumed by the figured elements of the organism." ”

“ In certain worn-out or highly differentiated cells a cell-division of this kind really seems to occur—the so-called 'direct' division—but in young cells, and indeed in all vigorous cells, the process, which looks simple, is, in reality, exceedingly complex. Not only is the structure of the nucleus incomparably more complex than was recognized a quarter of a century ago, but nature has placed within the cell a special and marvellously intricate apparatus, by means of which the component parts of the nucleus are divided between the two daughter-nuclei. ”

“ 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. ”

“ Here comparative research yields the important result that EVERY OVUM IS AT FIRST A SIMPLE CELL. I say this is very important, because our whole science of embryology now resolves itself into the problem: "How does the multicellular organism arise from the unicellular?" Every organic individual is at first a simple cell, and as such an elementary organism, or a unit of individuality. This cell produces a cluster of cells by segmentation, and from these develops the multicellular organism, or individual of higher rank. ”

“ For a stronger reason, we cannot imagine that any of the atoms of which any cell is composed, has these faculties. Nor can we conceive that a portion of these atoms, automatically, metamorphose themselves into a nucleolus, others into a nucleolinus, while the remainder continue to be a simple cell-body. ”

“ 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. ”

“ As, moreover, the first thing which a cell does when it grows is to divide, the whole proceeding looks rather futile. But ready-made protoplasm of an assimilable shape is rare, and it is not often that a cell, unless it be a plant or a parasite, finds itself in a substance which can be handed straight to the nucleus without further elaboration. Usually the cell has to discharge from itself a reagent, which will develop the right chemical qualities in the matter it wants to absorb. This substance is known as an enzyme, or ferment. ”

“ The nucleus, which in many cells is the largest and most central body, is of special importance. It contains a number of threadlike bodies, or chromosomes, that are the carriers of the cell’s heredity-controlling system. These contain granules of a material called chromatin, which is rich in a nucleic acid, DNA (deoxyribonucleic acid) . The chromosomes usually are not readily seen in the nucleus except when the cell, along with its nucleus, is dividing. When the nucleus is not dividing, a spherical body, the nucleolus, can be seen. (In some nuclei there may be more than one.) ”

“ The chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element of an organism which it excites." "We may infer that cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes, leaving behind the substance excited, now distinguished as cytoplasm." ”