Cell wall

“ Usually the cell-wall, when this is present, is a product of the cell’s secretive activity; sometimes, however, it appears to be formed by an actual conversion of the surface layer of the protoplasm, and retains the power of growth by “intussusception” like the rest of the protoplasm. Even when a limiting membrane is present, however, evidence is steadily accumulating to show that the cell is not an isolated physiological unit, but that, in the vast majority of cases, there is a protoplasmic continuity between the cells of the organism. ”

“ In this case the original cell wall remains as part of the wall of the daughter cells. Fission is the commonest form of cell multiplication throughout the vegetable kingdom.
The second form of cell division or internal cell division is shown at C. Here the protoplasm and nucleus repeatedly divide until a number of small cells are formed within the old one. These develop cell walls, and escape by the breaking of the old cell wall, which is left behind, and takes no part in the process. The cells thus formed are sometimes provided with two cilia, and are capable of active movement. ”

“ 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. ”

“ Bounding the ordinary plant-cell, there is a thin elastic wall composed of a substance called cellulose. This cell-wall forms a delicate sac within which there exists the living substance called protoplasm. It is this substance which is alive and works, and has really formed the cell-wall about itself. ”

“ In some cases the protoplasm at the circumference is so modified as to give the appearance of a limiting membrane called the cell wall. In brief, then, a cell is a mass of nucleated protoplasm; the nucleus may have a nucleolus, and the cell may be limited by a cell wall. Every tissue of the human body is formed through the agency of protoplasmic cells, although in most cases the changes they undergo are so great that little evidence remains of their existence. ”

“ B, cells of a fusiform or spindle shape found in the loose tissue under the skin and in other parts (called connective-tissue cells) ; C, cell having many processes or projections—such are found in connective tissue, D, primitive cells composed of protoplasm with nucleus, and having no cell wall. All are represented about 400 times their real size.
Some cells live a brief life of 12 to 24 hours, as is probably the case with many of the cells lining the alimentary canal; others may live for years, as do the cells of cartilage and bone. ”

“ Such a cell can and does grow when encysted: it grows even more readily in this state, since none of its energies are absorbed by the necessities of locomotion, etc. Growth leads, of course, to division: there is often an economy of wall-material by the formation of a mere party-wall dividing the cavity of the old cell-wall at its limit of growth into two new cavities of equal size. ”

“ A careful examination of our object with a fairly high magnification will show that, though the scraping was very gentle and quite painless, something has been removed from the mouth. We shall see a number of cells, somewhat overlapping like the tiles of a roof. They are almost colourless, and have ill-defined margins, certainly no cell wall, and each cell contains a dark spot near its centre. These cells from our own person will give us a good idea of the appearance of animal cells, for we are animals, much as we may dislike the idea. ”

“ 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 siliceous cell-wall is covered on the outside by a layer of protoplasm called the coleoderm. This layer may be quite thin and evident only when treated with fuchsin or Bismarck brown, or it may be of considerable thickness. The cell contains the cytoplasma, protoplasm, cell-sap, endochrome, pyrenoids, oil globules and nucleus, together with certain other less understood bodies. ”

“ Like other cells it has a cell wall, a cell substance with its linin and fluid portions, a nucleus surrounded by a membrane and containing a reticulum, a nucleolus and chromatic material, and lastly, a centrosome. Now such an egg is a complete cell, but it is not able to begin the process of division which shall give rise to a new individual until it has united with another cell of quite a different sort and commonly derived from a different individual called the male. ”

“ If instead of the whole protoplasm of the cell arranging itself into one mass, it divides into several, or if portions only of the protoplasm become marked out into new cells, in each case accompanied by rounding off and contraction, the new cells remaining free from one another, and usually each secreting a cell wall, then this process, whose relation to rejuvenescence is apparent, is called free-cell formation. ”

“ The cell utilizes a part of its food for its own growth; but, sooner or later, the growth ceases, and the cell, keeping the form and size it has acquired, becomes a permanent cell. It is round or oval, or resembles a many-sided crystal. Some cells become flat and square, like a tile; some put out rays, like a star, or form a zigzag, like the wall of a fortress; many lengthen themselves out. The inner structure, also, of the cell changes with age; the envelope, delicate and thin in youth, afterward receives accretions and ornaments. ”

“ In the first place the cell is a kind of inclosed sac, in which are found the elements of growth and life. Surrounding the cell is a thin wall known as the cell membrane. In plants this cell wall is composed of cellulose, a woody substance, which is thin and tender in green and growing plants, but hard and woody when the plant is mature. ”

“ It is absent in states with a distinct cell-wall to resist excessive turgescence, such as would lead to the rupture of a naked cell, and we conclude that its chief function is to prevent such turgescence in unprotected naked cells. ”

“ The Desmidiaceæ consist mostly of single cells (Pl. V. figs. 9, 16) ; and these consist of two equal halves or segments, as indicated either by a paleness of the endochrome or a deep constriction at the line of junction, which is called the suture. The cells are often elegantly lobed and cut, or spiny; and in many the surface exhibits minute markings, consisting of little protrusions of the cell-wall outwards, or inflations, as they are called. ”

“ Later, however, when a parent cell bursts, the newly developed cells no longer separate from it, but, by cohering to it and to each other, form a cluster of nucleated cells, while around this aggregation of units is formed a wall. ”

“ It seems reasonable to suppose that branching in this case simply means that one part of the cell- wall is weaker, less fitted to withstand internal tension, than the rest. If the cell- wall were uniform it would stretch uniformly, and the want of uniformity in the cell-wall may probably be traced to its being part of an aggregate. The string of cells is moored by one end, and any motion of the water will cause it to assume an oblique position. ”

“ The cells of the Florideæ, which are formed by the division of a mother-cell into two daughter-cells of unequal size, have always larger or smaller pits in the cell-walls, and the thin cell-wall separating two pits from each other is perforated by a number of small holes. ”

“ The cells of which they are built up are comparatively large, and, like almost all vegetable cells, are each inclosed in a wall of cellulose. The cellulose is perfectly transparent, and if the microscope, even with a low power, be brought to bear on one of these cells, a portion of its protoplasm may be seen in active rotation, flowing up one side of the long, tubular cell and down the other, and sweeping on with it such more solid particles as may become enveloped in its current. ”

“ Moreover, more cells were built, always taking the cross as the center of all things—six-sided cells, with a low, incomplete wall, or, rather, parapet, partitioning each off, to the number of about twenty-four cells in all. Each cell was closed, of course, at the top, the top being its floor, and open at the bottom, the bottom being, if I may so put it, the top; for, as has already been said, wasp cities are built upside-down, and everybody walks and hangs on his head, being so fitted for the purpose. ”

“ Below the tall columnar cells are four or five rows of nuclei which represent smaller, irregular cells, though the cell walls could not always be determined between the closely packed nuclei. No goblet cells are to be seen at any place.
Fig. 49. An outline of a transsection through the wall of the posterior region of the small intestine of the hibernating animal, under low magnification; lettering as in Figure 42. ”

“ The cells in some species are provided with a pedunculate operculum, by which it is intended to designate a process, which arising by a short tube from the anterior wall of the cell, immediately beyond the inner margin of the opening, projects forwards and bends over the front of the cell, expanding into a variously-formed limb, and serving as protection to the mouth of the cell in front. ”

“ In addition to the limit thus imposed upon the cytoplasmic area which can be effectually controlled by the nucleus, and the necessity for a minimum surface area to the protoplasmic mass, the advantages of the more or less complete subdivision of the living substance into—as far as their metabolism is concerned—semi-autonomous units, is indicated by the mechanical support derived from the specialized cell walls and turgescent cells of the plant, and the intercellular secretions of the animal tissues. ”

“ 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 inward201 at the middle, like an hour-glass, till the two sides meet. ”

“ 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 the cell house is the core of Sing Sing, so the cell structure is its core. In the exact centre of the building, steel within stone, six levels of cells rise, one level on another, climbing up almost to the roof, from which many hooded, round ventilators stare down like watchful eyes that never sleep. In each tier are two hundred cells, built back to back, each row of cells being faced by narrow iron balconies and reached by narrow wooden stairways. ”

“ The cells of the medullary rays are seen in one, two, or several rows, each cell having the form of a parallelopiped or ordinary brick—the bricks being supposed standing on their narrow sides and with the long axes directed radially. The walls in contact with the vascular bundles are thickened, and soon become woody and beset with simple pits; the cells contain protoplasm and nuclei, and in winter become filled to crowding with starch grains. ”

“ All manner of schemes of cell-formation were put forward during the ensuing years by a multitude of observers, and gained currency notwithstanding Von Mohl's reiterated contention that there are really but two ways in which the formation of new cells takes place—namely, "first, through division of older cells; secondly, through the formation of secondary cells lying free in the cavity of a cell." ”

“ As a matter of fact, it is not necessary for single cells to have this great length, because it is possible for them to be arranged end to end, making a path of living protoplasm consisting not of one cell but of a chain of them. If we look at a nerve cell under the microscope we see that it is made up of a little central mass of protoplasm to which has been given the name of “cell body.” From this cell body extends a tiny thread of living protoplasm. ”

“ Pray go on!"
"Well," he proceeded, "biology, as you know, starts with the single cell——"
"How do you spell it?" said Ellis, with shameless frivolity, "with a C or with an S?"
"Of these cells," continued Wilson, imperturbably, "every animal body is a compound or aggregation; the aggregation involving a progressive modification in the structure of each cell, the differentiation of groups of cells to perform special functions,—digestive, respiratory, and the rest,—and the subordination of each cell or group of cells to the whole. ”