Leclanché cell

“ 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. ”

“ Schleiden had first shown in 1838 that the body of any plant can be dissolved into tiny living corpuscles, which he called “cells,” because they often had the appearance of a filled honeycomb. A year later Schwann proved, in Johannes Müller’s laboratory, that the higher animal also is a product of these cells. The cell was recognised as the living unit that composed the oak and the rose, the elephant and the worm. Man himself, in fine, was but a pyramid of these cells—or, to speak more accurately (as each cell has its own life) , an immense community of cells, a cell-state. ”

“ But what threatens more 107absolute extinction of this argument still, all the German physiologists do not accept even the protoplasm-cell. Rindfleisch, for example, in his recently-published ‘Lehrbuch der pathologischen Gewebelehre’ speaks of the cell very much as we understand Virchow to have spoken of it. To him there is in the cell not only protoplasm but nucleus, and perhaps membrane as well. To him, too, the cell propagates itself quite as we have been hitherto fancying it to do, by division of the nucleus, increase of the protoplasm, and ultimate partition of the cell itself. ”

“ 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." ”

“ Now, the striking point in the description of the excretory cells in these organs, as described both by Claus and Spangenberg, is that they closely resemble the pipe-cells or solenocytes of Goodrich; each cell possesses a long tube-like projection, which opens on the surface. ”

“ 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. ”

“ In some cells, probably in all, the centrosome presides over division. Cells, however, do not always divide in the same way. Some simply lengthen, the nucleus also lengthening inside, become constricted in the middle like a dumb-bell, and separate. (See Diagram 15.)
Diagram 16.—Cell Division.
Others manage differently. In them the nucleus simply bursts, and turns its essential elements, a number—always a constant number—of coarse threads, adrift. Meanwhile, two centrosomes have moved to opposite ends of the cell, and there anchored themselves by fibrils ”

“ If it is absent, the cell loses its power of assimilation and growth, and soon dies. Haberlandt has shown that in plant cells, when any new formation of membrane is to take place in a given spot, the nucleus is found in its immediate vicinity; and Klebs found that only that portion of the protoplasm of a cell which contains the nucleus is capable of forming a cell-wall; whilst Townsend has further shown that if the non-nucleated mass is connected by strands of protoplasm to the nucleated mass, either of the same cell or of a neighbouring cell, it retains the power of forming a cell-membrane. ”

“ Nevertheless, it will be seen that protoplasm can only continue to exist in the form of a cell, since, unless thus organized, it can neither keep itself among favourable surroundings nor prepare fresh ingredients to make good its waste. If a cell be cut up into several pieces, these detached bits of protoplasm will live for a time; but death overtakes them as soon as they have used up their reserve material. When this is gone, they have to consume their own substance, a process which quickly proves fatal. ”

“ The fact that a double plasma membrane delimiting the daughter cells is first formed in cell division, each cell having its own membrane, and that if a cellulose wall is formed at all it appears later between these membranes, indicates that the wall is of secondary importance, which is further supported by the fact that in many cell divisions, as in all the prothallial cells of Zainia and Ginkgo, no cellulose walls are formed, the cells being delimited only by the plasma membranes. ”

“ The partial explanation came in 1838 with Schleiden and the following year with Schwann, who stated that animals and plants are built up of definite living units, the cells; that such units compose the tissues that had been determined by the physiologist Bichat, and that the organs are composed of definite layers of cells. The simplest animals, what we now call the Protozoa, were shown by Dujardin to be each composed of only a single cell. We define a cell as a particular mass of living substance regulated by a particular center, the nucleus. ”

“ A bone-cell cannot receive an impression, nor originate an idea, any more than a brain-cell can secrete bile. Each kind of cell has its own appointed duty. The most important duty that can possibly be allotted to any cell is obviously that of reproducing the individual for the purpose of continuing the race or species. So we find in higher animals that this function, like others, is relegated to a special set of cells also derived from the original single cell, and which are called “the germ-cells.” ”

“ Then the primary cell resolves itself into a cluster of peculiar secondary cells, each consisting of an elongated body containing an orange-coloured endochrome and a pair of long flagella, as seen in the antherozoids of the higher cryptogams. As the sperm-cells approach maturity the clusters may be seen to move within them; the bundles then separate and show an independent active movement while still within the cavity of the primary cell, and finally escape through a rupture in the cell-wall, rapidly diffusing themselves as they pass through the cavity. ”

“ 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." ”

“ During the period intermediate between stages I and K the notochord undergoes considerable transformations. Its cells cease to be flattened, and become irregularly polygonal, and appear but slightly more compressed in longitudinal sections than in transverse ones. The vacuolation of the cells proceeds rapidly, and there is left in each cell only a very thin layer of protoplasm around the nucleus. Each cell, as in the earlier stages, is bounded by a membrane-like wall. ”

“ We are thus brought back to the fundamental question as to how the germ-cells arise in the organism. Is it possible that the nucleoplasm of the germ-cell, with its immensely complex molecular structure, potentially containing all the specific peculiarities of an individual, can arise from the nucleoplasm of any of the body-cells,—a substance which, as we have just seen, has lost the power of originating any new kind of cell, because of the continual simplification of its structure during development? ”

“ The evidence points to the origin of all these free cells from germ-cells, and thus leads to the conception that the blastula stage of every embryo represents two kinds of cells, the one which will form the mortal host being the locomotor neuro-epithelial cell, the other the independent immortal symbiotic germ-cell. ”

“ The individuals—each known as a frustule—assume very various forms and may be unicellular or multicellular, but present no differentiation; many similar cells may be connected in chains, embedded in mucilaginous masses, or attached to mucilaginous stalks. The cells are bilateral or centric, often asymmetrical, slightly dorsiventral and have no cilia; those living in the free condition have the power of sliding upon a firm substratum. The cell contains 1 cell-nucleus and 1–2 plate-shaped or several disc-shaped chromatophores. ”

“ The external wall of cellulose renders it impossible for the confined protoplasm to emit, like a naked Amœba, pseudopodia from its outer side; but on the inner side there is no obstacle to the extension of the protoplasm, and here the cavity of the cell becomes more or less completely traversed by protoplasmic projections from the wall. These often stretch themselves out in the form of thin filaments, which, meeting with a neighboring one, become fused into it; they show currents of granules streaming along their length, and after a time become withdrawn and disappear. ”

“ Uskoff has in particular published experimental work in this province of hæmatology. This has led him to see in the white blood corpuscles the developmental series of one kind of cell, and to distinguish in it, three stages: (1) "young cells," which correspond to our lymphocytes; (2) "ripe cells" (globules mûrs) , large cells with fairly large and irregularly shaped nucleus, which are therefore our large mononuclear and transitional forms; (3) "old cells" (globules vieux) , which represent our polynuclear cells. The eosinophil cells are completely excluded from this classification. ”

“ Above is man, built of myriads of cells woven into the most ingenious tissues and the most perfect organs for each function of life; below we have the radiolarian, in which a single cell must discharge all the vital functions, because its whole body is merely one cell. But there is another wonder. This tiny particle of living slime, floating in the blue waves at Messina, hardly more visible than a drop of spittle, has a most remarkable quality. ”

“ It is, as a fact, altogether wanting in many, especially young, cells of the animal body. They are naked cells without any membrane. The distinguished anatomist also proved that the so-called "cell sap"—the real body of the cell—is not a simple fluid, but a viscous, albuminous substance, the independent movements of which had long been known in the rhizopods, and which the first to study it carefully, Felix Dujardin, had described as sarcode in 1835. ”

“ Others, on the contrary, are entirely useless to the cell in question, while many would cause considerable harm, if allowed to penetrate its wall cells. To these substances the single cell does not yield itself helplessly, but has at its disposal various arrangements for its own defence. It has, in the first place, a cell wall which is impermeable by many substances. Further, the cell, by means of different processes, is capable of altering substances, which may in any way be injurious to it, in such a way that the active group is rendered harmless. ”

“ 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. ”

“ 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. ”

“ X. Bichat accomplished a revolution when he decomposed the living body into tissues. His successors, advancing a step in the analysis, dissociated the tissues into elements. These elements, which one would have thought were infinitely varied, were reduced in their turn to one common prototype, the cell.
The living body, disaggregated by the histologist, resolves under the microscope into a dust, every grain of which is a cell. A cell is an anatomical element the constitution of which is the same from one part to the other of the same being, and from one being to another ”

“ The production of new individuals is accomplished by the division of the old cell into two. Thus it may be said that there is always a bit of the old cell remaining, though divided into fragments; and for this reason the Amœba-like Protozoans have been fancifully called "immortal."
Fig. 4.—Section, highly magnified, of a two-layered animal, Hydra (Grade II.) . Ec, outer layer of Ectoderm; En, inner layer of Endoderm; l, lamella dividing the two, represented by a line; n, nuclei of the cells; v, thin vacuoles of small interstitial cells; E, the Enteron or digestive cavity. ”

“ The definition of Verworn was reached by a series of steps representing the historical advance of knowledge regarding the cell. Schleiden and Schwann looked upon the cell as a hollow chamber having a cell-wall which had been formed around the nucleus; it was a great step when Schultze defined the cell in terms of living substance as "a globule of protoplasm surrounding a nucleus," and it is a still deeper level of analysis which gives us a discriminating definition like that of Verworn. ”