Animal cell

“ The animal cell, in its most complete form, is comparable in most parts of its structure to that of the plant, but differs from it in the entire absence of the 'cellulose wall' or of anything that represents it, the cell-contents being enclosed in only a single limitary membrane, the chemical composition of which, being albuminous, indicates its correspondence with the primordial utricle. ”

“ An organ may be composed of several tissues. Thus the heart has contractile tissue in its muscular walls, epithelial tissue lining its cavities, and skeletal tissue forming its framework. Still, notwithstanding their aggregation [21] into tissues and organs, it remains true that the body of one of the higher animals is composed of cells, together with certain cell-products, horny, calcareous, or other. The simplest animals, called protozoa, are, however, unicellular, each organism being constituted by a single cell. ”

“ While, however, the other animal cells in their transformations remain stationary here and there, either at the boundary line of worms, fishes, amphibia, reptiles, or mammals, the one cell which was destined to become man moves on to the stage of the tailed catarrhine apes, then of the tailless apes, and without staying here it irresistibly strides towards its original goal, and only stops where it is destined to stop. ”

“ Throughout the animal kingdom, germ-cells present the same external aspect when carefully examined with the microscope. No difference can be observed between the cells of the flowers of the oak and those of the apple, but the cells of the one always produce oak trees, while those of the other always produce apple trees. The same is true of the germs of animals, there being not the slightest apparent difference. We are unable to perceive how one cell should give origin to a dog, while another exactly like it becomes a man. ”

“ As is the case with vegetables, the animal structure is composed of cells, though they cannot be so easily traced as in the examples which we have already noticed. The young observer may readily perceive the animal cell, in its largest and simplest form, by placing a little of the yolk of egg under the microscope. CARTILAGE, or gristle, is easily seen to be composed of cells. The nails of the fingers afford good objects for the microscopist in search of animal cells. ”

“ The cell.—The characteristic thing about the body of Amœba and Paramœcium and the other "simplest animals"—for there are many members of the group of "simplest animals," or Protozoa—is that it is composed, for the animal's whole lifetime, of a single cell. A cell is the structural unit of the animal body. As [Pg 38] will be learned in the next exercise, the bodies of all other animals except the Protozoa, the simplest animals, are composed of many cells. ”

“ 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." ”

“ We see, then, that variation among these related cells has led to differences in size, in form, in transparency, and in function; while the cell-products have been differentiated into those which are of lifelong value, such as bone, cartilage, connective tissue, horn, chitin, etc., together with a variety of colouring matters; those which are of temporary value, such as the digestive secretions, fat, etc.; and those which are valueless or noxious, such as carbonic acid gas and urea, which are excreted as soon as possible. ”

“ 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 implies that the cells in question must have a structure which distinguishes them sharply from all other kinds of cells.
The view that each animal species is capable of building up complicated compounds of peculiar structure, and further, that every cell with special functions is formed of specially constructed components, is very often met with doubt. How is it possible for the animal and plant worlds to produce such an enormous number of different compounds? There would have to be formed millions and millions of different substances. ”

“ To say, however, that an animal’s body consists of cells is to take an entirely wrong starting-point. A cell is complete in itself, and can live if properly fed, even though separated from its neighbours. Many whole animals consist of only one cell. A cell is, moreover, capable of growing and dividing, thus giving rise to two cells with two nuclei, and it is only because cells find that it pays better not to separate, but to form masses and specialize at different kinds of work, that we have large animals composed of millions of cells like ourselves. ”

“ If our ideas concerning the structure of the particular cells of an organ, and the dependence of its functions on this peculiarity, prove correct, then it follows that, as we have already emphasized, each kind of cell must have at its disposal units of its own kind. We may then speak of substances that are "in harmony with" an organ, or even more precisely, "with the cells" ”

“ From the view-point of cell-differentiation, which seems to me the most tenable position, there does not seem any evidence for, or any probability of, the occurrence of any roundabout mode of development of the germinal cells which could enable them to pick up acquired characters en route.
(d) We come now to the contention that the organism, being one and continuous, if any member suffers, the germ suffers with it. The organs of the body are not isolated or insulated; the blood is a common medium; the nerves ramify everywhere ”

“ As far as anatomy and physiology alone can reveal, it is the result of millions of cells acting together that makes possible the existence of such a living, sentient, thinking creature as man or the highly intelligent elephant or any other multicellular animal. It is due to the mysterious powers of protoplasm that one little microscopic cell, like the fertilized ovum of a woman, is able to hold all the heritages of the race, and gradually unfold them as it builds up the body into myriads of diverse cells intimately associated together. ”

“ Such a cell contains nothing but that strange primitive life-substance named protoplasm, condensed at one point into a nucleus, and it is precisely of such cells that the bodies of all the animals we commonly know are made up; nevertheless an immense variety of creatures still exists, especially in the plankton of the sea, that, like those at the dawn of life, consist of one cell alone. ”

“ At first, we are told, our animal is only a simple cell, in fact that it is not a perfect cell, for as yet the cell-kernel or nucleus has not been separated from the cell-substance or protoplasm. When its limit of size has been reached it multiplies by self-division, or by simply breaking into parts, each part performing the same functions of nutrition and propagation as its predecessor. ”

“ In the former case we have the cells as yet free, while in the latter they are combined into tissues. From this point of view we may look upon the symbiosis of heterogeneous species of cells as a first experiment in the building up of a cell state. The single cells are still independent and their duties multifarious. There is no strong bond uniting the organisms into any one "organ," and yet they depend upon each other for mutual support. Unicellular beings begin to organize themselves into combinations. ”

“ The cell is the unit of growth. It is so with all forms of life—plant or animal, insect or bacterium. In the beginning the start is with a single cell, an egg, if you please. After fertilization has taken place, this single cell enlarges or grows. Many changes now occur, all rather rapidly, until the cell walls become too small, when it breaks apart and forms two cells just like the first used to be. This is known as cell division. As growth increases, the number of cells increases also—until in the end there are millions. ”

“ In the same animal the cells assume many different forms, having different physiological properties. These forms and properties are not modified by the vicinity of different cells. Within the organism each cell lives as if it were alone. If it were possible to isolate a cell of the human body and surround it by normal nutritive material, it would continue to live, to develop and reproduce itself, and carry on all its physiological functions exactly as before. ”

“ The ova of animals, the spores of plants, the isolated cells of lower organisms in general, all show the power of separate assimilation and development. "We must therefore, in general, ascribe to the cell an individual life, that is to say, the combination of the molecules in the single cell does suffice to produce the force whereby the cell is enabled to draw to itself new molecules. The ground of nutrition and growth lies not in the organism as a whole, but in the separate elementary parts, the cells. ”

“ Since this time the phenomenon has been observed with a great variety of cells other than red blood corpuscles and bacteria—leukocytes, spermatozoa, cells from liver, kidney, brain, epithelia, etc., protozoa, and many vegetable cells.
It is therefore a well-established fact that the proper injection of an animal with almost any cell foreign to it will lead to the blood of the animal injected acquiring the power to injure or destroy cells of the same kind as those introduced. ”

“ Such, stripped as far as possible of technicalities, are some of the facts concerning the behaviour of cells and their nuclei during the process of cell-multiplication. No good purpose would be subserved by pretending that we fully understand them. The splitting of the rods does indeed seem an efficient means to the end of securing a fair division of the nuclear substance, which, according to many biologists, is the organic bearer of hereditary qualities in the cells. ”

“ Sometimes the normally constructed cell is incapable of completing the otherwise normal decomposition of particular substances, so that materials appear in the blood which still show the characteristic features of the cell, from which they originated. ”

“ The true starting-point of the chick is a microscopic cell within the egg; and, when we look to other animals, we find that all of them, no matter how complex, start in the condition of a single microscopic cell and that, between that simple state and the fully formed animal, which is complex, there are many steps. Therefore, the adult stage of any animal represents the last step in a long series of modifications. ”

“ The isolation of the whole organism is immediatelv disturbed when disharmonious cells settle on any spot within the hitherto harmonious cell complex. From this moment the harmoniously organized cells of the tissues are subjected to the influence of a kind of cell which has an utterly strange organization of its own. These new cells have a characteristic metabolism corresponding with their whole structure and configuration, and this they bring with them definitely into the new organism. ”

“ 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 different mode of nutrition of animals [51] results in a totally different structural plan. Animal cells absorb material that is already organised, and that they may do so their cells are either quite naked, so affording an easy passage for solid particles, or they are clothed only by a thin membrane, through which solutions of slightly diffusible organic colloids may pass. ”

“ 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. ”

“ 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. ”

“ Therefore, unlike plants, multicellular animals display a compact structure with internal organs adapted to the different conditions which result from the method of nutrition peculiar to animals. A unicellular animal takes organic particles bodily into its protoplasm, and forming around them temporary cavities known as food vacuoles, treats them chemically. The multicellular animal has become shaped so as to enclose a space within its body, into which solid organic food-particles are carried and digested thereafter in a state of solution, to be shared by the single cells lining the cavity. ”