Unicellular organism

“ An instance of a unicellular organism is well seen in an Amoeba, or in one of the Foraminifera, classic examples for the study of undifferentiated protoplasm, which here composes the greater part of the body, while products of the formative activity of the protoplasm are seen in the external shell and in various internal granules and structures. As an example of a multicellular organism we may take the human body, built up of an immense number of living cells which produce, singly or in co-operation, a variety of substances and structures, each contributing to the functions of the body. ”

“ Nor can they be what we may call morphological units—the cells or protoplasts; because in the early stages of development the cells of one organism are indistinguishable from those of others, and because were cells the units of composition there could be no interpretation of what are called unicellular organisms—nothing to account for the innumerable varieties of them. Hence, of necessity, the structural elements of which each organism is built, being neither proteid molecules nor cells, must be something between them: probably some complex combination of different isomeric forms of proteids. ”

“ Probably unicellular organisms alone are wholly dependent upon themselves. They perform all the processes necessary for life independently of other cells, except when, as sometimes happens, a conjunction of these simple organisms rises to the level of a symbiosis. The latter, as we have already pointed out, must have a value corresponding exactly to the reciprocal interactions of the cells of the more highly organized forms of the vegetable and animal kingdoms. For there is no doubt that in plants, too, the cells have actively reciprocal relations. ”

“ 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. ”

“ 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. ”

“ The higher we climb in the scale of organization, particularly in the animal kingdom, the more do we meet with cells which are entrusted with special functions. For instance, we find cells which receive matter from the exterior. Others transform particular compounds into products of a special nature. Others, again, have the function of carrying the final products of metabolism to definite points for excretion.
A unicellular organism stands constantly in relation with numerous substances of the outer world, which differ from place to place and from time to time.. ”

“ For the protists, the unicellular organisms (without tissue) exhibit a much greater freedom and variety in the development of their fundamental forms than the histons, the multicellular tissue-forming organisms. In the protists (both protophyta [172] and protozoa) the constructive force of the elementary organism, the individual cell, determines the symmetry of the typical form and the special form of its supplementation; but in the histons (both metaphyta and metazoa) it is the plasticity of the tissue, made up of a number of socially combined cells, that determines this. ”

“ 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. In this way the animal body does not require so close a relation with the medium surrounding it; its food, the first requirement of an organism, is distributed to it from inside outwards. ”

“ In all animals and plants, above the lowest, the germ is a nucleated cell, using that term in its broadest sense; and the first step in the process of the evolution of the individual is the division of this cell into two or more portions. The process of division is repeated, until the organism, from being unicellular, becomes multicellular. The single cell becomes a cell-aggregate; and it is to the growth and metamorphosis of the cells of the cell-aggregate thus produced, that all the organs and tissues of the adult owe their origin. ”

“ If a unicellular organism, as described above, be placed in a vessel of sterilized water and left to live alone under otherwise ideal conditions, it will start dividing and multiplying, as though it meant to reproduce itself indefinitely. ”

“ All the other cells are capable of reproducing their like by that process of fission which, as we have seen, is very prevalent among unicellular organisms. Thus muscle, nerve, or skin cells can severally produce their like, but no muscle, nerve, or skin cell can produce a cell of an unlike kind; still less can it produce a whole organism, in which there are many different kinds of cells arranged in definite relations to one another. ”

“ Such a speck of living protoplasm as the amoeba is known to biologists as a cell; and of such cells or variations of them the structures of all plants and animals are built up, a plant or animal composed of a single cell being known as a unicellular organism, whereas a plant or animal composed of a plurality of associated cells is known as a multicellular organism. Since such unicellular organisms as the amœba are fairly observable, we may conveniently begin the study of life at that stage of evolution which they have reached. ”

“ 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. ”

“ 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 caryoplasm of the nucleus discharges the functions of reproduction and heredity; the cytoplasm of the cell-body accomplishes the metabolism, nutrition, and adaptation. Here we have, therefore, the first, oldest, and most important process of division of labor in the elementary organism. In the unicellular protists the organization rises in proportion to the differentiation of the various parts of the cell; in the tissue-forming histona it rises again in proportion to the distribution of work (or ergonomy) among the various organs. ”

“ 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. ”

“ We are acquainted with such contradictory facts, not perhaps among the unicellulars themselves, where it is difficult to procure direct proof, but in regard to the germ-cells of many-celled organisms which correspond to unicellular organisms. ”

“ But however large or complex such aggregation may be, it never forms sets of united cells or tissues. The whole of these lower creatures, therefore, may be spoken of as unicellular organisms; as though they may consist of many cells, those cells retain their individuality. Such creatures are all the lowest animals—those called Hypozoa [94] or Protozoa, and also the lowest cryptogamic [95] plants.
All other animals and all the higher plants are multicellular. ”

“ Without mitosis, or formation of threads, the homogeneous nucleus divides into two halves. These move away from each other, and become centres of attraction for the enveloping matter, the protoplasm. The same direct cleavage of the nuclei is also witnessed in the reproduction of many other protists, while other unicellular organisms show the indirect division of the cell. ”

“ Unicellular organisms in general, including the protozoa, the unicellular cryptogams, the various bacteria, and the free, isolated cells, spores, ova, etc. of higher organisms, are referable for the most part to a very small number of typical forms; but besides a certain number of others which may be so referable, though obscurely, there are obviously many others in which either no symmetry is to be recognized, or in which the form is clearly not one of equi­lib­rium. ”

“ 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. ”

“ This transformation of cells, at first all alike, into distinct orders of cells or tissues, whence different organs with different functions arise, is characteristic of all living creatures above those which each consist throughout life of one cell only.
We have seen that unicellular organisms may unite into a cylindrical or spheroidal colony, as in some Radiolaria, or into a spheroid of closely-adjusted cells, forming one layer, as in Volvox. ”

“ Probably I have now said enough to show that, remarkable and altogether exceptional as the properties of germ-cells of the multicellular organisms unquestionably show themselves to be, yet when these properties are traced back to their simplest beginnings in the unicellular organisms, they may fairly be regarded as fundamentally identical with the properties of living cells in general. ”

“ Throughout my argument the implication everywhere is that the physiological units are all of one kind; whereas Mr. Darwin regards his component units, or "gemmules," as being of innumerable unlike kinds. He supposes that every cell of every tissue gives off gemmules special to itself, and capable of developing into similar cells. We may here, in passing, note that this view implies a fundamental distinction between unicellular organisms and the component cells of multicellular organisms, which are otherwise homologous with them. ”

“ It is obvious that there are innumerable solitary plant-cells, and unicellular organisms in general, which, like the yeast-cell, do not correspond to any of the simple forms that may be generated under the influence of simple and homogeneous surface-tension ”

“ It is, then, the corpuscles of protoplasm, called cells (cellulæ) , which have so much interest for the physiologist, and these, like the cytods, may form independent organisms, which are then termed unicellular. Again, cells form other cells, and a multicellular organism results, and goes on increasing in geometrical progression. In the Vertebrata the cell retains its characteristic spheroidal shape, as seen in Fig. 423, and undergoes division by virtue of its living protoplasmic mass.
Fig. 423.
1. Newly formed cell structure; 2. Division of the nucleus; 3. ”

“ It has long been known, as we have seen, that unicellular organisms could go on dividing indefinitely and that germ plasm had a potential mundane immortality; but no one had suspected that highly organized and differentiated288 somatic cells, which had lost the power of producing the whole individual and could only produce cells of their own special tissue, had this power. Recent experiments, however, indicate that under certain highly elaborated conditions they, too, can be made to live and even grow indefinitely and that this growth can not only be observed but measured under the microscope. ”

“ And we need not therefore be surprised to find, as we do find, specks of protoplasm manifesting life, and yet showing no signs of organization. A further stage of evolution is reached when the imperfectly integrated molecules forming one of these minute aggregates, become more coherent; at the same time as they pass into a state of heterogeneity, gradually increasing in its definiteness. That is to say, we may look for the assumption by them, of some distinctions of parts, such as we find in cells and in what are called unicellular organisms. They cannot retain their primordial uniformity ”