Multicellular organism

“ 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. ”

“ 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. ”

“ We digressed from this study after having seen that all, even the most complex, multicellular plants and animals, in which the differentiation of the cells into a number of cell-groups with the most diverse functions has attained the highest degree of complexity, are able to produce special cells, the germ-cells, which have the power of reproducing from themselves another organism of the same species, and with the same complex structure. ”

“ In all cases, however, whether the cell-unit lives freely as a unicellular organism or forms an integral part of a multicellular individual, it exhibits in itself all the phenomena characteristic of living things. Each cell assimilates food material, whether this is obtained by its own activity, as in the majority of the protozoa, or is brought, as it were, to its own door by the blood stream, as in the higher Metazoa, and builds this food material into its own substance, a process accompanied by respiration and excretion and resulting in growth. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Both in our microscopic knowledge of the little and in our telescopic investigation of the great we have attained an invaluable insight that seemed inconceivable a hundred years ago. Improved methods of microscopic and biological research have not only revealed to us an invisible world of living things in the kingdom of the protists, full of an infinite wealth of forms, but they have taught us to recognize in the tiny cell the all-pervading “elementary organism” of whose social communities—the tissues—the body of every [Pg 3] multicellular plant and animal, even that of man, is composed. ”

“ The influence of the “cells” upon one another, the subordination of the cell’s growth, division and differentiation, to the requirements of the whole organism—seen in normal growth, but nowhere more strikingly than in development and regeneration,—is, however, very difficult of explanation in terms of the cell-theory as this was, until quite recently, generally understood. The very elaborate regional differentiation of the protoplasm often seen in the Protozoa sufficiently indicate that multicellular structure is no essential condition for complex regional differentiation. ”

“ 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. ”

“ 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. ”

“ Protoplasm lives and responds not only discretely in the lowest unit perceptible by the microscope, but collectively in the connected groups of these units called multicellular organisms, and in the disconnected groups of these organisms called species. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ All animals and plants begin life as single cells, which, in the vast majority of cases, are microscopic in size. Those that remain single cells, and dissociated throughout life, are called Unicellular Animals (Protozoa) and Plants (Protophyta) , and are to be seen mostly by the microscope alone; but those which, by multiplication and growth, form large numbers of cells that remain associated together as in the body of a bird or lizard, are called Multicellular Animals (Metazoa) or Plants (Metaphyta) . ”

“ The single cell of the simplest kind is able to exist as an independent organism. Many of the lowest plants and animals, and also many neutral protista (which are neither animals nor plants) , retain for life the character of a simple cell. ”

“ 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." ”

“ 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. ”

“ Doubtless there are, in an organism composed of cell groups, numerous kinds of cells which can live without having reciprocal relations with other cells, exactly in the same way as a single individual can isolate itself from its stock and still continue life for a certain time. ”

“ In these tiny Protozoa, those organisms which consist of one single cell only, the Amœbæ, as well as those consisting of several cells in a state of union, the Synamœbæ, are able to perform all the functions of animal life—cohesion, sensation, motion, digestion, and reproduction; but, as the organism becomes more and more complex, these different functions are shared among several groups of cells. ”

“ 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.. ”

“ Each separate cell of an organism would be in exactly the same state as a unicellular organism. Just as these have to make a selection from amongst the disharmonious substances by which they are continually bathed, so, too, would the cells of the body have to pick out the substances they need, according to the conditions presented. Not only would the work of the single cells be enormously increased, but also, without doubt, the mutual influence of different kinds of cells, by means of certain secretions, would be much hindered. ”

“ 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. ”

“ Inasmuch as the idea of an organic cell, as now adopted by histologists, rests on the presence of two different essential parts in this elementary organism, the internal cell and the external protoplasm, we must distinguish also two different kinds of elementary organisms: germless cytods, as moners for example, and the real germ-inclosing cells, which originate from the former by secreting in the interior of the small mass of protoplasm a true germ or nucleus. ”

“ 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. ”