Multicellularity

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ These phenomena, as we have seen, commence among unicellular organisms. In these they do not constitute reproduction, but the vital reënforcement of certain individuals. Conjugation takes place in a different manner in different cases.
For example, a unicellular animal applies itself against one of its fellows. The nucleus of each cell divides into two. Then the protoplasm of the two cells fuses over the whole surface of contact, and half the nucleus of the first cell penetrates the second cell, while half the nucleus of the latter enters the first cell. ”

“ Even the latest theory, developed especially by Ludwig in Germany—that protoplasm contains a separate enzyme for each separate function appointed to a particular task—amounts to nothing more than an analysis of the living organism.
The Formation of Multicellular Organisms
We cannot say that the cell is the element of life, because, in an absolute sense, it is not alive; it lives only when it constitutes an individual. Even the brain cells, the muscular fibres, the leucocytes, etc., are cells; but they do not live independently; their life depends upon the living individual that contains them. ”

“ Since nuclei depend upon the general protoplasm for their growth, they also become small in such cells. If this process of the transformation of protoplasm into differentiation products continues long enough it necessarily leads to the death of the cell, since the continued life of the cell depends upon the interaction between the general protoplasm and the nucleus. In cells laden with the products of differentiation, the power of regulation is first lost, then the power of division, and finally the power of assimilation; and this is normally followed by the senescence and death of the cells. ”

“ 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 new science of cytology has made us aware that the division of cells is not a passive or a simple process, but is extremely active, complex and varied. Living protoplasm is in motion, and the discovery that cell walls are not hermetically closed, but are perforated by delicate protoplasmic strands, lends strength to the belief of some biologists that protoplasm circulates, not only inside the individual cells, but through the entire organism. Conjugation may signify that such a circulation extends also throughout the species. ”

“ Thus it is just as inconceivable that either a homogeneous or a heterogeneous cell-colony possessing the physiological value of a multicellular individual should continue to grow to an unlimited extent by continued cell-division, as it is inconceivable that a unicellular being should increase in size to an unlimited extent. ”

“ Swarm-spores of algæ showed that protoplasm, when unenclosed in a cell-wall, can move about, direct its course, and change its shape. Knowledge of this fact did more than rectify the definition of the cell; it effaced one distinction between plants and animals, and gave a hint of the resemblance of primitive cells to such simple organisms as Amœba.
Martin Barry in 1843 announced that certain Protozoa (that name was not yet in use) are simple cells. He pointed out that they possess nuclei, like those of tissue-cells, and compared their increase by fission with the cleavage of the egg. ”

“ 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. ”

“ These artificial cells are sensible to most of the influences which affect living organisms. Like living cells they are influenced both in their organization and in their development by humidity, dryness, acidity, or alkalinity. They are also greatly affected by the addition of minute quantities of chemical substances either to the gelatinous blastema or to the drops which represent the primary nuclei. We may in this way obtain endless varieties, nuclei which are opaque or transparent, with or without a nucleolus, and cells containing homogeneous cytoplasm without a nucleus. ”

“ In the first place, regarding protoplasm as a chemical structure, why, if kept under favourable conditions, should it ever break down? We have no reason to suppose that it need. It is hard to see how the minute animals, consisting of only one cell, can die of old age, provided that no injurious influence be brought to bear upon them. When an individual has grown to a certain size, it divides in two, and each enters upon life afresh. Why, therefore, should not all the cells of the body continue to renew their youth? ”

“ 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. ”

“ The chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element of an organism which it excites." "We may infer that cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes, leaving behind the substance excited, now distinguished as cytoplasm." ”

“ If we take a survey of the assumptions we have been compelled to make from the standpoint of the theory to explain the development of germ-cells, budding, and regeneration, it would seem as if it were contradictory to assume that, on the one hand, complete germ-plasm should be given to certain cell-series as inactive accessory idioplasm, and, on the other, that very numerous cells, at least in the lower Metazoa, should have received the idioplasm of budding, and still more numerous cells that of regeneration. ”

“ 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. ”

“ 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. ”

“ If that is really so, the cell cannot possibly be the primitive organism; if it were, we should have a miracle at the beginning of organic life on the earth. The theory of natural evolution clearly and distinctly demands that the cell (in this sense) is a secondary development from a simpler, primary, elementary organism, a homogeneous cytode. There are still living to-day very simple protists which do not tally with this definition, and which I designated monera in 1866. As they must necessarily have preceded the real cells, they may also be called "precellular organisms." ”

“ 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." ”

“ Nevertheless he does so, by representing that a unicellular organism, even though it present such a considerable degree of organization as we meet with in the higher Protozoa, still resembles a germ-cell of a multicellular organism, in that it consists of all the essential constituents of a germ-cell, including germ-plasm in its nucleus. And inasmuch as a germ-cell is potentially immortal, so it must be with a unicellular organism ”

“ There are distinct and specific lines of cell-lineage in ontogenesis, so the theory states, along which the continuity of germ-protoplasm is kept up, which, in other words, lead from one egg to the other, whilst almost all other lines of cell-lineage end in “somatic” cells, which are doomed to death. What has been stated here is a fact in many cases of descriptive embryology, though it can hardly be said to be more than that. ”

“ 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 ”

“ Moreover, young cells are always more active than older ones, and they commonly possess either no cell wall or a very slight one, this being deposited as the cell becomes older and remaining long after it is dead. Such facts soon disproved the notion that the cell wall is a vital part of the cell, and a new conception took its place which was to have a more profound influence upon the study of living things than any discovery hitherto made. This was the formulation of the doctrine of the nature of protoplasm. ”