The unicellular organism can by its very nature transform itself into a multicellular organism only by the method of cell-division.
The multicellular organism was a colony of unicellular organisms, and its life was a sum of the lives of its constituent elements.
The morula, a compact mulberry-like congeries of segmentation-cells, corresponded to the synamoeba, or earliest association of undifferentiated amoeboid cells to form the first multicellular organism.
To Weismann we owe the knowledge of how it is that death intervenes when multicellular existence develops from unicellular.
III But we must proceed to examine the behavior of the various kinds of cells of which the various multicellular organisms are composed.
Unicellular forms can come only from preexisting cells of the same kind; and even the individual cells of a multicellular organism, when once differentiated, reproduce only other cells after their own kind.
By the vibrating movements of the cilia the entire multicellular body acquired a more rapid and stronger motion, and passed over from the creeping to the swimming mode of locomotion.
These are the earliest organs of the multicellular body, and the two cell layers of its enclosing wall, simple epithelia, are its earliest tissues; all the other organs and tissues are a later and secondary growth from these.
This connection is found, also, in the tissues of the multicellular animals and plants.
A sexual propagation by simple division is found in many of the multicellular species (for instance, in many cnidaria, corals, medusae, etc.
This anatomical knowledge is of extreme importance; and it is supplemented by the embryological discovery that each of the higher multicellular organisms is developed out of one simple cell, the impregnated ovum.
Individual development begins, in man and in all other multicellular animals, with the repeated segmentation of one simple cell.
This "tissue-soul" is the higher psychological function which gives physiological individuality to the compound multicellular organism as a true "cell-commonwealth.
This is emphatically true, whether it is a unicellular organism or a constituent element of a multicellular organism.
From coenobia or social unions of these afterwards arose the lowest histones, multicellular plants and animals.
As we have already seen, the result of the first division of labour among the homogeneous cells of the earliest multicellular animal body was the formation of an alimentary cavity.
The egg, in the modern physiological sense of the word, did not make its appearance until the descendants of the unicellular Protozoon had developed into multicellular animals, and these had undergone sexual differentiation.
The first cell-communities to be formed, which laid the early foundation of the higher multicellular body, must have consisted of homogeneous and simple amoeboid cells.
The Gastraea itself has originated from the simple multicellular vesicle, the Blastaea, and this in turn must have been evolved from the lowest circle of unicellular animals, to which we give the name of Protozoa.
The gastraea theory has now convinced us that all the Metazoa or multicellular animals can be traced to a common stem-form, the Gastraea.
It is clear that this morula reproduces for us to-day the simple structure of the multicellular animal that succeeded the unicellular amoeboid form in the early Laurentian period.
From it is developed in the same manner in all the Placentals, by repeated cleavage, a multicellular blastula.
It is an interesting fact that in the plant kingdom also the simple hollow sphere is found to be an elementary form of the multicellular organism.
This many-celled or multicellular condition of the body is true of all the animals except the simplest, the unicellular Protozoa.
Developing, in the case of multicellular organisms, from the same embryonic systems into which the secondary unit (gastrula or plant enbryo) differentiates.
A morphological term signifying development, in the case of multicellular organisms, from the same unit deme or unit of the inferior orders of individuality.
The ovum stands potentially for the entire organism--in other words, it has the faculty of building up out of itself the wholemulticellular body.
Hence, although the amoeba is nothing but a simple cell, it is evidently able to accomplish all the functions of the multicellular organism.
This cell produces a cluster of cells by segmentation, and from these develops the multicellular organism, or individual of higher rank.
Hence, in the fertilised egg which we eat daily, the yellow yelk is already a multicellular body.
These cells are separate living beings; they are the citizens of the State which the entire multicellular organism seems to be.
This ontogenetic process has a very great significance, and is the real starting-point of the construction of the multicellularanimal body.
I say this is very important, because our whole science of embryology now resolves itself into the problem: "How does the multicellularorganism arise from the unicellular?
Briefly, it was necessary to show that ALL the multicellular animals passed through these three stages, so that our biogenetic law would enable us to recognise them as reminiscences of ancestral forms.
We must, therefore, turn next to the question whether there are to-day any unicellular organisms, from the features of which we may draw some approximate conclusion as to the unicellular ancestors of the multicellular organisms.
In all these metazoa, or multicellular animals, the chief embryonic processes are substantially alike, although they often seem to a superficial observer to differ considerably.
According to this gastraea-theory there was originally in all the multicellular animals ONE ORGAN with the same structure and function.
In these cases, certain cells of a colony of unicellular plants or of the filaments of multicellular plants enlarge greatly and thicken their wall.
Of these the first three include multicellular plants, some of them of great size; the last three are unicellular organisms, with little in common with the rest excepting the possession of a brown colouring matter.
Such colonial forms as Hydrurus and Phaeocystis are supposed, however, to indicate a stage in the passage to the multicellular condition.
In all the multicellular plants of this group which have been adequately investigated, vegetative multiplication by means of what are known as hormogonia has been found to occur.
The multicellular species consist of filaments, branched or unbranched, which arise by the repeated divisions of the cells in parallel planes, no formation of mucilage occurring in the dividing walls.
Melobesia callithamnioides gives rise to multicellular propagula; (Griffithsia corallina is said to give rise to new individuals, by detaching portions of the thallus from the base of which new attachment organs have already arisen.
It is not easy in all cases to draw a distinction between a colony of planes and a multicellular individual.
The megaspore is filled with tissue as in typical Gymnosperms, and from some of the superficial cells 3 to 5 archegonia are developed, characterized by long multicellular necks.
Every living cell has psychic properties, and the psychic life of multicellular organisms is the sum-total of the psychic functions of the cells of which they are composed.
For example, in a species of fern from the French rocks there were multicellular hairs which looked like little stems of Equisetum owing to regular bands of teeth at the junctions of the cells.
Outer layer of cells, which forms a skin, in the multicellular plants.
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.
It is interesting to note that the main types of the unicellular animals are repeated again in the cells of different parts of the bodies of multicellular animals.
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.
Thus the link between the unicellular and the multicellular beings could be constituted through the intermediary of flagellated colonies on the one hand and, on the other hand, of beings similar to a phagocytella.
It was consequently considered as the primitive type of multicellular beings.
The question as to what are the ancestral forms ofmulticellular animals cannot be solved through direct observation, for there is a lacuna between them and unicellular beings, a lacuna which is due to the disappearance of intermediary forms.
The organism of multicellular animals possesses various cells which play the part of phagocytes.
He knew the life of unicellular beings and that of the lower multicellular organisms in their complete simplicity; he knew their mode of defence by ingestion and intracellular digestion.
Having arrived at this conclusion, Metchnikoff thought that the same mechanism of immunity must be found in other primitive and analogous cells, such as the phagocytes of multicellular beings.
The mechanism of immunity in protozoa could therefore really be compared with that of immunity in multicellular beings.
Therefore Metchnikoff supposed a priori that the phagocytes, being similar primitive cells of multicellular beings, would also react against poisons.
In the reproduction of multicellular organisms, one sees likewise but a continuation of the phenomena exhibited in Volvox.
These two kinds of reproductive cells in multicellular organisms are derived ordinarily from two separate individuals known as male and female, though there are some exceptions.
The footstalks of this latter species bearmulticellular hairs, which we have good reason to believe represent aborted tentacles.
In the lower part of the leaf, almost half the space on each side between the midrib and margin is destitute of glands; these being replaced by long, rather stiff, multicellular hairs, which intercross over the midrib.
They vary in development, and graduate, as Nitschke* states, and as I repeatedly observed, into the long multicellular hairs.
The leaves [page 351] are clothed with numerous multicellular hairs; some simply pointed; others bearing glandular heads, and differing much in length.
The petioles bear many multicellular hairs, some of which near the blade are surmounted, according to Nitschke, by a few rounded cells, which appear to be rudimentary glands.
On each side of the entrance from three to rarely seven long, multicellular bristles project out- FIG.
The long multicellular hairs are not so quickly affected.
The antennae, instead of projecting in front of the bladders, are curled under the valve, and are armed with twelve or fourteen extremely long multicellular bristles, generally arranged in pairs.
The antennae, which are united for a short distance at their bases, bear on their outer surfaces and summits numerous, long, multicellular hairs, surmounted by glands.
These are broad, flattened, and of large size; they bear on their margins multicellular hairs, surmounted by glands.
In this bladder, as well as in several others, there were some unicellular Algae, and one multicellular Alga, which no doubt had lived as intruders.
Or, in other words, that the production of these cell-seeds depends upon the adult condition of parent cells: not upon that of the multicellular organism as a whole.
The origin of stirp is to be found in the somatic tissues of the multicellular organisms themselves.
So that here we have another very possible means of communication between the germ-cells and the somatic-cells which together constitute a multicellular organism.
We may look in vain, he says, for any individual differences on the part of any multicellular organisms, which have been brought into existence independently of the blending of germ-plasms in a previous act of sexual union.
Lastly, and as regards the multicellular organisms, it is evident that Weismann’s essay On the Significance of Sexual Reproduction in the Theory of Natural Selection must be cancelled.
Hence the only causes of individual variation and of the origin of species in the unicellular organisms are the Lamarckian factors, just as in the multicellular the only cause of these things is natural selection.
Defn: Developing, in the case of multicellular organisms, from the same embryonic systems into which the secondary unit (gastrula or plant enbryo) differentiates.
Defn: A morphological term signifying development, in the case ofmulticellular organisms, from the same unit deme or unit of the inferior orders of individuality.
On the contrary, the ablest biologists still differ considerably as to the nature of the cell or the elementary individual, its relation to the whole of the multicellular organism, and so on.
The stable communities of cells which make up the body of the histona, or multicellular plants and animals, are called tissues (tela or hista).
In these there is not a real alternation of generations, as the multicellular tissue-forming organism develops directly from the impregnated ovum.
The sperm-cells and ova of the latter are redundant growth-products, which have the power of regenerating the whole multicellular organism.
However, we must not take this division too rigidly; and the absolute lack of a nucleus and tissue-formation separates the chromacea just as widely from the multicellular tissue-forming algae as the bacteria from the fungi.
This is clearly shown by the comparative biology of the protists, their unicellular organism presenting a much greater variety of stages of cell-organization than the subordinate tissue-cells in the bodies of the multicellular histona.
Among the multicellular metaphyta it is particularly the fungi that have taken to parasitism in various ways.
Weismann deduces from this a radical distinction between the unicellular and the multicellular organisms.
The third and highest stage of individuality to which the multicellular organism attains is the stock or colony (cormus).
Here the actual Bion (or the fully developed physiological individual) is not represented by the individual cells, but by the whole multicellular coenobium, which in each species has a definite form and size.
To what, when multicellular organisms began to specialize certain cells for reproduction, and these cells to mature themselves for fusion by throwing out half their chromosomes?
Multicellular organisms of more or less elaborate structure plainly cannot, without breaking up, fuse together like single cells.
We see the significance, then, of Weismann’s remark, “germ cells made their appearance along with the multicellular body.
In multicellular organisms, this single originating cell is usually formed by the fusion of two imperfect cells by what is indifferently called conjugation, sexual reproduction, or ‘amphimixis.
The reproduction of the constituent cells in the complex multicellular organism, during its natural growth or to make good the inevitable loss consequent on the wear and tear of life, is of the same character.
The animals whose bodies are formed of many cells in which there is a differentiation of structure and a specialization of function, are called metazoa, and the multicellular plants metaphyta.
As the different cells in the multicellular mass became associated into groups for different duties, the method of such division of labor was not alike in all machines.
It had been assumed that the body of the multicellular animal or plant was made of independent units.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "multicellular" in a variety of sentences. We hope that you will now be able to make sentences using this word.
