Multicellular organisms are organisms consisting of more than one cell.
The student of Nature wonders the more and is astonished the less, the more conversant he becomes with her operations; but of all the perennial miracles she offers to his inspection, perhaps the most worthy of admiration is the development of a plant or animal from its embryo. Huxley.
It has been pointed out that all organisms consist of one free living cell or of many cells, and some idea has been gained of unicellular forms from Sphaerella, Paramecium, and the Bacteria which were selected to illustrate various types of nutrition. We are now in a position to consider the origin and organization of the individual in the METAZOA and METAPHYTA, as the multicellular animals and plants are sometimes called.
Every individual, with exceptions to be noted later, begins its existence as a single cell which has been set free as such from the parent; or which has been formed at fertilization by the fusion of two cells, or gametes, each typically derived from a separate parent individual. The former is known as UNIPARENTAL, or ASEXUAL, reproduction and the latter as BIPARENTAL, or SEXUAL, reproduction. Both asexual and sexual methods are widespread among plants and animals, frequently alternating in regular sequence in the same species to give what is termed an ALTERNATION OF GENERATIONS.
The most remarkable fact about the reproductive cells is the inherent power of each to develop into a replica of the parent species from which it has separated. Both the spore and the zygote (fertilized egg) are set, one may say, to go through a series of changes which transform an apparently simple cell into an obviously complex multicellular plant or animal with all the tissues and organs characteristic of the species. It is important, at this point, to review the general method by which the development of the adult is accomplished.
Briefly, the modus operandi of development is cell division accompanied by differentiation. The spore (asexual) or the fertilized egg (sexual) by a succession of cell divisions, termed CLEAVAGE, passes from the single-cell stage to a two cell stage and then, with more or less regularity, to four-cell, eight cell, sixteen-cell stages, etc. If these cells separated after each division, the same general condition would obtain here which has been seen in the Protophyta and Protozoa, where each organism is a complete free-living cell. Or again, if cleavage merely resulted in a group of so many exactly similar cells, there would arise a colony of unicellular individuals rather than a multicellular organism.
Such colonial forms are, in fact, numerous among the lower plants and animals, and show nearly all grades of complexity from simple associations of a few identical cells, as for example in Spondylomorum, to groups of many thousands in which some of the individuals are specialized for certain functions.
(Fig. 17.) Volvox affords an instructive example of the latter condition. The majority of the cells, ten thousand or more, which form the relatively large spherical colony are FIG. 17. A simple colony of unicellular organisms (Spondylo morum) each of which carries on all the functions of nutrition and reproduction. Highly magnified.
(From Hegner, after Oltmanns.)
flagellated individuals each of which lives a practically in dependent existence in organic union with its fellows. The chief contribution of each of these cells to the economy of the whole results from the lashing of its flagella, which helps to propel the colony through the water. But, under certain conditions, some of the cells become specialized for reproduction and form new colonies which sooner or later are set free. Thus we have a foreshadowing of that differentiation and physiological division of labor between cells which is the most characteristic feature of the Metaphyta and Metazoa.
FIG. 18. Volvox globator, a large colony of flagellated unicellular or ganisms in which the various cells have become organically connected, and certain cells specialized for reproduction. A, mature colony (highly magnified) showing sperm, $ , and eggs, $ , in various stages of devel opment. B, four cells (more highly magnified) showing the connections between three 'somatic 1 cells, and the early differentiation of a repro ductive cell, rp; CD, contractile vacuole ; st, ' ey espot ' or stigma. (From Hegner, after Kolliker.)
However, in the developing multicellular organism cleav age results, sooner or later, in a body composed of cells which possess differentiations of one kind or another that adapt them for the special part they are destined to play in the economy of the individual. Thus cell division, involving differentiation, is the keynote of development in the higher plants and animals.
Among animals, for example, the cells which arise from the cleaving egg frequently become arranged so that they form the surface of a hollow sphere of cells known as a BLASTULA. All the cells at first appear essentially similar, but soon those at one side of the blastula become invaginated until the central cavity, termed the BLASTOCOEL, is largely obliterated. Accordingly there results the GASTRULA stage, which may be roughly compared to a sack, with an opening to the exterior termed the BLASTOPOBE, composed of two layers of cells. The outer layer is known as the ECTO DERM, and the inner, which lines the gastrula cavity (ENTERIC CAVITY), as the ENDODERM. The ectoderm comprises cells which are already somewhat differentiated among themselves for special purposes, but which, as a whole, form a primary tissue with general functions of its own, chiefly sensory and locomotor. Similarly the endoderm consists of cells which, as a group, form the nutritive cells of the embryonic animal.
In the gastrula stage of most animals, a third layer of cells arises typically from the endoderm and becomes disposed between the ectoderm and endoderm. This new middle layer is the MESODERM. In this way the so-called three PRIMARY GERM LAYERS are established which are characteris tic of the developing animal, and from these the specialized tissues which compose the various systems of organs of the