"An early step in the evolution of multicellular organisms was the association of unicellular organisms to form colonies. Myxobacteria stay together in loose colonies in which the digestive enzymes secreted by individual cells are pooled, thus increasing the efficiency of feeding (the "wolf-pack" effect). These cells represent a peak of social sophistication among procaryotes, for when food supplies are exhausted, the cells aggregate tightly together and form a multicellular fruiting body, within which the bacteria differentiate into spores that can survive even in extremely hostile conditions. When conditions are more favorable, the spores in a fruiting body germinate to produce a new swarm of bacteria.
Green algae eucaryotes exist as unicellular, colonial, or multicellular forms. Different species can be arranged in order of complexity, illustrating the kind of progression that probably occurred in the evolution of higher plants and animals. Unicellular green algae, such as Chlamydomonas, resemble flagellated protozoa except that they possess chloroplasts, which enable them to carry out photosynthesis. In closely related genera, groups of flagellated cells live in colonies held together by a matrix of extracellular molecules secreted by the cells themselves. The simplest species (those of the genus Gonium) have the form of a concave disc made of 4, 8, 16, or 32 cells. Their flagella beat independently, but since they are all oriented in the same direction, they are able to propel the colony through the water. Each cell is equivalent to every other, and each can divide to give rise to an entirely new colony. Larger colonies are found in other genera, the most spectacular being Volvox, some of whose species have as many as 50,000 or more cells linked together to form a hollow sphere. In Volvox the individual cells forming a colony are connected by fine cytoplasmic bridges so that the beating of their flagella is coordinated to propel the entire colony along like a rolling ball. Within the Volvox colony there is some division of labor among cells, with a small number of cells being specialized for reproduction and serving as precursors of new colonies. The other cells are so dependent on one another that they cannot live in isolation, and the organism dies if the colony is disrupted.
In some ways Volvox is more like a multicellular organism than a simple colony. All of its flagella beat in synchrony as it spins through the water, and the colony is structurally and functionally polarized and can swim toward a distant source of light. The reproductive cells are usually confined to one end of the colony, where they divide to form new miniature colonies, which are initially sheltered inside the parent sphere. Thus, in a primitive way, Volvox displays the two essential features of all multicellular organisms: its cells become specialized, and they cooperate. By specialization and cooperation the cells combine to form a coordinated single organism with more capabilities than any of its component parts.
Organized patterns of cell differentiation occur even in some procaryotes. For example, many kinds of cyanobacteria remain together after cell division, forming filamentous chains that can be as much as a meter in length. At regular intervals along the filament, individual cells take on a distinctive character and become able to incorporate atmospheric nitrogen into organic molecules. These few specialized cells perform nitrogen fixation for their neighbors and share the products with them. But eucaryotic cells appear to be very much better at this sort of organized division of labor; they, and not procaryotes, are the living units from which all the more complex multicellular organisms are constructed.
Multicellular organization depends on cohesion between cells. To form a multicellular organism, the cells must be somehow bound together, and eucaryotes have evolved a number of different ways to satisfy this need. In Volvox, as noted above, the cells do not separate entirely at cell division but remain connected by cytoplasmic bridges. In higher plants the cells not only remain connected by cytoplasmic bridges (called plasmodesmata), they also are imprisoned in a rigid honeycomb of chambers walled with cellulose that the cells themselves have secreted (cell walls).
Source: "From Single Cells to Multicellular Organisms," from Molecular Biology of the Cell, 3rd edn, 1994.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?db=Books&rid=cell.section.61