Many species of single-celled organisms exhibit different morphological forms. Individual cells may even exhibit different morphologies during the course of their biological life cycle. In times of environmental stress, some bacteria form endospores which are distinct from their normal free-living state. Some amoebae may differentiate into different cell types upon conglomeration with other amoebae of the same species, as in Dictyostelium discoideum.
All higher multicellularorganisms contain cells specialised for different functions. Most distinct cell types arise from a single totipotent cell that differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division). Multicellular organisms are composed of cells that fall into two fundamental types: germ cells and somatic cells. During development, somatic cells will become more specialized and form the three primary germ layers: ectoderm, mesoderm, and endoderm. After formation of the three germ layers, cells will continue to specialize until they reach a terminally differentiated state that is much more resistant to changes in cell type than its progenitors.