Abstract
The normal production of a multicellular organism from a single-celled zygote depends upon the processes of growth, differentiation, and morphogenesis. The classification of an organism as multicellular necessitates the involvement of the growth process, since multicellularity implies the production of additional cellular units. In general, growth is associated with an increase. The increase may involve mass, cell numbers, cell size, genetic material, or a combination of these. However, the developing organism is not merely composed of more cells or larger cells, but also of several kinds of cells, cells which are different both in structure and function. This variation among cells is due to the process of differentiation. Differentiation tends to proceed from an initial uniformity to an eventual diversity, from simplicity to complexity, from generality to specialization. The differential process allows for the physical and chemical heterogenity observed among the cells and tissues of an organism.