Cell division and cellular differentiation that results in conversion of an organism from or form to form.

For many multicellular eukartyotic organisms, including most animals and the sporophyte generation of plants and certain alage, development begins with the zygote and ends, approximately, with the form.

Other organisms, such as most multicellular fungi and plant or algae gametophytes, begin with a spore. Yet additional organisms "develop" from fragmented portions of the parental organism, with the quotes used because in this instance it is difficult to distinguish physiologically between development and tissue .

The process of development is highly physiologically involved, typically requiring both and among cells (i.e., cell-to-cell communication). The result is morphological modification of the growing of organism, literally resulting in the development of the organism's .

Typically distinction can be made between soma development versus germ-line development, giving rise to much of the body on the one hand and those cells that will give rise to the on the other. Growth may or may not cease over the course of development, but ultimately offspring, ideally, will be produced, thus signalling . Reproduction may take place over only a single () or instead over multiple reproductive episodes ().

Though it usually is much abbreviated, development can also occur within the context of unicellular organisms. This is seen either with changes in morphology that occur in predictable ways over the course of an organism's (e.g., ). Alternatively, unicellular organisms can form into colonies that, though not truly multicellular as commonly considered, nevertheless progress through the equivalent of , resulting in colony .

Generally the progression of forms in these latter cases are somewhat similar to that seen with , i.e., organisms or colonies are initiated with smaller forms that can be more adept than larger, more mature forms at from place to place. Growth involving cell division and/or mitosis leads to maturation and, generally, increase in size, such that the smaller, disseminating form is , thus completing the life cycle.

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See also embryogenesis, morphogenesis and organogenesis.