The basic shape, form and structure of an organism (whether fungal, plant or animal) does not arise all at once. Rather, the shape and form emerge as a result of a sequence of developmental adjustments. Each of these is usually irreversible within its morphogenetic sequence although often reversible by some gross disturbance; for example, differentiated cells being put into tissue culture, nuclear and cell transplants, regeneration after injury, etc. The whole process in which the final organisation and pattern of the organism is established is termed ‘morphogenesis’.

The most extensive research on the topic has been done with animals and from this a vocabulary has been established which describes morphogenetic events without pre-judging the mechanisms which may be involved (Slack, 1991). It is evident that as the embryonic organism develops towards adulthood, each intermediate state represents a reduction in developmental potential compared with the previous state. Each adjustment (or developmental ‘decision’) is made by cells already specified by earlier adjustments to belong to a particular developmental pathway. Consequently, developmental decisions are made from among progressively smaller numbers of alternatives until the particular structure to which the cell will contribute is finally determined. Classic embryological transplantation experiments revealed these states. Where the explant differentiated to a state representative of its old position then it was said to have been determined prior to transplantation.