3.5 Freedom and determination on the level of cells.
In embryogenesis a cell on its way to differentiation of function, may 'change it's mind' up to a certain point in its development. This happens for instance when, by migration, a cell arrives into another morphogenetic field than the one it grew up in. Since field organizers operate in progressively smaller partitions, cells at the boundaries of these domains will often be required to change their developmental pathways, because at the transitions it is uncertain to which domain a cell belongs. The initial field covering the whole embryo becomes partitioned into subfields corresponding to head, limbs etc. Then finer partitions arise within these domains generating vision-fields, olfactory fields etc. Reallocation of cells to make up for deficient fields in a damaged area is an instance of regenerative power.
Cells are continually tested whether the developmental pathway they follow is the correct one. Variations during genetic unfolding are compared with the exigencies of the immediate environment, that is the neighbouring cells which determine the morphogenetic field. Positive selection means that the developmental direction is reinforced. In the case of refusal the cell may move on until it finds a more reinforcing environment, or it may switch the direction of its developmental pathway, or it may do both. We have seen Darwinian evolution in action in the time-scale of cellular replication.