The basic purpose of the nervous system is to deal with the environment. The specialization of cells required for this purpose needs to be coordinated during development.
Development is controlled by two factors.
Early differentiation is stereotyped. It is a read out of the genetic information. Cell-cell interactions change the expression of genes. The visual system is an example of a system in which the environment affects its development. The development of proper connections between the thalamus and the visual cortex is highly dependent on visual inputs.