These tasks require circuitry of astonishing precision on an enormous scale, with a complexity far outweighing that of man-made microcircuits. Reforming those circuits in the adult brain, where the growing neurones would have to ignore countless millions of possible connections in order to find the right kind of neurone in the right part of the brain is just impossible. So the adult brain seems to prohibit the growth of neurones and the formation of new pathways. During foetal development the brain changes in several different ways. Massed movements of cells mould the adult brain's shape.
Cells replicate to produce neurones by the billion.
Chemical signals cause the cells to evolve into different types of neurone each secreting its own chemical messenger and responding in specific ways to the chemical messengers it encounters. The different cell types link up to form intricate circuits. And the huge numbers of neurones that fail to make enough connections commit suicide. But the changes must occur in order. Nerve cells can only make the right connections at the appropriate stages in the normal sequence of development.