<P> Typically, these neurodevelopmental processes can be broadly divided into two classes: activity - independent mechanisms and activity - dependent mechanisms . Activity - independent mechanisms are generally believed to occur as hardwired processes determined by genetic programs played out within individual neurons . These include differentiation, migration and axon guidance to their initial target areas . These processes are thought of as being independent of neural activity and sensory experience . Once axons reach their target areas, activity - dependent mechanisms come into play . Neural activity and sensory experience will mediate formation of new synapses, as well as synaptic plasticity, which will be responsible for refinement of the nascent neural circuits . </P> <Dl> <Dd> See embryogenesis for understanding the animal development up to this stage . </Dd> </Dl> <Dd> See embryogenesis for understanding the animal development up to this stage . </Dd> <P> Neurulation is the formation of the neural tube from the ectoderm of the embryo . It follows gastrulation in all vertebrates . </P>

When does the nervous system develop in an embryo