when do the initial stages of brain formation begin
Development of the nervous system in humans Late in the fourth week, the superior part of the neural tube flexes at the level of the future midbrainâ€”the mesencephalon. Above the mesencephalon is the prosencephalon (future forebrain) and beneath it is the rhombencephalon (future hindbrain). The optical vesicle (which will eventually become the optic nerve, retina and iris) forms at the basal plate of the prosencephalon.