<P> Late in the fourth week , the superior part of the neural tube flexes at the level of the future midbrain -- 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 . </P>
<P> Late in the fourth week , the superior part of the neural tube flexes at the level of the future midbrain -- 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 . </P>
when do the initial stages of brain formation begin
when do the initial stages of brain formation begin
Late in the fourth week
Late in the fourth week