<P> The spinal cord forms from the lower part of the neural tube . The wall of the neural tube consists of neuroepithelial cells, which differentiate into neuroblasts, forming the mantle layer (the gray matter). Nerve fibers emerge from these neuroblasts to form the marginal layer (the white matter). </P> <P> The ventral part of the mantle layer (the basal plates) forms the motor areas of the spinal cord, whilst the dorsal part (the alar plates) forms the sensory areas . Between the basal and alar plates is an intermediate layer that contains neurons of the autonomic nervous system . </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> <P> In the fifth week, the alar plate of the prosencephalon expands to form the cerebral hemispheres (the telencephalon). The basal plate becomes the diencephalon . </P>

When do the initial stages of brain formation begin