<P> After gastrulation the notochord--a flexible, rod - shaped body that runs along the back of the embryo--has been formed from the mesoderm . During the third week of gestation the notochord sends signals to the overlying ectoderm, inducing it to become neuroectoderm . This results in a strip of neuronal stem cells that runs along the back of the embryo . This strip is called the neural plate, and is the origin of the entire nervous system . The neural plate folds outwards to form the neural groove . Beginning in the future neck region, the neural folds of this groove close to create the neural tube (this form of neurulation is called primary neurulation). The ventral (front) part of the neural tube is called the basal plate; the dorsal (rear) part is called the alar plate . The hollow interior is called the neural canal . By the end of the fourth week of gestation, the open ends of the neural tube (the neuropores) close off . </P> <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>

When is the central nervous system fully developed