<P> At the beginning of the third week of development, the embryonic ectoderm forms a thickened strip called the neural plate . By the fourth week of development the neural plate has widened to give a broad cephalic end, a less broad middle part and a narrow caudal end . These swellings represent the beginnings of the forebrain, midbrain and hindbrain . Neural crest cells (derived from the ectoderm) populate the lateral edges of the plate at the neural folds . In the fourth week in the neurulation stage the neural plate folds and closes to form the neural tube, bringing together the neural crest cells at the neural crest . The neural crest runs the length of the tube with cranial neural crest cells at the cephalic end and caudal neural crest cells at the tail . Cells detach from the crest and migrate in a craniocaudal (head to tail) wave inside the tube . Cells at the cephalic end give rise to the brain, and cells at the caudal end give rise to the spinal cord . </P> <P> The tube flexes as it grows, forming the crescent - shaped cerebral hemispheres at the head . The cerebral hemispheres first appear on day 32 . Early in the fourth week the cephalic part bends sharply forward in a cephalic flexure . This flexed part becomes the forebrain (prosencephalon); the adjoining curving part becomes the midbrain (mesencephalon) and the part caudal to the flexure becomes the hindbrain (rhombencephalon). These three areas are formed as swellings known as the primitive vesicles . In the fifth week of developmement five brain vesicles have formed . The forebrain separates into two vesicles an anterior telencephalon and a posterior diencephalon . The telencephalon gives rise to the cerebral cortex, basal ganglia, and related structures . The diencephalon gives rise to the thalamus and hypothalamus . The hindbrain also splits into two areas--the metencephalon and the mylencephalon . The metencephalon gives rise to the cerebellum and pons . The myelencephalon gives rise to the medulla oblongata . Also during the fifth week, the brain divides into repeating segments called neuromeres . These are known as rhombomeres seen in the hindbrain . </P> <P> A characteristic of the brain is gyrification (wrinkling of the cortex). In the womb, the cortex starts off as smooth but starts to form fissures that begin to mark out the different lobes of the brain . Scientists do not have a clear answer as to why the cortex later wrinkles and folds, but the wrinkling and folding is associated with intelligence and neurological disorders . The fissures form as a result of the growing hemispheres that increase in size due to a sudden growth in cells of the grey matter . The underlying white matter does not grow at the same rate and the hemispheres are crowded into the small cranial vault . The first cleft to appear in the fourth month is the lateral cerebral fossa . The expanding caudal end of the hemisphere has to curve over in a forward direction to fit into the restricted space . This covers the fossa and turns it into a much deeper ridge known as the lateral sulcus and this marks out the temporal lobe . By the sixth month other sulci have formed that demarcate the frontal, parietal, and occipital lobes . A gene present in the human genome (ArhGAP11B) may play a major role in gyrification and encephalisation . </P> <P> The motor system of the brain is responsible for the generation and control of movement . Generated movements pass from the brain through nerves to motor neurons in the body, which control the action of muscles . The corticospinal tract carries movements from the brain, through the spinal cord, to the torso and limbs . The cranial nerves carry movements related to the eyes, mouth and face . </P>

What is the central part of the brain called