The central nervous system begins as single tube with wide opens at the ends called anterior and posterior neuropores. But as development continues they form what are called flexures. Two are ventral while one is dorsal.  The ventral ones are the cervical (between spinal cord and medulla) and mesencephalic flexures (placed in the midbrain). The dorsal flexure is the pontine flexure. Three primary brain vesicles are now recognisable and they are the forebrain, midbrain and hindbrain. They undergo considerable distortion and flexures until they reach their level of maturity.