The central nervous system appears at the beginning of the third week of embryonic development as a slipper-shaped plate of cells, called the neural plate. The other edges of the plate develop further, elevating to form the neural folds. As the folds become more elevated, they approach each other in the midline and eventually fuse, thus forming the neural tube. This fusion begins in the neck region and proceeds in a headward and tailward direction. Final closure of the tube occurs at approximately 25-27 days.

The upper (cranial) end of the neural tube has three dilations known as the primary brain vesicles. The prosencephalon, or forebrain, will eventually give rise to the cerebrum. The mesencephalon gives rise to structures collectively known as the midbrain, which include relay stations for thalamic sensory information related to auditory and visual information processing. The rhombencephalon, or hindbrain, consists of two parts, the metencephalon, which later forms the pons and cerebellum, and the myelencephalon, which forms the medulla oblongata (brainstem).

Most defects of the spinal cord result from abnormal closure of the neural folds in the third to fourth week of development. Spina bifida is one example of such a neural tube defect, which may also involve the covering of the spinal cord (meninges), vertebra, muscles, and skin. A number of closure defects can now be diagnosed prenatally by the detection of elevated levels of alpha-fetoprotein in the blood or amniotic fluid, or by ultrasound scanning. Recently, fetal surgery to close the defect prior to birth has been attempted, with good results.