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The neural developmental processes can be summarized by that the neural tissue, including the brain and spinal cord, arises from the ectodermal layer, which becomes identified in the embryonic disk during the second week after fertilization. The ectodermal tissue soon thickens to form the symmetrical neural plate, which is the forerunner of the brain and the spinal cord. By day 18 [Carnegie stage (CS) 8], a groove appears in the midline of the embryonic disk, which becomes deeper and longer due to the development of the bilateral neural folds along the lateral ends of the neural groove. By around day 20 [Carnegie stage (CS) 9], the primordia of the three primary brain vesicles (the forebrain of prosencephalon, the midbrain or mesencephalon and the rhombencephalon) are discernible as thickenings in the neural plate. The bilateral neural folds begin to fuse with each other at about 22 days [Carnegie stage (CS) 10]. This fusion initiates at the level of the future cervical region and extends to both cranial and caudal directions, to form a neural tube. The mode of fusion of neural folds is not the same in different species, and at least two initiation sites are recognized in human embryos. The openings at the rostral and caudal ends of the neural tube, i.e. the anterior and posterior neuropores, close at 24 days [Carnegie stage (CS) 11] and 28 days [Carnegie stage (CS) 12], respectively, and complete the formation of the neural tube. If the closure of the neuropores are disturbed, various forms of neural tube defects (NTD) can occur. Neural tube formation described above, which is accomplished by the fusion of neural folds, is called primary neurulation. On the other hand, the caudal end of the spinal cord is formed in a quite different fashion caudally to the posterior neuropore.