At about the third week of prenatal development, the ectoderm of the embryonic disk forms the neural plate, which folds at the edges into the neural tube (neuraxis). The middle pa is the first to become tubular, so that for some time the neural tube is open cranially and caudally. These openings are called the anterior and posterior neuropores, respectively. The fusion of two edges of the neural plate extends cranially and caudally. Ref: Clinical Neuroanatomy By Richard S. Snell, 2010, Page 14.