Neurulation begins in the third week of development and continues into the fourth week. The principal result of neurulation is the formation of the neural tube and neural crest cells.
The plate above depicts successive stages in the induction of the neural tube by the notochord. The lateral margins of the ectoderm that overlies the notochord become elevated at the neural folds. The neural folds approach each other in the midline where the fuse and sink below the surface of the ectoderm as the neural tube. At the same time, cells of the lateral margins of the neural folds split off to form the neural crest.
The animation depicts the formation of the neural folds, groove and neural tube in the third and fourth weeks of development. Make note of the fact that fusion of the neural folds begins in the middle of the embryo and proceeds simultaneously in a rostral and caudal direction.
By day 23, the neural tube is nearly complete. The unfused portions form the anterior and posterior neuropores. Closure of these will complete neural tube formation. The anterior neuropore generally closes by day 26 and the posterior neuropore closes by the end of the 4th week, day 28.
As the neural tube completes its fusion, blocks of mesoderm condense along side it. The are called somites and they contribute to the development of the vertebral column.