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Fig. 22. — Dorsal view of human embryo in the neural-plate stage. The amnion is opened, exposing the germinal plate lying directly on the yolk-sac. (Graf Spee, from Kollmann.) The further elevation of the edges and their approximation and fusion across the median line to form the neural tube is shown in four stages in Fig. 23. The comparison of these figures shows that the formation of the neural tube is most advanced in the middle of the germ plate corresponding to the junction of brain and spinal cord. From this region the differentiation and closure of the tube extends caudally and orally, the last portions to close being called the anterior and posterior neuropores (see Figs. 24 and 26). The process of closure, though it always begins in about the same region, shows some variation in the time of its occurrence. In Fig. 23, C, it is further advanced than in Fig. 23, D, which, judging from the number of somites, is the older embryo of the two.