The wall of the neural tube develops in the spinal cord in an almost homogeneous manner. However, there are zones within the brain with greater or less mitotic activity. Due to this, segmented bulges are formed perpendicular to the longitudinal axis. These are called neuromeres; or, more specifically for the rhombencephalon, rhombomeres. At S12, the following neuromeres can be seen in a rostral to caudal direction: 3 segments in the prosencephalon (prosomeres), one of them forming the telencephalon medium (T), 2 segments in the diencephalon (D1, D2), 2 segments in the mesencephalon (M1, M2) and 8 segments in the rhombencephalon (Rh1-Rh8). The Rh1-Rh7 rhombomeres develop from RhA-RhC, whereas Rh8 is unchanged from RhD and covers the area of the four occipital somites (S1-S4).