A transverse unitary subdivision of the neural tube that shares a common dorsoventral structure (floor, basal, alar, and roof plates), but each having differential molecular identities and fates; they comprise the secondary prosencephalon, diencephalon (prosomeres), the midbrain (mesomeres), and the hindbrain (rhombomeres). [ https://doi.org/10.1016/j.tins.2013.06.004 https://github.com/obophenotype/uberon/issues/318 ]