The status of the neural segment.

A crucial phase of development in the vertebrate rhombencephalon involves transient organization into segments. Recent studies on the diencephalon and telencephalon pose the question of whether segmentation might also play a role in the development of more rostral brain regions. Criteria for segmentation formulated for the hindbrain might be met by the diencephalon, although there is disagreement as to the number and arrangement of segmental units. In contrast to the hindbrain, these segments appear when neurogenesis has begun, and might represent definitive functional units. Regarding the telencephalon, it is at present unclear whether domains of gene expression are associated with other features that are characteristic of segmental development, or whether other mechanisms control specification of this region.