The brain in the second trimester, the subject of Volume
3, is nearing anatomical maturity throughout the brainstem. In contrast, the neurogenesis and neuronal migration are still in progress in the cerebral cortex and cerebellum. Consequently, the authors chose to focus on the migration, sojourning, and settling of the neuronal populations belonging to these immature structures.

These observations can help researchers develop a better understanding of normal brain development at this formative stage. In this volume, the authors offer consideration of a new concept regarding human cortical development: the identification of the stratified transitional field (STF), which continues to play an important role in later stages. Until recently regarded as simply an intermediate and transitional layer of little significance, modern imaging techniques have shown that it is in the STF where cortical neural connections are first specified. Among other salient points, the development of this field may well bring to light those disruptions during development that lead to cerebral palsy.