Part I Introduction
This Atlas focuses on the development of the human brain during the second trimester, and is Volume 3 in the Atlas of Human Central Nervous System Development series. Volume 1 (Bayer and Altman, 2002) provides a record of the development of the spinal cord. Volume 2 (Bayer and Altman, 2004a) is a record of brain development during the third trimester. We are nearly halfway finished with our journey through brain development. The brain specimens presented here are nearing anatomical maturity (at the light microscopic level) throughout the brainstem (diencephalon, midbrain, pons, and medulla) where only remnants of migrating streams of neurons and the germinal matrices remain. In contrast, the telencephalon (cerebral cortex, basal ganglia) contains many migrating neurons and active embryonic germinal matrices — the primary neuroepithelium and the secondary subventricular and subgranular zones. In addition, immature features predominate in the cerebellum; most of its neurons are still migrating and its secondary germinal matrix, the external germinal layer, is on the surface of the immature cortex. Embryonic structures predominate in the brains of all specimens to be illustrated in Volumes 4 and 5, and are the sole components of the youngest specimens.