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The Organization of the Mature Brain

This section considers some of the major foundational changes that occur during the embryonic period and early fetal period. In humans the embryonic period begins at conception and extends through GW8. By the end of the embryonic period the rudimentary structures of the brain and central nervous system are established and the major compartments of the central and peripheral nervous systems are defined. The early fetal period, which extends to approximately midgestation, is a critical period in the development of the neocortex. Most cortical neurons are generated by that time and many have migrated to their positions in the neocortex and have begun to from essential brain networks for information processing.

The First Step in Brain Development: Differentiation of the Neural Progenitor Cells

At the end of the second week after conception, the embryo is a simple, oval-shaped, two-layered structure. provides an overview of the major spatial dimensions of the embryo on embryonic day 13 note during the embryonic period age is often denoted by the number of days after conception, which is referred to as the embryonic day, thus gastrulation begins on embryonic dayorients the embryo within the context of the embryonic placenta, and Fig. 4shows how the embryonic spatial axes relate to the major spatial dimensions of the infant