Fig. 2. Timeline of murine nervous system development. Shortly after fertilization, the developing embryo undergoes gastrulation, a time where it subdivides without significant expansion of tissue and establishes anterior and posterior poles. In late gastrulation, neural induction begins with the specification of the Speamann’s organizer (SO) or Hensen’s node (HN). Neuralation follows gastrulation, establishing the embryonic neural axis with the formation of the neural groove on the dorsal ectodermal surface. By embryonic day (E) 8, the neural tube invaginates and nearly a day later fuses along the midline to form a complete neural tube. Around E10.5–E11, cells in various VZs (pink) begin to give rise to structures such as the retina, cerebral cortices, and the cerebellum. During this time, cycling cells progressively switch from symmetric to asymmetric division with at least one daughter cell permanently exiting the cell cycle. By birth most of the CNS, except for a few specialized regions including the rostral migratory stream, hippocampal dentate gyrus, granule cells of the cerebellum, and glial cells, enter a state of replicative quiescence. Bs, brainstem; D, diencephalon; Mb, midbrain; Sc, spinal cord. The photograph of neural invagination was reproduced with kind permission from a picture given by Gary Schoenwolf, University of Utah, Salt Lake City, Utah, to Dr. Sally Temple, Albany Medical College, Albany, NY (80) .