The Carnegie staging system is a method of staging embryonic development that evaluates the embryo's growth and differentiation in 23 stages. It supposes that human development averages 266 days, or 9.5 months. In the early stages of an embryo, a series of binary events take place in cell differentiation in order to establish cell lineage. Extraembrionic tissues are generated before embryonic tissue.

Embryonic development is apparent at 13-15 days after fertilization (stage 6). Stages 1-5 (days 1-7) are related to the proliferation of cells needed for implantation and extra embryonic tissue. At 14-15 days (stage 6b), intraembryonic cell populations migrate, producing morphological movements that yield a recognizable embryo. Cells from the primitive streak proliferate, providing a cell population that passes to the embryo. By days 19-21 (stage 9), the neural cell population is evident, resulting in the neurulation related processes. Neural place formation...