Carnegie stages

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo. In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo. The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos. In the human being only the first 60 days of development are covered; at that point the term embryo is usually replaced with the term fetus. It was based on work by Streeter (1942) and O'Rahilly and Müller (1987). The name 'Carnegie stages' comes from the Carnegie Institution of Washington. While the Carnegie stages provide a universal system for staging and comparing the embryonic development of most vertebrates, other systems are occasionally used for the common model organisms in developmental biology, such as the Hamburger–Hamilton stages in the chick. Days are approximate, and reflect the days since the last ovulation before pregnancy ('Postovulatory age').