The classification of embryo will based on age,size and phenotypic characters. The interlinks between these three factors allows to identify the embryonic carnegie stages. In 1942 Streeter initially studies on various organizational stages or termed as “horizons”.Later the same study was carried out by O’Rahilly and Muller in 1987 describe about embryonic stages or Carnegie stages. Owing to these stages the embryologists able to classify the embryo not just by its size but by using its age.
On observing the variations between internal and external embryonic development.The embryologists decided to divide the embryonic weeks (56 days) into 23 Carnegie stages. The beginning of feotal period after the 8th week i.e., the growth and maturation of organs takes place. Since the internal and external alterations are less observed . Therefore it is very difficult to divide the carnegie stages in an accurate form.
