embrüo arengu klassifitseerimise süsteem, The embryo can be classified according to its age, its size or its morphologic characteristics. The correlation between these three criterias will allow identifying the embryonic Carnegie stages. The sequence of appearance of the various structures in the development of the embryo remains always the same. Nevertheless embryos of the same age can be in various conditions of development. It is thus practical to divide the development into stages that take inner and outer morphologic signs into consideration. In this way precise statements can be made and various embryos can be compared with each other in a meaningful way. This separation into stages was originally developed by Streeter (1942) who termed the various organizational stages "horizons". Later this scheme was completed by O'Rahilly and Müller (1987) who spoke more simply of embryonic stages or Carnegie stages. These stages permit the embryologist to make a more precise classification than just using size would allow. Moreover, with the stage classification more precise statements can be made than is possible only using the age of the embryo (embryology.ch). The name "Carnegie stages" comes from the Carnegie Institution of Washington.