The internationally accepted Carnegie system of 23 developmental stages for the embryonic period proper (the first 8 post-ovulatory weeks) is outlined. At the end of this period the human embryo measures about 30 mm in C.-R. length. Some common misusages are noted, e.g., arabic numerals should be used for the states and the term "horizon" is obsolete; embryonic length is not a stage; STREETER's ages were based on the macaque and do not apply to the human. The introduction includes a discussion of the Carnegie Embryological Collection, the embryonic period, developmental stages, and finally a guide to embryonic staging in the human. An atlas of photographs of external views of embryos at the various Carnegie states is provided. Stages 1 to 8 (the presomite period, up to about 18 days) are described briefly and illustrated with photographs and photomicrographs. Stages 9 to 12 (the somitic period, about 20 to 26 days) and stages 13 to 23 (the postsomitic period, 4 to 8 post-ovulatory weeks) are summarized with reference to both external and internal development, and illustrated with photographs of the external appearances. In addition to (A) certain references cited in the introduction and in the text, the bibliography includes : (B) an annotated list of general works on staged human embryos, (C) an annotated list of selected examples of stages 1 to 15, and (D) an annotated list of references to information concerning staged embryos arranged according to the various systems of the body.