Early human development and the chief sources of information on staged human embryos.

In a brief historical survey, the importance of Wilhelm His, senior, to human embryology is emphasized. He provided the impetus to Mall to establish the Carnegie Embryological Collection, which serves as a 'Bureau of Standards' for early human development. The Carnegie system of 23 stages for the embryonic period proper (first 8 postovulatory wk) is outlined, and some common misusages are noted. Finally, because of the difficulty in tracking down data based on staged human embryos, an annotated list of more than 40 key references is provided.