Human prenatal development is divided into an embryonic period and a fetal period. The embryonic period begins with fertilization and ends eight weeks later. The staging of human embryos was introduced in 1914 by Franklin P. Mall at the Department of Embryology of the Carnegie Institution of Washington. Mall’s successor, George L. Streeter, later refined the classification of human embryos into 23 stages, or “developmental horizons”.
It is important to note that each of the 23 Carnegie stages represents an arbitrary point along the time-line of development, akin to a “freeze-frame” in a movie. The stages are based on a variety of morphological features and are independent of chronological age or size. As implied by the original term, “horizon”, the stages are not definitive steps, that when combined lead to a fully formed human fetus, but rather they are a series of events that must be completed during development. As the description of each stage is based on the features seen in an “average” embryo individual embryos may not fit exactly into a particular stage. There may also be considerable variation within a stage and overlap between stages.
Old site: http://virtualhumanembryo.lsuhsc.edu/HEIRLOOM/heirloom.htm
New site (under construction): http://virtualhumanembryo.lsuhsc.edu/