Staging tables for avian embryos: a little history.

Absolute time elapsed since fertilization, or hours' incubation, is not a good measure of the precise degree of development of an embryo because there is considerable variation. The chick embryo benefits from a detailed, well defined staging system introduced by Hamburger and Hamilton in 1951, perhaps the most precise and detailed available for any species. This paper briefly reviews the background and legacy of this table, including the remarkable work of its predecessors, Mathias Duval and Franz Keibel. It also begs the question of why the mouse embryo still lacks a similarly precise classification.