In 1951 Viktor Hamburger and Howard Hamilton published normal stages of chick embryogenesis that became a standard in the field and is still widely used today. “Ever since Aristotle ‘discovered’ the chick embryo,” they wrote, “as the ideal object for embryological studies, the embryos have been described in terms of the length of time for incubation, and this arbitrary method is still in general use…” (Howard and Hamilton 1951, p. 49). Hamburger and Hamilton recognized that the problem with staging embryos temporally was that the total length of development varied depending on factors such as breed or the temperature of incubation. Instead of using time intervals, they staged chick development based on morphological characteristics. This visual staging system, published as a series of micrographs and drawings of each stage, was critical for establishing the chick as an experimental model organism in the latter half of the 20th century.
Working out these stages required close study of the morphological details of chick embryos. Hamburger took detailed visual notes on parts of embryos that he ultimately used to stage them. For example, he closely studied the development of the limb buds, comparing and differentiating them based on subtle differences in shape. Hamburger also characterized the pharyngeal arches in embryos at various points in development. In his drawings, he followed their changing shape and created a labeling system for easy identification of each tissue fold. Many of these drawings were synthesized from observations of several embryos and were not direct representations of individual specimens.
Once they had a general idea of each stage, Hamburger and Hamilton generated images for their publication. This process involved finding embryos representative of each stage and then photographing them. Each specimen was photographed and printed several times to obtain a publishable shot that depicted important identifying features. In many cases, a photograph was paired with a simplified drawing to more clearly call attention to morphological details.