Hamburger–Hamilton stages

In developmental biology, the Hamburger–Hamilton stages (HH) are a series of 46 chronological stages in chick development, starting from laying of the egg and ending with a newly hatched chick. It is named for its creators, Viktor Hamburger and Howard L. Hamilton.

Chicken embryos are a useful model organism in experimental embryology for a number of reasons. Their domestication as poultry makes them more readily available than other vertebrates (such as mice), and being oviparous, the embryos are easily accessible. However, the rate of development can be affected by a range of factors; including the specific breed, the temperature of incubation, the delay between laying and incubation, and the time of year, raising the need to create a standardised system based on morphology rather than chronological age.

There had been a previous attempt to create a morphological system for staging chick development by the German embryologists Keibel and Abraham in 1900, but this system lacked detail and was not widely used, with most researchers relying on somite number or age to identify the stage of development. Hamburger and Hamilton aimed to provide a detailed description of developmental events, modelled on an earlier system for Axolotl by Harrison.

The Hamburger–Hamilton system provides advantages over the Carnegie system in that it allows the developing chick to be accurately characterized during all embryonic stages, and is used universally in chick embryology.

Chick embryos can be "staged" according to the different morphological landmarks. Although most organ systems have a stereotypical appearance at each stage, there are a few which particularly lend themselves to use in staging chick development.

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