Chick embryos are staged according to a series that is known as the Hamilton and Hamburger series. Each stage has been characterized and timed and is completely recognizable.

Numbered stages in chick embryo development. Stages 1 to 3 show a round cell, which begins to cleave in stage 3. Stages 4 to 13 show a tube like structure, which begins to develop darker regions as the stages advance. In stages 14 to 19, the side profile of the embryo becomes clearer, with a rounded portion that will be come the head and a tail like portion that begins curling inward. In stage 20, two limb buds, which are small, round protrusions, become visible. In stages 21 to 28, the limb buds get longer and a spot on the head portion that will become an eye begins to darken. The embryo gradually develops until the last stage shown, stage 35. At this stage the forelimbs have taken on the shape of wings, while the hindlimbs can be distinguished as legs with claw that form feet. A beak has also formed.

Figure: Hamburger and Hamilton described 46 distinct stages of the chicken embryo in "A series of normal stages in the development of the chick embryo". Journal of Morphology. 88 (1): 49–92.

Approximate timings are:

  • Stage 6, at 24 hours: headfold stage. No somites are yet visible, primitive streak at posterior half of embryo, neural plate at the anterior end.

  • Stage 7 – 14 are based on the number of pairs of somites visible.

  • Stage 13, at 48 hours: nineteen pairs of somites. The head is partly to fully turned to left. Cranial and cervical flexures make broad curves. Enlargement of the telencephalon. Atrioventricular canal indicated by constriction. Head-fold of amnion covers forebrain, midbrain and anterior part of the hindbrain.

  • Stages 15 - 35: the appearance of specific structures within the limbs and the length of the toes are used to define the stage.

  • Stages 36 - 46: these stages are defined by the development of the eyelids, feathers, and beak.