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.
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.