The chicken (Gallus gallus domesticus) is an important model organism for developmental biology.
Chickens were domesticated more than 8,000 years ago. The oldest reference to the chicken as a model organism is attributed to Aristotle who described the chicken embryo in his work Historia Animulum.
The chicken is a great model organism because the eggs are easily obtainable and it can be windowed to observe the living embryo and manipulate it. The relatively large size of the embryo simplifies observation of embryos.
By the time a hen’s egg is laid, the embryo has already passed through cleavage and begun gastrulation, forming a bilaminar blastoderm of about 60,000 cells. Development is virtually arrested at this stage if the egg is not incubated, but will recommence when the temperature is raised. The chick hatches 21 days after the egg was laid. The developmental stages of the chicken embryo are defined in the Hamburger-Hamilton series.