As long as the temperature of the egg remains below 25°C, the embryo inside it remains quiescent (this phase is called dormancy) since this temperature is that of physiological zero.
Temperatures above 25°C, provided by natural brooding or by means of an artificial incubator, allow embryonic development (which for hens is 21 days).
During embryonic development, the embryo undergoes important changes that can be divided into three phases. The first phase, which lasts 6 days, is called embryonic differentiation and is characterized by intense cellular migration (gastrulation) and the formation of the outline of all the vital organs (the heart begins to beat at the 44th hour of incubation). The second phase is the development of the organs and lasts from day 7 to day 16 of incubation.
The third phase, from day 17 to day 21 of incubation, is the maturation and preparation phase for hatching.
About 3 days after hatching, the embryo occupies the entire egg and begins autonomous pulmonary respiration (through the air chamber) and also the first vocalizations (internal peeping).
Hatching occurs on day 21 and the chick needs between 12 and 18 hours to emerge from the egg.