Chick embryo development has three phases, including early and middle phases when the chick's organs and systems develop and a third stage when the formed systems grow and mature. It takes 21 days from incubation to hatching of the chick.
Within 24 hours of incubation, the embryo begins to develop an immune system, head and beginnings of the eyes. During the second day, the heart forms, and the ears begin to emerge. By the time 42 hours passes since the beginning of incubation, the heart begins to beat.
On the third day, the legs and wings begin to develop. On the fifth day, the reproductive organs begin to form and differentiation of sex occurs. By the eighth day, the embryo shows signs of feather development.
At the end of two weeks, the embryo forms claws and rotates into proper position for breaking out of the shell of the egg. The egg tooth hardens to allow the chick to break out of the shell.
Between days 16 and 21, the claws and beak of the chicken become firm and resemble those of a hatched chick. The embryo absorbs the yolk sac into the stomach to allow for survival for up to 48 hours after hatching without food or water.
During this time the chick takes its first breath inside the shell of the egg, and may be heard peeping inside the egg. On the 21st day of incubation, the chicken emerges from the shell, and the egg tooth dries and falls off within the following 24 hours.