Egg Laying and Incubation

• Before a chicken egg is laid, fertilization has already taken place in which the egg and sperm become the parental cells. Division and growth of these living cells has begun. Cells segregate into groups that have special functions, otherwise known as tissues. Between the time that the egg is laid and incubation begins, there is no growth, and the embryo shows no signs of life.
  
  

Week One

• There is much activity inside the chick egg during the first seven days. Just 16 hours after the egg is laid, the embryo slightly resembles a chick. Within the next eight hours, the alimentary tract, the vertebral column, the beginning of the nervous system, the beginning of the head and the beginning of the eyes begin. On the second day, the heart begins to grow, as well as the ears. The heart also begins to beat. Day three brings the beginning of the nose, legs and wings. On the fourth day, the tongue begins to develop, followed by the reproductive organ formation and determination of the sex. Day six brings the beginning of the beak.
  
  

Week Two

• During the second week of incubation, the chick begins to grow feathers, and the chick's beak begins to harden. By the end of the second week, the embryo finds its way into a suitable position for breaking out of the shell. During this second week, the parts of the chick that started forming in week one continue to grow. During the growth period inside the egg, the chick gets food from the yolk sac and minerals from the shell of the egg.
  
  

Week Three

• During the final week of incubation, the chick's scales, claws and beak become hard and horny. The chick's beak naturally finds its way toward the air cell of the egg around day 17. On the 19th day, the yolk sac inside the egg begins to enter the chick's body cavity, completing the process on the following day. With the exception of the air cell, the embryo now takes up all of the room inside the egg. On the 21st day, the chick begins to peck at the eggshell and eventually makes its way out into the world.