How Does a Chicken Hatch from an Egg?
Inside a fertile chicken egg is a tiny cell called a germ cell? While the hen sits on the egg and keeps it warm, the germ cell divides into millions of cells to form the various parts of the chicken. The large yellow egg yolk and the thick clear liquid (or egg white) are not part of the chicken, but provide the nourishment for the chick that will grow from the germ cell. The chick grows until it fills the whole egg. In about 21 days, the chick is ready to come out of the egg. It cracks the shell with its beak and pushes out–the chick has “hatched”.
The process is slow and takes about a month of patience. During embryonic development, a spike forms on the beak of a chick either at the tip or on the upper side of the beak. This spike looks something like a small tooth and is called the egg tooth. In chicken eggs, it must develop prior to the nineteenth day of gestation since they break into the air sac at that time. They must break into the air sac because the pores in the eggshell no longer allow enough oxygen transfer for them to breathe. They will be hatching very soon at 21 days gestation. The egg tooth disappears by the time the chick is two weeks old.
Chicks are equipped during gestation with a special muscle on the back of their necks called a pipping muscle. The strength of the pipping muscle enables the fragile chick to beat against the hard calcium carbonate eggshell for hours until it finally breaks through and eventually emerges. Initially, this large neck muscle contracts to force the chick’s egg tooth through the egg’s inner membrane to access the air sac so it can breathe a day or two before hatching. This process is called internal pipping. Once the chick has broken through the inner membrane, if you listen carefully, you may hear it chirping. This temporary air supply gives the chick time to break through the eggshell.
A clutch of chicks may begin making clicking noises before actually hatching. This allows them to communicate with other chicks ready to hatch so that they can hatch at the same time. In the wild, this is a protective measure so the chicks can remain together. Once the chick breaks the internal membrane it immediately begins tapping at the eggshell until it breaks a small hole and begins to breathe outside air. This process is called external pipping. As the chick continues to hammer at the opening, you will begin to see the tip of the beak through the hole. The chick also turns and pushes in the shell; creating a crack that helps it open the shell and escape. The chick emerges wet and tired and may rest for a few hours until it dries.