Your hen will sit on her eggs and keep them warm and safe for approximately 21 days after she lays them before the eggs begin to hatch. Inside the eggs, the chicken embryos develop into fully formed baby chicks. Inside the shells with the embryos is everything they need to survive until hatching.
How Eggs Are Formed
A normal, healthy laying hen can produce one egg approximately every 24 hours during laying seasons. The egg is formed when the yolk, called an oocyte during this stage of development, releases from the hen's ovary and into the oviduct. If the hen has mated with a rooster, the oocyte becomes fertilized during its trip through the oviduct. It also develops the egg white and the shell of the egg during this time. When the egg is fully formed, the hen will lay it.
The chicken embryo inside the egg continues to develop physically after the egg has been laid. The embryo feeds on the yolk of the egg. The yolk of the egg provides plenty of nutrition to the developing chick. The yolk will fully sustain the chick until he is ready to hatch.
Importance of Fertilization
Hens lay eggs even if the eggs have not been fertilized by a rooster. If your hen has not been exposed to a rooster then you have no chance of a chick developing inside the egg. The embryo can not develop without having been fertilized.
Why Eggs Don't Hatch
If your eggs have been fertilized and you do not want chicks to develop inside the eggs, you will need to remove the eggs from the hen and refrigerate them shortly after they have been laid. Cold temperatures will stop the embryos from developing. If you do want chicks, but your hen does not sit on the eggs and the eggs become too cold, the eggs will not hatch. The embryos can not survive and develop if the climate is too cold or too dry.