A chicken egg, when fertilized, does have the makings to become a chicken. However, the eggs you buy from the grocery store are not fertilized and are therefore, simply an egg.
Chickens lay eggs like this: Female Chicken meets male chicken. The two chickens express their love and an egg is "planted"in the female chickens bum. Later, she tries to poo, but instead comes out...
21 days after the hen starts to stay on the clutch. A broody hen may take several days to gather enough eggs to brood so do not count the days she is not staying on the nest.