How do chicks develop in the egg?

Chickens form just like other animals, except they do it inside an egg instead of inside a womb. Cells inside the fertilized egg divide and differentiate to form the chick. It requires 21 days for proper incubation of a chicken. During the last few days before hatching, the remainder of the yolk is taken up inside the chick and this is its food source for it's first day or two outside the shell. A shell tooth is formed at the end of the beak, and this allows the chick to break through the shell. The shell tooth disappears soon after the chick emerges.