Chick embryos need proper heat, moisture and physical movement of the eggs in order to survive to hatching. During its 21 days of incubation, the chick will grow from the size of a pinhead to 1.5 oz.
Day After Fertilization
The embryo shows signs of resembling a chick by 16 hours old. By 24 hours, a vertebral column, nervous system, head and eyes begin to form.
Days Two Through Five
During this stage, the heart begins to beat and the legs and wings begin to form. By the end of day five, reproductive organ formation and gender differentiation begins.
Days Six Through 10
The beak and egg tooth are formed during this period. By day eight, feathers begin to form. Eggs should be turned until hatching to make growth uniform.
Days 11 through 17
The embryo begins getting into position for hatching. Beak scales and claws harden. The beak turns toward the air cell near the end of this stage.
Days 18 Through 20
The yolk sac draws into the unborn chick's body cavity to provide nutrients during its first days. The beak enters the air cell and the chick takes a breath.
Hatching at Day 21
With lungs functional, the chick must hatch to survive. After first pecking the shell, it will rest before breaking free completely in 10 to 20 hours.