Embryo fully covered with feathers. Albumen nearly gone and yolk increasingly important as nutrient. Day 17 Beak turns toward air cell, amniotic fluid decreases, and embryo begins preparation for hatching. Day 18 Growth of embryo nearly complete. Day 19 Yolk sac draws into body cavity through umbilicus.
Embryo occupies most of space within egg except air cell.
Day 20 Yolk sac completely drawn into body cavity. Embryo becomes chick, breaks amnion, starts breathing air in air cell Allantois ceases to function and starts to dry up.