Development Stages of the Chicken Embryo The fertilized blastodisc is now called the blastoderm. It grows to become the embryo and as it grows the primary source of food is the yolk. Waste products like urea collect in a sack called the allantois. The exchange of oxygen and carbon dioxide occur through the eggshell, the chorion lines the inside surface of the egg and is connected to the blood vessels of the embryo. The embryo develops inside the egg for 21 days until a chick hatches out of its eggshell.