Day 2: The Heart of the Matter

The Egg Yolk

On the second day of incubation a blood ring appears around the area opaca. The question mark shape in the middle of the ring is the embryo. By 24 hours, pockets of blood cells and developing blood vessels, called blood islands, begin to appear on the surface of the yolk along the outer edges of the area opaca. These are called the vitelline vessels. At the same time, the heart and vessels within the chick embryo have begun to develop. The omphalomesenteric veins can be seen branching out from the embryo in this photo of a 43 hour chick embryo. The vitelline vessels and the branches of the omphalomesenteric arteries and veins grow toward each other. When they meet, channels open and circulation from the yolk to the embryo begins. Vitelline circulation is responsible for bringing food from the yolk to the embryo.

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