Day 2: The Heart of the Matter

The Blood Vessels Get Ready

The circulatory system of the embryo is organized into three arcs:

Vitelline Circulation

The vitelline vessels form a layer of capillaries over the surface of the yolk, which absorb food and water from the yolk and carry it back to the embryo. While the embryo is still young, it relies on the vitelline vessels to also absorb oxygen, which has passed through the porous shell and diffused through layers of albumen. The vitelline vessels start out as blood islands on the surface of the yolk. Channels form around the blood islands and eventually enclose them to become vessels. At the same time, the omphalomesenteric veins begin to grow from the caudal end of the heart and omphalomesenteric ateries branch from the embryonic aorta. The vitelline vessels and the omphalomesenteric vessels grow toward each other. By 33-35 hours they have connected.

Intra-Embryonic Circulation

The major vessels which supply the embryo itself also start to form as the heart prepares to begin pumping. The ventral aorta is an extension of the truncus ateriosus. It supplies blood to all the major arteries of the embryo's body. Blood from the body returns to the heart through the cardinal veins, which empty into the sinus venosa.

Allantoic Circulation

By the third or fouth day of incubation, the chick embryo needs more oxygen than can be supplied by only the vitelline capillaries, and the embryo begins to rely on the allantoic circulation. The allantois is an extra-embryonic membrane that begins to grow from the hindgut of the chick embryo around 66 hours. It remains attached to the embryo by the allantoic stalk. As it grows with the chick embryo, the allantois is pressed against the inside of the egg shell. Here its capillaries can easily absorb oxygen passing through the pores of the shell in exchange for carbon dioxide and other waste products from the embryo. The allantois is supplied by the allantoic artery which branches off the aorta near the allantoic stalk. Oxygenated blood from the allantois returns to the heart through the allantoic vein. The allantoic vein empties into the sinous venosus.

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