The allantois is the sac-like extraembryonic membrane that removes waste from the embryo. As the embryo grows, so does the allantois, eventually pressing against the chorion. This allows the allantois to exchange oxygen and carbon dioxide with the air outside the eggshell. In addition, the allantois also serves as a disposal site for uric acid.
The yolk sac is the extraembryonic membrane that surrounds the egg yolk. The yolk sac has a well-developed vascular system that transports nutrients from the egg yolk to the developing embryo.
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