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Section through hen's egg after 10 days' incubation. The living cells in the egg divide to make the tissues and organs of the young birds. The yolk provides the food for this and the albumen is a source of both food and water. The eggshell and shell membranes are permeable, and oxygen diffuses into the air space, being absorbed by part of the network of capillaries which spread out over the yolk and over a special sac, the allantois, which has become attached to the air space. The blood carries the oxygen to the embryo. Carbon dioxide is eliminated by the reverse process through the eggshell.
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