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It takes just 21 days for an egg to go from just laid to newly hatched chick, and a lot goes on in just the first week. Look closely and you’ll find blood vessels, a backbone, wing buds, eyes, a brain, and—throbbing prominently by day 5 or so—a beating heart.
The life of a chicken embryo begins inside the hen, before the egg is laid. A rooster deposits sperm into the hen’s reproductive tract. A sperm unites with a yolk cell, forming an embryo. The hard shell then grows around the yolk before it is laid.
After an egg is laid, an incubated egg develops quickly. What starts as a white spot on day one develops visible blood vessels on day two. Four or five days after being laid, the embryo has a visible backbone and a beating heart. After six or seven days, the wing buds, eyes, and brain are visible.
Zoomable High-Resolution Image of a Chicken Embryo. © GigaMicro.
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