WEBVTT

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Between 3 and 4 weeks,

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the body plan emerges

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as the brain,

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spinal cord,

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and heart of the embryo

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are easily identified

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alongside the yolk sac.

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Rapid growth causes folding

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of the relatively flat embryo.

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This process incorporates

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part of the yolk sac

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into the lining

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of the digestive system

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and forms the chest

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and abdominal cavities

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of the developing human.