WEBVTT

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By 1 week,

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cells of the inner cell mass

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form two layers called

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the hypoblast

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and epiblast.

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The hypoblast gives rise

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to the yolk sac,

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which is one of the structures
through which

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the mother supplies nutrients

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to the early embryo.

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Cells from the epiblast form

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a membrane called the amnion,

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within which the embryo

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and later the fetus

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develop until birth.

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By approximately 2 1/2 weeks,

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the epiblast has formed

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3 specialized tissues,

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or germ layers,

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called ectoderm,

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endoderm,

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and mesoderm.

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Ectoderm gives rise

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to numerous structures

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including the brain,

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spinal cord,

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nerves,

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skin,

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nails,

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and hair.

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Endoderm produces the lining
of the respiratory system

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and digestive tract,

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and generates
portions of major organs

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such as the liver

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and pancreas.

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Mesoderm forms the heart,

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kidneys,

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bones,

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cartilage,

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muscles,

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blood cells,

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and other structures.