Trophoblast is the layer of extraembryonic ectoderm which nourishes the embryo or develops into fetal membranes with nutritive functions. In mammals, during the second week of development, the trophoblast differentiates into __________.
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syncytiotrophoblast
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ectoderm
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endoderm
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intraembryonic mesoderm
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glochidium
A
Correct answer
Explanation
The trophoblast gives rise to both the syncytiotrophoblast and the cytotrophoblast, as well as the extraembryonic mesoderm. The ectoderm is a derivative of the epiblast, and the secondary yolk sac comes from endoderm cells that line the exocoelomic cavity or the primitive yolk sac.