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原来，在受精卵着床于子宫内膜之后，胚胎的部分胚膜和相邻的母体子宫内膜就会逐渐融合形成胎盘。胎盘与胚胎之间又会逐渐形成包裹着脐动脉、脐静脉、脐尿管的管状结构，这个结构就是脐带，我们可以把脐带想象成一根包裹成不同管线的电缆。
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