细胞是人体内结构和功能的基本单位，在成年人体内大约有40万亿到60万亿个细胞，包含了200多种不同的细胞类型。然而如此庞大的细胞群体在发育的最早期阶段是由一个单细胞——受精卵发育而来。精子与卵子在输卵管里奇迹般地会合后，形成一个受精卵，生命开始了。
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