During development of the embryo it goes from being a single cell, the fertilized egg, to being a multi-cellular organism with billions of cells in it.

胚胎的发育,始于一个单一细胞,即受精卵,直到形成一个以十亿计数量的细胞构成的,多细胞有机体

耶鲁公开课 - 生物医学工程探索课程节选