What's a physical mechanism that could lead to differences between these two cells at this very early stage of development?

什么样的生理机制导致细胞,在发育伊始就产生这些差异

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This surface layer thickens at an early stage in a mouse after 8 days of development.

发育8天后，老鼠早期胚胎的,外胚层开始增厚。

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