当一个生物产生时，胚胎将形成三个胚层外胚层、内胚层和中胚层，然后再由这些胚层形成机体的组织和器官。
When an animal develops, embryos form three cellular or germ layers - ectoderm, endoderm and mesoderm - which then go on to form the body's organs and tissues.
在外胚层和内胚层之间是中胚层，它形成肌肉、生殖腺、血管和排泄器官。
Between the ectoderm and endoderm is mesoderm, which forms the muscles, reproductive organs, blood vessels and excretory organs.
在外胚层和内胚层之间是中胚层，它形成肌肉、生殖腺、血管和排泄器官。
Between the ectoderm and endoderm is mesoderm, which forms the muscles, reproductive organs, blood vessels and excretory organs.
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