Any of three cellular layers, the ectoderm, endoderm, or mesoderm, into which most animal embryos differentiate and from which the organs and tissues of the body develop through further differentiation.胚层：外胚层、内胚层或中胚层三个细胞层的任一个，多数动物的胚胎先分化成胚层，而后通过进一步分化发育成器官和组织