How does a metazoan body become multicellular?

In sexual reproduction, male gamete (sperm) and female gamete (ovum) fuse to form unicellular
zygote. The zygote undergoes mitosis to form multicellular embryo. All the cells of an early embryo are similar. As the embryonic cells further divide, they get differentiated and by the end of embryonic development all varieties of the cells are formed in the young adult body.