Question: do all cells in a multicellular organism have the same function

Answer: No, In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare (e.g. vertebrates, arthropods, Volvox), as great part of species have the capacity for somatic embryogenesis (e.g. land plants, most algae, many invertebrates).