An embryonic precursor of a skeletal muscle fibre. During development in the early embryo, numerous individual precursor cells, called myoblasts, proliferate and migrate to sites where muscle formation will take place. There they fuse to form myotubes, each containing numerous nuclei and bounded by a single plasma membrane. The process of differentiation continues as the myotube manufactures the characteristic muscle proteins that will form the contractile myofibrils and eventually matures into a muscle fibre.