The myoblasts become spindle-shaped and begin to express a muscle-specific protein. At S19-20, the mononucleated myoblasts accumulate and fuse together to form polynucleated cylindrical myotubes, in which the nuclei are arranged in a chain located in the centre and myofilaments are located in the periphery, near the plasma membrane.
At week 9, the primordia of most muscle groups are already present. Contractile proteins are synthesized and the primordia of neuromuscular junctions begin to appear. From week 10, the neuromuscular junctions become functional. The muscle cells react to impulses from motorneurons and contract. The transformation of myotubes into muscle fibres is thought to occur under the influence of the contractile activity. The myofilaments move towards the centre of the myotubes and the nuclei move towards the periphery. In this way, the adult structure of the muscle fibre is formed. The transverse lines (Z-bands) delimiting the sarcomeres become visible under the light microscope.