myotube

Elongated multinucleate cells (three or more nuclei) that contain some peripherally located myofibrils. They are formed in vivo or i n vitro by the fusion of myoblasts
and eventually develop into mature muscle fibres that have peripherally
located nuclei and most of their cytoplasm filled with myofibrils. In
fact, there is no very clear distinction between myotubes and muscle
fibres proper.

http://www.mblab.gla.ac.uk/~julian/dict2.cgi?4299 

myotubes of regenerating mammalian skeletal (striated) muscle lunder the microscope. They closely resemble the myotubes found in embryonic muscle. Notice the numerous, closely packed and centrally positioned nuclei. In
adult mammalian muscle fibers (fiber = cell in this context) the nuclei
are situated at the periphery, just under what is called the sarcolemma
-- a strongly reinforced, highly specialized membrane-shell on the cell
boundary. The nuclei migrate to the periphery as differentiation
progresses.

http://www.indiana.edu/~pietsch/myotubes.html