Recent observations are considered to provide further evidence for an abnormality involving motoneurones in DMD. The dystrophic process appears to take place in two stages of which the first occurs during early embryonic life. This stage is thought to involve faulty inductive actions of the neural tube upon mesoderm and upon itself. The neural consequences vary among individuals and are manifested as mental retardation, EEG abnormalities and losses of functioning motor units. While the first two abnormalities are non-progressive, a further loss of motor units, associated with striking reductions in the numbers of excitable muscle fibers, takes place in trunk and large limb muscles at 9-12 years. The latter process, the cause of which is uncertain, constitutes the second stage of DMD.