Pelizaeus (1885) described a family of which five members exhibited a rather uniform clinical symptomatology, consisting of tremor of the head, nystagmus, spastic paraplegia, etc. He thought he was dealing with an atypical familial form of disseminated sclerosis. Twenty-five years later Merzbacher (1910) investigated the same family and found the symptoms in twelve members, representing four generations. Symptomatology and clinical course of the condition was, in all patients, approximately identical: onset within the first months of life, rapid progression within the first six years, slowing down later considerably, death after twenty to thirty years due to intercurrent disease. The symptoms were tremor of the head, nystagmus; later paraplegia, ataxia, visual and auditory disturbances, involuntary movements, etc. The emotional and intellectual sphere remained comparatively unimpaired. The heredity was found to be recessive: the mothers, though themselves spared by the disease, pass it on to their sons (though among the twelve cases of Merzbacher there were also two females). In the one case investigated histopathologically by Merzbacher a diffuse demyelinization was found, particularly marked within the occipital region, distributed symmetrically in both the hemispheres and characterized by the persistence of myelin islands, often around vessels, which gave the condition the appearance of a tiger's skin. Merzbacher thought he was dealing with an hereditary aplasia of axis cylinders and myelin fibres, and called the condition aplasia axialis extra-corticalis congenita.