(1) The ganglion cell disease of amaurotic idiocy is characterized by demineralization, that of tuberous sclerosis by hypermineralization of the cytoplasm. In this respect the ganglion cell disease of amaurotic idiocy aligns itself with other degenerative conditions; that of tuberous sclerosis with other conditions of a blastomatous or neoplastic nature. The actively proliferating reactive glia cells in amaurotic idiocy are hypermineralized, like all actively growing tissues.(2) The demineralization of the diseased ganglion cells in amaurotic idiocy is of especial interest in view of the associated increase in lipoid material. In our studies thus far, it appears that wherever lipoid is accumulated, normally or pathologically, heat resistant mineral ashes are absent. This is well exemplified by the lipoid body of the normal myelin sheath, which is completely lacking in heat resistant mineral ash.4(3) The "periventricular spongioblastomatous hamartoma," presumably an attenuated form of tuberous sclerosis, which had been described by one of us 10 and three new cases of which are reported here, shows gross hypermineralization which corresponds to that generally found in tumors and other actively growing tissues.(4) The mineral ash picture of the brain in "Mongolian idiocy" (Langdon-Down's disease) does not show significant alterations from the normal.Abstract Teaser