Recent studies of the fine structure of the cranial meanings of laboratory animals and man have shown that there is no subdural space. The latter is formed artificially by the tendency of meningeal tissues to cleave along a collagen-free zone, the dura-arachnoid interface layer. This layer is composed of an outer zone of dural border cells and an inner arachnoid barrier layer. The fine structure of nine arachnoid cysts was studied to determine the derivation of the cyst's wall from the various components of normal human meanings. A cleaved dura-arachnoid interface layer covered only the dome of the cyst where the latter had abutted the dura mater. The interface layer did not partake in forming the cyst's wall. The dominant phenomenon of the cyst's wall was an absence of the normal trabeculation of the subarachnoid space, the trabecules being replaced by tightly packed collagen fibrils and a few scattered cells in between. Some cells were layered discontinuously at the inner face of the cyst wall, but there was no organized inner lining. No evidence was found for either a tight sealing of the extracellular spaces in the cyst's wall, nor for the existence of an active transcellular fluid movement.