Since the classical experiments of D a n d y and Blackfan ( 5 ) it has been generally recognized that the chorioid plexuses are the principal sources of cerebrospinal fluid. Controversy still exists, however, concerning the method of production of the fluid. I t has been held by some authorities that the cerebrospinal fluid is actively secreted by the epithelial cells of the plexuses and by others that the process is one of selective filtration. A further problem is presented by the inability

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... some substances which circulate freely in the blood stream to enter the cerebrospinal fluid. This finding has led to the concept of a blood-cerebrospinal fluid barrier whose principal locus must reside in the chorioid plexus. The work reported in this communication was carried out with the idea that any attempt to elucidate problems concerning the chorioid plexus demanded as a prerequisite an examination of the fine structure of the plexus using higher resolution than is possible with light microscopy.