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 of 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. Material and MethodsYoung rabbits, 24 days old, were employed in the investigation. The rabbits were killed by a blow and the brain exposed by removal of the vault of the skull. Under a binocular microscope the chorioid plexuses of the lateral ventricles were exposed, removed, and placed in 1 per cent osmium tetroxide buffered to a pH of 7.4 (14). The average time between the death of the animal and the immersion of the plexuses in the osmium tetroxide was 7 minutes.The plexuses were left in the fixative for periods varying from 1 to 3 hours. They were then washed in Ringer's solution for 20 minutes and dehydrated by passage through an alcohol series (20 minutes in each grade). The specimens were agitated frequently during these treatments. The plexuses were then impregnated for 1 hour with n-butyl methacrylate monomer, containing 5 per cent methyl methacrylate and finally embedded by polymerisation at 45°C. in the presence of 1 per cent benzoyl peroxide as catalyst. Sections were cut on a microtome modified according to the description by Hodge, Huxley, and Spiro (11), and were collected on grids coated with carbon film (1) as the supporting membrane. The sections were examined in a Siemens Elmiskop I electron microscope fitted with a 25/~ objective * This investigation has been aided by a grant from the Nuffield Foundation.

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