Abstract
The fine structure of the capillaries, arterioles, and venules of the human and monkey retina are described. The capillaries consist of endothelial tubes without fenestrations and a basement membrane in which pericytes are embedded. The structure of endothelial cells indicates that they are very active. Some difference in composition of the basement membrane material on the outer and inner surface of the pericytes may exist. The outer layers were vacuolated in the adult but not in young human or monkey material. This may represent a change due to age. The cytoplasmic organellesof the pericytes are concentrated on the outer (glial) side. Fibrillar material was found in theircytoplasm somewhat similar to that in smooth muscle. The endothelial cells in the monkey containedan unusual cytoplasmic organelles xoith a highly organized and complicated internal structure. These are related to other cytoplasmic membrane structures.