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Abstract
Arterial annuli occur at the junction of side-arm branches of retinal arteries in a number of species, and we have studied them in the owl monkey, cat, dog, rat, and pig. They were initially described as being PAS-positive, but they can also be demonstrated with the Massons, elastic-Van Gieson's, and Gridley's quadruple stains. In most instances the annuli were hypercellular, and some of the cells had small dark nuclei which appeared to be different from those of surrounding cells. There is no evidence that annidi contain muscular or elastic tissue components; most likely they are composed of basement membrane or immature collagen. Man and rhesus monkey do not have arteriolar annuli, nor do they show hypercellularity at sites of arterial side-arms.