Anomalous development of retinal vessels.

MICHAELSON (1954), writing on the retinal blood vessels in man, stated that "an artery crossing an artery, or a vein crossing a vein, has not once been reported, or been personally observed". This is in keeping with the present hypothesis that the development of retinal vessels in vertebrates is stimulated by a local intraretinal factor. In man, each quadrant of the fundus accumulates, during development, sufficient of this factor to attract to itself a vessel complex. Thereafter, it affects maximally the nearest budding vessel, thus explaining the rarity of an artery crossing an artery, or a vein crossing a vein. The two following cases show respectively a retinal vein crossing a retinal vein and a retinal artery crossing a retinal artery.