The part played by undue persistence of portions of the fetal vascular system of the eye has long been recognized in considering the factors involved in the production of congenital anomalies. Indeed, one of the most striking phenomena in the whole developmental history of the mammalian eye is the elaborate formation of a complicated transient vascular system which disappears in adult life. These vessels are peculiar to mammals, only rudiments of them occurring in animals lower in the scale. They reach a very high stage of development in man, and their occasional persistence or abnormality is to be expected and is well known. The transient vascular system referred to comprises the hyaloid artery and its branches (the outer and the inner set of the vasa hyaloidea propria and the terminal branches of the hyaloid on the back of the lens), the posterior vascular capsule of the lens, the lateral or