The fetal retina contains some neuroblasts almost as soon as the optic vesicle has invaginated to incorporate the lens vesicle, and is thereby made highly radiosensitive. In the mouse the optic vesicle is first seen at nine days gestation, in the rat about nine and a half days, in the monkey probably about 18, and the human by 25 days. Soon thereafter the vesicle makes contact with the head ectoderm and begins to invaginate to form the optic cup, into which the lens vesicle is pinched off from the head ectoderm. From this time until late in gestation retinal primordia are transforming through neuroblasts to neurons to give rise to the nuclear layers of the formed retina. Thus, the eye is radiovulnerable for an extensive portion of the gestation period, as will be demonstrated in this study, for it is the neuroblast (or any differentiating cell) that is so particularly