左眼のないウシガエル

An adult bull frog，Rana catesbeiana，with abnormality of the eye was collected in a stream in the vicinity of Shimane University，Matsue city. The abnormality was characterized by complete absence of the left eye．The skin covering the orbital part on the eyeless side was continuous : no differentiation of the eyelids，cornea and the nictitating membrane was observed but the sikn possessed several holds near the posterior border of the eye orbit．Constituents of the eyeball and the eye muscle were not detectable under the skin covering the orbit. Anatomical observations revealed that the optic nerve on the eyeless side was conspicuously slender and transparent. The ill-developed optic nerve ended in a part of dura mater encephali and falied to reach the eye orbit．From histological observations，the optic nerve on the eyeless side was no more than a thin strand of loose fibrous tissue which extended a short way from the brain. The optic nerve on the side of the normal eye was normal．The brain was totally asymmetrical and especially the abnormality of the right optic lobe was remarkably observed. The embryological causes of the absence of the eye were discussed.