PHOTOREVERSAL OF ABNORMAL MORPHOGENESIS IN SEA‐URCHIN EMBRYOS CAUSED BY UV‐IRRADIATION

— Morphological abnormalities induced by UV-irradiation of 8- or 16-cell-stage embryos of the sea-urchin, Hemicentrotus pulchenimus, and their photoreversal were studied. UV-irradiation of the animal hemisphere of embryos caused the formation of exogastrulae, while that of the vegetal hemisphere caused the formation of permanent blastulae. These UV-induced morphological abnormalities were photoreversed when the UV-irradiated embryos were subsequently illuminated with visible light, so that the UV-irradiated embryos developed into normal pluteus-larvae. When UV-irradiated embryos were illuminated with visible light up to the onset of the DNA-synthesis phase of the following cell cycle, the UV-induced morphological abnormalities were photoreversed almost completely. The effectiveness of an exposure to visible light declined thereafter and was subsequently completely lost.