The effect of barbituric acid (BA), which is a structural analogue of uracil, has been studied on the development ofPlanorbis exustus. 2. Cleavage is not affected by BA at any concentration that has been tried. At 0.06 M, BA is found to stop development of the embryos at the early gastrula stage. Some of the arrested embryos subsequently exogastrulated. The block of development cannot be removed by transferiing them to normal medium. The chemical at concentrations ranging from 0.005 to 0.04 M cause a general retardation in development. It conspicuously prolongs the duration of development between the trochophore and the veliger stages. Reduction or absence of shell occurs when animals metamorphose in the presence of BA. 3. The results seem to indicate that in the molluscan species studied there is no RNA synthesis during the cleavage phase, that morphogenesis stops when the synthesis of RNA is blocked by the use of BA, that catabolism is predominant in the pre-trochophore phase. The irreversible block imposed by BA (0.06 M) at gastrulation is interpreted to suggest that the inhibitor acts on some nuclear RNA.
