The effects of 8-azaguanine, a purine analogue, 2-thiouracil, a pyrimidine analogue and ethionine, an amino-acid analogue, on the various stages of cleavage ofPlanorbis exustus have been studied. The development of eggs was inhibited by these analogues. 2. The effects of these analogues are not reversible: The effects produced by 8-azaguanine, 2-thiouracil and ethionine were not prevented by guanosine, uracil and methionine. 3. Other effects of these analogues have been briefly, discussed and they were compared with those of other agents modifiying the development ofPlanorbis eggs. 4. It is suggested that these analogues bring about their effects by interfering with RNA and protein synthesis.
