Abstract
When D. excentricus eggs are permitted to cleave in 0.1[image] mercaptoethanol in sea water and then returned to normal sea water, a large proportion of the embryos develops as twins, producing normal twin plutei. The effectiveness of the treatment is restricted to the short period during which the 1st cleavage furrows are forming. If the treatment is repeated at the time of the 2d cleavage, quadruplets are produced. The results are discussed in terms of the significance of thiol groups of proteins for the interaction of blastomeres.