The A and C blastomeres at the 4 cell stage ofIlyanassa obsoletawere deleted and the resulting BD half embryos were cultured until the age of seven or eight days. The embryos were observed daily for developmental progress. The week-old larvae were fixed and closely examined for the presence of velar lobes, velar pigment, everted stomodeum, eyes, foot, operculum, statocysts, external shell, internal birefringent masses, and gut pigment. These traits were then assessed for their ability to develop in a mosaic and/or regulative manner. Both mosaic and regulative developmental controls function in the organogenesis of allIlyanassalarval organs, with one exception. Since a fully normal intestine was observed in larvae, the fate of the endodermal derivatives from the 4d cell are strictly mosaic. It is clear that the D quadrant is necessary for the normal development of bilateral organs, even though it does not directly contribute to them, and it can regulate the production of these organs when the A, B, or, C quadrant is missing. In addition, more right than left eyes and statocysts were present in BD halves. This may be explained by the presence of cytoplasm in the D quadrant which determines the right side of the larvae. This cytoplasm, present in the CD cell at the 2 cell stage, is partitioned into both the C and D cells at the next division, but normally only effects development of the C quadrant.