Regulative and mosaic development ofIlyanassa obsoletaembryos lacking the A and C quadrants

Abstract

Summary The A and C blastomeres at the 4 cell stage of Ilyanassa obsoleta were 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 all Ilyanassa larval 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...