Practice question

Sea urchin blastomeres at four-cell stage possess extraordinary regulative potential because fate depends on conditional interactions rather than fixed determinants. Isolated blastomere initiates compensatory signaling through nuclear accumulation of beta-catenin, activation of Delta-Notch and Nodal pathways, respecifying missing micromere functions and re-establishing endomesoderm gene regulatory network including pmar1 and foxA. Resulting larva is proportionate and fully patterned with gut, skeleton and ciliated band despite originating from single cell, albeit smaller. This experiment by Hans Driesch challenged preformationism and demonstrated embryo as dynamic self-organizing system capable of regulation.