Practice question

Ventral marginal blastomeres normally receive Wnt antagonists and retain active GSK-3 degrading beta-catenin via Axin complex, leading to ventral mesoderm forming blood and mesothelium. Exogenous beta-catenin mRNA injected ventrally bypasses degradation, translocates nucleus, binds TCF3 activating Siamois, Twin and Xnr genes ectopically, creating second Nieuwkoop center and consequently second Spemann organizer expressing Goosecoid and Chordin. This ectopic organizer secretes BMP antagonists inducing secondary neural axis with duplicated notochord and somites, producing conjoined twin embryo joined ventrally. Experiments demonstrate beta-catenin sufficiency for axis induction and respecification.