Practice question

Classic organizer experiment by Waddington grafted Hensen's node from donor chick embryo into lateral area pellucida of host embryo. Donor node retained autonomy expressing Chordin, Noggin, Shh and Goosecoid, antagonizing BMP in surrounding host epiblast inducing ectopic neural plate marked Sox2 and ectopic notochord. Host developed secondary embryonic axis complete with somites and neural tube parallel to primary axis. Result proved avian node equivalent to Spemann organizer, capable of neural induction and dorsal mesoderm patterning, demonstrating conserved mechanism of secondary axis induction via BMP antagonism across vertebrates.