Practice question

Avian functional equivalent of amphibian dorsal blastopore lip identified by Waddington via transplantation assays. Hensen's node located at anterior tip of primitive streak expresses organizer markers goosecoid, chordin, noggin, FoxA2 and Shh, homologous to Spemann organizer transcriptome. When transplanted to lateral epiblast it induces secondary embryonic axis with notochord and neural plate recruiting host cells. It also directs left-right asymmetry via Shh and secretes BMP antagonists neuralizing ectoderm. Hypoblast, primitive groove and streak itself lack full organizer inductive capability. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.