Practice question

Primitive groove is a midline depression along primitive streak where epiblast undergoes epithelial-mesenchymal transition regulated by SNAI2, downregulates E-cadherin, ingresses to replace hypoblast and become mesoderm and definitive endoderm. Functionally it serves as portal for internalization, comparable to amphibian blastopore through which bottle cells apically constrict and involuting marginal zone mesoderm enters to form archenteron. Both represent blastoporal openings for gastrulation; chick groove is linear due to massive yolk constraint while frog blastopore is circular, but molecular regulation by Brachyury and Nodal is homologous.