Practice question

Comparative embryology recognizes primitive groove in chick as homologue of amphibian blastopore, both marking site where surface cells internalize to form internal germ layers. In chick epithelial cells ingress through groove, in amphibian marginal zone involutes over blastopore lip forming archenteron roof. Both structures appear dorsally, express Brachyury and organizer markers, involve involution/ingression movements, and connect outside to interior cavity. Archenteron, neural tube and notochord are later derivatives not homologous groove structures. Therefore primitive groove equivalent to blastopore reflecting conserved gastrulation topology. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.