Practice question

In sea urchins gastrulation begins with ingression of primary mesenchyme cells derived from large micromeres entering blastocoel. Vegetal plate then invaginates to form primitive gut archenteron, and opening at vegetal pole where invagination occurs becomes blastopore surrounded by circumblastoporal cells. Since deuterostome fate dictates blastopore persists as anus, initial blastopore remains as larval anus while mouth forms secondarily where archenteron contacts stomodeum. Therefore among early gastrula landmarks, anus represents first functional structure derived directly from blastopore, reflecting ancestral deuterostome patterning principles.