Practice question

Avian eggs accumulate enormous yolk mass vegetally, leaving only a small ooplasmic cap called blastodisc at the animal pole where nucleus and organelles reside. Yolk physically prevents cytokinesis through vegetal region, so cleavage furrows remain confined to blastodisc, leaving yolk undivided. This incomplete division is termed meroblastic. Because daughter cells spread as a flat disc atop yolk rather than cutting whole egg, pattern is discoidal. Contrasting with holoblastic cleavage in isolecithal mammalian or amphibian eggs, discoidal meroblastic cleavage enables formation of multilayered blastoderm with epiblast, hypoblast and subgerminal cavity essential for later gastrulation.