blastodisc

Blastodisc is a disc-shaped region on the surface of certain eggs, most notably in birds and other amniotes, where early embryonic development begins. It forms after discoidal, meroblastic cleavage that occurs in yolk-rich eggs, producing a small disk of cells atop the yolk rather than a fully divided embryo.

In birds, the blastodisc comprises two layers—the epiblast and the hypoblast—separated by a transient subgerminal cavity. The central area pellucida within the disc is the portion where the embryo develops; the surrounding area opaca contributes mainly to extraembryonic tissues such as the yolk sac and associated membranes. As development proceeds, the blastoderm undergoes gastrulation, giving rise to the germ layers, while the yolk supplies nutrition.

The term blastodisc is also used in some other animal groups to describe a patch of cytoplasm at the embryo’s surface during early cleavage. In insects such as Drosophila, for example, the blastodisc can refer to the disk-shaped region containing nuclei during the syncytial blastoderm stage, which later cellularizes to form the cellular blastoderm and ultimately the embryo.

Overall, the blastodisc denotes a regional embryo-forming disk in eggs with restricted, disc-like cleavage, serving as the focal point from which the embryo and certain extraembryonic structures arise.