blastodisk

Blastodisk is a disk-shaped region on the surface of certain yolk-rich eggs, notably in birds and some reptiles, where early embryonic cell divisions occur. In telolecithal eggs, cleavage is discoidal, taking place only in this limited area on top of the yolk, while the surrounding yolk remains undivided.

During development, the blastodisk forms a two-layered blastoderm as cells divide within this disk. The upper layer becomes the epiblast, which gives rise to the embryo proper, while the lower layer is the hypoblast. The region outside the blastodisk, known as the area opaca, does not participate in initial cleavage and contributes to surrounding extraembryonic tissues. As development proceeds, gastrulation and subsequent morphogenesis reorganize these cells to establish the body plan.

The blastodisk thus functions as the primary site of embryogenesis in these eggs, with nutrients provided by the yolk. Its organization and development are central topics in comparative embryology, illustrating how different patterns of cleavage influence early formation in vertebrates and invertebrates.

In practical terms, the blastodisk is a foundational concept for understanding discoidal cleavage and the progression from a disk-shaped blastoderm to a structured embryo. Because the process varies among species, the term is most often used in discussions of avian and reptilian embryology, where the disk-like region determines the initial arrangement of embryonic and extraembryonic tissues.