germinal disc

The germinal disc grows to envelop and eventually absorb the yolk, forming the embryo.

Prior to fertilization, the yolk together with the germinal disc is a single cell, one of the few single cells that can be seen by the naked eye.

The germinal disc, also called the blastodisc, is a small, circular, white spot (approximately 2-3 mm across) on the surface of the yellow yolk of a bird's egg.

During embryonic development, transformation of the flat, two-layer germinal disc into a three-dimensional body depends on transition of some cells from an epithelial phenotype to spindle-shaped cells with motile behaviour, a mesenchymal phenotype.