Frog embryo development. Light microscope time-lapse footage of a frog blastula folding inwards to form a gastrula. After fertilisation, an egg divides multiple times to form a blastula, a hollow sphere made of a single layer of cells. Gastrulation occurs when the single-layered blastula folds inwards and reorganises into a three-layered structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm. Subsequently, individual organs begin to differentiate within the germ layers as the embryo develops into a tadpole.