Describe how the three layers of the embryo are formed from a single layer of cells in the blastula.

In some animals that lay eggs, such as frogs, gastrulation is accomplished when the hollow blastula invaginates to form an inner layer of cells called the endoderm and an outer layer of cells called the ectoderm ... These three layers, the endoderm, the ectoderm and the mesoderm, are called the primary germ layers.