Final answer:
The endodermal and ectodermal germ layers form during gastrulation, a stage following the blastula in embryonic development. It sets the foundation for later organ development.
Step-by-step explanation:
The formation of endodermal and ectodermal germ layers occurs at gastrulation. After the blastula stage, which is typically a hollow ball of cells, gastrulation begins. This crucial phase involves the reorganization of cells in the blastula to form the three primary germ layers. The ectoderm gives rise to structures such as the nervous system and skin, while the endoderm develops into internal glands and organs. Gastrulation is vital for setting the stage for organogenesis, the process where organs develop from these foundational layers.