At which stage of development are the three germ layers first present?

The presence of the three germ layers—ectoderm, mesoderm, and endoderm—first occurs during the gastrulation stage of embryonic development. This developmental phase is characterized by a significant reorganization of the blastoderm, which leads to the formation of these germ layers that will subsequently give rise to various tissues and organs of the body.

During gastrulation, cells migrate and differentiate to establish these foundational layers. The ectoderm will develop into structures such as the skin and nervous system, the mesoderm will form muscles, bones, and the circulatory system, and the endoderm will contribute to the gastrointestinal tract and other internal organs. Prior to gastrulation, in stages like the zygote and morula, the embryo is not yet organized into these specific germ layers, and during the blastocyst stage, implantation occurs, but the differentiation into the three germ layers hasn’t yet taken place. Therefore, the correct answer is that the three germ layers are first present during the gastrula stage.