Which process directly establishes the three germ layers in vertebrate embryos?

• A. Neurulation
• B. Cleavage
• C. Gastrulation
• D. Blastula Formation

Answer: (C)

During gastrulation, the embryo reorganizes from a single-layered structure into three distinct germ layers: ectoderm, mesoderm, and endoderm. This cell choreography sets up the body plan, because each layer will give rise to different tissues and organs—ectoderm forms the skin and nervous system, mesoderm becomes muscle, bone, and the circulatory system, and endoderm lines the gut and associated organs. Neurulation occurs afterward, shaping the nervous system from ectoderm, while cleavage is the early rapid cell division that creates more cells without growth, and blastula formation is the hollow ball formed at that stage. So the process that directly establishes the three germ layers is gastrulation.