ACT Science Exam Prep

Answer: E - Organ differentiation in an embryo occurs during the gastrula stage. It is during gastrulation that the three germ layers arise.

Key Takeaway: The process of gastrulation results in the formation of the three germ layers, ectoderm endoderm, and mesoderm. It is these three layers that give rise to all the tissues and organs of the body. The endoderm forms the tissues of the digestive system as well as the lungs and thyroid. The ectoderm forms the central nervous system, epidermis, hair, and mammary glands. The mesoderm forms the skeleton, skeletal muscle, heart, blood, and spleen.