Final answer:
The first major event in organogenesis is neurulation, where the central nervous system begins to form. Gastrulation precedes neurulation and leads to the development of the three primary germ layers. Hence, the answer is B) gastrulation.
Step-by-step explanation:
The first major event in organogenesis is neurulation. Neurulation is an embryonic process during which structures that will become the central nervous system start to form. Following gastrulation, specialized neuroectodermal tissues thicken into a neural plate, which then folds to form the neural tube. This tube will give rise to the spinal cord, while the anterior part develops into the brain.
Organogenesis is the development of organ rudiments from the three primary germ layers: ectoderm, mesoderm, and endoderm. After the process of neurulation, somites form next to the neural tube, and these will differentiate into components of the axial skeleton, skeletal muscle, and dermis.
Therefore, the correct answer to the question 'The first major event in organogenesis is ___' is B) gastrulation, which leads to the formation of the three germ layers that facilitate organogenesis.