Final answer:
chordin mRNA is first expressed in the region that will become the notochord, which forms the central axis of the animal body.
Step-by-step explanation:
chordin mRNA is first expressed in the region that will become the notochord in vertebrates. The notochord is a rod-shaped structure that forms the central axis of the animal body. It plays a crucial role in the development of the vertebrate body plan, particularly in the formation of the neural tube and the mesoderm.