In chordates, what structure extends most of the length of the body during development?

• A. Nerve cord
• B. Spine
• C. Tail
• D. Notochord

Answer: (D)

The notochord runs along nearly the entire length of the body during development, providing axial support and guiding the formation of surrounding tissues. This flexible rod is a hallmark of chordates and helps organize the development of the nervous system and muscles as the embryo grows. In many vertebrates, the notochord is later replaced by the vertebral column, but its presence throughout development is what establishes the elongated body plan characteristic of chordates. The dorsal nerve cord, while important and extending along much of the body, is the nervous system structure rather than the primary axial support. A tail is a posterior extension and does not describe the length of the entire body. The spine forms later in development, not along the full length in the same way the notochord does.