What structure is indicated by the arrow "3"?

Question

BlackTom AI · Accepted Answer

To interpret the labeled diagram, I’ll evaluate what each potential structure corresponds to in a chordate embryo.
Option: post-anal tail — This is the extended tail region beyond the anal opening. It is not a distinct internal rod and is usually a continuation of the body, not a compact midline structure marked by an arrow.
Option: intestine — The gut tube would be a hollow tube running along the ventral midline, connected to the stomach and intestines. The arrow in this position is not pointing at the gut tube; it points to a compact rod-like structure.
Option: nerve cord — The dorsal nerve cord lies just above the notochord in the embryo and is also a hollow tubular structure. If the arrow were at the dorsal nerve cord, we’d expect it to point at a tube-like structure near the dorsal midline. Here, the arrow labeled 3 designates a rod-like structure between the gut and the nerve cord, not the nerve cord itself.
Option: notochord — In chordate embryos, the notochord is a flexible, rod-shaped support that runs along the dorsal midline between the gut tube (ventral) and the nerve cord (dorsal). It provides axial support and is classically depicted as a distinct midline rod in diagrams of early development. The position and morphology match a central, rod-like structure labeled by arrow 3.
Option: myotome (muscle segment) — Myotomes are blocks of segmented muscle derived from somites, located lateral to the neural tube and not as a single midline rod. They are typically seen as paired blocks rather than a single central rod.
Option: pharyngeal arch — Pharyngeal arches appear in the head region as segmented external-gacial structures and arches, not as a midline rod along the trunk. They would be located more anteriorly and laterally, not along the midline where arrow 3 points.
Option: atriopore — The atriopore is an opening associated with early chordate circulatory or pharyngeal structures, not a central midline rod. It would not correspond to the labeled straight, elongated structure in the middle of the trunk.
Overall, the arrow 3 points to a central, rod-like structure along the dorsal midline consistent with the notochord, distinguishing it from the other options based on position, morphology, and developmental role.

W25 BIOL D006A 02Y Form/Funct In The Biolgcl Wrld Animal Review Test

What structure is indicated by the arrow "3"?

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