Which condition is associated with bilateral symmetry beginning to set in?

• A. White
• B. Gray
• C. Yellow
• D. Red

Answer: (B)

The main idea here is when the body first shows mirror-image organization on the two sides. That early bilateral symmetry becomes visible in the nervous system as paired gray matter regions appear on both sides of the central axis. Gray matter contains neuronal cell bodies, so these symmetrical clusters on the left and right clearly mark the left–right pairing that defines bilateral symmetry. White matter, made of myelinated fibers, surrounds these regions and isn’t the feature that first signals the bilateral arrangement. Since gray matter highlights the mirrored, paired structures, it’s the best cue for this stage of development.