A 55-year-old man has a 5-cm submucosal jejunal mass and is…
A 55-year-old man has a 5-cm submucosal jejunal mass and is resected. The specimen shows nests and trabeculae of round cells with pink granular cytoplasm and salt-and-pepper chromatin. His serum 5-hydroxyindoleacetic acid (5-HIAA) level is markedly increased. Which of the following embryologic derivatives is most likely related to the cells in the mass?