Embryogenesis and Germ Layer Formation (2C) Practice Test

In an early embryo, investigators observe that a subset of cells transitions from a tightly packed epithelial organization at the surface to a more migratory, loosely associated population that occupies a middle position between outer and inner layers. Later, these cells contribute to supportive and contractile tissues. Based on the passage, which germ layer identity is most consistent with the described middle population?

Baseline: After gastrulation, ectoderm is generally outer, endoderm inner, and mesoderm occupies an intermediate position and contributes to muscle and connective tissues.

A chick embryo is examined at a time when three germ layers are present. A micropipette delivers a lineage tracer to the innermost epithelial layer. Later, tracer is found in glandular tissue that secretes digestive enzymes into the gut lumen. Based on the passage, which tissue is most likely labeled?