The archenteron of gastrula is lined by ............... (primary germ layer).

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structure of Gastrula**: The gastrula is a developmental stage in embryogenesis that follows the blastula stage. It is characterized by the formation of three primary germ layers: ectoderm, mesoderm, and endoderm.

2. **Identifying the Archenteron**: The archenteron is the primitive gut that forms during the gastrulation process. It is a hollow structure that eventually develops into the digestive tract of the organism.

3. **Determining the Lining of the Archenteron**: The archenteron is lined by one of the three primary germ layers. In this case, the layer that lines the archenteron is the endoderm.

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