The ............... is the cavity of gastrula.

Explanation

To answer the question "The ............... is the cavity of gastrula," we need to identify the specific term that refers to the cavity within the gastrula stage of embryonic development.

### Step-by-Step Solution:

1. **Understanding Gastrulation**: Gastrulation is a crucial phase in embryonic development where the single-layered blastula reorganizes into a multi-layered structure known as the gastrula.

2. **Identifying the Layers**: The gastrula consists of three primary germ layers:
- **Ectoderm**: The outer layer.
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