________ stimulates contraction of the female reproductive tract and helps in the fusion of male and female gametes.

Explanation

### Step-by-Step Solution:

1. **Identify the Function**: The question asks for a substance that stimulates contraction of the female reproductive tract and aids in the fusion of male and female gametes.

2. **Recall Relevant Terms**: In the context of human reproduction, hormones and substances that play a role in the reproductive process need to be considered.

3. **Recognize Prostaglandins**: Prostaglandins are known to stimulate contractions in the female reproductive tract. They are a group of lipid-derived hormones that play a crucial role in various physiological processes, including reproduction.

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