When and where do chorionic villi appear in humans ? State their function.

Explanation

### Step-by-Step Solution:

1. **Definition of Chorionic Villi**:
- Chorionic villi are finger-like projections that sprout from the chorion, which is the outermost membrane surrounding the developing embryo.

2. **When do Chorionic Villi Appear?**:
- Chorionic villi appear after the implantation of the embryo into the uterine wall. This typically occurs about 6 to 10 days after fertilization.

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