Why is yolk-sac not found in human female?

Explanation

**Step-by-Step Solution:**

1. **Understanding the Yolk Sac**: The yolk sac is an extra-embryonic structure that is typically found in the early stages of development in many vertebrates, including some mammals. It serves as a source of nutrients for the developing embryo before the placenta is fully formed.

2. **Presence in Early Pregnancy**: In human females, the yolk sac can be observed during the early stages of pregnancy, particularly around the fifth week, through ultrasound imaging. It acts as an initial indicator of pregnancy.

3. **Role of the Yolk Sac**: The primary function of the yolk sac is to provide nutrients to the embryo until the placenta is developed enough to take over this role.

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