The event that triggers the formation of ootid is

Options

A. binding of sperm head to receptors ( ZP3) on zona pollucida
B. Entry of sperm into secondary oocyte
C. Cortical reaction
D. Entry of sperm into fallopian tube

Answer

To determine the event that triggers the formation of ootid, we need to understand the process of oogenesis and the specific stages involved in the development of the oocyte.

### Step-by-Step Solution:

1. **Understanding Oogenesis**:
- Oogenesis is the process of egg (ovum) formation in females. It begins with the primary oocyte, which is a diploid (2N) cell.

**Hint**: Remember that oogenesis involves several stages, starting from the primary oocyte.

2. **Meiosis in Oogenesis**:
- The primary oocyte undergoes meiosis, which consists of two meiotic divisions. The first meiotic division results in a secondary oocyte and a polar body. The secondary oocyte is haploid (N).

**Hint**: Focus on the meiotic divisions and their outcomes in the oogenesis process.

3. **Completion of Meiosis II**:
- The secondary oocyte is arrested in the metaphase of the second meiotic division until fertilization occurs. Upon fertilization, the secondary oocyte completes meiosis II, resulting in the formation of an ootid and another polar body.

**Hint**: Consider what happens during fertilization and how it relates to the completion of meiosis.

4. **Triggering Event**:
- The event that triggers the completion of meiosis II and the formation of the ootid is the entry of the sperm into the secondary oocyte. This entry leads to the completion of the second meiotic division.

**Hint**: Identify the specific action that initiates the final steps of oocyte maturation.

5. **Conclusion**:
- Therefore, the event that triggers the formation of the ootid is the entry of sperm into the secondary oocyte.

### Final Answer:
The event that triggers the formation of ootid is the entry of sperm into the secondary oocyte.

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