which of the following is correct regarding cleavage division?

Options

A. cleavage divisions are resulting in formation of blastomeres
B. cleavage divisions in mammals are slow and synchronous
C. during cleavage there is increase in DNA :cytoplasm ratio
D. zona pellucida breaks after 3rd cleavage division

Answer

To solve the question regarding cleavage division, we will analyze each statement provided in the options and determine their correctness based on our understanding of cleavage division in embryology.

### Step-by-Step Solution:

1. **Understanding Cleavage Division**:
- Cleavage division refers to the series of rapid cell divisions that occur in the zygote after fertilization. These divisions lead to the formation of smaller cells called blastomeres.

**Hint**: Remember that cleavage is the initial stage of embryonic development following fertilization.

2. **Analyzing the First Statement**:
- The first statement claims that cleavage division results in the formation of blastomeres. This is correct because during cleavage, the zygote divides into multiple smaller cells known as blastomeres.

**Hint**: Focus on the definition of cleavage and its outcome in terms of cell division.

3. **Analyzing the Second Statement**:
- The second statement suggests that cleavage division in mammals is slow and synchronized. While it is true that cleavage in mammals is slow, it is actually asynchronous, meaning that the divisions do not occur at the same time. Therefore, this statement is incorrect.

**Hint**: Distinguish between synchronous and asynchronous divisions in mammalian cleavage.

4. **Analyzing the Third Statement**:
- The third statement indicates that during cleavage, there is an increase in the DNA to cytoplasm ratio. However, during cleavage, the amount of cytoplasm remains relatively constant while the number of nuclei (due to cell division) increases. Thus, this statement is also incorrect.

**Hint**: Consider how cell division affects the ratio of DNA to cytoplasm during cleavage.

5. **Analyzing the Fourth Statement**:
- The fourth statement claims that the zona pellucida breaks after the third cleavage. In reality, the zona pellucida remains intact until the completion of cleavage, at which point it breaks down. Therefore, this statement is incorrect.

**Hint**: Recall the role of the zona pellucida and when it breaks down in relation to cleavage stages.

6. **Conclusion**:
- After analyzing all the statements, we find that only the first statement is correct regarding cleavage division.

### Final Answer:
The correct answer is the first statement: "Cleavage division results in the formation of blastomeres."