In frog equal holoblastic cleavage is not found due to

Options

A. A dark pigment in animal pole
B. Heavy yolk in vegetal pole
C. Yolk concentrated in center of egg
D. Nucleus is asymmetrical in position

Explanation

**Step-by-Step Solution:**

1. **Understanding Cleavage Types**: Cleavage in embryonic development can be classified into different types based on the distribution of yolk in the egg. Holoblastic cleavage is where the entire egg is divided into smaller cells, but it can be equal or unequal depending on yolk distribution.

2. **Identifying the Egg Structure**: In frogs, the egg has a distinct structure with a dark pigment located at the animal pole and a heavy concentration of yolk at the vegetal pole. This uneven distribution of yolk is crucial for understanding the type of cleavage that occurs.

3. **Effect of Yolk on Cleavage**: The presence of heavy yolk at the vegetal pole affects the cleavage process. The yolk is dense and does not divide easily, which hinders the formation of cleavage furrows. This results in unequal division of the egg.

4. **Consequences of Uneven Yolk Distribution**: Due to the concentration of yolk at the vegetal pole, the cells that form in this region become larger compared to those in the animal pole. This leads to unequal cleavage, where the cleavage furrows do not divide the egg equally.

5. **Conclusion**: Therefore, the reason why equal holoblastic cleavage is not found in frogs is primarily due to the heavy yolk present at the vegetal pole, which interferes with the cleavage process.

**Final Answer**: The heavy yolk in the vegetal pole.

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