The developing embryo Is a hollow sphere of cells, one cell thick during the ____ stage

Options

A. cleavage
B. polar body
C. gastrula
D. blastula.

Explanation

To solve the question regarding the stage of the developing embryo that is a hollow sphere of cells, one cell thick, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Stages of Embryonic Development**:
- The early stages of embryonic development include cleavage, morula, blastula, and gastrula.
- Each stage has distinct characteristics and structures.

2. **Identifying the Cleavage Stage**:
- The cleavage stage involves the division of the zygote into smaller cells called blastomeres.
- During this stage, the cells are not yet organized into a hollow structure; hence, this option is incorrect.

3. **Considering the Polar Body**:
- Polar bodies are byproducts of oocyte division and do not contribute to the structure of the embryo.
- They provide nutrition to the developing egg but do not form a hollow sphere of cells. Thus, this option is also incorrect.

4. **Examining the Gastrula Stage**:
- The gastrula stage is characterized by the formation of germ layers (ectoderm, endoderm, and mesoderm).
- At this stage, the embryo is no longer a hollow sphere but has differentiated layers, making this option incorrect as well.

5. **Identifying the Blastula Stage**:
- The blastula stage follows the morula stage and is characterized by a hollow sphere of cells, one cell thick.
- This structure is known as the blastocyst in mammals, and it is indeed the correct answer to the question.

6. **Conclusion**:
- The developing embryo is a hollow sphere of cells, one cell thick during the **blastula** stage.

### Final Answer:
The correct answer is **blastula**.