Read the passage and answer the questions:
Implantation is the embedding of the blastocyst into the endometrium of the uterus. Blastocyst is a ball of cells with a large, fluid-filled cavity called the blastocoel. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and inner cell mass. With the formation of blastocoel, morula is converted to blastula which is called blastocyst in mammals.
The process that transforms the embryo into a three-layered stage is called.....

Options

A. Blastulation
B. Cleavage
C. Gastrulation
D. Organogenesis

Explanation

To answer the question regarding the process that transforms the embryo into a three-layered stage, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Terms**: First, we need to understand the key terms mentioned in the passage. The passage discusses the blastocyst, which is a stage of embryonic development characterized by a ball of cells with a fluid-filled cavity (blastocoel).

2. **Identifying the Stages of Development**: The passage mentions the transition from morula to blastula, which is referred to as the blastocyst in mammals. The blastula stage is important as it precedes the formation of the three-layered structure.

3. **Recognizing the Process**: The question specifically asks about the process that transforms the embryo into a three-layered structure. This three-layered stage is known as the gastrula.

4. **Defining Gastrulation**: The process that leads to the formation of the gastrula is called gastrulation. During gastrulation, the cells of the blastula reorganize to form three distinct layers: the ectoderm, mesoderm, and endoderm.

5. **Selecting the Correct Answer**: From the options provided:
- **Blastulation** refers to the formation of the blastula.
- **Cleavage** refers to the series of cell divisions that occur after fertilization.
- **Gastrulation** is the correct term for the process that transforms the embryo into a three-layered stage.
- **Organogenesis** is the process of organ formation that occurs after gastrulation.

Thus, the correct answer is **gastrulation**.

### Final Answer:
The process that transforms the embryo into a three-layered stage is called **gastrulation**.

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