The first phase of embryonic development is:

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding Embryonic Development**: The question asks about the first phase of embryonic development, which is also known as embryogenesis. This process involves several stages that lead to the formation of a fully developed embryo.

2. **Identifying the Options**: The options provided are:
- Option 1: Cleavage
- Option 2: Blastulation
- Option 3: Gastrulation
- Option 4: Placentation

3. **Analyzing Option 1 - Cleavage**: Cleavage is the initial phase of embryonic development. It involves the rapid division of the fertilized egg (zygote) into smaller cells called blastomeres. This process occurs without significant growth, leading to the formation of a multicellular structure.

4. **Analyzing Option 2 - Blastulation**: Blastulation follows cleavage. During this phase, the cells continue to divide and reorganize to form a hollow sphere of cells known as the blastula. This stage comes after cleavage, so it cannot be the first phase.

5. **Analyzing Option 3 - Gastrulation**: Gastrulation occurs after blastulation. It is the process where the blastula reorganizes into a three-layered structure called the gastrula, which leads to the formation of germ layers. This is also not the first phase.

6. **Analyzing Option 4 - Placentation**: Placentation refers to the formation and structure of the placenta, which occurs later in embryonic development. Thus, it is not relevant to the question about the first phase.

7. **Conclusion**: The first phase of embryonic development is indeed cleavage, as it is the stage where the zygote undergoes rapid cell division.

**Final Answer**: The first phase of embryonic development is cleavage.

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