During the development of embryo, which of the following occurs first?

Options

A. Differentiation of organ
B. Differentiation of tissue
C. differentiation of organ system
D. Differentiation of cells

Explanation

To determine which process occurs first during the development of an embryo, we can break down the stages of embryonic development step by step:

1. **Fertilization**: The process begins with fertilization, where the male and female gametes (sperm and egg) fuse to form a zygote. This is the very first step in embryonic development.

2. **Cleavage**: After fertilization, the zygote undergoes a series of rapid cell divisions known as cleavage. This process does not increase the overall size of the embryo but increases the number of cells.

3. **Differentiation of Cells**: Following cleavage, the cells begin to differentiate. This means that the cells start to take on specific roles and characteristics, leading to the formation of different types of cells.

4. **Differentiation of Tissues**: As cells continue to differentiate, they group together to form tissues. Tissues are collections of similar cells that perform a specific function.

5. **Differentiation of Organs**: The differentiated tissues then combine to form organs, which are structures that perform specific functions within the body.

6. **Differentiation of Organ Systems**: Finally, the organs work together to form organ systems, which carry out complex functions necessary for the survival of the organism.

Given the options provided in the question, the first occurrence during embryonic development is the **differentiation of cells**.

### Final Answer:
The correct answer is **differentiation of cells** (Option 4).