Define the following :
Germ layers .

Explanation

### Step-by-Step Text Solution:

1. **Definition of Germ Layers**: Germ layers are the primary layers of cells in an embryo that develop into different tissues and organs of an organism. They are crucial in the early stages of development.

2. **Types of Germ Layers**: There are three main types of germ layers:
- **Ectoderm**: This is the outermost layer. It gives rise to structures such as the skin, brain, and spinal cord.
- **Mesoderm**: This is the middle layer. It develops into various systems including the circulatory system (blood vessels), muscles, and bones.
- **Endoderm**: This is the innermost layer. It forms the lining of the digestive tract and other internal organs.
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