The structure which differentiate gastrula from blastula

Options

A. 3 germ layers
B. Micromeres
C. Blastocoel
D. None of these

Explanation

### Step-by-Step Solution:

1. **Understanding the Stages of Development**:
- The development of an embryo progresses through various stages, including the blastula and gastrula stages.
- The blastula is characterized as a hollow sphere of cells, while the gastrula is the next stage in development.

2. **Identifying Key Features**:
- The blastula stage consists of a single layer of cells surrounding a fluid-filled cavity called the blastocoel.
- The gastrula stage, however, is more complex and involves the formation of three distinct germ layers.

3. **Defining Germ Layers**:
- The three germ layers formed during the gastrula stage are:
- **Ectoderm**: The outer layer, which eventually develops into the skin and nervous system.
- **Mesoderm**: The middle layer, which forms muscles, bones, and the circulatory system.
- **Endoderm**: The inner layer, which develops into internal organs such as the digestive tract and lungs.

4. **Conclusion**:
- The primary structure that differentiates the gastrula from the blastula is the presence of these three germ layers (ectoderm, mesoderm, and endoderm).
- Therefore, the correct answer to the question is "3 germ layers."