An undefined or undifferentiated fibrillar nucleus is seen in

Options

A. Eukaryotic cells
B. Prokaryotic cells
C. Cells of higher organisms
D. Cells of higher plants

Explanation

To answer the question "An undefined or undifferentiated fibrillar nucleus is seen in," we need to analyze the characteristics of different types of cells and their nuclei.

### Step-by-Step Solution:

1. **Understanding Nucleus Types**:
- The nucleus can be classified into two main types based on the presence of a nuclear membrane: eukaryotic and prokaryotic.
- Eukaryotic cells have a defined nucleus that is enclosed by a nuclear membrane.
- Prokaryotic cells, on the other hand, lack a true nucleus and have a region called the nucleoid where the genetic material is located.

2. **Identifying the Nucleus in Prokaryotes**:
- In prokaryotic cells, the genetic material is not enclosed within a membrane. This results in what is described as an "undefined" or "undifferentiated" nucleus.
- The nucleoid in prokaryotes is a fibrillar structure where the DNA is concentrated, but it does not have the organized structure of a eukaryotic nucleus.

3. **Comparing with Other Cell Types**:
- In higher organisms and higher plants, the cells are eukaryotic and contain a well-defined nucleus with a double nuclear membrane.
- Therefore, the nuclei in these cells are not undefined or undifferentiated.

4. **Conclusion**:
- Based on the characteristics discussed, the correct answer to the question is that an undefined or undifferentiated fibrillar nucleus is seen in **prokaryotic cells**.

### Final Answer:
The correct answer is **prokaryotic cells**.