**A Dikaryon is formed when two haploid cells do not fuse immediately.**

**Explanation:**

**Definition of Dikaryon:**
A dikaryon is a cell or organism that contains two genetically distinct haploid nuclei, each with its own distinct nucleus. These two haploid nuclei are derived from the fusion of two compatible haploid cells but do not fuse immediately after fusion. Dikaryotic cells are commonly found in fungi.

**Formation of Dikaryon:**

1. **Haploid Cells:**
- A dikaryon is formed from two haploid cells. Haploid cells contain only one set of chromosomes, representing half the genetic material required for normal cell function.
- In fungi, haploid cells are produced through the process of meiosis, where a diploid cell undergoes two rounds of cell division to produce four haploid cells.

2. **Fusion of Haploid Cells:**
- In certain fungi, two compatible haploid cells come into contact with each other.
- These haploid cells may be of the same mating type (e.g., "+" or "-") or different mating types.
- When these haploid cells come into contact, they undergo a process called plasmogamy, where their cytoplasms fuse together. However, their nuclei remain separate.

3. **Delayed Nuclear Fusion:**
- After plasmogamy, instead of immediately fusing their nuclei together, the two haploid nuclei remain separate in the dikaryotic cell.
- This delayed fusion of nuclei leads to the formation of a dikaryon.
- The two haploid nuclei in the dikaryotic cell are genetically distinct, each with its own set of chromosomes.

4. **Dikaryotic Stage:**
- The dikaryotic cell, containing two distinct haploid nuclei, enters a dikaryotic stage.
- During this stage, the two haploid nuclei coexist and function together within the same cytoplasm.
- This allows for genetic recombination and exchange of genetic material between the two nuclei.

5. **Karyogamy:**
- Eventually, under certain conditions, the two haploid nuclei within the dikaryotic cell may fuse together.
- This fusion of nuclei is called karyogamy and results in the formation of a diploid cell.
- The diploid cell can then undergo meiosis to produce haploid cells, completing the life cycle of the fungus.

**Conclusion:**
A dikaryon is formed when two haploid cells do not fuse immediately after plasmogamy. The delayed fusion of nuclei leads to the formation of a dikaryotic cell, which contains two genetically distinct haploid nuclei within the same cytoplasm.