Which one of the following statement is correct :

Options

A. Stem cells are specialized cells
B. There is no evidence of the existence of gills during embryogenesis of mammals
C. All plant and animal cells are totipotent
D. Ontogeny repeats phylogeny

Answer

To solve the question "Which one of the following statements is correct?", we will analyze each option provided.

### Step-by-Step Solution:

1. **Analyze Option 1: "Stem cells are specialized cells."**
- Stem cells are not specialized; they are undifferentiated cells with the potential to develop into various specialized cell types. Therefore, this statement is incorrect.

2. **Analyze Option 2: "There is no evidence of existence of gills during the embryogenesis of mammals."**
- This statement is also incorrect. Research shows that mammalian embryos exhibit structures known as pharyngeal arches, which are analogous to gills in fish during their development.

3. **Analyze Option 3: "All plant and animal cells are totipotent."**
- This statement is incorrect as well. Totipotent cells are those that can give rise to an entire organism, such as the fertilized egg. Not all plant and animal cells have this capability; most are differentiated and specialized.

4. **Analyze Option 4: "Ontogeny repeats phylogeny."**
- This statement is correct. It suggests that the development of an organism (ontogeny) reflects the evolutionary history (phylogeny) of that organism. This concept is often summarized in the phrase "ontogeny recapitulates phylogeny."

### Conclusion:
The correct answer is **Option 4: "Ontogeny repeats phylogeny."**