Which one of the following statements is correct?

Options

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

Explanation

To determine which statement is correct, we will analyze each of the provided statements one by one.

### Step-by-Step Solution:

1. **Analyze the First Statement:**
- The first statement refers to "ontogeny recapitulates phylogeny," a phrase coined by Ernst Haeckel.
- This concept suggests that the development of an organism (ontogeny) reflects its evolutionary history (phylogeny) and includes intermediate forms of its ancestors.
- **Conclusion:** This statement is correct.

2. **Analyze the Second Statement:**
- The second statement claims that "stem cells are specialized cells."
- However, stem cells are actually unspecialized cells that have the potential to differentiate into various specialized cell types.
- **Conclusion:** This statement is incorrect.

3. **Analyze the Third Statement:**
- The third statement mentions that "there is no evidence of the existence of gills during the embryogenesis of mammals."
- In fact, mammalian embryos do exhibit structures known as pharyngeal arches, which are analogous to gills in fish, during early development.
- **Conclusion:** This statement is incorrect.

4. **Analyze the Fourth Statement:**
- The fourth statement asserts that "all plant and animal cells are totipotent."
- Totipotent cells are those that can give rise to an entire organism, and not all plant or animal cells have this capability. Only certain cells can become totipotent under specific conditions.
- **Conclusion:** This statement is incorrect.

### Final Conclusion:
- The only correct statement among the options provided is the first one: "ontogeny recapitulates phylogeny."