Which one of the following statements is correct?

Options

A. Stem cells are specialised 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

Explanation

To solve the question "Which one of the following statements is correct?", we need to analyze each of the provided statements carefully.

### Step-by-Step Solution:

1. **Evaluate the First Statement:**
- The first statement claims that "stem cells are specialized cells."
- Stem cells are actually non-differentiated cells that have the potential to develop into specialized cells. They are not specialized themselves; they are capable of dividing and giving rise to various cell types.
- **Conclusion:** This statement is **incorrect**.

2. **Evaluate the Second Statement:**
- The second statement asserts that "there is no evidence of existence of gills during the embryogenesis of animals."
- In fact, during the embryonic development of many animals, including humans, there are stages where structures resembling gills (pharyngeal gill slits) are present.
- **Conclusion:** This statement is **incorrect**.

3. **Evaluate the Third Statement:**
- The third statement states that "all plant and animal cells are totipotent."
- This is not true; only certain cells, like the zygote and some early embryonic cells, are totipotent. Most plant and animal cells are not totipotent and cannot give rise to a whole organism.
- **Conclusion:** This statement is **incorrect**.

4. **Evaluate the Fourth Statement:**
- The fourth statement claims that "ontogeny repeats phylogeny," which is a concept proposed by Ernst Haeckel.
- This statement suggests that the developmental stages (ontogeny) of an organism reflect its evolutionary history (phylogeny). This idea is known as the biogenetic law.
- **Conclusion:** This statement is **correct**.

### Final Answer:
The correct statement is **option number 4**: "ontogeny repeats phylogeny."

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