Which one of the following statement is correct ?

Options

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

Answer

To solve the question "Which one of the following statement is correct?", we will analyze each statement provided in the options and determine their validity.

### Step-by-Step Solution:

1. **Understanding Ontogeny and Phylogeny**:
- **Ontogeny** refers to the development of an individual organism, particularly the embryonic development.
- **Phylogeny** refers to the evolutionary history and relationships among species, essentially the ancestral lineage.

**Hint**: Remember that ontogeny is about individual development, while phylogeny is about evolutionary history.

2. **Evaluating the First Statement**:
- The first statement claims that "ontogeny repeats phylogeny." This is based on the biogenetic law which suggests that the development of an embryo (ontogeny) reflects the evolutionary history (phylogeny) of that organism.
- This statement is generally accepted as correct in biological sciences.

**Hint**: Think about how embryonic development can show features that are reminiscent of ancestral forms.

3. **Evaluating the Second Statement**:
- The second statement claims that "stem cells are specialized cells." This is incorrect because stem cells are undifferentiated and have the potential to develop into various specialized cell types, but they are not specialized themselves.

**Hint**: Recall the definition of stem cells and their role in differentiation.

4. **Evaluating the Third Statement**:
- The third statement asserts that "there is no evidence of existence of gills during embryogenesis of mammals." This is also incorrect, as mammalian embryos do exhibit gill arches during certain stages of development, which are remnants of ancestral gills.

**Hint**: Consider the embryonic development stages of mammals and the presence of structures that indicate evolutionary traits.

5. **Evaluating the Fourth Statement**:
- The fourth statement claims that "all plants and animals are totipotent." This is incorrect because while some plant cells can be totipotent (able to develop into a complete organism), most animal cells are not totipotent.

**Hint**: Differentiate between plant and animal cell capabilities regarding totipotency.

6. **Conclusion**:
- After evaluating all statements, the only correct statement is the first one: "ontogeny repeats phylogeny."

### Final Answer:
The correct statement is: **"Ontogeny repeats phylogeny."**