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 and anlmal cells are totipotent.

Answer

To determine which statement is correct among the given options, let's analyze each statement step by step.

### Step 1: Understand the first statement - "Ontogeny repeats phylogeny."
- **Explanation**: This statement is related to the Recapitulation Theory of Biogenetic Law. According to this theory, the development of an embryo (ontogeny) reflects the evolutionary history of the species (phylogeny). In simpler terms, the embryonic development stages of an organism resemble the adult forms of its evolutionary ancestors.
- **Conclusion**: This statement is considered correct.

### Step 2: Analyze the second statement - "Stem cells are specialized cells."
- **Explanation**: Stem cells are unique because they have the ability to differentiate into various specialized cell types. However, stem cells themselves are not specialized; they are undifferentiated cells that can give rise to specialized cells.
- **Conclusion**: This statement is incorrect.

### Step 3: Evaluate the third statement - "There is no evidence of existence of gills during embryogenesis of mammals."
- **Explanation**: During the embryonic development of mammals, there is indeed evidence of structures that resemble gills, known as pharyngeal arches. These structures indicate a common ancestry with aquatic organisms.
- **Conclusion**: This statement is incorrect.

### Step 4: Consider the fourth statement - "All plants and animal cells are totipotent."
- **Explanation**: Totipotent cells are those that can develop into any cell type, including an entire organism. However, not all plant and animal cells possess this capability; only certain cells, like the fertilized egg and some early embryonic cells, are considered totipotent.
- **Conclusion**: This statement is incorrect.

### Final Conclusion:
After analyzing all the statements, the only correct statement is:
- **"Ontogeny repeats phylogeny."**