In which of the following fertilized eggs are covered by hard calcareous shell?

Options

A. Frog
B. Reptiles and birds
C. Mammals
D. Frog and Toad

Answer

To answer the question regarding which fertilized eggs are covered by a hard calcareous shell, we can analyze the options based on the characteristics of the eggs produced by different organisms.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about fertilized eggs that have a hard calcareous shell, which is made of calcium carbonate. This type of shell is typically found in certain animal groups.

2. **Analyzing Frog Eggs**:
- Frog eggs are surrounded by a jelly-like substance, not a hard shell.
- The jelly serves to protect the eggs from the environment but does not provide a hard calcareous covering.
- **Conclusion**: Frog eggs do not have a hard calcareous shell.

3. **Analyzing Reptile and Bird Eggs**:
- Reptiles and birds lay eggs that are indeed covered by a hard shell made of calcium carbonate.
- This hard shell protects the developing embryo inside and is a characteristic feature of their reproductive strategy.
- **Conclusion**: Reptile and bird eggs have a hard calcareous shell.

4. **Analyzing Mammal Reproduction**:
- Most mammals do not lay eggs; instead, they give birth to live young.
- The few exceptions, like monotremes (e.g., platypus), do lay eggs, but these eggs do not have a hard calcareous shell.
- **Conclusion**: Mammals do not produce fertilized eggs with a hard calcareous shell.

5. **Final Conclusion**:
- Based on the analysis, the only group among the options that has fertilized eggs covered by a hard calcareous shell is reptiles and birds. Therefore, the correct answer is reptiles and birds.

### Answer:
The fertilized eggs covered by a hard calcareous shell are found in **reptiles and birds**.

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