AIIMS 2003 Zoology Embryonic Development Assertion Reason Question

To tackle the given question on holoblastic cleavage and egg types in mammals, we need to analyze both the assertion and the reason provided.

Assertion

Holoblastic cleavage with almost equal sized blastomeres is a characteristic of placental animals.

Explanation of Assertion: Holoblastic cleavage refers to a type of cleavage where the entire egg is divided into smaller cells known as blastomeres. In placental animals (such as humans), the cleavage is typically holoblastic and results in blastomeres that are roughly equal in size. This type of cleavage allows for more uniform distribution of cytoplasmic constituents and is essential for proper embryonic development.

Reason

Eggs of most mammals, including humans, are of centrolecithal type.

Explanation of Reason: This statement is incorrect. The eggs of most mammals, including humans, are classified as isolecithal rather than centrolecithal. Isolecithal eggs have a small amount of uniformly distributed yolk, which leads to holoblastic cleavage. In contrast, centrolecithal eggs have a large amount of yolk concentrated at the center of the egg, which typically leads to incomplete cleavage (meroblastic cleavage).

Conclusion about Correct Answer

Given the analysis:

• The assertion is true because it correctly describes the cleavage pattern in placental animals.
• The reason is false because it inaccurately categorizes mammalian eggs as centrolecithal.

Correct Answer

Thus, the correct answer is C) Assertion is true but Reason is false.

Clarifications for Other Options:

• Option A: Both Assertion and Reason are true and Reason is the correct explanation of Assertion. (Incorrect because the Reason is false.)
• Option B: Both Assertion and Reason are true but Reason is not the correct explanation of Assertion. (Incorrect since the Reason is false.)
• Option D: Both Assertion and Reason are false. (Incorrect as the Assertion is true.)

Summary

In summary, the assertion correctly states a characteristic of placental animals, while the reason misclassifies the type of eggs in mammals. This distinction is crucial for understanding embryonic development in different species.