Question

Cleavage in mammals is

• A. holoblastic equal
• B. holoblastic unequal
• C. superficial
• D. discoidal.

Answer 1

The Correct Option is B

The question concerns the type of cleavage that occurs in mammals. Let's break down the concept to understand the correct answer.

Cleavage in Biology:

Cleavage is the process of cell division in the early embryo. In the animal kingdom, cleavage patterns can vary significantly and are categorized based on how the egg divides.

Types of Cleavage:

• Holoblastic Cleavage: This type of cleavage is where the entire egg is divided into smaller cells. There are two main types of holoblastic cleavage:
  • Equal: The egg divides into cells of approximately equal size.
  • Unequal: The egg divides into cells of unequal size.
• Meroblastic Cleavage: This involves partial cleavage where only a part of the egg is divided.
  • Discoidal: Cleavage is restricted to a small disc of cytoplasm on the egg surface.
  • Superficial: Cleavage occurs only in the outer cytoplasmic layer.

Cleavage in Mammals:

In mammals, the cleavage is holoblastic unequal. This is because the egg of mammals contains a small amount of yolk, and the division results in different-sized cells - micromeres and macromeres. This pattern allows for efficient utilization of even a small amount of yolk.

Explanation of Options:

• Holoblastic equal: Not applicable as mammalian cells divide into unequal sizes.
• Holoblastic unequal: Correct answer as this describes the strategy where the complete egg divides into unequal cells, which is characteristic of mammalian embryos.
• Superficial: Incorrect for mammals as it refers to cleavage restricted to the outer layer, typical in some insects.
• Discoidal: Incorrect for mammals, common in birds and reptiles.

Conclusion:

In summary, the type of cleavage that occurs in mammals is holoblastic unequal. This accommodates the unique characteristics of the mammalian egg and results in the differentiation necessary for complex development.