6. Centrolecithal eggs show
   (1) superficial cleavage
   (2) displaced radial cleavage
   (3) bilateral cleavage
   (4) discoidal cleavage

Introduction

Centrolecithal eggs are a type of egg where the yolk is centrally located within the cytoplasm. This yolk arrangement impacts the pattern of cleavage during early embryonic development. Centrolecithal eggs are commonly found in many arthropods, and their cleavage pattern is distinct due to the large yolk mass at the center.

What is Cleavage in Centrolecithal Eggs?

• Because the yolk is concentrated in the center, cell division cannot occur throughout the egg as it would hinder progress.

• Instead, cleavage is meroblastic, meaning partial cleavage.

• Specifically, centrolecithal eggs undergo superficial cleavage, where cleavage divisions occur only in the peripheral cytoplasm surrounding the central yolk.

• During early development, multiple nuclei form inside the cytoplasm, and cytokinesis partitions these on the surface, producing many cells around the yolk.

Biological Importance of Superficial Cleavage

• This cleavage allows rapid nuclear division without disrupting the large central yolk, which serves as a nutrient reservoir.

• The peripheral layer of cells formed eventually contributes to the embryo’s body, while the yolk provides nourishment.

• Superficial cleavage accommodates the yolk size to facilitate efficient embryonic development in yolk-heavy eggs.

Contrast with Other Cleavage Types

• Displaced radial cleavage is seen in mesolecithal eggs like amphibians with moderate yolk.

• Bilateral cleavage occurs in some tunicates and mollusks, defining symmetry differently.

• Discoidal cleavage is characteristic of telolecithal eggs (birds and reptiles) where cleavage is confined to a small disc of cytoplasm at the animal pole.

Conclusion

Centrolecithal eggs exhibit superficial cleavage, a specialized meroblastic cleavage confined to the peripheral cytoplasm around a central yolk. This pattern is crucial for accommodating the large yolk while allowing early embryonic cell divisions, especially in arthropods.

This article provides insight into the unique cleavage pattern of centrolecithal eggs, emphasizing the importance of yolk distribution in embryogenesis.

Final Answer:
(1) superficial cleavage