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How does yolk cause cleavage to become partial in birds, reptiles, and many fishes?

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embryos of different animals — Animal development The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.

In animals such as birds, reptiles, and many fishes, the presence of a large, dense mass of yolk significantly influences the process of cleavage. Because these eggs are megalecithal—containing a large amount of yolk—the yolk acts as a physical barrier that hinders the cytoplasmic movements required for mitosis.1

This obstruction retards or entirely suppresses the onset of cleavage within the yolk-heavy regions of the egg. Consequently, cleavage becomes partial, or meroblastic, meaning only a portion of the egg material is subdivided into cells, while the remainder serves as a nutrient mass for the developing embryo.1

In these specific groups, this results in discoidal cleavage, where cell division is restricted to a small disk of cytoplasm located at the animal pole. The bulk of the yolky egg material remains uncleaved, and the resulting embryo forms as a disk-shaped group of cells resting atop the yolk mass.12