Influence on Cleavage Pattern:
* Amount of Yolk: The amount of yolk in an egg determines the pattern of cleavage.
* Microlecithal (little yolk): Eggs with very little yolk, like those of sea urchins and mammals, undergo holoblastic cleavage. This means the entire egg divides into smaller cells.
* Mesolecithal (moderate yolk): Eggs with moderate yolk, like those of amphibians, undergo holoblastic cleavage, but it is unequal. The yolk-rich vegetal pole divides more slowly, leading to larger cells at the vegetal pole and smaller cells at the animal pole.
* Macrolecithal (lots of yolk): Eggs with a lot of yolk, like those of birds and reptiles, undergo meroblastic cleavage. Only a small region of cytoplasm at the animal pole divides, forming a disc of cells called the blastodisc. The yolk remains undivided.
Impact on Cleavage Rate:
* Yolk as an Obstacle: Yolk is dense and inert, making it difficult for the cleavage furrows to penetrate. This slows down the rate of cleavage in eggs with a lot of yolk.
Consequences for Development:
* Cell Size and Number: The amount of yolk influences the size and number of cells produced during cleavage. More yolk leads to fewer but larger cells.
* Blastula Formation: The pattern of cleavage determines the structure of the blastula, the early embryonic stage. In holoblastic cleavage, the blastula is a hollow sphere. In meroblastic cleavage, the blastula is a disc of cells sitting on top of the yolk.
* Developmental Potential: The differences in cell size and distribution of yolk create regional differences in the embryo. This contributes to the establishment of the body axes and the developmental potential of different regions.
Examples:
* Sea urchins: With little yolk, they undergo rapid holoblastic cleavage, resulting in a hollow blastula.
* Frogs: With moderate yolk, they undergo slower holoblastic cleavage with unequal cell sizes, resulting in a blastula with a distinct animal and vegetal pole.
* Chickens: With a lot of yolk, they undergo meroblastic cleavage, resulting in a blastodisc that eventually forms the embryo.
In conclusion, the yolk plays a crucial role in shaping the pattern and rate of cleavage, which ultimately influences the early developmental processes and the formation of the embryo.