Question

The rate of cleavage in a Zygote depends upon

A. Amount of yolk

B. Amount of cytoplasm

C. Size of a nucleus

D. All of the above

Answer 1

Hint: Blastomeres are cells derived from cleavage. They shape a morula that is a compact mass of cells. Morula also forms a blastula, which is the final stage of the cleavage. Cleavage can be of two forms, namely holoblastic (total or complete cleavage) or meroblastic (also called partial cleavage).

Complete answer:

Cleavage is the repeated mitotic division of the zygote resulting in an increasing number of cells. The daughter cells generated by these divisions are termed blastomeres. 

Cleavage involves the division of the embryo at a different stage of development. The yolk is being deposited on the vegetal pole. It provides food for the developing embryo. Yolk prevents mitotic division, so the deutoplasm divides slowly in contrast to the cytoplasm of the animal pole. The amount of yolk deposited increases, the amount of active cytoplasm steadily decreases, and the position of the nucleus influences and prevents mitosis.

Cleavage, in developmental biology, is referred to as the division of cells at an early embryonic level. After cleavage, fertilization occurs which involves the complex activation of cyclin-dependent kinase. In most organisms, rapid cell cycles are carried out without significant overall growth. As a consequence, clusters of cells are formed of the same size as the original zygote. The amount of yolk present in the zygote is determined by the cleavage. The yolk is present in much smaller amounts in placental mammals, thus, in that case, the mother provides nourishment for the embryo developing, and therefore the zygote moves through the holoblastic cleavage. On the other hand, in species such as birds, more than enough yolk is present to feed the embryo for its growth, which is why they move through meroblastic cleavage.

So, choice A- Amount of yolk is the correct answer.

Note: The vegetable pole is the pole of the egg with the maximum concentration of yolk, while the animal pole consists of the lowest concentration of yolk. Cell division and cleavage are not the same as in cleavage, but the nuclear mass and number of cells increase but the cytoplasmic mass remains unchanged.