Question

The type of cleavage in a zygote depends upon
A. Amount of yolk
B. Amount of cytoplasm
C. Size of nucleus
D. All of the above

Answer 1

Hint: Blastomeres are the cells derived from cleavage. They form a morula which is a compact mass of cells. Morula further forms blastula which is the end stage of cleavage. Cleavage can be of two types namely, holoblastic (which is total or entire cleavage) or it may be meroblastic (also called partial cleavage).

Complete answer:
Cleavage, in developmental biology, is referred to as the division of cells in early embryonic stages. After cleavage, fertilization occurs which involves cyclin dependent kinase complex activation. In the majority of species, rapid cell cycles are carried out with no considerable overall growth. As a result clusters of cells are produced having size same as that of original zygote. The cleavage is determined by the amount of yolk present in the zygote. Yolk is present in very less amount in placental mammals therefore in that case mother provides nourishment for the developing embryo, hence the zygote goes through holoblastic cleavage. On the other hand, in species like birds, more than sufficient amount of yolk is present for providing nourishment to the embryo for their development, hence they go through meroblastic cleavage.

So, the correct answer is option “A” that is the type of cleavage in a zygote depending upon the amount of yolk.

Note: Vegetal pole is the pole of the egg having highest yolk concentration while animal pole consists of minimum concentration of yolk. Also, cell division and cleavage are not the same as in cleavage, however the nuclear mass and number of cells increases but the cytoplasmic mass remains constant.