ANATOMY

Detailed explanation-1: -(B) Holoblastic unequal: It is the cleavage which produces unequal size of blastomeres. Example: frog egg, which contains a fair amount of yolk towards the vegetal pole. Because of the concentration of the yolk at the vegetal pole, the blastomeres formed in a different size.

Detailed explanation-2: -Because there is a moderate amount of yolk and unequal distribution of yolk present in the frog egg, the cleavage is of the holoblastic unequal type.

Detailed explanation-3: -Cleavage in most frog and salamander embryos is radially symmetrical and holoblastic, just like echinoderm cleavage. The amphibian egg, however, contains much more yolk. This yolk, which is concentrated in the vegetal hemisphere, is an impediment to cleavage.

Detailed explanation-4: -Humans and Marsupials are an example of unequal holoblastic cleavage. The mode of cleavage is determined by the amount of yolk and it’s distribution. The first cleavage occurs in human zygotes about 30 hours after fertilization.

Detailed explanation-5: -The first cleavage occurs shortly after the zygote nucleus forms. A furrow appears that runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the gray crescent. This divides the egg into two halves forming the 2-cell stage.