Question

Cleavage in human is
A. Equal
B. Unequal
C. Superficial
D. None of the above

Answer 1

Hint: The fusion of cells in the early embryo stage is known as cleavage. Cleavage results in a solid ball of cells known as morula having 8-16 cells. In mammals humans, the rate and pattern of cleavage are non-specific.

Complete answer:
The cleavage in which the egg is divided unequally is called an unequal cleavage. 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. It is holoblastic (complete) and gives rise to the blastomeres; one of which is slightly larger than the other.
i. Holoblastic: The cleavage in which the segmentation line passes through the entire egg, dividing it completely. It occurs in alecithal, microlecithal and mesolecithal eggs. Holoblastic cleavage is of two types:
a. Equal Holoblastic: In this process, the egg is divided into two equal halves.
b. Meroblastic cleavage: The lines of segmentation do not completely pass through the egg and remain confined to a part of the egg. Such type of cleavage is found in megalecithal eggs as the yolk provides resistance to the cleavage. Insects, birds, reptiles are examples of meroblastic cleavage.
ii. Superficial cleavage occurs in centrolecithal eggs of insects.
So, the correct answer is option B.

Note: All cleavage divisions are mitotic and resultant daughter cells are blastomeres and the total size and volume of the embryo remain the same because in cleavage divisions interphase is without growth phase. During cleavage, the size of blastomeres keeps on decreasing, as there is no growth of blastomeres.