Question

Cleavage in mammals is:
A. Holoblastic equal
B. Holoblastic unequal
C. Superficial
D. Discoidal

Answer 1

Hint: In mammals the cells which are formed after cleavage are of the same size and appearance. These eggs have less amount of yolk in the cell i.e. they undergo Holoblastic cleavage. So cleavage in mammals is Holoblastic equal.

Complete answer:
The rapid and multiple mitotic cell divisions are called cleavage. When the cleavage occurs the cells do not grow in size, they only increase their number. Cleavage can take place in two ways: Holoblastic (total) cleavage or meroblastic (partial) cleavage. The type of cleavage is decided by the amount of yolk in the eggs. In case of placental mammals where nourishment is provided by the mother’s body, the eggs have a very less yolk and undergo holoblastic cleavage. On the other hand the other species, such as birds, which have a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.

If we talk about superficial cleavage, it is a type of meroblastic cleavage where mitosis occurs without cytokinesis, resulting in production of many nuclei, which migrate to the periphery of a centrolecithal egg. A centrolecithal egg contains the yolk located in the center. Ex: insects

In case of Discoidal cleavage which occurs in birds, fishes and reptiles, this is a meroblastic cleavage where the cytoplasmic divisions are restricted to the animal pole, resulting in the formation of a disc of cells confined at the animal pole or on top of the yolk.

Note: During the cleavage process, the cells of embryo divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells to form a mass of cells which further gets differentiated into specialized cells to form organism.