Question
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Holoblastic cleavage occurs in
A. Insects
B. Frog
C. Birds
D. Mammals

Answer
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Hint: During the process of sexual reproduction after the fusion of male and female gametes, a zygote is formed. The zygote undergoes repeated divisions to grow into a complete organism. The early division of cells in a growing embryo after fertilization is known as cleavage of egg.

Complete answer:
The process of cell division that occurs in a developing zygote at the early stages is known as cleavage. The zygote undergoes rapid cell cycles and it produces a cluster of cells. The cells derived from cleavage are called blastomeres.

The egg yolk, which is a significant portion of the egg and is necessary for the growing embryo. It contains important proteins and choline. The egg yolk is suspended in the portion of the egg white in the form of one or even two spiral bands of tissue that is known as the chalazae. The amount of yolk present in the eggs are significant for the process of cleavage. On the basis of the amount of yolk present in the egg, the cleavage can be of two types which are holoblastic cleavage and meroblastic cleavage.

Meroblastic cleavage occurs in the eggs which contains dense yolk throughout the eggs. The amount of yolk is more in quantity. This type of eggs undergoes incomplete cleavage or incomplete division. In the eggs of fishes, reptiles and birds, the egg yolk is concentrated at one end or pole of the egg. In these types of eggs, the division is incomplete and due to cleavage a disc of cells are formed.

In most of the insect groups, the eggs undergo superficial type of meroblastic cleavage. In these eggs, the yolk is concentrated in the centre and is surrounded by a net like structure.

In mammalian eggs, the quantity of yolk is very less and are isolecithal in nature. The yolk is small in amount and is evenly distributed. In holoblastic cleavage, total or complete cleavage of the egg and it leads to the formation of blastomeres of equal sizes.

Hence, The correct answer is option (D).

Note: In eggs that contain less amount of yolk, the cleavage pattern of the zygote is holoblastic in nature. In this type of cleavage, the yolk is divided into equal, distinct and separate cells which are known as blastomeres. In mammals, the amount of yolk is very less and is centralised in the middle portion of the egg.