Question

Type of cleavage found in mammals is
a. Holoblastic
b. Meroblastic
c. Superficial
d. None of the above

Answer 1

Hint: During cleavage, the cells split without an increase in mass. A large single-celled zygote divides into numerous smaller cells. In placental mammals where nutrition is provided by the mother's body, the eggs have a very little amount of yolk and go through this specific type of cleavage.

Complete answer:
The cleavages result in a condensed mass of blastomeres called a morula. It gets molded into a blastula. Though the wall of the blastula is called the blastoderm, the middle cavity is called the blastocoel.

In holoblastic cleavage furrow divides the egg fully and the blastomeres formed may be equal or unequal in amount. It happens in alecithal or microlecithal eggs. Normally, the mammalian egg is alecithal, meaning no yolk.

Hence, The correct answer is option (A).

Additional information:
Types of cleavages:
- In microlecithal and isolecithal eggs, cleavage ultimately leads to the configuration of blastomeres of the same size. This is known as Equal holoblastic cleavage. Eg: Amphioxus and placental mammals.
- In mesolecithal and telolecithal eggs, cleavage leads to the formation of blastomeres of unequal size and called Unequal holoblastic cleavage. Among the blastomeres, there are numerous small-sized micromeres and not many large-sized macromeres.
- Meroblastic cleavage - The cleavage furrows are limited to the active cytoplasm found either in the animal pole (macrolecithal egg) or surrounding the egg (centrolecithal egg).

Note: The process of cleavage occurs through repetitive mitotic divisions. This division leads to the formation of blastomeres. The mitotic process is very rapid. As the cleavage progresses, the resulting daughter cells, namely the blastomeres get lesser in size. During cleavage, there is no expansion in the blastomeres. The whole size and volume of the embryo remain the same.