Question

Cleavage in mammals :

• A. Holoblastic equal
• B. Holoblastic unequal
• C. Superficial
• D. Discoidal

Answer 1

The correct option is A Holoblastic equal

The process of cleavage or cellulation happens through repeated mitotic divisions. These divisions result in cells called blastomeres. The mitotic process is very rapid. As the cleavage progresses the resultant daughter cells, namely the blastomeres get reduced in size. During cleavage, there is no growth in the blastomeres. The total size and volume of the embryo remain the same. The cleavages result in a compact mass of blastomeres called morula. It gets transformed into a blastula. While the wall of the blastula is called the blastoderm, the central cavity is called the blastocoel. Types of cleavages:

• Equal holoblastic cleavage - In microlecithal and isolecithal eggs, cleavage leads to the formation of blastomeres of equal size. Eg: Amphioxus and placental mammals.
• Unequal holoblastic cleavage - In mesolecithal and telolocithal eggs, cleavage leads to the formation of blastomeres of unequal size. Among the blastomeres, there are many small-sized micromeres and a few large-sized macromeres.
• Meroblastic cleavage - In this type the cleavage furrows are restricted to the active cytoplasm found either in the animal pole (macrolecithal egg) or superficially surrounding the egg (centrolecithal egg).

So, the correct answer is 'Holoblastic equal'.