Question

Assertion: Holoblastic cleavage with almost equal sized blastomeres is a characteristics feature of placental mammals
Reason: Egg of most mammals including humans are of centrolecithal type
A. Both assertion and reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct and reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Both assertion and reason are incorrect.

Answer 1

Hint: The process of subdivision of the ovum into smaller cells is called cleavage . These cells are called blastomeres. As cleavage proceeds the ovum comes to have 16 cells. It now looks like a mulberry and is called the morula. There are different types of cleavage based on the amount of yolk present in the egg.

Complete Answer:
Holoblastic cleavage: When cleavage furrow passes through the egg completely. The whole egg divides. This is known as the Holoblastic cleavage. It is found in all the eggs except megalecithal eggs. The whole egg divides into blastomeres. No part of the egg remains undivided.
It is of 2 types :-
(a) Equal holoblastic cleavage - In those eggs, in which the amount of yolk is less and it is distributed evenly in the egg, cleavage occurs in the whole egg, blastomeres are of the same size. All the parts of the egg show the same rate of cleavage.
Example: It is found in microlecithal and isolecithal eggs.

(b) Unequal holoblastic cleavage - In those eggs in which the amount of yolk is medium and it is distributed unevenly in the egg. The blastomeres are bigger and less in number where the concentration of yolk is higher in the egg. The part of egg which contains a small amount of yolk, blastomeres here formed are smaller and more in number. The bigger blastomeres are called megameres and smaller blastomeres are called micromeres.
Examples: Unequal holoblastic cleavage is present in mesolecithal and telolecithal eggs and human eggs.
The eggs of most mammals including humans are alecithal, with negligible amount of yolk. In Centrolecithal eggs yolk is present at the center of the egg with cytoplasm around it.

Thus the correct answer is option C: Assertion is correct but reason is incorrect.

Note: The term 'Cleavage' was given by "Von Baer".
- In fertilized egg or activated egg, the egg undergoes repeated cell divisions which occur rapidly, producing a multicellular structure without changing its size. All these rapid mitotic cell divisions are collectively called cleavage or segmentation. Due to the process of cleavage, a single celled zygote, through successive mitotic cell divisions, changes into a complex multicellular structure. The total size of the embryo remains the same. Though the number of blastomeres as a result of mitotic cell divisions increases, the size of blastomeres gradually decreases as compared to parent cells.