Question

In the following question, a statement of assertion (A) is followed by a statement of reason (R ).
A: Cleavage in the human zygote is holoblastic.
R: Human egg contains an abundant amount of yolk in its cytoplasm.

• A. Both assertion and reason are true and reason is the correct explanation of assertion.
  
• B. Both assertion and reason are true but reason is not the correct explanation of assertion.
  
• C. Assertion is correct, but the reason is incorrect
  
• D. Both assertion and reason are incorrect
  

Answer 1

The correct option is C Assertion is correct, but the reason is incorrect


The mitotic division within the developing zygote is called cleavage. There are different patterns of the cleavage of the zygote in different animals. Common ones are holoblastic cleavage and meroblastic cleavage. The pattern of embryonic cleavage is determined by the position of the mitotic spindle and by the amount and distribution of yolk in the cytoplasm. The yolk is an adaptation of those animals that go isolated from any food supply from the mother body. The animals like placentals have a very small amount of yolk in their egg cytoplasm as they get the nourishment directly from the mother’s body through the umbilical cord. Zygote with relatively little yolk isolecithal(evenly distributed yolk) or mesolecithal (moderate amount of yolk, unevenly distributed) eggs cleaves by the holoblastic method. In holoblastic cleavage, the zygote and blastomeres are completely divided during the cleavage which results in the doubling of blastomeres with each cleavage. This is generally seen in humans, sea urchins etc.
The incomplete cleavage in telolecithal (uneven distribution of yolk in the cytoplasm of ovum) or megalecithal eggs (the amount of yolk present is very large) of animals, such as birds and reptiles is called the meroblastic cleavage.

Figure: Cleavage in embryo