Question

What is true about cleavage in the fertilized egg of humans?
(a) Meroblastic
(b) Starts when egg reaches the uterus
(c) Starts in the fallopian tube
(d) It is identical to normal mitosis

Answer 1

Hint: In fertilisation, the fusion of male gamete and female gamete forms zygote. It is a diploid unicellular cell which will give rise to a multicellular foetus in the female’s body by a series of successive divisions called ‘cleavage.’

Complete answer:
After fertilisation, a series of rapid mitotic divisions called cleavage takes place. These mitotic divisions are unlike any normal mitosis because only the number of cells is increasing without any increase in the mass. Thus, no significant growth can be observed in cleavage except the division of the cytoplasm of zygote equally among numerous smaller cells or morula. Little or no yolk is present in humans and thus the cleavage type is holoblastic i.e cytokinesis divides the zygote completely into two halves and so on. In humans, cleavage begins in the fallopian tube and morula is achieved after the 4th day of fertilisation.
- Fertilisation occurs in the isthmus portion of the fallopian tube.
- It is followed by cleavage of zygote.
- Cleavage transforms the zygote into a morula. It is characterised by a solid mass of cells with 16- 32 cells.
- This will lead to a blastula stage or hollow mass of cells. A fluid- filled cavity inside the ball of cells is called the blastocoel surrounded by an outer layer ‘trophoblast’ and an inner layer ‘inner cell mass.’
- The trophoblast layer extends into the endometrial lining of the uterus by breaking down the molecules of the endometrium.
- This is then followed by the gastrulation stage wherein the three germ layers are originated by the rearrangement of cells into endoderm, mesoderm and ectoderm. And this ball of cells is known as gastrula.
- After implantation, the placenta is formed.
So, the correct answer is ‘ Starts in the fallopian tube.’

Note:
The placenta acts as a barrier between the foetus and the mother. Although the blood of foetus and mother do not mix or blend, the placenta acts as an ultrafilter where soluble nutrients, minerals, hormones, antibodies, can cross over and pass onto the foetus. It also helps in the exchange of gases between the two as well as the elimination of nitrogenous wastes from the foetus.