Question

What is true for cleavage: -
A. Size of embryo increases
B. Size of cells decreases
C. Size of cells increases
D. Size of embryo decreases

Answer 1

Hint: Reproduction is a process in which organisms form offspring. The processes in reproduction are pre fertilization, fertilization and post fertilization events. Cleavage is a post fertilization event that occurs after the formation of Zygote.

Complete Answer:
- The mitotic division starts as a zygote moves through the isthmus of the oviduct called oviduct towards the uterus.Due to cleavage the zygote forms 2, 4, 8 ,16 daughter cells called blastomeres.
- The embryo with 8 to 16 blastomeres is called a morula. The morula continuously divides and transforms into blastocyst as it moves further into the uterus.
- The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner layer group of cells attached to the trophoblast are called the inner cell mass.
- The trophoblast layer then gets attached to the endometrium and the inner cell wall gets differentiated as the embryo.
- After attachment, the uterine cell divides rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation and it leads to pregnancy. So in cleavage the size of the cell decreases.

So the correct answer is (B).

Some additional information:
- Parturition is the process of delivery of foetus.
- Lactation is the production of milk by mammary glands by the end of pregnancy.

Note: The process of childbirth also known as parturition is induced by a complex neuroendocrine mechanism involving cortisol, estrogen and oxytocin.