Question

Explain the process of early cleavage till the formation of morula.

Answer 1

Definition: The process of rapid mitotic division of single celled zygote to form hollow, spherical, multicellular blastula is called cleavage. 

Cleavage occurs during its passage through the Fallopian tube to the uterus. In humans, cleavage is complete or holoblastic and equal.

1^st Cleavage:
It is completed about 30 hours after fertilization.
It is vertical, i.e. it takes place from animal pole to vegetal pole.
Two daughter cells or blastomeres are formed. 
2^nd Cleavage:
It is completed about 60 hours after fertilization.
The second cleavage is also vertical, but at right angle to the first one.
The second cleavage occurs first in one blastomere resulting in  transitory three-celled stage followed by four cell stage. 
3^rd Cleavage:
It takes place about 3 days (72 hours) after fertilization.
It is longitudinal and horizontal and forms 8 cells.
The cleavage continues and results in the formation of a solid mass of cells known as Morula.

Morula:
Morula is about 16 to 32 celled stage, which looks like a small mulberry. It reaches the uterus 4 - 6 days after fertilization. It is still surrounded by Zona pellucida. Zona pellucida prevents implantation at abnormal site. It does not expose trophoblast cells till it reaches implantation site.