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

Explanation

(1) Cleavage is a rapid mitotic division of zygote to form a blastula.
(2) Immediately after fertilization, the zygote starts cleavage which consists of repeated divisions.
(3) Cleavage converts the zygote into a mass of cells called morulla.
(4) It takes place during the passage of zygote through the fallopian tube to the uterus.
(5) In human beings, cleavage is said to be holoblastic and equal and indeterminate.

(6) Cleavage divisions are rapid with short interphase. There is no time for cells to grow in size therefore cells becomes progressively smaller.
(7) The resulting daughter cells formed by cleavage are called blastomeres. Cleavage shows faster synthesis of DNA.
(8) First cleavage `rarr` vertical `rarr` thirty hours after fertilization `rarr` 2 blastomeres formed.
Second cleavage `rarr` vertical `rarr` at right angles to the first `rarr` 60 hours after fertilization.
Third cleavage `rarr` horizontal/latitudinal `rarr` at right angles to the first two cleavages `rarr` eight-celled stage `rarr` seventy-two hours after fertilization.
(9) Successive divisions produce a solid ball of cells called morula of 16 cells. It consists of an outer layer of smaller clearer cells and an inner mass of larger cells.
(10) Morula reaches the uterus about 4 - 6 days after fertilization.