Write short notes on the following:
(a) Formation of blastocyst. (b) Implantation of blastocyst.

Explanation

Formation of blastocyst While travelling towards uterus, the zygote undergoes holoblastic, radial and unequal cleavage to form a solid ball of about 8 to 16 cells, called morula, which resembles a " little mulberry ". The cells are called blastomeres, and are of 2 types: peripheral, small and transparent micromeres, and central and larger macromeres. At 16 to 32-celled stage, it enters the uterus on fourth day after fertilization. Then it transforms into a hollow spherical structure called blastocyst. It is formed of outer, single layered trophoblast, and a mass of undifferentiated cells, called inner cell mass or embryonal knob, attached on one side of trophoblast. Cavity of blastocyst is called blastocoel and is filled with a nutritive fluid secreted by endometrial glands of endometrium.