Explanation

Implantation (i) Zygote divides rapidly by mitotic division. This is called cleavage. As a result 2, 4, 8, 16 daughter cells are produced which are termed as blastomeres (ii) Embryo with 8-16 blastomeres is called a morula (iii) The morula changes into a large mass of cells called blastocyst, which passes further into the uterus (iv) Blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called inner cell mass (v) The trophoblast layer gets attached to the cells of the endometrium and the inner cell mass gives rise to the embryo (vi) The cells of endometrium divide rapidly and cover the blastocyst (vii) So, the blastocyst gets embedded in the endometrium of the uterus. This is called implantation, which leads to pregnancy