Describe the process of implantation of embryo.

The mitotic division starts as the zygote moves through the isthmus of the oviduct called cleavage towards the uterus and forms $2,4,8,16$ daughter cells called blastomeres.

The embryo with $8$ to $16$ blastomeres is called a morula.

The morula continues to divide and further moves into the uterus and transforms into blastocyst.

The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and inner group of cells attached to trophoblast called the inner cell mass.

All these changes occur in a week's time.

The creation of blastocyst located in morula is done by cells of trophoblast.

Enzymes are secreted from the cells of endoderm which includes some tissues and blood vessels of walls of uterus and makes implantation possible.

The inner wall of uterus develops and partially surrounds embryo. This process is called implantation.

$\because$ The uterine cells divide rapidly and covers the blastocyst. It is called implantation and it leads to pregnancy.