The mitotic division (cleavage) starts as the zygote moves through the isthmus towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres.
The embryo with 8-16 blastomeres is called a morula.
Morula continues to divide and transforms into blastocyst.
The blastomeres in the blastocyst are arranged into an outer layer (trophoblast) and an inner group of cells (inner cell mass) attached to trophoblast.
The trophoblast layer then gets attached to endometrium and the inner cell mass gets differentiated to 3 germ layers (outer ectoderm, middle mesoderm & inner endoderm).
This 3-layered structure (gastrula) forms the embryo.
After attachment, uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst becomes embedded in the endometrium. This is called implantation.
