Embryonic Development: Cleavage & Formation of Blastocyst -
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.
When the blastomeres divide completely the cleavage is called holoblastic cleavage.
The embryo with 8 to 16 blastomeres is called a morula.
The embryo gradually becomes a 64-celled structure.
A cavity is now formed within the cell mass. This cavity is called the blastocyst cavity or blastocoel.
The embryo is now termed the blastocyst.
The blastocyst is made up of an outer layer of cells called the trophoblast or trophectoderm and an inner cell mass or embryoblast.
The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo.