Once the sperm cell has penetrated the membrane of the egg cell and the chromosomes from each have intermingled, the cell is known as a zygote.The zygote, divide by mitosis within an hour or two of their formation. Cleavage continues until a solid ball of cells is produced termed a morula. The center of the morula subsequently hollows and filled with fluid then morula is called a blastocyst.
The blastocyst consists of two layers of cells (blastomeres):
1) The trophoblast - some of these cells will turn into the chorion--the fetal portion of the placenta. This layer of trophoblastic cells plays an important role in forming a protective shell around the blastocyst and also plays an important role in initiation of implantation due to the sticky character of these cells.
2) the inner cell mass - develops into the embryo and extra embryonic membranes