The first cleavage is a normal meridonal division. During the second cleavage, however, one of the two blastomeres divides meridonally and the other equitorally. The blastomeres can divide at different times which makes it possible for them to have an odd number of cells. In addition, the mammalian genome becomes active during early cleavage. Compaction occurs during the third cleavage.
Cells maximize contact with each other and begin to spread on each other.
Later in cleavage the embryo divides its cells, with most becoming the trophoblast, an external group of cells that produces no embryonic structures. The remaining inner cells form the inner cell mass, which are all pluripotent and will give rise to the actual embryo. Invagination, the infolding of a region of cells, occurs at the blastopore which is directly opposite the point of sperm entry. As the cells continue to invaginate, a gastrocoel is created which overtakes the former blastocoel that was there. At this point, the 3 layers of germ cells can be recognized, endoderm, mesoderm, and ectoderm.