Definition of Cleavage
A newly fertilized ovum with polar bodies in the zona pellucida on the left. Once cleavage begins, the cells double in number with each division. A zygote at the four cell stage on the right. Note that in early cleavage, the zygote is still contained within the zona pellucida.
Compaction occurs when the zygote reaches the nine cell stage and the blastomeres change shape and tightly align themselves with one another.
This allows for greater cell to cell interaction and subsequent re-aggregation of the cells into inner and outer cell masses.
It is probably mediated by adhesion glycoproteins.