Embryo Compacting

[gengwall]

I know at about four days after fertilization the group of basically autonomous blastomeres suddenly becomes interconnected chemically and physically by "compacting". My question is, does anyone know what triggers this? I know there are genes being turned on and off, but there must be something that causes them to do so. Do we know if it is a timing thing; a critical mass of cells thing; a chemical clock thing?