Compaction. In mammalian embryos such as the mouse, the fertilized egg divides three times to form eight loosely packed cells, which become tightly adherent in a process known as compaction. In the late 1970 s, several laboratories made antibodies against mouse cell surface proteins. The antibodies prevented compaction, as did removal of $\mathrm{Ca}^{2+}$ from the medium. What sort of protein do the antibodies probably recognize, and why?