Depending on the strain of mice, an average of 8-12 eggs is ovulated per cycle. Fertilization takes place in oviduct around midpoint of dark cycle. The second meiotic division is completed at fertilization and the second polar body is extruded. Zona pellucida - a transparent membrane which protects the egg and later the embryo. Figure 2.20 - Mouse Life Cycle.
Cleavage occurs in oviduct.
Cleavage in the mouse is slow compared to the chick. After the 8-cell stage, the cells of the embryo undergo compaction . During compaction, cells move closer together - so from the outside the whole embryo appears smaller, but the actual area of cell surface contact. It is at the 16-cell stage that cells first become polarized. The blastomeres flatten and increase their surface contact with each other.