This is much smaller than the Xenopus egg, which is 1-2 mm. The mouse egg is ovulated from the ovary around midpoint of dark cycle. Upon ovulation the egg completes meiosis I and extrudes the first polar body. The ovulated egg enters the infundibulum of the oviduct. 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.