Figure 5. Calculation method for measuring the size of each part of the oocyte. ZP: Zona pellucida. PS: perivitelline space. PB: 1st polar body. Diameter of cytoplasm (A) (A1+A2) / 2. Inner diameter of zona pellucida (B) = (B1+B2) / 2. Outer diameter of zona pellucida (C) = (C1+C2) / 2. Thickness of zona pellucida = (C B) / 2. Size of perivitelline space = (B A) / 2. Yoshida & Niimura (2010) with permission.