1. Laboratory of Animal Cell BiologyNational Institute of Agrobiological Sciences Ikenodai 2, Kukizaki‐machi Inashiki‐gun Ibaraki 305–0901 Japan
2. Laboratory of Reproductive Biology and TechnologyNational Institute of Agrobiological Sciences Ikenodai 2, Kukizaki‐machi Inashiki‐gun Ibaraki 305–0901 Japan