Parthenogenetic or gynogenetic embryos, in which development is initiated only by the nucleus derived from the female parent without the nucleus of the male parent, fail to develop normally and stop developing at an early embryogenic stage. Elucidating the cause of this was very important to fully understand the mechanism of development. In the two papers presented here, we will read about early studies that addressed this difficult question. At the same time, we will also consider the factors that seriously inhibit the research.