In most single-celled flagellates, the female and the male or the + and the gametes, respectively have the same size. This state is called isogamy. Primitive multicellular organisms in contrast display a tendency towards gametes of different size. The female gamete is larger than the male gamete, a state called heterogamy or anisogamy. In oogamy, the female gamete, also called egg or oogonium has no flagellum. Its nucleus is surrounded by a voluminous plasma containing a lot of nutriments. The male gamete is small, mobile, and contains only a small amount of plasma, just enough to provide the energy required for the movements.
Related Theory
The evolution towards oogamy is accompanied by the evolution of gametophytes that again protect the development of the gametes. While the male gametes leave the gametophyte after ripening, the egg cell becomes the embryo after fertilization that remains in the protection of the gametophyte. The progression from isogamy to oogamy occurred several times in the course of evolution. It exists in the plant kingdom, the animal kingdom, and with fungi.